Chapter 3 Fluid flow evolution in the Albanide fold- and-thrust … 3: Fluid...48 Chapter 3 3.2...

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Isotope analysis of fluid inclusions

de Graaf, S.

2018

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citation for published version (APA)de Graaf, S. (2018). Isotope analysis of fluid inclusions: Into subsurface fluid flow and coral calcification.

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Fluid flow evolution in the Albanide fold-and-thrust belt

Tectonic forces generated during thrust emplacement along active margins may drive complex fluid flow patterns in fold-and-thrust belts and foreland basins. In the Albanide fold-and-thrust belt, fracture-controlled fluid flow led to the development of calcite vein networks in a sequence of naturally fractured Cretaceous to Eocene carbonate rocks. Fluid inclusion isotope data of these calcite veins demonstrate that fluids circulating in the carbonates were derived from an underlying reservoir that consisted of a mixture of meteoric water and evolved marine fluids, probably sourced from deep-seated evaporites. The meteoric fluids infiltrated in the hinterland before being driven outward into the foreland basin and ascended as soon as fracturing induced a sufficient increase in permeability. The contribution of fluids derived from evaporites increases towards the thrust front in association with elevated deformation, which is expected to be the main driver of expulsing connate waters from the evaporites. Structural and petrographic observations provide time constraints for the various phases of fracture infilling and reveal an increasing dominance of meteoric water in the system through time as migration pathways shortened and marine formation fluids were progressively flushed out. Similar fluid flow evolutions have previously been recorded in various fold-and-thrust belt settings elsewhere in the world.

Based on:

De Graaf, S., Nooitgedacht, C. W., Le Goff, J., Van der Lubbe, H. J. L., Vonhof, H. B., and Reijmer, J. J. G. (accepted for publication in AAPG Bulletin) Fluid flow evolution in the Albanide fold-and-thrust belt: Insights from δ2H and δ18O isotope ratios of fluid inclusions

Nooitgedacht, C. W., De Graaf, S., Van der Lubbe, H. J. L., Vonhof, H. B., Le Goff, J., and Reijmer, J. J. G. (in prep.) Tracking fluid migration in the external Albanides using hydrogen and oxygen isotopes of fluid inclusions in calcite veins

3Chapter

Chapter 346

3.1 IntroductionFold-and-thrust tectonics is commonplace along convergent plate margins

and is associated with the development of intricate fracture and fault networks (Roure and Sassi, 1995). The structural complexity of fold-and-thrust belts opens up a wide array of conceivable fluid flow patterns (Travé et al., 2007; Morley et al., 2011; Lacroix et al., 2014). Tectonic loading due to thrust emplacement, for instance, may instigate basin-ward expulsion of fluids along large-scale migration pathways like faults or basement detachments (Oliver, 1986; Ge and Garven, 1992). The timing of fluid flow and circulating fluid types may differ considerably between foreland fold-and-thrust belts depending on their stratigraphic architecture and structural evolution (Ferket et al., 2003; Schneider et al., 2004; Barbier et al., 2012; Crognier et al., 2018). Predicting the evolution and interplay of deformation and fluid flow in foreland fold-and-thrust belts remains complicated, while it is of key importance when assessing the potential existence of aquifers or petroleum-bearing reservoirs (Agosta et al., 2010; Morley et al., 2011). Carbonate rock formations are of particular interest since the permeability of lithified limestone is generally governed by brittle deformation structures (i.e., fracture and faults) due to limited matrix porosities (Nelson, 2001).

In this Chapter, fluid inclusion isotope data of calcite veins are used to delineate the fluid flow system of the Albanide foreland fold-and-thrust belt. Prominent calcite veining in the Albanide fold-and-thrust belt exists in fracture networks in Cretaceous to Eocene Ionian Basin carbonates, which are the main reservoir rocks in the area (Velaj et al., 1999) and an analogue to producing reservoirs in the Adriatic Sea and onshore Italy (Cazzola and Soudet, 1993; Zappaterra, 1994). The carbonate rocks are mainly composed of calciclastic gravity flows and mass transport deposits (Rubert et al., 2012; Le Goff et al., 2015). Events of fracturing provided discrete pathways facilitating the episodic migration of fluids and governing the emplacement of abundant hydrocarbon accumulations in the area (Graham Wall et al., 2006). Previous vein-based fluid flow reconstructions in the Ionian Zone focused on the areas of Kremenara (Van Geet et al., 2002; Swennen et al., 2003), Shpiragu (Graham Wall et al., 2006), Kelçyrë (Vilasi et al., 2009) and Saranda (Lacombe et al., 2009). These previous researches inferred multiple fluid flow episodes throughout the evolution of the foreland fold-and-thrust belt. Although the sequence of fluid flow and vein cementation events has quite elaborately been studied, the precise origin and migration pathways of fluids requires further constraints, especially for the early-stage fluid system.

Excellent outcrops of Cretaceous to Eocene carbonate rocks are present in the Mali Gjere mountain ridge, which is a topographic expression of the Kurveleshi thrust sheet in the Ionian Zone of south Albania (Figure 3.1). In addition to studying veins from the Mali Gjere, calcite veins from a set of outcrops from the two other main thrust units in the Ionian Zone – the Çika and the Berati belt – were analyzed

Fluid flow evolution in the Albanide fold-and-thrust belt 47

Bodrishtë

JorgucatMuzinë

Frashtan

Terihat

Vanister

Mashkullorë

14

41

22

15

20

18

20

Greece

10 km

Pre-Adriatic Depression

Kruja ZoneKrasta Zone

Sazani Zone

Mirdita Zone

Albanian alps

Kora

bi

Korça Basin

Tirana

Çika belt

Kurveleshi beltBerati belt

Ionian Zone

Montenegro Kosovo

Macedonia

Greece

Legend

Cretaceous

Jurassic

Triassic

Paleocene

Neogene

Quaternary

N

Research area

Sarandë

Mali GjereÇika belt Berati belt

4 km1 km

Jurassic-Cretaceous carbonates and shales

Flysch and molasse cover

SW NEA B

Kurveleshi belt

Mali G

jere

Çika Belt

Berati Belt

Gjirokastër

Kurveleshi Belt

A

B

Triassic evaporites

Delvina oil field

A B

C

Ionian Sea

20˚ 00’ 20˚ 15’

39˚50’

40˚00’

Figure 3.1 (A) Map showing the main structural units of Albania (Moisiu and Gurabardhi, 2004) with the research area indicated by the black square. (B) Geological map of southern Albania after Moisiu and Gurabardhi (2004). Seven outcrop locations (Mashkullorë, Vanister, Terihat, Frashtan, Muzinë, Jorgucat and Bodrishtë) were studied along the Mali Gjere mountain ridge, which exposes Cretaceous to Eocene carbonate rocks within the Kurveleshi thrust sheet. Red lines depict faults. (C) Simplified geological cross-section through southern Albania after Velaj (2015), indicated by the line A-B in the geological map.

to assess spatial variations in the fluid flow evolution throughout the entire fold-and-thrust belt. Besides fluid inclusion isotope analysis, complementary petrographic and δ18O and δ13C isotope analyses of the calcite vein cements were carried out. An analysis of the arrangement and cross-cutting relations of distinct structural sets was performed to acquire a chronological framework for the various generations of fracture-infilling calcite.

Chapter 348

3.2 Background3.2.1 Geological evolution of the Ionian ZoneThe Ionian Zone is one of the structural domains that makes up the Al-

banides (Figure 3.1) and refers to an assembly of folds and thrusts of Ionian Basin deposits situated in the southwest of Albania (Meço and Aliaj, 2000; Robertson and Shallo, 2000). Deposition in the Ionian Basin was closely linked to the structural evolution of the Adriatic plate (Channell et al., 1979). In Triassic times, at least 2000 m of evaporites and shallow-water carbonates were deposited in an epicontinental marine environment (Vlahović et al., 2005; Karakitsios, 2013). Subsequent exten-sional tectonics within the Adriatic plate from the Late Triassic onward led to the development of rift basins in a horst and graben system. The Ionian Basin was such an intracontinental rift basin and formed between the Apulian and Kruja carbonate platforms (Robertson and Shallo, 2000; Heba and Prichonnet, 2009).

The syn-rift sequence in the Ionian Basin from the Jurassic until the Oligocene is characterized by sediment input from the bordering carbonate platforms (Hairabian et al., 2015; Le Goff et al., 2015). The sequence of Cretaceous to Eocene carbonate gravity flows that is presently exposed along the Mali Gjere mountain ridge exhibits a thickness of 900-1350 m (Velaj, 2015) and is mainly composed of decimeter to meter-scale strata of fine-grained mud- to wackestones with minor amounts (< 2%) of planktic foraminifera. Uplift of the hinterland provoked by the Alpine Orogeny led to an increasing input of terrigenous material into the basin and the deposition of up to 2000 m of Oligocene flysch, Neogene molasse, and Quaternary sand- and siltstones on top of the Ionian Basin carbonates (Meço and Aliaj, 2000).

The position of the Ionian Zone in the southern extremity of the Alpine orogenic belt was crucial for its structural development from the Oligocene onward (Robertson and Shallo, 2000; Nieuwland et al., 2001). Southwest-verging fold-and-thrust tectonics involving an intricate network of structural elements (e.g., ramp anticlines, back-thrusts, strike-slip faults and salt diapirs) caused a regional uplift and the formation of three main NNW-striking anticlinal thrust sheets: the Çika, Kurveleshi and Berati belt. Triassic evaporites and to lesser extent Early Jurassic Posidonia shales act as decollement levels facilitating thin-skinned thrusting (Velaj, 2002; Swennen et al., 2003). The Ionian Zone is bordered by the Vlora-Elbasan lineament in the north and extends towards the south into the Hellenides (Neumann and Zacher, 2004; Karakitsios, 2013).

3.2.2 Fluid flow in the Cretaceous to Eocene carbonatesPrimary porosity of the Cretaceous to Eocene carbonate rocks is low (0-1%)

due to compaction and calcite pore infill during early marine diagenesis (Van Geet et al., 2002; Vilasi et al., 2006; Dewever et al., 2007). Multiple events of brittle deforma-tion induced episodic increases in rock permeability and circulation of fluids in the


Cretaceous to Eocene carbonates (Graham Wall et al., 2006). Episodes of fluid flow occurred before and during the main phase of folding and thrusting (Swennen et al., 2000). Fluids were overpressured and buffered by the host-rock during the early pre-folding fracture infilling events (Van Geet et al., 2002). Meteoric fluids played an important role in vein cementation events during the late stage, which is also associated with the development of karst networks (Vilasi et al., 2006). Evidence for leaching of evaporites indicates that fluids were supplied along deep-seated decolle-ment levels (Van Geet et al., 2002; Vilasi et al., 2009).

Although burial histories may have varied throughout the entire Ionian Basin, peak burial of the carbonates is estimated to have been reached at depths of 2-4 km and temperatures of 80-100˚C in Early Miocene times (Rigakis and Karakitsios, 1998; Roure et al., 2004; Lacombe et al., 2009; Vilasi et al., 2009). Brittle deformation structures provided reservoir qualities to the Cretaceous to Eocene carbonates (Velaj et al., 1999). Hydrocarbon charge in the Late Miocene and Pliocene was derived from underlying Triassic to Jurassic shales and filled structural traps in strongly deformed parts in the north of the Ionian Zone (Zappaterra, 1994; Prifti and Muska, 2013). The Oligocene flysch deposits that overlie the carbonate reservoir rocks act as an effective seal (Velaj, 2015). Although the sealing cover has largely been removed in the central part of the Ionian Zone, hydrocarbon accumulations may still occur in structural traps associated with the impermeable Triassic evaporites (Vilasi et al., 2009). The Ionian Zone is renowned for hosting the most important petroleum accumulations in Albania (Curi, 1993; Velaj, 2015).

3.3 MethodologyAn evaluation of the structural development of the naturally fractured

Cretaceous to Eocene carbonate rocks from the Mali Gjere may allow for establishing chronological relationships of distinct generations of calcite fracture infill and tracking the hydrogeological system through time. Orientations of fractures and stylolites were measured in seven distinct outcrops (Figure 3.1B; Table 3.1) using an iPad equipped with GeoID (Lee, 2014), a professional application for measuring geological structures. Fractures were subdivided in barren ones (joints) and infill-bearing ones (veins), with the extent of fracture infill being documented for the veins (i.e., infill width).

The studied outcrops are all part of the upper part of the sequence of Cretaceous to Eocene calciturbidites (i.e., Upper Cretaceous to Eocene). An outcropping pavement or vertical section of typically 30-100 m2 (depending on structure density) was delimited and the structures within it were documented. Measurements were plotted in equal-area lower hemisphere projections using OSXStereonet (Cardozo and Allmendinger, 2013) to discern different structural sets. Abutment relationships and physical intersections between structural sets, referred to as cross-cutting relationships, were documented to determine relative time constraints for the structural sets.

Thirty-four hand samples of veins were retrieved from the same outcrops studied in the structural analysis. Collected vein samples cover the various phases of

Chapter 350

fracture infilling recognized in the Mali Gjere. Isotope analyses of calcite vein cements and fluid inclusion water were performed following the analytical protocols as described in Chapter 1. Apart from the isotope analyses, thin sections were produced from 32 vein samples for petrographic analysis of vein textures, fluid inclusion assemblages and cross-cutting relationships. The petrographic study was performed using a polarizing microscope (Nikon Polarizing Microscope Eclipse 50i POL) and a confocal laser scanning microscope (Olympus Confocal Microscope - FV3000).

NNWSSE

064/18

20 m

NNWSSE

Figure 3.2 Interpretation of fracture patterns in a gently dipping pavement in the Mali Gjere (39˚59’19.76”N, 20˚13’4.84”E). Recent dissolution activity along fracture planes enhanced the visibility of their traces, which may be continuous over tens of meters. Two systematic sets predominate in this particular outcrop: a pervasive WNW-ESE pre-folding set (blue) and a set of NE-SW syn-folding fractures (black). Measured fracture orientations are presented in the lower hemisphere stereonet projection with thick lines representing averages for the pre-folding (blue) and syn-folding (black) set.

Table 3.1 Geographic coordinates and bedding orientations (dip direction/dip angle) per outcrop location (see Figure 3.1B)

Outcrop Coordinates BeddingBodrishtë 39°53'57.93"N 20°18'24.56"E 013/14Jorgucat 39°55'49.92"N 20°16'18.84"E 068/41Muzinë 39°56'39.72"N 20°13'53.26"E 059/22Frashtan 39°58'16.99"N 20°14'1.52"E 055/20Terihat 39˚59'25.84"N 20˚12'58.35"E 064/18Vanister 40°0'50.97"N 20°11'38.05"E 036/20Mashkullorë 40°6'58.54"N 20°5'27.59"E 037/15


3.4 Results3.4.1 Structural elementsBedding in the Mali Gjere is consistently northeast dipping (Table 3.1) and

devoid of major faulting and folding (Figure 3.1B). Across the seven studied outcrops, the orientations of in total 362 structural elements were measured. Orientations of fractures exhibit a high degree of systematics (Figure 3.2) and are organized in four recurring sets (Figure 3.3). An extensive network of WNW-ESE oriented opening-mode fractures is most prominent and recognized in every outcrop. Substantial calcite fracture infill of up to 2 cm thick occurs in 82% of the fractures that belong to this structural population. In the outcrops of Bodrishtë, Terihat and Vanister, another fracture set with similar infill characteristics is present at a slightly different angle (i.e., more E-W orientation).

A third set of opening-mode fractures is oriented NNE-SSW, which is approximately perpendicular to the WNW-ESE fracture set. Although NNE-SSW fracturing is less pervasive, it is recognized in all outcrops except Terihat. Calcite infill occurs in 75% of the NNE-SSW fractures and is usually thinner (< 0.5 cm) compared to the WNW-ESE fractures. Fractures of the WNW-ESE, E-W and NNE-SSW sets display an orthogonal relationship to the bedding. A fourth deformational event recognized in the Mali Gjere is characterized by sub-vertical NE-SW oriented

n = 54Bodrishtë Jorgucat

n = 41Muzinën = 10

Frashtann = 38

Terihatn = 46

Vanistern = 84

Mashkullorën = 75

Pre-folding 1

Pre-folding 2

Syn-folding

Stylolite (plane)

Pole to fracture plane

Fracture sets

Figure 3.3 Orientations of fracture planes (poles) and tectonic stylolitic surfaces (dashed great circles) presented in contoured lower hemisphere stereonet projections for the seven outcrops studied. Squares represent averages of opening-mode fracture sets; different colors correspond to the distinct structural sets. Interpretation of structural sets is based on structure orientations and cross-cutting relationships. Bedding-parallel burial stylolites are plentiful in all outcrops, but not included in the stereonet projections for clarity. Structures that are identified to be pre-folding were rotated back to their original position at the time of development (i.e., before inclination of bedding).

Chapter 352

Figure 3.4 Compilation of small-scale structures observed in the field. (A) Cross-cutting relationships of three sets of pure-opening mode veins in Frashtan. (B) A WNW-ESE oriented vein that is cut by an NNE-SSW oriented vein in Bodrishtë. (C) A WNW-ESE oriented vein that is dissolved by a bedding-parallel burial stylolite in Vanister. (D) An NNW-SSE striking tectonic stylolite that is oriented vertically with respect to the present-day surface in Bodrishtë.

fractures. Fractures related to this stress field were not identified in every outcrop and are predominantly barren; calcite infill occurs in 37% of the fractures and is typically thin (< 0.2 cm).

Bedding-parallel stylolites are abundant in all outcrops. Besides, two sets of tectonic stylolites were recorded. One set strikes approximately NNW-SSE and displays a perpendicular relationship to the surface (Figure 3.4D). This set is well-defined and documented in all outcrops except Muzinë, being especially pervasive in the outcrops of Jorgucat and Mashkullorë. A second set of tectonic stylolites is oriented NE-SW and only observed in the outcrops of Mashkullorë and Bodrishtë.

Across the outcrops, 33 cross-cutting relations between the various structural sets were recorded, in addition to countless relationships involving burial stylolites (Figure 3.4). WNW-ESE and E-W fractures are systematically cross-cut by the other two fracture sets. The major portion of these fractures (70%) post-date burial stylolites (Figure 3.4C), whereas fractures of the NNE-SSW set display a more ambiguous relationship with burial stylolites. NE-SW fractures systematically cut through the WNW-ESE, E-W and NNE-SSW fracture sets and post-date burial stylolites. Cross-cutting relations between fractures and tectonic stylolites are scarce. Only the outcrops of Mashkullorë and Terihat present clear evidence of veins of both the WNW-ESE and NNE-SSW set being dissolved by – and thus predate – NNW-SSE oriented tectonic stylolites.

A B

DC

273/

69

147/86

228/67

WN

W-ESE

NNE-SSW


3.4.2 Vein petrographyMineral infill in fractures related to the three main deformational phases in

the Mali Gjere is uniquely composed of calcite. Bitumen staining, although common in fractures in the north of the Kurveleshi belt (Graham Wall et al., 2006), is not observed in fractures from the Mali Gjere. Calcite fracture infill is typically blocky (Figure 3.5A-B) with well-defined growth zones (Figure 3.5I), while the infill of a minor number of samples exhibits a syntaxial texture (Figure 3.5F). A coarsening inward trend is common with rims of small crystals (

Chapter 354

z = 4-12 μm

z = 10 μm

z = 7-9

μm

z = 6-

11 μm

z = 5-

6 μm

50 μm

Gas phase

A B

C

0.5 cm

10 μm

(< 20%) in two other veins (Mu4 and Fr2). The vein from which Te6 and Te7 were sampled is hosted in a chert concretion and displays a blackish color with poorly defined crystals when studied under a polarizing microscope (Figure 3.5H).

Deformational calcite twinning and banded textures are common in veins from all generations. Banding may be minor (Figure 3.5D) or highly frequent consisting of innumerable generations separated by thin bands of host rock (Figure 3.5E). Vein walls are in general matching (Figure 3.5A, 3.5C and 3.5D). Clear indications of shear were not observed in the infill of any of the fracture generations. Brecciated textures with shards of host rock dispersed in the calcite fracture infill occur in three WNW-ESE and one NNE-SSW vein (Figure 3.5G). Stylolitic surfaces developed along the walls of three E-W to WNW-ESE oriented veins (Figure 3.5B).

Veins retrieved from the Mali Gjere are dominated by primary fluid inclusions. Primary fluid inclusions are typically small (< 2 mm) and mostly occur as isolated inclusions in clusters within single crystals (Figure 3.6A-B) or organized along growth bands (Figure 3.5I). Although primary fluid inclusions are mostly monophase, larger inclusions may be two-phase with a low vapor-liquid ratio (< 0.1; Figure 3.6A). Primary fluid inclusions exhibit an equidimensional rounded or rectangular shape and do not display signs of post-trapping modifications (e.g., decrepitation or leakage). Secondary monophase fluid inclusions, which are arranged along trails that are unrelated to the crystal growth faces (Shelton and Orville, 1980; Smith and Evans, 1984; Bodnar, 2003), can reach a size of up to 10 mm and are less common than the primary ones.

Figure 3.6 Petrographic appearance of fluid inclusions hosted in calcite veins from the Mali Gjere. (A) Compiled image of a 3D confocal surface scanning analysis (up to 14 mm depth) of vein Va2 showing the distribution of primary fluid inclusions. Some of the larger inclusions display a minor gas phase (see inset). The scanning depth (z) is given for the different panels in the image. (B) Confocal image of all-liquid primary fluid inclusions in Bo6. (C) Petrographic view of secondary fluid inclusion trails (next to the red lines) in vein Te3.


3.4.3 Isotope dataFluid inclusion isotope data were obtained for 26 calcite vein samples. The

δ18Ow and δ2Hw isotope signatures hold a significant positive correlation (Figure 3.7) and range from -7.8 to 1.1‰ for δ18Ow and -56.0 to -14.7‰ for δ2Hw (Table 3.2). Isotope data plot either onto or to the right of the Global Meteoric Water Line (GMWL), which expresses the isotope fractionation of terrestrial meteoric waters as a global average (Craig, 1961). Veins belonging to the WNW-ESE, E-W and NNE-SSW fracture sets display higher isotope values than veins hosted by NE-SW fractures.

Complementary carbon and oxygen isotope data were obtained for the calcite of 36 vein samples, as well as for five host rock samples. Average values of the measurements are presented in Table 3.2. Acquired isotope values of the veins range from -6.5 to 0.9‰ for δ18Oc and from -3.0 to 3.0‰ for δ13Cc (Figure 3.8). Host rock samples display consistent isotope values throughout the various outcrops (-2.3 to -1.5‰ for δ18Oc and from 1.1 to 2.7‰ for δ13Cc). Carbon isotope values of the veins closely resemble those of the host rock, apart from a number of samples from Frashtan, Bodrishtë and Mashkullorë, which display a depletion in 13C. Veins are mostly depleted in 18O with respect to the host rock (Figure 3.8). Especially veins from the Frashtan outcrop display remarkably low δ18Oc values.

-70

-60

-50

-40

-30

-20

-10

0

10

-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4

2 Hw

(‰ v

s. V

SMO

W)

18Ow (‰ vs. VSMOW)

Seawater

GMWL

2

Figure 3.7 Fluid inclusion isotope data of calcite veins from the Mali Gjere. Fluid inclusion water displays a distinct correlation in the δ18Ow-δ2Hw space that is at a lower slope than the GMWL. See Figure 3.10 for interpretation of the data.

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ldin

g 1

1.6

-2.6

-0.8

-28.

3O

ne g

ener

ation

of b

lock

y ca

lcite

JG1b

Jorg

ucat

291/

51Pr

e-fo

ldin

g 2

1.8

-3.7

1.1

-23.

8Bl

ocky

cal

cite

exh

ibiti

ng c

urve

d de

form

ation

al

twin

sJG

2Jo

rguc

at31

0/70

?1.

8-4

.0-1

.0-1

9.6

MU

1M

uzin

ë22

4/72

Pre-

fold

ing

12.

9-1

.6-0

.7-2

9.6

Vein

com

pose

d of

blo

cky

calc

ite w

ith c

lear

twin

de

velo

pmen

tM

U2

Muz

inë

224/

72Pr

e-fo

ldin

g 1

2.8

-1.5

Bloc

ky c

alci

te w

ith b

recc

iate

d ho

st ro

ck

frag

men

tsM

U3

Muz

inë

214/

71Pr

e-fo

ldin

g 1

2.9

-1.7

Vein

ban

ding

. Blo

cky

calc

ite in

fill w

ith c

lear

twin

la

mel

lae

MU

4M

uzin

ë32

3/78

Pre-

fold

ing

2-0

.4-2

4.4

Mos

tly b

lock

y ca

lcite

with

a c

oars

enin

g in

war

d tr

end.

Par

t (±

20%

) is

fine-

grai

ned

and

diso

rgan

ized

(rec

ryst

alliz

ed)

MU

5M

uzin

ë33

0/76

Pre-

fold

ing

22.

81.

0-5

.4-4

0.3

One

gen

erati

on o

f cle

an b

lock

y ca

lcite

MU

6M

uzin

ë22

1/66

Pre-

fold

ing

13.

0-3

.2Bl

ocky

cal

cite

. A s

tylo

litic

surf

ace

deve

lope

d on

on

e ve

in w

all

FR1

Fras

htan

273/

69Pr

e-fo

ldin

g 2

1.5

-5.1

-3.7

-41.

9O

ne g

ener

ation

of b

lock

y ca

lcite

with

inte

nse

curv

ed tw

in la

mel

lae

FR2

Fras

htan

308/

85Sy

n-fo

ldin

g-0

.7-6

.5-7

.8-4

7.9

Mos

tly b

lock

y ca

lcite

. Par

t (±

10%

) is

fine-

grai

ned

and

diso

rgan

ized

(rec

ryst

alliz

ed)

FR3.

1Fr

asht

an32

7/81

Syn-

fold

ing

0.5

-5.3

-6.9

-56.

0

FR3.

2Fr

asht

an32

7/81

Syn-

fold

ing

0.5

-5.2

-6.4

-46.

3

FR4

Fras

htan

264/

67Pr

e-fo

ldin

g 2

1.5

-4.8

-3.2

-34.

1Sy

ntax

ial c

alci

te in

fill w

ith m

inor

dev

elop

men

t of

twin

lam

ella

eTE

1Te

rihat

144/

88Sy

n-fo

ldin

g1.

3-2

.8-4

.2-2

0.7

Fine

gra

ined

dis

orga

nize

d ca

lcite

(r

ecry

stal

lizati

on)

TE2

Terih

at07

9/11

Syn-

fold

ing

1.9

-3.3

Band

ed v

ein

with

a s

ynta

xial

to b

lock

y ca

lcite

in

fill.

Min

or d

evel

opm

ent o

f tw

in la

mel

lae

TE3

Terih

at16

8/80

Pre-

fold

ing

11.

8-3

.40.

3-2

5.8

Coar

se b

lock

y ca

lcite

cry

stal

s. P

erva

sive

twin

la

mel

lae.

Sty

loliti

c su

rfac

es o

n th

e ve

in w

alls

TE4

Terih

at30

6/79

Syn-

fold

ing

1.5

-1.1

Min

or v

ein

band

ing.

Blo

cky

calc

ite

TE5

Terih

at17

8/78

Pre-

fold

ing

11.

9-2

.5-0

.1-2

7.9

Bloc

ky c

alci

te in

fill w

ith c

lear

twin

lam

ella

e

TE6

Terih

at26

3/40

?1.

9-4

.3-0

.1-1

6.8

Blac

kish

and

cha

otic

infil

l. Ve

in h

oste

d in

che

rt

TE7

Terih

at26

3/40

?1.

8-4

.5-1

.2-2

8.8

Blac

kish

and

cha

otic

infil

l. Ve

in h

oste

d in

che

rt

VA1

Vani

ster

149/

78Sy

n-fo

ldin

g1.

1-2

.2-5

.7-4

1.9

Band

ed v

ein

with

syn

taxi

al to

blo

cky

calc

ite

VA2

Vani

ster

349/

80Pr

e-fo

ldin

g 1

1.8

-1.2

Bloc

ky c

alci

te w

ith a

coa

rsen

ing

inw

ard

tren

d,

band

ed a

ppea

ranc

e

VA3

Vani

ster

146/

84Sy

n-fo

ldin

g2.

3-0

.3-5

.7-3

7.7

Hig

h fr

eque

ncy

vein

ban

ding

VA8

Vani

ster

280/

80Pr

e-fo

ldin

g 2

1.6

-3.0

-1.5

-14.

7Sy

ntax

ial t

o bl

ocky

cal

cite

infil

l

VA9

Vani

ster

171/

80Pr

e-fo

ldin

g 1

1.7

-2.1

0.7

-22.

4O

ne g

ener

ation

of b

lock

y ca

lcite

with

a

coar

seni

ng in

war

d tr

end

Inte

nse

curv

ed

defo

rmati

onal

twin

ning

MA1

Mas

hkul

lorë

211/

73Pr

e-fo

ldin

g 1

-3.0

-2.1

-2.3

-36.

4O

ne g

ener

ation

of b

lock

y ca

lcite

MA2

Mas

hkul

lorë

218/

61Pr

e-fo

ldin

g 1

1.5

-2.3

-5.5

-40.

2O

ne g

ener

ation

of b

lock

y ca

lcite

Hos

t roc

k Bo

dris

htë

1.6

-1.5

Hos

t roc

k M

uzin

ë2.

7-1

.8

Hos

t roc

k Fr

asht

an1.

5-1

.7

Hos

t roc

k Te

rihat

1.7

-1.9

Hos

t roc

k Va

nist

er1.

1-2

.3


Tab

le 3

.2

Ove

rvie

w o

f iso

tope

dat

a an

d pe

trog

raph

ic o

bser

vatio

ns o

f cal

cite

vei

n sa

mpl

es a

nd fi

ve h

ost

rock

sam

ples

. The

thr

ee p

hase

s of

frac

ture

in

fillin

g ar

e co

vere

d, w

ith ‘p

re-fo

ldin

g 1’

ref

erri

ng to

the

set o

f ear

ly b

uria

l WN

W-E

SE a

nd E

-W o

rien

ted

vein

s, ‘p

re-fo

ldin

g 2’

to s

ubse

quen

t NN

E-SS

W

orie

nted

vei

ns a

nd ‘s

yn-fo

ldin

g’ t

o th

e la

test

gen

erat

ion

of N

E-SW

ori

ente

d ve

ins

rela

ted

to L

PS. V

ein

orie

ntat

ions

are

as

mea

sure

d in

the

fiel

d an

d gi

ven

in t

he ‘d

ip d

irec

tion/

dip

angl

e’ c

onve

ntio

n.

Sam

ple

Out

crop

Orie

ntat

ion

Frac

ture

set

Cal

cite

Fl

uid

incl

usio

nsPe

trog

raph

ic c

hara

cter

istic

s

(‰ v

s. V

PDB

)(‰

vs.

VSM

OW

)

δ13 C

cδ1

8 Oc

δ18 O

wδ2

Hw

BO1

Bodr

isht

ë24

0/90

Pre-

fold

ing

12.

20.

1H

igh-

freq

uenc

y ve

in b

andi

ng w

ith b

recc

iate

d sh

ards

of h

ost r

ock

BO2

Bodr

isht

ë24

8/77

Pre-

fold

ing

11.

1-0

.7O

ne g

ener

ation

of c

lean

blo

cky

calc

ite

BO4

Bodr

isht

ë18

4/75

Pre-

fold

ing

1-2

.8-0

.8-2

.0-3

6.8

Bloc

ky a

nd s

omew

hat m

essy

cal

cite

infil

l

BO5

Bodr

isht

ë19

5/72

Pre-

fold

ing

11.

3-1

.5-0

.4-2

6.0

One

gen

erati

on o

f cal

cite

infil

l with

a c

oars

e bl

ocky

to s

ynta

xial

text

ure.

Bre

ccia

ted

host

rock

fr

agm

ents

and

inte

nse

(cur

ved)

cal

cite

twin

ning

. St

ylol

itic

surf

ace

alon

g on

e ve

in w

all

BO6.

2aBo

dris

htë

281/

79Pr

e-fo

ldin

g 2

0.2

-0.8

Brec

ciat

ed s

hard

s of

hos

t roc

k in

a b

lock

y ca

lcite

in

fill

BO6.

2bBo

dris

htë

190/

78Pr

e-fo

ldin

g 1

1.5

-2.2

One

gen

erati

on o

f blo

cky

calc

ite w

ith c

lear

ca

lcite

twin

ning

BO6.

3Bo

dris

htë

184/

75Pr

e-fo

ldin

g 1

0.8

-1.3

-3.3

-35.

9O

ne g

ener

ation

of b

lock

y ca

lcite

with

inte

nse

twin

dev

elop

men

tBO

7Bo

dris

htë

183/

80Pr

e-fo

ldin

g 1

0.4

-0.6

-2.6

-35.

1O

ne g

ener

ation

of b

lock

y ca

lcite

JG1a

Jorg

ucat

191/

56Pr

e-fo

ldin

g 1

1.6

-2.6

-0.8

-28.

3O

ne g

ener

ation

of b

lock

y ca

lcite

JG1b

Jorg

ucat

291/

51Pr

e-fo

ldin

g 2

1.8

-3.7

1.1

-23.

8Bl

ocky

cal

cite

exh

ibiti

ng c

urve

d de

form

ation

al

twin

sJG

2Jo

rguc

at31

0/70

?1.

8-4

.0-1

.0-1

9.6

MU

1M

uzin

ë22

4/72

Pre-

fold

ing

12.

9-1

.6-0

.7-2

9.6

Vein

com

pose

d of

blo

cky

calc

ite w

ith c

lear

twin

de

velo

pmen

tM

U2

Muz

inë

224/

72Pr

e-fo

ldin

g 1

2.8

-1.5

Bloc

ky c

alci

te w

ith b

recc

iate

d ho

st ro

ck

frag

men

tsM

U3

Muz

inë

214/

71Pr

e-fo

ldin

g 1

2.9

-1.7

Vein

ban

ding

. Blo

cky

calc

ite in

fill w

ith c

lear

twin

la

mel

lae

MU

4M

uzin

ë32

3/78

Pre-

fold

ing

2-0

.4-2

4.4

Mos

tly b

lock

y ca

lcite

with

a c

oars

enin

g in

war

d tr

end.

Par

t (±

20%

) is

fine-

grai

ned

and

diso

rgan

ized

(rec

ryst

alliz

ed)

MU

5M

uzin

ë33

0/76

Pre-

fold

ing

22.

81.

0-5

.4-4

0.3

One

gen

erati

on o

f cle

an b

lock

y ca

lcite

MU

6M

uzin

ë22

1/66

Pre-

fold

ing

13.

0-3

.2Bl

ocky

cal

cite

. A s

tylo

litic

surf

ace

deve

lope

d on

on

e ve

in w

all

FR1

Fras

htan

273/

69Pr

e-fo

ldin

g 2

1.5

-5.1

-3.7

-41.

9O

ne g

ener

ation

of b

lock

y ca

lcite

with

inte

nse

curv

ed tw

in la

mel

lae

FR2

Fras

htan

308/

85Sy

n-fo

ldin

g-0

.7-6

.5-7

.8-4

7.9

Mos

tly b

lock

y ca

lcite

. Par

t (±

10%

) is

fine-

grai

ned

and

diso

rgan

ized

(rec

ryst

alliz

ed)

FR3.

1Fr

asht

an32

7/81

Syn-

fold

ing

0.5

-5.3

-6.9

-56.

0

FR3.

2Fr

asht

an32

7/81

Syn-

fold

ing

0.5

-5.2

-6.4

-46.

3

FR4

Fras

htan

264/

67Pr

e-fo

ldin

g 2

1.5

-4.8

-3.2

-34.

1Sy

ntax

ial c

alci

te in

fill w

ith m

inor

dev

elop

men

t of

twin

lam

ella

eTE

1Te

rihat

144/

88Sy

n-fo

ldin

g1.

3-2

.8-4

.2-2

0.7

Fine

gra

ined

dis

orga

nize

d ca

lcite

(r

ecry

stal

lizati

on)

TE2

Terih

at07

9/11

Syn-

fold

ing

1.9

-3.3

Band

ed v

ein

with

a s

ynta

xial

to b

lock

y ca

lcite

in

fill.

Min

or d

evel

opm

ent o

f tw

in la

mel

lae

TE3

Terih

at16

8/80

Pre-

fold

ing

11.

8-3

.40.

3-2

5.8

Coar

se b

lock

y ca

lcite

cry

stal

s. P

erva

sive

twin

la

mel

lae.

Sty

loliti

c su

rfac

es o

n th

e ve

in w

alls

TE4

Terih

at30

6/79

Syn-

fold

ing

1.5

-1.1

Min

or v

ein

band

ing.

Blo

cky

calc

ite

TE5

Terih

at17

8/78

Pre-

fold

ing

11.

9-2

.5-0

.1-2

7.9

Bloc

ky c

alci

te in

fill w

ith c

lear

twin

lam

ella

e

TE6

Terih

at26

3/40

?1.

9-4

.3-0

.1-1

6.8

Blac

kish

and

cha

otic

infil

l. Ve

in h

oste

d in

che

rt

TE7

Terih

at26

3/40

?1.

8-4

.5-1

.2-2

8.8

Blac

kish

and

cha

otic

infil

l. Ve

in h

oste

d in

che

rt

VA1

Vani

ster

149/

78Sy

n-fo

ldin

g1.

1-2

.2-5

.7-4

1.9

Band

ed v

ein

with

syn

taxi

al to

blo

cky

calc

ite

VA2

Vani

ster

349/

80Pr

e-fo

ldin

g 1

1.8

-1.2

Bloc

ky c

alci

te w

ith a

coa

rsen

ing

inw

ard

tren

d,

band

ed a

ppea

ranc

e

VA3

Vani

ster

146/

84Sy

n-fo

ldin

g2.

3-0

.3-5

.7-3

7.7

Hig

h fr

eque

ncy

vein

ban

ding

VA8

Vani

ster

280/

80Pr

e-fo

ldin

g 2

1.6

-3.0

-1.5

-14.

7Sy

ntax

ial t

o bl

ocky

cal

cite

infil

l

VA9

Vani

ster

171/

80Pr

e-fo

ldin

g 1

1.7

-2.1

0.7

-22.

4O

ne g

ener

ation

of b

lock

y ca

lcite

with

a

coar

seni

ng in

war

d tr

end

Inte

nse

curv

ed

defo

rmati

onal

twin

ning

MA1

Mas

hkul

lorë

211/

73Pr

e-fo

ldin

g 1

-3.0

-2.1

-2.3

-36.

4O

ne g

ener

ation

of b

lock

y ca

lcite

MA2

Mas

hkul

lorë

218/

61Pr

e-fo

ldin

g 1

1.5

-2.3

-5.5

-40.

2O

ne g

ener

ation

of b

lock

y ca

lcite

Hos

t roc

k Bo

dris

htë

1.6

-1.5

Hos

t roc

k M

uzin

ë2.

7-1

.8

Hos

t roc

k Fr

asht

an1.

5-1

.7

Hos

t roc

k Te

rihat

1.7

-1.9

Hos

t roc

k Va

nist

er1.

1-2

.3

Chapter 358

-4

-3

-2

-1

0

1

2

3

4

-7 -6 -5 -4 -3 -2 -1 0 1 2

13C c

(‰ v

s. V

PDB

)

18Oc (‰ vs. VPDB)

Muzinë

Terihat

Frashtan

Vanister

Bodrishtë

Mashkullorë

Jorgucat

Met

eoric

and/o

r temperature effect

External carbon(hydrocarbons?)

WRI or high-δ18Oevaporation fluids

Hostrock

Figure 3.8 Carbon and oxygen isotope data of vein and host rock samples from different outcrop locations along the Mali Gjere. Squares and crosses represent vein and host rock samples, respectively, and correspond in color per outcrop. Fluids acquired DIC through host rock dissolution as demonstrated by the similarity in δ13Cc between veins and host rock. Depletions in 18O with respect to host rock are related to variable degrees of fluid-DIC oxygen isotope equilibration under meteoric conditions and/or elevated temperatures. Water-rock interactions (WRI) or vein precipitation from isotopically enriched evaporation fluids could explain enrichments in 18O with respect to the host rock.

3.5 Discussion3.5.1 Structural evolution of the Mali Gjere3.5.1.1 Early-burial fracturing eventsEarly burial diagenesis caused a loss of porosity and permeability of the

Cretaceous to Eocene carbonate rocks deposited in the Ionian Basin (Van Geet et al., 2002; Vilasi et al., 2006). Multiple events of fracturing were subsequently key for causing periodic increases in rock permeability (Figure 3.9). Cross-cutting relationships between the different structural sets that are present in the Mali Gjere reveal a clear sequence of deformational events. Bedding-parallel burial stylolites are especially useful time markers due to their development before the onset of intense thrusting in the Oligocene (Swennen et al., 2000). Fractures that are oriented WNW-


ESE and the minor set of E-W fractures are interpreted to have formed first because they systematically pre-date the other fracture sets and burial stylolites. Although the precise chronology of the WNW-ESE and E-W fractures is unclear, their similar orientations and infill characteristics support a similar time of formation. Because these fractures systematically pre-date burial stylolites, they are interpreted to have formed during early burial in a foreland basin setting, which is in line with their bedding-orthogonal position.

NNE-SSW oriented tensile fractures typically post-date the WNW-ESE fractures and pre-date NE-SW fractures. Cross-cutting relationships between the NNE-SSW fracture generation and burial stylolites are ambiguous, suggesting that they formed synchronously under a vertically-oriented s1. Along with their layer-orthogonal orientation, this suggests that the NNE-SSW fractures developed at a later stage during burial compared to the first phase of brittle deformation, but still before inclination of bedding with the carbonates being situated in a foreland basin setting. In the structural analysis (Figure 3.3), orientations of fractures and stylolites of the pre-folding fracture sets were returned to their original position at the time of formation (i.e., before inclination of bedding) to account for slight variations in bedding orientation throughout the Mali Gjere.

3.5.1.2 Burial fracturing in foreland basinsThe orientations of the two main pre-folding fracture assemblages (WNW-

ESE and NNE-SSW) are similar to orientations of pre-folding fractures that were previously recognized elsewhere in the Ionian Zone (Graham Wall et al., 2006; Lacombe et al., 2009; Vilasi et al., 2009), suggesting that a regionally uniform stress regime persisted in the foreland basin. Syn-subsidence fracturing is an important process in structurally shaping carbonate rocks in a wide array of settings, ranging from shallow-water carbonate platforms (Bertotti et al., 2017; Lavenu and Lamarche, 2017; Nooitgedacht et al., 2018) to basinal carbonate deposits (Vitale et al., 2012). Pervasive fracturing during early burial in carbonates is facilitated by the rapid embrittlement of carbonate rocks (Lamarche et al., 2012; McNeill and Eberli, 2012; Lavenu et al., 2014). Rock failure may occur even in the absence of tectonic stresses as soon as the overburden-induced stress is sufficiently high, which may occur already before complete lithification at shallow burial depths below 500 m (Lavenu and Lamarche, 2017).

Pinpointing the nature of the controlling stresses during syn-burial fractur-ing of the Ionian Basin carbonates is difficult due to the complexity of fold-and-thrust belt systems. The architecture of the collisional system ranging from the foreland basin to the orogenic wedge may encompass lateral variations in stress regimes. Whereas compressional stresses prevail within the fold-and-thrust belt, the foreland basin is typically dominated by flexural stresses that are related to lithospheric bend-ing provoked by tectonic loading in the hinterland (Beaumont, 1981; DeCelles and Giles, 1996; Ferket et al., 2003). Strata in a foreland basin may experience switches in stress fields as the thrust front advances, and different organizations of structural

Chapter 360

elements may be expected depending on the position of the rock within the devel-oping fold-and-thrust system (Tavani et al., 2015).

The main pre-folding fracture sets are roughly orthogonal (WNW-ESE and NNE-SSW). Orthogonal fracture assemblages are typical for the early orogenic deformation phase in foreland fold-and-thrust belts and thought to be associated with perpendicular stretching directions throughout the foreland basin (Tavani et al., 2015; Figure 3.11). In the forebulge and outermost part of the foredeep, flexural stresses related to bending of the lithosphere predominate (Bradley and Kidd, 1991), which typically leads to the development of longitudinal (i.e., parallel to the trend of the foreland basin) bed-perpendicular opening joints (Billi and Salvini, 2003; Lash and Engelder, 2007).

As rocks migrate towards the thrust front, transverse (i.e., perpendicular to the trend of the foreland basin) joints become more common and develop as a second early orogenic assemblage (Tavani et al., 2015). Transverse joints develop in association with along-strike curving and stretching of the foredeep close to the thrust front, which may yield a s3 that is oriented parallel to the foredeep trend (Quintà and Tavani, 2012). Although transverse joints are most common close to the thrust front, they may also form as cross-joints in the flexuring regime where longitudinal joints predominate (Destro, 1995; Quintà and Tavani, 2012). Since the first syn-burial assemblage of WNW-ESE fractures is roughly parallel to the trend of the Mali Gjere, it could possibly represent a longitudinal fracture set. The subsequent orthogonal NNE-SSW fracture set would then represent a set of transverse joints. We do stress that his interpretation must be treated with caution due to the complexity of fracture networks in fold-and-thrust belt settings.

Fracturing

Early diagenesis

Fracture infilling (i.e., fluid flow)

Burial stylolitization

Tectonic stylolitization

K Paleocene Eocene Oligocene Miocene < 5 Ma

Pre-folding Syn-folding

Buri

al c

urve

dept

hD

epos

ition

~3km

Figure 3.9 Paragenetic sequence showing the three events of fracturing and fluid flow that affected the Cretaceous to Eocene carbonate sequence. The time spans shown are approximations. Fracturing and fluid flow were most intense during the first deformational event, taking place in the early burial stage. Intensities of small-scale fracturing and fluid flow decrease through time.


3.5.1.3 LPS fracturing and tectonic stylolitizationNE-SW oriented fractures abut on and cross-cut the other fracture sets and

are, therefore, interpreted to have formed last. They post-date burial stylolites, and their orientation is parallel to the direction of tectonic transport during thrusting in the Ionian Zone (Velaj et al., 1999). Hence, NE-SW fractures are most likely associated with layer-parallel shortening (LPS) from the Oligocene onward. When the rocks approach the thrust front, the tectonic stress component increases and the s1 shifts from a vertical position to a horizontal position perpendicular to the direction of tectonic transport, which leads to the development of sub-vertical extensional joints parallel to the s1 (Tavani et al., 2015). Syn-folding fractures related to NE-SW oriented LPS are recognized throughout the entire Ionian Zone (Graham Wall et al., 2006; Lacombe et al., 2009).The typically barren aspect and low density of the NE-SW fractures might indicate that no interconnected fracture network was established that could facilitate fluid flow.

Tectonic stylolites encountered in the Mali Gjere are mostly perpendicular to the NE-SW fractures, pointing to a cogenetic relation (i.e., formed under the same stress regime with a sub-horizontal s1) (Fletcher and Pollard, 1981). The fact that tectonic stylolites are sub-vertical with respect to the surface (Figure 3.4D) supports a late genesis after tilting of bedding during thrust emplacement. Tectonic stylolitization may be an effective deformation mechanism for accommodating strain during LPS in carbonate rocks (Alvarez et al., 1978; Evans et al., 2003). Inherited veins and joints that are perpendicular to the shortening direction (longitudinal) may be reactivated as preferential sites for the development of tectonic stylolites and facilitate LPS (Figure 3.5B; Railsback and Andrews, 1995).

In the outcrop of Mashkullorë, a clearly defined set of NE-SW oriented tectonic stylolites deviates from the predominant orientation of tectonic stylolites. This set is perpendicular to the WNW-ESE fracture set; however, a relation would be improbable considering the extensional regime in which these fractures probably formed. Because the bedding is horizontal at Mashkullorë, it cannot be established whether these stylolites are related to a pre- or post-folding tectonic event. Tectonic stylolites recorded by (Vilasi et al., 2009) in outcrops near Kremenara, Saranda and Kelçyra in the Ionian Zone fall apart in the same two directions as we observe. However, also in this study, there was no convincing evidence for the timing and origin of the NE-SW oriented stylolite set.

3.5.2 Fluid characteristics3.5.2.1 Fluid inclusion isotope dataPetrographic analyses indicate that vein cements are largely original (e.g.,

syntaxial structures, vein banding, inward coarsening of crystals) and dominated by primary fluid inclusions. The only exceptions include i) the disorganized infill of sample Te1, which is probably the result of recrystallization in relation to late-stage reactivation during active thrusting, and ii) the blackish infill of the vein of

Chapter 362

samples Te6 and Te7; also the nature and timing of this chert-hosted vein remained unclear. Even though fluid inclusion water in the remaining vein samples seems to be predominantly of primary origin, certain post-entrapment alteration processes may drive shifts in fluid inclusion isotope signatures. In-situ isotope alteration of fluid inclusion water in high-temperature settings may occur due to bulk diffusion of water (Bakker, 2009) or oxygen isotope exchange between entrapped fluids and host minerals (Rye and O’Neil, 1968). However, at the estimated precipitation temperatures of veins in the Albanides of below 70˚C (Swennen et al., 2000; Vilasi et al., 2009), the rates of these processes are thought to be too low to cause significant alteration (Rye and O’Neil, 1968; Bakker, 2009). Various studies indeed demonstrated that isotope signatures of fluid inclusion water may be preserved in ancient vein systems over geologic time scales (Vityk et al., 1993; Naden et al., 2003; De Graaf et al., 2017).

Isotope data of vein-hosted fluid inclusions from the Mali Gjere exhibit a positive correlation between δ2Hw and δ18Ow with lowest values matching a meteoric fluid endmember (Figure 3.10). A hypothesis featuring the mixing of meteoric water with an isotopically enriched end-member would most readily explain this isotope pattern. The meteoric recharge would have isotope values of approximately -7.5‰ for δ18Ow and -50‰ for δ2Hw. Calcite isotope data could hold an inverted J-shape towards low δ18Oc and δ13Cc values (Figure 3.8), which would support meteoric

-70

-60

-50

-40

-30

-20

-10

0

10

-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

2 Hw

(‰ v

s. V

SMO

W)


Pre-folding 1

Pre-folding 2

Syn-folding

Unknown

Seawater

Evaporation

GMWL

TE1; recrystallizedsee figure 6C

Mixing tre

nd

2

Evaporite

endmemb

er

Meteoricendmember

High-T WRI

Figure 3.10 Fluid inclusion isotope data of calcite veins from the Mali Gjere reveal the isotope evolution of circulating fluids in the Albanide fold-and-thrust belt. Early pre-folding veins probably precipitated from a mixture of meteoric water and evolved marine formation fluids, whereas later stage syn-folding veins precipitated predom-inantly from the meteoric fluid endmember. Sample TE1 deviates from the isotope evolution due to post-depositional alteration (Figure 3.5C).


diagenetic conditions (Lohmann, 1988). Based on isotope data of early-stage calcite veins that are presently exposed in the adjacent Berati thrust sheet, (Vilasi et al., 2009) also inferred that meteoric-derived fluids could be circulating in the Cretaceous to Eocene carbonates already during the burial stage.

Exact isotope values of the other fluid end-member cannot be constrained but would lie somewhere along the extension of the isotope evolution trend in Figure 3.10. This fluid end-member most probably corresponds to marine formation or connate water that experienced isotope enrichment due to either sub-aerial evaporation in a shallow-water environment or high-temperature water-rock interaction (WRI) at depth (Horita, 2005). Evaporated marine fluids could have been sourced from formation waters trapped in Triassic evaporites, which are abundantly present in the subsurface. 87Sr/86Sr isotope data of calcite veins in Cretaceous to Eocene carbonate rocks from the adjacent Berati thrust sheet indicate that interaction between evaporites and fluids occurred already during the earliest flow events (Vilasi et al., 2009). The minor enrichments in δ18Oc with respect to host rock that some vein calcites display could also be the result of interaction of paleo-fluids with evaporites (Swennen et al., 2000; Vilasi et al., 2009). Interaction with evaporites is still observed in the salinity signature of present-day groundwater in the Albanides (Eftimi and Frashëri, 2016). Positive correlations between δ18O and δ2H similar to those observed in Figure 3.10 are common for groundwater reservoirs in modern sedimentary basins and are often explained by mixing of meteoric water with saline formation waters (e.g., Knauth et al., 1980; Moldovanyi et al., 1993; Taylor, 1997; Sanjuan et al., 2016).

Alternative explanations for the isotope data are less plausible. Whereas progressive isotope enrichments may result from boiling (steam loss) of subsurface fluids, this is only feasible in in high-temperature fluids (>> 100˚C) (Truesdell et al., 1977; Darling and Ármannsson, 1989). Precipitation temperatures of veins in the Kurveleshi belt are relatively low (< 70˚C; Swennen et al., 2000; Van Geet et al., 2002), suggesting that fluids in the Albanides did not reach sufficiently great depths during migration for a significant effect of such Rayleigh-type fractionation processes. Moreover, fluid inclusions do not reveal signs of boiling or effervescence (i.e., co-existing gas and liquid inclusions; Moncada et al., 2012). Membrane filtration in clay-rich sediments or exchange reactions with hydroxyl-bearing clay minerals could also force minor modifications of δ18O and δ2H in groundwater (Taylor, 1997; Horita, 2005), but are unlikely to account for the strong isotope variations observed. Hydrogen isotopes, in particular, are assumed to remain largely unaffected by WRI (Clayton et al., 1966; Ohba and Matsuo, 1988).

For being a bulk fluid inclusion isotope study, the extent of infiltration of secondary fluids and their effect on the observed isotope signatures cannot be precisely determined. Hence, one hypothesis that cannot be completely excluded is that the trend (partly) reflects the admixture of secondary meteoric fluids to a primary fluid inclusion content with a marine signature. However, clear evidence for abundant secondary fluid inclusion trails is absent in thin sections, whereas the samples with lowest isotope values would have their fluid inclusion content almost completely replaced in this scenario. Altogether, a fluid mixing system is most likely to underlie the recorded fluid inclusion isotope variations.

Chapter 364

3.5.3 Oxygen isotope fractionation systematicsFrom thermodynamic laws, it follows that fluid temperatures in an equili-

brated system are constrained by: 1) the δ18Oc of precipitating calcite; and 2) the δ18Ow of the mineral-forming fluid (Kim and O’Neil, 1997; Zheng, 2011). The combination of δ18Ow and δ18Oc may thus provide meaningful precipitation temperatures, when assuming equilibrium precipitation. As for the veins studied here, oxygen isotope fractionation factors correspond to a temperature range of -12 to 39˚C with a mean of 16˚C (Kim and O’Neil, 1997). Whereas the upper temperature limit could theo-retically agree with shallow-crustal fluids, the lowest temperatures are unrealistically low for the paleo-fluid system.

Experimental equations for mineral-water oxygen isotope fractionation have previously been identified to produce unrealistically low temperatures in other natural systems (Yan et al., 2012). Even in the slowly growing calcites of Devils Hole – considered a natural laboratory – temperatures calculated using the experimental equation of Kim and O’Neil (1997) are 8˚C lower than actual precipitation tem-peratures (Coplen, 2007). Non-equilibrium fractionation of oxygen isotopes may be commonplace for calcite precipitation in natural systems (Watkins et al., 2014). As for the veins in this study, the calculated precipitation temperatures below 0˚C could point to a system that was not in isotope equilibrium either. A possible driving force behind non-equilibrium kinetic effects on oxygen isotope fractionation in vein systems could be high precipitation rates, which may lead to considerable 18O enrichments in calcite cements (Scholz et al., 2009; Gabitov et al., 2012).

In ancient vein systems, however, part of the apparent temperature discrep-ancy could also be due to the late infiltration of (meteoric) waters that are depleted in 18O with respect to the primary fluid inclusion content. Such waters may infiltrate through recrystallization or micro-fracturing under deformational stresses, as evi-denced by secondary fluid inclusion trails. Besides, certain chemical parameters may also put a control on oxygen isotope fractionation apart from temperature and cause an apparent out-of-equilibrium system, including pH (Beck et al., 2005; Dietzel et al., 2009) and fluid salinity (Truesdell, 1974; Hu and Clayton, 2003). Altogether, the precise reasons for the anomalous fractionation factors remain unclear. In any case, oxygen isotope fractionation factors between bulk fluid inclusion water and calcite from this study are not suitable for establishing reliable temperature and depth estimates for vein precipitation.

3.5.4 Fluid flow in the Albanide fold-and-thrust beltVein-forming fluids in the Ionian Zone are composed of a mix of meteoric

and marine formation/connate waters. The resemblance in δ13Cc between veins and host rock indicates that mineralizing fluids acquired dissolved inorganic carbon (DIC) species mainly through dissolution (e.g., stylolitization) of the hosting Cretaceous to Eocene carbonate rocks. Depletion in 13C with respect to host rock as demonstrated by some vein samples could be related to an input of carbon from


external sources (e.g., hydrocarbons). Degradation of organic matter may also introduce 13C-depleted methane and CO2 into circulating fluids and significantly lower δ13Cc signatures (Stahl, 1979; McManus and Hanor, 1988; Prikryl et al., 1988).

Observed vein banding probably results from a crack-and-seal type of infilling, in which fractures repeatedly break open along the vein walls (Ramsay, 1980). Vein banding and syntaxial cements (Figure 3.5F), which were observed in fractures of all generations, suggests that fluid flow and fracture infilling occurred concurrently with fracture opening for each phase of deformation. Matching vein walls (i.e., devoid of significant dissolution; Figure 3.5A, 3.5C and 3.5D) point to a short residence time of fluids in the fracture networks and a transient character of fluid flow.

Brecciated textures are common in early burial veins (Figure 3.5G) and indicate that high hydraulic pressures exerted by underlying fluid reservoirs may have facilitated fracturing (Roberts and Nunn, 1995; Cosgrove, 2001; Sibson, 2003).

SW NEForebulge Foredeep Orogenic wedge

σ1

σ3σ2

σ1

σ2σ3

σ2

σ1σ3

-7.5‰ δ18O-50‰ δ2H

1

23

2 31

Figure 3.11 Schematic representation of large-scale fluid flow patterns in the Albanide fold-and-thrust belt. Meteoric water is charged in topographically high areas of the orogenic wedge and driven basin-ward due to compressional tectonic forces and the elevated position of the hydraulic head. Meteoric fluids ascend as soon as the fracture-controlled permeability of the Cretaceous to Eocene carbonate rocks was sufficiently high. The carbonates experienced various stress regimes from the moment of deposition until incorporation into the fold-and-thrust belt. Foreland basins are typically characterized by the development of opening-mode fractures parallel and perpendicular to the trend of the foreland (Tavani et al., 2015; blocks 1-2). Late-stage fractures develop parallel to the direction of tectonic transport and are associated with orthogonal tectonic stylolites (block 3). The vertical scale is exaggerated in the figure.

Chapter 366

Overpressured fluid reservoirs may be installed under an increasing overburden in the case of the pre-folding fluid system. Regional compressional stresses related to thrusting may also contribute to elevating hydraulic pressures in confined fluid reservoirs (Oliver, 1986; Ge and Garven, 1989). High fluid pressures during vein formation in the Cretaceous to Eocene carbonates of the Ionian Zone were also identified by (Van Geet et al., 2002; Lacombe et al., 2009; Vilasi et al., 2009). The evidence for hydraulic overpressures suggests that fluids were derived from a deeper-seated reservoir. Lateral flow of meteoric water through this reservoir from a terrestrial recharge zone is indispensable because the carbonates were being covered by Oligocene flysch deposits in an underfilled foreland basin during the syn-subsidence events of fluid flow. Pinpointing the rock formation(s) that accommodated lateral flow of fluids is complicated as multiple deep-seated porous formations exist in the Ionian Zone including Triassic breccias and Jurassic limestones (Zelilidis et al., 2015).

We propose a model with gravity-driven basin-ward flow of rainwater that is charged in topographic highs shaped by active fold-and-thrust tectonics in the hinterland (Figure 3.11). Large-scale compression induced by advancing thrust tectonics may elevate pore pressures in the foreland to facilitate basin-ward flow of groundwater (Oliver, 1986; Ge and Garven, 1989). Basin-ward expulsion of fluids is commonly observed in fold-and-thrust belt settings worldwide (Machel and Cavell, 1999; Ferket et al., 2003; Lynch and Van der Pluijm, 2017). Meteoric fluids that are charged in topographic highs in the hinterland may be transported far into the foreland basin as demonstrated in vein-based studies in various fold-and-thrust belt settings, including the Appalachians (Evans and Battles, 1999; Kirkwood et al., 2000), the Rocky Mountains (Beaudoin et al., 2011, 2014) and the Mexican fold-and-thrust belt (Fitz-Diaz et al., 2011). Similar to what we infer for the Albanides, the charged meteoric fluids in these studies are identified to mix at depth with saline formation fluids and deposit vein cements upon fracturing in relation to lithospheric flexure. In the present-day Ionian Zone, gravity-driven flow and upwelling of meteoric waters is still recognized (Eftimi and Frashëri, 2016), indicating that the large-scale fluid flow system with outflowing meteoric water has remained temporally stable in the Albanide fold-and-thrust belt.

Early stage pre-folding veins and late stage syn-folding veins plot in distinct fields along the observed fluid inclusion isotope trend (Figure 3.10). The meteoric water endmember becomes progressively more dominant through time, demonstrating that marine formation waters were gradually flushed out of the system due to the continued throughput of meteoric recharge. Progressive shortening of fluid migration pathways as the carbonate rocks advanced towards the thrust front (i.e., approached the zone of meteoric recharge) may have contributed to the increasing dominance of meteoric fluids through time. The inferred evolution of fluids is supported by overall lower fluid inclusion salinities of late stage veins in the Ionian Zone (Vilasi et al., 2009). A fluid evolution with meteoric waters gaining in dominance through time was also inferred for the Mexican fold-and-thrust belt by (Fitz-Diaz et al., 2011), who recorded a similar decrease in δ2H of circulating fluids from the earliest to the latest stages of deformation.


3.6 Spatial fluid variations throughout the Ionian Zone

Fluids circulating in fracture networks within the Kurveleshi belt display a clear temporal evolution with meteoric fluids gaining in dominance through time. To assess spatial variations in fluid flow pathways throughout the entire fold-and-thrust belt setting, the study area was expanded in the work of Nooigedacht et al. (in prep.) to the adjacent Çika and Berati belts, which are along with the Kurveleshi belt the main structural units of the Ionian Zone. In total, vein and fracture assemblages were studied in twelve outcrops of Ionian Basin sediments spread along the Çika and Berati belts (Figure 3.12). The sedimentological aspect of the rocks is highly consistent throughout the entire Ionian Zone (i.e., low-permeability fine-grained calciturbidites). Fracture patterns within the studied outcrops in the Çika and Berati

Shalës (SL)

Sopik (Sp)

Ksamil (Ks)

Metoq (MQ)

Sarandë (Sa)

Shën Vasil (SV)

Borsh (Br)

Porto Palermo (PP)

Sheper (Sh)

Kelçyrë (Ke)

Skore (Sk)

Poliçan (Po)

Podgoran (Pd)

Çika

Kurvel

eshi

Berati

N

20 kmIonian Sea

Greece

Figure 3.12 In addition to the seven outcrops studied in the Kurveleshi belt, we studied veins in eight outcrops in the external-most Çika belt and six outcrops in the internal-most Berati belt. The abbreviations used for the outcrops are given in parentheses. The map view is slightly enlarged compared to Figure 3.1B.

Chapter 368

belt reflect the same stress fields as inferred for the Kurveleshi belt, supporting the notion that regional forces controlled deformation of the Ionian Zone carbonates. A total of 39 vein samples were collected from the Çika and Berati belt for isotope analyses; a summary of the δ2Hw and δ18Ow measurements is presented in Table 3.3.

When fluid inclusion isotope data of early burial veins are plotted per thrust sheet, a remarkable grouping shows up (Figure 3.13). Fluids in the external Çika belt are dominated by the meteoric fluid end-member, while vein-forming fluids in the internal Berati belt are shifted considerably more towards the evaporite end-member. The isotope composition of fluids in the Kurveleshi belt in turn is intermediate between the bordering Çika and Berati belts. This trend probably reflects a gradual increase of the evaporite signal towards the thrust front, where tectonic forces and deformation are elevated, causing widespread expulsion of connate waters from the evaporites. An obvious implication is that the basin-ward flow of meteoric waters was not guided through the evaporites, but rather through a shallower overlying aquifer, which experienced episodic inputs of upwelling fluids from the evaporites.

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2 Hw

(‰ v

s. V

SMO

W)


Çika Belt

Kurveleshi Belt

Berati Belt

Seawater

GMWL

Figure 3.13 Fluid inclusion isotope signatures of pre-folding veins from the three thrust belts in the Ionian Zone. Fluids in the external Çika belt are shifted towards the meteoric side of the mixing system, while the internal Berati belt is dominated by connate seawater evaporation fluids. Vein-forming fluids in the Kurveleshi belt are intermediate. The data reveal an increase of the evaporite signal towards the thrust front.


Table 3.3 Fluid inclusion isotope signatures of veins in the Ionian Basin carbonates of the Berati and Çika thrust sheets. Vein orientations are given in the ‘dip direction/dip angle’ convention.

Berati Belt Çika Belt

Sample Orientation Fracture set δ18Ow δ2Hw Sample Orientation Fracture set δ18Ow δ2HwSk1 027/46 Pre-folding 1 1.1 -20.2 SL1 210/35 Pre-folding 1 -3.7 -30.2Po2 176/51 Pre-folding 1 0.5 -15.4 SL2 210/35 Pre-folding 1 -3.5 -42.4Po3a 188/75 Pre-folding 1 -0.2 -28.1 SL3 210/35 Pre-folding 1 -1.9 -26.5Po3b 313/24 Pre-folding 2 -1.1 -26.2 SL4 210/35 Pre-folding 1 -3.2 -35.1Po4 195/75 Pre-folding 1 -0.8 -14.7 Sp2 236/68 Pre-folding 1 -3.8 -36.5Sh1 013/70 Pre-folding 1 -1.8 -27.7 Sp3 244/66 Pre-folding 1 -4.2 -23.5Sh3 017/67 Pre-folding 1 1.4 -25.9 KS1 241/75 Pre-folding 2 -4.7 -29.2Sh4 021/80 Pre-folding 1 2.2 -18.7 KS3 237/86 Pre-folding 2 -2.3 -29.7Sh5 023/77 Pre-folding 1 0.6 -23.8 MQ1 297/66 Pre-folding 2 -4.8 -32.5Sh6 339/49 Pre-folding 1 1.8 -24.4 MQ2 240/50 Pre-folding 1 -4.8 -34.4Sh7 312/50 Pre-folding 1 -1.4 -28.3 MQ3 218/65 Pre-folding 1 -5.2 -35.7Sh8 021/31 Pre-folding 1 -0.1 -20.2 Sa1 213/38 Pre-folding 1 -2.0 -22.5Sh9 348/26 Pre-folding 1 -0.9 -28.2 Sa2a 270/48 Pre-folding 2 -3.0 -33.1Ke1 265/65 Pre-folding 2 2.5 -32.4 Sa2b 185/48 Pre-folding 1 -1.6 -18.0Ke3 216/65 Pre-folding 1 -0.2 -26.9 Sa3 213/38 Pre-folding 1 -3.1 -13.9Ke4 281/66 Pre-folding 2 3.2 -27.2 Sa4 175/48 Pre-folding 1 -1.0 -14.8Ke5 210/51 Pre-folding 1 -2.5 -35.2 Sa5 186/40 Pre-folding 1 -3.0 -30.0Ke6 180/64 Pre-folding 1 1.7 -33.3 SV1 310/45 Pre-folding 1 -3.8 -39.0Ke7a 226/49 Pre-folding 1 2.8 -27.4 SV2 333/54 Pre-folding 1 -2.3 -37.9Ke7b 226/49 Pre-folding 1 0.3 -23.8 Br1 209/39 Pre-folding 1 -0.8 -19.3Ke7c 180/48 Pre-folding 1 1.4 -24.7 Br2 207/57 Pre-folding 1 -2.1 -31.1Pd1.1 261/53 Pre-folding 2 2.1 -22.1 Br3 028/67 Pre-folding 2 -3.6 -27.6Pd1.2 261/53 Pre-folding 2 1.4 -13.4 PP1 206/69 Pre-folding 1 -3.0 -14.8Pd1.3 261/? Pre-folding 2 -2.7 -34.2Pd2 191/70 Pre-folding 1 0.8 -19.9Pd4 265/36 Pre-folding 2 -4.2 -29.2

Since syn-folding fractures with substantial calcite infill are scarce in the Çika and Berati belts, there is not sufficient fluid inclusion isotope data of the syn-folding stage to establish spatial variations in fluid flow during active thrusting. During this stage, however, variable fluid isotope signatures might be expected in relation to the compartmentalization of the fold-and-thrust belt. Each thrust unit may constitute a separate hydrogeological unit with distinct fluid flow characteristics.

Although the thrust sheets are all positioned within the same fold-and-thrust complex and subject to the same regional tectonic forces, the composition of circulating fluids may vary considerably between them. Along with the clear temporal evolution in fluid flow patterns as revealed from the Kurveleshi belt, this demonstrates that the prediction of fluid flow in fold-and-thrust belt settings is highly challenging. Nevertheless, the general evolution from large-scale basin-ward migration pathways and pervasive fluid flow during early burial to more localized flow of meteoric fluids during later stage tectonic activity may be widely applicable for carbonate rock formations in fold-and-thrust belt settings.

Chapter 370

3.7 ConclusionsFluid inclusion isotope data of calcite veins hosted in carbonate rocks

from the Ionian Zone of Albania provide unique insight into the evolution of fluid migration pathways throughout the development of the Albanide fold-and-thrust belt. Fluid flow in the studied carbonate rocks is closely associated with events of brittle deformation, which establish discrete pathways (i.e., fractures) allowing for fluids to migrate. Fluids circulating within the carbonates were episodically supplied through upwelling from an underlying confined reservoir. This groundwater reservoir probably comprised a mixture of meteoric water (-7.5‰ for δ18Ow and -50‰ for δ2Hw) and evolved marine waters derived from deep-seated evaporite occurrences. The contribution of fluids derived from the evaporites is higher towards the thrust front in association with elevated deformation, which is expected to be the main driver for expulsing connate waters from the evaporites.

The topographically high hinterland of the orogenic wedge most likely formed a zone of meteoric recharge that fed the deep-seated fluid reservoir. Charged rainwater flowed outward into the foreland basin driven by gravity and gradually flushed out the marine formation waters. As a consequence, fluid inclusions contained in late stage veins, which formed during active thrusting, are dominated by the meteoric water endmember. The model for fluid flow in the Albanide foreland fold-and-thrust belt characterized by the continued basinward expulsion of meteoric waters from an early stage onward applies to many fold-and-thrust belt settings worldwide. A pronounced water-DIC oxygen isotope disequilibrium precludes the use of δ18Ow/δ18Oc pairs as a paleothermometer for vein precipitation. The disequilibrium could be related to kinetic isotope fractionation during vein precipitation and/or secondary infiltration of surface waters.

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