STRUCTURAL EVOLUTION OF THE ANTI-ATLAS DOMAIN :AN OVERVIEW pour Yves Missenard
le géoparc du jbel bani - tata

STRUCTURAL EVOLUTION OF THE ANTI-ATLAS DOMAIN :AN OVERVIEW pour Yves Missenard

The  Anti-Atlas represents the  most important  segments  of  the  major Pan-African  (≈0.5Ga) belt  system  of North  Africa. This orogen  exposed  in  a  series  of  sporadic  SW–NEtrending  outcrops over  700km  across  southern Morocco, and reach ≈150km  wide  in  the central  part, west  of  Ouarzazate (Figure G1 & G2).Scattered outcrops of related rocks are described in NW Algeria which indicate that the belt continues from SE Morocco southeastwards with a NW–SEtrend.

The Anti-Atlas orogen comprises two main sequences of rocks: a metamorphic basement of  Palaeoproterozoic (2Ga)  age  and  the  Neoproterozoic  rocks.  The Palaeoproterozoic  rocks  form the northern margin of the West African Craton mainly outcropping in the Reguibate Shield in Mauritania And Algeria.    The    Palaeoproterozoic    basement    is    exposed    in a    series    of    uplifted    inliers (‘‘boutonnieres’’) surrounded by  the Neoproterozoic  rocks  that  were  locally  deformed with  the basement during the Pan African Orogeny. with  the during the Pan African Orogeny.

Paleoproterozoic rocks and Eburnian orogeny

The  oldest  rocks  of  Morocco, Archean  in  age3  Ga(Montero  et  al,  2014) crop  out  in the southernmost Reguibat E shield which  formsthe  northern  part the  West  African  Craton  (WAC).Further north,  in  the  Anti-Atlas  chain,  the  basement  units  are  Paleoproterozoic  (Taznakht  Group, former” PI”)cropping only in the south of the Anti-Atlas Major Fault (AAMF). They are metamorphic  rocks (greenschist to amphibolite facies) intruded by peraluminous and calc-alkaline granitoids dated around  2Ga(Thomas  et  al,  2002).The  corresponding  tectono-magmatic  and  metamorphic  events are assigned to the Eburnian/Birimian orogeny.

Neoproterozoic rocks and Pan-African orogeny

As    everywhere    in    the    WAC,    Mesoproterozoic    rocks    are    lacking    in    the    Anti-Atlas. Neoproterozoic Formations overlie directly the Paleoproterozoic basement. The Neoproterozoic/Paleoproterozoic  boundary  is  generallya  tectonic  contact  (thrust,  strike-slip  or detachment  faults),  and  seldom  a  stratigraphic  contact  (Tizi  n’Taghatine).  Globally,  the Neoproterozoic formations record three main stages of the Pan-African cycle (figureG4):

I .The  early  Neoproterozoic platform development  is marked  by  the  accumulation  of  thousands of  meters  of  quartzites  and  stromatolitic  limestones  (Taghdoute  Group,  former  “PII”), intruded  by  doleritic  dykes  and  gabbroic  intrusions.  These  rocks  are  associated  with  the rifting  of  the  WAC  margin, broadly  contemporaneous  with  the  oceanic  accretion  further north (760 Ma), witnessed along the AAMF by the Bou Azzer-El Graara and Siroua ophiolitic sequences (Bou Azzer Group).

II .Oceanic closure and subsequent Pan-African collision are associated with oceanic subduction along the northern margin of the WAC, ending with the  accretion of oceanic arc formations (figureG4).The reported “blueschist facies” mineral associations in the Bou  Azzer  inlier are controversial.  The  polarity  of  the  subduction  remains  also  matter  of  debates;  the  same  is true  for  the  real  location  of  the  northern  edge  of  the  WAC. The  oblique  Pan-African collision(655 Ma to 640 Ma) generated south-verging thrust sheets onto the cratonic margin. South  of  the  AAMF,  the  main  Pan-African  phase  is  recorded  by  low  grade recrystallizations, synmetamorphic  folds  and  various  ductile  and  brittle  structures  in  the  Taghdout  Group series.

III The  Late  to  Post  Pan-African extensional  event  is  recorded  by  the  extensive  volcanic  and volcano clastic series of the Ouarzazate Group (former “PIII”, 580 Ma to 560Ma), interbedded with  subaerial  to  lacustrine  deposits,  which  unconformably  overlie  the  Eburnian  and/or  Pan-African basement units. The Ouarzazate Group shows abrupt variations of thickness and facies controlled  by  extensional  tectonic  activity.  Various  high-K  calc-alkaline  to  alkaline  plutons emplaced   within   the   Ouarzazate   Group,   coeval   with   the   volcanic   rocks   of   comparable chemistry.

FigureG4: Generalized lithostratigraphic column for the Anti-Atlas Pan-African orogen. “PI” = “XI”, etc. are the classical stratigraphic symbols used on Anti-Atlas geological maps. HKCA: High-K calc-alkaline (granitoids) after Thomas et al. (2004), Gasquet et al. (2005) and Liégeois et al.2006 in Youbi et al., 2013

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