incipient mantle delamination active tectonics and crustal thickening in northern morocco insights from gravity data and numerical modeling abstract the betic rif orocline surrounding the alboran sea the westernmost tip of the mediterranean sea accommodates the nw se convergence between the nubia and eurasia plates recent gps observations indicate a 4 mm yr sw motion of the rif mountains relative to stable nubia incompatible with a simple two plate model new gravity data acquired in this study define a pronounced negative bouguer anomaly south of the rif interpreted as a 40 km thick crust in a state of non isostatic equilibrium we study the correlation between these present day kinematic and geodynamic processes using a finite element code to model in 2 d the first order behavior of a lithosphere affected by a downward normal traction representing the pull of a high density body in the upper mantle we show that intermediate viscosities for the lower crust and uppermost mantle 10 21 10 22 pa s allow an efficient coupling between the mantle and the base of the brittle crust thus enabling 1 the conversion of vertical movement resulting from the downward traction to horizontal movement and 2 shortening in the brittle upper crust our results show that incipient delamination of the nubian continental lithosphere linked to slab pull can explain the present day abnormal tectonics contribute to the gravity anomaly observed in northern morocco and give insight into recent tectonics in the western mediterranean region

subduction and volcanism in the iberia north africa collision zone from tomographic images of the upper mantle abstract new tomographic images of the upper mantle beneath the westernmost mediterranean suggest that the evolution of the region experienced two subduction related episodes first subduction of oceanic and or extended continental lithosphere now located mainly beneath the betics at depths greater than 400 km took place on a nw se oriented subduction zone this was followed by a slab tear process that initiated in the east and propagated to the west leading to westward slab rollback and possibly lower crustal delamination the current position of the slab tear is located approximately at 4 w and to the west of this location the subducted lithosphere is still attached to the surface along the gibraltar arc our new p wave velocity model is able to image the attached subducted lithosphere as a narrow high velocity body extending to shallow depths coinciding with the region of maximum curvature of the gibraltar arc the occurrence of intermediate depth earthquakes and anomalously thick crust this thick crust has a large influence in the measured teleseismic travel time residuals and therefore in the obtained p wave tomographic model we show that removing the effects of the thick crust significantly improves the shallow images of the slab and therefore the interpretations based on the seismic structure

active surface deformation and sub lithospheric processes in the western mediterranean constrained by numerical models we present the results of dynamic modeling of the western mediterranean that accounts for observed global positioning system gps surface deformation of the alboran sea and surrounding rif and betic mountains as the result of the combined effects of relative motion of the eurasian and nubian plates low strength in the alboran sea region and sub lithospheric processes occurring beneath the external rif domain assuming that the lithosphere behaves elastically over the short time period of the gps observations an elastic plate model is considered in our study including an area of weak lithosphere factor of 10 centered on the alboran sea and in which lateral boundary conditions consist of the nubia eurasia oblique convergence sub crustal processes are modeled by application of a horizontal traction on a small area patch at the base of the elastic plate our modeling studies demonstrate the need for sub crustal or sub lithospheric southwestward directed forcing to account for observed southwestward motion of the rif and betic domains based on the location orientation and small area of the traction patch we hypothesize that forcing is associated with delamination and rollback of the subducted african continental lithospheric mantle beneath the external rif zone due to the pull of the oceanic part of the western mediterranean slab a dynamic process that may be similar to that where the over riding plate is driven toward the subduction zone during slab rollback

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

the permian moradi formation of northern niger paleosol morphology petrography and mineralogy abstract three basic paleosol morphologies named type a type b and type c are described from the middle upper permian strata of the moradi formation tim mersoi basin northern niger the moradi formation is a typical alluvial redbed succession dominated by red mudrocks with fine to coarse grained pebbly channel sandstones and matrix breccias type a paleosols are hosted by well sorted fine to medium grained trough cross bedded and massive sandstones and preserve abundant vertical to horizontal micritic and microspar calcite tubules interpreted as rhizoliths lateral variability of rhizoliths in type a paleosols and their close association with fluvial channel fill sediments suggests they are the roots of grove stands of phreatophytic vegetation that grew within unstable anabranching stream systems type b paleosols are hosted by mudrocks and preserve well developed ped structure abundant micritic calcite nodules and vertically stacked micritic calcite nodular bodies as well as rare calcite with satin spar texture interpreted as a pseudomorphic replacement of pedogenic gypsum the morphology of type b paleosols suggests they were formed in well drained floodplain deposits on stable landforms type c paleosols are similar to type b but preserve pedogenic structures indicative of soil volume expansion and contraction as well as more abundant stage ii pedogenic carbonate nodules the morphology of type c paleosols suggests that they developed periodically rather than seasonally in poorly drained deposits that nevertheless occupied a relatively stable part of the landscape such as the plains flanking ephemeral lakes or sabkhas x ray diffraction analysis of the the range of morphologies petrographic textures and mineralogy of the paleosol profiles indicates semi arid to hyper arid climatic setting this paleoclimatic reconstruction is in agreement with middle and late permian conceptual paleoclimate models and quantitative general circulation models nevertheless and in spite of an arid climate moradi paleosols and their host strata also indicate a relatively shallow groundwater table importantly this shallow groundwater resource undoubtedly helped to support the moderately diverse fossil vertebrate assemblage and large stature macrophytes preserved in the moradi formation

the quest for the africa eurasia plate boundary west of the strait of gibraltar the missing link in the plate boundary between eurasia and africa in the central atlantic is presented and discussed a set of almost linear and sub parallel dextral strike slip faults the swim1 faults that form a narrow band of deformation over a length of 600 km coincident with a small circle centred on the pole of rotation of africa with respect to eurasia was mapped using a new swath bathymetry compilation available in the area offshore sw portugal these faults connect the gloria fault to the rif tell fault zone two segments of the plate boundary between africa and eurasia the swim faults cut across the gulf of cadiz in the atlantic ocean where the 1755 great lisbon earthquake m 8 5 8 7 and tsunami were generated providing a new insight on its source location

puzzling features of western mediterranean tectonics explained by slab dragging the recent tectonic evolution of the western mediterranean region is enigmatic the causes for the closure of the moroccan marine gateway prior to the messinian salinity crisis for the ongoing shortening of the moroccan rif and for the origin of the seismogenic trans alboran shear zone and eastern betics extension are unclear these puzzling tectonic features cannot be fully explained by subduction of the east dipping gibraltar slab in the context of the regional relative plate motion frame here we use a combination of geological and geodetic data as well as three dimensional numerical modelling of subduction to show that these unusual tectonic features could be the consequence of slab dragging the north to north eastward dragging of the gibraltar slab by the absolute motion of the african plate comparison of our model results to patterns of deformation in the western mediterranean constrained by geological and geodetic data confirm that slab dragging provides a plausible mechanism for the observed deformation our results imply that the impact of absolute plate motion on subduction is identifiable from crustal observations identifying such signatures elsewhere may improve the mantle reference frame and provide insights on subduction evolution and associated crustal deformation

geodynamic evolution of the lithosphere and upper mantle beneath the alboran region of the western mediterranean constraints from travel time tomography an edited version of this paper was published by the american geophysical union copyright 2000 agu see also http www agu org pubs crossref 2000 2000jb900024 shtml http atlas geo cornell edu morocco publications calvert2000 htm

mantle structure under gibraltar constrained by dispersion of body waves 1 westudytheafrica iberiaplateboundaryinthevicinity of gibraltar numerous models have been proposed for that region throughout the last decades proposing mechanisms that range widely from continental delamination convective removal to subduction of oceanic lithosphere to better constrain upper mantle structure under the region we study waveforms of p waves that traverse the alboran sea region between spain and morocco these show dispersive behavior which together with early arrival times confirms the presence of an anomalous upper mantle structure under the alboran sea the dispersion is consistent with that expected from subducted lithosphere waveforms of body waves therefore provide a way to better constrain the elusive mantle structure and dynamics of the alboran sea region citation bokelmann g and e maufroy 2007 mantle structure under gibraltar constrained by dispersion of body waves geophys res lett 34 l22305 doi 10 1029 2007gl030964

ongoing lithospheric removal in the western mediterranean evidence from ps receiver functions and thermobarometry of neogene basalts picasso project the western mediterranean tectonic system consists of the betic mountains in southern spain and the rif mountains in northern morocco curved around the back arc extensional alboran basin multiple tectonic models have been developed to explain the coeval compressional and extensional tectonic processes that have affected the western mediterranean since the oligocene in order to provide constraints on these evolutionary models we use ps teleseismic receiver functions rf thermobarometric analyses of post oligocene basalts and previous teleseismic tomography images to investigate the lithospheric structure of the region ps rfs were calculated using seismic data from 239 broadband seismic stations in southern iberia and northern morocco and thermobarometric analysis was performed on 19 volcanic samples distributed throughout the region the rf images reveal a highly variable moho depth 25 to 55 km as well as a strong positive sub moho horizon between 45 and 80 km depth beneath the central betic and rif mountains which we interpret to be the top of the previously imaged alboran sea slab thermobarometric constraints from magmas in the eastern betics and rif indicate mantle melting depths between 40 and 60 km typical of melting depths beneath mid oceanic ridges where little to no lithosphere exists together the rf and thermobarometric data suggest ongoing and recent slab detachment resulting from delamination of the continental lithosphere

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

plate rigidity inversion in southern california using interseismic gps velocity field summary this paper presents an inversion method using the interseismic velocity field to determine effective rigidity of the lithosphere the method is based on the minimization of a cost function defined as the quadratic measure of the difference between measured and modelled velocity fields on a discrete set of points the continuous mapping of the rigidity is fulfilled with a limited set of parameters and the forward solution is achieved using a plane stress finite element code the computation of the cost function gradient in the parameters space allows one to iteratively find the best parameters set through a suitable optimization algorithm we first design a benchmark including an abrupt rigidity variation that cannot be described by a continuous function for such a case we show that increasing the number of parameters is a way to accurately describe sharp variations of the rigidity map then we use a dense gps velocity field over the southwestern united states to estimate the corresponding rigidity variations for different spatial resolutions of the parameters grid we analyse the conceptual and practical difficulties associated with our methodology finally rigidity maps obtained by our inversion method in southwestern united states and particularly across the san andreas fault system are reviewed and compared to current plate rigidity estimates and geophysical data over this area

gps constraints on africa nubia and arabia plate motions summary we use continuously recording gps cgps and survey mode gps sgps observations to determine euler vectors for relative motion of the african nubian arabian and eurasian plates we present a well constrained eurasia nubia euler vector derived from 23 igs sites in europe and four cgps and three sgps sites on the nubian plate 0 95 4 8 n 21 8 4 3 e 0 06 0 005 myr 1 we see no significant 1m m yr 1 internal deformation of the nubian plate the gps nubian eurasian euler vector differs significantly from nuvel 1a 21 0 4 2 n 20 6 0 6 e 0 12 0 015 myr 1 implying more westward motion of africa relative to eurasia and slower convergence in the eastern mediterranean the arabia eurasia and arabia nubia gps euler vectors are less well determined based on only one cgps and three sgps sites on the arabian plate the preliminary arabia eurasia and arabia nubia euler vectors are 27 4 1 0 n 18 4 2 5 e 0 40 0 04 myr 1 and 30 5 1 0 n 25 7 2 3 e 0 37 0 04 myr 1 respectively the gps arabia nubia euler vector differs significantly from nuvel 1a 24 1 1 7 n 24 0 3 5 e 0 40 0 05 myr 1 but is statistically consistent at the 95 per cent confidence level with the revised euler vector reported by chu gordon based on a re evaluation of magnetic anomalies in the red sea 31 5 1 2 n 23 0 2 7 e 0 40 0 05 myr 1 the motion implied in the gulf of aqaba and on the dead sea fault dsf by the new gps nubia arabia euler vector i e ignoring possible sinai block motion and possible internal plate deformation grades from pure left lateral strike slip in the gulf and on the southern dsf with increasing compression on the central and northern dsf with relative motion increasing from 5 6 to 7 5 mm yr 1 1m m yr 1 from south to north along the northern dsf i e north of the lebanon restraining bend motion is partitioned between 6 1m m yr 1 left lateral motion parallel to the fault trace and 4 1m m yr 1 faultnormal compression relative motions on other plate boundaries including the anatolian and aegean microplates derived from the gps euler vectors agree qualitatively with the sense of motion indicated by focal mechanisms for large crustal earthquakes m 6 where data are available on fault slip rates on plate bounding faults north anatolian fault east anatolian fault dead sea fault red sea rift they are generally lower than but not significantly different from the full plate motion estimates suggesting that the majority of relative plate motion is accommodated on these structures

active tectonics of the western mediterranean geodetic evidence for rollback of a delaminated subcontinental lithospheric slab beneath the rif mountains morocco surface deformation in morocco derived from five years of global positioning system gps survey observations of a 22 station network four continuously recording gps cgps stations and four international gnss service igs stations in iberia indicates roughly southward motion 3 mm yr of the rif mountains morocco relative to stable africa motion of the rif is approximately normal to the direction of africa eurasia relative motion which is predominantly strike slip and results in shortening of the rif and subsequent crustal extension of the adjacent alboran sea region the sense and the n s asymmetry of the observed deformation i e no evidence for north directed shortening in the betic mountains north of the alboran sea cannot be easily explained in terms of crustal plate interactions suggesting that dynamic processes below the crust are driving the recent geologic evolution of the western mediterranean the model that best fits the observations involves delamination and southward rollback of the african lithospheric mantle under the alboran and rif domains

modern tectonic stress field in the mediterranean region evidence for variation in stress directions at different scales summary a compilation of more than one thousand stress indicators which include in situ stress measurements focal mechanisms microtectonic and other geological data allowed us to reconstruct the modern stress field in the mediterranean region and the surrounding area average stress directions at different scales have been reconstructed by means of a linear interpolation method this method takes into account the distribution scale and quality of stress data the results of the interpolation at plate scale allow us to recognize slightly deformed regions such as the northwestern european platform where average maximum horizontal stress direction is oriented roughly nnw sse subparallel to absolute and relative plate velocity directions other regions such as the caucasus alps and pyrenees where recent tectonic deformation and seismicity are present display important variations of stress directions the reconstruction of the average stress directions at different scales within the french alps pointed out that the average stress field pattern may vary from one scale to another nevertheless variations of stress directions at a given scale are consistent with the kinematics of faults of the same scale

crustal structure and gravity anomalies beneath the rif northern morocco implications for the current tectonics of the alboran region we analyse bouguer anomaly data and previously published moho depths estimated from receiver functions in order to determine the amount of isostatic compensation or uncompensation of the rif topography in northern morocco we use moho depth variations extracted from receiver function analyses to predict synthetic bouguer anomalies that are then compared to observed bouguer anomaly we find that moho depth variations due to isostatic compensation of topographic and or intracrustal loads do not match moho depth estimates obtained from receiver function analyses the isostatic misfit map evidences excess crustal root as large as 10 km in the western part of the study area whereas a missing crustal root of 5 km appears east of 4 3 e this excess root missing topography correlates with the presence of a dense mantle lid the noticeable southwestward drift of the western rif area and with a current surface uplift we propose that a delaminated mantle lid progressively detaching westward or southwestward from the overlying crust is responsible for viscous flow of the ductile lower crust beneath the rif area this gives rise to isostatic uplift and westward drift due to viscous coupling at the upper lower crust boundary at the same time the presence of this dense sinking mantle lid causes a negative dynamic topography which explains why the observed topography is too low compared to the crustal thickness

crustal resistivity structure of the southwestern transect of the rif cordillera morocco a ne sw magnetotelluric 110 km long profile including 18 sites was acquired across the western rifcordillera along the eurasian african plate boundary allowing to constrain its poorly known deepstructure it extends from the internal zones close to the alboran coast crossing the external zones andup to the gharb foreland basin the periods recorded range from 0 001 s to 1000 s the combination ofmagnetotelluric data with available geological data provides new insight regarding the relationship

evidence for elevated alkalinity in the glacial southern ocean an increase in whole ocean alkalinity during glacial periods could account in part for the drawdown of atmospheric co2 into the ocean such an increase was inevitable due to the near elimination of shelf area for the burial of coral reef alkalinity we present evidence based on downcore measurements of benthic foraminiferal b ca and mg ca from a core in the weddell sea that the deep ocean carbonate ion concentration co32 was elevated by similar to 25 mu mol kg during each glacial period of the last 800 kyr the heterogeneity of the preservation histories in the different ocean basins reflects control of the carbonate chemistry of the deep glacial ocean in the atlantic and pacific by the changing ventilation and chemistry of weddell sea waters these waters are more corrosive than interglacial northern sourced waters but not as undersaturated as interglacial southern sourced waters our inferred increase in whole ocean alkalinity can be reconciled with reconstructions of glacial saturation horizon depth and the carbonate budget if carbonate burial rates also increased above the saturation horizon as a result of enhanced pelagic calcification the weddell records display low co32 during deglaciations and peak interglacial warmth coincident with maxima in percent caco3 in the atlantic and pacific oceans should the burial rate of alkalinity in the more alkaline glacial deep waters outstrip the rate of alkalinity supply then pelagic carbonate production by the coccolithophores at the end of the glacial maximum could drive a decrease in ocean co32 and act to trigger the deglacial rise in pco 2

new gps constraints on active deformation along the africa iberia plate boundary abstract we use velocities from 65 continuous stations and 31 survey mode gps sites as well as kinematic modeling to investigate present day deformation along the africa iberia plate boundary zone in the western mediterranean region the gps velocity field shows southwestward motion of the central part of the rif mountains in northern morocco with respect to africa varying between 3 5 and 4 0 mm yr consistent with prior published results stations in the southwestern part of the betic mountains of southern spain move west southwest with respect to eurasia 2 3 mm yr the western component of betics motion is consistent with partial transfer of nubia eurasia plate motion into the southern betics the southward component of betics motion with respect to iberia is kinematically consistent with south to southwest motion of the rif mountains with respect to africa we use block modeling constrained by mapped surface faults and seismicity to estimate the geometry and rates of strain accumulation on plate boundary structures our preferred plate boundary geometry includes one block between iberia and africa including the sw betics alboran sea and central rif this geometry provides a good fit to the observed motions suggesting a wide transpressive boundary in the westernmost mediterranean with deformation mainly accommodated by the gloria azores fault system to the west and the rif tell lineament to the east block boundaries encompass aspects of earlier interpretations suggesting three main deformation styles i extension along the ne sw trending trans alboran shear zone ii dextral strike slip in the betics corresponding to a well defined e w seismic lineament and iii right lateral strike slip motion extending west to the azores and right lateral motion with compression extending east along the algerian tell we interpret differential motion in the rif alboran betic system to be driven both by surface processes related the africa eurasia oblique convergence and sub crustal dynamic processes associated with the long history of subduction of the neotethys ocean lithosphere the dextral slip identified in the betic mountains in southern spain may be related to the offshore fault that produced the great 1755 lisbon earthquake and as such may represent a significant seismic hazard for the west mediterranean region

absolute plate motions and regional subduction evolution we investigate the influence of absolute plate motion on regional 3 d evolution of subduction using numerical thermomechanical modeling building on our previous work we explore the potential impact of four different absolute plate motion frames on subduction evolution in the western mediterranean region during the last 35 my one frame is data based and derived from the global moving hotspot reference frame gmhrf of doubrovine et al 2012 and three are invented frames a motion frame in which the african plate motion is twice that in the gmhrf and two frames in which either the african plate or the iberian continent is assumed fixed to the mantle the relative africa iberia convergence is the same in all frames all motion frames result in distinctly different 3 d subduction evolution showing a critical dependence of slab morphology evolution on absolute plate motion we attribute this to slab dragging through the mantle forced by the absolute motion of the subducting plate which causes additional viscous resistance affecting subduction evolution we observed a strong correlation between increase in northward africa motion and decrease in the speed of westward slab rollback along the african margin we relate this to increased mantle resistance against slab dragging providing new insight into propagation and dynamics of subduction transform edge propagator step faults our results demonstrate a large sensitivity of 3 d slab evolution to the absolute motion of the subducting plate which inversely suggests that detailed modeling of natural subduction may provide novel constraints on absolute plate motions

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

gps constraints on continental deformation in the africa arabia eurasia continental collision zone and implications for the dynamics of plate interactions 1 the gps derived velocity field 1988 2005 for the zone of interaction of the arabian african nubian somalian and eurasian plates indicates counterclockwise rotation of a broad area of the earth s surface including the arabian plate adjacent parts of the zagros and central iran turkey and the aegean peloponnesus relative to eurasia at rates in the range of 20 30 mm yr this relatively rapid motion occurs within the framework of the slow moving 5 mm yr relative motions eurasian nubian and somalian plates the circulatory pattern of motion increases in rate toward the hellenic trench system we develop an elastic block model to constrain present day plate motions relative euler vectors regional deformation within the interplate zone and slip rates for major faults substantial areas of continental lithosphere within the region of plate interaction show coherent motion with internal deformations below 1 2 mm yr including central and eastern anatolia turkey the southwestern aegean peloponnesus the lesser caucasus and central iran geodetic slip rates for major block bounding structures are mostly comparable to geologic rates estimated for the most recent geological period 3 5 myr we find that the convergence of arabia with eurasia is accommodated in large part by lateral transport within the interior part of the collision zone and lithospheric shortening along the caucasus and zagros mountain belts around the periphery of the collision zone in addition we find that the principal boundary between the westerly moving anatolian plate and arabia east anatolian fault is presently characterized by pure left lateral strike slip with no fault normal convergence this implies that extrusion is not presently inducing westward motion of anatolia on the basis of the observed kinematics we hypothesize that deformation in the africa arabia eurasia collision zone is driven in large part by rollback of the subducting african lithosphere beneath the hellenic and cyprus trenches aided by slab pull on the southeastern side of the subducting arabian plate along the makran subduction zone we further suggest that the separation of arabia from africa is a response to plate motions induced by active subduction

how much nubia eurasia convergence is accommodated by the ne end of the eastern betic shear zone se spain constraints from gps velocities this research was funded by the spanish ministry of economy and competitiveness research project cgl2011 30153 c02 02 university of alicante research project vigrob053 university of jaen paiuja 2019 2020 ceactierra and rnm282 research group of junta de andalucia

analysis of continental midcrustal strain localization induced by microfracturing and reaction softening 1 low angle extensional shear zones which often characterize the brittle ductile transition of the continental crust are seen here to result from strain localization the potentially destabilizing deformation mechanism is assumed to be the progressive transformation of fractured coarse feldspar grains into white mica as observed in the east tenda shear zone alpine corsica the coupling between microfracturing and feldspar to mica reaction is coeval with strain localization that occurred in that field case at a depth close to 15 km this reaction is proposed as the main destabilizing factor responsible for the onset of localization with feldspar having a stationary dislocation creep flow stress larger than mica to test this hypothesis a rheological model is constructed based on the field observations for a mixture of three phases mica quartz and feldspar deforming at a common strain rate the phase concentrations change with time according to the feldspar to mica reaction which takes place only if feldspar grains are fractured a condition detected with the mohr coulomb criterion the tendency for the strain to localize is assessed by numerical means for the structure composed of an upper crust gliding rigidly over the lower crust which sustains an overall simple shear the onset of strain localization is defined by an increase of at least two orders of magnitude in strain rate over part of the lower crust the upper crust gliding velocity has to be increased by at least a factor of 5 for localization to occur the time lapse for this velocity change determines the depth of the shear zone 15 17 km the kinetics of the metamorphic reaction and the final amount of white mica control its width 1 4 km the time of the shear zone formation is less than half a million years