ORIGINAL PAPER

Transpressional tectonics and nappe stacking along the SouthernVariscan Front of Morocco

Andrea Cerrina Feroni Æ Alessandro Ellero ÆMarco G. Malusa Æ Giovanni Musumeci ÆGiuseppe Ottria Æ Riccardo Polino Æ Leonardo Leoni

Received: 20 February 2008 / Accepted: 10 May 2009 / Published online: 30 May 2009

� Springer-Verlag 2009

Abstract The Southern Variscan Front in the Tinerhir

area involves Palaeozoic allochthonous units (Ouaklim and

Tilouine units) thrust onto the northern edge of the West

African Craton during late Carboniferous time. Illite

crystallinity data highlight an anchizonal grade for the

Ouaklim Unit, and a diagenesis-anchizone transition for the

Tilouine Unit during deformation phase D1. The tectonic

stack is crosscut by major dextral reverse faults bounding

E–W trending domains of dominant shortening deforma-

tion (central domain) and strike-slip deformation (northern

and southern domains), later segmented by a network of

post-Variscan faults. This complex deformation pattern is

the result of kinematic partitioning of dextral transpression

along the Southern Variscan Front, coeval with the

Neovariscan (300–290 Ma) oblique convergence observed

at the scale of the whole Moroccan Variscides. Partitioning

of dextral transpression described in the Tinerhir area is

consistent with dextral wrench faulting along the Tizi n’

Test Fault, and with Appalachian-style south-directed

thrusting in the Tinerhir and Bechar-Bou Arfa areas.

Keywords Variscan tectonics � Transpressional

deformation � Strain partitioning � Illite crystallinity �Southern Variscan Front � Eastern Anti-Atlas � Morocco

Introduction

The Variscan orogenic system derives from late Palaeozoic

oblique convergence between Gondwana and Laurussia

(Gleizes et al. 1998; Shelley and Bossiere 2000; Matte

2001). Extensively exposed in Morocco, it consists of

Gondwanan crustal domains showing contrasting defor-

mation and metamorphic characters. In the southernmost

Moroccan Variscan Belt, the Meseta metamorphic units

are juxtaposed to the northern edge of the West African

Craton along the Southern Variscan Front (Fig. 1). The

peculiar tectonic setting of this area is classically inter-

preted as an Appalachian-type fold belt (Michard et al.

1982; Helg et al. 2004; Toto et al. 2008), but reconciling

its kinematic characters with a simple compressional

deformation model is not straightforward. Major right–

lateral displacements of late Variscan age, inferred

for instance along the Tizi n’Test Fault (Mattauer et al.

1972; Pique and Michard 1989), contrast with the coeval

low-angle thrusting documented in the Tinerhir region

(Michard et al. 1982) and with the complex deforma-

tion pattern observed in the Tamlelt inlier (Houari and

Hoepffner 2003). Deformation along the Southern Vari-

scan Front, and the relative position of the Meseta block

with respect to the stable West African Craton, are

thus key points for reliable Palaeozoic geodynamic

reconstructions.

A. Cerrina Feroni � A. Ellero � G. Musumeci � G. Ottria (&)

CNR Istituto di Geoscienze e Georisorse, Via S. Maria 53,

Pisa 56126, Italy

e-mail: ottria@dst.unipi.it

M. G. Malusa

Dipartimento di Scienze Geologiche e Geotecnologie,

Universita di Milano-Bicocca, Piazza della Scienza,

4, Milano 20126, Italy

G. Musumeci � L. Leoni

Dipartimento di Scienze della Terra, Universita di Pisa,

Via S. Maria 53, Pisa 56126, Italy

M. G. Malusa � R. Polino

CNR Istituto di Geoscienze e Georisorse,

Via Valperga Caluso 35, Torino 10123, Italy

123

Int J Earth Sci (Geol Rundsch) (2010) 99:1111–1122

DOI 10.1007/s00531-009-0449-x

According to classical works (e.g. Michard et al. 1982),

the Southern Variscan Front in the Tinerhir area would be

located north of the Saghro inlier, a window of Pre-

cambrian basement covered by autochthonous sediments of

Palaeozoic age. Recent fission track data on apatite dem-

onstrate the occurrence of Variscan nappes, now eroded,

above the Saghro inlier (Malusa et al. 2007a). The South-

ern Variscan Front would be thus located further south than

previously assumed (cf. Hoepffner et al. 2006).

In recent times, the Tinerhir area has been the subject

of detailed field surveys for the realization of the new

1/50,000 geological map of Morocco (e.g. El Boukhari

et al. 2007a; Malusa et al. 2007b; Schiavo et al. 2007a).

This paper integrates the results of geological mapping, and

the apatite fission-track data set, with a detailed structural

analysis and new illite crystallinity data from the Palaeo-

zoic units exposed between the Saghro inlier and the South

Atlas Fault (Fig. 1). It illustrates a Variscan tectonics

dominated by transpressional deformation partitioned

within E–W structural domains, and shows that the allo-

chthonous units postulated atop the Saghro inlier are still

preserved on its northeastern edge. An updated tectonic

model for this sector of the Southern Variscan Front, taking

into account transpressional deformation mechanisms, is

therefore proposed and discussed within the regional tec-

tonic framework of the North African Variscan Belt.

Geological outline of the Tinerhir area

Stratigraphic setting

The Precambrian basement of the Saghro inlier com-

prises Neoproterozoic sedimentary and magmatic rocks

with Pan-African metamorphic overprint (Choubert 1963;

Gasquet et al. 2005). The Palaeozoic cover consists of

Cambrian-to-Carboniferous sedimentary rocks (Fig. 2),

comprising terrigenous clastics and minor carbonates

deposited in an epicontinental marine environment (Du

Dresnay et al. 1988; El Boukhari et al. 2007a, b; Malusa

et al. 2007b; Schiavo et al. 2007a). Lower Cambrian

sedimentation starts with conglomerates and sandstones

infilling an uneven Precambrian morphology, followed

by alternating shales and carbonates (‘‘Serie schisto-

calcaire’’), and by the ‘‘Gres Terminaux’’ sandstones

(Buggisch and Siegert 1988; Landing et al. 2006). The

onset of a second sedimentary cycle of Middle Cambrian

age is marked by transgressive carbonate breccias

(‘‘Breche a Micmacca’’), followed by fossiliferous green

shales (‘‘Schistes a Paradoxides’’) and by the ‘‘Gres du

Tabanit’’ sandstones (Alvaro and Clausen 2006). After a

Late Cambrian hiatus, clastic sedimentation resumes in

Ordovician time with thick alternations of shale and

cliff-forming sandstones referred to as ‘‘1st Bani’’ and

Fig. 1 (a) Tectonic setting of Morocco; APDTZ Atlas Palaeozoic dextral transform zone from Houari and Hoepffner (2003). (b) Geological

sketch map of the study area; EHF El Hart n’Iamine Fault, SAF South Atlas Fault, TBF Tizi n’Boujou Fault, TZF Tazlourt Fault

1112 Int J Earth Sci (Geol Rundsch) (2010) 99:1111–1122

123

‘‘2nd Bani’’ (Destombes 1976; Hamoumi et al. 1994).

The 2nd Bani sandstone records the sea-level fall during

the Late Ordovician glaciation (Ouanaimi 1998), and is

followed by transgressive Silurian black shales and by

upper Silurian–Devonian carbonates (Hollard 1981). The

Carboniferous succession in the study area comprises the

Aıt Yalla Formation (Tournasian–lower Visean) and the

Tinerhir Formation (upper Visean–Westphalian) (El

Boukhari et al. 2007a; Malusa et al. 2007b; Schiavo

et al. 2007a). The Aıt Yalla Formation is characterized

to the east by pelites and sandstones, and to the west by

the ‘‘Schistes a blocs’’ complex (Hindermeyer 1955;

Michard et al. 1982). The latter shows chaotic sedi-

mentary facies, characterized by blocks of Ordovician

sandstones, Silurian black shales, and Devonian lime-

stones embedded in pelites (Schiavo et al. 2007a), and

interpreted as debris-flow deposits. The Tinerhir Forma-

tion consists of flysch-type strata (Hindermeyer 1955),

interpreted either as turbidites (Michard et al. 1982;

Graham and Sevastopulo 2007) or as deltaic deposits

(Soualhine et al. 2003), overlain by platform limestones

and conglomerates (Schiavo et al. 2007b).

Upper Triassic–lower Liassic tholeiitic dykes crosscut

the Precambrian basement and the Palaeozoic succession

(Sebai et al. 1991; Knight et al. 2004). Palaeozoic strata are

unconformably overlain by a Cretaceous–Neogene suc-

cession starting with Cenomanian alluvial deposits

(Hindermeyer et al. 1977; Schiavo et al. 2007a). Recent

magmatic activity includes Pliocene–Quaternary pyroxene

nephelinites (Berrahma et al. 1993).

The Variscan nappe stack in the Tinerhir area

The complete Palaeozoic authochtonous sequence charac-

terizes only the eastern side of the Saghro inlier (Fig. 1),

where it unconformably covers the underlying basement

(El Boukhari et al. 2007b; Malusa et al. 2007a). On the

northern side, in the Tinerhir area, most of the Palaeozoic

sequences are detached from their basement and constitute

two allochthonous units referred to as Ouaklim and Tilo-

uine units. They form a structural stack overthrusted onto

the authochtonous cover (Figs. 2, 3) and dissected by E- to

NE-trending steeply dipping faults.

The Tilouine Unit and the structurally lower Ouaklim

Unit, which is exposed within tectonic windows, show

different stratigraphic sequences, deformation styles and

metamorphic overprint. The stratigraphic succession of the

Tilouine Unit contains the same Palaeozoic formations

observed on the eastern side of the inlier, starting from the

Schistes a Paradoxides, but its overall thickness is lesser

(Fig. 2). In this unit, the Aıt Yalla Formation mainly

consists of pelites. The stratigraphic succession of the

Ouaklim Unit is even thinner and dismembered in distinct

tectonic elements by subsidiary faults. Devonian lime-

stones are lacking, and the Aıt Yalla Formation in this unit

mainly consists of ‘‘Schistes a blocs’’ deposits.

Deformation history

The autochthonous sedimentary cover of the Saghro inlier,

as other Palaeozoic successions described across the Anti-

Atlas belt (e.g. Helg et al. 2004), is mildly deformed by

gentle regional scale anticlines and synclines with near-

vertical axial planes. Two generations of folds, forming

dome-and-basin interference patterns, were recognized.

E–W trending folds are generally superposed onto N–S

folds (Dal Piaz et al. 2007; El Boukhari et al. 2007c).

In the allochthonous Ouaklim and Tilouine units,

deformation is instead more complex and intense. Poly-

phase Variscan deformation (D1 and D2 phases) results in

superposed fold systems and fault zones, which are cross-

cut by later faults related to a post-Variscan deformation

history (see Malusa et al. 2007a for details on the post-

Variscan evolution of the area).

Deformation phase D1

The D1 structures mainly correspond to low-angle thrust

planes that do not involve the Precambrian basement. This

thin-skinned tectonic phase led to nappe stacking and

southward thrusting of allochthonous units onto the Saghro

inlier. The Tilouine Thrust, the main D1 tectonic structure

in the study area (TIT in Figs. 3, 4), dips gently to the NW

Fig. 2 Stratigraphic columns in the Tilouine and Ouaklim units and

in the autochthonous succession. a Volcanoclastic rocks, b conglom-

erates and sandstones, c shales and carbonates, d sandstones (‘‘Gres

Terminaux’’), e carbonate breccias (‘‘Breche a Micmacca’’), f shales

(‘‘Schistes a Paradoxides’’), g sandstones (‘‘Gres du Tabanit’’), O1shales, O2 sandstones (‘‘1st Bani’’), O3 shales, O4 sandstones (‘‘2nd

Bani’’), C1 chaotic complex and pelites, C2 turbiditic sandstones. PRPrecambrian, Lower Cambrian; chiefly Middle Cambrian, OOrdovician, S Silurian, D Devonian, C Carboniferous

Int J Earth Sci (Geol Rundsch) (2010) 99:1111–1122 1113

123

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1114 Int J Earth Sci (Geol Rundsch) (2010) 99:1111–1122

123

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Int J Earth Sci (Geol Rundsch) (2010) 99:1111–1122 1115

123

and juxtaposes the Tilouine Unit onto the Ouaklim

Unit. This thrust is best exposed near the village of

Tilouine, where it separates Ordovician formations of the

Tilouine Unit, in the hanging wall, from the strongly

foliated lower Carboniferous Aıt Yalla Formation of the

Ouaklim Unit, in the footwall (Figs. 4, 5a). The thrust

plane consists of a centimetre- to decimetre-thick fault

zone, crosscutting foliations in both hanging wall and

footwall rocks (Fig. 5b). Slickenside striae plunge toward

NW, documenting a dominant top-to-SE reverse

motion coupled with slight dextral transcurrence

(Fig. 6a).

West of the village of Ouaklim, the Tilouine Thrust

branches off within the Ouaklim Unit, displacing hanging

wall Ordovician rocks over a footwall containing Carbon-

iferous rocks (Fig. 4b).

Thrusting of the Tilouine Unit onto the Ouaklim Unit

is associated with F1 folds, mainly developed in the

Ouaklim Unit. F1 folds are tight to isoclinal and mostly

preserved as rootless hinges (Fig. 5c). They have a

penetrative axial-plane cleavage (S1) characterized by re-

crystallization of fine-grained white mica and chlorite.

This cleavage represents the main fabric at the outcrop

scale both in the pelitic sequences (e.g. in the Schistes a

Paradoxides) and in the pelitic interlayers of the sand-

stone sequences (e.g. in the Gres du Tabanit and in the

Tinerhir Formation) (Fig. 5d). Otherwise, in sandstone

layers (e.g. in the 1st Bani and 2nd Bani formations), the

S1 cleavage is poorly developed and the sedimentary

bedding is crosscut by centimetre-scale shear zones with

calcite fibres indicating a top-to-SE sense of shear,

consistent with SE-ward thrusting.

Fig. 5 Structural features of

deformation phase D1. aOrdovician sandstones of the

Tilouine Unit (O3) thrust above

the Carboniferous chaotic

complex of the Ouaklim Unit

(C1) along the Tilouine Thrust.

b Details of the Tilouine Thrust

of a, indicating a top-to-SE

sense of shear. c F1 rootless

hinge zone and associated S1

axial-plane foliation in Middle

Cambrian ‘‘Schistes a

Paradoxides’’ (Ouaklim Unit,

southern domain). d S0

(bedding)–S1 (foliation)

relationships in the pelitic flysch

of the Tinerhir Formation

(Ouaklim Unit, southern

domain)

Fig. 6 Structural data from the study area (equal-area stereographic

projections, lower hemisphere)

1116 Int J Earth Sci (Geol Rundsch) (2010) 99:1111–1122

123

Deformation phase D2

The thick-skinned D2 deformation phase is expressed by an

association of folds and steeply dipping fault zones. The

latter, representing the most apparent D2 structures at map

scale, are referred to as El Hart n’Iamine Fault and Tazlourt

Fault in Figs. 3 and 4. They are two steeply dipping E–W

trending fault zones, tens of kilometres in length that

crosscut the Tilouine Thrust and other D1 structures. The

N-dipping El Hart n’Iamine Fault, exposed on the northern

part of the study area, juxtaposes the Silurian-Carbonifer-

ous succession of the Ouaklim Unit against the Ordovician-

Carboniferous formations of the Tilouine Unit (Fig. 4c).

The Tazlourt Fault runs predominantly within the Ouaklin

Unit (Fig. 7a), and in the easternmost side thrusts the

Ouaklim Unit onto the Tilouine Unit (Fig. 4c). Second-

order faults related to the Tazlourt Fault led to tectonic

slicing within the Ouaklim Unit, with the decollement

plane at the level of the lower Ordovician shales (O1;

Fig. 4a, b). The El Hart n’Iamine and Tazlourt faults bear

oblique to up-dip slickenside striae, indicating dominant

dextral strike-slip coupled with top-to-S reverse motion

(Figs. 6b, 7b). Further south, the northern boundary of the

Saghro inlier corresponds to another fault zone, consisting

of steep NNW-dipping D2 fault planes (Figs. 3, 4). This

fault zone juxtaposes Middle Cambrian rocks ascribed to

the Tilouine and Ouaklim units against the Saghro base-

ment and its autochtonous sedimentary cover. In places, it

is truncated by later faults (Fig. 4c). Oblique and up-dip

slickenside striae indicate oblique-reverse motion, with

strong partitioning of reverse and right–lateral slip

throughout the fault zone (Fig. 7c).

These major D2 faults define three structural domains,

referred to as northern, central and southern domains,

Fig. 7 Structural features of

deformation phase D2.

a Tazlourt Fault (TZF), marking

the boundary between the

central and southern domain.

b D2 fault plane bearing oblique

slickenlines (L2) with a dextral

strike-slip movement (Ouaklim

Unit, southern domain). c D2

fault plane with dip-slip

slickenlines (L2) indicating a

top-to-the-SE sense of

movement (Ouaklim Unit,

southern domain).

d Hectometre-scale SW-plunging

F2 fold developed in alternating

Ordovician sandstones and

shales (Ouaklim Unit, central

domain). e Type-3 interference

structure (Ramsay, 1967)

between F1 and F2 folds in

pelites of the Carboniferous

flysch (Ouaklim Unit, northern

domain). f Mesoscale duplex

structure along a bedding

parallel shear zone (Ouaklim

Unit, southern domain)

Int J Earth Sci (Geol Rundsch) (2010) 99:1111–1122 1117

123

which are characterized by differences in D2 deformation

(Fig. 8). Folding structures are dominant in the central

domain, where the F2 upright open folds have NE–SW to

E–W strike, decametre to hectometre wavelength, and are

gently overturned toward the south (Figs. 4, 6c, 7d). Their

axial-plane foliation (S2 in Fig. 6d) ranges from a spaced

crenulation cleavage in sandstones, to a well-developed

slaty cleavage in pelites, and bear no evidence of meta-

morphic crystallization. Superposition of D2 onto D1 folds

produces type-3 interference structures (Ramsay 1967),

widespread at the outcrop scale (Fig. 7e). At the regional

scale, the S2 foliation and the F2 axial planes in these three

domains define a sigmoidal pattern, pointing to a non-

coaxial deformation regime (Fig. 8).

Fault zones are dominant in the northern and southern

domains, where tight F2 folds of metre to decametre wave-

length are strictly associated to fault planes. These structures

are mostly developed in the northern domain, where steep N-

dipping fault zones are marked by metre-thick cataclastic

bands and by strongly deformed slices of Silurian black

shales, often occurring as pinched anticlines within the

Tinerhir Formation. Oblique to up-dip slickenside striae are

consistent with dextral-oblique displacement.

The F2 folds are strictly associated with strike-slip and

thrust faults bounding tectonic slices of strongly deformed

rocks (e.g. Silurian black shales). Folds associated with D2

thrust faults are SE-verging ramp anticlines with a steeply

dipping axial-plane foliation, best developed in pelites.

Bedding parallel shear zones and mesoscale duplex struc-

tures were also detected in the field (Fig. 7f).

Age constraints on D1 and D2 deformation

The youngest deposits deformed by D1 and D2 structures

belong to the Tinerhir Formation (upper Visean–

Westphalian). The Tinerhir Formation is crosscut by

undeformed Upper Triassic–lower Liassic tholeiitic dykes,

and unconformably overlain by Upper Cretaceous deposits.

Therefore, the age of D1–D2 deformation is constrained

between late Carboniferous and Early Triassic time. Thus,

it is consistent with a Neovariscan evolution (300-290 Ma),

as already proposed by Michard et al. (1982) and Pique and

Michard (1989) (Westphalian phase). Cooling paths mod-

elled from apatite fission-track data show no differential

exhumation across D1 or D2 structures during the Permo-

Triassic time (Malusa et al. 2007a), suggesting a late

Carboniferous age for both D1 and D2 deformation phases.

Alpine tectonics, related to Cenozoic deformation in the

Atlas belt, is attested by faults dissecting the Variscan

structures. The main post-Variscan fault in the study area is

the ENE–WSW Merouane Fault (Figs. 3, 4), which is

associated with NE–SW and NNE–SSW minor faults cut-

ting the northern boundary of the Saghro inlier. Further

south, major post-Variscan faults are the Isk n’Izekelli, the

Tizi n’ Boujou and the Bou Larhzazil-Tinifift faults. These

structures record left-lateral transtension coeval with the

Mesozoic rifting, followed by late Neogene right-lateral

transpression related to the Atlasic orogeny (Malusa et al.

2007a).

Metamorphic conditions

The metamorphic grade during deformation phase D1 was

investigated through the analysis of illite crystallinity on 61

metapelite samples from the Ouaklim and Tilouine units

(Fig. 3). The overall bulk mineralogy is very similar in the

samples from both the units. Assemblages typically consist

of quartz, more abundant in coarser samples, illite

(muscovite), low amount of feldspars and subordinate

Fig. 8 Tectonic sketch map of

the E–W trending domains

bounded by D2 first-order strike-

slip faults and related foliation

and axial-plane trajectories.

Same acronyms as in Fig. 3

1118 Int J Earth Sci (Geol Rundsch) (2010) 99:1111–1122

123

chlorite. The clay fraction (60–70%) consists of illitic

material (illite ± I/S mixed layers), subordinate chlorite

and sporadic kaolinite. Moderate amounts of smectite,

probably of secondary origin, was observed. I/S mixed

layers are normally more abundant in the Tilouine Unit

than in the Ouaklim Unit.

Histograms in Fig. 9 show the distribution of KI index

values, along with standard deviation (Kubler 1984, 1990).

Average KI values are 0.31 ± 0.05�D2h in the Ouaklim

Unit and 0.41 ± 0.09�D2h in the Tilouine Unit. On the

basis of KI values characteristic of the diagenesis/anchiz-

one boundary (0.42�D2h) and of the anchizone/epizone

boundary (0.25�D2h), our results point to a middle anchi-

zonal grade for the Ouaklim Unit and to diagenesis/an-

chizone transition for the Tilouine Unit during deformation

phase D1, which correspond to a metamorphic temperature

of about 250–300�C for Ouaklim Unit and of 200–250�C

for Tilouine Unit.

Discussion

The new structural and illite crystallinity data from the

Tinerhir area show that the Palaeozoic sequences exposed

along the Southern Variscan Front are dominantly allo-

chthonous and detached from their basement. The lack of

Lower Cambrian strata in these sequences may result from

tectonic excision, rather than depositional hiatus. The

autochthonous Palaeozoic cover crops out extensively only

south of the Tizi n’ Boujou Fault, which overprints the

Southern Variscan Front between the Saghro and the

Ougnat inliers. This reconstruction is consistent with that

proposed by Malusa et al. (2007a), who postulated the

occurrence of Variscan allochthonous units above the

northern portion of the Saghro inlier. We show that these

allochthonous units, eroded above the Saghro basement,

are still preserved between the Saghro and Ougnat inliers,

and that the Southern Variscan Front is located in a

southernmost position with respect to classical recon-

structions (e.g. Michard et al. 1982).

Polyphase deformation and strain partitioning

The following tectonic evolution for the Southern Variscan

Front in the Tinerhir area is delineated by data reported in

this work (Fig. 10):

– An early thin-skinned deformation phase (D1) produced

a southward-facing nappe stack with associated folding

and S1 cleavage (Fig. 10a), developed under very low-

grade metamorphic conditions. This nappe stack was

overthrust onto the Precambrian basement and its

Palaeozoic autochtonous cover. The slight difference

in metamorphic conditions between the upper Tilouine

Unit (diagenesis/anchizone transition) and the lower

Ouaklim Unit (anchizone) may result from their

different position in the D1 nappe pile.

Fig. 9 Illite ‘‘cristallinity’’ index (KI) in the Ouaklim and Tilouine

units. KI boundaries between metamorphic zones after Kubler (1984,

1990). X average value, r standard deviation, N number of study

samples. Sample location in Fig. 3

Fig. 10 Interpretative model (not to scale) for the tectonic evolution

of the Tinerhir area. a Thin-skinned D1 deformation phase: develop-

ment of the structural stack with the Tilouine Unit overthrust onto the

Ouaklim Unit along the Tilouine Thrust (TIT). These allochthonous

units overthrust the autochtonous cover. b Thick-skinned D2 defor-

mation phase: the D1 nappe pile is disrupted by steeply dipping

transpressional faults, leading to the development of fault-bound

domains (EHF El Hart n’Iamine Fault, TZF Tazlourt Fault)

Int J Earth Sci (Geol Rundsch) (2010) 99:1111–1122 1119

123

– A second thick-skinned deformation phase (D2) led to

the development of large-scale antiforms and synforms.

The D1 nappe pile was disrupted by D2 structures,

producing three elongated E–W trending domains

delimited by steeply dipping transpressional faults

(Fig. 10b). D2 deformation mainly took place in the

northern and southern domains, where structural data

highlight a dominant dextral shear component, with

development of strike-slip fault-bounded slices, thrust-

related folds, upright foliation and widespread bedding

parallel shear zones. All these structures bear kinematic

indicators that indicate a dextral-oblique component of

movement, suggesting orogen-parallel strike-slip for

the northern and southern domains. In the central

domain, D2 structures are mainly represented by E–W-

striking south-verging folds, suggesting orogen-normal

shortening. Therefore, the structural pattern of the

Tinerhir area derives from a polyphase evolution where

the D2 deformation phase developed in a transpressional

tectonic setting with dextral strike-slip displacements.

This interpretation confirms the occurrence of a late

Variscan nappe stack in the Tinerhir region, as already

proposed by Michard et al. (1982), but some important

differences with respect to previous interpretations also

arise. In fact, on the northern side of Saghro inlier, the

autochthonous cover is poorly exposed, and most of the

autochthonous cover described by Michard et al. (1982) is

now interpreted as allochthonous, belonging to the Tilouine

and Ouaklim units. Deformation history that was respon-

sible for thrusting according to the above authors is clearly

polyphase. The Palaeozoic authoctonous cover crops out

extensively only south of the Tizi n’Boujou Fault, which

re-activated the Southern Variscan Front since the Meso-

zoic time.

The Southern Variscan Front as a late Variscan

transpressional zone

The kinematic partitioning of oblique convergence into

complex transpressional zones (e.g. Oldow et al. 1990;

Tikoff and Teyssier 1994; Jones and Tanner 1995; Dewey

et al. 1998) has been described in many orogenic belts and

invoked to explain their structural complexity (Brun and

Burg 1982; Hubbard and Mancktelow 1992; Cerrina Feroni

et al. 2004; Tavarnelli et al. 2004; Malusa et al. 2009).

The structural pattern in the Tinerhir area results from a

polyphase Neovariscan evolution where D2 transpressional

deformation was partitioned between major E–W trending

domains of dominant orogen-normal shortening (central

domain) and domains of dominant orogen-parallel strike-

slip (northern and southern domains). This deformation

pattern allows a direct correlation with the right-lateral Tizi

n’Test Fault, to the west, and bears many similarities with

that described by Houari and Hoepffner (2003) in the

Tamlelt inlier, to the east. In this latter area, thrusting is in

fact combined with strike-slip faulting, SSE-facing over-

turned folds are associated with E–W dextral shear zones,

and slaty cleavage developed under weak metamorphic

conditions. We can conclude that the Southern Variscan

Front does not represent a simple Appalachian-type fore-

land thrust belt (Michard et al. 1982), but has to be

regarded as a late Variscan transpressional zone where

orogen-normal shortening was followed by right-lateral

oblique movements. The latter are consistent with the late

Variscan strike-slip tectonics observed along the Tizi

n’Test Fault, which accommodated a dextral displacement

of some hundreds of kilometres at the boundary between

the Southern Meseta and the West African Craton

(Mattauer et al. 1972). Within this framework, the palae-

ogeographic restoration of fault blocks exposed in the

study area is not straightforward.

The structural pattern proposed for the Tinerhir sector

can be tentatively fit in the geodynamic model proposed by

Hoepffner et al. (2006) for the Mesetan orogen (Fig. 11). In

this reconstruction, the Tinerhir area is located in the

Southern Zone of Hoepffner et al. (2006), which marks the

boundary between the Mesetan and Saharan domains. The

Southern Zone is limited to the north by the Atlas Palae-

ozoic Transform Zone, and to the south by the Southern

Variscan Front, representing a regional scale transpres-

sional shear zone dissecting the southern Meseta during

late Carboniferous time. Within this framework, it is pos-

sible to reconcile gently dipping thrust faults and steeply

dipping transpressional shear zones in a single evolution

model. According to our reconstruction, the Tilouine and

Fig. 11 Geodynamic model for the Southern Variscan Front within

the framework of the Moroccan Variscan Orogen (deformation

pattern in the Meseta block modified after Hoepffner et al. 2006). CBMeseta coastal block, CZ Meseta central zone, EZ Meseta eastern

zone, APTZ Atlas Palaeozoic transform zone, EHF El Hart n’Iamine

Fault, TIT Tilouine thrust, TZF Tazlourt fault

1120 Int J Earth Sci (Geol Rundsch) (2010) 99:1111–1122

123

the Ouaklim units would represent laterally extruded thrust

sheets with flower geometry dissected by strike-slip

deformation (Fig. 11). Their roots are probably located

west of the study area, along the Tizi n’Test Fault or

beneath the Meso-Cenozoic sedimentary sequences

exposed in the Atlas region.

Conclusions

The Southern Variscan Front of Marocco, described so far

as an Appalachian-type foreland fold belt, reveals more

complex deformation patterns. Allochthonous Palaeozoic

sequences are stacked onto the northern margin of the West

African Craton along south-facing low-angle thrusts. These

sequences are cut by steeply dipping right-lateral faults,

which bound major E–W trending domains of dominant

pure shear and simple shear. All these structures point to a

late Carboniferous transpressional tectonics along the

boundary of the West African Craton and can be inter-

preted in the light of a Neovariscan oblique convergence at

the scale of the whole Moroccan Variscides. They are

consistent with the deformation history in the Tamlelt inlier

and in the whole Meseta block and allow correlations

between the Southern Variscan Front and the Tizi n’Test

Fault.

Acknowledgments This work was carried out within the framework

of the CNR Research Project ‘‘Ruolo delle strutture transpressive nel

sistema orogenico circummediterraneo’’. We thank A. Michard for his

precious advice, P. Pertusati, L. Baidder and F. Ghiselli for the

stimulating discussions in the field, and C. Ribecai and G. Sbrana for

preparing illite samples. The manuscript benefited from detailed and

constructive reviews by K. Hefferan and C. Hoepffner.

Appendix

Illite ‘‘crystallinity’’ index (KI or Kubler Index) of the 10 A

reflection was determined on oriented aggregates of\2 lm

fraction using a Philips PW1710 automatic diffractometer

equipped with a long-focus Cu Tube. The diffractometer was

set as follows: Cu Ka Ni-filtered radiation; 40 kV; 20 mA;

slits: 1� divergence and scatter, 0.2 mm receiving; scan

speed: 0.5�2h/min; step size of 0.02�2h; a counting time of

2 s for each step. Oriented aggregates were processed both in

air-dried and glycolated states, from 3 to 30�2h under the

same experimental conditions, to determine clay mineral

association in different lithologies. Bulk-rock mineral

assemblages were also assessed from whole rock powder in

the range of 4–60�2h. The amount of clay on each slide was at

least 3 mg cm-2 (Lezzerini et al. 1995). The KI index (half-

height peak width expressed as �D2h) values were measured

on chart-strip XRD patterns obtained in air-dried condition

(AD) and calibrated to Kubler’s standards. KI indexes were

also measured on the glycolated state (EG).

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