crustal versus mantle core complexes deep crustal and mantle rocks are exhumed in core complex mode of extension in three types of structures metamorphic core complexes oceanic core complexes and magma poor passive margins using available analogue and numerical models and their comparison with natural examples the present paper reviews the mechanical processes involved in these different types of extensional setting three main aspects are considered i the primary role of lithosphere rheology ii the lithosphere scale patterns of progressive deformation that lead to the exhumation of deep metamorphic or mantle rocks and iii the initiation and development of detachment zones crustal core complexes develop in continental lithospheres whose moho temperature is higher than 750 c with upper crust dominated strength profiles contrary to what is commonly believed it is argued from analogue and numerical models that detachments that accommodate exhumation of core complexes do not initiate at the onset of extension but in the course of progressive extension when the exhuming ductile crust reaches the surface in models convex upward detachments result from a rolling hinge process mantle core complexes develop in either the oceanic lithosphere at slow and ultra slow spreading ridges or in continental lithospheres whose initial moho temperature is lower than 750 c with sub moho mantle dominated strength profiles it is argued that the mechanism of mantle exhumation at passive margins is a nearly symmetrical necking process at lithosphere scale without major and permanent detachment except if strong strain localization could occur in the lithosphere mantle distributed crustal extension by upper crust faulting above a decollement along the ductile crust increases toward the rift axis up to crustal breakup mantle rocks exhume in the zone of crustal breakup accommodated by conjugate mantle shear zones that migrate with the rift axis during increasing extension

fault reactivation control on normal fault growth an experimental study field studies frequently emphasize how fault reactivation is involved in the deformation of the upper crust however this phenomenon is generally neglected except in inversion models in analogue and numerical models performed to study fault network growth using sand silicon analogue models we show how pre existing discontinuities can control the geometry and evolution of a younger fault network the models show that the reactivation of pre existing discontinuities and their orientation control i the evolution of the main fault orientation distribution through time ii the geometry of relay fault zones iii the geometry of small scale faulting and iv the geometry and location of fault controlled basins and depocenters these results are in good agreement with natural fault networks observed in both the gulf of suez and lake tanganyika they demonstrate that heterogeneities such as pre existing faults should be included in models designed to understand the behavior and the tectonic evolution of sedimentary basins

new models for evolution of magma poor rifted margins based on a review of data and concepts from west iberia and the alps direct observation and extensive sampling in ancient margins exposed in the alps combined with drill hole and geophysical data from the present day iberia margin result in new concepts for the strain evolution and near surface response to lithospheric rupturing at magma poor rifted margins this paper reviews data and tectonic concepts derived from these two margins and proposes that extension leading to thinning and final rupturing of the continental lithosphere is accommodated by three fault systems each of them characterized by a specific temporal and spatial evolution during rifting of the margin by its fault geometry and its surface response the data presented in this paper suggest that margin architecture and distribution of rift structures within the future margin are controlled first by inherited heterogeneities within the lithosphere leading to a contrasting behaviour of the future distal and proximal margins during an initial stage of rifting the place of final break up appears to be determined early in the evolution of the margin and occurs where the crust has been thinned during a first stage to less than 10 kilometres during final break up the rheology of the extending lithosphere is controlled by the thermal structure related to the rise of the asthenosphere and by serpentinization and magmatic processes

oceanic corrugated surfaces and the strength of the axial lithosphere at slow spreading ridges abstract we analyse the topography and gravity signature of 39 corrugated surfaces formed over the past 26 myrs in the footwall of axial detachment faults at the eastern southwest indian ridge these corrugated surfaces appear to have formed at a melt supply about half the global melt supply average for mid ocean ridges and we find that their presently elevated topography relative to adjacent non corrugated seafloor was mostly acquired at the end of their formation at the termination stage this configuration which also characterizes many off axis corrugated surfaces in other oceans suggests that the plate flexural rigidity was very low during the formation of the corrugated surface and increased significantly at the termination stage following buck 1988 we hypothesize that stresses related to bending of the plate cause internal deformation and damage in the footwall of the fault which is associated with weakening as a possible mechanism for enhanced footwall weakening while corrugated surfaces form we propose the formation of weak shear zones coated with hydrous minerals such as talc amphibole chlorite and serpentine in mantle derived ultramafics next to gabbro intrusions if this hypothesis is correct the amount of footwall weakening and roll over along axial detachment faults at slow spreading ridges may be controlled both by access to hydrothermal fluids in the footwall of the detachment and by the abundance and distribution of gabbros intrusions in exhumed ultramafics

eduction extension and exhumation of ultrahigh pressure rocks in metamorphic core complexes due to subduction initiation the controversy over the exhumation of ultrahigh pressure uhp rocks centers on whether it involves rising of pieces of crust detached from subducted continental lithosphere or an entire subducted plate that undergoes eduction i e reverse subduction we present a new thermomechanical model of continental subduction showing that these apparently contrasting mechanisms can occur together crust subducted deep enough is heated and weakened causing limited diapiric rise while crust subducted to shallower depths retains strength and is exhumed only by eduction the model also shows for the first time how eduction followed by seafloor spreading can occur in a zone of regional convergence this occurs spontaneously when subduction of buoyant crust causes a subduction zone to lock up in one place causing a new subduction zone to form in another the model is consistent with many features of the youngest region of uhp rock exhumation on earth the d entrecasteaux islands uhp exhumation and the amount of regional extension as well as the seismic structure around the islands can be explained by eduction ductile flow fabrics seen on the islands would result from exhumation of the most deeply subducted crust heated enough to undergo partial melting reversal of motion on the north dipping continental subduction zone required by this model was likely triggered by initiation of the new britain trench as suggested previously our model implies that the crust of goodenough basin south of the islands was exhumed by eduction in the last 5 ma and this hypothesis can be tested by drilling

continuous exhumation of mantle derived rocks at the southwest indian ridge for 11 million years the global mid ocean ridge system where tectonic plates diverge is traditionally thought of as the largest single volcanic feature on the earth yet wide expanses of smooth sea floor in the easternmost part of the southwest indian ridge in the indian ocean lacks the hummocky morphology that is typical for submarine volcanism at other slow spreading ridges the sea floor can extend by faulting the existing lithosphere along only one side of the ridge axis however the smooth sea floor in the easternmost southwest indian ridge also lacks the corrugated texture created by such faulting instead the sea floor is smooth on both sides of the ridge axis and is thought to be composed of altered mantle derived rocks here we use side scan sonar to image the sea floor and dredge samples to analyse the composition of two sections of the southwest indian ridge between 62 050 e and 64 400 e where the sea floor formed over the past 11 million years we show that the smooth floor is almost entirely composed of seawater altered mantle derived rocks that were brought to the surface by large detachment faults on both sides of the ridge axis faulting accommodates almost 100 of plate divergence and the detachment faults have repeatedly flipped polarity we suggest that this tectonic process could also explain the exhumation of mantle derived rocks at the magma poor margins of rifted continents

thermobaricity in the transition zones between alpha and beta oceans abstractthe role of the ocean in earth s climate is fundamentally influenced by the locally dominant stratifying property heat or salt which in turn can be used to categorize the ocean into three classes alpha beta and transition zone oceans alpha and beta oceans are regions where the stratification is permanently set by heat and salt respectively transition zone oceans exist between alpha and beta oceans and are regions where the stratification is seasonally or intermittently set by heat or salt despite their large ranges of temperature and salinity transition zone oceans are the most weakly stratified regions of the upper oceans making them ideal locations for thermobaric effects arising from the nonlinear equation of state of seawater here a novel definition and quantification of alpha beta and transition zone oceans is presented and used to analyze 4 years 2010 13 of hydrographic data developed from the argo profiling float array two types of thermobaric instabilities are defined and

deformation associated with the denudation of mantle derived rocks at the mid atlantic ridge 13 15 n the role of magmatic injections and hydrothermal alteration 1 outcrops of deeply derived ultramafic rocks and gabbros are widespread along slow spreading ridges where they are exposed in the footwall of detachment faults we report on the microstructural and petrological characteristics of a large number of samples from ultramafic exposures in the walls of the mid atlantic ridge mar axial valley at three distinct locations at lat 13 n and 14 45 n one of these locations corresponds to the footwall beneath a corrugated paleo fault surface bearing in mind that dredging and rov sampling may not preserve the most fragile lithologies fault gouges this study allows us to document a sequence of deformation and the magmatic and hydrothermal history recorded in the footwall within a few hundred meters of the axial detachment fault at the three sampled locations we find that tremolitic amphiboles have localized deformation in the ultramafic rocks prior to the onset of serpentinization we interpret these tremolites as hydrothermal alteration products after evolved gabbroic rocks intruded into the peridotites we also document two types of brittle deformation in the ultramafic rocks which we infer could produce the sustained low magnitude seismicity recorded at ridge axis detachment faults the first type of brittle deformation affects fresh peridotite and is associated with the injection of the evolved gabbroic melts and the second type affects serpentinized peridotites and is associated with the injection of si rich hydrothermal fluids that promote talc crystallization leading to strain localization in thin talc shear zones we also observed chlorite serpentine shear zones but did not identify samples with serpentine only shear zones although the proportion of magmatic injections in the ultramafic rocks is variable these characteristics are found at each investigated location and are therefore proposed as fundamental components of the deformation in the footwall of the detachment faults associated with denudation of mantle derived rocks at the mar

on the rotation and frictional lock up of normal faults explaining the dip distribution of normal fault earthquakes and resolving the low angle normal fault paradox abstract classical rock mechanics predicts that normal faults should form at 60 65 rotating to 30 40 before locking so widespread slip at around 20 is considered paradoxical furthermore the dip distribution of normal fault earthquakes has a distinct unexplained peak at 45 for both problems some combination of low friction high fluid pressures stress rotation and efficiency of low angle slip have been suggested but provide at best a partial solution a simple quantitative model for normal fault rotation iterating between slip on faults and distributed strain predicts that faults spend more time at lower angles combining this result with mohr coulomb analysis of the range where seismogenic normal faults should lock up predicts the dip distribution of seismogenic normal faults and matches both the range and 45 peak observed for normal fault earthquakes as even the lowest dips of normal fault ruptures fall within the limits of fault reactivation slip on seismogenic low angle normal faults is not paradoxical a new mohr griffith solution to the limits of fault reactivation extends the analysis into the top few km where low angle slip is best constrained by field observations and shows that low cohesion normal faults can remain active at

continental break up of the south china sea stalled by far field compression the outcome of decades of two dimensional modelling of lithosphere deformation under extension is that mechanical coupling between the continental crust and the underlying mantle controls how a continent breaks apart to form a new ocean however geological observations unequivocally show that continental break up propagates in the third dimension at rates that do not scale with the rate of opening here we perform three dimensional numerical simulations and compare them with observations from the south china sea to show that tectonic loading in the direction of propagation exerts a first order control on these propagation rates the simulations show that in the absence of compression in that direction continental break up propagates fast forming narrow continental margins independently of the coupling when compression is applied propagation stagnates forming v shaped oceanic basins and wide margins changes in out of plane loading therefore explain the alternation of fast propagation and relative stagnation these new dynamic constraints suggest that the west to east topographic gradient across the indochinese peninsula prevented continental break up propagation through the 1 000 km wide continental rift of the central and west basin of the south china sea until the direction of stretching changed 23 million years ago resulting in bypassing and acceleration of continental break up propagation tectonic loading in the direction of propagation exerts an important control on the propagation of continental break up according to three dimensional simulations of the south china sea

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

dependence of seismic coupling on normal fault style along the northern mid atlantic ridge while normal faults are essential in shaping the seafloor formed at slow spreading mid ocean ridges information on their behavior on short seismic cycle time scales is limited here we combine catalogs of hydro acoustically and teleseismically recorded earthquakes to characterize the state of seismic coupling along the northern mid atlantic ridge mar between 12 and 35on along this portion of the mar axis tectonic extension is either taken up by steep conjugate faults that outline well defined ridge parallel abyssal hills or dominantly by a large offset detachment fault on one side of the axis we investigate variations in seismicity and seismic moment release rates across thirty ridge sections that can be clearly characterized either as abyssal hill or detachment bearing we find that detachment bearing sections are associated with significantly greater seismicity and moment release rates than abyssal hill bearing sections but show variability that may reflect the along axis extent of individual detachment faults overall the measured seismic moment release rates fail to account for the long term fault slip rates this apparent seismic deficit could indicate a mixed mode of fault slip where earthquakes only account for 10 30 of offset build up at abyssal hill faults while the rest is accommodated by some form of transient aseismic creep we find this seismic coupling fraction to be significantly greater 40 60 at individual detachment systems which is somewhat at odds with the common inference that detachment faults can sustain long lived localized strain because they are weak we therefore propose alternative interpretations for seismic coupling based on dynamic friction theory this article is protected by copyright all rights reserved

estimation of subsurface temperature anomaly in the indian ocean during recent global surface warming hiatus from satellite measurements a support vector machine approach abstract estimating the thermal information in the subsurface and deeper ocean from satellite measurements over large basin wide scale is important but also challenging this paper proposes a support vector machine svm method to estimate subsurface temperature anomaly sta in the indian ocean from a suite of satellite remote sensing measurements including sea surface temperature anomaly ssta sea surface height anomaly ssha and sea surface salinity anomaly sssa the svm estimation of sta features the inclusion of in situ argo sta data for training and testing svm one of the most popular machine learning methods can well estimate the sta in the upper 1000 m of the indian ocean from satellite measurements of sea surface parameters ssta ssha and sssa as input attributes for svm the results based on the common svm application of support vector regression svr were validated for accuracy and reliability using the argo sta data both mse and r 2 for performance measures are improved after including sssa for svr mse decreased by 12 and r 2 increased by 11 on average the results showed that sssa in addition to ssta and ssha is a useful parameter that can help detect and describe the deeper ocean thermal structure as well as improve the sta estimation accuracy moreover our method can provide a useful technique for studying subsurface and deeper ocean thermal variability which has played an important role in recent global surface warming hiatus since 1998 from satellite measurements in large basin wide scale

the 3 d geometry of detachment faulting at mid ocean ridges seismic images of cretaceous slow spreading crust from the eastern central atlantic provide new constraints on the process of seafloor spreading and the importance of detachment faulting the seismic depth sections image detachment faults that appear to have exhumed footwall massifs of similar geometry to massifs of plutonic and mantle rocks mapped at the present mid atlantic ridge the detachments are consistent with the structure and microearthquakes of the mid atlantic ridge at 26 n with the footwall rotation inferred from paleomagnetic data and with numerical modeling of oceanic detachments other seismically imaged detachments have similar dimensions and geometry but are covered by a layer of small fault blocks the detachment types differ in whether or not the fault locks up in the subsurface probably controlled by the fault strength the elastic thickness and whether the exhumed footwall is partly covered by basalts toward the segment middle decreasing mantle serpentinization decreasing elastic thickness and thicker median valley basalts all increase the likelihood that the fault locks up and a new fault propagates upwards from the still active root zone transferring a slice of the hanging wall to the footwall to be rafted with the footwall out of the median valley as a result an oceanic detachment fault exhuming the footwall at a segment end to form an oceanic core complex may disappear laterally beneath rafted blocks detachment faulting may be more widespread at slow spreading ridges than interpreted from seafloor mapping

oceanic detachment faults generate compression in extension in extensional geologic systems such as mid ocean ridges deformation is typically accommodated by slip on normal faults where material is pulled apart under tension and stress is released by rupture during earthquakes and magmatic accretion however at slowly spreading mid ocean ridges where the tectonic plates move apart at rates 244 000 events detected along 25 km of the ridge axis to depths of 10 km below seafloor surprisingly the majority of these were reverse faulting events restricted to depths of 3 7 km below seafloor these reverse events delineate a band of intense compressional seismicity located adjacent to a zone of deeper extensional events this deformation pattern is consistent with flexural models of plate bending during lithospheric accretion our results indicate that the lower portion of the detachment footwall experiences compressive stresses and deforms internally as the fault rolls over to low angles before emerging at the seafloor these compressive stresses trigger reverse faulting even though the detachment itself is an extensional system

vertical deformation associated with normal fault systems evolved over coseismic postseismic and multiseismic periods vertical deformation of extensional provinces varies significantly and in seemingly contradictory ways sparse but robust geodetic seismic and geologic observations in the basin and range province of the western united states indicate that immediately after an earthquake vertical change primarily occurs as subsidence of the normal fault hanging wall a few decades later a 100 km wide zone is symmetrically uplifted the preserved topography of long term rifting shows bent and tilted footwall flanks rising high above deep basins we develop finite element models subjected to extensional and gravitational forces to study time varying deformation associated with normal faulting we replicate observations with a model that has a weak upper mantle overlain by a stronger lower crust and a breakable elastic upper crust a 60 dipping normal fault cuts through the upper crust and extends through the lower crust to simulate an underlying shear zone stretching the model under gravity demonstrates that asymmetric slip via collapse of the hanging wall is a natural consequence of coseismic deformation focused flow in the upper mantle imposed by deformation of the lower crust localizes uplift under the footwall the breakable upper crust is a necessary model feature to replicate footwall bending over the observed width 10 km which is predicted to take place within 1 2 decades after each large earthquake thus the best preserved topographic signature of rifting is expected to occur early in the postseismic period the relatively stronger lower crust in our models is necessary to replicate broader postseismic uplift that is observed geodetically in subsequent decades

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

flipping detachments the kinematics of ultraslow spreading ridges abstract although the seafloor spreading hess initially proposed was a virtually amagmatic process little attention has been paid to that possibility since we construct a kinematic framework for virtually amagmatic and magma poor hess style seafloor spreading and successfully apply it to processes operating at the southwest indian ridge swir the kinematic model is based on symmetric divergence about a rift axis at depth with a repeating cycle in which a fault propagates up from the rift axis develops into a detachment fault accommodating the plate divergence migrates beyond the rift axis and is abandoned when a new fault propagates up through the footwall from the rift axis we rigorously explore the controls on the depth dip and timing of fault initiation and abandonment and use the kinematic framework to reconstruct the evolution of smooth mantle dominated seafloor at the swir through symmetric divergence about a fixed rift axis the model predicts the development of successive detachments of flipping polarity as observed each rooting along a narrow and fixed rift axis at 20 km depth the base of the seismically defined brittle lithosphere the detachments root at 80 consistent with constraints on seismicity defined detachment orientation at oceanic core complexes and exhume mantle based on the continuity of basement ridges of magnetic anomalies and of the seismic activity at the base of the lithosphere it appears that these exhumation detachments transition laterally into rafting detachments transporting fault bounded volcanic slices up and away from the spreading axis to form the rougher volcanic seafloor found between mantle dominated domains the kinematic framework shows that increased magmatic divergence requires the detachments to root at shallower depths consistent with the seismicity defined shallowing of the base of the brittle lithosphere moving along the ridge axis towards the volcanic centres only in the immediate vicinity of volcanic centres where the seismicity dies out may magmatism dominate we conclude that detachment tectonics dominate the process of ultraslow seafloor spreading as well as much of slow seafloor spreading totalling about one third of the global ridge system and present the first 3d tectonic model for ultraslow seafloor spreading

the effects of lower crustal strength and preexisting midcrustal shear zones on the formation of continental core complexes and low angle normal faults to investigate the formation of core complexes and low angle normal faults we devise thermo mechanical simulations on a simplified wedge like orogenic hinterland that has initial topography moho relief and a preexisting mid crustal shear zone that can accommodate shear at very low angles 20 we mainly vary the strength of the lower crust and the frictional strength of the preexisting mid crustal shear zone we find that the strength of the lower crust and the existence and strength of a preexisting shear zone significantly affect the formation and evolution of core complexes with increasing lower crustal strength we recognize varying extensional features with decreasing exhumation rate these are characterized by bivergent metamorphic massifs classic cordilleran metamorphic core complexes multiple consecutive core complexes or boudinage structures and a flexural core complex underlined by a large subsurface low angle detachment fault with a small convex curvature topographic loading and mantle buoyancy forces together with divergent boundaries drive a regional lower crustal flow that leads to the exhumation of the lower crust where intensive upper crustal faulting induces strong unloading the detachment fault is a decoupling zone that accommodates large displacement and accumulates sustained shear strain at very low angle between upper and lower crust though the regional stress is largely andersonian we find non andersonian stress in regions adjacent to the preexisting shear zone and those with high topographic gradient our new models provide a view that is generally consistent with geological and geophysical observations on how core complexes form and evolve

formation of metamorphic core complexes in non over thickened continental crust a case study of liaodong peninsula east asia pre thickened hot orogenic crust is often considered a necessary condition for the formation of continental metamorphic core complexes mccs however the discovery of mccs in the liaodong peninsula where the crust has a normal thickness 35 km challenges the universality of this scenario therefore we implement a series of 2 d numerical thermo mechanical modeling experiments in which we investigate the conditions of mcc formation in normal crusts as well as the relationships between the underlying mechanisms and the syn rift basin evolution in these experiments we explore the impact of the lithostratigraphic and thermo rheological structure of the crust we also examine the lithosphere thickness strain softening extension rate and surface erosion sedimentation processes the experiments demonstrate that high thermal gradients and crustal heterogeneities result only in a symmetric spreading dome which is geometrically incompatible with the observations of the mccs in the liaodong peninsula according to our further findings the strain softening should play a key role in the development of asymmetric strain localization and domal topography uplift while synchronous surface erosion controls the polarity of the syn rift basin the synthetic model data are compatible with the geological observations and cooling history based on the thermo chronology for the eastern part of the east asia during the late mesozoic to the early cenozoic the model predicted p t t paths are essentially different from those inferred for the other known mccs confirming the exceptional character of the mcc formation in the wide rift system of the east asia

a deep seismic investigation of the flemish cap margin implications for the origin of deep reflectivity and evidence for asymmetric break up between newfoundland and iberia summary seismic reflection and refraction data were acquired along the southeast margin of flemish cap at a position conjugate to drilling and geophysical surveys across the galicia bank margin the data document first order asymmetry during final break up between newfoundland and iberia an abrupt necking profile of continental crust observed off flemish cap contrasts strongly with gradual tapering on the conjugate margin there is no evidence beneath flemish cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong s like reflection which indicates that this mode of extension occurred only on the galicia bank margin compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the flemish cap margin instead anomalously thin 3 4 km thick oceanic crust is observed this crust is highly tectonized and broken up by high angle normal faulting the thin crust and rift structures that resemble the abandoned spreading centre in the labrador sea suggest that initial seafloor spreading was affected by processes observed in present day ultra slow spreading environments landwards flemish cap is underlain by a highly reflective lower crust the reflectivity most likely originates from older palaeozoic orogenic structures that are unrelated to extension and break up tectonics

on spreading modes and magma supply at slow and ultraslow mid ocean ridges abstract the ultraslow eastern southwest indian ridge swir offers an opportunity to study the effect of magma supply on an ultraslow mid ocean ridge starting from quasi melt free detachment dominated spreading and transitioning to volcanic spreading as one nears prominent axial volcanos detachments in the quasi melt free mode extend along axis 60 to 95 km and have a lifetime of 0 6 to 1 5 myrs they cut into their predecessor s footwall with an opposite polarity causing part of the footwall lithosphere to experience further deformation hydrothermal alteration sparse magmatism and possibly thermal rejuvenation in a hanging wall position the accretion of the oceanic lithosphere in this context therefore occurs in two distinct stages over the lifetime of two successive detachment faults we examine the transition from this nearly amagmatic detachment dominated mode to the more common volcanic mode of spreading showing that it occurs along axis over distances 30 km it involves a significant thinning of the axial lithosphere and a gradual decrease of the amount of tectonic displacement on faults as the magmatic contribution to the divergence of the two plates increases we develop a conceptual model of this transition in which magma plays a double role it fills the space between the diverging plates thus reducing the need for displacement along faults and it modifies the thermal state and the rheology of the plate boundary affecting its thickness and its tectonic response to plate divergence based on a comparison of the ultraslow eastern swir with the faster spreading mid atlantic ridge we show that the activation of the volcanic or of the detachment dominated modes of spreading is connected with the volume of magma supplied per increment of plate separation over a range of axial lithosphere thickness and therefore over a range of the m ratio defined by buck et al 2005 as the relative contribution of magma and faults to plate divergence m is smaller for a given volume of melt per increment of plate separation if the plate is thicker we therefore propose that m does not fully explain the variability in faulting styles observed at slow and ultraslow ridges and propose that rheological changes induced by magma also play a key role melt itself is weak hydrothermally altered gabbro peridotite mixtures are weak and melt heat sustains more vigorous hydrothermal circulation resulting in contrasted potentials for strain localization footwall flexure on faults and the development of detachment faults

tectonic nature and seismic response of top basement detachment faults in magma poor rifted margins 1 top basement detachment faults tbdf represent exhumed segments of downward concave detachments accommodating tens of kilometers of offset tbdfs are key structures of magma poor rifted margins but are difficult to identify in seismic profiles because they coincide with top basement reflections we have constructed realistic seismic models of an alpine example which were used to simulate full marine seismic surveys and to identify the seismic fingerprint of tbdfs in cmp stacked and time migrated sections minor extensional allochthons which locally cap tbdfs give rise to pronounced diffractions which are in stark contrast to continuous reflections from uncapped segments of tbdfs in time migrated synthetic sections we find a continuous and constant amplitude reflection alternating with a laterally discontinuous reflection of variable amplitude these diagnostics were also identified in the lusigal 12 seismic profile imaging the hobby high detachment a tbdf in the iberia abyssal plain using these diagnostics small extensional allochthons and high angle faults cutting the hobby high detachment are observed in lusigal 12 and the entire sedimentary cover can be shown to be postrift

granite intrusion in a metamorphic core complex the example of the mykonos laccolith cyclades greece abstract the aegean domain is a well suited place to study the formation of metamorphic core complex mcc and to investigate the role of syn tectonic granites on their development in the northern cyclades the mykonos delos rhenia mcc is characterized by the intrusion of a kilometer scale late miocene pluton of i type granitoids within a migmatitic gneiss dome new combined ams anisotropy of magnetic susceptibility and microstructural studies on the mykonos granitoids together with recently published thermochronological data allow us to use the granitoids as strain markers the mykonos granitoids form a laccolith like intrusion with a n70 e long axis the laccolith is strongly asymmetric with an outlying root zone to the sw and a major body mainly developed to the ne the laccolith construction is due to successive pulses of more or less differentiated magma that intruded the cycladic blueschist unit the attitude of stretching markers suggests an important about 60 vertical axis local rotation phenomenon in the cycladic upper crust during the exhumation of the mykonos mcc structural data suggest a four stage evolution of the mykonos mcc i a first stage characterized by flat shearing toward the n ne and by the formation of a domal structure in migmatitic paragneisses with multi scale generation of folds with axes either perpendicular or parallel to the regional stretching as a result of the interplay between regional n20 e directed extension and ew shortening ii a second stage marked by the emplacement of the mykonos laccolith at 13 5 0 3 ma at the top of the migmatitic paragneisses iii the third stage corresponding to the development of protomylonitic foliations and lineations in the whole laccolith in high to medium temperature conditions and iv the late stage marked by an acceleration of the exhumation of the mykonos mcc this exhumation was accommodated by important rotations of upper crustal blocks during the end of the exhumation processes around 10 ma deformation localized at the top of the laccolith in semi ductile conditions and then in brittle conditions in the major detachment plane our study shows that the cycladic plutonism event had no role on the initiation of the mcc however the geometry of the mykonos intrusion supports that the magmas are sucked into the direction of regional extension and that the intrusion of magmas has caused an acceleration of the last stages of the mcc development this acceleration was marked by a very fast exhumation of the laccolith after its emplacement

fuji dome a large detachment fault near 64 e on the very slow spreading southwest indian ridge a continuous domed detachment surface fuji dome has been imaged on the very slow spreading southwest indian ridge using deep towed side scan sonar and has been investigated by manned submersible and sea surface geophysics the dome is morphologically similar to other oceanic detachments core complexes or mega mullions in addition to bathymetric mullions observed in ship borne bathymetry finer scale spreading parallel striations were imaged with the side scan on the detachment surface metabasalt crops out near the termination probably as part of a thin fault sliver gabbro and troctolite probably crop out near the summit of the dome the rest of the detachment surface is covered with sediment and rubble which is basaltic except for a single sample of serpentinite most of the detachment surface dips toward the ridge axis at 10 20 but near the breakaway it is strongly rotated outward and dips away from the axis at up to 40 normal undeformed volcanic seafloor crops out adjacent to the detachment modeling of sea surface magnetic data suggest the detachment was active from 1 95 ma for about 1 ma during a period of reduced and asymmetric magmatic accretion modeling of sea surface and seafloor gravity requires laterally fairly uniform but high density material under the dome and precludes steeply dipping contacts between bodies with large density contrasts at shallow levels under the dome

on the formation of oceanic detachment faults and their influence on intra oceanic subduction initiation 3d thermomechanical modeling abstract extensional detachment faults which have been widely documented in slow spreading and ultraslow spreading ridges on earth can effectively localize deformation due to their weakness after the onset of oceanic closure these weak oceanic detachments may directly control the nucleation of a subduction zone parallel to the former mid ocean ridge as is suggested for the neotethys in middle jurassic times so far this hypothesis has only been tested by 2d numerical models whereas the geometry of detachment faults is intrinsically three dimensional here we conducted a series of 3d numerical thermomechanical experiments in order to investigate the formation of detachment faults in slow oceanic spreading systems and their subsequent response upon inversion from oceanic spreading to convergence numerical results show that during the oceanic spreading stage the formation of detachment faults strongly depends on the magnitude of the healing rate of faulted rocks in the oceanic lithosphere that reflects the stability of hydrated minerals along fractured rocks the detachment faults formed in our 3d numerical models deviate from the rolling hinge model of oceanic detachment faulting where fault footwalls are rotated and oceanic core complexes are thereby formed our results accentuate that the controlling physical parameters for the development of oceanic core complexes and detachment faults can differ and that their coupled development in nature remains a key target for future research upon modeled transition to compression previously formed asymmetric spreading patterns are prone to asymmetric inversion where one oceanic plate thrusts under the other our results suggest that detachment faults accommodate significant amounts of shortening during the initiation of oceanic closure but in contrast to the previously proposed simple conceptual model no direct inversion of a single detachment fault into an incipient subduction zone is observed instead a widespread interaction of multiple detachment faults occurs after the onset of convergence ultimately the nascent subduction zone cuts through the base of several pre existing detachment faults thereby forming an initial accretionary wedge in the incipient fore arc

dynamics and structural development of metamorphic core complexes the development of metamorphic core complexes mccs in a thickened continental lithosphere is studied using fully coupled thermomechanical numerical code accounting for elastic brittle ductile properties of constituent rocks for mccs to develop the middle lower crust and the sub moho mantle are required to be weak enough to flow laterally so as to simultaneously feed the exhuming dome and enable the moho to keep a flat geometry the conditions are satisfied with initial moho temperatures of 800 c or higher with crustal thicknesses of 45 km or greater and with initial effective viscosities lower than 1020 pa s and 1022 pa s in the lower crust and the underlying mantle respectively a compositional mainly density anomaly with the properties of granite is placed centrally in the crust to localize strain at the onset of deformation during a first stage of upper crust necking the deformation pattern is relatively symmetrical and dominated by graben formation in the upper crust when the first ductile layers reach the surface a second stage of dome amplification and widening occurs dome amplification is accommodated by horizontal flow in the ductile crust giving an early symmetrical pattern of conjugate shear zones with no obvious detachment zone the system then rapidly becomes asymmetric with the localization of a detachment zone along one dome limb further accommodating dome widening thus the exhumation process of a metamorphic dome results in the progressive development of a detachment zone depending on initial moho temperature the detachment zone can migrate in space or die out and be replaced by a new one with an opposite dip

tectonic structure evolution and the nature of oceanic core complexes and their detachment fault zones 13 20 n and 13 30 n mid atlantic ridge microbathymetry data in situ observations and sampling along the 13 20 n and 13 20 n oceanic core complexes occs reveal mechanisms of detachment fault denudation at the seafloor links between tectonic extension and mass wasting and expose the nature of corrugations ubiquitous at occs in the initial stages of detachment faulting and high angle fault scarps show extensive mass wasting that reduces their slope flexural rotation further lowers scarp slope hinders mass wasting resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface extension and drag along the fault plane uplifts a wedge of hangingwall material apron the detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble while local slumping emplaces rubble ridges overlying corrugations the detachment fault zone is a set of anostomosed slip planes elongated in the along extension direction slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar fault planes with extension parallel stria are exposed along corrugation flanks where the rubble cover is shed detachment fault rocks are primarily basalt fault breccia at 13 20 n occ and gabbro and peridotite at 13 30 n demonstrating that brittle strain localization in shallow lithosphere form corrugations regardless of lithologies in the detachment zone finally faulting and volcanism dismember the 13 30 n occ with widespread present and past hydrothermal activity semenov fields while the irinovskoe hydrothermal field at the 13 20 n core complex suggests a magmatic source within the footwall these results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution

magmatism at the west iberia non volcanic rifted continental margin evidence from analyses of magnetic anomalies we discuss the magmatic development of the west iberia non volcanic rifted continental margin in the north atlantic ocean so called non volcanic rifted continental margins are characterized by a lack of syn rift magmatism and are considered to be largely amagmatic however this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously we concentrate our attention on the recently described zone of exhumed continental mantle zecm to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the zecm leading to the onset of seafloor spreading we use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide angle seismic experiments presented elsewhere forward and inverse modelling of a sea surface magnetic anomaly chart and of surface and deep towed magnetometer profiles shows that anomalies within the zecm differ in trend amplitude and source type from those in the adjacent oceanic crust and thinned continental crust the zecm anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin we interpret such bodies as syn extensional intrusions that increase in volume oceanward they eventually merge in the vicinity of a margin parallel basement peridotite ridge to give rise to a continuous crust that records reversals in the earth s magnetic field from the time of anomaly m4 n m5 r i e to mark the onset of seafloor spreading we find no evidence for anomalies formed by seafloor spreading at either slow or ultraslow rates before m5 r 128 ma

downward continued multichannel seismic refraction analysis of atlantis massif oceanic core complex 30 n mid atlantic ridge 1 detailed seismic refraction results show striking lateral and vertical variability of velocity structure within the atlantis massif oceanic core complex occ contrasting notably with its conjugate ridge flank multichannel seismic mcs data are downward continued using the synthetic on bottom experiment sobe method providing unprecedented detail in tomographic models of the p wave velocity structure to subseafloor depths of up to 1 5 km velocities can vary up to 3 km s over several hundred meters and unusually high velocities 5 km s are found immediately beneath the seafloor in key regions correlation within situand dredged rock samples video and records from submersible dives and a 1 415 km drill core allow us to infer dominant lithologies a high velocity body ies found to shoal near to the seafloor in multiple locations is interpreted as gabbro and is displaced along isochrons within the occ indicating a propagating magmatic source as the origin for this pluton s the western two thirds of the southern ridge is capped in serpentinite that may extend nearly to the base of our ray coverage the distribution of inferred serpentinite indicates that the gabbroic pluton s was emplaced into a dominantly peridotitic host rock presumably the mantle host rock was later altered via seawater penetration along the detachment zone which controlled development of the occ the asymmetric distribution of seismic velocities and morphology of atlantis massif are consistent with a detachment fault with a component of dip to the southeast the lowest velocities observed atop the eastern central dome and conjugate crust are most likely volcanics here an updated model of the magmatic and extensional faulting processes at atlantis massif is deduced from the seismic results contributing more generally to understanding the processes controlling the formation of heterogeneous lithosphere at slow rate spreading centers

metamorphic core complexes windows into the mechanics and rheology of the crust metamorphic core complexes are products of normal fault displacements sufficient to exhume rocks from below the brittle ductile transition these faults detachments may initiate within the brittle crust at steep angles but they sole into the ductile middle crust and during displacement rotate to gentler dips due to hanging wall extension the exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping low friction shallow segments of the detachment metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress strain localization and strain rate with decreasing temperature providing a window into physical conditions and deformational processes in the mid crust the metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault in addition to those accompanying exhumation these processes may include diapiric emplacement of gneiss domes or exhumation in a subduction channel and may not be directly related to formation of the core complex factors favouring core complex formation are high gravitational potential energy of the extending crust weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence

heterogeneous and asymmetric crustal accretion new constraints from multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 36 15 n at slow spreading mid ocean ridges crustal accretion style can vary significantly along and across ridge segments in magma poor regions seafloor spreading can be accommodated largely by tectonic processes however the internal structure and formation mechanism of such highly tectonized crust are not fully understood we analyze multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 35 40 n 36 40 n a section of the ridge that shows diverse accretion styles we identify volcanic tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain t and the fraction of magmatic accretion m estimated t values range from 0 2 0 4 on ridge segments to 0 6 0 8 at the rainbow nontransform discontinuity ntd at segment ends t is asymmetric reflecting asymmetries in accretion rate topography and faulting between inside and outside offset corners detachment faults have formed preferentially at inside corners where tectonic strain is higher we identify at least two oceanic core complexes on the fossil trace of the ntd in addition to the rainbow massif which occupies the offset today a gravity high and low magnetization suggest that the rainbow massif which hosts a high temperature hydrothermal system was uplifted by a west dipping detachment fault asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis which may have caused emplacement of magma into the footwall hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep penetrating second generation faults

azimuthal seismic anisotropy in a zone of exhumed continental mantle west iberia margin summary p wave azimuthal seismic anisotropy of the uppermost mantle has been shown to indicate directions of extension both recent and fossil in oceanic and continental settings we have determined the p wave anisotropy of a zone of exhumed continental mantle beneath the southern iberia abyssal plain as a possible indication of the initial direction of separation of iberia from north america in a region where there are no magnetic lineations or fracture zone lineations adjacent to the margin with which to constrain this direction over 10 000 airgun shots were recorded by 11 ocean bottom seismometers hydrophones from which 24 900 arrivals that had traversed unaltered weakly serpentinized upper mantle were picked complete 360 azimuthal coverage was attained between the shots and receivers for ranges up to 55 km p wave raypaths through the zone of exhumed continental mantle were ray traced through a velocity model that incorporated the 3 d relief of the seabed and acoustic basement time residuals with respect to this model were fitted using an azimuthally dependent function small anisotropy 7 per cent with a fast direction of 143 was determined to exist between 3 1 and 6 7 km below the top of the basement although the fast direction suggests that the post breakup mantle stretching between iberia and north america was northwest southeast this hypothesis is hard to reconcile with other observations instead we conclude that motion along concave downward faults during the exhumation of the upper mantle could have been sufficient to change an initially roughly east west margin normal fast direction to the 143 azimuth observed today the degree of serpentinization was estimated to decrease from 20 to 0 per cent between 3 and 7 km depth into the basement in the zone of exhumed continental mantle the degree of upper mantle anisotropy is similar to that observed in other oceanic and continental settings where similar degrees of serpentinization are inferred from mean p wave velocities

along strike upper plate deformation in response to metamorphic core complex emplacement se arizona abstract metamorphic core complexes represent concentrated zones of crustal extension universally the hanging walls of these systems are extensively fractured attesting to the significant horizontal extension above the evolving detachment zone recent analysis of a large grid of 2 d seismic reflection lines within the tucson basin of southeastern arizona has facilitated a nearly three dimensional interpretation of subsurface features related to cenozoic crustal extension within the northern tucson basin the catalina detachment fault dips 23 35 to the southwest from the western flank of the catalina rincon metamorphic core complex in the southern portion of the basin the ne trending santa rita normal fault dips 15 20 to the northwest from the western flank of the santa rita mountains and is cut by the catalina detachment beneath the central tucson basin the orientation of the santa rita fault is problematical in that its orientation is nearly perpendicular to extension directions in the region while geologic and seismic reflection evidence indicates that the catalina detachment and santa rita fault were active synchronously one possible explanation is that the santa rita fault is accommodating along strike upper plate deformation in response to core complex emplacement to test this we employed a finite element modeling approach a two dimensional model consisting of a homogenous elastic material undergoing uniform extension is used to study changes in stresses and displacements in proximity to a zone of weakness representing a detachment fault in the upper crust away from the detachment fault primary principal stresses are oriented parallel to the regional extension direction as expected near the detachment extension in the system produces a rotation of the primary principal stresses of nearly 60 with respect to the regional stress field around the end of the fault additionally the model predicts mechanical failure of the upper plate of the detachment system to the south of the catalina core complex this model supports our interpretation that the orientation and early displacement noted on the santa rita fault is the result of a perturbation in the regional stress field caused by the catalina detachment and the associated brittle failure of the upper plate from the extreme crustal extension associated with core complex emplacement

cooling history of atlantis bank oceanic core complex evidence for hydrothermal activity 2 6 ma off axis 1 we report 26 u th he zircon ages from atlantis bank southwest indian ridge which constrain time scales and rates of lower crustal cooling in ultraslow spreading oceanic crust in this setting samples from the detachment fault surface indicate that denuded oceanic crust cooled rapidly 1200 c ma consistent with existing models for the cooling of oceanic crust u th he zircon ages from samples collected along n s and e w trending faults scarps record young ages inconsistent with standard cooling models for lower oceanic crust these samples have a mean u th he zircon age 2 6 ma younger than their corresponding igneous crystallization ages and record cooling through 200 c well outside the rift valley similar anomalously young ages are recorded by zircon sphene and apatite fission track data from odp hole 735b we interpret these young ages as recording an off axis thermal heating event associated with localized high temperature 300 c hydrothermal fluid flow resulting from underplated mafic magmas

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

stacked gabbro units and intervening mantle a detailed look at a section of iodp leg 305 hole u1309d hole u1309d integrated ocean drilling program iodp legs 304 305 penetrated 1415 m into the seafloor of the atlantis massif an oceanic core complex at 30 n mid atlantic ridge more than 96 of all recovered rocks are gabbroic on the basis of a mineral chemical overview we suggest that between 800 and 1100 m below sea floor mbsf a magmatic unit occurs ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part below 1235 mbsf massive gabbronorites oxide gabbros were drilled and they may represent the roof of an underlying magmatic unit the focus here is on the zone where both units interact and screens totaling 50 m of a microstructurally distinct olivine rich troctolite occur we argue that the olivine rich troctolite is a former mantle rock which was converted to a crust mantle transition zone dunite at the base of the upper magmatic unit later as melts derived from the lower magmatic unit percolated through it it was equilibrated to a more evolved chemistry and transformed to a fine grained olivine rich troctolite our main arguments against a possible cumulate nature of the olivine rich troctolite are the lack of a systematic downhole trend in compatible elements within the olivine rich troctolite its distinctly fine grained microstructure the high cr content of cpx and its ni rich olivine composition the high nio for a given mg mg fe in the olivine rich troctolite can be modeled by simple equilibration of relict mantle olivine with a mildly evolved melt evidence for the percolation of evolved melts through the olivine rich troctolites are ti rich interstitial pyroxenes and as inclusions in cr spinel highly evolved amphiboles and orthopyroxenes plus the occurrence of millimeter scale noritic veins the percolation by evolved melts would also be the major difference to otherwise conceptually similar rocks from the ophiolitic crust mantle transition zone

magmatic tectonic conditions for hydrothermal venting on an ultraslow spread oceanic core complex hydrothermal venting an important cooling mechanism of the earth supports a diverse array of seafloor and sub seafloor ecosystems that are sustained by large thermal and chemical fluxes vents have been found along even the slowest and coldest spreading centers calling into question the driving heat source for these vents the ultraslow spreading mid cayman spreading center in the caribbean sea which hosts the axial flank von damm vent field vdvf provides an opportunity to probe the mechanisms for venting at ultraslow spreading rates using active source seismic data from the 2015 cayseis cayman seismic experiment we determined the seismic velocities in the large massif beneath the vdvf we propose that this massif was produced by a pulse of on axis magmatism at ca 2 ma which was then followed by exhumation cooling and fracturing a low seismic velocity anomaly 5 km below the vdvf is evidence for either a cracking front mining lithospheric heat or intrusive magmatic sills both of which could drive ongoing deep hydrothermal fluid circulation we conclude that the transient magmatism and variable crustal thickness at ultraslow spreading centers create conditions for long lived hydrothermal venting that may be widespread and other vdvf like vents may be common in these areas

controls on the thermomechanical evolution of hyperextended lithosphere at magma poor rifted margins the example of espirito santo and the kwanza basins high quality long offset seismic data from many distal rifted margins show evidence for hyper extended 10 km thick crust direct observation of such domains is challenging as they lie at great water depth buried beneath thick sedimentary sequences and formed by rock assemblages that are hydrated and geophysically indistinguishable only a few drill holes have penetrated basement at ultradistal rifted margins these observations together with outcrops of preserved analogs exposed in collisional orogens suggest that the complex interaction of detachment faults rooted in a subhorizontal shear zone in the hyperextended crust or in the serpentinized mantle controls the formation of the ocean continent transition while depth dependent thinning controls the early phases of rifting conforming to classical rift models we still have a superficial understanding of how normal faults and subhorizontal shear zones form and evolve during rifting and lithospheric breakup here we develop a rheological parameterization to simulate the formation of and slip on large offset normal faults rooted in growing brittle to ductile shear zones the evolution of these structures leads to the creation of a hyperextended crust and eventually exhumed serpentinized mantle we also propose a simplified formulation to simulate magmatic underplating and seafloor spreading the resulting numerical models provide a self consistent picture for the evolution of magma poor rifted margins from initiation of rifting to seafloor spreading the model results are compared with first order observations of the kwanza and espirito santo conjugate margins in the south atlantic as well as of magma poor margins globally

finite thickness of shear bands in frictional viscoplasticity and implications for lithosphere dynamics permanent deformations in the lithosphere can occur in the brittle as well as in the ductile domain for this reason the inclusion of viscous creep and frictional plastic deformation is essential for geodynamic models however most currently available models of frictional plasticity are rate independent and therefore do not incorporate an internal length scale which is an indispensible element for imposing a finite width of localized shear zones therefore in computations of localization either analytical or numerical a width of the shear zone is obtained which tends to zero in numerical computations this manifests itself in a severe mesh sensitivity moreover convergence of the global iterative procedure to solve the non linear processes is adversely affected which negatively affects the reliability and the quality of predictions the viscosity which is inherent in deformation processes in the lithosphere can in principle remedy this mesh sensitivity however elasto visco plastic models which are commonly used in geodynamics assume a series arrangement of rheological elements maxwell type approach which does not introduce an internal length scale here we confirm that a different rheological arrangement which put a damper in parallel to the plastic slider kelvin type approach introduces an internal length scale as a result pressure and strain and strain rate profiles across the shear bands converge to finite values upon decreasing the grid spacing we demonstrate that this holds for non associated plasticity with constant frictional properties and with material softening with respect to cohesion finally the introduction of kelvin type viscoplasticity also significantly improves the global convergence of non linear solvers