le bassin de tizi n test haut atlas maroc exemple d evolution d un segment oblique au rift de l atlantique central au trias detailed mapping completed by a microtectonic study of the tizi n test triassic basin located along the tizi n test fault zone in the moroccan high atlas has allowed us to improve the knowledge on the geometry of the structures and the activity of faults during the triassic extensional events related to the rifting of the central atlantic the latter are reflected by the development of a rift at present inverted and deformed by the collision of africa and europe comprising kilometric scale grabens and half grabens bounded by major faults trending ene wsw with a dip towards the nnw and a dip slip syndepositional motion inverse analysis of fault slickenside populations shows a heterogeneous triassic state of stress however in the most significant measurement sites the maximum horizontal stress 1 is vertical while the minimum stress 3 is horizontal with a nw se trend the strike slip component appears to be very small during the triassic a noticeable fact because of the obliquity of the basin w

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

subduction kinematics and dynamic constraints the kinematics of subduction zones shows a variety of settings that can provide clues for dynamic understandings two reference frames are used here to describe the simple 2d kinematics of subduction zones in the first the upper plate is assumed fixed whereas in the second frame upper and lower plates move relative to the mantle relative to a fixed point in the upper plate u the transient subduction hinge h can converge diverge or be stationary similarly the lower plate l can converge diverge or be stationary the subduction rate vs is given by the velocity of the hinge h minus the velocity of the lower plate l vs vh vl the subduction rate 1 increases when h diverges and 2 decreases when h converges combining the different movements at least 14 kinematic settings can be distinguished along the subduction zones variable settings can coexist even along a single subduction zone as shown for the 5 different cases occurring along the apennines subduction zone apart from few exceptions the subduction hinge converges toward the upper plate more frequently along e or ne directed subduction zone whereas it mainly diverges from the upper plate along w directed subduction zones accompanying backarc extension before collision orogen growth occurs mostly at the expenses of the upper plate shortening along e ne directed subduction zones whereas the accretionary prism of w directed subduction zones increases at the expenses of the shallow layers of the lower plate the convergence shortening ratio is 1 along e or ne directed subduction zones whereas it is 1 along accretionary prisms of w directed subduction zones backarc spreading forms in two settings along the w directed subduction zones it is determined by the hinge divergence relative to the upper plate minus the volume of the accretionary prism or in case of scarce or no accretion minus the volume of the asthenospheric intrusion at the subduction hinge since the volume of the accretionary prism is proportional to the depth of the decollement plane the backarc rifting is inversely proportional to the depth of the decollement on the other hand along e or ne directed subduction zones few backarc basins form e g aegean andaman and can be explained by the velocity gradient within the hangingwall lithosphere separated into two plates when referring to the mantle the kinematics of subduction zones can be computed either in the deep or in the shallow hotspot reference frames the subduction hinge is mostly stationary being the slab anchored to the mantle along w directed subduction zones whereas it moves w or sw ward along e or ne directed subduction zones surprisingly along e or ne directed subduction zones the slab moves out of the mantle i e the slab slips relative to the mantle opposite to the subduction direction kinematically this subduction occurs because the upper plate overrides the lower plate pushing it down into the mantle as an example the hellenic slab moves out relative to the mantle i e sw ward opposite to its subduction direction both in the deep and shallow hotspot reference frames in the shallow hotspot reference frame upper and lower plates move westward relative to the mantle along all subduction zones this kinematic observation casts serious doubts on the slab negative buoyancy as the primary driving mechanism of subduction and plate motions w directed subduction zones rather provide about 2 3 times larger volumes of lithosphere re entering into the mantle and the slab is pushed down this opposite behavior is consistent with the down dip extension seismicity along e ne directed subduction zones and the frequent down dip compression along the w directed subduction zones subduction kinematics shows that plate velocity is not dictated by the rate of subduction along the w directed subduction zones the rate of subduction is rather controlled i by the hinge migration due to the slab interaction with the easterly trending horizontal mantle wind along the global tectonic mainstream ii by the far field plate velocities and iii by the value of negative buoyancy of the slab relative to the country mantle alternatively e ne nne directed subduction zones have rates of sinking chiefly determined i by the far field velocity of plates and ii by the value of negative buoyancy of the slab relative to the country mantle along this type of subduction the subduction hinge generally advances e ne ward toward the upper plate decreasing the subduction rate but it moves w sw ward relative to the mantle all this indicates that subduction zones have different origin as a function of their geographic polarity and the subduction process is more a passive feature rather than being the driving mechanism of plate motions a rotational component combined with mantle density and viscosity anisotropies seems more plausible for generating the global tuning in the asymmetry of subduction zones

triassic early liassic extensional systems and their tertiary inversion essaouira basin morocco abstract nne ssw striking late triassic early liassic half grabens of the essaouira basin and the adjacent high atlas are linked by roughly ew striking transfer faults they are overlain by a wide considerably less faulted evaporitic sag basin and extensive basalt flows a pre pliensbachian unconformity truncates the triassic early liassic sequences on the e and ne margin of the basin and is overlain by a mid liassic to cretaceous platform sequence in the cap sim necnafa depression jurassic and early cretaceous salt tectonics caused widespread salt withdrawal structures tertiary nnw to sse compression caused the inversion of ne and nne striking structures and or the formation of salt anticlines the same compression formed ew striking reverse faults and anticlines suggesting the reactivation of late triassic early liassic transfer faults

kinematic significance of sediment filled fissures in the north mountain basalt fundy rift basin nova scotia canada abstract numerous sediment filled fissures are present near the upper surface of the north mountain basalt 202 ma of the fundy basin which formed during triassic jurassic rifting the fissures are infilled with the same clastic material comprising the basal parts of the overlying earliest jurassic age sedimentary formations which accumulated less than 200 k y after the cooling of the basalt nearly all of the 1368 fissures measured in this study are planar to sub planar and are sub vertical after correcting the basalt flows to paleo horizontal the fissures typically formed along preexisting columnar joints within the basalt northeast striking fissures generally define a preferred orientation and commonly are wider than other fissures the sediment filled fissures therefore indicate earliest jurassic age nw se extension this extension direction is consistent with that indicated by the ne striking shelburne diabase dike the attitude of mesoscopic faults and the geometry of the fundy rift basin thus sediment filled fissures by themselves can serve as useful kinematic indicators and place tight constraints on the relative timing of extension

muddy and dolomitic rip up clasts in triassic fluvial sandstones origin and impact on potential reservoir properties argana basin morocco abstract the significance of rip up clasts as sandstone framework grains is frequently neglected in the literature being considered as accessory components in bulk sandstone composition however this study highlights the great value of muddy and dolomitic rip up clast occurrence as a information source about low preservation potential from floodplain deposits and b key element controlling host sandstone diagenetic evolution and thus ultimate reservoir quality high resolution petrographic analysis on triassic fluvial sandstones from argana basin t6 and t7 t8 units highlights the significance of different types of rip up clasts as intrabasinal framework components of continental sediments from arid climates on the basis of their composition and ductility three main types are distinguished a muddy rip up clasts b dolomitic muddy rip up clasts and c dolomite crystalline rip up clasts spatial distribution of different types is strongly facies related according to grain size origin of rip up clasts is related to erosion of coeval phreatic dolocretes in different development stages and associated muddy floodplain sediments cloudy cores with abundant inclusions and clear outer rims of dolomite crystals suggest a first replacive and a subsequent displacive growth respectively dolomite crystals are almost stoichiometric this composition is very similar to that of early sandstone dolomite cement supporting phreatic dolocretes as dolomite origin in both situations sandstone diagenesis is dominated by mechanical compaction and dolomite cementation a direct correlation exists between 1 muddy rip up clast abundance and early reduction of primary porosity by compaction with irreversible loss of intergranular volume igv and 2 occurrence of dolomitic rip up clasts and dolomite cement nucleation in host sandstone occluding adjacent pores but preserving igv both processes affect reservoir quality by generation of vertical and 3d fluid flow baffles and barriers that compartmentalize the reservoir these findings may provide quantitative useful data for the better understanding of reservoir quality in analogous hydrocarbon bearing basins such as the bay of fundy nova scotia canada

finite frequency rayleigh wave tomography of the western mediterranean mapping its lithospheric structure 1 we present a 3 d shear wave velocity model for the crust and upper mantle of the western mediterranean from rayleigh wave tomography we analyzed the fundamental mode in the 20 167 s period band 6 0 50 0 mhz from earthquakes recorded by a number of temporary and permanent seismograph arrays using the two plane wave method we obtained phase velocity dispersion curves that were inverted for an isotropic vs model that extends from the southern iberian massif across the gibraltar arc and the atlas mountains to the saharan craton the area of the western mediterranean that we have studied has been the site of complex subduction slab rollback and simultaneous compression and extension during african european convergence since the oligocene the shear velocity model shows high velocities beneath the rif from 65 km depth and beneath the granada basin from 70 km depth that extend beneath the alboran domain to more than 250 km depth which we interpret as a near vertical slab dangling from beneath the western alboran sea the slab appears to be attached to the crust beneath the rif and possibly beneath the granada basin and sierra nevada where low shear velocities 3 8 km s are mapped to 55 km depth the attached slab is pulling down the gibraltar arc crust thickening it and removing the continental margin lithospheric mantle beneath both iberia and morocco as it descends into the deeper mantle thin lithosphere is indicated by very low upper mantle velocities beneath the alboran sea above and east of the dangling slab and beneath the cenozoic volcanics

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

diachronous rifting drifting and inversion on the passive margin of central eastern north america an analog for other passive margins integration of new data with existing information indicates that the tectonic development of the passive margin of eastern north america between the carolina trough and scotian basin was considerably more complex than the classic two stage rift drift model first the transition from rifting to drifting was diachronous in the southeastern united states the rift drift transition occurred after the late triassic synrift deposition and before eastern north america magmatism in the earliest jurassic 200 ma in maritime canada the rift drift transition occurred after eastern north america magmatic activity and synrift deposition in the early jurassic and before postrift deposition in the early middle jurassic 185 ma second the deformational regime changed substantially after rifting on both the southern and northern segments of the margin generally northwest southeast postrift shortening replaced northwest southeast synrift extension northeast striking reverse faults formed and many of the rift basin boundary faults had reverse displacements in the southeastern united states the change in the deformational regime occurred in the late triassic early jurassic during the rift drift transition simultaneously diabase sills and dikes many striking nearly perpendicular to the trend of the rift basins intruded the continental crust and a massive wedge of volcanic or volcaniclastic rocks developed near the continent ocean boundary in maritime canada the change in the deformational regime occurred during or after the early jurassic and before or during the early cretaceous that is during the rift drift transition or early stages of sea floor spreading

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

triassic liassic western moroccan synrift basins in relation to the central atlantic opening abstract the upper triassic lower liassic sequence of the atlantic margin of morocco is primarily composed of fine grained detrital sedimentary rocks containing basaltic intercalations its age is carnian to hettangian well data outcrop observations and seismic sequence analysis suggest that rifting started earlier carnian in the eastern and southern parts of the margin and later proceeded towards the west and the north resulting in the formation of the central atlantic rift on the moroccan margin the rift was composed of i eastern basins relatively narrow and bounded by e dipping faults which are reactivated hercynian structures ii a central horst and iii western basins bounded by w dipping faults the 020 trending basins are separated by w dipping faults the 020 trending basins are separated by 070 striking transfer faults estimations of the amount of lithospheric extension along the moroccan external rift system show a thinning ratio decreasing northward and probably associated to a necking of the lithosphere rift evolution on the part of the moroccan margin shows good correlation to the conjugate north american margin

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

tilted blocks pattern paleostress orientation and amount of extension related to triassic early rifting of the central atlantic in the amzri area argana basin morocco abstract in the amzri area argana basin morocco the tilted blocks pattern within triassic redbeds appears to be related to a nw se extension during the early rifting of the central atlantic the amount of extension measured on these structures reaches 15

cyclicity of triassic to lower jurassic continental red beds of the argana valley morocco implications for palaeoclimate and basin evolution abstract cyclical playa deposits form a prominent part of the continental clastic succession of the argana valley western high atlas of morocco the red beds formed in triassic to lower jurassic times during rifting of the north american and african plates detailed stratigraphic work revealed asymmetrical metre scale cycles in mudstone dominated successions that constitute the intermediate and upper portion of the basin fill sedimentary cycles commonly comprise ephemeral lake shales at the base playa mudflat mudstones in the intermediate part and both fluvial and aeolian sandstones at the top cycles of the aglegal member t4 are mainly characterized by analcime rich playa mudflat deposits displaying features indicative of palaeo vertisols cycles of the sidi mansour t7 and hasseine t8 members are represented by ephemeral lake or dry playa mudflat deposits grading into saline mudflat mudstones that are overlain by sheet flow and minor aeolian sandstones sedimentary cycles of the imerhrane member t9 comprise sheet delta sandstones at the base overlain by ephemeral lake and dry playa mudflat mudstones upper cycle portions consist of aeolian dune and sand sheet sandstones cyclicity is attributed to palaeoclimatic and associated palaeohydrological fluctuations probably within the milankovitch frequency band sedimentary facies of each cycle provide evidence for a gradual climatic drying with time the asymmetrical drying upward cycles possibly record the precession cycle since they are modulated by symmetrical i e oscillating drying wetting cycles of a lower frequency indicative of eccentricity a long term change in palaeoclimate ranging from semi arid conditions with seasonal precipitation t4 towards an arid non seasonal climate t8 is preserved within the cyclical units spanning a time interval of several million years individual cycles can be traced laterally for tens of kilometres correlative sections are almost identical in thickness suggesting a phase of tectonic quiescence during their deposition sediment accumulation was not restricted to small scale grabens but took place continuously throughout the exposed area of the argana valley implying that accommodation space has not been controlled by syndepositional tectonism

transtensional arm of the early mesozoic fundy rift basin penecontemporaneous faulting and sedimentation formed as a major right lateral fault zone during paleozoic collisional orogenies and reactivated as a left oblique system during the early mesozoic the east striking minas fault zone of atlantic canada controlled adjacent sedimentation in the fundy rift basin producing a series of synsedimentary microbasins northeast striking boundary faults of the fundy basin underwent mostly early mesozoic normal slip and are reactivated paleozoic thrusts the adjacent basin has a much thicker section transverse folds and synthetic rider blocks contrasts in structural and stratigraphic styles are a response to local deformation controlled by reactivated fault zones of differing orientation under consistent northwest southeast early mesozoic extension rather than responses to a sequence of changing stress patterns

post gondwana geomorphic evolution of southwestern africa implications for the controls on landscape development from observations and numerical experiments the relationship between morphology and surficial geology is used to quantify the denudation that has occurred across southwestern africa since the fragmentation of gondwana during the early mesozoic two main points emerge significant denudation of the order of kilometers is widespread except in the kalahari region of the continental interior the denudation is systematically distributed so that the continental exterior catchment draining directly to the cape basin is denuded to a greater depth than the interior catchment inland of the great escarpment the analysis also implies that the majority of the denudation occurred before the beginning of the cenozoic for both the exterior and interior catchments existing models of landscape development are reviewed and implications of the denudation chronology are incorporated into a revised conceptual model this revision implies that the primary effect of rifting on the subsequent landscape evolution is that it generates two distinct drainage regimes a marginal upwarp or rift flank uplift separates rejuvenated rivers that drain into the subsiding rift from rivers in the continental interior that are deflected but not rejuvenated the two catchments evolve independently unless they are integrated by breaching of the marginal upwarp if this occurs the exterior baselevel is communicated to the interior catchment that is denuded accordingly denudation rates generally decrease as the margin evolves and this decrease is reinforced by the exposure of substrate that is resistant to denudation and or a change to a more arid climate the observations do not reveal a particular style of smaller scale landscape evolution such as escarpment retreat that is responsible for the differential denudation across the region it is proposed that numerical model experiments which reflect the observational insights at the large scale may identify the smaller scale controls on escarpment development if the model and natural systems are analogous four numerical experiments are presented in which the roles of antecedent topography resistant substrate climate change and lowering the baselevel of the interior catchment are investigated for an initially high elevation margin bordered by an escarpment the model results suggest several styles of landscape evolution that are compatible with the observations escarpments may retreat in a regular manner but they also degrade and are destroyed only to reform at the drainage divide between exterior and interior catchments

oblique extension in the jurassic trough of the central and eastern high atlas morocco a field structural analysis has been carried out in the high atlas mountains of morocco paleostress solutions provide information on the successive orientations of major stress patterns during mesozoic and cenozoic time an initial rift stage in the early jurassic is characterized by normal oblique faults bounding tilted blocks which are associated with tensional paleostress patterns whose minimum component trends west northwest east southeast faults parallel to this direction are interpreted as paleotransfer faults we assume that the divergent motion responsible for the opening of the rift system in the early jurassic was oriented west northwest east southeast subparallel to the paleotransfer faults and the trend of the minimum component a of the paleostress field the east west trending jurassic central eastem high atlas rift opened obliquely and not in a pure strike slip stress regime along east west striking faults as previously proposed a later stage of rifting middle jurassic is characterized by large normal faults and is supposed to accompany local movements probably due to gravity the uplift of the high atlas belt occurred mainly during the cenozoic period as a consequence of a north south to northwest southeast directed compression related to collision between europe and africa rcsumc une analyse structurale sur le terrain a kt6 effectuk dans les montagnes du haut atlas marocain les solutions du problbme des palkocontraintes renseignent sur les orientations successives des principaux modes de distribution des contraintes durant les pkriodes du mksozoique et du ctnozoique un stade initial de rift est apparu au jurassique prkcoce il est caractkrisk par des failles normales obliques dtveloppces en bordure de blocs basculcs elles sont associces au rkgirne des palhontraintes extensives avec une composante minimale orientce ouest nord ouest est sud est les failles paralltles i cette direction sont interprktks come des failles de palcotransfert nous supposons que le mouvement de divergence responsable de l ouverture du systkme de rift au jurassique prccoce ctait orient6 ouest nord ouest est sud est subparalltle aux failles de palkotransfert et a la direction de la composante minimale a3 du champ de pal6ocontraintes le rift jurassique de direction est ouest de la rcgion centrale orientale du haut atlas fut overt obliquement contrairement l hypothtse proposk antkrieurement d un regime coulissant de contrainte pure agissant le long des failles orientkes est ouest le stade tardif de distension jurassique moyen est caractcrisk par de grandes failles normales on assume qu il chit accompagnk de mouvements locaux probablement d origine gravitaire le soultvement de la ceinture du haut atlas a kt6 rkalisk principalement durant la pcriode du ccnozoique comme la cons6quence de la compression orientke nord sud 9 nord ouest sud est associce b la collision entre 1 europe et 1 afrique traduit par la rkdacti

tectonic evolution of the fundy rift basin canada evidence of extension and shortening during passive margin development seismic and field data show that the fundy rift basin of southeastern canada experienced two distinct episodes of deformation during mesozoic time the first episode during middle triassic to early jurassic time was extensional rifting associated with nw se extension reactivated ne trending paleozoic compressional structures as normal faults forming the northwestern boundary faults of the fundy basin displacements on the low angle boundary faults locally exceeded 10 km rifting also reactivated east trending paleozoic compressional structures as oblique slip faults with normal and sinistral strike slip components forming the northern boundary faults of the fundy basin several kilometers of sediments and lava flows filled the basin during rifting the second deformational episode occurred during or after early jurassic time and probably before or during early cretaceous time inversion associated with nw se shortening occurred along all faulted margins of the fundy basin the northwestern boundary faults experienced several kilometers of reverse displacement broad anticlines developed within their hanging walls and the fundy basin acquired its synclinal form the northern boundary faults of the fundy basin became oblique slip faults with reverse and dextral strike slip components gentle synclines tight anticlines and faults with reverse separation deformed the synrift strata near the northern margin of the fundy basin neither collision nor subduction zones existed near the fundy basin during mesozoic time hence we believe that tectonic processes associated with seafloor spreading e g incipient ridge push forces continental resistance to plate motion produced the shortening in the fundy basin shortening occurred during the transition from rifting to drifting as north america separated from northern africa and or during the early stages of drifting as the seafloor spreading centers of the north atlantic propagated northward

a record of appalachian denudation in postrift mesozoic and cenozoic sedimentary deposits of the u s middle atlantic continental margin abstract the complex interplay between source terrain uplift basin subsidence paleoclimatic shifts and sea level change left an extensive sedimentary record in the contiguous offshore basins of the u s middle atlantic margin salisbury embayment baltimore canyon trough and hatteras basin isopach maps of 23 postrift lower jurassic to quaternary a allostratigraphic units coupled with a revised stratigraphic framework reveal that tectonism by regulating sediment supply accumulation rate dominated the interplay of forcing mechanisms tectonic pulses are evidenced by abruptly accelerated sediment accumulation marked latitudinal shifts in the location of depocenters and regional changes in lithofacies relatively rapid tectonic subsidence during the early and middle jurassic history of the basins may have enhanced sediment accumulation rates beginning in the late jurassic however subsidence rates decreased significantly though occasional short pulses of subsidence may have effected relative sea level rises sea level change heavily influenced the distribution and redistribution of sediments one they reached the basins and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins we conclude that source terrains of the central appalachian highlands were tectonically uplifted intensely weathered and rapidly eroded three times since the late triassic 1 early to middle jurassic aalenian to callovian 2 mid early cretaceous barremian and 3 late cenozoic middle miocene intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces geomorphic features commonly reported to characterize the central appalachian highlands this series of three irregularly spaced tectonic quiescent cycles does not however match the traditional four cycle concept of post triassic appalachian peneplanation

the characteristics of carbonate system recovery during a relatively dry event in a mixed carbonate siliciclastic environment in the pelsonian middle triassic proximal marginal marine basins a case study from the tropical tethyan northwest gondwana margins abstract eight early mid pelsonian middle triassic marine sedimentary successions an outcrop and subsurface sections of the ra af formation from the northwest gondwana margins facing the southeast corner of the tropical western tethyan seaway assigned to the southern intertropical convergence zone itcz south israel levant basin north arabian plate are the focus of this work the successions especially with the exclusive outcrop at har arief are a golden opportunity to observe the still connected transitional zone between the huge siliciclastic domains derived from gondwana hinterland parts north arabian plate and the open tropical tethyan marine carbonate domain and their mutual relationships this multidiscipline research tracks a carbonate recovery attempt in a dominant siliciclastic system the recovery effort was characterized by starved sedimentary conditions basin constrained oxygen stressed environments and low diverse autochthonous burrowed sediments barren of other faunal remains low diverse fossiliferous allochthonous beds transported from higher sites on the slopes revealed differential distributions of echinoids mollusks and especially foraminifera large pilammina pilamminella versus small glomospirids these variances were attributed to different environmental conditions on the slopes of each basin type and illuminate some constraints on their distribution pattern and climate affinity in the triassic southern itcz mixed carbonate siliciclastic sedimentary system the carbonate recovery phase is ascribed to a relatively dry period in the hinterland rather than to sea level rise this dry period which interrupted the relatively humid climate in the northern tropical gondwana during the pelsonian is represented by the constant although with some perturbations siliciclastic sedimentary system that prevailed in the southeastern corner of the tethys seaway during the anisian insights from the symmetric and asymmetric correlation of climate fluctuations in the hinterlands of the southeastern european basins northern hemisphere and this work from the southern hemisphere both embracing basins ascribed to the western tropical tethyan seaway illuminate the possible effects of the ictz behavior pattern and possible extra tropical forces on the climate variations

subsidence history in basins of northern algeria abstract the tellian foreland in algeria represents a part of the southern tethyan margin during the mesozoic its tectonic evolution includes a rifting stage during the triassic and liassic times characterised by tilted blocks and early diapiric events during the liassic a post rift regime from middle jurassic up to the late cretaceous and basin inversion during the tertiary related to the african and european plates convergence the subsidence modelling supported by surface and subsurface data integration emphasises different subsidence phases 1 liassic subsidence phase under tethyan and atlantic control related to crustal thinning 2 late jurassic cretaceous and tertiary subsidence phases where the first one is linked to diapiric events and the second in eastern algeria to the effects of the rifting event developed more easterly in the gulf of gabes and sirte the last one is flexural and occurs in foreland basins during the tertiary taking into account the subsidence record of each structural domain a correlation between tectonic events and the subsidence phases can be established

alteration in the irdp drill hole compared with other drill holes in iceland the overall alteration pattern in the drill hole at reydarfjordur is very similar to alteration patterns observed in icelandic geothermal areas and in low grade metamorphosed basalts in deep crustal sections elsewhere in iceland however more detail is obtained by the study of the irdp drill core than by study of drill cuttings sampled in previous drill holes in iceland a comparatively high fossil thermal gradient is obtained at reydarfjordur by a combination of mineral stability data and the observed occurence of secondary minerals this high gradient is consistent with the measured dike dilation at the drill site and the location of the drill site adjacent to a central volcano

benue trough and benue chain it is suggested that the benue trough may have been initiated by the extension of the equatorial oceanic fracture zones onto the continent during the early stages of the separation of africa and south america a transcurrent movement along deep seated faults of the basement is responsible for the structure of the trough after a period of extension characterized by the sedimentary infilling of the trough and igneous activity the trough is turned into a chain through a short phase of compression a discrete pattern of en echelon folds and the strain pattern deduced from the analysis of large scale and microfracturing correspond to a n s to n160 e direction of compression coupled to a sinistral transcurrent movement along a ne sw direction the difference in age of the folding phase in the northern and southern parts of the chain indicates a diachronous nature for this tectonic event in relation to the wrench regime the tertiary evolution of the trough marked by a tensional regime resulting in an e w extension is to be related to the cameroon volcanic line

denudation rates for the southern drakensberg escarpment se africa derived from in situ produced cosmogenic 36c1 initial results cosmogenic 36 c1 concentrations in basalt samples from the drakensberg escarpment on the se african passive margin give quantitative estimates of denudation and scarp retreat rates over the 10 4 10 6 year timespan addressed by these data the calculated escarpment retreat rate has been 50 95 m ma 1 and the average summit denudation rate 6 m ma 1 the scarp retreat rate is an order of magnitude less than previously suggested the rate of summit lowering is sufficient to prevent the long term intact survival of erosion cycle surfaces formed in the mesozoic that were previously inferred for this region

synsedimentary faulting and palaeocurrent patterns in the triassic sandstones of the high atlas morocco the influence of syndepositional fault patterns on palaeocurrents is demonstrated in fluvial to shallow marine sandstones of upper triassic basins in the high atlas the synsedimentary nature of faults is deduced from hydroplastic slickensides thickness variations due to block tilting and dislocation of layers next to the fault scarp on a regional scale it is shown that the major normal fault trend of n050 070 controlled the overall palaeocurrent pattern which was directed towards the west southwest i e in the direction of the future atlantic ocean some anomalies in the palaeocurrent pattern could be related to an increase in subsidence which induced a general coarsening of sediment towards the top of the triassic

thermal evolution of rifted continental margins new evidence from fission tracks in basement apatites from southeastern australia the palaeozoic granitic basement of southeastern australia shows a regular pattern of apatite fission track ages which is dominated by a rapid decrease in apparent age towards the rifted continental margin apparent ages away from the margin vary between about 230 and 360 myr but drop sharply to minimum values of around 80 myr near the coast just south of the sydney basin and 150 175 myr elsewhere along the coast fission track length information allows the age pattern to be interpreted as the result of partial to complete resetting of the older ages by a widespread thermal event at about 80 100 myr this event accompanied the rift phase of passive margin development and peaked just before the onset of sea floor spreading in the adjacent tasman sea the zone of reduced apatite ages is deeply embayed around the gippsland basin supporting the suggestion that it originated as a cretaceous failed arm of the tasman rift system the thermal event has affected apatites up to about 130 km from the margin and has produced a characteristic set of bimodal track length distributions in apatites with partially reset ages the bimodality gradually disappears towards the oldest apatite ages which have negatively skewed unimodal distributions the youngest apatites also tend towards unimodal length distributions indicating that thermal annealing of pre existing tracks in these samples went almost to completion maximum temperatures reached in samples presently exposed in the coastal area were thus probably about 110 120 c thermal activity was accompanied by considerable uplift across the region followed by gradual erosion to expose the present land surface post breakup erosion was generally greater towards the new coastline with near coastal samples being previously buried a probable 1 5 2 5 km

denudational history along a transect across the drakensberg escarpment of southern africa derived from apatite fission track thermochronology 1 the denudational history of a 500 km long transect across the drakensberg escarpment on the high elevation passive margin of se africa is quantified on the basis of thermal history modeling of apatite fission track data for 15 deep borehole samples supplemented by an additional 10 outcrop samples a minimum of 4 5 km of denudation since formation of the margin 130 myr ago is estimated for the coastal zone with a marked early cretaceous episode of accelerated denudation broadly coincident with continental breakup samples from the swartberg borehole sw 1 67 located 30 km seaward of the present position of the drakensberg escarpment indicate a total depth of denudation of 3 1 1 2 km since 91 myr with a phase of accelerated denudation of 2 1 0 9 km at a mean rate of 95 43 m myr between 91 and 69 myr samples from the ladybrand borehole la 1 68 west of the lesotho highlands indicate 1 7 0 5 km of denudation since 78 myr with a phase of accelerated denudation at 82 43 m myr from 78 to 64 myr average denudation rates declined to about 10 m myr during much of the tertiary although the apatite fission track data do not provide any direct constraints on the denudational history of the lesotho highlands interpolation between the two boreholes constrained by geological evidence and extrapolated in situ produced cosmogenic 36cl derived denudation rate estimates suggests a pattern of denudation compatible with numerical modeling studies of escarpment evolution involving rapid river incision seaward of a preexisting inland drainage divide these patterns of denudation are incompatible with constant retreat of the drakensberg escarpment from an initial position near the present coast we suggest that the drakensberg escarpment formed by rapid post breakup river incision seaward of a preexisting drainage divide located just east of the present escarpment location and became pinned at this divide with subsequent retreat rates of only 100 200 m myr

the origin of intraplate deformation in the atlas system of western and central algeria from jurassic rifting to cenozoic quaternary inversion abstract analysis of petroleum exploration data supplemented by paleostress data enabled discussion of the origin of the deformation in the western and central saharan atlas algeria this intraplate area has recorded the breakup of pangea late triassic the opening of the maghrebian tethys since the dogger and subsequently its closure oligocene to present however the two periods of strong coupling between europe and africa late lutetian and pleistocene which correspond to rapid uplifts of the atlas system and important deformations are not collision related they can be correlated to the beginning and the end of the development of the western mediterranean sea

timing and structural relations of regional zeolite zones in basalts of the east greenland continental margin zeolite isograds delineating regionally extensive metamorphic mineral zones occur throughout most of the early tertiary flood basalts in central east greenland mineralogic zones defined by isograds are essentially uniform in thickness throughout the province parallel paleotopographic surfaces and transgress lava stratigraphy the distribution of index zeolite minerals is consistent with a thermal gradient of 40 5 c km and heat flow of 1 7 0 5 heat flow units zeolite facies metamorphism occurred rapidly during and just after the bulk of volcanism 57 55 ma and prior to hydrothermal alteration associated with emplacement of the ca 55 ma skaergaard intrusion and mafic dikes of the coastal dike swarm consequently zeolite isograds are temporal markers for distinguishing synvolcanic and postvolcanic deformation events discordance between dips of zeolite isograds and lava flows suggests a pre metamorphic and probably synvolcanic component to tilting of the lava pile later deformation likely associated with dikes and faults near the continental margin tilts the zeolite isograds 2 5 se

quantifying passive margin denudation and landscape development using a combined fission track thermochronology and cosmogenic isotope analysis approach abstract a new approach to empirically constraining numerical models of denudation and landscape development across passive continental margins is presented involving a combination of apatite fission track thermochronology and cosmogenic isotope analysis this integrated strategy is applied to the namibian sector of the south west african margin where the conventional landscape evolution model of inland retreat of an escarpment initiated at the coast at break up implies a mean rate of retreat of 1 km ma 1 the thermochronological and cosmogenic isotope data are incompatible with a simple escarpment retreat model since they show that denudation rates oceanward of the present escarpment position have been low 1 since the end of the eocene and the estimated rate of escarpment retreat has been only 10 m ma 1 this low rate of retreat is however consistent with numerical landscape evolution models where the escarpment is pinned at an inland drainage divide

crustal deformation and submarine canyon incision in a meso cenozoic first order transfer zone sw iberia north atlantic ocean using a dense grid of high quality 2d seismic profiles dredge and outcrop data the offshore prolongation of a first order transfer zone the messejana plasencia fault zone mpfz is explained within the context of oblique rifting between southwest iberia newfoundland and west tethys the offshore mpfz is shown to comprise a 5 10 km wide region of deformation oblique to the continental margin reactivated in the mesozoic as part of a wider transcurrent domain the sao vicente sub basin here the geometry of faults and strata denotes the generation of a pull apart basin during the jurassic early cretaceous in contrast its late cretaceous to cenozoic evolution favours left lateral transpression during the counter clockwise rotation and eastward migration of iberia towards its present position erosion was subsequently enhanced in the sao vicente sub basin due to the reactivation of syn rift structures by documenting the evolution of the sao vicente sub basin and adjacent mpfz this work demonstrates 1 the temporal and spatial scales in which first order transfer zones accommodate crustal movements during continental rifting and subsequent inversion episodes 2 the generation of an extensive region of strain accommodation in the vicinity of the mpfz an observation with profound implications to future palaeogeographic reconstructions of the north atlantic ocean and 3 that the sao vicente canyon the physiographic expression of the mpfz incised the margin as early as the latest cretaceous paleocene synchronously with the onset of tectonic uplift in southwest iberia in such a setting the sao vicente sub basin and mpfz formed important by pass corridors for sediment sourced from proximal areas of the margin at present the mpfz comprises a complex releasing restraining bend accommodating important vertical and horizontal movements in southwest iberia based on earthquake data from similar transfer zones the mpfz should be able to generate large magnitude earthquakes and potentially destructive tsunamis

resolution of the age structure of the detrital zircon populations of two lower cretaceous sandstones from the weald of england by fission track dating modes in the frequency of distribution of fission track ages obtained from detrital zircon grains may prove characteristic of individual sandstone bodies supporting the identification of the sources from which a particular flow of sedimentary detritus was derived and thus allowing new inferences to be made concerning palaeogeography a computer program has been written and used to identify modes in the zircon fission track age distribution within two lower cretaceous sandstone samples from the weald of southern england pronounced modes appear in one rock around 119 ma 160 ma 243 ma and 309 ma and in the other around 141 ma 175 ma 257 to 277 ma and 394 to 453 ma the geological implications of these quite dissimilar zircon age spectra are discussed it is concluded that they support the palaeogeographical models of allen 1981 and indicate that the provenance of the first sample from the top ashdown sandstone member at dallington in east sussex was almost entirely southerly while that of the second from the netherside sand member at northchapel in west sussex was more varied but predominantly westerly and northerly

early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate abstract along the southeast african continental margin a deep sedimentary basin and marginal ridge bordering the agulhas fracture zone formed coevally with south american counterparts underlying the falkland plateau extension in both the southern outeniqua and the falkland basin is related to large angle 100 rotation of the lafonia falkland islands block in middle late jurassic times during attempted propagation of the proto indian ocean across the active circum pacific margin of gondwanaland in the early cretaceous the jurassic microplate boundaries were partly reactivated and partly transected by a right lateral shear along the 1200 km long falkland agulhas transform fault preceded by substantial vorticity of the surrounding crustal blocks this boundary segment separating the south american and african plates was probably generated by complex geodynamic interactions between oceanic plate subduction continental to back arc rift propagation and the impingement of deep mantle plumes on the lithosphere

rift controlled fluvial tidal transitional series in the oukai meden sandstones high atlas of marrakesh morocco abstract the carnian formation of the oukai meden sandstones was deposited in the northwestern corner of the african craton along tectonic troughs grouped as a wsw ene rift system on the southwestern margin of the tethys red sandstones represent mainly distal braided stream deposits where very extensive sand flats with 3d dunes 2d sand waves and high regime horizontally laminated sandstones predominate the similarities in orientation between current patterns and trough border faulting the sudden large variations in the thickness of deposits and the occurrence of lateral alluvial fans along the marginal faults are all evidence of the tectonic control of sedimentation the middle part of this clastic continental rift series includes finer facies containing marine reworked or in situ fossils and tracks as well as sedimentary structures interpreted as being of tidal origin especially overlapping s shaped sets the episodic marine influxes that occur within these continental rift deposits are thought to result from the substantial downdropping of tectonic blocks and the drowning of fluviatile facies by the peritethyan marine domain to the northeast the development of marine facies was of relatively short duration and figures little in the sedimentary record butit is evidence of the earliest mesozoic onlaps of the tethys over the african craton in a carnian rift

the neogene quaternary evolution of the guercif basin morocco reconstructed from seismic line interpretation abstract the neogene guercif basin of northeastern morocco is the eastern continuation of the rifean foredeep from which it has been separated by the late neogene uplift of the tazzeka spur of the middle atlas a dense grid more than 1200 km of seismic reflection profiles across the basin acquired during various surveys 1977 1982 1984 1985 the drilling of several wells used to calibrate the seismic profiles in particular those affecting the neogene quaternary sequence as well as the available superficial geological data allow us to describe the late tertiary quaternary evolution of this basin the guercif basin is superimposed on late cretaceous eocene inversion structures involving jurassic half grabens which are the downplunge northeastern continuation of the middle atlas extensive pre tortonian erosion mainly due to tectonic uplift occurred prior to the main neogene subsidence event which was due to the emplacement of the rifean nappes minor early tortonian extension is observed but by the end of the tortonian an overall compressional regime began which continued into the plio quaternary and involves minor reactivation of the pre existing inversion structures as well as the formation of narrow salt cored anticlines the same compressional regime is also responsible for the renewed uplift of the middle atlas and the masgout uplift to the north of the basin

mid jurassic events in the western high atlas morocco angular unconformities time gaps and progressive increase of thickness in the mesozoic sedimentary pile of the western high atlas morocco point to mid jurassic tectonic pulses in this area red beds middle jurassic in age were deposited in the nearshore agadir essaouira basin in contrast to transgressive tendencies of the eustatic curve these phenomena are easily explained by uplift and erosion in the central part of the mountain belt even in the middle jurassic thus a rift stage permian to early jurassic in the tectonic history of the western high atlas was followed by a gulf stage late jurassic to middle cretaceous until later the high mountain range generated by inversion in late cretaceous and neogene to recent times tensional forces that created the mobile rift belt were superimposed by vertical uplift during the middle jurassic that gave rise to a nne ssw stretching high even in the present day high mountain range this area called the ancient massif is strongly elevated thus the evaluation of the geological history of the western high atlas seems much complicated

the inverted triassic rift of the marrakech high atlas a reappraisal of basin geometries and faulting histories the high atlas of morocco is an aborted rift developed during the triassic jurassic and moderately inverted during the cenozoic the marrakech high atlas with large exposures of basement and triassic early syn rift deposits is ideal to investigate the geometries of the deepest parts of a rift constituting a good analogue for pre salt domains it allows unraveling geometries and kinematics of the extensional and compressional structures and the influence that they exert over one another a detailed structural study of the main triassic basins and basin margin faults of the marrakech high atlas shows that only a few rift faults were reactivated during the cenozoic compressional stage in contrast to previous interpretations and emphasizes that fault reactivation cannot be taken for granted in inverted rift systems preserved extensional features demonstrate a dominant dip slip opening kinematics with strike slip playing a minor role at variance to models proposing a major strike slip component along the main basin bounding faults including faults belonging to the tizi n test fault zone a new middle triassic paleogeographic reconstruction shows that the marrakech high atlas was a narrow and segmented orthogonal rift sub perpendicular to the main regional extension direction which was nw se in contrast to the central and eastern segments of the atlas rift which developed obliquely this difference in orientation is attributed to the indented ouzellarh precambrian salient part of the west african craton which deflected the general rift trend in the area evidencing the major role of inherited lithospheric anisotropies in rift direction and evolution as for the cenozoic inversion total orogenic shortening is moderate 16 and appears accommodated by basement involved large scale folding and by newly formed shortcut and by pass thrusting with rare left lateral strike slip indicators triassic faults commonly acted as buttresses

geophysical evidence of cretaceous volcanics in logone birni basin northern cameroon central africa and consequences for the west and central african rift system abstract detailed analyses and interpretation realized by combining existing 2d reflection seismic and gravity magnetic data of the logone birni basin lbb in the west and central african rift system wcas have revealed the distribution of the main buried volcanic bodies as well as their relationships with the structural and tectonic evolution of this basin the volcanic activity in the lbb is restricted to the cretaceous period three main volcanic episodes are identified and are associated to the neocomian late albian and cenomanian turonian rifting phases respectively the volcanic bodies within the lower cretaceous are either lying directly on basement or are mainly interbedded with the contemporaneous sediments whereas the upper cretaceous bodies are morphologically expressed in the forms of dykes and sills the volcanic activity was more intense in the western region of the central lbb zina sub basin along the cameroon nigeria border whereas it was scanty and scattered in the other parts of the basin the main volcanic dykes are found on the flanks of the major faults bounding basement horsts or in crestal positions in association with syndepositional faults although wcas is associated with large amount of crustal extension and minor volcanism the intense volcanic activity observed in lbb during the cretaceous suggests that the intrusive zone during this period was confined to the basement beneath the study area flanked respectively to the north south and southwest by the lake chad poli and chum triple junctions tensional stresses generated by this localized domal uplift accounts for most of the observed tectonic structures where major faults transected the entire lithosphere thus providing conduits for magma migration

early cretaceous uplift and erosion of the northern appalachian basin new york based on apatite fission track analysis abstract the thermal history of outcropping devonian sediments of the northern appalachian basin new york has been investigated using fission track analysis of detrital apatites from 57 sandstone samples based on lengths and apparent age measurements using fission tracks in apatite it is concluded that lower devonian sediments presently at the surface in the catskill region were cooled rapidly from temperatures higher than about 110 c during early cretaceous times 120 140 ma ago in the western part of new york wellsville buffalo data from late devonian sediments are consistent with cooling at the same time as that identified for the catskill region but from lower temperatures in the range of approximately 80 110 c the maximum temperature these sediments experienced since deposition for a pre uplift paleogeothermal gradient of 25 35 c km the confined track length data indicates uplift and erosion of 2 3 km for western new york and greater than 3 4 km for the catskill region a differential uplift pattern which is consistent with the historical stratigraphic data from the region this conclusion is at variance with earlier interpretations put forth by others rapid broad scale uplift and erosion of the scale identified imply that large volumes of sediment could have been supplied from the northern appalachian basin during the early cretaceous this timing for the dominant post devonian cooling phase in the basin is not accounted for by recent models of the tectonic evolution of the appalachian orogen but is compatible with the change from carbonate to siliciclastic deposition in the atlantic coastal plain it is suggested that this style of broad regional uplift without significant deformation is characteristic of a tectonic regime associated with and subsequent to continental rifting apatite fission track analysis is shown to be a basic tool in providing fundamental limits for thermal history assessment in regional tectonic problems

escarpment evolution on high elevation rifted margins insights derived from a surface processes model that combines diffusion advection and reaction experiments with a surface processes model of large scale 1 1000 km long term 1 100 m y erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent the model describes changes in topographic form as a result of simultaneous short and long range mass transport representing hillslope diffusive processes and fluvial transport advection respectively fluvial entrainment is modeled as a first order kinetic reaction which reflects the credibility of the substrate and therefore the fluvial system is not necessarily carrying at capacity one dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are 1 antecedent topography drainage 2 the timescale or equivalently a length scale in the fluvial entrainment reaction 3 the flexural response of the lithosphere to denudation and 4 the relative efficiencies of the short and long range transport processes when rainfall and substrate lithology are uniform a significant amount of discharge draining over the escarpment top causes it to degrade only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions an additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment this corresponds to a substrate that is hard to detach by fluvial erosion and therefore to fluvial erosion that is not transport limited continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide it is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology low flexural rigidities promote steep and slowly retreating escarpments for given rainfall and substrate conditions the morphology of a retreating model escarpment is determined by the ratio of the short range diffusive and long range advective transport efficiencies a low ratio which is interpreted to correspond to a relatively arid climate and weathering limited conditions promotes steep sharp topped escarpments with straight main slopes and escarpment retreat occurs over a wide range of height scales a high ratio interpreted to correspond to a more humid temperate climate produces a convex upper slope and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock however resistant caprocks are not an essential requirement for model scarps to retreat we conclude that the inferred controls and model behavior are both consistent with the present day morphology of rifted continental margins and with modern conceptual models of landscape evolution

mid triassic to early liassic clastic evaporitic deposits over the maghreb platform abstract the development of triassic to lower liassic clastic evaporitic series over the epicratonic maghreb platform is closely associated with the eastern opening of a tethyan marine domain between africa and europe west of the platform morocco became separated from north america in late triassic times by rifting along the axis of the proto atlantic ocean in addition ne sw and ene wsw trending atlasic half grabens formed essentially in morocco as part of a late triassic early liassic atlas rifting episode this is the tectonic context in which the red bed to evaporite sequences were deposited a first depositional pattern is illustrated by the areally extensive onlapping of upper triassic clastic evaporitic series over the algerian saharan platform from the carnian to the norian distal mudflats and evaporites graded westward i e landward into proximal fluvial facies seven stratigraphic cycles formed part of a general base level rise of a major retrogradational clastic to evaporitic cycle indicative of general tectonic subsidence of the algerian platform as a whole the evaporites are thus interpreted as markers of persistent onlapping occurring at the edge of the basin as clastic influxes gradually diminished a second depositional pattern is provided by ladinian to rhaetian clastic evaporitic carbonate transgressive sequences of the atlasic domain in tunisia these terminate characteristically in marine facies in this domain intense tectonic activity was consistently offset by a high rate of evaporitic or clastic sedimentation in shallow water environments a third depositional pattern is illustrated in the moroccan high atlas in the central part of this range carnian red conglomerates and sheet like sandstones of a distal braided fluvial plain were deposited in tectonic troughs that linked together as part of a rift system episodic connections occurred with the tethyan domain to the northeast in the western high atlas an upper carnian norian sequence several hundred meters thick displays a general trend from clastics to evaporites reflecting the development of arid paleoclimatic conditions the moroccan meseta provides examples of clastic evaporitic depositional patterns within intracratonic troughs clastic filling of rift induced troughs in the oujda mountains exhibits significant facies and thickness variations ranging from local proximal alluvial and distal anastomosed channel systems through to dominant mudflat systems in the 1000 m thick series of the 60 km long khemisset basin facies range from red beds located mainly along the basin margins to salt dominated deposits in the basin center increasing accommodation controlled the deposition of evaporites and the maximum flooding surface is to be found in typical halite deposits with interbedded bands of clays at that time greater tectonic subsidence rates allowed more marked episodic marine incursions clastic evaporitic series from the passive margin of the moroccan atlantic coast are known only from bore holes and onshore and offshore seismic data recent papers have reported interesting sections across strongly subsiding grabens or tilted blocks related to predominantly ssw nne faults

the moroccan anti atlas the west african craton passive margin with limited pan african activity implications for the northern limit of the craton abstract the moroccan anti atlas region located south of the south atlas fault has been viewed traditionally as containing two segments separated by the anti atlas major fault these two segments are said to consist of a 600 700 ma pan african segment located in the northeast and b 2 ga eburnian segment situated to the southwest on the basis of observations in the zenaga and saghro inliers and of a recent literature review we suggest that this subdivision is inappropriate in that eburnian and pan african materials occur throughout the anti atlas region the entire anti atlas is underlain by eburnian crust unconformably overlain by a lower neoproterozoic passive margin allochthonous pan african ocean crustal slices were thrust onto the west african craton wac passive margin sequence 685 ma ago as a result of pan african accretion tectonics high level high k calc alkaline and alkaline granitoids locally intruded the anti atlas sequence as a whole at the end of the pan african orogeny at 585 560 ma the intervening 100 m y interval was marked by quiescence this succession of events can be related to the behaviour of one single rigid cratonic passive margin during an orogeny and correlated to the pan african events that occurred to the east in the tuareg shield and to the north in avalonian terranes this model implies that the actual northern limit of the wac is located at the south atlas fault saf and not at the anti atlas major fault aamf we propose that the aamf corresponds to the southwestern boundary of an aulacogen that formed along the northern margin of the wac during early neoproterozoic times this is consistent with the development of the gourma aulacogen on the eastern side of the wac this model further suggests that the northeastern boundary of the wac occurs north and not south of ougarta algeria support for this model is provided by geological and geophysical evidence

a sedimentological model for the continental upper triassic tadrart ouadou sandstone member recording an interplay of climate and tectonics argana valley south west morocco the continental upper triassic tadrart ouadou sandstone member was deposited in an extensional setting on the pangaean continent strongly influenced by a low latitude climatic regime 10 to 20 north complex interaction of basin subsidence and climatically driven processes led to high facies variability and a lack of correlatable units across the argana valley exposures a process orientated approach integrating detailed facies with architectural element analysis was undertaken which resulted in a multistage depositional model for the tadrart ouadou sandstone member the basin scale model shows that basal alluvial fan and braided river systems are confined to the centre of the argana valley exposures aeolian deposits occur throughout the sequence but dominate in the north after a phase of playa deposition prominent basin wide fluvial incision of up to 8 m marks the onset of perennial fluvial flow these well sorted internally complex and locally highly amalgamated fluvial sandstones are widespread throughout the basin and are focused in a north to south south west flowing channel system after a final stage of aeolian sedimentation sandstone deposition of the tadrart ouadou sandstone member in the argana valley is terminated rapidly by the onlap of lacustrine mudstones of the sidi mansour member the study revealed that except for one pronounced period of perennial conditions sedimentation is controlled largely by ephemeral fluvial flow alternating ground water tables deflation processes and periods with limited periodic local run off the study highlights that facies architecture in the basin is the result of complex interaction of local syn sedimentary tectonics and the climatic regime within the basin but also the climate of the catchment area to the east the data suggest a proximal to mid distal basin setting in the rain shadow to the west of a mountain range massif ancien which exerted a strong control on the depositional environments of triassic deposits exposed in this part of south west morocco

syn and postrift evolution of the el jadida agadir basin morocco constraints for the rifting models of the central atlantic the el jadida agadir basin shows an evolution related to that of the central atlantic rifting history the synrift period late early triassic to earliest jurassic is dominated by deposition of red clastic sediments and evaporites which are overlain by or intercalated with basalt flows two intense extensional phases are recognized the first probably in middle triassic times is oriented north northwest south southeast and resulted in the formation of east northeast west southwest oriented grabens and half grabens the second a widespread northwest southeast trending extension is ladinian to earliest liassic and induced the formation of westward dipping half grabens extensional tectonics initiated in the southern part of the basin and migrated northwards contrary to classical ideas proposing rifting from north to south the postrift evolution is characterized by jurassic to eocene subsidence tectonism appears to have been restricted to a narrow area close to the present day coast subsidence h

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

role of synsedimentary strike slip faults in the formation of moroccan triassic basins upper triassic sandstone and siltstone infilled basins in the massif ancien of the high atlas morocco are distributed along a complex east northeast trough at the boundary between the hercynian orogen and the west african craton the formation of these basins involved a dense pattern of pre existing faults hydroplastic slickensides which affect nonlithified sediments help identify not only dip slip but also strike slip faults together with other tectonic and sedimentary structures the directional distribution of these faults leads to a model in which extension varies from north northwest during lower unit sandstone infilling to northwest during upper unit siltstone infilling the latter direction is compatible with the formation of the lower liassic intrusional doleritic dikes of the anti atlas

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data