Tectonics and geochronology of the Rehamna massif (Morocco ... · Michard et al., 2008 Introduction...

Francis Chopin (Czech Geological Survey, UdS, UNS, now at the University of Helsinki)

Michel Corsini (University of Nice)

Karel Schulmann (Czech Geological Survey and University of Strasbourg)

Mohamed El Houicha (Chouaib Doukkali University, Morocco)

Jean-François Ghienne (University of Strasbourg)

Landscape in the Rehamna massif

Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian−Variscan Orogeny

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Helsinki Seminar 27 February 2014

[email protected]

Introduction Helsinki Seminar 27 February 2014 [email protected]

Elevation map of Morocco and neighbouring countries

from GTOPO30 database

Michard et al., 2008

Present topography = Cenozoic Alpine belt (i.e 40 Ma to present): Maghrebides (Rif):

parautochthonous and allochthonous

Atlas: intracontinental belts

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The post-variscan unconformity on top of a Variscan granite from the High Moulouya massif

Helsinki Seminar 27 February 2014 [email protected]

Michard et al., 2008

Introduction

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The post-variscan unconformity on top of Variscan basement (Eastern Rehamna, Daalat unit)

Upper Cretaceous cover

Variscan basement

Upper Cretaceous cover

Photo: F. Chopin

Introduction

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Extension of the Paleozoic and Precambrian and Paleozoic outcrops

modified from Piqué and Michard (1989), in Michard et al., 2008

Introduction

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Image Credit: Ray Troll’s (http://www.trollart.com/)

Alpine Orogeny (Atlas, Rif…)

Alleghanian−Variscan Orogeny (Meseta, Appalachian)

Svecofennian orogeny

Karelian craton

Introduction

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Nance et al., 2008

Alleghanian−Variscan Orogeny

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Lower Permian

Palaeogeographic reconstructions Helsinki Seminar 27 February 2014 [email protected]

Cocks & Torsvik, 2006

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Helsinki Seminar 27 February 2014 [email protected] Palaeogeographic reconstructions


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Objectives

• Tectonic at the Gondwana margin

• Time scale of tectonic and magmatic events

• Geodynamic of the Alleghanian−Variscan belt


Photo: F. Chopin

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The Morocco variscides

• South-Western branch of the Variscan Belt at the NW Gondwana margin

• A late Carboniferous to early Permian orogen reworking Early Carboniferous intracontinental basins and its basement


Chopin et al., 2014

Modified after Hoeppfner et al., 2005 and Michard et al., 2008

542

488

443

416

359

299

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Western Meseta

The Rehamna massif Helsinki Seminar 27 February 2014 [email protected]

Lower metam. unit

Upper metam. unit

Wegmann ,1935 12

Chopin et al., 2014

The Rehamna massif Helsinki Seminar 27 February 2014 [email protected]

10

00

m

Compilation: Chopin et al., 2014

Chopin et al., 2014

Lower metam unit

• Moderatly dipping to subhorizontal metamorphic foliation

• Top to the SW−SSW shearing Diot, 1989

L1

Quartz CPO

D1: structure and kinematic Helsinki Seminar 27 February 2014 [email protected]

Chopin et al., 2014

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Photo: F.Chopin

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Photo: F.Chopin

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Photo: F.Chopin

Baguette Express – © Susan Eby Glass

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Lower metamorphic unit:

E-W trending dome

Upper metam unit 2km

Lower metam unit

E

F

G

Upper metamorphic unit:

Fold and thrust belt

F

G

E

D1 to D2: structures Helsinki Seminar 27 February 2014 [email protected]

Chopin et al., 2014

Helsinki Seminar 27 February 2014 [email protected] D1 to D2: structures

Photo: F.Chopin

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Photo: F.Chopin

S2 β2


40Ar/39Ar Geochronology

Photo: Chrystele Verati

Photo: Nevada Isotope Geochronology Lab

Before measurement • Crushing, cleaning of selected samples • Hand-picking of single grains (e.g. micas, amphibole) • Irradiation (Nuclear reactor at the McMaster University in Hamilton) Measurement • Step heating using a CO2 Synrad laser • Isotopic measurements using mass spectrometer





Before measurement • Crushing, cleaning of selected samples • Hand-picking of single grains (e.g. micas, amphibole) • Irradiation (Nuclear reactor at the McMaster University in Hamilton) Measurement • Step heating using a CO2 Synrad laser • Isotopic measurements using mass spectrometer



Relevant 40Ar/39Ar Plateau age: 1) at least 70% of released 39Ar 2) at least three successive steps in the plateau 3) the integrated age of the plateau should agree with each apparent age of the plateau within

a 1σ error confidence interval

McDougall and Harrison, 1999

McDougall and Harrison, 1999

Diagram illustrating accumulation of radiogenic argon in an igneous rock as it cools from magmatic temperatures

Undisturbed Loss Excess

40Ar/39Ar age spectra

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D1 to D2: geochronology

1 km

1km

309.43 ± 7.16 Ma

170

190

210

230

250

270

290

310

330

10 20 30 40 50 60 70 80 90

Cumulative 39Ar Released (%)

Biotite R061A

Ag

e(M

a)

Sidi Ali (North)

Mica schist (St + Grt)


Chopin et al., 2014

Interpretation: time of cooling during syn-convergent doming 23

309.4±7.1 Ma

309.4±7.1 Ma

Cumulative 39Ar Released (%)Cumulative 39Ar Released (%)

301.10 ± 6.30 Ma

170

190

210

230

250

270

290

310

330

10 20 30 40 50 60 70 80 90

Muscovite R147

Lalla Titaf (East)

Mica schist (Ms + Bt)

D1 to D2: geochronology

1 km

1km


Chopin et al., 2014 Interpretation: time of cooling during syn-convergent doming 24

301.1±6.3 Ma

301.1±6.3 Ma

D3 structures

• Folding of the Coastal block (Cambrian) • Curvature of the upper metam anticline • Folding of isograds • Development of a circular subdome


Chopin et al., 2014

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D3 structures

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S1 S3

A

B

C

WNW-ESE compression

reworking of the upper and lower metamorphic units

WNW

WNW

WNW

ESE

ESE

ESE

A

B

C


Chopin et al., 2014

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D3 structures Helsinki Seminar 27 February 2014 [email protected]

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S3 β3

WNW ESE

D3 geochronology

285.46 ± 5.64 Ma

170

190

210

230

250

270

290

310

330

10 20 30 40 50 60 70 80 90

Muscovite 11-01B


Sidi Ali Dôme

Ag

e(M

a)

Orthogneiss


Chopin et al., 2014

Interpretation: resseting of Argon system during D3 28

285.5±5.6 Ma

285.5±5.6 Ma

284.57 ± 6.39 Ma

170

190

210

230

250

270

290

310

330

10 20 30 40 50 60 70 80 90

Muscovite 12-02C

Lalla Tittaf (West)

Age

(Ma)


Metarhyolite

D3 geochronology

WNW ESE


Chopin et al., 2014

Interpretation: resseting of Argon system during D3 29

284.6±6.4 Ma

284.6±6.4 Ma

D3 syn tectonic magmatism

WNW ESE

Associated dykes 285,4 ± 6,1 Ma (U-Pb zircon, Guerrot in Baudin et al., 2003)

284.16 ± 5.54 Ma

170

190

210

230

250

270

290

310

330

10 20 30 40 50 60 70 80 90

Muscovite R159

Rais El Biod (border)

Ag

e(M

a)


Syn tectonic


Chopin et al., 2014

Interpretation: time of crystallization of the syntectonically (D3) emplaced granite 30

284.16 ± 6.54

284.2±5.5 Ma

Late variscan magmatism

274.85 ± 5.43 Ma

170

190

210

230

250

270

290

310

330

10 20 30 40 50 60 70 80 90Cumulative 39Ar Released (%)

Biotite (R166)

Sebt de Brikiine

Late/post tectonic

Similar to the datation made by Tisserand, 1977 (recalculated by Michard et al., 1982): 278 ±2 Ma (Rb/Sr)


Chopin et al., 2014

Interpretation: emplacement of the late/post tectonic batholith

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274.8±5.4 Ma

Interpretation of orogenic fabrics

Late Carboniferous to Early Permian (315−290 Ma)

D1: top to the SSW stacking D2: NNE−SSW syn convergent doming

Early Permian (290−280 Ma)

+ post tectonic

Magmatism (275 Ma)

D3: WNW directed compression


Chopin et al., 2014

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Cocks & Torsvik, 2006 33


Alleghanian−Variscan Orogeny Helsinki Seminar 27 February 2014 [email protected]

Gutiérrez-Alonso et al., 2012 Buckling an orogen: The Cantabrian Orocline GSA Today, v. 22, no. 7, doi: 10.1130/GSATG141A.1.

34 Gutiérrez-Alonso et al., 2012


Gutiérrez-Alonso et al., 2012

Buckling an orogen: The Cantabrian Orocline

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Chopin et al., 2014


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Engelder and Whitaker, 2006

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Study of cleat (natural fractures in coal) and joint in the Alleghanian orogeny

Terry Engelder, 2008, AAPG presentation


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Older, deeper coal, Appalachian Wide Field Stress cleat joint Terry Engelder, 2008, AAPG presentation


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Younger, shallower coal: Alleghanian trending cleat and joint

Terry Engelder, 2008, AAPG presentation


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Engelder and Whitaker, 2006

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Hatcher, 2002 Hatcher, 2010

Helsinki Seminar 27 February 2014 [email protected] Palaeogeographic reconstructions


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Alleghanian−Variscan Orogeny

Paleogeographic reconstructions after Cocks & Torsvik [2006] at 310 and 280 Ma.

Tectonic datas from this study (Morocco) and

bibliography


Chopin et al., 2014

310 Ma

280 Ma

Chopin et al., 2014

Conclusion

• Late Carb. to Early Permian (315−290 Ma): SSW directed compression

• Early Permian (290−275 Ma): WNW directed compression + Magm

• Late/post tectonic magmatism at 275 Ma

Major change in tectonic regime in Morocco

Correlation with Alleghanian−Variscan belt

• 315−290 Ma: convergence of the Gondwana landmass with the European Variscan collage to the north. At the same time the eastern margin of Laurentia was undergoing dextral strike slip deformation

• 290−275 Ma: Head-on collision between Gondwana and Laurentia while Europe was subject to major dextral wrench deformation


Photo: F.Chopin

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Landscape and peoples Helsinki Seminar 27 February 2014 [email protected]

Photo: F.Chopin


Landscape and peoples

48 Photo: F.Chopin


Landscape and peoples

Photo: F.Chopin

Tectonics and geochronology of the Rehamna massif (Morocco ... · Michard et al., 2008 Introduction...

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