Evolving anatomy of a high pressure carbonate reservoir (I ...€¦ · 107 valleys in the Natih...

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Evolving anatomy of a high pressure carbonate 1

reservoir (I) – Field observations from Jabal Shams, 2

Oman Mountains 3

4

Marc Holland1, Janos L. Urai1, Emanuel J. M. Willemse2 5

6

1Geologie – Endogene Dynamik, RWTH Aachen, Lochnerstrasse 4-20, D-7

52066 Aachen, Germany 8

2Shell Exploration and Production Company, New Orleans, U.S.A. 9

10

Abstract 11

12

We studied exceptional outcrops of an exhumed high pressure cell in a Cretaceous 13

carbonate reservoir on the southern flank of Jabal Shams/ Oman. This more than 2 14

km thick sediment pile develops a complex and rapidly changing anisotropy, due to 15

mechanical stratigraphy and several generations of pervasive regional fault and 16

fracture sets which were healed by calcite cement before the next set faults or 17

fractures formed. 18

Burial extension within a high fluid-pressure environment led to the formation of four 19

fracture generations by an anticlockwise rotating stress field. This was followed by 20

bedding parallel shear under lithostatic fluid pressure conditions. The high pressure 21

cell was drained along dilatant normal faults which were also repeatedly cemented 22

and reactivated. 23


The rapidly changing mechanical anisotropy in combination with a chemically 24

reactive system form a complex feedback system in which the mechanical strength, 25

strain and the permeability structure undergo major changes in this coupled THM 26

(thermal, hydraulic, mechanical) system. 27

Introduction 28

29

The Oman Mountains offer exceptional outcrop quality over large areas allowing 30

access to structures at a variety of scales. Our study is located on the southern flank of 31

Jabal Shams where several generations of regional vein sets are exposed (Hilgers et 32

al. 2006a). We carried out a detailed structural study integrated with high-resolution 33

satellite images (Holland et al. this volume), field campaigns and the analyses of 34

samples. The studied area exposes Mesozoic carbonate rocks that have been subject to 35

a multiphase deformation at a depth of several kilometers (Al-Wardi 2006, Beurrier et 36

al. 1986, Breton et al. 2004, Glennie 2005, Glennie et al. 1974, Hilgers et al. 2006a, 37

Searle 2007). 38

The structural inventory incorporates veins, joints and normal faults that were studied 39

in the field and mapped on the satellite image for a regional study of these meso-scale 40

features and their spatial distribution (de Keijzer et al. 2007). This is discussed in two 41

parts: The main aim of this first part is to describe and classify the structural elements 42

of the field area. These micro- and mesoscale field observations are used to obtain 43

detailed structural analyses and the temporal sequence. The information is used to 44

discuss aspects of TMH processes (Thermal, Mechanical and Hydraulic) in this high-45

pressure cell (Barton et al. 1985, e.g. Bradley 1975, Bradley 1994, Engelder 1990, 46

Nguyen & Selvadurai 1998, Noorishad et al. 1984, Olsson & Barton 2001, Ortoleva 47


1995, Tsang et al. 2004, Tsang et al. 2000). The sequence of events as well and the 48

corresponding boundary conditions are derived from the fieldwork. 49

The second part of the study (Holland et al. this volume) focuses on the interpretation 50

of satellite images and the spatial distribution of the structures discussed in this first 51

part. The detailed observations of the fieldwork are used to guide the interpretation of 52

the satellite image, which shows the distribution of faults and fractures on a regional 53

scale and discusses whether the observations can be extended laterally. 54

55

56

Geological outline 57

58

Figure 1

59

The study area is located on the southwest flank of Jabal Shams (Figure 2, Figure 3), 60

the highest peak of the Jabal Akhdar domal structure. The Jabal Akhdar dome defines 61

the central part of the Oman Mountains, which extend from the Musandam Peninsula 62

in the north of the Sultanate down to the Batain coast in the southeast of the sultanate 63

(Figure 1). 64

This mountain belt is part of the Alpine-Himalayan chain that formed during 65

northeast-directed subduction and accretion of the Arabic continental plate below the 66

Eurasian plate (Al-Wardi 2006, Beurrier et al. 1986, Breton et al. 2004, El-Shazly et 67

al. 2001, Glennie 2005, Glennie et al. 1974, Hilgers et al. 2006a, Loosveld et al. 1996, 68

Searle 2007). Details of this major geodynamic event such as the extent and number 69

of microplates, the timing of the deformation, the strain partitioning, the direction of 70


the subduction and details on the exhumation are still subject of controversial 71

discussions (Al-Wardi & Butler 2006, Breton et al. 2004, El-Shazly et al. 2001, 72

Glennie 2005, Gray et al. 2005a, Gray et al. 2005b, Gray & Miller 2000, Miller et al. 73

2002, Miller et al. 1999, Pillevuit et al. 1997, Searle 2007, Searle et al. 2005, Searle et 74

al. 2004, Warren & Miller 2007, Warren et al. 2003, Wilson 2000). 75

76

Figure 2

77

Orogenesis started with the intra-oceanic subduction (Breton et al. 2004) in 78

Cenomanian, which led to the emplacement of two major nappes (Breton et al. 2004, 79

El-Shazly et al. 2001, Glennie 2005). These nappes (called the ‘Allochthonous’) 80

comprise the volcano-sedimentary Hawasina complex and the Semail Ophiolites as 81

well as exotic blocks of distal origin (Figure 2) (Beurrier et al. 1986, Glennie et al. 82

1974, Pillevuit et al. 1997, Searle 2007). 83

84

The so-called ‘Autochthon’ comprises the strata below these nappes that was 85

deposited prior to the thrust sheet emplacement and consequently has been affected by 86

the deformation. The ‘NeoAutochthon’ defines the strata deposited after the nappes’ 87

emplacement (Al-Wardi 2006, Breton et al. 2004, Glennie 2005, Glennie et al. 1974, 88

Searle 2007). 89

90

Figure 3

91


The field area exposes the youngest members of the autochthonous unit (Figure 2). 92

Deposited during the breakup of Gondwana a passive margin of the Arabian plate 93

(Breton et al. 2004, Glennie 2005, Loosveld et al. 1996, Searle 2007, Sharland et al. 94

2004) with a relatively continuous sedimentation was present in the region of the 95

Oman Mountains (Hughes Clarke 1988, Searle 2007, Sharland et al. 2004, Ziegler 96

2001). From Thitonian to Turonian a transition from a deepwater facies to an open-97

marine carbonate shelf and shallow marine carbonate platform took place reflected by 98

the Kahmah group and Wasia group (Figure 4). These comprise predominantly 99

carbonates and minor shales (Hughes Clarke 1988, Sharland et al. 2004, Ziegler 100

2001). 101

102

Figure 4

103

After the onset of subduction the carbonate sequence (Kahmah and Wasia group) was 104

uplifted. The uplift is believed to be a response to the flexural bending of the foreland. 105

This uplift in the Throninan (the ‘Wasia-Aruma break’) is documented by incised 106

valleys in the Natih formation (El-Shazly et al. 2001, Filbrandt et al. 2006, Loosveld 107

et al. 1996, Patton & O'Conner 1988, Searle 2007, Warbuton et al. 1990). 108

109

Ongoing subduction in the north led to shallow thrusting and the emplacement of the 110

two autochthonous nappes, renewed burial of the Autochthonous (El-Shazly et al. 111

2001) and the deposition of the Aruma group. The 2.5 km thick carbonate stack 112

(Hilgers et al. 2006a) was subsequently overthrust by the Hawasina and the Semail 113

nappes (Figure 2). 114


The southwest verging nappe emplacement ended either in the early Campanian or 115

Maastrichtian (Breton et al. 2004, El-Shazly et al. 2001, Glennie 2005, Searle 2007) 116

as the thrusting processes ceased when buoyant continental crust stopped the 117

subduction process (El-Shazly et al. 2001). 118

119

The uplift and folding of this part of the Oman Mountains that also forms the present 120

day structure of the Jabal Akhdar dome (Figure 3) is however a result of younger 121

events associated with the subduction beneath the Makran continental margin of Iran 122

(Figure 1) (Breton et al. 2004, El-Shazly et al. 2001, Glennie 2005, Searle 2007). 123

Although localized isostatic uplift and gravitational sliding continued throughout 124

early Tertiary, the main phase of uplift started 4 Ma ago and continues until today 125

leaving a juvenile topography (Figure 3) (Breton et al. 2004, Glennie 2005, Kusky et 126

al. 2005, Searle 2007). 127

128

The regional metamorphic grade in the Oman Mountains as a result of the collision 129

and obduction rises towards the northeast with peak conditions in the region of the 130

Saih Hatat culmination (Figure 2) that shows carpholite, blueshist and eclogite facies 131

rocks (Breton et al. 2004, El-Shazly et al. 2001, Miller et al. 2002, Searle 2007, Searle 132

et al. 2004, Warren & Miller 2007). The southern flank of Jabal Shams as a distal part 133

is however situated in the anchizone (Breton et al. 2004). It shows incipient cleavage 134

in argillaceous units and the onset of pressure solution processes in the carbonates 135

(Al-Wardi 2006, Breton et al. 2004). 136

137

138


Location and conditions of the field area 139

140

The field area is located in the Oman Mountains on the southern flank of its highest 141

peak - Jabal Shams. Jabal Shams is a part of the Jabal Akhdar domal structure; one of 142

a small number of large anticlinal structures (Figure 1, Figure 2, Figure 3). 143

144

Our primary working area is located in and around Wadi Nakhr - the “grand canyon “ 145

of Oman - near the town Al Hamra (Figure 3). The area is 17.5 km in an East/West 146

direction and 5.5 km in North/South direction, covering primarily 75 km2. The 147

morphology is that of an enormous dip-slope on the southern limb the Jabal Akhdar 148

dome (Figure 3). 149

The elevation of the field area ranges from approximately 800 m at Wadi Ghool at the 150

southern margin of the field area to over 2,000 m at the small village Al Hayl at the 151

western rim of the Wadi Nakhr canyon. Wadi Nakhr is the largest of several deep 152

canyons that cut the slope in a predominantly North/South direction. These sub-153

parallel cuts offer impressive vertical profiles. The tallest section at Wadi Nakhr 154

offers a continuous vertical exposure of approximately 1 km showing rocks of the 155

Wasia, Kahmah and Sahtan group (Figure 5, Figure 6, Figure 22, Figure 25a). 156

The good outcrop quality in the area is a result of massive flash floods from late 157

Mioscene to early Pleistocene (Rodgers & Gunatilaka 2003) and the recent arid 158

conditions. The arid conditions during most of the year prevent extensive vegetation 159

and the formation of soil. Short but heavy annual rainfall in the spring’s rain season 160

commonly form flash floods cleaning the surfaces free of dust and debris. Within the 161

juvenile topography of Jabal Shams, sediment accumulation is therefore only present 162

in flat segments of Wadis or in moulds formed by differential erosion. The differential 163


erosion of ‘soft’ rocks leads to the exposure of predominantly competent units on the 164

surface. The softer rocks are exposed mainly in profiles along the wadi walls (Figure 165

6). 166

The clean outcrop surfaces make this area highly suitable for remote sensing work 167

while the profile cuts of the wadis provide additional information about the vertical 168

framework (Figure 6). 169

170

171

Figure 5

172

Figure 6

Lithology 173

174

The lithology exposed in the field area belongs predominantly to the autochthonous 175

Hajar supergroup exposing the Mesozoic Sahtan, Kahmah and Wasia groups (Figure 176

4, Figure 5) (Beurrier et al. 1986, Glennie 2005, Glennie et al. 1974). Outcrops of the 177

younger Muti formation of the Aruma group can be found just outside the field area. 178

The lithological column is exposed from the Rayda Formation (Berriasian) over Salil, 179

Habshan, Lekhwair, Kharaib, Shu’aiba and Nahr Umr to the Natih formation (Albian 180

to Turonian) at the top (Hughes Clarke 1988, Sharland et al. 2004). Spanning more 181

than a kilometer, the complete column is exposed at the western wall of Wadi Nakhr 182

(Figure 4, Figure 5). 183

184

Sahtan group 185


The oldest strata exposed in the field area belong to the Sahtan group: Rust-brown 186

shaly units of the Dhruma formation and bluish carbonates of the Mafraq formation 187

are found in the cliffs of the deepest sections of the wadis and in a footwall section of 188

a large fault in the Northeast of the field area (Figure 4, Figure 5) (Hughes Clarke 189

1988). 190

191

192

Kahmah group 193

More common throughout cliff and surface outcrops are however the Kahmah and the 194

Wasia groups. The Kahmah group is a thick carbonate sequence. In its upper part it 195

shows a transition from deeper marine pelagic to shallow marine limestones (Al-196

Wardi 2006, Hughes Clarke 1988) interpreted as a “regressive mega-sequence” 197

comprising a highstand systems tract (Pratt & Smewing 1993). It is the lateral 198

equivalent of the economically important Thamama Group of the U.A.E. with some 199

differences in the lower parts (Hughes Clarke 1988, Pratt & Smewing 1993, Sharland 200

et al. 2004). 201

Resistant to weathering and erosion the upper formations of the Kahmah group make 202

up the bulk of the surface exposures of the eastern part of the study area (Figure 6). 203

204

205

Rayda formation 206

The Rayda formation as the oldest formation of the Kahmah group can be recognized 207

in the cliff profile by overlaying the steep cliffs of the subsequent Sahtan group 208

(Figure 5). The Rayda formation shows prominent stacks of thinly bedded bright, 209

porcellanitic limestones and argillaceous carbonates (Figure 24d) (Al-Wardi 2006, 210


Hughes Clarke 1988). It displays a condensed deep water unit with pelagic biota 211

abundant in radiolaria (Pratt & Smewing 1993). Its elements are similar in thickness 212

forming a regular stack of thin predominantly weathering resistant beds. This 213

formation is exposed in deeper parts of several wadis as well as on the Northeast part 214

in the footwall block of a major normal fault (Figure 24d, e). 215

216

217

Salil formation 218

Compared to the regular bedding of the Rayda formation, the overlying Salil 219

formation is more heterogeneous. The alternating thin limestones, argillaceous 220

limestones and marls that dominate the Salil formation are interpreted as a 221

periplatform facies by Pratt and Smewing (1993) The differences in the weathering 222

characteristics lead to an irregular cliff profile bound to the top by the “clean” 223

carbonate of the Habshan formation (Figure 4, Figure 5, Figure 25b) 224

225

226

Habshan formation 227

The Habshan formation consists of predominantly bioclastic grainstones that form 228

prominent, more than 100 m tall cliff faces in the lower Kahmah group (Figure 5). 229

The Habshan formation can be easily recognized due to the weathering contrast to the 230

under and overlying formations. Outcrops of the Habshan in the study area are limited 231

to cliff faces. The Habshan formation represents a regional platform carbonate 232

(Hillgärtner et al. 2003) with massive beds. 233

234

235


Lekhwair formation 236

The facies changes in the next younger Lekhwair formation, which shows 237

heterogeneous bed assemblages (Figure 4, Figure 5, Figure 25b). The Lekhwair 238

formation represents sedimentary cycles of three main facies comprising argillaceous 239

limestones, marls but also cleaner wackestones and grainstones (Hughes Clarke 240

1988). The meter to decimeter stacking pattern shows a shallowing upward trend 241

(Hillgärtner et al. 2003). The relative strength of the individual materials as well as 242

the thickness of the single beds strongly varies. 243

244

245

Kharaib formation 246

An additional distinct cliff face is formed by the Kharaib formation. Consisting of an 247

essentially clean carbonate the Kharaib forms two almost equally sized massive beds 248

(Figure 4, Figure 5, Figure 26). It was deposited in a low angle carbonate ramp system 249

in a shallow water environment (Buchem et al. 2002). Similar to the Shu’aiba the 250

formation is characterized by the benthic foraminifera Orbitolina and abundant rudists 251

(Buchem et al. 2002). 252

253

254

Hawar Member and Shu’aiba formation 255

A few meter thick argillaceous carbonate is the weak interlayer separating the Kharaib 256

cliff from the overlaying Shu’aiba cliff (Figure 4, Figure 5, Figure 6b): The Hawar 257

member was interpreted as a protected lagoon facies by Buchem et al. (2002). Its low 258

resistance to weathering leads commonly to a gentle slope in the otherwise steep cliff 259

faces of the underlying and overlying formations. 260


The overlying Shu’aiba formation in contrast consists of dominantly bioturbated 261

packstone (Pratt & Smewing 1993) and represents an aggradational carbonate 262

platform (Hillgärtner et al. 2003). The Shu’aiba’s high overall resistance to 263

weathering creates a homogeneous appearing cliff of approximately 100 m height 264

(Figure 5, Figure 6, Figure 22). This massive element is easily recognized in the 265

overall cliff profile. The Shu’aiba formation forms most of the surface outcrop in the 266

eastern part of the field area. On a regional scale, the Shu’aiba formation forms 267

prolific petroleum systems throughout the Middle East region and is therefore well 268

characterized (Buchem et al. 2002). 269

270

271

Wasia group 272

The Wasia group (Figure 4) was formed in shallow water conditions of a subtidal 273

environment. It is described by some authors as highstand systems tracts after a 274

transgression event that separates the two groups (Pratt & Smewing 1993). The 275

Wasia’s contact to the lower Shu’aiba formation consequently shows local evidence 276

of subaerial exposure during the stratigraphic break (Buchem et al. 2002). 277

278

279

Nahr Umr formation 280

The contact of the Kahmah and the younger Wasia group is defined by the sharp 281

contact of the Shu’aiba to the prominent Nahr Umr formation (Figure 5, Figure 6b). 282

The Nahr Umr is known as a prominent clay bearing seal for a number of reservoirs 283

throughout the Middle East region, whereas the formation here comprises marls and 284

argillaceous limestone with minor intercalations of more competent beds (Figure 5, 285


Figure 6b, Figure 22). (Hughes Clarke 1988, Sharland et al. 2004) The depositional 286

setting of this formation is described as a moderately deep shelf (Pratt & Smewing 287

1993). The Nahr Umr is one of the thickest formations exposed in the field area. Its 288

low resistance to weathering however reduces the outcrops to a relatively small area 289

protected from erosion by a footwall block of a major normal fault (Fig. 6b). Other 290

outcrops of this formation are limited to gentle slopes between the steep cliff faces of 291

the overlying Natih and underlying Shu’aiba formation (Figure 22). The slope 292

exposures are however often covered by debris and provide limited structural 293

information. 294

295

296

Natih formation 297

The youngest strata within the field area belong to the massive Natih formation 298

(Figure 5, Figure 22). The Natih is several hundreds of meters thick and divided 299

informal into subdivisions from ‘g’ at the base to ‘a’ at the top to indicate different 300

members (Hughes Clarke 1988). The Natih formation consists of argillaceous 301

wackstone, bioclastic packstone and grainstone deposited in a shallow shelf 302

environment (Pratt & Smewing 1993). The members ‘e’ and ‘a/b’ form impressive 303

cliffs of many 10’s of meters, whereas the ‘c’ and ‘d’ members are prone to erosion 304

leading to gentle slopes. The bulk of the surface outcrops of the western part and the 305

southernmost margin of the field area expose members of this formation. Its rather 306

homogenous appearance in the field limits the ability to distinguish its members. 307

308

309


Methods 310

311

This study was based on satellite imagery and field observations. Multispectral 312

Landsat scenes as well as a high-resolution Quickbird satellite image provided the 313

basis for the field work (Holland et al. this volume). The study of the satellite images 314

as well as a high resolution DEM and air photo prior to the fieldwork showed the 315

overall distribution of strata and the structural elements as described in the second 316

part of this study (Holland et al. this volume). The fieldwork focused on the visit of 317

pre-selected areas and transects. For navigation and direct quality check map printouts 318

as well as a GPS connected handheld computer allowed the collection of geo-319

referenced information. 320

Several hundred commented waypoints as well as a few thousand geo-referenced 321

digital photographs were taken together with structural measurements that are 322

integrated in a GIS system. The database of the photographs was used to correlate the 323

field elements with the information displayed on the satellite images (Holland et al. 324

this volume). 325

Structural measurements, overprinting relationships, macroscopic observations as 326

well as the documentation of the profile views with sketches and photographs are 327

incorporated into the database. 328

329

The fieldwork complements the satellite data by providing vertical profiles of wadi 330

walls (Figure 5, Figure 22, Figure 26). From these fault geometries were sketched and 331

photographed (Figure 16, Figure 26) Two persons estimated the fault offsets 332

independently by correlating offset beds. Homogeneous units – like the Natih – are 333


difficult to correlate within small wadis due to the lack of marker beds. Fault offsets 334

that exceed the height of the outcrop limit the interpretation as well. 335

336

Micro- and macro scaled observations on the fracture population were carried out to 337

derive the temporal relationship of structures (Figure 11, Figure 14, Figure 17a). The 338

observations on the internal structure of the fractures and faults like offset 339

relationships, fragmentation, and segmentation as well as the types of cement are vital 340

to understand the temporal relationships. 341

342

343

Structural elements 344

345

Figure 7

346

The emphasis of this study is the characterization of the structural elements, 347

predominantly brittle deformation fractures and faults (Figure 6b). 348

The fracture density throughout the entire field area is very high (Holland et al. this 349

volume). The massive population incorporates hairline fractures, joints, shear 350

fractures and shear veins, veins and faults. 351

352

We use the term fracture in a general sense to describe any discontinuity within a rock 353

mass formed as a response to stress (Bonnet et al. 2001). If an isolated fracture shows 354

a very small opening component we use the term hairline fracture (Figure 29b). 355

Systematic fractures with a pure opening mode component are called joints and can 356


be cemented (Figure 7) or uncemented (Figure 28). A clear distinction between joint 357

and fault is not always possible as we observe reactivated joints and hybrid fractures 358

(Hancock 1985). The term vein is used to generally describe a fracture filled with 359

mineral different than the rock mass it is embedded in (Bonnet et al. 2001) (Figure 7). 360

If a shear component is present the feature is labeled as a shear fracture (Figure 8, 361

Figure 9). 362

363

The vast majority of the fractures shows an open-mode component, in most cases 364

cemented with white calcite. This bright cement forms a high optical contrast to the 365

overall dark colored carbonates making the cemented fractures easy to recognize in 366

the field as well as on the Quickbird satellite image (Holland et al. this volume) 367

Based on cross-cutting relationships and stable isotope analyses Hilgers et al. (2006a). 368

distinguished in total seven different generations of veins for an interpretation of the 369

palaeo-stress regime of the same region Whereas Hilgers et al. worked predominantly 370

on profile views we incorporate profile and surface data from virtually the entire 371

study area to form a much more extensive data set to study the orientation of the stress 372

field (Holland et al. this volume). Our classification is based on temporal relationships 373

and fracture morphologies described in the following. 374

375

376

Veins, normal to bedding (v ⊥ s0) 377

378

A prominent group of fractures comprises veins of different strike directions that are 379

oriented normal to the bedding. These predominantly dilatant fractures are cemented 380

with bright calcite. They are most frequent in massive carbonate beds. 381


382

Veins in vertical segments of styolites 383

One group of veins is formed in the steep limbs of styolites. Described in detail by 384

Hilgers et. al (2006a), these bedding perpendicular veins have rather small 385

dimensions, dictated by the morphology of the styolites. This group is rare within the 386

field area and found only in exposures of rocks of the Sahtan group, exposed in the 387

deepest sections of Wadi Nakhr. 388

389

390

Veins with straight segments 391

A much more common type of vein is characterized by straight segments and 392

apertures that can be as large as 10 cm (Figure 7). These veins are filled with coarse-393

grained calcite cement. Thick veins of this group are commonly sub-parallel to each 394

other and up to few 10’s of meters long. The majority of these veins are found in 395

massive carbonate beds that are intercalated in weaker strata (Figure 7b, e, g). The 396

vertical dimension of the fractures is in many cases limited to the bed thickness as 397

they terminate at the bedding interfaces. Within a single bed the spacing of these 398

veins appears to be rather constant (Figure 7a), whereas the spacing in the adjacent 399

layers can be much different (Figure 7c, e, f), even if they show similar thicknesses. 400

The same applies for the apertures of the veins, suggesting that there is no consistent 401

relationship of vein spacing to the bed thickness (Bai & Pollard 2000). 402

Layers with similar thickness may show very distinct fracture patterns as e.g. spacing 403

may differ by a factor five among similar beds within a single outcrop (Figure 7e, f). 404

405


The veins of this group are entirely cemented with white calcite, forming blocky 406

crystals. In some veins broken lensoid host rock fragments are embedded in the 407

cement (Figure 9a). The cement of these regular veins is predominantly white but 408

occasionally colored yellowish on the scale of an outcrop. The latter may be a result 409

of a later staining, e.g. due to the weathering of Fe-bearing material of adjacent beds. 410

The morphology and spacing pattern of this group is the most regular pattern among 411

the veins when observed in outcrop scale. 412

413

414

Figure 8

415

Vein terminations 416

A range of different vein terminations was identified over the group. 417

Some veins show straight segments over a large length but terminate with splays, 418

wing cracks or they transfer into en-échelon segments (Figure 8). This shows the 419

presence of an additional in-plane component. Although these shear veins may form 420

regular arrays they are commonly solitary structures parallel to regular veins (Figure 421

8c, e). In the latter case the shear-veins are more pronounced due to larger apertures 422

and longer traces as opposed to the other veins. 423

The larger aperture veins of this group are not necessarily stratabound; they can cut 424

across several beds. The cement of these veins is similar to the previously described 425

type: White blocky calcite with common inclusions of host rock flakes. 426

427

Branching and braided systems 428


Branching and braided systems of joints form another group, found usually within a 429

sub-parallel population of more regular distributed veins or cemented hairline 430

fractures (Figure 8b, d, f). The apertures of the braided veins are in the order of 431

centimeters or smaller. The cement of the braided system is similar to the blocky 432

cement described earlier. Due to the braided character the spacing and the aperture of 433

the irregular veins are highly variable (Figure 8d, f). This group shows – as an effect 434

of the branching – variances in strike directions as well as non-linear segments. The 435

fracture walls are less regular as compared to the previous groups. 436

437

A significant increase in complexity within this group is seen in some cases at step-438

over segments between adjacent fractures. Here the veins are broken down into 439

numerous small strands (Figure 8f, Figure 9e). Similar shapes are seen at vein 440

terminations, as the vein may feather into arrays of irregular shaped strands. The 441

termination may also gradually taper into the rock. 442

443

En-échelon systems 444

The en-échelon veins form separate systems in a more organized fashion. The 445

individual en-échelon segments are commonly aligned over distances of several 446

meters (Figure 8e, Figure 9c). These segments may form isolated strands or conjugate 447

sets of strands (Figure 9d) both in the horizontal as well as in the vertical direction. 448

The aperture of the individual segments rarely exceeds 3 cm. 449

450

Figure 9

451


Figure 10

452

Joint orientations 453

The orientation of the previously described veins were measured in the field and 454

interpreted on the satellite image (Holland et al. this volume). Plotted into a rose 455

diagram a distribution into four major groups is apparent. The mean strike directions 456

of the groups are approximately 000°, 045°, 090° and 130° (Figure 10) 457

458

The 130° striking fracture set is the most prominent on the satellite image and in the 459

field. Sub-parallel straight joints and cemented hairline fractures commonly belong to 460

this group, showing also the longest fracture traces. In contrast the 000° and 090° sets 461

show the sharpest distribution. The 000° set is however commonly altered due to the 462

fractures striking down-slope leading to preferential physical erosion of this group. 463

464

Figure 11

465

Figure 12

466

467

Overprinting relations 468

A relative chronology of the fractures in these groups is not always entirely clear 469

(Figure 11, Figure 12): Abutting is not observed among all systematic veins and the 470

crosscutting relationships are not always easy to interpret (Figure 12). Curving of a 471

fracture set towards other sets was not observed in the field area. Since the temporal 472


relationship cannot be based on abutting we used offset relationships along shear 473

veins and the relationship of crosscutting cements to determine the chronological 474

order (Figure 11, Figure 12). The offset across shear veins is unambigiuous. The 475

cement generations are more difficult to interpret (Figure 12a): 476

If a rock body with veins is fractured and filled with a homogenous material that is 477

optically distinct from the previous veins the relationship is easy to recognize (Figure 478

12b). It seems however, that some newly cemented fractures show an effect, where 479

the precipitated material mimics the fracture walls. This heterogeneous precipitation 480

makes it in some cases difficult to determine the temporal relationship (Figure 12b). 481

482

Figure 12a shows such an example. At first glance at the overview picture the white 483

vertical vein seems to cut the horizontal one. The detailed view in Figure 12a shows 484

however that the reddish rim of the vertical vein is disrupted by the horizontal 485

fracture. A robust distinction is in this case not possible. Similar examples are 486

common among the veins sets and may easily lead to a misinterpretation of the 487

relative chronology. 488

489

Figure 13

490

We determined the offset relationships of the joints from photographs and field 491

observations listed in Figure 13. Listed per line, the upper part of the figure displays 492

fracture sets measured on photographs, the lower part displays field notes. 493

The fracture strike readings are plotted into the graph with a color marker. 494

495


The observations from one outcrop pertain two or three generations of fractures at the 496

same time at most. The relative succession within the individual outcrops are listed in 497

the left part of Figure 13 color- coding the corresponding fractures with three colors. 498

None of the studied outcrops displays all generations necessary to define a consistent 499

temporal relation. 500

501

For this purpose the second part (Figure 13b) uses five instead of three age classes. 502

The five classes use the same relative chronology but a different grouping resulting in 503

much more ordered pattern. 504

Based on this division the oldest veins commonly strike in the 000º direction (Figure 505

13, dark green). The second set is striking into the 130º direction followed by the 090º 506

and then the 045º directions. Offset is common within the latter two groups marked 507

with the letter ‘o’. The youngest members (Figure 13b, red) are distributed among all 508

the previous groups and might represent reactivation features or interpretation errors. 509

510

511

Bedding parallel veins (v || s0) 512

513

Figure 14

514

Non-stratabound veins of the previously described set are overprinted by shear zones 515

parallel to the bedding (Figure 14). Striations and offset relationships show a top east 516

and top to north movement (Al-Wardi 2006, Al-Wardi & Butler 2006, Breton et al. 517

2004, Searle 2007). The overall distribution of these shear zones within the vertical 518


column is rather heterogeneous. It is developed locally either on the interface of 519

competent layers or within the incompetent layers themselves. 520

521

Figure 15

522

Shear at bedding interfaces 523

Bedding interfaces in many stratigraphic levels acted as slip planes. This is shown by 524

blocky calcite forming layer-parallel veins of up to several centimeters thick (Figure 525

14, Figure 15b). In other cases, these veins are fibrous, with quartz and calcite fibres, 526

indicating a different mode of precipitation. These sub-horizontal veins show offset 527

relationships and sense of shear indicators with a top to North to top to East direction 528

(Al-Wardi 2006, Al-Wardi & Butler 2006, Hilgers et al. 2006a). Local imbrications of 529

these veins are found as well as the incorporation of host rock fragments that are 530

oriented parallel to the bedding (Figure 15, Figure 17a). Locally the shear movement 531

is also observed along more irregular bedding surfaces or along the clay seams of 532

incipient styolites (Figure 15a). The slip on these irregular surfaces leads to the 533

formation of dilatant jogs (Koehn & Passchier 2000). The jogs are cemented and 534

show embedded host rocks fragments (Figure 15a, b). The formation of the slip planes 535

at bedding interface is commonly linked to cleaner carbonate beds, whereas the 536

argillaceous beds tend to localize the slip within the beds themselves. 537

538

539

Shear within beds 540

Within argillaceous units (Figure 15c-h) the bedding-parallel shear develops 541

additional deformation structures. The clay rich layers may form s/c textures (Figure 542


15g) or disjunctive cleavage (Figure 15c, d). Additional elements of the shear zones 543

include sigmoidal clasts incorporating quartz (Figure 15e, f), pyrite cubes with 544

pressure shadows, as well as rotated pinch and swell veins (Figure 15c, d). Although 545

the strain is largely concentrated in the argillaceous layers upper more competent 546

layers may locally show boudinage (Figure 15e, f). 547

548

Most of the layer parallel shear was observed in the profiles of the central Wadi 549

Nakhr within the lower part of the stratigraphic column as well as in the topmost 550

Natih formation. The localization within the column seems to be influenced by the 551

layer succession: Pure carbonate stacks commonly show multiple shear beds at the 552

interfaces of the layers (Figure 15a, b), whereas argillaceous members localize the 553

deformation stronger towards single planes resulting in a higher degree of damage. 554

555

556

Normal faults 557

558

Figure 16

559

The study area exposes a large number of normal faults with offsets up to 560

approximately 500 m (Figure 16). Although these are predominantly normal faults, 561

oblique slickensides are commonly observed (Al-Wardi 2006, Al-Wardi & Butler 562

2006, Filbrandt et al. 2006, Hilgers et al. 2006a, Searle 2007). On the cemented fault 563

planes striations can show several generations as well as different directions 564

indicating a complex history. 565


566

The vertical offset on the faults reaches up to several hundreds of meters as estimated 567

for the largest fault in the northeast part of the field area. 568

A temporal relationship of the normal faults to the previously described structures is 569

derived from outcrops in Wadi Nakhr, where a number of small offset normal faults 570

cut and offset the layer parallel shear veins (Figure 17a). 571

572

The fault orientation as derived from the interpretation of the satellite image (Holland 573

et al. this volume) shows a prevailing strike of the fault segments from 090° to 120°. 574

The directions of these mature faults are sub-parallel to the fold axis of the Jabal 575

Akhdar dome. 576

577

We discuss a division of the faults into groups related to the throw magnitude. 578

579

Figure 17

580

Incipient faults (throw < 0.5 m) 581

582

A large number of incipient faults (Figure 17) are interpreted to nucleate along the 583

cemented joint planes formed perpendicular to the bedding striking 090° and 130° 584

(Figure 17e). 585

586

The non-systematic joints perpendicular to the bedding and incipient faults closely 587

relate to each other. Both, the joints and the faults show an identical strike direction, 588


both are largely dilatant and both features are cemented limiting a clear distinction. 589

When nucleating along the joints, the steep joint segments within the competent units 590

become interconnected and increased throw forms trough-going fault strands. These 591

strands show commonly refraction at bedding interfaces (Ferrill & Morris 2003, 592

Hancock 1985). Observed in thinly bedded units the incipient faults with up to a few 593

centimeters throw may develop dilatant jogs (Figure 17f) (Ferrill & Morris 2003, 594

Hancock 1985), which are - similar to the joints - entirely filled with coarse-grained 595

cement. 596

597

Other structures in incipient faults are tension gashes (Figure 17b, c) or en-échelon 598

segments and conjugate sets (Figure 17b, d). These structures indicate the reactivation 599

of previous structures or the impact of a mechanical anisotropy within the system. The 600

en-échelon systems are commonly observed in massive carbonate beds closely 601

associated with normal fault zones. The en-échelon structures form in some cases as 602

conjugate sets (Figure 17 b, d) and are commonly found in zones of low displacement. 603

With progressive offset these structures forms normal faults. 604

605

606

Figure 18

607

Figure 19

608

Small to medium offset faults (0.5-10 m throw) 609

610


With larger offsets in the order of 0.5 to 10 m the internal structure of the fault zone 611

changes. The end members in this group can be described as follows: 612

613

Fault segments with localized displacement 614

Often observed in profile view, medium offset normal faults show sharp linear fault 615

planes as the first end-member (Figure 18). These straight segments are 616

predominantly exposed in massive cliff sections. The fault cores of these sections are 617

commonly narrow; they are cemented and show intense fragmentation of the cement. 618

Embedded fragments consist of blocky calcite or broken calcite rhombohedra. With 619

more intense fragmentation the fault core can show calcite-cemented breccias (Figure 620

18b). 621

In contrast to the uniform white calcite cement of the previously described veins, the 622

cement of the faults can have brown, gray, yellow or red colors suggesting other 623

genetic conditions or other fluid sources (Hilgers et al. 2006a). Incorporated host rock 624

and angular cement fragments are more common within the fault’s cement (Figure 625

20). 626

With this end-member the fault core or the zone with fragmentation and cementation 627

is usually a few centimeters to 10’s of centimeters thick concentrating the throw. This 628

leaves the damage zone very narrow, as the surrounding host rock remains virtually 629

intact. 630

631

632

Fault segments with distributed throw 633

In plane view on competent carbonate beds another morphology is common (Figure 634

19). Here the fault throw seems to be distributed over a wider zone comprising an 635


anastomosing network of fractures. A single strand may concentrate the throw and 636

show characteristics of a fault core and a large number of associated fractures define a 637

wide deformation zone (Figure 19a). The fault core strands have larger apertures 638

compared to the other fractures in the system and show a larger degree of damage as 639

well as greater variety in color. The aperture of the fractures can be several 10’s of 640

centimeters, filled with calcite cement. 641

The major faults of such a zone run sub-parallel and may branch to interconnect 642

gradually with the less prominent fractures of the accompanied deformation zone 643

(Figure 19b, d, e). The latter fractures of the deformation zone have smaller apertures 644

and show commonly the white blocky calcite cement without the discoloring or the 645

intense fragmentation (Figure 19a). These fractures are formed predominantly in the 646

direction of the previously described joints. The spacing of the fractures within the 647

“fault corridor” is dense and irregular. Shear indicators and reactivation features are 648

frequently present. These incorporate splays and horsetail fractures, en-échelon 649

segments, riedel structures and conjugate sets of riedels. Flakes of host rock material 650

are also found throughout these sets (Figure 20). 651

652

Figure 20

653

Figure 21

654

Calcite Rhombohedra 655

Indirectly associated with these intermediate fault zones are occurrences of large 656

calcite rhombohedra (Figure 21) that we find along the traces of such faults. These 657


massive idiomorphic crystals with lengths of more than 60 cm are found in different 658

parts of the field area. The rhombohedra commonly show different zonation of clear 659

calcite, yellow, white or gray colors. In some specimens styolites developed parallel 660

to the crystal shape. Occasionally weathered rhombohedra are stained with malachite. 661

662

663

Large offset faults 664

665

The largest vertical offsets are observed at two major fault zones, one in the northeast 666

and one in the southeast of the mapping area (Figure 16). Both fault systems strike 667

sub-parallel to the fold axis of the Jabal Akhdar domal structure, showing an overall 668

strike direction of approximately 100° to 110°. The largest offsets within the study 669

area are at the eastern boundary and are in the order of several 100’s of meters, 670

diminishing towards the west. 671

672

Figure 22

673

The Southern fault 674

One of the two major systems is exposed in the southern part of the field area. 675

Surface exposures in the eastern part of this southern fault zone show the Natih 676

formation offset against the Shu’aiba and Nahr Umr formation. The throw is mainly 677

localized to a major fault zone until it crosses Wadi Nakhr and branches towards a 678

number of strands on the western side of the canyon (Figure 16, Figure 22). As the 679


throw is distributed the total offset diminishes towards the west where the southern 680

fault dies out. 681

682

In a profile view exposed at the Wadi Nakhr’s western wall the fault system shows 683

prominent horst and graben assemblages (Figure 22). A major horst structure can be 684

seen in the deeper lying Kahmah group. The throw distribution is more complex in 685

the upper Natih formation where the fault zone is spread to an array of horst and 686

graben structures. The Nahr Umr formation in-between forms more a monocline 687

rather than an array of faults. 688

689

Figure 23

690

The general outcrop conditions in the East along this major fault zone are poor due to 691

the easily eroded Nahr Umr formation exposed or offset by this fault. 692

Numerous wadis pond against the Natih formation on the hanging wall block and 693

accumulate soil and debris onto the eroded fault zone. Some wadis are redirected by 694

the fault, cutting deep into the exposed Nahr Umr formation or cut into the fault zone 695

itself. Some sections in the eastern part of the fault are however preserved and 696

exposed along a road cut (Figure 23). Here a fault zone several 10’s of meters width 697

shows extensive deformation. The massive zone is made of a heterogeneous assembly 698

of competent material “floating” in a clay-rich matrix; Material probably derived from 699

the argillaceous Nahr Umr formation. The high degree of damage is linked to 700

discoloring in reddish and yellowish colors. 701

702

703


Figure 24

704

The large Northern fault 705

The other major fault zone is sub-parallel and exposed a few kilometers to the north 706

(Figure 16). This fault shows the largest offset at the eastern boundary diminishing the 707

throw towards the west, where it seems to bifurcate into another system. Surface 708

exposures in the east show the Kahmah group offset against itself or against 709

formations of the Sahtan group (Figure 24). With the fault zone being located in more 710

competent layers the deformation zone is narrower as compared to the southern fault. 711

The fault core in the eastern part consists of a heavily damaged and cemented zone of 712

several meters width (Figure 24). With a higher resistance to weathering, the fault 713

core forms a topographic ridge locally more than 6 m tall (Figure 24a). Profile views 714

on the large-offset section in the eastern part of the fault show that the wide fault core 715

spans the entire exposed profile (Figure 24c, d). Figure 24d shows e.g. massive drag 716

folds that are formed in the thin interbedded sequence of the Rayda formation 717

exposed in the footwall block. 718

Towards the west branching of the fault zone becomes more important and the size of 719

the fault cores diminish. The anatomizing character however leads to a dense network 720

of cemented fractures forming a wide damage zones within this fault system. 721

Towards Wadi Nakhr the major faults splits at the surface into smaller strands with 722

offsets in the order of several ten’s of meters as observed in the 1 km tall profile at the 723

western wall of the canyon (Figure 25, Figure 26). Similar to the profile of the 724

southern fault the Nahr Umr formation deforms by forming a monocline (Figure 22, 725

Figure 26). Underneath throw is localized to single fault planes in the thick carbonate 726

layers of the Kahmah group. Within the uppermost section of the Kahmah group the 727


single-strand fault splays towards the contact to the Nahr Umr to translate into the 728

monocline structure (Figure 26). 729

730

Figure 25

731

Figure 26

732

733

Ramp structures 734

735

Figure 27

736

Compression related structures like ramps and duplexes that are north/west directed 737

are described by several authors in the greater area (Al-Wardi 2006, Breton et al. 738

2004, Hilgers et al. 2006a, Searle 2007). These are reported to overprint the normal 739

faults that are previously described. Within our field area we find only one example of 740

a duplex structure (Figure 27), which is directed south/west. Located within the Natih 741

formation on the Southern part of the field area the isolated structure forms a 742

topographic ridge that can be traced a few kilometers. The ridge is oriented parallel to 743

the slope and therefore parallel to the fold axis of the Jabal Akhdar dome (Figure 744

27c). The temporal relationship as well as the significance of this duplex structure 745

remains unclear, as we did not study this structure in detail yet. The jointing pattern 746

within the ramp remains normal to the bedding suggesting that the ramp was formed 747


after the formation of the veins. More work is needed to put this south/west directed 748

ramp into the context of the structural framework. 749

750

751

Uncemented and partly cemented joints 752

753

Figure 28

754

In addition to the cemented fractures the area exposes a large amount of uncemented 755

or partly cemented joints observed in profile view as well as in plane view (Figure 756

28). The orientation of these fractures is identical to the group of cemented joints that 757

are approximately normal to the bedding as they show all four strike directions. 758

In places where this jointing is developed systematically, the spacing of the joints is 759

smaller than compared to the spacing of its massive cemented counterparts. Plane-760

view observations show the formation of dense joint patches (Figure 28, Figure 29). 761

762

Within the heterogeneously distributed patches the spacing can be as little as a few 763

centimeters. Abutting within the patches is present but shows no clear relationship 764

suggesting that the joints were formed simultaneously (Figure 28e, f, Figure 29). 765

766

Figure 29

767

768


Neotectonic faulting 769

770

A last fracturing stage is evident by small-scale offsets observed in the cemented 771

gravels of fluvial terraces within Wadi Nakhr indicating recent tectonic activity 772

(Figure 30). In agreement with Kusky et al. (2005), the observed fracture network 773

strikes approximately in a north/south direction following the orientation of the large 774

canyon. Kusky et al. observed a north/south trending fault cutting some cobbles in the 775

cemented conglomerate of a terrace while bypassing others cobbles. 776

777

Figure 30

778

779

Interpretation of the field data 780

781

This carbonate stack of the Autochthonous shows evidence for a multiphase 782

deformation visible by different sets of fractures and deformation structures. 783

784

Vein sets normal to bedding (v1 ⊥ s0) 785

The first generation of structures are veins normal to bedding (Figure 32b-e, Figure 7, 786

Figure 8), with large apertures and mostly clean blocky calcite cement. Fibrous veins 787

that could indicate a slow fracture opening were not found. The blocky calcite 788

suggests that the calcite precipitated in open voids. 789

790


Abutting was never observed among veins of this group and none of the fractures are 791

systematically curved. This indicates that cementation healed the fractured rock 792

repeatedly, restoring most of its mechanical strength prior to the next fracturing event. 793

The cyclic fracturing-healing model is a mechanism to produce joint spacings much 794

smaller than bedding thickness (Figure 7). These super-saturated beds were formed by 795

many different stages of fracturing and healing rather than in a single event. An 796

alternative explanation of the formation of the joints involves large differential 797

stresses. The conditions of such an environment would promote the lack of 798

mechanical interaction such as curving. 799

800

The age relationships of in total four strike direction classes are reasonably consistent. 801

We interpret the data presented in Figure 13 on overprinting relationships from 42 802

outcrops imply four vein generations of which the oldest strikes north/south. This set 803

is overprinted by the most prominent set that strikes approximately 804

northwest/southeast dominated by long, wide and straight veins. This in turn is 805

overprinted by an east/west striking and then a northeast/southwest striking group. 806

The east/west striking group commonly offsets older veins, and is interpreted to be 807

partly reactivated by the normal faulting processes at a later stage. The systematic 808

anticlockwise rotation of progressively younger veins is interpreted to represent a 809

corresponding rotation of the minimum principle stress, from east/west, to 810

northeast/southwest, to north/south towards northwest/southeast by in total 135˚. 811

Since the age relationship presented in this study is based only on a small sample size 812

(<50 outcrops) a second interpretation cannot be ruled out yet. The four fracture 813

groups may alternatively form conjugate sets. Grouping the N/S and E/W as one set 814

and the NW/SE and NE/SW as the other would relate to a 45˚ rotation of the 815


minimum principle stress. To test the latter case a much larger regional study on the 816

overprinting relationships is required. 817

818

The apertures and spacing of the straight, sub-parallel veins are reasonably consistent 819

within one layer on the scale of an outcrop, but can be quite different in neighboring 820

layers of equal thickness (Figure 7e) suggesting that layer thickness is not the only 821

parameter controlling the spacing and aperture. Additional parameters could be (i) 822

tensile strength of the layer, (ii) rheological contrast to neighboring weak layers, (iii) 823

permeability structure of the system to affect the buildup and release of local fluid 824

pressures (Bai 2000a, Bai 2000b, Bai & Pollard 1998, Engelder 1987, Engelder 1990, 825

Pollard 1979, 1987, Zoback 1978) 826

827

Many of the veins contain indicators for repeated fracturing and sealing, under 828

progressively changing stress (Figure 9). Wall rock flakes are commonly embedded in 829

the veins, with their long axis predominantly parallel to the vein wall in agreement 830

with the model of a fast restoration of the strength (Holland et al. in preparation). 831

832

The orientation of the fractures perpendicular to the bedding as well as the prominent 833

open-mode component indicates that the veins have formed in response to low 834

differential stresses and tensile minimum effective stress. 835

836

837

Duplex structures (d*) 838


The surface exposure of the duplex structure (Figure 32f, Figure 27) on the southern 839

flank was needs more study. We interpret this ramp to postdate v1, as these are rotated 840

by the ramp, and to occur before the bedding-parallel, top-NE directed shear. 841

842

843

Structures parallel to bedding (d2 || s0, v2 || s0) 844

Veins and deformation structures that are sub parallel to bedding (Figure 32g, Figure 845

14) overprint v1, indicating a change of the stress field. Fluid pressures close to 846

lithostatic, were proposed for this phase by (Hilgers et al. 2006a). Strain accumulated 847

in this stage is partitioned heterogeneously in the vertical column. Argillaceous units 848

and bedding interfaces tend to localize strain (Figure 15). In contrast to the southwest-849

directed emplacement of the Hawasina and Semail ophiolite, the striations, S/C 850

textures, the rotation of pinch-and-swell veins as well as offsets all document a top to 851

north, and top to east movement (Al-Wardi 2006, Al-Wardi & Butler 2006, Breton et 852

al. 2004, Searle 2007). The cleavage seen within the argillaceous units indicates the 853

transition towards ductile behavior. 854

855

856

Normal faulting (d3) 857

Small offset normal faults exposed in the walls of Wadi Nakhr overprint v1 and v2, 858

indicating another major change of the stress field (Figure 32h, Figure 17). Faults 859

commonly start with arrays of en-échelon veins, a prominent opening-mode 860

component and massive cementation, indicating high fluid pressure conditions and a 861

chemically reactive environment in the early stages of the normal faulting. Incipient 862

normal faults offset these arrays of en-échelon veins (Figure 17c). Isotope 863


measurements by Hilgers et al. (2006a) show until this stage a rock-buffered isotope 864

signature, whereas with progressive normal faulting meteoric signatures become more 865

common indicating the formation of effective fluid conduits (Hilgers et al. 2006a). 866

867

Other faults are interpreted to have nucleated along to the veins of the 090º and 130º- 868

striking set of the first generation (v1). This is based on the observation of some 869

normal fault zones consisting of long, parallel veins, with brecciated vein cement, 870

(Figure 19a and d), and on the presence of long, straight veins in the complex damage 871

zone of normal faults with a few m offset (Figure 19b and d). 872

873

874

Therefore we infer the presence of a weak mechanical anisotropy during nucleation of 875

the normal faults, both in horizontal (veins) and vertical direction (due to mechanical 876

stratigraphy). 877

Progressive (normal to strike slip) motion on these faults led to the formation of 878

dilatant jogs (Figure 17f), to zones of extensive fragmentation (Figure 19b, d, e) and 879

re-cementing. This organizes the damage zone into a mature fault zone that strikes at 880

100º-110º, at a 10-20° angle to the veins. 881

The evolution of such a mechanical system can be expected to be very complex as the 882

fractures (zones of weakness) are rapidly cemented, restoring mechanical strength and 883

re-sealing the fracture for fluid flow. Cemented aggregates of host rock fragments, 884

broken and re-cemented calcite crystals indicate that the mechanical properties as well 885

as the transport properties of the fault system changed frequently throughout time 886

(Figure 20). Progressive movement on these faults, up to offsets of several hundred 887


meters tends to make the fault zones wider and more complex, but without major 888

change in structural elements. 889

890

Figure 31

891

892

Many observations indicate a systematic change in fault zone structure as a function 893

of position in the mechanical stratigraphy, and are therefore dependent on where the 894

observations have been made. 895

896

Massive carbonate layers commonly contain single, steep fault planes and very 897

narrow damage zones in the center (Figure 18, Figure 31). When this layer is in 898

contact with thick argillaceous (weak) beds, these often develop a gradual monocline 899

rather than localized offset. The transition between the monocline in the soft layer and 900

the single fault plane in the hard layer contains a number of splays (Figure 18d, Figure 901

22, Figure 26). 902

903

Outcrop observations in profile view are commonly done on hard cliffs of wadi walls 904

(Figure 18, Figure 31) exposing the single-plane section (except the relay structure of 905

Figure 25d), whereas map view observations come from the top surfaces of hard 906

layers, on which the splayed faults are exposed (Figure 19, Figure 31). 907

908

909

Calcite Rhombohedra 910


The growth of the massive calcite rhombohedra (Figure 21) is interpreted to have 911

taken place in dilatant jogs of up to 60 cm length, in open space. Changes in color and 912

growth zonation indicate changes in fluid composition or other external conditions 913

(Hilgers et al. 2006b, Nollet et al. 2005, 2006). 914

915

916

Un- and partly cemented joints and neotectonics (d4). 917

The uncemented joints exposed in plane and profile view all strike into the directions 918

of the v1 group (Figure 32i, Figure 28, Figure 29). They generally have higher 919

densities and locally much higher densities. In some cases beds with open joints are 920

on top of beds with cemented hairline fractures with the same orientation. This 921

indicates that some open joints have formed as a result of dissolution and weathering. 922

Another explanation could be that the joints were formed as relaxation fractures at 923

exhumation, which would explain the ubiquitous presence. In the latter case, the 924

denser spacing could reflect a change of the elastic properties of the rock as an effect 925

of its P/T path. 926

927

The influence of the neotectonic movements is a possible contributor as well (Figure 928

30). Fractures in cemented terraces inside Wadi Nakhr indicate recent tectonic 929

movement that presumably contributed to the erosion of the Wadi Nakhr canyon in 930

the Pleistocene (Kusky et al. 2005, Rodgers & Gunatilaka 2003). 931

932

933

Summary 934

935


Figure 32

936

In summary, detailed field observations in excellent exposures provide the basis for a 937

consistent and robust model of the multiphase evolution of the Jabal Shams high 938

pressure cell in accordance with the work of Hilgers et al. (2006a). This evolution is 939

illustrated by the schematic drawing shown in Figure 32. The earliest structures v1 are 940

a series of anticlockwise rotating tension veins. The first of these formed in a 941

north/south trending direction (Figure 32b), followed by a set striking approximately 942

130º (Figure 32c), 090º (Figure 32d) and 045º (Figure 32e). All vein sets are 943

perpendicular to the bedding, show large apertures and blocky calcite cement. The 944

geometry of the fractures show no signs for interaction between the fracture sets in 945

terms of abutting or curving. Rapid sealing of the fractures, restoring tensile strength 946

is interpreted to be the major cause for the dense spacing of this pattern. Veins of this 947

stage show a rock-buffered isotopic signature (Hilgers et al. 2006a) consistent with 948

the limited vertical extent and the rapid sealing of the fractures. The tensile effective 949

stresses required for the formation of this regional vein system may have been formed 950

in response to overpressure build-up during burial, perhaps in combination with outer-951

arc extension during emplacement of the Hawasina/Semail nappe. 952

953

The position of the isolated ramp structure (d*) is not yet understood in detail. The 954

joints in the ramp are normal to bedding, suggesting that the ramp postdates the 955

jointing process. The S to SW vergence of the ramp could indicate its relation to the 956

emplacement of the Hawasina and Semail nappes. Figure 32f, is our best guess of the 957

evolution of the ramp. 958

959


The next stage is bedding parallel shear (d2), which indicates a major change of the 960

effective stress tensor (Hilgers et al. 2006a). Bedding parallel veins carbonates 961

indicate fluid pressures close to lithostatic. Arrays of re-cemented rock flakes show 962

that also in this case cementation repeatedly restored the strength during deformation. 963

The shear movement is top to north and top to east (Figure 32e). This direction - 964

opposite to the nappes' emplacement - suggests that these shear zones are formed after 965

the nappe emplacement. An event like this is discussed in detail by Al-Wardi and 966

Butler (2006). 967

968

The next major change in effective stress under continuously high fluid pressures led 969

to strongly dilatant normal- to strike slip faults with a distinct isotopic signature 970

indicating meteoric influence, draining the high-pressure system (Hilgers et al. 971

2006a)(d3) that offset the bedding-parallel shear zones (Figure 32g). The faults 972

nucleated as en-échelon vein sets or along the pre-existing veins in the 090º and 130º 973

strike direction. This means that these faults cannot be simply used to infer the 974

principle stress directions, because of the anisotropy. 975

This weak lateral mechanical anisotropy, combined with the mechanical layering, 976

produces distinct fault zone structures as shown in Figure 31. Important elements are 977

refraction at bedding planes and splaying of single fault strands towards the contact 978

with weaker material (Figure 31). 979

980

Also in this stage of evolution the environment was chemically active and led to 981

healing of the fault gouge, as shown by several generations of deformed, brecciated 982

fault cement, cemented in turn by less deformed calcite. 983

984


985

The high-pressure cell of Jabal Shams proves to be a natural example to show 986

thermal, hydraulic, mechanical processes (e.g. Bradley 1975, Noorishad et al. 1984, 987

Olsson & Barton 2001, Tsang et al. 2004, Zoback 1978). The high fluid pressures and 988

the repeated cementation of the fracture system led to the formation of a complex 989

fracture network. Its transport properties as well as the mechanical strength of the 990

system are constantly changing as the calcite cement seals the fractures. 991

992

993


Acknowledgements 994

The authors would like to thank Henk Droste and Pascal Richard for their 995

contributions in respect to the local geology. Erik Wanningen and Johannes 996

Schoenherr were invaluable for the success of the fieldwork. Klaus Reicherter is 997

thanked for spotting the neotectonic faults within Wadi Nakhr. This project was 998

funded by Shell International Exploration and Production B.V. 999


- Figure captions - 1000

1001

Figure 1- Falsecolor Landsat images showing the complex geology of the Oman 1002

Mountains (Images from Global Landcover Facility). 1003


Figure 2 - Landsat scene of the Northeast part of the Oman Mountains. The 1004

prominent elements are the higher metamorphic Saih Hatat culmination in the 1005

Northeast and the Jabal Akhdar domal structure with lower metamorphic conditions. 1006

Surface exposure of the Semail ophiolite is visible at many parts of the image. The 1007

white box on the southern flank of the anticline marks the field area (Landsat ETM 1008

scene (NASA Landsat Program 2000); Band 3,2,1 as RGB, pansharpened and 1009

manually stretched. UTM Grid 40N; WGS-84). 1010


Figure 3 - Perspective view of the Landsat image draped on an ASTER DEM. The 1011

view is to the NE and shows the topography without vertical exaggeration. The field 1012

area on the southern flank of the Jabal is cut by numerous valleys that offer profile 1013

views up to a kilometer height (Landsat ETM scene (NASA Landsat Program 2000); 1014

Band 3,2,1 as RGB; Pan-sharpened and with manual stretch; refer to previous image 1015

for scale) 1016


Figure 4 - Simplified lithological column of the strata exposed on the western wall of 1017

Wadi Nakhr showing the resistance to weathering as a measure for the relative 1018

strength. The stacking pattern impacts the cliff profile of many wadis (Simplified 1019

representation of the bed stacking, Not to scale). 1020


Figure 5 - Collage of photos taken from the East flank of Wadi Nakhr showing the 1021

complete lithological column exposed at the western Wall of the canyon. Note how 1022

the stacking of the formations influences the cliff profile (View towards the W., 1023

perspective distortion applies; Column height is more than 1,000 m) 1024


Figure 6 - (A) View across Wadi Nakhr towards the E. The flank shows exposures of 1025

the Shu’aiba formation. In the cliff profile the Hawar member and the Kharaib 1026

formation are exposed as well as underlying formations. The height of the topmost 1027

Shu’aiba cliff is approximately 100 m. 1028

(B) More detailed view onto the Shu’aiba formation exposed on the eastern flank of 1029

the mapping area. The dense fracture pattern is visible by slightly brighter streaks. In 1030

the distance the softer Nahr Umr formation is exposed. (View towards the E, 1031

thickness of the Shu’aiba Formation is approximately 100 m). 1032


Figure 7 - Figure collection shows outcrops with veins perpendicular to the bedding. 1033

(A) Isolated competent bed within the Nahr Umr formation shows veins with 1034

apertures more than 10 cm. The layer-confined fractures show a spacing of 1035

approximately 2 m. (B) Small-scale layer confined veins in a small competent bed 1036

sandwiched between argillaceous materials. Note the short length of the features as 1037

well as their dense spacing. (C) View onto a bedding surface. The fractured patch is a 1038

remnant of the overlying layer. Although this layer is intensively fractured, no 1039

fractures are found in the underlying material implying that brittle deformation 1040

occurred in the top layer and more ductile deformation is the lower layer. (D) 1041

Exposed carbonate bed with different sets of veins and joints. Cementation, spacing 1042

and apertures differ between the different sets. (E) Profile view of layer-confined 1043

veins in competent units sandwiched between argillaceous units (coin for scale). (D) 1044

Overview of previous image showing differences in the spacing between neighboring 1045

layers (Natih formation). (G) Bedding surface with systematic and non-systematic 1046

veins. The massive veins are sub-parallel and differ in spacing among the different 1047

exposed beds in the middle left of the picture (Nahr Umr formation). 1048


Figure 8 - Unsystematic veins occur either in bundles or in an isolated manner. The 1049

cemented features commonly show long continuous, linear segments. Terminations 1050

are commonly splays, branches or en-échelon segments indicating in plane 1051

movement. (A) Long linear veins with variance in spacing. (B) Branching vein 1052

network with massive apertures along individual strands. (C) Linear vein segment 1053

with splays at its termination. Note the presence of subparallel hairline fractures. (D) 1054

Complex vein network on polished surface with multiple directions. (E) Massive vein 1055

subparallel to hairline fractures shows massive aperture and en-échelon termination. 1056

(F) Vein arrays with braided terminations 1057


Figure 9 - Variations on the vein’s internal structures and the vein’s morphology: (A) 1058

Embedded rock fragments in a massive vein indicating repeated stages of failure and 1059

healing. (B) Two small linear vein segments with abundant splay fractures indicating 1060

a dextral component. (C) Linear array of riedel structures (Shoe tip for scale). (D) 1061

Conjugate set of riedel structures (Coin for scale). (E) The feathered fractures at the 1062

terminations of these riedels indicate multiple fracture and healing phases (Pen for 1063

scale). 1064


Figure 10 - Directional statistics on the joint population as derived from the satellite 1065

image. (Left) Rose diagram showing the four main strike directions. (Middle) 1066

Directional plot of the maximum length shows significantly longer joints in the 130°-1067

140° strike direction which could misinterpret faults. (Right) Directional plot of the 1068

mean joint length. 1069


Figure 11 - Examples for overprinting relationships: (A) A thin vein striking 1070

approximately 45º offsets two 130º striking veins. (B) Clear overprinting relationships 1071

on this outcrop due to the offset of numerous veins. (C) Surface outcrop with three 1072

visible joint directions. 1073


Figure 12 - Example of unclear overprinting relationships: (A) Two veins cut each 1074

other. Even at higher magnification it is not clear which fracture predates the other. 1075

The brighter cement of the horizontal fracture is continuous, but also the reddish rim 1076

of the vertical vein can be seen in the intersection. (B) This effect could be explained 1077

by preferential cementation that mimics the material of the wall (lower case). (C) 1078

Bounding white rims of the horizontal vein are continuous. In the intersection 1079

however are remnants of the oblique vein. 1080


Figure 13 - Table to organize the temporal overprinting relationships of the veins 1081

derived from more than 40 outcrops. The individual outcrops are listed by row (A) 1082

The relative age is presented by three colors in strike bins of 10°. ‘O’ indicates if an 1083

offset is apparent. Since more absolute groups are present than relative groups a 1084

second figure is necessary: (B) The order of the left figure is maintained but more 1085

classes are introduced. With five absolute classes the temporal relationship suggests 1086

the 0° class to be oldest, followed by the 130° set, the 90-100° set and the 45° set. The 1087

red color indicates the youngest set scattered into different strike directions (See text 1088

for details). 1089


Figure 14 - The bedding perpendicular veins (d1) are cut and offset by layer parallel 1090

veins. The latter are heavily cemented with bright calcite (Pocket knife for scale). 1091


Figure 15 - Features of the bedding parallel shear all with a top to N movement. 1092

Marked images are mirrored for dextral impression: (A) Horizontal pull-apart 1093

structures due to bedding parallel shear along an irregular clay seam of an incipient 1094

styolite. Image is mirrored to for dextral shear. (B) Stack of cemented segments 1095

formed within horizontal pull-apart. (C) Array of pinch-and-swell veins in an 1096

argillaceous unit. The latter shows the development of cleavage. (D) Close-up image 1097

of the latter formation. (E) This clay unit localizes the strain; note the boudinage of 1098

the overlying carbonate. (F) Close-up image of the latter showing sigmoidal Qz clasts 1099

and intense deformation. (G) S/C textures within clay rich unit. (H) Heavily deformed 1100

and cemented deformation zone within a horizontal shear zone. Image is approx. 70 1101

cm across. 1102


Figure 16 - Fault map of the mapping area with markers for the estimated offset. Two 1103

major faults (shaded) cut the field area in the eastern part with several hundred 1104

meters offset, whereas the majority of the faults are much smaller (UTM-40N, WGS-1105

84, grid spacing is 5,000 m) 1106


Figure 17 - Low offset normal faults and associated structures: (A) Normal fault 1107

cutting the bedding parallel veins. The fault plane is cemented. (B) Aligned tension 1108

gashes show nucleation of incipient faulting. (C) Tension gashes in a normal fault 1109

indicate the transition from brittle to ductile deformation. (D) Conjugate set of 1110

cemented fractures as a precursor of fault nucleation. (E) Reactivation of joints as 1111

first fault nucleation in an area with a distinct mechanical anisotropy. (D) Dilatant 1112

jog as a result of fault plane refraction. 1113


Figure 18 - Intermediate offset faults: (A) Normal fault with a sharply localized fault 1114

plane. (B) Detailed view of the latter with person for scale. (C, D) Normal fault with a 1115

sharply localized fault plane. The detailed view shows an increased degree of damage 1116

in the strong layers as the fault splays at the interface to weaker layers. (E) Steeply 1117

dipping fault with sharp localization. (F) Fault cutting the Natih formation changes 1118

from a single strand system to a braided system within the central interbedded 1119

section. 1120


Figure 19 - Normal fault systems with wide deformation zones: 1121

(A) This fault shows a thick fault core with a well-defined breccia, localizing the 1122

throw. Some deformation is however distributed to neighboring joint planes or veins. 1123

Red discoloring is present among some of the strands. (B, D, E) Massive deformation 1124

zone of a medium offset fault. The strain is largely delocalized to form a complex 1125

network of fractures using some of the preexisting joint planes. (C) Fault zone with 1126

intense fragmentation and cementation. 1127


Figure 20 - Detailed images of fault structures: (A) Broken and re-cemented calcite 1128

crystals forming a breccia with dark host rock fragments and a reddish cement. (Scale 1129

bar shows centimeter increments). (B) Small normal fault with multiple cement 1130

materials. (C) Dextral shear vein with blocky calcite in the central part documenting 1131

a stage with a massive open-mode component. (D) Thick cemented zone with 1132

numerous embedded rock flakes indicating repeated failure and healing stages. 1133


Figure 21 - Found along the traces of medium faults, zones of poor outcrop 1134

conditions expose massive calcite rhombohedra with more than half a meter length. 1135

The idiomorphic crystals commonly show color zonations and occasionally styolites. 1136

Image C shows a massive cemented block in a normal fault segment of a small graben 1137

system. 1138


Figure 22 - View across Wadi Nakhr onto the western cliff face exposing the southern 1139

major fault. Showing a single horst structure at the base (Shu’aiba and Kharaib 1140

formations), the fault segments into a number of horst and graben structures in the 1141

Natih formation at the surface. The intercalated Nahr Umr formation shows less 1142

defined fault planes. (View to W, Cliff face is approx. 400-500 m tall) 1143


Figure 23- Road cut exposure in the East of the field area showing the deformation 1144

zone of the southern fault. A clay rich zone of few 10’s of meters width shows intense 1145

deformation and mixing as this section offsets the Natih against the Shu’aiba 1146

formation. Presumably the material belongs to the intercalated Nahr Umr formation. 1147


Figure 24 - Images related to the northern major fault: (A) Topographic ridge made 1148

entirely of the massively cemented deformation zone of the northern fault. (B) View 1149

along the backside of the deformation zone. Note that the background shows the drag 1150

folds of the Rayda formation. No weathering resistant ridge is developed there. (C) 1151

View towards the W onto the massive deformation zone of the northern fault. The zone 1152

of drag folds and higher deformation is some 10’s of meters wide. Note how the 1153

density of the vegetation increases within this zone suggesting a higher permeability. 1154

(D) Drag fold of the Rayda formation at the northern fault. (E) Quickbird subset to 1155

indicate the location of the shown images (Quickbird panchromatic band; UTM-40N, 1156

WGS-84). 1157


Figure 25- Multidimensional image set on the northern major fault on the W-wall of 1158

Wadi Nakhr (A). The normal fault has an offset of approx. 60 m branching to a few 1159

strands in the bottom part (B). Here (C) the fault shows sub-parallel fault planes 1160

rotating blocks, deforming intercalated layers with drag folds and intense 1161

fragmentation (D, Person for scale in the right fault strand). 1162


Figure 26 - Interpreted cliff exposure of the faults of the northern fault zone exposed 1163

at the western wall of Wadi Nakhr. The vertical exposure spans several formations 1164

and shows a straight segment where the thick stack of the Shu’aiba and Kharaib 1165

formation are offset against each other. The fault however splays towards the contact 1166

to the upper Nahr Umr formation that deforms more like a monocline (Perspective 1167

distortion applies; see Figure 4 for formation thicknesses’). 1168


Figure 27- (A, B) Small-scaled ramp structure on the southern flank of the anticline. 1169

The ramp forms a topographic ridge, which can be traced a few hundred meters (Both 1170

views are to the SE). Presumably the entire ridge is formed by this ramp (Aerial photo 1171

draped on DEM, View to E, image is approximately 2 km wide) 1172


Figure 28- Partly cemented and uncemented joints. Within the field area a large 1173

number of joints with no, or only little cement is found. Abutting is commonly found 1174

among these fractures and the strike directions correspond to the early veins. (A) 1175

Irregular joint set with different strike directions in a carbonate bed with cemented 1176

fractures. (B) Cliff exposures show the intense jointing in virtually every layer. Note 1177

that the spacing is smaller as the bed thickness. (C) Orthogonal joint set showing only 1178

two strike directions. (D) Densely jointed cliff faces. (E) Patch with high joint density, 1179

note three dominant strike directions and remnants of cement. (F) Outcrop surface 1180

with dense joint pattern with three strike directions (View to the W). 1181


Figure 29 - (A) Within some of the high-density joint patches remnants of cement are 1182

found. (B) The irregular patches in this case are accompanied with deeper lying beds 1183

that are unjointed In the latter material cemented hairline fractures suggest that the 1184

overlying joints are a formed exclusively by dissolution or insolation. 1185


Figure 30 - Neotectonic features: (A) Fracture systems as such on the shown image 1186

cut the cemented terraces in Wadi Nakhr indicating recent tectonic movement. (B) 1187

The orientation of the fractures corresponds with the axis of Wadi Nakhr suggesting 1188

that the formation of this large valley was dictated by these fracture systems. 1189


Figure 31 – The normal faults develop single strand systems in thick competent layers 1190

(brown) and monoclines in softer layers (gray). Between these end-members a 1191

splayed system develops. Field observations on the normal faults are commonly made 1192

within the wadis exposing predominantly the competent layers, whereas the surface 1193

observations are made on the exposed top of the latter layers. 1194


Figure 32 - The evolution of the regional fracture network is interpreted to be a result 1195

of a multiphase deformation: The predominantly carbonate rock material (A) formed 1196

sets of joints with prominent apertures. (B, C, D, E) These fractures are formed 1197

perpendicular to the bedding probably as a response to high fluid pressures. The 1198

open-mode fractures are effectively cemented with white calcite. An isolated ramp 1199

structure is interpreted to have formed next with a top to S/SW movement. The role 1200

and the temporal relationship are not yet clear (F). Bedding parallel shear with a top 1201

to N, NE movement postdates the bedding perpendicular veins (G) forming layer 1202

parallel veins and argillaceous shear zones. Normal faulting (H) develops next 1203

nucleating partly along the weak anisotropy of the veins. The normal fault system 1204

forms anastomosing networks striking approximately 110°. (I) Exhumation and 1205

exposure to weathering lead to the opening of joints (Simplified sketch, not to scale; 1206

arrow points N). 1207


- References - 1208

Al-Wardi, M. 2006. Structural evolution of the Jebel Akhdar culmination and its 1209 implications for exhumation processes in the northern Oman Mountains, 1210 University of Leeds. 1211

Al-Wardi, M. & Butler, R. W. H. 2006. Constrictional extensional tectonics in the 1212 northern Oman Mountains, its role in culmination development and the 1213 exhumation of the subducted Arabian margin. In: Deformation of the 1214 Continental Crust: The Legacy of Mike Coward (edited by Ries, A. C., Butler, 1215 R. W. H. & Graham, R. H.) 272. Geological Society of London, 187-202. 1216

Bai, T. 2000a. Explanation for fracture spacing in layered materials. In: Nature 1217 London (edited by Pollard, D. D. & Gao, H.) 403. Macmillan Journals: 1218 London, United Kingdom, United Kingdom, 753. 1219

Bai, T. 2000b. Fracture spacing in layered rocks; a new explanation based on the 1220 stress transition. In: Journal of Structural Geology (edited by Pollard, D. D.) 1221 22. Pergamon: Oxford-New York, International, International, 43. 1222

Bai, T. & Pollard, D. D. 1998. Mechanical prediction of fracture aperture in layered 1223 rocks under extension. Eos, Transactions, American Geophysical Union 79, 1224 812. 1225

Bai, T. & Pollard, D. D. 2000. Closely spaced fractures in layered rocks: initiation 1226 mechanism and propagation kinematics. Journal of Structural Geology 22, 1227 1409-1425. 1228

Barton, N., Bandis, S. & Bakhtar, K. 1985. Strength, deformation and conductivity 1229 coupling of rock joints. International Journal of Rock Mechanics and Mining 1230 Science & Geomechanics Abstracts 22(3), 121. 1231

Beurrier, M., Bechennec, F., Hutin, G. & Rabu, D. 1986. Rustaq, Geological Map 1232 Oman 1:100,000; Sheet NF40-3D. Ministry of Petroleum and Minerals, 1233 Sultanate of Oman. 1234

Bonnet, E., Bour, O., Olding, N. E., Davy, P., Main, I., Cowie, P. & Berkowitz, B. 1235 2001. Scaling of fracture systems in geological media. Reviews of Geophysics 1236 39(3), 347-383. 1237

Bradley, J. S. 1975. Abnormal formation pressure. In: AAPG Bulletin 59. American 1238 Association of Petroleum Geologists: Tulsa, OK, United States, United States, 1239 957. 1240

Bradley, J. S. 1994. Pressure compartments in sedimentary basins; a review. In: 1241 AAPG Memoir (edited by Powley, D. E.) 61. American Association of 1242 Petroleum Geologists: Tulsa, OK, United States, United States, 3. 1243

Breton, J.-P., Béchennec, F., Le Métour, J., Moen-Maurel, L. & Razin, P. 2004. 1244 Eoalpine (Cretaceous) evolution of the Oman Tethyan continental margin: 1245 Insights from a structural field study in Jabal Akdhar (Oman mountains). 1246 GeoArabia 9(2), 1-18. 1247

Buchem, F. S. P. v., Pittet, B., Hillgärtner, H., Grötsch, J., Mansouri, A. I. A., Billing, 1248 I. M., Droste, H. H. J., Oterdoom, W. H. & Steenwinkel, M. v. 2002. High-1249 resolution Sequence Stratigraphic Architecture of Barremian/Aptian 1250 Carbonate Systems in Northern Oman and the United Arab Emirates (Kharaib 1251 and Shu'aiba Formations). GeoArabia 7(3), 461-501. 1252

de Keijzer, M., Hillgartner, H., al Dhahab, S. & Rawnsley, K. 2007. A surface–1253 subsurface study of reservoir-scale fracture heterogeneities in Cretaceous 1254 carbonates, North Oman. In: Fractured Reservoirs (edited by Lonergan, L., 1255


Jolly, R. J. H., Rawnsley, K. & Sanderso, D. J.). Geological Society of London 1256 Special Publications. 1257

El-Shazly, A. K., Bröcker, M., Hacker, B. & Calvert, A. 2001. Formation and 1258 exhumation of blueshists and eclogites from NE Oman: New perspectives 1259 from Rb-SR and 40Ar/39Ar dating. J. metamorphic Geol. 19, 233-248. 1260

Engelder, T. 1987. Joints and shear fractures in rock. Acad. Press: London, United 1261 Kingdom, United Kingdom, 27. 1262

Engelder, T. 1990. Natural hydraulic fracturing (edited by Lacazette, A.). A. A. 1263 Balkema: Rotterdam, Netherlands, Netherlands, 35. 1264

Ferrill, D. A. & Morris, A. P. 2003. Dilational normal faults. Journal of Structural 1265 Geology 25(2), 183-196. 1266

Filbrandt, J. B., Al-Dhahab, S., Al-Habsy, A., Harris, K., Keating, J., Al-Mahruqi, S., 1267 Ozkaya, S. I., Richard, P. D. & Robertson, T. 2006. Kinematic interpretation 1268 and structural evolution of North Oman, Block 6, since the Late Cretaceaous 1269 and implications for timing of hydrocarbon migration into Creaceous 1270 reservoirs. GeoArabia 11(1), 97-140. 1271

Glennie, K. W. 2005. The Geology of the Oman Mountains - An outline of their 1272 origin. Scientific Press Ltd. 1273

Glennie, K. W., Boeuf, M. G. A., Hughes Clarke, M. W., Moody-Stuart, M., Pilaar, 1274 W. F. H. & Reinhardt, B. M. 1974. Geology of the Oman Mountains (Parts 1,2 1275 and 3). Verhandelingen Koninklijk Nerderlands Geologie en Mijnbouw 1276 Genootschap 31, 423 p. 1277

Gray, D. R., Gregory, R. T., Amstrong, R. A., Richards, I. J. & Miller, J. M. 2005a. 1278 Age and stratigraphic relationships of structurally deepest level rocks, Oman 1279 Mountains> U-Pb SHRIMP evidence for Late Carboniferous Neothethys 1280 rifting. Journal of Structural Geology 27, 611-626. 1281

Gray, D. R., Gregory, R. T. & Miller, J. M. 2005b. Comment on "Structural 1282 evolution, metamorphism and restoration of the Arabian continental margin, 1283 Saih Hatat region, Oman Mountains" by M.P. Searle et al. Journal of 1284 Structural Geology 27, 371-374. 1285

Gray, D. R. & Miller, J. M. 2000. A new structural profile along the Muscat-Ibra 1286 transect, Oman: Implications for emplacement of the Samail ophiolite. In: 1287 Ophiolites and Oceanic Crust: New Insights from Field Studies and the Ocean 1288 Drilling Programme (edited by Dilek, Y., Moores, E. M., Elthon, D. & 1289 Nicholas, A.) Special Paper, vol. 349. Geological Society, America, 513-523. 1290

Hancock, P. L. 1985. Brittle microtectonics: principles and practice. Journal of 1291 Structural Geology 7(3-4), 437-457. 1292

Hilgers, C., Kirschner, D. L., Breton, J.-P. & Urai, J. L. 2006a. Fracture sealing in a 1293 regional, high-pressure cell in Jabal Akhdar, Oman mountains - first results. 1294 Geofluids 6(2). 1295

Hilgers, C., Nollet, S., Schoenherr, J. & Urai, J. L. 2006b. Paleo-overpressure 1296 formation and dissipation in reservoir rocks. Oil Gas European magazine 2, 1297 68-73. 1298

Hillgärtner, H., van Buchem, F. S. P., Gaumet, F., Razin, P., Pittet, B., Grötch, J. & 1299 Droste, H. H. J. 2003. The Barremian-Aptian Evolution of the Eastern Arabian 1300 Carbonate Platform Margin (Northern Oman). Journal of Sedimentary 1301 Research 73(5), 756-773. 1302

Holland, M., Saxena, N. & Urai, J. L. this volume. Evolving anatomy of a high 1303 pressure carbonate reservoir (II) – Interpretation of remote sensing data of 1304 Jabal Shams, Oman Mountains. GeoArabia this volume. 1305


Holland, M., Urai, J. L. & Schoenherr, J. in preparation. Anastomosing Crack-Seal 1306 Patterns - A special form of Zebra carbonate. 1307

Hughes Clarke, M. W. 1988. Stratigraphy and rock unit nomenclature in the oil-1308 producing area of interior Oman. Journal of Petroleum Geology 11(1), 5-60. 1309

Koehn, D. & Passchier, C. W. 2000. Shear sense indicators in striped bedding-veins. 1310 Journal of Structural Geology 22(8), 1141-1151. 1311

Kusky, T., Robinson, C. & El-Baz, F. 2005. Tertiary-Quaternary faulting and uplift in 1312 the norhern Oman Hajar Mountains. Journal of the Geological Society 162(5), 1313 871-888. 1314

Loosveld, R. J. H., Bell, A. & Terken, J. J. M. 1996. The Tectonic Evolution of 1315 Interior Oman. GeoArabia 1(1), 28-51. 1316

Miller, J. M., Gray, D. R. & Gregory, R. T. 2002. Geometry and significance of 1317 internal windows and regional sioclinal folds in northeast Saih Hatat, 1318 Sultanate of Oman. Journal of Structural Geology 24, 459-386. 1319

Miller, J. M., Gregory, R. T., Gray, D. R. & Foster, D. A. 1999. Geological and 1320 geochronological constraints on the exhumation of a high-pressure 1321 metamorphic terrane, Oman. In: Exhumation Processes: Normal Faulting, 1322 Ductile Flow and Erosion (edited by Ring, U., Brandon, M. T., Lister, G. S. & 1323 Willet, S. D.) Special Publications, vol. 154. Geological Society, London, 1324 241-260. 1325

NASA Landsat Program. 2000. ETM+, WRS-2, Path 158, Row 44; 038-450. USGS, 1326 Sioux Falls. 1327

Nguyen, T. S. & Selvadurai, A. P. S. 1998. A model for coupled mechanical and 1328 hydraulic behaviour of a rock joint. International Journal for Numerical and 1329 Analytical Methods in Geomechanics 22(1), 29-48. 1330

Nollet, S., Hilgers, C. & Urai, J. L. 2005. Sealing of fluid pathways in overpressure 1331 cells - a case study form the Buntsandstein in the Lower Saxony Basin (NW 1332 Germany). International Journal of Earth Sciences 94, 1039-1055. 1333

Nollet, S., Hilgers, C. & Urai, J. L. 2006. Experimental study of polycrystal growth 1334 from an advecting supersaturated fluid in a model fracture. Geofluids 6, 185-1335 200. 1336

Noorishad, J., Tsang, C. F. & Witherspoon, P. A. 1984. Coupled thermal-hydraulic-1337 mechanical phenomena in saturated fractured porous rocks: numerical 1338 approach. J. Geophys. Res.; Vol/Issue: 89:B12, Pages: 10,365-10,373. 1339

Olsson, R. & Barton, N. 2001. An improved model for hydromechanical coupling 1340 during shearing of rock joints. International Journal of Rock Mechanics and 1341 Mining Sciences 38(3), 317. 1342

Ortoleva, P. 1995. Genesis and dynamics of basin compartments and seals. In: 1343 American Journal of Science (edited by Al-Shaieb, Z. & Puckette, J.) 295. 1344 Kline Geology Laboratory, Yale University: New Haven, CT, United States, 1345 United States, 345. 1346

Patton, T. L. & O'Conner, S. 1988. Cretaceous flexural history of the northern Oman 1347 Mountains foredeep, United Arab Emirates. Bulletin of American Association 1348 of Petroleum Geologists 72, 797-807. 1349

Pillevuit, A., Marcoux, J., Stampfli, G. & Baud, A. 1997. The Oman Exotics: a key to 1350 the understanding of the Neotethyan geodynamic evolution. Geodinamica 1351 Acta 10(5), 209-238. 1352

Pollard, D. D. 1979. On the mechanical interaction between a fluid-filled fracture and 1353 the Earth's surface. In: Tectonophysics (edited by Holzhausen, G.) 53. 1354 Elsevier: Amsterdam, Netherlands, Netherlands, 27. 1355


Pollard, D. D. 1987. Elementary fracture mechanics applied to the structural 1356 interpretation of dykes. In: Special Paper - Geological Association of Canada 1357 34. Geological Association of Canada: Toronto, ON, Canada, Canada, 5. 1358

Pratt, B. R. & Smewing, J., D. 1993. Early Cretaceous platform-margin configuration 1359 and evolution in the central Oman Mountains, Arabian Peninsula. AAPG 1360 Memoir 56, 201-212. 1361

Rodgers, D. W. & Gunatilaka, A. 2003. Bajada formation by monsoonal erosion of a 1362 subaerial forebulge, Sultanate of Oman. Sedimentary Geology 154(3-4), 127. 1363

Searle, M. P. 2007. Structural geometry, style and timinig of deformation in the 1364 Hawasina Window, Al Jabal al Akhdar and Saih Hatat culminations, Oman 1365 Mountains. GeoArabia 12(2), 99-130. 1366

Searle, M. P., Warren, C. J. & Parrish, R. R. 2005. Reply to: Comment by Gray, 1367 Gregory and Miller on "Structural evolution, metamorphism and restoration of 1368 the Arabian continental margin, Saih Hatat region, Oman Mountains". Journal 1369 of Structural Geology 27, 375-377. 1370

Searle, M. P., Warren, C. J., Waters, D. J. & Parrish, R. R. 2004. Structural evolution, 1371 metamorphism and restoration of the Arabian continental margin, Saih Hatat 1372 region, Oman Mountains. Journal of Structural Geology 26(3), 451-473. 1373

Sharland, P., Casey, D. M., Davies, R. B., Simmons, M. B. & Sutcliffe, O. E. 2004. 1374 Arabian Plate Sequence Stratigraphy. GeoArabia 9(1), 199-214. 1375

Tsang, C.-F., Stephansson, O., Kautsky, F., Jing, L. & Ove, S. 2004. Coupled thm 1376 processes in geological systems and the decovalex project. In: Elsevier Geo-1377 Engineering Book Series Volume 2. Elsevier Science Ltd, 3. 1378

Tsang, C. F., Stephansson, O. & Hudson, J. A. 2000. A discussion of thermo-hydro-1379 mechanical (THM) processes associated with nuclear waste repositories. 1380 International Journal of Rock Mechanics and Mining Sciences 37(1-2), 397. 1381

Warbuton, J., Burnhill, T. J., Graham, R. H. & Isaac, K. P. 1990. The evolution of the 1382 Oman Mountains foreland basin. In: The Geology and Tectonics of the Oman 1383 Region (edited by Robertson, A. H. F., Searle, M. P. & Ries, A. C.) Special 1384 Publication no. 49. Geological Society of London, 419-427. 1385

Warren, C. J. & Miller, J. M. 2007. Stuctural and stratigraphic controls on the origin 1386 and tectonic history of a subducted continental margin, Oman. Journal of 1387 Structural Geology 29, 541-558. 1388

Warren, C. J., Parrish, R. R., Searle, M. P. & Waters, D. J. 2003. Dating the 1389 subduction of the Arabian continental margin beneath the Semail ophiolite. 1390 Oman. Geology 31(10), 889-892. 1391

Wilson, H. H. 2000. The age of the Hawasina and other problems of Oman Mountain 1392 Geology. Journal of Petroleum Geology 23(3), 345-362. 1393

Ziegler, M. A. 2001. Late Permian to Holocene Paleofacies Evolution of the Arabian 1394 Plate and its Hydrocarbon Occurrences. GeoArabia 6(3), 445-503. 1395

Zoback, M. D. 1978. Hydraulic fracture propagation and the interpretation of 1396 pressure; time records for in situ stress determinations. Proceedings - 1397 Symposium on Rock Mechanics, 14. 1398

1399 1400

54°0'0"E

54°0'0"E

56°0'0"E

56°0'0"E 58°0'0"E

58°0'0"E

60°0'0"E

60°0'0"E

22°0'0"N 22°0'0"N

24°0'0"N 24°0'0"N

26°0'0"N 26°0'0"N

28°0'0"N 28°0'0"N

100

Kilometers

OMAN

U.A.E.

IRAN

Gulf of Oman

Arabian Gulf

Dubai

Abu Dhabi

Masqat

Musandam

Om

an

Mountains

Batain

520000.000000

520000.000000

540000.000000

540000.000000

560000.000000

560000.000000

580000.000000

580000.000000

600000.000000

600000.000000

620000.000000

620000.000000

640000.000000

640000.000000

660000.000000

660000.000000

2540000

.00

00

00

2540000

.00

00

00

2560000

.00

00

00

2560000

.00

00

00

2580000

.00

00

00

2580000

.00

00

00

2600000

.00

00

00

2600000

.00

00

00

2620000

.00

00

00

2620000

.00

00

00

0 16Kilometers

Masqat

Jebel Akhdar Antiform

Gulf of Oman

SemailOphiolithe

Nizwa

Saih Hatat

Lim

it o

f th

e La

nd

sat s

cen

e

Al Hamra

Ghool

‘Oman Exotic

s’ Semail ophiolite nappe

Nakhr

Field area

Jebel Shams

Tanuf

100 m

Rayda Fm.

Salil Fm.

Habshan Fm.

Lekhwair Fm.

Lower Kharaib Fm.

Hawar Mbr.

Shuaiba Fm.

Nahr Umr Fm.

Natih Fm.

Upper Kharaib Fm.

e

f+g

c+d

b

a

Simplified lithological column Simplified infered cliff profile

94 Ma WA

SIA

GR

OU

PK

AH

MA

H G

RO

UP

SAHTAN GROUP

146 Ma

136 Ma

135 Ma

129 Ma

126 Ma

123 Ma

121 Ma

113 Ma

112 Ma

103 Ma

96 Ma

S/SW N/NE

NATIH

c,d

e

fg

NAHR UMR

SHUAIBA

Hawar Mbr.

upper KHARAIB

lower KHARAIB

LEKHWAIR

HABSHAN

SALIL

RAYDAH

SHATAN Group

Shuaiba Fm

Hawar Mbr.

Nahr Umr. Fm

Shuaiba Fm.

A

B

A B

C D

E F

G

2 m

2 m

2 cm

0.4 m10 cm

5 cm

0.7 m

A B

C D

E F

0.5 m

5 cm

1 m

A B

C D

E

10 cm

5000

10000

1500030

60

90

120

150

180

0

Joint orientation (n=161,097)

100

200

300

400

50030

60

90

120

150

180

0

Max length [m]

10

20

30

4030

60

90

120

150

180

0

Mean length [m]

A

B

C

vein

rock matrix

fracturingcementation

A

B

C

O O

O O O

O O

O O

O O

O O

O O

O O

O O

O O

O O

O O

O O

O O

O O

O O

O O

O O

O O

YoungerYoungest

Older

Oldest

18060 90 120 1500 3018060 90 120 1500 30

Relative order Absolute order (5 groups)

A B

M

M

A B

C D

E F

G H

MM

A

B C

D

E

F

15 cm

30 m

10 cm

1.5 m 10 cm

20 m

A B

C D

F

E

2 m

1.5 m

2 m6 m

A B

C

D

E

A B

C D

A

B

C

KHARAIB

HAWAR MBR.

SHUAIBA

NAHR UMR

NATIH

S

N

A

B

DC

528000.000000

528000.000000

529000.000000

529000.000000

2563000

.000000

2563000

.000000

0 200 400

Meters

A B

C D

E

S NA

B C

D

SHUAIBAHawar Mbr.

Upper KHARAIB

Lower KHARAIB

LEKHWAIR

HABSHAN

NAHR UMR

NATIH

S N

A

B

C

A B

C D

E F

Evolving anatomy of a high pressure carbonate reservoir (I ...€¦ · 107 valleys in the Natih...

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