strain localization at the margins of strong lithospheric domains insights from analog models the lateral variation of the mechanical properties of continental lithosphere is an important factor controlling the localization of deformation and thus the deformation history and geometry of intraplate mountain belts a series of three layer lithospheric scale analog models with a strong domain sd embedded at various depths are presented to investigate the development of topography and deformation patterns by having lateral heterogeneities within a weak continental lithosphere the experiments performed at a constant velocity and under normal gravity indicate that the presence or absence of the sd controls whether deformation is localized or distributed at a lithospheric scale deformation and topography localize above the edges of the sd while the sd region itself is characterized by minor amounts of surficial deformation and topography the depth of the sd within the ductile crust ductile mantle lithosphere or both controls the pattern of deformation and thus the topography the presence of a sd in the ductile crust or in the mantle results in limited surficial topographic effects but large variations in the moho topography strong moho deflection occurs when the sd is in the ductile crust while the moho remains almost flat when the sd is in the mantle when the sd occupies the ductile lithosphere the sd is tilted these analog experiments provide insights into intraplate strain localization and could in particular explain the topography around the tarim basin a lithospheric scale heterogeneity north of the india asia collision zone

numerical modelling of arc continent collision application to taiwan two dimensional finite element numerical modelling is applied to study the deformation and failure of the overriding plate during arc continent collision continental margin subduction this plate has elasto plastic rheology with strain weakening and contains a volcanic arc with thinned and weakened lithosphere the overriding plate deforms due to the normal and tangential stresses applied along the interplate surface these boundary conditions represent the friction and pressure between the plates the latter is due to the flexural rigidity of the subducting lithosphere and the buoyancy force generated by the subducting continental crust the modelling shows that continental margin subduction results in increasing compression and failure of the overriding plate which occurs along the surface dipping under the arc in either of two possible directions the failure mode is largely controlled by the two competitive factors the rigidity of the subducting plate and thickness gradient of the subducted continental crust a high rigidity favors failure along an ocean vergent fault which is followed by a subduction reversal while a high thickness gradient favors failure in the opposite direction which is followed by a fore arc block underthrusting beneath the arc both scenarios seem to have natural analogs we consider one of them the ongoing arc continent collision in taiwan and argue that this process occurs according to the second scenario corresponding to the fore arc underthrusting wavelet statistical analysis of the seismicity distribution to the south of taiwan has clearly displayed a shallow 0 40 km zone of high density coherently distributed seismicity beneath the luzon arc this zone interpreted as a lithospheric scale fault dips from the forearc basin to the east and corresponds to the initiation of the forearc block subduction a self consistent combined mechanical gravity topography model is used to see whether failure of the overriding plate to the south of taiwan can be captured by this model by tuning diverent controlling parameters we did not succeed in obtaining realistic topography and gravity field in a model where failure of the overriding plate was not allowed introduction of this failure and underthrusting of the forearc block under the luzon arc allowed us to fit both topography and gravity data 2000 elsevier science b v all rights reserved

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean example of the western pyrenees 1 in this paper we describe the tectonosedimentary evolution and its subsequent inversion of a basin that underwent extreme crustal thinning in a transtensional setting ahead of a propagating ocean in the western pyrenees the labourd mauleon area situated in the western pyrenees at the termination of the v shaped bay of biscay is an ideal natural laboratory to study how such complex basins evolve in time and space because of a mild inversion of the basin during pyrenean compression the rift structures and their relations to basement rocks and sediments are exposed and can be directly studied in the field the basin shows a complex polyphase evolution that starts with left lateral dominated transtension in latest jurassic early aptian time this event is overprinted by a late aptian early albian extension that is related to the counterclockwise rotation of iberia away from europe leading to the opening of the bay of biscay during this stage the late triassic to jurassic carbonate platform was stretched salt migrated and detachment faults exhumed upper and lower crustal and mantle rocks to the seafloor the final structure of the basin resembles a sag basin floored by exhumed rocks overlain by extensional allochthons and compartmentalized by n40 to n60 transfer faults the sedimentary architecture is characterized by late aptian synrift sediments e g urgonian limestones that were deposited in fault bounded basins and are overlain by thick latest aptian to albo cenomanian sediments e g flysch noir that define a sag sequence the complex tectonosedimentary evolution of the basin is associated with salt tectonics and overprinted by a major magmatic thermal event that postdates mantle exhumation

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

the pyrenean architecture as revealed by teleseismic p to s converted waves recorded along two dense transects between 2011 and 2013 two dense transects were deployed across the central and western pyrenees to get better constraints on the deep lithospheric architecture and discriminate the competing models of the structure and formation of the pyrenees each transect recorded the regional and global seismicity during a period of approximately 1 yr here we exploit the records of teleseismic compressional waves and of their conversions to shear waves on internal discontinuities in order to map lithospheric interfaces beneath the two transects the migrated sections obtained by performing common conversion point stacks are in remarkable agreement with the results of the ecors pyrenees and ecors arzacq deep seismic surveys however the migrations of converted waves reveal new details of the deep lithospheric architecture that could not be seen with the active source experiments the new images provide clear and definite evidence for the subduction of a thinned iberian crust down to at least 70 km depth a result that has important implications for the formation of the pyrenees the subduction of the iberian lithosphere leads to reconsider the amount of convergence between iberia and europe during the cenozoic a recent regional p wave tomography relying on the data of the pyrope and iberarray temporary experiments revealed the segmentation of lithospheric structures by inherited hercynian ne sw transfer faults that were reactivated during the albian rifting our migration images are consistent with this model and give further support to the idea that the pyrenees were produced by the tectonic inversion of a segmented hyperextended rift that was buried by subduction beneath the european plate

the role of the bay of biscay mesozoic extensional structure in the configuration of the pyrenean orogen constraints from the marconi deep seismic reflection survey 1 seismic interpretation of the marconi deep seismic survey enables recognition of the upper crustal structure of the eastern part of the bay of biscay and the main features of its alpine geodynamic evolution the new data denotes that two domains with different pyrenean and north foreland structures exist in the bay of biscay in the eastern or basque parentis domain the north pyrenean front is located close to the spanish coast and the northern foreland of the pyrenees is constituted by a continental crust thinned by a north dipping fault that induced the formation of the early cretaceous parentis basin in the western or cantabrian domain the north pyrenean front is shifted to the north and deforms a narrower and deeper foreland basin which lies on the top of a transitional crust formed from the exhumation of lithospheric mantle along a south dipping extensional low angle fault during the early cretaceous the transition between these two domains corresponds to a soft transfer zone linking the shifted north pyrenean fronts and a north to wnw directed thrust that places the continental crust of the landes plateau over the transitional crust of the bay of biscay abyssal plain comparison between this structure and regional data enables characterization of the extensional rift system developed between iberia and eurasia during the late jurassic and cretaceous and recognizes that this rift system controlled not only the location and features of the pyrenean thrust sheets but also the overall structure of this orogen

direct response of small non glaciated headwater catchments to late quaternary climate change the valle de la fueva southern pyrenees abstract topographic depressions in mountain regions function as sediment traps and their morpho sedimentary archives provide important insight into erosion and sediment dynamics in mountain ranges in order to establish the direct drivers and timing of erosion phases in the southern central pyrenees small non glaciated headwater catchments in the highly erodible valle de la fueva drainage basin were investigated as to their morphogenetic development geomorphic mapping revealed three morphogenetic complexes associated with the valle de la fueva drainage basin development i high lying talus plateaus and lateral slopes ii extensive glacis and pedimentation surfaces and iii deeply incised ravines with late stage fluvial cut terraces the talus slopes are the oldest preserved geomorphic units of the valle de la fueva and potentially denote the initial drainage basin topography talus bedrock and boulder exposure ages cluster yet between 21 and 40 10be ka indicating that the carbonate dominated talus surfaces were rejuvenated during the last glacial cycle mis 4 mis 2 the younger morphogenetic complexes ii iii point to successive basin erosion stages from diffusive surface pedimentation under cold climate conditions ii to rapid linear fluvial dissection and late stage terrace formation iii denoting the abandonment of pediments as actively eroding surfaces 10be depth profiles of glacis cover sediments yielded consistent late mis 2 exposure ages the abandoning of pediments marks the onset of rapid fluvial downcutting of deep ravines iii around the holocene climate transition which involved substantial sediment routing and basin scale erosion lower fluvial cut terraces denote the termination of intense catchment erosion at around 14 8 10be ka and minor fluvial incision during middle late holocene this research shows that small non glaciated mountain catchments feature valuable high resolution morpho sedimentary archives that are useful for assessing the direct impact of major climate transitions on surface processes and landscape transformation in mountain regions improvements in quantifying the timing and rates of paleo surface processes are vital for enhancing regional glaciofluvial chronologies and large scale basin erosion models that are possibly afflicted with time averaging biases

a new hybrid framework for simulating hypervelocity asteroid impacts and gravitational reaccumulation abstract we present a hybrid approach for simulating hypervelocity impacts onto asteroids the overall system response is separated into two stages based on their different characteristic timescales first the short timescale fragmentation phase is simulated using a modified version of the tonge ramesh material model implemented in a material point method framework then a consistent hand off to an n body gravity code is formulated to execute the long timescale gravitational reaccumulation calculation we demonstrate this hybrid approach by considering the 5 km s head on impact of a 1 21 km diameter basalt impactor on a 25 km diameter target asteroid the impact event resulted in the fragmentation but not complete disruption of the entire target a granular core is observed at the end of the fragmentation simulations which acts as a gravity well over which reaccumulation occurs in the n body simulations our results suggest that disruption thresholds for rocky asteroids are higher when energy dissipating mechanisms such as granular flow and pore collapse are included

control of lithosphere rheology on subduction polarity at initiation insights from 3d analogue modelling abstract a series of analogue experiments was used to explore how lateral variations in lithosphere rheology may result in a subduction polarity reversal lithosphere models made of sand and silicone putty were designed to represent a plate convergence setting varying laterally from continent continent to ocean continent several experiments were performed to test the effects of strength profile variations in the converging plates the modelling outcomes categorized into models that develop with and without a subduction polarity reversal while a subduction polarity reversal was the most prevalent experiment outcome the results suggest that upper mantle strength is the dominant factor in determining whether a subduction polarity shift occurs or not a weak upper mantle layer promotes obduction of the oceanic lithosphere in the western segment of the model in model iterations where the upper mantle of the continental lithosphere was stronger than the oceanic lithosphere a reversal in subduction polarity from north dipping in the east to south dipping in the west occurred the experiment design was inspired by the iberian eurasian plate convergence and may provide a simple regional scale explanation for the deformation patterns along the pyrenean and cantabrian orogen

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

plume like upper mantle instabilities drive subduction initiation 1 the control of upper mantle lithosphere interactions mli on compressional tectonics is not well resolved this applies to the role of mli in triggering of subduction initiation or lithosphere mantle lm downwellings we present results of thermo mechanically thermo dynamically coupled numerical experiments that are consistent with an array of recent geophysical constraints on lithosphere and upper mantle rheology and structure we demonstrate that mli can lead to initiation of continental lithosphere subduction inducing its spontaneous downthrusting to depths of 300 500 km upon plume impingement of the lithosphere this downthrusting is pre conditioned by rheological stratification of visco elasto plastic lithosphere and its free surface the subsequent evolution of the slab is governed by phase changes and its interactions with the surrounding mantle we demonstrate that the mode of mli is strongly affected by the lateral heterogeneities and the presence of suture zones in the lithosphere

new evidence of magmatic diapirs in the intermediate crust under the dead sea israel simultaneous inversion of p wave arrivals of 262 local earthquakes recorded in the region of the dead sea was carried out to model the velocity stratification in the underlying crust and uppermost mantle the resulting tomographic model delineates several large magmatic domes rising from the lower crust at depth of approximately 20 km and ascending more than 12 km into the overlying intermediate crust commonly the domal ascent took place along the boundary faults of the dead sea rift the domes are elliptical in shape measuring nearly 5 15 km in diameter and are spaced about 20 km apart earthquake foci are abundant around the diapirs but rare within them the occurrence of these magmatic diapirs under the dead sea implies anomalously high temperatures with metastable stratification in the lower crust and suggests magmatic upwelling

impact of arc continent collision on the conditions of burial and exhumation of uhp lt rocks experimental and numerical modelling abstract a 2 d physical and finite element numerical modelling of arc continent collision was performed to study the deformation and failure of the overriding lithosphere the experimental technique allowed us to model the whole subduction collision process from oceanic subduction to deep subduction of the continental crust with the numerical approach we have modelled the deformation of the overriding plate only through initial stages of its failure and studied the influence of different parameters on this process the results obtained by both techniques are coherent and mutually complementary they show that the failure of the overriding plate is physically quite plausible or even inevitable during subduction the conditions for such a failure the weakening of this plate are prepared during oceanic subduction the weakening occurs due to the interaction between the subducting lithosphere and the asthenosphere in the mantle corner between the two plates and due to back arc spreading in oceanic subduction zones with a compressional regime no back arc opening thick and strong back arc lithosphere the weakest zone is volcanic arc area when weakening becomes sufficient during subduction the lithosphere fails in this area the failure occurs along a fault dipping under the arc in either of two possible directions and results either in subduction reversal or subduction of the fore arc almost half of the presently active subduction zones are characterised by a tensional subduction regime with back arc spreading in such subduction zones the weakest place is not the volcanic arc but the back arc spreading centre when a subduction regime changes from tensional to compressional failure occurs in the vicinity of the extinct spreading centre this process can occur during oceanic subduction again along either a trench vergent or trenchward dipping fault but the formation of a trench verging fault is most likely in this latter case which is a principal subject of our study the failure is followed by partial subduction of the arc plate complete subduction occurs during arc continent collision subduction of the continental margin when tectonic compression of the lithosphere increases rapidly and becomes sufficient to push the arc plate into the mantle the arc itself can be subducted completely or be partially or completely scraped off and accreted a deeply subducted material including continental margin is preserved at relatively low temperatures between the lithospheric mantle and the cold subducted arc plate to about 150 km depth subduction of the arc plate is a major phenomenon which affects all processes associated with continental subduction from deep burial and hp lt metamorphism to exhumation of subducted material does this process occur in nature future investigations will allow us to answer this question in this paper we analyse the conditions of emplacement of a very young oceanic lithosphere samail ophiolite on the continental crust in oman in the late cretaceous and argue that this lithosphere formed in a back arc basin it reached and overthrust the arabian continent after complete subduction of the arc plate

lithospheric scale structures from the perspective of analogue continental collision abstract analogue models were employed to investigate continental collision addressing the roles of 1 a suture zone separating different crustal blocks 2 mid crustal weak layers and 3 mantle strengths these models confirmed that low amplitude lithospheric and crustal buckling is the primary response to shortening with a wavelength mainly controlled by the strong upper crust and or lithospheric mantle with increasing shortening bivergent thrusts formed across the brittle layers equivalent to upper crust at inflection points of the buckles while ductile layers lower crust and mantle continued being folded the resulting geometries displayed pop ups and pop downs above the boundaries sutures between blocks further shortening led to wider thrust bounded deformation zones belts in front of the stronger blocks identified as effective indenters during shortening upward extrusion into belts allowed exhumation of deep material weak layers enhanced strain localisation these modelling results have similarity in large scale structures of modern orogens and are relevant to the understanding of natural collisional processes

seismic tomography of the earth s mantle the analysis of information contained in seismograms is the most promis ing tool for retrieving the three dimensional structure of the earth s mantle and thereby improving our understanding of its dynamics the procedure through which three dimensional images of different parts of the earth s interior arc obtained by inversion of a set of observables measured on the surface of the earth is commonly termed tomography by analogy with the medical procedure with which it shares many aspects of methodology but with much less favorable experimental conditions ideally one would like to be able to model entire seismograms over a wide frequency band for a large number of uniformly distributed sources and stations in order to obtain an optimal illumination of the inhomogeneities appropriate for tomographic inversion in practice a number of difficulties and limitations arise onc is the nonuniform distribution of sources and receivers seis micity is concentrated at plate boundaries and the distribution of con tinental and island areas leaves large portions of the oceans unin strumented another difficulty is our limited ability to accurately forward model the effects of wave propagation in a realistic earth which presents departures from a spherically symmetric referencc model of both elastic anc1astic and isotropic anisotropic nature this is why until now the simplest kind of tomography travel time tomography has prevailed relying on simplifying assumptions concerning the interaction of asphericity with propagating seismic waves ray theory and the laws of geometrical optics in their

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

analogue modelling of continental collision influence of plate coupling on mantle lithosphere subduction crustal deformation and surface topography abstract the role of the plate boundary in a continental collisional setting is investigated by lithospheric scale analogue models key variables in this study are the degree of coupling at the plate interface and along the moho of the lower plate as well as the geometry of the plate contact they control the onset of intra plate deformation orogenic architecture amount of mantle lithosphere subduction and basin development in all experiments deformation initiates at the plate interface by the formation of a pop up structure a vertical plate boundary with respect to the shortening direction results in buckling of the lithosphere whereas experiments with an inclined plate boundary show underthrusting and foreland basin development without orogenic wedge formation continental collision and coinciding mantle lithosphere subduction may occur only if the lower crust of the foreland plate is weak enough promoting crust mantle decoupling during decoupling the weak lower crust beneath the orogen thickens significantly by ductile flow as it detaches from the down going mantle lithosphere this lower crustal thickening effects the distribution of upper crustal deformation and topography subduction of weak lower crust is favored when the weak plate interface has a significant thickness 15 km in nature and a high amount of shortening is applied increasing coupling at the plate interface through time leads to intra plate deformation by thickening and gentle folding and influences surface uplift and subsidence above the plate interface the transition from a mechanically decoupled plate boundary with a significant amount of mantle lithosphere subduction towards stronger plate coupling resulting in intra plate deformation and topography development can be recorded in for instance the caucasus the colombian cordillera the pyrenees and the alps thickening of the lower crust as portrayed for the western alps does not demand a strong frictional type behavior of the lower crust but can also be the consequence of ductile processes

the northern red sea and the dead sea rift abstract the results of geophysical measurements at sea when combined with mesostructural measurements on land demonstrate systematic extensional features in the region of the northern red sea and the dead sea rift rifts and diapirs were encountered in the northern red sea showing an arcuate trend whereas the faulted continental margins at that region are oriented nw se the rifts and the diapirs changed their trend northwards from nnw sse to n s systems of rifts and diapirs are considered to be extensional features therefore the tectonic extensional axis probably changed its orientation accordingly a series of rifts and diapirs was found also in the gulf of elat trending approximately parallel to the coastlines and is interpreted to be the northern extension of the red sea rift and diapir system mesostructural measurements that were carried out in the western flank of the dead sea rift indicated further evidence for structural extension trending e w to ene wsw in addition to measurements of left lateral offset of approximately 10 km all these data when combined with the geophysical measurements of the characteristics of the earth crust underneath the dead sea rift suggest a tectonic regime of oblique extension in the region of the red sea and the dead sea rift this tectonic regime indicates that the red sea spreading center extends northwards along the dead sea rift and the clockwise change of the axial trend imposed a sinistral offset to the tectonic extension thus it is suggested that the dead sea rift is an incipient oblique spreading center

lithospheric memory state of stress and rheology neotectonic controls on europe s intraplate continental topography abstract to date research on neotectonics and related continental topography development has mostly focused on active plate boundaries characterized by generally high deformation rates the intraplate sedimentary basins and rifts of the northern alpine foreland are associated with a much higher level of neotectonic activity than hitherto assumed seismicity and stress indicator data combined with geodetic and geomorphologic observations demonstrate that europe s intraplate lithosphere is being actively deformed this has major implications for the assessment of its natural hazards and environmental degradation the lithosphere of the northern alpine foreland has undergone a polyphase evolution with an intense interplay between upper mantle thermal perturbations and stress induced intraplate deformation that points to the importance of lithospheric folding of the thermally weakened lithosphere in this paper we address relationships between deeper lithospheric processes neotectonics and surface processes in the northern alpine foreland with special emphasis on tectonically induced topography the objectives are to quantify the effects of ongoing alpine collision and atlantic ridge push on the intraplate deformation in europe and its impact on topography evolution and related natural hazards this paper reviews the four dimensional topographic evolution of the european lithosphere through a multi disciplinary approach linking geology geophysics and geotechnology until now research on neotectonics and related topography development of intraplate regions has received little attention our study examines a number of selected natural laboratories in continental europe from orogen through platform to continental margin these natural laboratories include the carpathians pannonian system the northwest european platform iberia and the atlantic continental margin we focus on lithosphere memory and neotectonics with special attention to the thermo mechanical structure of the lithosphere mechanisms of large scale intraplate deformation late neogene anomalies in subsidence and uplift and links with surface processes and topography evolution

subduction related structures in the north iberian margin the oblique convergence of eurasia and iberia since the early cretaceous caused the formation of the pyrenean intracontinental collisional orogen in the east and progressed to continent ocean collision with subduction of the bay of biscay oceanic crust beneath the north iberian margin in the west two deep multichannel seismic profiles iam 12 and escin 4 integrated with gravity modeling and other geological and geophysical data provide the crustal scale architecture of this margin and its tectonic evolution during the convergence the north iberian margin is modeled with a south or south southeast dipping oceanic crust beneath the outer part of the continental shelf mesozoic basins on the shelf were inverted during the tertiary and compressional activity continued until recent times in the escin 4 section while a shallower probably neogene age basin is subjected to active recent erosion in the iam 12 section in the oceanic areas a marginal trough deepens and widens toward the east as a result of the regional east dip of the oceanic basement the accretionary prism increases in size from west to east 18 56 km and its internal structure and morphology varies along strike the prism is buried by postcorivergence sediments in both sections and in the iam 12 section appears to have been active at least during lutetian to burdigalian times the crustal scale structure of the north iberian margin is that of an arrested subduction zone in which a remnant oceanic basin was being consumed near two continental plates that collided obliquely

subduction initiation along the inherited weakness zone at the edge of a slab insights from numerical models summary a preexisting weakness zone in the lithosphere is required to initiate subduction here we focus on a new type of weakness zone a subduction transform edge propagator step fault which is inherited from a tear along the edge of a slab using coupled thermal mechanical models we show that step fault perpendicular convergence results in a dipping shear zone in any tectonic setting at a continental margin this shear zone dips towards the continent which is an excellent starting condition for ocean continent subduction if far field convergence persists step faults become new subduction boundaries the trench moves landward during the earliest stages of convergence when slab pull becomes a dominant driving force after 80 km convergence trench roll back commences the initial geometry and mechanical properties of the sub crustal step fault zone affect the results subduction initiation is facilitated by a wide 100 km and low viscosity weakness zone incipient subduction is easier for young oceanic lithosphere due to its lower flexural rigidity and is insensitive to the far field convergence rate as step faults are commonly associated with young oceanic lithosphere subduction initiation is thus relatively easy along them of particular interest are continent ocean margins where step faulting has occurred

rupture creep lamellae and crocodiles happenings in the continental crust from the observation of certain seismic reflectivity patterns in phanerozoic continental crusts a relationship is suggested between 1 rupture and seismicity generally restricted to the upper crust 2 crocodile tectonics a kind of vertical indentation observed by diverging plane reflectors preferably in the middle crust 3 creep and the formation of reflecting lamellae in the lower crust based on these observations and on viscosity depth estimates a conceptual model for the above mentioned structures is presented which is also compatible with observations in older crusts boundary conditions are given for rupture as well as for lamellae forming creep

mantle exhumation crustal denudation and gravity tectonics during cretaceous rifting in the pyrenean realm sw europe insights from the geological setting of the lherzolite bodies 1 the pyrenean peridotites lherzolites form numerous small bodies of subcontinental mantle a few meters to 3 km across exposed within the narrow north pyrenean zone npz of mesozoic sediments paralleling the north pyrenean fault recent studies have shown that mantle exhumation occurred along the future npz during the formation of the albian cenomanian pyrenean basins in relation with detachment tectonics this paper reviews the geological setting of the pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the bearn region only two types of geological settings have to be distinguished among the pyrenean ultramafic bodies in the first type sedimented type or s type the lherzolites occur as clasts of various sizes ranging from millimetric grains to hectometric olistoliths within monogenic or polymictic debris flow deposits of cretaceous age reworking mesozoic sediments in dominant proportions as observed around the lherz body in the second type tectonic type or t type the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed triassic rocks and have fault contacts with units of deformed jurassic and lower cretaceous sediments belonging to the cover of the npz these deformed mesozoic formations are not older that the aptian early albian they are unconformably overlain by the albian cenomanian flysch formations and have experienced high temperature low pressure mid cretaceous metamorphism at variable grades such a tectonic setting characterizes most of the lherzolite bodies exposed in the western pyrenees these geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the mesozoic cover in the preorogenic evolution of the pyrenean realm in the light of such evidence a simple model of basin development can be inferred involving extreme thinning of the crust and mantle uprising along a major detachment fault we demonstrate coeval development of a crust mantle detachment fault and generalized gravitational sliding of the mesozoic cover along low angle faults involving triassic salt deposits as a tectonic sole this model accounts for the basic characteristics of the precollisional rift evolution in the pyrenean realm

analogue modelling of the initiation of subduction summary we report the results of experiments on the initiation of subduction using stratified analogue models in a large centrifuge where the experiments are driven only by the enhanced gravity of the centrifuge without the effect of external lateral stresses the scaled density of the stratified components resembles that of the asthenosphere and the continental and oceanic lithospheres the experiments demonstrate that under the effect of enhanced gravity the layers simulating the oceanic lithosphere detach from the front of the continental lithosphere and plunge under it pushing the more pliable asthenosphere downwards simultaneously the continental lithosphere is thrust over the downgoing slab where friction is low the continental lithosphere extends considerably so that the ductile continental lithosphere is exposed in some places the rate of thrusting of the experimental continental slab over the oceanic one as well as the amount of extension of the overriding slab and the extent of the rollback of the subduction zone that follows the initial lithospheric detachment are controlled by friction and density differences between the subducting and the overthrust slabs the analogue experiments emphasize the role of lateral density variations in incipient subduction and the effect of differential seismic friction along the subduction plane on the evolution of subduction zones their shape and the evolution of their backarc basins the morphological resemblance of the experimental results to various subduction systems seems to support their applicability to real subduction systems

styles of post subduction collisional orogeny influence of convergence velocity crustal rheology and radiogenic heat production abstract a number of models of continental collision already exist but the role of foregoing oceanic plate subduction or variable coupling between plates still remains to be explored in addition heat generation by radioactive decay may be quite variable for example our geochemical data from low to high grade metamorphic rocks of the lesser and higher himalayan sequences reveal measured heat production values on the order 4 5 w m 3 which is considerably higher than normal inferred upper crustal values 3 therefore we performed 2d numerical experiments of oceanic continental subduction followed by continental collision for varying continental lower crustal strength convergence velocity and radiogenic heating rate oceanic subduction produces thinning and upwarping of the overriding lithospheric wedge such as is observed in andean type collisional settings cascadia and central andes after collision two main orogenic styles develop depending on the values of key parameters either a slow convergence rates weak shear heating and consequently strong rheological coupling between plates and or a weak lower crust generate a two sided thick and narrow symmetrical collisional zone or b high convergence rates strong shear heating and therefore weak rheological coupling between plates and a strong lower crust produce a one sided thin and broad asymmetrical collisional zone in this case a vertical lithospheric body divides two upper crustal wedges that develop distinct deformation patterns in both cases enhanced radiogenic heating favors the formation of midcrustal partially melted channels that propagate toward the surface and the pro foreland partial melting triggers lateral extrusion of metamorphic rocks from different depths 15 23 kbar 700 c on top of coherent lp lt units the geometry of metamorphic isograds inverted shapes of p t paths at different grades and exhumation rates are similar to those observed in parts of the himalayan chain indicating that the models capture the essence of real systems the model results demonstrate that internal heat production rate is a crucial parameter in determining the timing of slab breakoff polarity reversal and exhumation of the inverted metamorphic sequence with respect to the timing of partial melting late stages of some numerical experiments showed interesting geodynamical evolution of the collisional zone with the appearance of polarity reversal roll over of both plates and roll back of the subducting slab irrespective of these details hundreds of km of continental lithosphere are recycled into the mantle in all models supporting the idea that continental crustal consumption is indeed a common and considerable process during collision this affects inferences about extents of convergence and or shortening during orogenesis moreover crustal recycling into the deep mantle during collisional orogeny needs to be considered in models of crustal growth and evolution

switches in subduction direction and the lateral termination of mountain belts pyrenees cantabrian transition spain the switch in subduction polarity at the transition from the pyrenees to the cantabrians implies an abrupt change in the gross direction of plate movement the pyrenees formed in response to the northward subduction of continental crust beneath europe during the palaeogene whereas the cantabrians developed during the southward subduction of biscay oceanic crust beneath spain as yet no attempt has been made to explain the mechanism and geometry of this apparent switch in plate kinematics the transition from northward to southward subduction occurs across a discrete lineament the pamplona fault oriented transversely with respect to the long axis of the adjacent mountain belts regional mapping reveals a 180 switch in the facing direction of thrusts and associated folds across the pamplona fault balanced structural cross sections constructed either side of the pamplona fault exhibit marked contrasts in bulk shortening depth to detachment gross structural geometry and the thickness and completeness of the pre orogenic mesozoic succession the pyrenean cantabrian transition zone is thus envisioned as a fundamental crustal lineament of pre orogenic origin a new model for the cenozoic tectonic evolution of north spain has the pamplona fault separating a series of transtensional basins in the pyrenees from a passive margin in the cantabrians subsequent plate collision between spain and europe reactivated the faults that controlled pre orogenic 9syn rift9 sedimentation in the present pyrenean area in the cantabrians however plate collision signalled a shift from a passive margin to an active margin consuming biscay oceanic crust beneath a northern spain subduction zone an implication of this model is that the foreland crust was affected variably by 9backarc9 heating during the early stages of spain europe collision this is expressed by the changing depth of the ebro basin the foreland basin that developed in response to pyrenean cantabrian orogenesis whose thickness of the tertiary succession varies laterally according to changes in flexural rigidity

the tectonics of the timna region in southern israel and the evolution of the dead sea rift abstract the timna region is located in the western margin of the southern arava valley at a junction between two regional orthogonal fault systems the dead sea rift and the temed fault the dead sea rift trends n s and extends from the northern red sea to southern anatolia and the temed fault trends e w from the gulf of suez to the arava valley structural analysis of the timna region showed the effects of two tectonic phases that were active there since the miocene the first tectonic phase formed the dextral strike slip temed fault and two systems of secondary faults trending nw se and ne sw the second tectonic phase downfaulted the rift floor and uplifted the rift shoulders simultaneously forming a regional n s trending joint system along the rift shoulder subsequent sinistral strike slip faulting along the rift caused a drag on the eastern part of the timna region and the resulting 2 5 km northward displacement reactivated pre existing faults and formed several rifts and uplifted blocks a correlation of the structural data of the timna region with recently acquired geophysical structural and stratigraphic findings from various places along the dead sea rift indicates that the first tectonic phase in timna is the erythrean phase this phase formed systems of e w trending faults in the negev and northern sinai as well as nw se trending faults in saudi arabia jordan israel and egypt this tectonic phase was active in the miocene and is associated with the first evolutionary phase of the red sea spreading center during this phase the spreading center extended northwards towards the suez rift the second tectonic phase in timna is the levantine phase that formed the dead sea rift and its uplifted shoulders this phase has been active since the pleistocene and is associated with the second tectonic phase of the red sea spreading the structural and geophysical data suggest that the dead sea rift is an incipient spreading center extending from the red sea whereas the suez rift was not reactivated during this phase the change of trend of the northern red sea spreading center from nw se to n s deformed the east shoulder of the southern dead sea rift so that a limited sinistral strike slip displacement occurred recent seismic activity indicates that the evolution of the dead sea rift is presently active

gravity influenced brittle ductile deformation and growth faulting in the lithosphere during collision results from laboratory experiments 1 because of the competition between brittle and ductile rheologies and their interplay with tectonic and buoyancy forces lithospheric deformation results in very contrasting styles in continental collision especially with unconfined boundaries deformation can be either homogeneously distributed or localized on complex fault patterns and different deformation modes such as contraction extension and strike slip interfere using scaled lithospheric analog experiments made of dry sand silicone putties and dense honey we investigate the mechanical parameters that control the deformation style in colliding systems with a particular focus on the roles of buoyancy and brittle ductile coupling the analysis of tens of experiments shows that the principal deformation features depend on two main parameters a brittle to ductile strength ratio g which controls deformation localization at the largest scale and a buoyancy to strength ratio ar which fixes the relative amount of contractional extensional and strike slip structures strain localization occurs only for g larger than a critical value 0 5 and the range of g values over which the transition from nonlocalized to localized deformation occurs is small the three main deformation regimes contraction strike slip and extension which coexist in most of the collision experiments occur in relative proportions that depend mainly on ar and on the nature of the boundary conditions

seismic evidence of alpine crustal thickening and wedging from the western pyrenees to the cantabrian mountains north iberia 1 the alpine collision between the iberian and european plates resulted in a complex crustal structure beneath the northern iberian peninsula as revealed from a new set of seismic refraction wide angle reflection profiles the study area is characterized by two major e w ranges the cantabrian mountains and the pyrenees which are relayed to the south by the iberian chain important variations in crustal thickness and velocity distribution are found in a 560 km long e w transect in contrast to the typical 30 to 32 km thick european variscan crust of the western end of the profile a continuous alpine crustal root is evidenced from the cantabrian mountains to the central pyrenees with a moho depth of 46 48 km locally rising to 40 km depth in between beneath the basque cantabrian basin another outstanding feature is the inferred presence of high velocities of 6 40 6 75 km s at midcrustal depths which can be associated with portions of a lower crustal wedge from the northern european domain indenting the southern iberian crust during the alpine stage of compression this indentation produces the delamination of the iberian crust with northward underthrusting of its lower half and the consequent crustal thickening the indenting wedge has a discontinuous presence along the longitudinal section as it was controlled and or affected by n s to ne sw structures further evidence of the northward subduction of the iberian plate is provided by another profile toward the aquitaine basin while a n s profile across the iberian chain reveals an alpine midcrustal thickening beneath this belt with moho depths of 42 km

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

crustal structure and orogenic material budget in the west central pyrenees surface and subsurface data are combined to construct a crustal scale cross section of the western central pyrenees france and spain at the boundary between the european and iberian plates the position of moho reflections in the ecors arzacq reflection profile suggests a tectonic wedge of european crust and upper mantle had indented the iberian plate at lower crustal levels the european wedge is overlain by an upper iberian wedge thus constituting a double stacked wedge geometry the upper wedge was delaminated and deformed giving rise to the pyrenean orogenic prism manifested as a bivergent fan in upper crustal levels the underthrust lower iberian plate has been imaged to depths of 55 60 km but crustal budget considerations based on a palinspastic reconstruction require that this continental root subducted to depths up to 90 km total orogenic contraction calculated from surface structures is about 75 80 km which was accomplished between the latest cretaceous and the early miocene at an averaged rate of 1 2 mm yr consistent with these moderate values exhumation of the orogen is much less than in the more shortened eastern parts of the range paleozoic basement and preorogenic mesozoic rocks are little eroded and much of the exhumation involved cannibalization of early foreland basins which once covered the entire poorly emergent orogen at this transect this resulted in a continuous process of sediment recycling and coupled with a considerable lateral arrival of material a bulk negative erosion sedimentation budget