tectonique et venues volcaniques en contexte de collision exemple du massif neogene du siroua atlas marocain effets combines d une transformante et de la suture panafricaine the late miocene early pliocene siroua strato volcano is made of particular hyperalkaline rocks it lies between the high atlas and the anti atlas in a collisional zone related to the continental subduction of the african plate under the moroccan meseta our field observations and analyses of spot landsat mss and dem digital elevation model imagery have permitted mapping of faults joints and volcanic edifices the elongate shape of volcanoes and linear clusters of adjacent edifices together with their relationships with faults show that magma ascent was favored by tectonic crustal scale open fractures essentially tension fractures tail cracks and open faults these fractures together with other nonvolcanic narrow nne striking troughs provide valuable information on the regional deformation since the late miocene the shortening extension type strain which is responsible for the open fractures is situated near the azdem transform a zone of active faults striking nne parallel to the co

jurassic penetrative deformation and cenozoic uplift in the central high atlas morocco a tectonic model structural and orogenic inversions the central high atlas is characterized by a relatively mild deformation with a local development of a more or less penetrative cleavage in the axial part of the belt locally this cleavage and the plutonic bodies which outcrop in the core of most of the north east trending anticlines ridges with intrusions are overlain by a disconformable sequence that comprises red clastic deposits which are capped by cenomano turonian limestones the age of these red deposits are bracketted between bathonian to mid cretaceous therefore the spatial relationship of the cleavage with mafic intrusions in the axial part of the central high atlas suggests that it developed contemporaneously with the emplacement of magmas probably during mid jurassic times during this period the structural inversion occurred from a transtensional regime to a transpressive regime there are few indications that an important shortening persisted within the axial part of the central high atlas after the end of the jurassic along the northern and the southern borders of the belt however cretaceous and cenozoic strata were deposited their sedimentary facies bear witness of successive stages of the atlas evolution especially the rapidly coarsening up of the clastic deposits since the oligocene in these marginal basins indicates the onset of the rising up of the belt i e the orogenic inversion

intermediate depth seismicity in the western mediterranean unrelated to subduction of oceanic lithosphere there are three zones in the gibraltar area where intermediate depth earthquakes occur down to 150 km two of these the gulf of cadiz and the western alboran sea are oceanic areas which could be associated with the subduction of oceanic lithosphere our present measurements of arrival times of p and s waves show however that the third zone beneath the high atlas is a typical intracontinental chain where it is not likely that oceanic lithosphere has been subducted during the past 20 million years

evolution of the ridges of midelt errachidia section in the high atlas revealed by paleomagnetic data new paleomagnetic data 43 sites from mesozoic sediments are contributed in this work verifying the presence of a pervasive syntectonic early cretaceous remagnetization in the easternmost area of the moroccan high atlas using the small circle intersection method we have calculated the characteristic remagnetization direction dec 337 3 inc 38 4 that fits with a 100 ma age according to the apparent polar wander path of africa the paleomagnetic vectors of remagnetization are used to obtain the geometry during the remagnetization stage 100ma of one of the most renowned geological cross sections of the high atlas the midelt errachidia profile the partial restoration of the cross section at 100ma allows us to determine the dips of the beds at the remagnetization stage in five structures ridges or anticlines our results indicate that the five ridges that configure the midelt errachidia profile were initiated to different degrees prior to wholesale compressive deformation during the cenozoic this configuration can be explained according to two different scenarios that we discuss in this paper transpression and diapirism the geological model obtained both at present and at 100ma indicates the existence of a mesozoic cover substantially decolled from the paleozoic basement what strongly contrasts with previously published transects of the same area

deep rooted thick skinned model for the high atlas mountains morocco implications for the structural inheritance of the southern tethys passive margin a re interpretation of the deep structure of the high atlas is presented through integration of geophysical and geological data highlighting the architectural significance of the southern tethys palaeomargin previous crustal models suggest the occurrence of a flat intra crustal detachment at a depth of 20 km a zone where surface thrusts merge and below which the lower high atlas crust appears continuous however within this study seismic refraction data electrical resistivity surveys and gravity modelling all appear to detect a jump in crustal thickness between the high atlas and the northern plains we interpret these data as penetration by thrusts within the south atlasic fault zone through the lower crust to offset the moho in accordance with a deep rooted thick skinned tectonic model the south atlasic fault zone corresponds to a series of crustal scale structures inherited from a series earlier orogenic events pan african hercynian and atlasic and thus is sufficiently weak to create separate crustal blocks in the northern part of the stable african plate this structural configuration was achieved during cenozoic plate collision between africa and europe culminating in tectonic inversion of mesozoic extensional basins linked to the development of tethys and opening of the atlantic the inherited structures typically comprise normal faults and tilted blocks affecting the upper crust and crustal scale detachments affecting the lower crust 2006 elsevier ltd all rights reserved

transient fluvial incision as an indicator of active faulting and plio quaternary uplift of the moroccan high atlas one of the challenges facing earth scientists is to determine the extent to which geomorphic features can be used to extract tectonic signals from landscapes here we quantitatively analyse the long profiles of rivers that drain southwards across the south atlas fault saf a thrust fault that forms the southern margin of the high atlas mountains in morocco to derive new data on the late cenozoic activity of this fault system river long profiles were extracted for 32 major rivers flowing southwards into the ouarzazate basin of these eleven exhibit concave up river profiles with a mean concavity of 0 64 and normalized steepness indices in the range 47 5 219 0 m 0 9 by contrast 21 rivers exhibit at least one knickpoint upstream of the thrust front knickpoint height varies from 100 1300 m with calculated incision at the range bounding fault ranging from 80 900 m despite the drainage areas upstream of the knickpoint ranging over several orders of magnitude in map view knickpoint locations generally plot along sub parallel lines and there are no obvious relationships with lithological units for knickpoints exhibiting slope break morphology channel reaches below slope break knickpoints have higher mean concavities 0 76 than above the knickpoint indicative this observation combined with a lithological or river capture origin for the knickpoints having been ruled out suggests that an increase in uplift rate along a planar fault zone during the plio quaternary caused the initiation of the transient response i e knickpoint formation to a change in base level observed in the river profiles this uplift event can be correlated to the convective removal of the lithospheric root to the atlas mountains resulting in the anomalously high topography at the present day

early jurassic normal faulting in a carbonate extensional basin characterization of tectonically driven platform drowning high atlas rift morocco abstract this paper describes a tectonostratigraphic model of the synrift evolution of the early jurassic high atlas rift of morocco the model is constrained by mapping of a set of inverted extensional blocks by facies analysis of carbonate platform and turbiditic to hemipelagic synrift deposits and by high resolution n 100 ka biostratigraphy of the early jurassic succession the chronostratigraphic packages of the high atlas of rich vary significantly in thickness facies and architecture from one tectonic block to another our study shows how synrift strain varied in space and time over a long time interval 14 ma around the high atlas rift initially in sinemurian time the high atlas rift was affected by low strain normal faulting that controlled the growth of an extensive low gradient carbonate platform except in the northern domain towards the rift axis where hemipelagic deposition related to high rate faulting prevailed subsequently in carixian domerian time a rapid increase in accommodation space and block subsidence caused by high strain normal faulting brought about localized drowning of the carbonate platform and the development of calciturbidites and of starved deposits towards the rift axis during this interval high strain upper crustal normal faulting migrated rapidly over a period of 5 ma towards the rift periphery

neogene exhumation of the marrakech high atlas morocco recorded by apatite fission track analysis apatite fission track aft data have been obtained along a traverse across the marrakech high atlas to constrain its tectono thermal evolution aft ages vary between 212 15 ma and 20 4 ma an early miocene aft age accompanied by long mean track length from the central part of the chain has been interpreted as the timing of the main inversion of this region with the creation of relief because of the shortening induced by the interplay between the african and eurasian plates thermal modelling of samples collected south of the south atlas fault zone indicates a middle late miocene or even later cooling that has been attributed to the component of the uplift of the chain related to the thermal anomaly present beneath the atlas mountains

on the tectonic origin of iberian topography the present day topography of the iberian peninsula can be considered as the result of the mesozoiccenozo ic tectonic evolution of the iberian plate including rifting and basin formation during the mesozoic and compression and mountain building processes at the borders and inner part of the plate during the tertiary followed by neogene rifting on the mediterranean side and surface processes acting during the quaternary the northern central part of iberia corresponding to the geological units of the duero basin the iberian chain and the central system shows a mean elevation close to one thousand meters above sea level in average some hundreds of meters higher than the southern half of the iberian plate this elevated area corresponds to i the top of sedimentation in tertiary terrestrial endorheic sedimentary basins paleogene and neogene and ii planation surfaces developed on paleozoic and mesozoic rocks of the mountain chains surrounding the tertiary sedimentary basins both types of surfaces can be found in continuity along the margins of some of the tertiary basins the bouguer anomaly map of the iberian peninsula indicates negative anomalies related to thickening of the continental crust correlations of elevation to crustal thickness and elevation to bouguer anomalies indicate that the dierent landscape units within the iberian plate can be ascribed to dierent patterns 1 the negative bouguer anomaly in the iberian plate shows a rough correlation with elevation the most important gravity anomalies being linked to the iberian chain 2 most part of the so called iberian meseta is linked to intermediate elevation areas with crustal thickening this pattern can be applied to the two main intraplate mountain chains iberian chain and central system 3 the main mountain chains pyrenees and betics show a direct correlation between crustal thickness and elevation with higher elevation crustal thickness ratio for the central systemvs the betics and the pyrenees other features of the iberian topography namely the longitudinal pro le of the main rivers in the iberian peninsula and the distribution of present day endorheic areas are consistent with the tertiary tectonic evolution and the change from an endorheic to an exorheic regime during the late neogene and the quaternary some of the problems involving the timing and development of the iberian meseta can be analysed considering the youngest reference level constituted by the shallow marine upper cretaceous limestones that indicates strong dierences induced by i the overall tertiary and recent compression in the iberian plate responsible for dierences in elevation of the reference level of more than 6 km between the mountain chains and the endorheic basins and ii the eect of neogene extension in the mediterranean margin responsible for lowering several thousands of meters toward the east and uplift of rift shoulders a part of the recent uplift within the iberian plate can be attributed o sostatic uplift in zones of crustal thickening

jurassic sedimentary record and tectonic evolution of the northwestern corner of africa abstract the history of the opening seaway from the westernmost tethys to the central atlantic is traced by the analysis of the sedimentary facies development in the external rif basin of northern morocco and the geological and seismic data from the moroccan atlantic continental margin in the rif basin after the early sinemurian sedimentary facies dated by ammonites foraminifers and brachiopods indicate a progression of rapid subsidence resulting from extensional tectonic tilted blocks escarpment fault breccias neptunian dykes etc from the n and ne to the s and sw from the toarcian to the bajocian deltas progress from the w and sw into the rides sud rifaines realm from the late bathonian to the oxfordian deep sea fans develop in the external rif during the same period deltaic sediments fill in the middle atlas basin of eastern morocco and progress into the external rif the top of the jurassic is characterised by carbonate deposits at the northwestern corner of africa the subsidence of the sedimentary basins by rifting is initiated in the late triassic however at the mazagan transect of the atlantic continental margin the tectonic pattern characteristic of a passive continental margin appears clearly only in the early jurassic at the foot of the mazagan escarpment the sedimentary record shows a foundering of the first bloc during early to middle lias a thermal uplift phase is indicated by emersion of the african margin shoulder in late liassic and thermal relaxation starts in the middle jurassic the morphology of this transect compared with the conjugate side of the american continent is most easily explained by the uniform sense simple shear model

the carnian pluvial event in western europe new data from iberia and correlation with the western neotethys and eastern north america nw africa regions abstract at the beginning of the late triassic a period of increased precipitation occurred over large tracts of central pangea the carnian pluvial event it was a short lived event 1 ma coeval with a marked sea level fall and active rifting new and classical sections of carnian age in the iberian peninsula have been studied and a new correlation scheme is proposed for the iberian domain integrating field data and subsurface electric log data this is the first time that the portuguese and spanish late triassic carnian sediments are correlated the carnian pluvial event is identified in the iberian peninsula as a continental clastic wedge manuel fm and coeval sediments in between marine evaporite formations during a marked sea level drop event its age is considered to be julian early carnian by palynological and paleogeographic considerations a new correlation of the late triassic sediments of the iberian peninsula with coeval sediments of western europe nw africa and eastern north america is proposed and a new paleogeographical map of central eastern pangea for this period is presented in order to demonstrate that the geographical extent of the siliciclastic sediments related to the carnian pluvial event was much larger than accepted before

new application of gis and statistical analysis in mapping the distribution of quaternary calcrete tensift al haouz area central morocco abstract quaternary calcretes are widespread in soils of many regions of the world especially in semi arid and arid areas their development has been studied worldwide for more than a century involving various disciplines and resulting in large amounts of data concerning their evolution in this article the geographical distribution of calcrete has been studied by combining geographical information systems gis and statistical analysis in this paper we present a qualitative approach that incorporates topographic geological hydrogeological and climatic data as well as field observations after getting all four parameters they were divided into classes and an index value has been assigned to each of them according to influence degree on calcrete development finally all the parameters are superimposed to prepare the final map of calcrete distribution the multivariate statistical analysis techniques used were principal component analysis and cluster analysis both methods were performed to discuss the potential relations among the investigated parametres caco3 palygorskite slope and calcrete evolution the qualitative approach adopted provided a distribution map predicting the possible areas where different calcrete forms might occur and allowed to elucidate the degree of influence of environmental factors on the evolution of calcrete the accuracy of this method has been estimated by comparing it with 52 cross section data from the field this approach proved to be successful in this study the results of statistical analysis confirm the results obtained using gis method and highlight the influence of the studied environmental factors on the spatial distribution of different types of calcrete

spatial and temporal distribution of ooids along a jurassic carbonate ramp amellago outcrop transect high atlas morocco carbonate ramp systems are widespread throughout the geological record but very few areas have seismic scale continuous and structurally undeformed outcrops that allow reliable interpretation of facies distributions and stacking patterns the amellago outcrop shows the detailed depositional and stratigraphic relationships of an ooid dominated ramp system that is almost completely exposed along a dip profile 37 km long and 1000 m thick in the lower to middle jurassic of the southern high atlas morocco ammonite and brachiopod fauna provide excellent biostratigraphic control on small scale stacking patterns at amellago the evolution of depositional environments is evident at different scales of space and time during this period of tectonic quiescence dominated by thermal subsidence an important observation is that the amellago ramp system contains micrite rich ooid free intervals that alternate with ooid rich intervals the ooid rich intervals are mainly in the late transgressive and highstand system tracts whereas the ooid free intervals occur in the early transgressive phase more than 25 such alternations were recorded in high frequency cycles and at the scale of one large cycle at the aalenian bajocian transition these compositional changes and the associated different ramp geometries are interpreted to result from the combined effects of eustatic sea level and climatic changes

major fracture zones of morocco the south atlas and the transalboran fault systems within the fracturai pattern of morocco 35 45 and 70 90 directions are predominant most of the faults originated already during the upper palaeozoic and were repeatedly reactivated later on two of the large fracture systems are considered here the south atlas fault system saf is composed of different faults with directions varying between 45 90 they were not functionally connected to the southern hinge of the high atlas until its miocene uplift today it seems to be inactive on the whole both the tectonic data and the facies distribution of mesozoic strata contradict clearly the interpretation that the saf is a huge wrench fault or even the mesozoic plate margin of africa the sw striking transalboran fault system taf is still active it crosses morocco from melilla to agadir showing again singular faults which are unified to extended lineaments only in a few areas the intervals between the faults are bridged by linear arrangements of earthquake hypocenters sinistral massflow within the deeper parts of the lithosphere seems to be compensated at the surface by movements along pre existing faults for the moroccan segment of the taf a sinsitral displacement is testified at least since the oligocene while the northeastern segments from the rif to spain did not even originate until the pliocene this would mean that sinistral shear was transmitted from africa into the accreted alboran and iberia blocks after the miocene collision

transpressional structuring of the high atlas belt morocco abstract the high atlas belt of morocco is a doubly vergent intracontinental belt formed during cenozoic convergence between the african and eurasian plates this belt is characterized by the simultaneous occurrence of high topographic elevation minor crustal thickening and weak tectonic shortening which are commonly explained by models of inversion tectonics and mantle upwelling this paper aims to test the consistence of this type of model presenting the results of a multi scale tectonic study including mapping of major faults and fault slip data inversion from key areas homogeneously distributed through the entire high atlas our data highlight kinematic components parallel to the orogen trend supporting a transpressional structuring integration between new tectonic data and available geophysical and thermochronologic constraints has allowed to extend to depth the main faults detected at the surface resulting in the overall geometry of a positive flower structure this scenario is the result of a long history of transpressional and transtensional reactivations of major lithospheric faults since the break up of pangea lithospheric scale faults isolate independently behaving lithospheric blocks characterized by different exhumation rates in this framework the compressive component of the transpressional geodynamic regime can produce the high topographic elevation of the high atlas belt without crustal thickening

extensional detachment faulting in the evolution of a tethys passive continental margin eastern alps switzerland the austroalpine nappes of switzerland represent an exhumed and tectonically imbricated segment of the passive continental margin of the jurassic tethys within one of these nappes err nappe part of an upper crustal extensional detachment is preserved indicating that the thinning of the crust was achieved by non uniform extension the mesozoic age of the detachment is shown by comparison between its associated cataclasites and identical cataclasites that are found as redeposited components in middle jurassic sedimentary breccias this low angle detachment within the basement was kinematically linked to synsedimentary high angle normal faults at the surface the area of the err nappe belongs in terms of jurassic paleogeography to the most distal part of the continental margin where both low and high angle normal faults dipped oceanward that is west to northwest in present day coordinates in the more proximal part of the margin however the high angle normal faults dipped eastward toward the continent ammonite stratigraphy within the sediment prisms adjacent to the faults gives evidence for an early jurassic age of the faulting in the proximal part of the margin whereas in the distal part faulting occurred during latest early to middle jurassic time we therefore propose that the jurassic extension of the crust which finally led to the opening of the piemont ligurian ocean was achieved by two fault systems which differ in geometry fault orientation and age these fault systems were composed of a basal low angle detachment with high angle normal faults above the orientation of the older eastward dipping detachment was prone for reactivation during early alpine crustal shortening the present day thrust contact between the two major tectonic units of the area the lower austroalpine and the central austroalpine nappe complex therefore might correspond to an eastward dipping jurassic low angle normal fault

mantle heat flow and geotherms for the main geologic domains in morocco thermal and deep seismic soundings data are used to study the dependence between the compressional pn velocity and the surface heat flow or the temperature at the moho discontinuity in morocco this correlation indicates a significant decrease in pn velocity where high heat flow and moho temperature are observed this result is consistent with respect to other regions of the world crustal heat generation models and geotherms are constructed for the major moroccan geological domains extending from the precambrian units in the south to the alpine units in the north the crustal contribution in surface heat flow is on average 35 mwm 2 with high values of 41 42 mwm 2 in the western and eastern meseta where hercynian granite intrusions could enrich the crust in radioactive heat sources high mantle heat flow values are obtained beneath the alboran neogene basin 62 mwm 2 the rif 47 mwm 2 the middle atlas 41 mwm 2 and the south atlantic margin 40 mwm 2 where the crust is thinned by an extensional tectonic regime despite their similar formation context the intra continental belts of the middle and the high atlas show different geothermal field components a lithospheric heating process in the middle atlas could be the result of a plio quaternary basaltic volcanism finally the precambrian basement of the anti atlas like all the west african shield is a stable domain showing the lowest subsurface temperatures

kinematics of large scale tip line folds from the high atlas thrust belt morocco abstract the kinematics of several tip line folds situated along the south atlas front near goulmima morocco is established using both forward modelling and analysis of syn folding deformation from the sahara foreland to the atlas the two successive major structures forming the mountain front in this area fit geometrically with a generalised fault propagation fold model and a model of fault propagation fold altered by late transport on the upper flat respectively field evidence shows that the two folds are developed more or less synchronously the analysis of minor structures helps address the question of migrating hinges vs fixed hinges during the fold s growth

crustal structure along a traverse across the middle and high atlas mountains derived from seismic refraction studies in 1986 explosion seismic investigations have been carried out along a traverse of about 350 km length running from nnw to sse crossing the high and middle atlas two further profiles run e w through the middle and nw se through the high atlas

kinematic and thermal evolution of the moroccan rifted continental margin doukkala high atlas transect 1 the atlantic passive margin of morocco developed during mesozoic times in association with the opening of the central atlantic and the alpine tethys extensional basins formed along the future continental margin and in the atlas rift system in alpine times this system was inverted to form the high and middle atlas fold and thrust belts to provide a quantitative kinematic analysis of the evolution of the rifted margin we present a crustal section crossing the atlantic margin in the region of the doukkala basin the meseta and the atlas system we construct a post rift upper crustal section compensating for tertiary to present vertical movements and horizontal deformations and we conduct numerical modeling to test quantitative relations between amounts and distribution of thinning and related vertical movements rifting along the transect began in the late triassic and ended with the appearance of oceanic crust at 175 ma subsidence possibly related to crustal thinning continued in the atlas rift in the middle jurassic the numerical models confirm that the margin experienced a polyphase rifting history the lithosphere along the transect preserved some strength throughout rifting with the effective elastic thickness corresponding to an isotherm of 450 c a mid crustal level of necking of 15 km characterized the pre rift lithosphere

primary vs secondary curved fold axes deciphering the origin of the ait attab syncline moroccan high atlas using paleomagnetic data abstract the ait attab syncline located in the central high atlas displays a curved geometry in plan view and is considered as one of the most spectacular fold shapes in the central high atlasic belt we conducted a paleomagnetic study in jurassic cretaceous red beds to investigate the origin of this geometry the natural remanent magnetization nrm is dominated by a secondary magnetization carried by haematite with unvarying normal polarity that has been dated at about 100 ma the regional fold test performed in both limbs of the syncline is positive and the paleomagnetic vectors after tectonic correction are parallel throughout the curvature indicating a negative oroclinal bending test these results are inconsistent with previous works that consider the bent geometry of this syncline to result from subsequent distortion of originally ne sw trending structures by rotation about a vertical axis we interpret the nrm data to demonstrate that the changing trend of the ait attab syncline is a primary feature resulting from the influence of pre existing ne sw and e w striking extensional faults that developed during a strike slip regime paleomagnetic results also reveal that the tilting observed in the sampled red beds is post albian probably linked to the cenozoic inversion of the high atlasic belt

development of quaternary calcrete in the tensift al haouz area central morocco characterization and environmental significance abstract this study utilizes features of calcrete to elucidate the main environmental factors that affect its formation and distribution in the tensift al haouz area central morocco one of semi arid regions where calcrete is ubiquitous and occurs in a variety of forms more than hundred calcretes profiles were examined and described by field observations selected samples were analyzed by optical microscopy om scanning electron microscopy sem x ray diffraction xrd and chemical analysis xrf two types of evolution and genesis are evidenced 1 calcrete of pedogenic origin in the central and eastern part of the studied area developed in the vadose zone with an upward gradational maturity pattern ranging from incipient nodular to concretionary and to hardpan 2 combination of groundwater and pedogenic origin in the western part the paired calcrete profiles consist from bottom to top of massive calcrete suggested to have a groundwater origin it might be formed under wet to semi arid climatic conditions and the gravel hardpan situated at the top suggested to have developed in the vadose zone by pedogenic processes under arid climate in this study we conclude that the vertical and lateral distribution of calcrete is related to differences in the topography the nature of the bedrock the texture of parent material and the climate

palaeoenvironmental and tectonic significance of miocene lacustrine and palustrine carbonates ait kandoula formation in the ouarzazate foreland basin morocco abstract the ouarzazate basin is the southern foreland basin to the high atlas mountains in morocco the sedimentary fill records a sequence dating from the eocene to pleistocene that records the interplay between tectonics and climate this study presents the first stable isotope and facies analyses of the middle to late miocene ait ibrirn lacustrine member ait kandoula formation these data test whether the basin was internally draining and enable the development of palaeoenvironmental models for the middle to late miocene five sedimentary facies of lacustrine and palustrine limestones are interbeddded with extensive sequences of palaeosols and fluvial sandstones and conglomerates often associated with evaporite gypsum development these facies can be divided into two facies associations related to water depth and sub aerial exposure within the basin in the serravalian and early tortonian shallow water successions dominate the stratigraphy typical of underfilled foreland basin settings furthermore carbonate 18o and 13c isotopes from the sections show covariance confirming that these carbonates were deposited within a hydrologically closed basin however late tortonian to messinian carbonates do not demonstrate the covariance typical of endorheic basins additionally the facies association indicates the presence of deeper water lake systems demonstrating that the basin was externally draining at this time these results question the established view of tectonic stagnation in the late miocene and suggest that the cenozoic sediments of the ouarzazate basin contain a rich and untapped record of climate change and tectonic evolution on the edge of the sahara desert

evolution of jurassic carbonate platforms in an active transtensional rift high atlas of morocco carbonate platforms developed during the early and middle jurassic on the margins of two seaways that are now exposed in the middle and high atlas morocco the seaways occupied transtensional rifts which by early jurassic had formed the segmented terrain on which the carbonate platforms established two episodes of platform development occurred in the high atlas both recording the effects of tectonic eustatic and sedimentologic controls on platform trends morphologies and internal stratigraphy lias sinemurian to domerian and lias dogger aalenian to bajocian along the borders of mesetas platform margins became established over basin bounding faults whereas platforms within the seaway developed on pre existing or active structural highs fault scarp margins olistolith and mega breccia emplacement and drowned or foundered margins record the tectonic contribution to shaping the margins platform interior stratigraphy contains third and fourth order depositional sequences that reflect multiple sea level fluctuations during a gradual sea level rise structural highs in the interior of the seaways some remaining active during the lias were capped or fringed by platform carbonates or in deeper water were covered by sponge algal buildups basin lows filled with turbidites and limestone marl cycles carbonate alluvial fans and neptunian dikes are associated with syntectonic wrenching within the seaway the liassicmore platform was inundated by a toarcian drowning event with shale deposition occurring across the southern platform within the basin and locally condensed sequences forming on intra basin highs lias dogger platforms ceased with the transition to continental sedimentation marking the end of the high atlas seaway less

inversion tectonics and the evolution of the high atlas mountains morocco based on a geological geophysical transect an edited version of this paper was published by the american geophysical union agu copyright 1998 agu see also http www agu org pubs crossref 1999 1998tc900015 shtml http atlas geo cornell edu morocco publications beauchamp1999 htm

crustal shortening and vertical strain partitioning in the middle atlas mountains of morocco the ne sw trending middle atlas mountains of morocco are obliquely oriented within the late cenozoic regional stress field resulting in deformation that is partitioned into strike slip faulting and thrust related folding in the central middle atlas thrusting is confined to a 20 km wide fold belt between two relatively rigid crustal blocks that are obliquely converging we suggest that in addition to strain partitioning observed in plan view a partitioning of deformation between the upper and lower crust may be necessary to reconcile estimated crustal thickening and horizontal shortening within the fold belt cross section balancing based on field observations demonstrates a relatively modest amount of cenozoic horizontal shortening 4 7 km normal to the fold belt producing 800 m of structural relief yet geophysical data suggest this contraction has not produced a significant crustal root beneath the fold belt that is the belt does not appear to be isostatically compensated assuming all horizontal shortening was accommodated by crustal thickening beneath the fold belt implies much greater thickening than is suggested by constraints on the preshortened crustal thickness it thus appears that thickening does not accommodate all of the contraction we suggest one possible solution the upper crust shortens by thickening faulting and folding whereas the lower crust deforms laterally

joint inversion of local regional and teleseismic data for crustal thickness in the eurasia africa plate boundary region summary a new map for the moho discontinuity eam02 in the eurasia africa plate boundary region is presented reliable results have also been obtained for the southern and eastern mediterranean basin the northern african coasts and the eastern atlantic ocean regions only occasionally considered in studies on the mediterranean region the moho topography model is derived from two independent sets of constraints information contained in the fundamental and highermode rayleigh waves obtained from waveform modelling is used to constrain the moho depth between estimates of crustal thickness taken from published reflection and refraction surveys gravity studies and receiver function analysis strong lateral variations in the moho topography have been observed in the mediterranean sea confirming the complex evolution of this plate boundary region in the west the moho discontinuity has been found at 15 20 km depth suggesting extended and at least in some locations oceanic crust while in the east the crust is on average 25 30 km thick there it is interpreted either as mesozoic oceanic or thinned precambrian continental crust covered by thick sedimentary deposits standard continental crust 30 35 km is observed along the eastern part of the northern african coast while to the west a rapid change from a relatively deep moho down to 42 km below the atlas mountain range to the thin crust of the southwestern mediterranean sea has been found the crust beneath the eastern north atlantic ocean can be up to 5 km thicker compared with standard oceanic crust 6 km the crust has been interpreted to be heterogeneous as a consequence of irregular magma supply at the mid atlantic ridge in addition serpentinization of the submoho mantle could contribute to the imaging of apparently anomalous thick oceanic crust in europe the presence of crustal roots 45 km beneath the major mountain belts has been confirmed while thin crust 25 km has been found beneath extensional basins comparing the obtained moho topography and moho depth computed assuming isostatic compensation at 60 km depth shows that most of the mediterranean and eastern atlantic region appears to be in isostatic equilibrium the large positive residuals observed for the eastern mediterranean are likely to be due to overestimating crustal thickness owing to the thick sediment deposits present

geometry and kinematics of the south atlas front algeria and tunisia abstract the south atlas front is the structural boundary between the sahara platform and the atlas mountains it is frequently indicated as a foreland dipping monocline known as the sahara flexure earlier work generally considered that such a cover structure resulted from direct inversion of basement faults this assumption is not applicable to the studied area which is situated close to the algeria tunisia border our study demonstrates a thin skinned style characterised by ramp related folds ten parallel balanced cross sections give an interpretation for the south atlas front from negrine to gafsa about 100 km along strike the geometry at depth of each cross section is defined by forward modelling of each individual anticline the depth to detachment is defined using the geometric characteristics of the fault propagation folds well exposed along the deformation front in all sections the modelling is consistent with basal decollements located within cretaceous stratigraphic levels lateral changes of the tectonic style accommodated by tear faults are observed they seem related to lateral variations in the efficiency of the based decollement faults inducing either a jump of the decollement to another level or the development of en echelon folds the modelling predicts small rotations of cover sheets separated by large scale tear faults

focal mechanisms of the atlas earthquakes and tectonic implications the determination of the focal mechanisms of the strongest shocks in the atlas chain and the canary islands area reveals that the present day tectonics along this range is a compression reflected either by reverse or dextral strike slip faulting along the ene wsw trends all the solutions are consistent with 1 the other focal mechanism solutions determined further north in the azores gibraltar alboran rif and tell areas 2 the nnw to nw trending compression inferred from other methods such as neotectonics in situ stress field measurements and plate motions

the southern moroccan passive continental margin an example of differentiated long term landscape evolution in gondwana abstract the ene trending anti atlas mountain belt reveals variation in recent topography with elevations of 3 km indicating a differentiated long term landscape evolution to unravel the subsidence and exhumation history low temperature thermochronology has been applied on 21 samples from the precambrian basement of the irherm kerdous ifni and bas dr a inlier thermochronological data suggest a common geological evolution of the western anti atlas zircon fission track ages from 287 23 to 331 24 ma point to a main exhumation in the late carboniferous early permian assigned to the variscan folding and post folding erosion zircon u th he 205 287 ma and apatite fission track 120 219 ma data are related to the central atlantic rift and drift stage as the anti atlas formed the exhumation shoulder of the rift system the rock uplift and exhumation phase lasted up to the early cretaceous whereby 9 km of precambrian palaeozoic overburden has been eroded thermal modelling reveals a minor sedimentary burial 1 5 2 5 km throughout the cretaceous in accordance with the widespread transgression across the northern africa in the cenomanian turonian apatite u th sm he ages between 49 3 and 164 10 ma confirm recent minor exhumation in the western anti atlas with an increasing impact of the atlasian orogeny and potentially a mantle plume anomaly from southwest to northeast reflecting the differentiated topography in the area

electrical resistivity structure of the eastern moroccan atlas system and its tectonic implications the electrical resistivity structure of the crust and upper mantle of the atlas mountain system was studied using magnetotelluric and geomagnetic deep soundings field experiments were done in eastern morocco along a traverse from the anti atlas to the rides rifaines in two campaigns in 1983 and 1988

from central atlantic continental rift to neogene uplift western anti atlas morocco terra nova 23 35 41 2011 abstract to put constraints on the mesozoic to recent growth of the anti atlas system we investigated the temperature time history of rocks by applying extensive low temperature thermochronological analysis to three precambrian inliers along the coast and 250 km into the interior bedrocks yield old u th he ages on zircon 248 193 ma and apatite 150 50 ma and also fission track ages of 173 121 ma on apatite these datasets are interpreted as recording passive margin upward movements from central atlantic rifting until the early cretaceous a phase of sedimentary burial was evidenced for the cretaceous eocene the extension of this thin 1 5 km basin is loosely constrained but can be extended to the western regions of northern africa effects of the existing thermal perturbation of lithospheric origin 100 km below the atlas show that the 120 60 c isotherms are not much deflected large scale uplift has possibly occurred in the western anti atlas since c 30 ma and is associated with a mean denudation rate of 0 08 km ma 1

simulation of vatnajokull ice cap dynamics 1 we apply a coupled model of ice sheet dynamics and subglacial hydrology to investigate the dynamics and future evolution of the vatnajokull ice cap iceland in this paper we describe a new theoretical approach to introducing longitudinal stress coupling in the ice dynamics solution and we analyze our ability to simulate the main features of vatnajokull with and without longitudinal stress effects equilibrium ice cap configurations exist for vatnajokull but under a narrow range of climatic boundary conditions equilibrium reconstructions have an average ice thickness greater than what is observed at vatnajokull consistent with our inability to capture surge dynamics in vatnajokull s outlet glaciers hydrological regulation of basal flow longitudinal stress coupling and a simple parameterization of the subglacial heat flux from vatnajokull s geothermal cauldrons all help to reduce average ice thickness in the equilibrium reconstructions but cases that reproduce the present day ice volume have an ice cap area that is 5 10 less than the actual ice cap present day reconstructions that adopt a realistic climate spin up for the period 1600 1990 provide improved fits to the modern day ice cap geometry this indicates that climatic disequilibrium also plays a significant role in dictating vatnajokull s morphology simulations for the period 1600 2300 illustrate that air temperature is the dominant control on vatnajokull s volume and area longitudinal stress coupling and hydrological coupling both increase vatnajokull s sensitivity to future warming

post variscan evolution of the anti atlas belt of morocco constrained from low temperature geochronology the anti atlas belt of morocco extends ene wsw over more than 600 km from the atlantic margin in the west to the interior of the african plate in the east it exhibits precambrian rocks outcropping as basement inliers and surrounded by marine ediacaran cambrian sequences around the axis of the mountain range the belt which has for a long time been interpreted as of variscan age is now revealed to have experienced major vertical movements through mesozoic and cenozoic times thereby the anti atlas domain appears to be affected by two episodes of exhumation separated by an episode of subsidence the initial episode occurred in the late triassic and led to the exhumation of 7 5 10 5 km of crustal rocks by the end of the middle jurassic ca 160 150 ma the following phase resulted in 1 3 km of basement subsidence and occurred during the late jurassic and most of the early cretaceous the basement rocks were then slowly brought to the surface after experiencing 2 3 5 km of exhumation throughout the late cretaceous and the cenozoic the timing of these episodes of exhumation and subsidence coincides with major tectonic and thermal events in relation with the evolution of the atlantic and tethys oceans indicating that the effects of their rifting and drifting extended beyond their presumed margins

basin inversion in tectonic wedges insights from analogue modelling and the alpine carpathian fold and thrust belt abstract this work simulates the inversion and incorporation of sub thrust segmented half graben basins into tectonic wedges by means of sandbox analogue models segmented half graben basins striking at 90 45 and 15 to the extension direction were created first and then shortened using different angles for the basal detachment and topographic slope a shallow viscous polymer layer located above the half graben basin was included in one of the models the experiments were analysed using time lapse photography topography laser scans and image based 3d voxels the results indicate a deformation sequence characterised by layer parallel compaction fault reactivation thrust propagation and related folding fault reactivation was associated with the layer parallel compaction accomplished by slip along the basal detachment prior to and in between pulses of thrusting results reveal the fundamental control imposed by the vertical load of the tectonic wedge and its integrated strength profile in the inversion of sub thrust basins results are compared to the alpine carpathian fold and thrust belt

widespread cretaceous secondary magnetization in the high atlas morocco a common origin for the cretaceous remagnetizations in the western tethys in this paper we present the first palaeomagnetic data 51 sites for mesozoic lower middle jurassic sediments of the moroccan central high atlas and address the study of a widespread remagnetization the remagnetization is characterized by a very stable component with systematic normal polarity carried by magnetite the relationship between the magnetic properties and location within the basin suggests that the acquisition of the remagnetization is controlled by basin geometry fold tests indicate that the overprint acquisition is syn folding in some structures but clearly predates the tertiary compressional stage using the small circle intersection method we have calculated the remagnetization direction d 336 4 i 29 2 comparison with the global apparent polar wander path indicates that the remagnetization was acquired during the late cretaceous probably cenomanian considering both the basinal confinement of remagnetization and the connection with other remagnetization events in the western tethys we propose a scenario explaining widespread remagnetizations in the region concerning both basin scale conditions mainly related to sediment thickness and a regional scale thermal event acting as catalyst of remagnetizations in those sedimentary basins that satisfy the basin scale conditions

structural development of the taza guercif basin as a constraint for the middle atlas shear zone tectonic evolution abstract the neogene taza guercif basin in north eastern morocco is located in correspondence of the eastern termination of the rifian arc and is aligned along with the middle atlas shear zone since tortonian the tectonic evolution of the taza guercif basin was tied to repeated advancements of the rifian front and to the transpressive tectonics of the middle atlas shear zone the major structures in this basin consist of thrust related anticlines aligned along with the middle atlas structures and second order normal faults elongated according to the rifian trend the latter are well developed mainly in the north western sector of the basin bab stout area apart from the bab stout area the basin is dominated by compressive and transpressive structures guercif transpressive area the shortening directions responsible for the development of the major and minor compressive structures in the basin vary from nnw to nne but no superposition relationships have been found this change of the paleo stress field orientations is here ascribed to a progressive deformation under the constant push between africa and europe an evolutionary model for the development of these two structural areas in relation to the tectonics of the rif and of the middle atlas is here proposed in this model the sinistral transpressive middle atlas shear zone is regarded as the boundary between the moroccan meseta microplate to the west and the african indenter to the east which is moving faster to the north

glacio isostatic deformation around the vatnajokull ice cap iceland induced by recent climate warming gps observations and finite element modeling 1 glaciers in iceland began retreating around 1890 and since then the vatnajokull ice cap has lost over 400 km 3 of ice the associated unloading of the crust induces a glacio isostatic respo

crustal structure beneath broad band seismic stations in the mediterranean region summary we have analysed receiver functions to derive simple models for crustal structure below 12 broad band seismological stations from the midsea project and 5 permanent broad band stations in the mediterranean region including northern africa to determine an accurate moho depth we have reduced the trade off between crustal velocities and discontinuity depth using a new grid search method which is an extension of recently published methods to determine crustal thickness in this method the best fitting synthetic receiver function containing both the direct conversion and the reverberated phases is identified on a model grid of varying moho depth and varying poisson s ratio the values we found for moho depth range from around 20 km for intra oceanic islands and extended continental margins to near 45 km in regions where the eurasian and african continents have collided more detailed waveform modelling shows that all receiver functions can be well fit using a 2 or 3 layer model containing a sedimentary layer and or a mid crustal discontinuity on comparing our results with moho maps inferred from interpolated reflection and refraction data we find that for some regions the agreement between our receiver function analysis and existing moho maps is very good while for other regions our observations deviate from the interpolated map values and extend beyond the geographic bounds of these maps

post glacial rebound and asthenosphere viscosity in iceland during the weichselian glaciation iceland was covered with an ice cap which caused downward flexure of the earth s surface the post glacial rebound in iceland was very rapid being completed in about 1 000 years the length of this time interval constrains the maximum value of asthenosphere viscosity in iceland to be 1 times 10 sup 19 pa s or less further clarification of the ice retreat and uplift history may reveal lower viscosity current changes in the mass balance of icelandic glaciers must lead to measurable elevation changes considering this low viscosity expected current elevation changes around the vatnajoekull ice cap are of the order of 1 cm per year due to mass balance change in this century

cenozoic sedimentary dynamics of the ouarzazate foreland basin central high atlas mountains morocco cenozoic continental sedimentary deposits of the southern atlas named imerhane group crop out a in the ouarzazate foreland basin between the precambrian basement of the anti atlas and the uplifted limestone dominated high atlas and b in the ait kandoula and ait seddrat nappes where jurassic strata detached from the basement have been thrust southwards over the ouarzazate basin new biostratigraphic and geochronological data constraining the final eocene marine regression the characterization of the new ait ouglif detrital formation presumed to be of oligocene age and the new stratigraphic division proposed for the continental imerhane group clarify the major tectonogenetic alpidic movements of the central high atlas range four continental formations are identified at regional scale their emplacement was governed principally by tectonic but also by eustatic controls the hadida and ait arbi formations upper eocene record the major paleogene regression they are composed of margino littoral facies coastal sabkhas and fluviatile systems and reflect incipient erosion of the underlying strata and renewed fluvial drainage the ait ouglif formation presumed oligocene had not been characterized before it frequently overlies all earlier formations with an angular unconformity it includes siliciclastic alluvial deposits and is composed predominantly of numerous thin fining upward cycles the ait kandoula formation miocene pliocene is discordant extensive and represents a thick coarsening upward megasequence it is composed of palustro lacustrine deposits in a context of alluvial plain with localized sabkhas giving way to alluvial fans and fluviatile environments the upper conglomeratic formation quaternary is the trace of a vast conglomeratic pediment forming an alluvial plain and terraces the second and third formations correspond to two megasequences engendered by the uplift of the central high atlas in two major compressive phases during late oligocene and miocene pliocene times these two geodynamic events were separated by a tectonically calm phase materialized by palustro lacustrine sedimentation gorler et al 1988 tectono sedimentary analysis of the two megasequences shows that the basin structure and depositional processes were controlled by the compressive tectonic context generated by the collision of north africa and iberia in tertiary times jacobshagen et al 1988 the quaternary formation was apparently controlled by a tectonic continuum and by climatic variations