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

seismic imaging of crust beneath the dharwar craton india from ambient noise and teleseismic receiver function modelling we use cross correlation of continuous 18 months 2009 february to 2010 august ambient noise data recorded over 35 broad band seismographs in the archean dharwar craton and the adjoining granulite terrain to generate rayleigh wave group velocity maps in the period 5 28 s this is supplemented with longer period data 40 70 s from earthquake source combined group velocity measurement was inverted jointly with the teleseismic receiver functions obtained at 50 stations includes 15 stations operated during 1998 2002 to produce shear velocity image of the crust the velocity image reveals thinner crust 34 38 km in the late archean 2 7 ga eastern dharwar craton edc while all other terrains mid archean and proterozoic have crustal thickness from 40 to over 50 km the mid archean 3 36 ga greenstone belt of the western dharwar craton wdc has the thickest crust 50 km the average crustal vs beneath the edc is 3 70 3 78 km s 1 as compared to 3 80 3 95 km s 1 beneath the wdc we observe significant lateral variation in the thickness of lower crust vs 3 8 4 2 km s 1 10 15 km in the edc compared to 20 30 km in the wdc the lowermost part of the crust vs 4 0 km s 1 is thin 5 km beneath the edc in contrast to more thickness 10 27 km beneath the wdc our analysis suggests intermediate composition for the crust beneath the edc similar to those for other cratons in contrast the mid archean exposed wdc crust has more mafic composition and exceptional thickness a scenario at variance with the global observations we interpret this thick mafic crust to represent undeformed geological segment of 3 36 ga the edc with a nearly flat moho felsic to intermediate composition of crust and thin basal layer may represent a reworked terrain during the late archean

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

crustal structure beneath the rif cordillera north morocco from the rifsis wide angle reflection seismic experiment the different geodynamic models proposed since the late 1990s to account for the complex evolution of the gibraltar arc system lack definite constraints on the crustal structure of the rif orogen here we present the first well resolved p wave velocity crustal models of the rif cordillera and its southern continuation toward the atlas made using controlled source seismic data two 300 km long wide angle reflection profiles crossed the rif along ns and ew trends the profiles recorded simultaneously five land explosions of 1tn each using 850 high frequency seismometers the crustal structure revealed from 2 d forward modeling delineates a complex laterally varying crustal structure below the rif domains the most surprising feature seen on both profiles is a 50 km deep crustal root localized beneath the external rif to the east the crust thins rapidly by 20 km across the nekkor fault indicating that the fault is a crustal scale feature on the ns profile the crust thins more gradually to 40 km thickness beneath middle atlas and 42 km beneath the betics these new seismic results are in overall agreement with regional trends of bouguer gravity and are consistent with recent receiver function estimates of crustal thickness the complex crustal structure of the rif orogen in the gibraltar arc is a consequence of the miocene collision between the iberian and african plates both the abrupt change in crustal thickness at the nekkor fault and the unexpectedly deep rif crustal root can be attributed to interaction of the subducting alboran slab with the north african passive margin at late oligocene early miocene times

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

a possible lower crustal flow channel in the middle urals based on reflection seismic data a possible lower crustal flow channel in the middle urals based on reflection seismic data

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

understanding the kinematic evolution and genesis of a back arc continental sag basin the neogene evolution of the transylvanian basin abstract formation and evolution of back arc basins is related to the interplay between subduction and convergence velocities during the formation of an orogenic chain sedimentary basins lacking fault systems that are genetically associated can be locally observed in the hinterland of the main orogenic belt in the usual location of a back arc basin these are interesting features potentially driven by a number of contrasting mechanisms such as lithospheric traction forces driven by plate boundary processes lateral migration of lower crustal material during orogenesis or sub lithospheric processes such as the rapid retreat of subducted slabs the transylvanian basin is a miocene basin that evolved at the rear of the main carpathians arc during the subduction of a slab attached to stable europe the pannonian basin is a typical continental extensional back arc domain to the east transylvania accommodated up to 3 5 km of middle upper miocene sediments deposited in a period coeval with the shortening in the neighbouring carpathians the basin fill was subsequently exhumed and partly eroded when the carpathians collided with the european craton in late miocene times this was associated with contractional deformation and widespread salt diapirism that forms the bulk of miocene deformation observed inside the basin our study shows that no significant normal faults are present inside the basin that could have accommodated 3 5 km subsidence in terms of upper crustal extension furthermore the basin kinematics cannot be linked directly with any other mechanical process explaining the observed miocene subsidence therefore we discuss several mechanisms that may explain regional scale observations and we conclude that the miocene transylvanian basin most likely formed in response to an asymmetric simple shear extension assisted by erosion and or lower crustal flow the upper crustal effects were localized in the neighbouring pannonian basin

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

sediment loading on the western platform of the new zealand continent implications for the strength of a continental margin abstract mechanical properties of the new zealand lithosphere are determined by forward modelling the gravity and isostatic response of a plio pleistocene sediment loaded shelf margin the giant foreset formation on the western platform of new zealand is a 2 km thick 250 km long and 100 km wide sediment package both the downward flexed pliocene miocene boundary beneath the package and a 35 50 mgal peak to trough gravity anomaly over the sediment package are best matched with a flexural rigidity in the region of 5 6 10 22 1 5 10 23 n m effective elastic thickness t e 20 28 km the giant foresets are a particularly important load to model because unlike most sediment loaded passive margins there is a time gap of about 70 80 ma between rifting and subsequent deposition of the sedimentary load furthermore the width of the giant foreset load is sufficiently small to afford a reasonable resolution of t e

evidence for slab rollback in westernmost mediterranean from improved upper mantle imaging this work is funded by the picasso project nsf grant ear 0808939 the deployment and data processing for spanish stations was funded by consolider ingenio 2010 project topo iberia csd2006 00041 as well as alert es cgl2010 19803 c03 02

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

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

lithospheric pressure depth relationship in compressive regions of thickened crust we investigate the pressure distribution with depth in regions undergoing horizontal shortening and experiencing crustal thickening both analytically and numerically our results show that in a convergent tectonic setting pressure can be considerably higher than lithostatic the pressure resulting from the weight of the overburden increases in pressure with respect to lithostatic conditions result from both the contribution of horizontal stresses and the flexural vertical loads the latter generated by the deflection of the upper crust and of the mantle because of the presence of topographic relief and a root respectively the contribution of horizontal stresses is particularly relevant to the upper crust and uppermost mantle where rocks are thought to deform brittlely in these domains pressure gradients twice lithostatic can be achieved the contribution of horizontal stresses is less important in the ductile domains as diverential stresses are progressively relaxed nevertheless the evects are still noteworthy especially close to the brittle ductile transition flexural vertical loads generated by the deflection of the upper crust and lithospheric mantle are relevant for rocks of the weaker lower crust as a result of the combination of the two mechanisms the pressure gradient varies vertically through the lithosphere ranging from negative inverted gradients to gradients up to several times the lithostatic gradient the pressure values range from one to two times the lithostatic values 1rgz to 2rgz

lower crustal flow in an extensional setting constraints from the halloran hills region eastern mojave desert california postextensional uplift and tilting of quaternary lake sediments in the halloran hills eastern mojave desert california suggest that regional scale flow within the lower crust is occurring in response to tectonic loads created during upper crustal extension in late miocene time the concept that significant flow within the lower crust may occur for millions of years after the cessation of extension indicates that topographic gradients can be modified by regional scale flow within the lower crust and that direct constraints on lower crustal flow can be obtained by quantitative analysis of rates of postextensional surface tilting an analytic solution developed in this paper combines viscous flow in the lower crust flexural bending within the midcrust and isostasy into an invertible linear differential equation that describes the topographic response to upper crustal extension assuming that there was no topographic relief across the halloran hills region prior to extension inversion of modern topographic data indicates that crustal thinning above the halloran hills detachment increases westward with a listric geometry and that little crustal thinning has occurred east of the western flank of clark mountain in good agreement with the known geometry of the fault and the location of its breakaway zone incorporation of quaternary tilting data indicates that the modern viscosity beneath the halloran hills assuming a 10 km lower crustal channel is a maximum of 1019 pa s and that the viscosity of the lower crust has decreased by at least 1 to 2 orders of magnitude since 8 ma this corresponds to a temperature increase of at least 75 to 100 c at the moho in our opinion the most likely source of this temperature increase is a regional scale thermal event within the underlying mantle and diffusion of heat upward into the lower crust if correct these results have important implications for the way in which crustal extension is linked to mantle heating within the basin and range province

western mediterranean subcontinental mantle emplacement by continental margin obduction obduction is the tectonic process that results from thrusting of an oceanic lithosphere section ophiolite onto a continent in contrast thrusting of subcontinental mantle observed in several mountain belts remains a major unknown of plate tectonics in the western mediterranean the ronda and beni bousera peridotites are the largest worldwide subcontinental mantle exposure from a geological point of view the ronda peridotites are exhumed by hyperstretching of the continental lithosphere in a back arc immediately followed by thrusting explaining their present day position inside the alboran crust using 2 d and 3 d modeling of new gravimetric data combined with local seismic tomography we show that the ronda peridotites are rooted inside the alboran mantle along the entire gibraltar arc on these bases we propose that the emplacement of the ronda peridotites occurred in a back arc setting and corresponds to the thrusting of an entire hyper stretched continental margin onto a continent a process that we define as continental margin obduction this results from two successive deformation events continental upper plate extension driven by slab roll back immediately followed by upper plate shortening likely triggered when a buoyant continental domain enters the subduction we propose that this process affected the entire western alboran domain with peridotite bodies embedded within the crust along the whole gibraltar arc we suggest that other examples such as the alps ivrea mantle body could likely represent continental margin obduction at the onset of continental collision

conditions for flow in the continental crust 1 earthquake focal depth distributions and effective elastic thickness variations on the continents suggest that in old cold regions such as shields the lower continental crust can be stronger than the upper mantle and imply that the upper mantle can flow even when the lower crust does not however in other places substantial flow in the lower crust does occur particularly near extensional metamorphic core complexes we suggest that the lower crust only flows when its viscosity is significantly reduced either by heating from igneous intrusions or by the addition of water in both cases the important characteristics of the flow are a consequence of the limited time over which the viscosity reduction occurs the timescale is controlled either by conductive cooling of intrusions or by the separation of melt from its matrix which removes the water

prograding coastal facies associations in the vryheid formation permian at effingham quarries near durban south africa abstract this paper describes and interprets a flat lying sandstone siltstone sequence 70 m thick in three disused quarries the beds comprise the lowest part of the vryheid formation middle ecca in the durban vicinity the sequence is conveniently divisible into two parts the lower division constitutes a prograding beach barrier association while the upper one represents a back barrier lagoonal complex fourteen sedimentary facies are described and interpreted to represent a range of depositional environments including open water shelf silts sandy shoreface and littoral deposits organic rich muds and peats of lagoonal origin a tidal inlet washover fans and a fluvial channel sand among the conclusions reached are that the local middle ecca coastline extended in a northwest to southeast direction and that progradation was towards the southwest that the coastline was microtidal and that stormy conditions were common with prevalent palaeowinds from the northwest the absence of invertebrate body fossils in these strata is attributed to penecontemporaneous solution of shelly remains this is the first time that a coastal sequence has been identified on the southeast margin of the main karoo basin of south africa

the nature of crustal boundaries combined interpretation of wide angle and normal incidence seismic data abstract after a few seconds two way traveltime normal incidence seismic reflection sections are composed mainly of assemblages of short reflections very rarely are seen continuous reflections that might correspond to the moho or a mid crustal discontinuity the inferred continuity of these boundaries has traditionally come from refraction seismology there is now a body of high quality coincident wide angle and normal incidence seismic data that have been recorded with 50 100 m shot spacing and with high frequency sources e g mobil babel the complexity and characteristics of the wide angle arrivals seen on these data suggest that they do not originate from continuous boundaries it is suggested that these arrivals are reflections from the same assemblage of short length reflectors that are responsible for normal incidence reflections seismic velocities below the middle crust may 1 1 change corresponding to normal incidence reflectivity or 2 2 generally increase with depth with localised sills or lens structures of different velocity accounting for the observed reflections wide angle arrivals that have traditionally been identified as reflections from crustal boundaries e g the mid crust and moho and which were considered indicative of a sharp velocity discontinuity from continuous boundaries may instead result from a concentration of lamellae

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

the mechanics of continental extension and sedimentary basin formation a simple shear pure shear flexural cantilever model abstract deep seismic reflection data and earthquake seismology show the fundamental importance of major crustal faults during extension of continental lithosphere controlling the development of rifted sedimentary basins major basement faults are generally planar and extend down to 10 15 km depth corresponding to the base of the seismogenic layer beneath this depth deformation gives way to distributed ductile shear in the lower crust and lithospheric mantle a mathematical model is described of the geometric thermal and flexural isostatic response of the lithosphere to extension by planar faulting of the upper crust and plastic distributed deformation in the lower crust and mantle during faulting upper crustal footwall and hanging wall blocks behave as two mutually self supporting flexural cantilevers their response to isostatic forces induced by extension generates footwall uplift and hanging wall collapse for extension on multiple faults interference of footwall uplift and hanging wall collapse gives rise to the familiar block rotations of rift tectonics the flexural cantilever model is able to predict crustal structure and sedimentary basin geometry for faults of arbitrary spacing horizontal displacement and polarity the effects of fault spacing fault polarity and the amount of extension as well as erosion of footwall uplift on crustal structure and basin geometry during both syn rift and post rift stages of basin evolution are explored models of passive rifting involving lithosphere extension in response to the build up of far field tectonic forces and decompression induced partial melting of the upper asthenosphere and lower lithosphere appear to conform to the main geological observations the role of active rifting driven by locally derived deviatoric tensional forces generated by mantle plumes remains uncertain

thermomechanical structure of european continental lithosphere constraints from rheological profiles and eet estimates summary the eet equivalent elastic thickness of the lithosphere is a measure of the integrated lithospheric strength it is directly related to the mechanical thickness and rheology of the crustal and mantle lithosphere we present a comparison of eet estimates and strength profiles based on the extrapolation of rock mechanics data for different parts of the european and eurasian continental lithosphere we discuss the temporal and spatial variations in the mechanical thickness and strength inferred from data for precambrian segments of europe s lithosphere variscan europe and the alpine collision belt this analysis demonstrates important spatial and temporal variations in lithospheric rigidity for orogenic belts and sedimentary basins in eastern and western europe and asian parts of eurasia the eet estimates based on synthetic rheological profiles constrained by newly available geophysical data are consistent with the estimates of eet derived from flexural studies of sedimentary basins forelands and orogenic belts these rheological profiles suggest weakening of the major parts of the european and eurasian continental lithosphere by decoupling of the crustal and upper mantle parts a comparison with the seismicity depth distribution for some selected sites suggests that the intra plate seismicity is essentially restricted to the upper crustal parts of europe s lithosphere providing additional support to the notion of the decoupled lithosphere the presence of intra plate stress fields can explain a significant part of the observed variations in eet estimates within individual thermotectonic age groups a comparison of wavelengths of crustal and lithospheric folding with observations shows these wavelengths to be consistent with estimates of eet inferred from the rheological response of basins and orogens at more moderate levels of intra plate stress

does the great glen fault really disrupt moho and upper mantle structure many previous interpretations of deep seismic reflection profiles across the great glen strike slip fault system have postulated that the fault penetrates the crust and upper mantle as a vertical plane that sharply offsets the moho discontinuity such an interpretation has become a general feature of more recent regional syntheses of the deep geology of the british isles reprocessing of portions of four profiles across the fault improves the resolution of lower crustal and upper mantle structure and calls into question the moho step interpretation diffraction analysis and seismic migration applied to reflection data north of shetland across the northward continuation of the great glen fault walls boundary fault indicate a narrow synform or keel developed on the moho directly beneath the fault this keel is itself underlain by a possible mantle reflection that is cut off by the downward projection of the fault north and west of ireland analyses of amplitudes frequencies and the geometrical behavior of reflections upon migration show that structures previously interpreted as moho steps may be better explained as distinct packages of reflectivity that are uppermost mantle or possibly out of the plane in origin in one location north of ireland where some three dimensional control exists the vertically downward projection of the great glen fault intersects without disrupting dipping structure in the upper mantle this observation leads to a model for displacement on the fault system in which motion is laterally transferred along a dipping ramp or blind thrust in the uppermost mantle somewhat analogous to models developed for the san andreas fault that indicate displacement along the fault to be laterally offset within the middle crust one of the principal conclusions of this study that major vertical steps on the moho beneath the great glen fault are difficult to support from the available seismic data is consistent with theologically based studies which predict that moho topography such as vertical steps is unlikely to be preserved over long periods of geologic time

slab behaviour and its surface expression new insights from gravity modelling in the se carpathians abstract we use lithosphere scale gravity models to calculate gravity anomalies resulting from oceanic subduction continental collision slab steepening delamination and break off local isostasy was assumed for determining vertical movements caused by mass changes related to these tectonic processes our results show that subduction is accompanied by basin subsidence on the upper plate caused by the heavy lithospheric root of the subducting slab the basin evolution goes parallel with the slab evolution and shows considerable modifications when the processes at depth change slab steepening delamination break off characteristic gravity anomaly curves were acquired for the different tectonic scenarios these curves together with other data e g basin evolution on the upper and the lower plate were used for the reconstruction of the tectonic evolution of the se carpathians which includes tertiary subduction and collision followed by slab steepening and delamination

moho offsets beneath the western ghat and the contact of archean crusts of dharwar craton india abstract we present the moho depth variation along a 600 km long profile from the west to the east coast of south india covering the passive continental margin and the western ghat escarpment created during india madagascar separation at 85 ma archean western and eastern dharwar craton and proterozoic basin the image is generated through three different approaches h vp vs stacking common conversion point ccp migration and inversion of teleseismic receiver functions at 38 locations the moho depth along the profile varies smoothly between 34 and 41 km except beneath the western ghat and at the contact of east and west dharwar craton where it is offset by up to 8 km the study suggests i the possible differential uplift of the western ghat as a consequence of india madagascar separation and the prominent role of deep crustal structure in the location of the escarpment compared to the surface process and ii presence of long lived steeply dipping fault separating the two distinct archean crustal blocks indicative of mechanically strong continental lithosphere beneath the dharwar craton

lower crustal flow and the role of shear in basin subsidence an example from the dead sea basin abstract we interpret large scale subsidence 5 6 km depth with little attendant brittle deformation in the southern dead sea basin a large pull apart basin along the dead sea transform plate boundary to indicate lower crustal thinning due to lower crustal flow along axis flow within the lower crust could be induced by the reduction of overburden pressure in the central dead sea basin where brittle extensional deformation is observed using a channel flow approximation we estimate that lower crustal flow would occur within the time frame of basin subsidence if the viscosity is 7 1019 1 1021 pa s a value compatible with the normal heat flow in the region lower crustal viscosity due to the strain rate associated with basin extension is estimated to be similar to or smaller than the viscosity required for a channel flow however the viscosity under the basin may be reduced to 5 1017 5 1019 pa s by the enhanced strain rate due to lateral shear along the transform plate boundary thus lower crustal flow facilitated by shear may be a viable mechanism to enlarge basins and modify other topographic features even in the absence of underlying thermal anomalies

sediment supply from the betic rif orogen to basins through neogene abstract we present a quantification of total and partial divided by time slices sedimentary volumes in the neogene basins of the betic rif orogen these basins include the alboran sea the intramontane basins the guadalquivir and rharb foreland basins and the atlantic margin of the gibraltar arc the total volume of neogene sediments deposited in these basins is 209 000 km3 and is equally distributed between the internal alboran basin and intramontane basins and the external basins foreland basins and atlantic margin the largest volumes are recorded by the alboran basin 89 600 km3 and the atlantic margin 81 600 km3 the guadalquivir and rharb basins amount 14 000 km3 and 14 550 km3 respectively whereas the intramontane basins record 9235 km3 calculated mean sediment accumulation rates for the early middle miocene show an outstanding asymmetry between the alboran basin 0 24 mm yr and the foreland basins 0 06 0 07 mm yr and the atlantic margin 0 03 mm yr during the late miocene sedimentation rates range between 0 17 and 0 18 mm yr recorded in the alboran basin and 0 04 mm yr in the intramontane basins in the pliocene quaternary the highest sedimentation rates are recorded in the atlantic margin reaching 0 22 mm yr sedimentary contribution shows similar values for the inner and outer basins with a generalized increase from late miocene to present from 3500 to 6500 km3 my interestingly the alboran basin records the maximum sedimentary contribution during the late miocene 5500 km3 my whereas the atlantic margin does during the pliocene quaternary 6600 km3 my the spatial and time variability of the sediment supply from the betic rif orogen to basins is closely related to the morphotectonic evolution of the region the high sedimentation rates obtained in the alboran basin during the early middle miocene are related to active extensional tectonics which produced narrow and deep basins in its western domain the highest sedimentary contribution in this basin as well as in the foreland and intramontane basins is recorded during the late miocene due to the uplift of wide areas of the betics and rif chains the analysis of the sedimentary supply also evidences strong relationships with the post tortonian crustal thickening and coeval topographic amplification that occurred in the central betics and rif with the concomitant evolution of the drainage network showing the fluvial capture of some internal basins by rivers draining to the atlantic ocean the ancestral guadalquivir

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

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

three dimensional mapping of seismic reflections from the crust and upper mantle northwest of scotland abstract a survey area 190 km by 110 km to the northwest of scotland on the uk continental shelf has been the site of intense exploration using the deep seismic reflection technique a closely spaced grid of deep reflection data totalling 1600 line km has been acquired by the birps group between 1981 and 1987 the majority of the data were recorded to 15 s two way travel time 50 km depth profiles with recording times up to 60 s 230 km depth were also acquired the survey includes a two ship synthetic aperture profile the slave line which synthesises a 0 16 km offset cdp profile the main features revealed and consistently imaged across the survey area by this combined dataset are an upper crust containing half graben style basins bounded to the west by planar faults a reflective lower crust and dipping and sub horizontal high amplitude reflections within the mantle three dimensional mapping of the dipping reflectors shows a complex package of reflections which updomes into the lower crust in the central area of the survey disrupting the moho and continuing into the uppermost mantle where it thins to a couplet of reflections in the north and south of the survey area this structure is observed only in the mantle to a depth of 80 km this dataset is unique in deep seismic reflection profiling in terms of the quality of the image returned the great depth to which reflections are imaged and the intensity of surveying of the lower crust and upper mantle the continuity and strength of the reflectors seen on these records show that even at the great depths at which these structures exist the seismic reflection technique is able to image them as high amplitude coherent and highly continuous reflections the reflections are interpreted as shear zones or faults within the lower crust and upper mantle this indicates that in this part of the world at least both the lower crust and upper mantle are able to sustain discrete zones of deformation

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

observation of present day tectonic motions in the southeastern carpathians results of the ises crc 461 gps measurements results are presented from gps measurements performed in the southeastern carpathians between 1997 and 2004 data from 25 stations observed during 13 campaigns were analyzed by the department of earth observation and space systems deos of delft university of technology the repeatabilities of the solutions are on the order of 1 4 mm for the horizontal and 4 8 mm for the vertical component the resulting velocity estimates have an uncertainty of b1 mm yr and b3 mm yr respectively the region southeast of the carpathian bend zone shows a horizontal movement towards sse of 2 5 mm yr while the transylvanian basin shows very small motions with respect to eurasia the vertical velocity field indicates the existence of uplift and subsidence domains in the se carpathians in good agreement with pliocene quaternary orogen and basin studies another 29 gps stations installed in the last 3 yr will generate a denser velocity field in the coming years for this region d 2005 elsevier b v all rights reserved

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

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

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