hierarchical segmentation of the malawi rift the influence of inherited lithospheric heterogeneity and kinematics in the evolution of continental rifts we used detailed analysis of shuttle radar topography mission digital elevation model and observations from aeromagnetic data to examine the influence of inherited lithospheric heterogeneity and kinematics in the segmentation of largely amagmatic continental rifts we focused on the cenozoic malawi rift which represents the southern extension of the western branch of the east african rift system this north trending rift traverses precambrian and paleozoic mesozoic structures of different orientations we found that the rift can be hierarchically divided into first order and second order segments in the first order segmentation we divided the rift into northern central and southern sections in its northern section the rift follows paleoproterozoic and neoproterozoic terrains with structural grain that favored the localization of extension within well developed border faults the central section occurs within mesoproterozoic neoproterozoic terrain with regional structures oblique to the rift extent we propose that the lack of inherited lithospheric heterogeneity favoring extension localization resulted in the development of the rift in this section as a shallow graben with undeveloped border faults in the southern section mesoproterozoic neoproterozoic rocks were reactivated and developed the border faults in the second order segmentation only observed in the northern section we divided the section into five segments that approximate four half grabens asymmetrical grabens with alternating polarities the change of polarity coincides with flip over full grabens occurring within overlap zones associated with 150 km long alternating border faults segments the inherited lithospheric heterogeneity played the major role in facilitating the segmentation of the malawi rift during its opening resulting from extension

upper limits of power law creep of rocks at depths below a few kilometers most rocks probably flow in the steady state dominantly by diffusion assisted dislocation creep mechanisms whereby strain rate is proportional to a low power of deviatoric stress at high stress levels for metals ceramics and rocks the power law breaks down plb to some other strain rate stress functional relationship and this paper explores four methods by which the breakdown stress b may be estimated for metals alloys and ceramics b 10 3 for non silicate rocks b 5 10 3 and for silicates b 10 2 for silicates the low stress exponent power law breaks down at high stresses to an exponential dependence of strain rate upon stress or to a dorn law the governing flow mechanisms generally being lattice resistance controlled dislocation glide and mechanical twinning plb breakdown stresses for silicates are nearly strain rate insensitive and extrapolated values of b are so high as to suggest that low temperature high stress plasticity may be suppressed entirely in geological deformations plb for olivine rich rocks is discussed in some detail and new three dimensional deformation mechanism surfaces for wet and dry olivine polycrystals are presented

the rift to break up evolution of the gulf of aden insights from 3d numerical lithospheric scale modelling abstract the gulf of aden provides an ideal setting to study oblique rifting since numerous structural data are available onshore and offshore recent surveys showed that the spatio temporal evolution of the gulf of aden rift system is dominated by three fault orientations displacement orthogonal wsw rift parallel wnw and an intermediate e w trend the oldest parts of the rift that are exposed onshore feature displacement orthogonal and intermediate directions whereas the subsequently active necking zone involves mainly rift parallel faults the final rift phase recorded at the distal margin is characterised by displacement orthogonal and intermediate fault orientations we investigate the evolution of the gulf of aden from rift initiation to break up by means of 3d numerical experiments on lithospheric scale we apply the finite element model slim3d which includes realistic elasto visco plastic rheology and a free surface despite recent advances 3d numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved we address this issue by proposing a simple post processing method that uses the surface stress tensor to evaluate stress regime extensional strike slip compressional and preferred fault azimuth the described method is applicable to any geodynamic model and easy to introduce our model reproduces the observed fault pattern of the gulf of aden and illustrates how multiple fault directions arise from the interaction of local and far field tectonic stresses in an evolving rift system the numerical simulations robustly feature intermediate faults during the initial rift phase followed by rift parallel normal faulting at the rift flanks and strike slip faults in the central part of the rift system upon break up displacement orthogonal as well as intermediate faults occur this study corroborates and extends findings from previous analogue experiments of oblique rifting on lithospheric scale and allows new insights in the timing of fault successions of the gulf of aden and continental rifts in general

did hot high heat producing granites determine the location of the oslo rift abstract the late carboniferous early permian oslo rift formed in apparently cold stable lithosphere of the fennoscandian shield in a tensional stress regime widely documented in northwest europe at that time the rift formed obliquely to older crustal structures that display only limited permian reactivation and although numerical modelling suggests that the present day lithospheric structure would serve to focus tensional stresses in the oslo region the assumption that no lithospheric evolution has occurred since the palaeozoic is by no means obvious here i show that up to 5 km thick regional scale late to post sveconorwegian granites in the vicinity of the oslo rift with heat production rates averaging ca 5 w m3 nearly three times higher than the surrounding sveconorwegian gneisses would have increased the temperature in the lower crust and lithospheric mantle by up to 100 c resulting in significant thermal weakening of the lithosphere in this area given a tensional stress regime weakening by these high heat producing element granites would have made the oslo area a favoured site for passive rifting and may have been a first order parameter locating rifting to this part of the fennoscandian shield the thermo rheological effects of such granites must be considered along with other factors in future models of initial rift mechanisms in the oslo rift and probably in other rifts elsewhere

three dimensional laboratory modelling of rifting application to the baikal rift russia abstract continental rifting is treated as a mechanical instability developing under horizontal tectonic tension the instability results in strain localization and the formation of a neck which is interpreted as a rift zone at the scale of the whole lithospheric plate this process occurs in plane stress conditions and can therefore be modelled to a first approximation by a one layer plate whose properties represent integral over thickness properties of the real lithosphere we have designed a scaled experimental single layer lithosphere model having elasto plastic rheology and lying upon a liquid substratum to study its behaviour under axial horizontal tension in a homogeneous plate the instability develops along a linear zone oriented at an angle of 60 to the tension axis this orientation is preserved even when the divergent displacement of the plate boundaries is not plain parallel but rotational in the latter case the strain localization zone is rapidly propagating when the plate length to width ratio is less than 2 5 the necking develops along two branches conjugated at an angle of about 120 which is frequently observed in actual rift systems if the model contains a local weak zone hot spot or fault zone the rift junction is located at this zone in the lithospheric models comprising strong cratonic and weak segments strain localization depends on the configuration of the boundary between different lithospheres the necking starts to form within the weak segment in the vicinity of the cratonic promontories and propagates in opposite directions again at an angle of ca 60 to the tension axis in the models containing both a strong lithosphere and local weak zones the rift configuration depends on their shape and relative positions with necking always going through the weak zones in a set of models we have reproduced the geometry of the boundary between the siberian craton and the thermally much younger 100 ma sayan baikal lithosphere in the baikal rift area in these models we were able to obtain the well known three branch configuration of the baikal rift system only by introducing a weak zone in the area of lake baikal such a zone simulates the paleozoic suture existing in this area as in nature two wide outer branches eastern and western are oblique to the regional tension axis whereas the central one is narrow and orthogonal to the tension direction in nature and in the model rifting starts in the central branch corresponding to lake baikal the modelling also predicts the formation of a fourth oblique ns trending branch to the south of baikal although poorly expressed in the field this branch has some seismotectonic and magmatic manifestations the orientations of all four branches with respect to each other and with respect to the regional tension direction are remarkably similar in nature and in the model

interactions between propagating rotational rifts and linear rheological heterogeneities insights from three dimensional laboratory experiments the lateral propagation of rifts is a consequence of the relative divergence of lithospheric plates about a pole of rotation modern and ancient examples of rifts are known to overprint pre existing linear anisotropies in the crust and lithosphere such as lithospheric boundaries crustal sutures and thermal anomalies here we investigate how propagating rifts interact with pre existing structures by using three dimensional analogue experiments with rotational extensional boundary conditions and variably oriented linear weak zones in the lithospheric mantle when linear weaknesses are oriented at low angles to the rift axis early strain localization occurs in narrow domains which merge at later stages resulting in continental break up by unzipping strong strain partitioning is observed when the linear heterogeneity is oriented at high angles with respect to the rift axis in these experiments early sub parallel v shaped basins propagate toward the pole of rotation until they are abandoned and strain is transferred entirely to structures developed in the vicinity of the strongly oblique weak lithosphere zone boundary the experimental results are characterized in terms of their evolution patterns of strain localization and surface topography as a function of the lithospheric heterogeneity obliquity angle comparison of the experiments to ancient and modern examples in nature may help to elucidate the common but still poorly understood process of propagating rift lithospheric heterogeneity interaction

control of the rheological structure of the lithosphere on the inward migration of tectonic activity during continental rifting analogue models are used to analyse the parameters controlling the evolution of extensional deformation in continental rifts models are deformed in a centrifuge and simulate the continental lithosphere floating and extending above a low viscosity asthenosphere model results reproduce the typical evolution of deformation during continental narrow rifting with early activation of large boundary faults and basin subsidence followed by their abandonment and localization of tectonic activity in internal faults near the centre of the rift the experiments document the strong influence exerted by the thickness of both brittle and ductile crustal layers and syn rift sediment accumulation on the evolution of deformation namely on the amount of bulk extension preceding inward fault migration thin upper and or lower crust and absent or low syn rift sedimentation promote a rapid abandonment of boundary faults and a transition to in rift fault development for low amounts of extension conversely thick upper and or lower crust and high syn rift sediment accumulation favour prolonged slip on boundary faults and delayed development of internal faulting the experimental results suggest that the inward migration of faulting during extension of continental lithosphere results from the interplay between the ductile stresses acting at the base of the upper crust and the total resistance of this brittle layer

modeling suggests that oblique extension facilitates rifting and continental break up 1 in many cases the initial stage of continental break up was and is associated with oblique rifting that includes break up in the southern and equatorial atlantic separation from eastern and western gondwana as well as many recent rift systems like gulf of california ethiopia rift and dead sea fault using a simple analytic mechanical model and advanced numerical thermomechanical modeling techniques we investigate the influence of oblique extension on the required tectonic force in a three dimensional setting while magmatic processes have been already suggested to affect rift evolution we show that additional mechanisms emerge due to the three dimensionality of an extensional system focusing on non magmatic rift settings we find that oblique extension significantly facilitates the rift process this is due to the fact that oblique deformation requires less force in order to reach the plastic yield limit than rift perpendicular extension the model shows that in the case of two competing non magmatic rifts with one perpendicular and one oblique to the direction of extension but otherwise having identical properties the oblique rift zone is mechanically preferred and thus attracts more strain

why do continents break up parallel to ancient orogenic belts the frequently observed parallelism between rifts and the pre existing orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents we propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropie strength in the lithosphere that guides the propagation of continental rifts subcrustal mantle mechanical anisotropy is supported by i the anisotropie strength of olivine ii an ubiquitous tectonic fabric in exposed mantle rocks and iii measurements of seismic and electrical anisotropy during major episodes of continent rifting parallel to orogenic belts ocean opening through rifting and continent break up is frequently related to the occurrence ofhotspots there is howevcr a discrepancy between hot spots acting as pin point sources of heat and the linear extent of rifts over thou sands ofkilometres moreover rifts tend to parallel pre existing orogenic fab

lithospheric structure of the malawi rift implications for magma poor rifting processes this project was supported by the national science foundation continental dynamics nsf cd grant ear 1255233 and office of international and integrative activities oiia grant iia 1358150 this project is also supported by the nsf earthcube integration grant 1740627 we thank the computational infrastructure for geodynamics geodynamics org which is funded by the national science foundation under award ear 0949446 and ear 1550901 for supporting the development of advanced solver for problems in earth s convection

effect of lithosphere thickness heterogeneities in controlling rift localization numerical modeling of the oslo graben 1 we present 2 d thermo mechanical and numerical modeling at lithospheric scale to assess the impact of pre existing lithosphere thickness contrast on rift localization the results of the modeling suggest that thickness heterogeneity was responsible for focussing rifting and passive asthenospheric upwelling in the permo carboniferous oslo graben at the western margin of the fennoscandian shield these findings are consistent with inferences from recent studies showing that both the crust mantle boundary and the base of the lithosphere are and were deeper at the eastern margin of the oslo graben resulting in contrasting crust and lithosphere thickness

control of rift obliquity on the evolution and segmentation of the main ethiopian rift the main ethiopian rift has evolved in two stages with successive activation of differently oriented fault systems analogue modelling of the earth s lithosphere demonstrates that such a rift evolution requires neither magma weakening nor a change in plate kinematics the main ethiopian rift is an active rift in the break up stage and it marks the incipient boundary between the nubia and somalia plates1 rifting started with the activation of large boundary faults and diffuse volcanism followed by focused magmatism and faulting in the rift floor along step like segments oblique to the rift axis that now act as a protoridge for future seafloor spreading2 this concentration of volcano tectonic activity has been thought to be either magma assisted2 3 or controlled by a change in rift kinematics with a late oblique rifting phase that would have caused the development of the step like fault segments that focused magma upwelling4 5 geodetic6 7 seismic8 and stress field9 data confirm current oblique rifting kinematics but plate kinematics models do not predict a change in nubia somalia motion in the past 11 million years10 here i use lithospheric scale analogue models of oblique rifting to analyse the development of the main ethiopian rift i find that neither magma weakening nor a change in plate kinematics are required to simulate a two phase evolution with successive activation of differently oriented fault systems i conclude that rift evolution and segmentation are controlled by rift obliquity independent of magmatic processes

the deep structure and reactivation of the kyrgyz tien shan modelling the past to better constrain the present abstract in the tien shan belt cenozoic to active deformation is guided by structures and rheological contrasts partly inherited from the paleozoic variscan orogeny and reactivated by the india asia collision cenozoic deformation net estimates are constrained by lithospheric scale geological cross sections which rely on reconstructions of the paleozoic central asian orogenic belt caob however several geodynamic scenarii have been proposed for its formation including a south or a north dipping subduction which are still debated due to lack of constraints here we designed numerical experiments to test different hypotheses about the initial geometry and the effective rheology of the paleo sutures and especially the one located between the tien shan and the tarim basin where most cenozoic deformation localizes the different geometries of paleozoic structures are used as input variables in the thermo mechanical models which are then run forward in time after a finite amount of shortening the structures that develop self consistently out of the proposed heterogeneities are then compared to the current cenozoic finite and active deformations we find that a crustal south dipping suture zone lying on a resistant lithospheric mantle best explains the localization of the deformation the current geometry of the structures and the moho depth variations using the model results we propose a mechanically consistent depth interpolated crustal cross section of the kyrgyz tien shan which incorporates both geological and geophysical data

modelling the extension of heterogeneous hot lithosphere abstract the consequences of weak heterogeneities in the extension of soft and hot lithosphere without significant previous crustal thickening has been analysed in a series of centrifuge models the experiments examined the effects of i the location of heterogeneities in the ductile crust and or in the lithospheric mantle and ii their orientation perpendicular or oblique to the direction of bulk extension the observed deformation patterns are all relevant to the so called wide rifting mode of extension weak zones located in the ductile crust exert a more pronounced influence on localisation of deformation in the brittle layer than those located in the lithospheric mantle the former localise faulting in the brittle crust whereas the latter tend to distribute faulting over a wider area this latter behaviour depends in turn upon the decoupling provided by the ductile crust localised thinning in the brittle crust is accompanied by ductile doming of both crust and mantle domains of maximum thinning in the brittle crust and ductile crust and mantle are in opposition lateral differences in brittle crust thinning are accommodated by lateral flow in the ductile crust and mantle this contrasts with cold and strong lithospheres whose high strength sub moho mantle triggers a necking instability at the lithosphere scale this also differs from the extension of thickened hot and soft lithospheres whose ductile crust is thick enough to give birth to metamorphic core complexes thus for the given lithospheric rheology the models have relevance to backarc type extensional systems such as the aegean and the tyrrhenian domains

rotation narrowing and preferential reactivation of brittle structures during oblique rifting abstract occurrence of multiple faults populations with contrasting orientations in oblique continental rifts and passive margins has long sparked debate about relative timing of deformation events and tectonic interpretations here we use high resolution three dimensional thermo mechanical numerical modeling to characterize the evolution of the structural style associated with varying geometries of oblique rifting in a layered continental lithosphere automatic analysis of the distribution of active extensional shear zones at the surface of the model demonstrates a characteristic sequence of deformation phase 1 with initial localization of deformation and development of wide moderately oblique en echelon grabens limited by extensional shear zones oriented close to orthogonal to 3 trend subsequent widening of the grabens is accompanied by progressive rotation of the phase 1 extensional shear zones to an orientation sub orthogonal to the plate motion direction phase 2 is characterized by narrowing of active deformation resulting from thinning of the continental mantle lithosphere and development of a second generation of extensional shear zones during phase 2 deformation localizes both on plate motion direction orthogonal structures that reactivate rotated phase 1 shear zones and on new moderately oblique structures parallel to 2 finally phase 3 consists in the oblique rupture of the continental lithosphere and produces an oceanic domain where oblique ridge segments are linked with highly oblique accommodation zones we conclude that while new structures form and trend parallel to 2 in an oblique rift progressive rotation and long term slip along phase 1 structures promotes orthorhombic fault systems which accommodate upper crustal extension and control oblique passive margin architecture the distribution orientation and evolution of frictional plastic structures observed in our models consistent with documented fault populations in the main ethiopian rift and the gulf of aden conjugate passive margins both of which developed in moderately oblique extensional settings

successive orthogonal and oblique extension episodes in a rift zone laboratory experiments with application to the ethiopian rift small scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique type 1 and from oblique to orthogonal type 2 in both the type 1 and type 2 models faults produced during the first stage strongly control fault development during the second stage in type 1 models the oblique faults developed during the second oblique phase are confined within a first phase graben whereas in type 2 models the oblique faults produced during the first phase continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks type 1 models are compared with the structural setting of the ethiopian rift the evolution of the rift is related to a recent extensional event whose principal direction of stretching trends at around 50 to preexisting major normal faults type 1 laboratory models are fairly comparable to the northern sector of the ethiopian rift referred to here as mer they account for both the development of the en echelon oblique faults of the wonji fault belt and the sinistral shear gradient running parallel to the eastern border of the mer which formed during an oblique rifting extension the statistical analysis of the whole ethiopian rift fault pattern by reference to the experimental data allows the determination of a n100 n110 mean direction of stretching

lateral crustal variation and post rift magmatism in the northeastern south china sea determined by wide angle seismic data abstract the northeastern margin of the south china sea experienced continental rifting followed by magmatism in the cenozoic this evolution has strongly influenced the heterogeneity of its crustal structure to gain insights into the spatial variation in crustal structure data from 30 ocean bottom seismometers were used to forward model ten p wave velocity models along both dip and strike directions the cenozoic strata 1 7 3 5 km s the mesozoic strata 3 5 5 5 km s the crystalline crust 5 5 7 5 km s and high velocity lower crust 7 0 7 5 km s were identified based on p wave velocities overall the crust thins from 25 km on the continental shelf to 15 km on the slope with thicker crust underneath the dongsha basin a mesozoic basin but thinner crust underneath the pre cenozoic nw trending fault zones this difference in thickness is possibly controlled by different strength of inherited structures i e the dongsha basin and the fault zones furthermore a widespread high velocity lower crust layer which resulted from the intrusion of a mafic sill complex was observed over the shelf to slope mafic intrusions within the lower crust have uplifted the caprock on the shelf and magma eruptions have formed many post rift seamounts on the slope the evidence suggests that post rift magmatism overprinted the vertical crustal structure we therefore propose that the heterogeneous crustal structures in this passive margin are an integrated result of segmental extension followed by magmatic overprinting

physical analog modeling of pull apart basin evolution abstract pull apart basins are structural depressions formed by localized extension along strike slip fault systems typically at releasing bends or steps in the fundamental strike slip system analog modeling is used to evaluate the sequence of structural evolution of pull apart basins and factors that control their degree of structural asymmetry and geometry basin evolution internal structure and overall symmetry are investigated for oblique releasing step angles and with varying rates of displacement between brittle and ductile crust on opposing sides of the strike slip system while maintaining relative rates between brittle crust on opposing sides of the basin pull apart basin evolution is separated into three stages incipient early and mature incipient pull apart basins are characterized by formation of a normal fault bounded graben or half graben parallel to the oblique step between main strike slip zones in the early stage of formation additional normal faults form toward the basin interior from the original bounding faults and cross basin strike slip faults cut diagonally across the basin interior basin bounding normal fault systems are characterized by lateral variations of fault throw and localized relay ramps in the mature stage of evolution strike slip and normal faults join to completely bound the pull apart basin analog model results indicate that displacement associated with cross basin faults causes development of a through going strike slip fault that links the two main strike slip displacement zones ultimately resulting in a decline in normal fault activity asymmetric symmetric and hybrid pull apart basins all follow the same overall deformation sequence just described the asymmetry of a pull apart basin is controlled by the degree of decoupling between brittle and ductile crust beneath the two crustal blocks in relative motion this is modeled by maintaining a constant relative rate of motion between opposing fault blocks in all models but varying the rate between the blocks and the fixed model basement absolute rate models in which one side of the detachment is fixed with respect to the basement form asymmetric pull apart basins defined by a half graben with the master fault on the mobile side in models where opposing sides of the fault system are equally decoupled from the basement symmetrical pull apart basins form defined by horst and graben structures and master fault dominance switching sides along the length of the basin fault segmentation associated with relatively immature pull apart basins may be capable of arresting earthquake rupture late linkage of main strike slip zones by a cross basin fault could extend potential rupture area dramatically increasing the possibility for large magnitude earthquakes

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

role of weak zone orientation in continental lithosphere extension 1 lithosphere extension and continental breakup axes are often sub parallel to orogenic belts and suture zones in an attempt to understand the relation between weak zone orientation and extension direction a 3 d numerical model is developed with which the following aspects are studied the relation between weak zone axis and axis of maximum crustal deformation the amount and distribution of crustal thinning and subsequent implications for rift a symmetry the results suggest that upon oblique extension rifts develop within the weak zone that individually cross the inherited structure while as a group they follow the weak trend this results in an alternating rift asymmetry the pattern of crustal thinning topography and thermal structure are not symmetric around the rift axes of the major rift zones and change along axis over such a rift this is a possible explanation for alternating asymmetric rift structures such as observed in the east african rift system

tectonic inheritance and continental rift architecture numerical and analogue models of the east african rift system 1 the western branch of the east african rift is composed of an arcuate succession of elongate asymmetric basins which differ in terms of interaction geometry fault architecture and kinematics and patterns of uplift subsidence and erosion sedimentation the basins are located within proterozoic mobile belts at the edge of the strong tanzanian craton surface geology suggests that the geometry of these weak zones is an important parameter in controlling rift development and architecture although other processes have been proposed in this study we use lithosphere scale numerical models and crustal scale analogue experiments to shed light on the relations between preexisting structures and rift architecture results illustrate that on a regional scale rift localization within the mobile belts at the curved craton s western border results in an arcuate rift system which implies that under a constant extensional stress field part of the western branch experienced orthogonal extension and part oblique extension largest depocenters are predicted to form mostly orthogonal to the extension direction and smaller depocenters will form along the oblique parts of the rift the varying extension direction along the rift zone furthermore results in lengthwise varying rift asymmetry segmentation characteristics and border fault architecture trend length and kinematics analogue models predict that discrete upper crustal fabrics may influence the location of accommodation zones and control the architecture of extension related faults at a local scale models support that fabric reactivation is responsible for the oblique slip kinematics on faults and for the development of z shaped or arcuate normal faults typically documented in nature

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

tectonic influences on the quaternary drainage evolution on the north western margin of the french central massif the creuse valley example abstract this work examines the links between tectonics and fluvial dynamics on the north western margin of the french central massif geomorphological and sedimentological analyses of detrital deposits were carried out as the basis for correlating the different formations and for reconstructing the palaeodrainage in the creuse basin cross sections of the valleys longitudinal profiles of terraces and glacis indicate post depositional deformation of about 50 m amplitude uplift north and south of gueret and in the eguzon district these deformations are related to the central massif uplift of 50 m above the paris basin since 1 1 ma ois 32 the creuse incision varies from 140 m in the crozant anticline to 60 m in the paris basin an upper pliocene palaeochannel located on the sedelle ardentes issoudun lineament is evidenced by andalusite coming from the fougeres unit the reactivation of the transverse faults between le pin and le menoux has distorted the middle terraces the main knick points are mainly due to tectonics and have been persistent in the landscape since the upper pliocene their recession rate is controlled both by discharge and lithology tributary streams exhibit strong convexities indicating that the erosional response to pliocene uplift has not yet propagated into upland surfaces

how preexisting weaknesses control the style of continental breakup continental breakup is a highly variable process differences occur in the relative timing and extent of volcanism uplift and graben formation as well as in the mode and amount of continental extension before breakup we propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends continental lithosphere is viewed as a composite material composed of two strong layers one in the upper mantle and one in the middle crust finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust this leads to early melt segregation and volcanism margin uplift during the late stages of the extension process and relatively narrow symmetrical extended margins in contrast failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle this produces initial graben formation cooling in the lower crust and upper mantle and broad asymmetrical extended margins volcanism only occurs late in the process failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup

from continental rifting to seafloor spreading insight from 3d thermo mechanical modeling abstract continental rifting to seafloor spreading is a continuous process and rifting history influences the following spreading process however the complete process is scarcely simulated using 3d thermo mechanical coupled visco plastic numerical models we investigate the complete extension process and the inheritance of continental rifting in oceanic spreading our modeling results show that the initial continental lithosphere rheological coupling decoupling at the moho affects oceanic spreading in two manners 1 coupled model a strong lower crust mechanically couples upper crust and upper mantle lithosphere generates large lithospheric shear zones and fast rifting which promotes symmetric oceanic accretion i e oceanic crust growth and leads to a relatively straight oceanic ridge while 2 decoupled model a weak ductile lower crust mechanically decouples upper crust and upper mantle lithosphere generates separate crustal and mantle shear zones and favors asymmetric oceanic accretion involving development of active detachment faults with 3d features complex ridge geometries e g overlapping ridge segments and curved ridges are generated in the decoupled models two types of detachment faults termed continental and oceanic detachment faults are established in the coupled and decoupled models respectively continental detachment faults are generated through rotation of high angle normal faults during rifting and terminated by magmatism during continental breakup oceanic detachment faults form in oceanic crust in the late rifting early spreading stage and dominates asymmetric oceanic accretion the life cycle of oceanic detachment faults has been revealed in this study

transition from continental break up to punctiform seafloor spreading how fast symmetric and magmatic 1 we present a comparison between numerical and analogue models focusing on the role of inherited lithospheric structures in influencing the process of continental break up our results highlight that the presence of pre existing anisotropies localizes strain and favors continental break up and formation of a new ocean for a fixed strain rate the pre rift lithosphere configuration influences rift duration melt production and width and symmetry of the continental margin pair model results show a mainly two phase tectonic history from continental extension to oceanization in the first phase extension affects contemporaneously the whole rift structure while in the second phase asthenosphere upwelling occurs into punctiform regularly spaced spots sequentially propagating in an extension orthogonal direction index terms 8109 tectonophysics continental tectonics extensional 0905 8120 tectonophysics dynamics of lithosphere and mantle general 8150 tectonophysics plate boundary general 3040 citation corti g j van wijk m bonini d sokoutis s cloetingh f innocenti and p manetti transition from continental break up to punctiform seafloor spreading how fast symmetric and magmatic geophys res lett 30 12 1604 doi 10 1029 2003gl017374 2003

slip history of the la cruz fault development of a late miocene transform in response to increased rift obliquity in the northern gulf of california abstract the gulf of california rift has accommodated oblique divergence of the pacific and north america plates in northwestern mexico since miocene time due to its infancy its rifted margins preserve a rare onshore record of early continental break up processes and an opportunity to investigate the role of rift obliquity in strain localization we map rift related structures and syn tectonic basins on southern isla tiburon a proximal onshore exposure of the rifted north america margin we integrate analysis and geochronology of syn tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity on southern isla tiburon an early phase of extension initiated between 19 17 ma and 12 2 ma subsequently these normal faults and related basins were cut by the la cruz strike slip fault and buried by deposits of the la cruz basin an elongate fault controlled trough coextensive with the la cruz fault crosscutting relationships show that the nw striking la cruz fault accrued 5 2 km of dextral slip 8 4 ma the la cruz fault and parallel tiburon transform were kinematically linked to detachment faulting that accommodated latest miocene to pliocene oblique opening of the offshore upper tiburon pull apart basin the onset of strike slip faulting on isla tiburon was synchronous with the 8 6 ma onset of transform faulting and basin formation along 1000 km of the reconstructed pacific north america plate boundary this transition coincides with the commencement of a clockwise azimuthal shift in pacific north america relative plate motion that increased the obliquity of the gulf of california rift and formed the gulf of california shear zone the record from the proto gulf of california illustrates how highly oblique rift geometries where transform faults are kinematically linked to pull apart basins enhance the ability of continental lithosphere to rupture and ultimately hasten the formation of new oceanic rift basins

continent scale strike slip on a low angle fault beneath new zealand s southern alps implications for crustal thickening in oblique collision zones new zealand s southern alps lie adjacent to the continent scale dextral strike slip alpine fault on the boundary between the pacific and australian plates we show with a simple 2 d model of crustal balancing that the observed crustal root and erosion expressed as equivalent crustal shortening is up to twice that predicted by the orthogonal plate convergence since 11 ma and even since 23 ma when the alpine fault formed we consider two explanations for this involving a strong component of motion along the length of the plate boundary zone geophysical data indicate that the alpine fault has a listric geometry flattening at mid crustal levels and has accommodated sideways underthrusting of australian plate crust beneath pacific plate crust the geometry of the crustal root together with plate reconstructions requires the underthrust crust to be the hyperextended part of an asymmetric rift system which formed over 500 km farther south during the eocene the narrow remnant part today forms the western margin of the campbell plateau at 10 ma the hyperextended margin underwent shallow subduction in the puysegur subduction zone and then was dragged over 300 km along the length of the southern alps beneath a low angle 20 section of the alpine fault we speculate that prior to 10 ma more distributed lower crustal shortening and thickening occurred beneath the southern alps accommodating southward extrusion of continental crust in the northern part of the plate boundary zone providing a mechanism for clockwise rotation of the hikurangi margin

transtensional faulting patterns ranging from pull apart basins to transform continental margins an experimental investigation abstract seventeen small scale experiments were performed to study the deformation induced in brittle ductile models by a releasing stepover between two transform faults and by intersection between transform and divergent plate boundaries in both cases faulting depends on the rheological layering presence and strength of a ductile layer at depth and the width of the basal stepover successive types of pull apart basins are observed in the first set of experiments which are compared with natural examples firstly a lazy z shaped basin appears which is bounded by y faults above transform boundaries and r faults propagating from each corner of the stepover then r faults replace r faults leading to a rhomb shaped graben at every stage the basin length to width ratio remains between 2 2 and 3 8 suggesting that scale independence of pull apart basins is related to the geometrical shape of bounding faults in a second set of experiments deformation at the end of a transform boundary is characterized by a horsetail splay bounding a divergent basin within the horsetail splay block tilting and block rotation about vertical axes lead to a surface slope perpendicular to the slope of the divergent basin a feature that can be compared with marginal ridges at transform margins

inward migration of faulting during continental rifting effects of pre existing lithospheric structure and extension rate abstract lithospheric scale analogue models are used to analyse the parameters controlling the typical evolution of deformation during continental narrow rifting characterized by early activation of large boundary faults and basin subsidence followed by localization of tectonic activity in internal faults at the rift axis integration of current and previous experiments shows that the evolution of deformation in particular the amount of extension needed for the abandonment of boundary faults and migration of deformation to in rift faults is dependent on at least five boundary conditions i thickness of brittle layers including syn rift sediments ii thickness of ductile layers iii extension rate iv width of the weak zone localizing extension and v rift obliquity with respect to the extension direction an increase in the amount of extension corresponding to the inward migration of faulting i e a longer phase of slip on boundary faults is observed for a an increase in the thickness of both brittle and ductile crustal layers and syn rift sediment accumulation b a decrease in extension rate and width of the weak zone and c a decrease in rift obliquity a unified account of these correlations is presented based on the hypothesis that fault migration occurs when boundary faults can no longer accommodate the imposed bulk extension leading to time space variations of internal strain and strain rate and consequently stress in the ductile layers which overcome the total resistance of brittle layers to thoroughgoing faulting

quantifying the thermo mechanical impact of plume arrival on continental break up abstract the arrival of a plume head at earth s continental lithosphere is often considered to be an important factor for continental break up however the impact of plume impingement on strength and duration of a rift remains unclear in this study we quantify the mechanical and thermal influence of a plume i e lithosphere erosion on continental break up to do that we apply the three dimensional numerical code slim3d that features realistic elasto visco plastic rheology we model the thermo mechanical response of a segment of earth s lithosphere that is affected both by extension as well as plume related lithosphere erosion in order to evaluate the influence on the overall force budget we find that lithosphere erosion leads to a moderate lithospheric strength reduction of several tn m in a force limited environment however this strength reduction may have strong influence on the timing of continental break up or it may even control whether continental break up takes place at all additional reduction of the lithospheric strength is likely due to the massive emplacement of dikes that follows intensive melting within the plume head

rift segment interaction in orthogonal and rotational extension experiments implications for the large scale development of rift systems abstract during extension of the continental lithosphere rift basins develop these are often initially offset and must interact and connect in order to create a continuous rift system that may ultimately achieve break up when simulating extensional tectonics and rift interaction structures analogue and numerical modellers often apply a continuous extension rate along the strike of a rift or rift system yet in nature significant extension velocity variations occur along rifts and plate boundaries as a natural consequence of tectonic plates moving apart about a pole of rotation resulting in rotational extension and associated rift propagation and structural gradients here we present various analogue tectonic experiments to assess rift interaction structures forming in orthogonal extension settings versus rotational extension settings our modelling efforts show that rotational extension and orthogonal extension produce significantly different large scale structures rotational extension can cause important variations in rift maturity between rift segments delay rift interaction zone development and make rift segments propagate in opposite directions still local features in a rotational extension system can often be regarded as evolving in an orthogonal extension setting furthermore we find that various degrees of rift underlap produce three basic modes of rift linkage structures low underlap distance high angle experiments develop rift pass structures with increasing underlap distance ca 40 transfer zone basins develop high degrees of underlap 30 tend to result in en echelon sub basins our results match with data from previous modelling efforts and natural examples we furthermore propose a large scale tectonic scenario for the east african rift system based on rotational extension and associated rift propagation these insights may also be applicable when studying other large scale rift systems

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

continental rifting parallel to ancient collisional belts an effect of the mechanical anisotropy of the lithospheric mantle analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process rift propagation is not random but tends to follow the trend of the orogenic fabric of the plates systematically reactivating ancient lithospheric structures continental rifts often display a clear component of strike slip deformation in particular in the early rifting stage moreover although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts these observations suggest that the deformation of the lithosphere especially during rifting is controlled by its preexisting structure on the other hand 1 the plasticity anisotropy of olivine single crystal and aggregates 2 the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs and 3 seismic anisotropy data which require a tectonic fabric in the upper mantle coherent over large areas suggest that preservation within the lithospheric mantle of a lattice preferred orientation lpo of olivine crystals may induce a large scale mechanical anisotropy of the lithospheric mantle we use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process we assess the deformation of an anisotropic continental lithosphere in response to an axi symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head model results show that a lpo induced mechanical anisotropy of the lithospheric mantle may result in directional softening leading to heterogeneous deformation during continental rifting this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique 30 55 to the preexisting mantle fabric this directional softening associated with olivine lpo frozen in the lithospheric mantle may also guide the propagation of the initial instability that will follow the preexisting structural trend the preexisting mantle fabric also controls the deformation regime imposing a strong strike slip shear component a lpoinduced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting structural inheritance the plume rift paradox and the onset of transtension within continental rifts fl 2001 elsevier science b v all rights reserved

patterns of continental extension along the conjugate margins of the central and north atlantic oceans and labrador sea the anomalously thin crust and subsidence history of passive margins suggest that significant extension occurs within continental lithosphere prior to the onset of seafloor spreading we estimate the amount of continental extension prior to seafloor spreading along the conjugate margins of the central and north atlantic oceans and labrador sea from the total tectonic subsidence of the margins in the central atlantic basin the integrated continental extension varies from 225 km for the break between the southern appalachian segment of the north american margin and africa to over 600 km for the break between the blake plateau region of north america and the cape verde terrace region of africa the original width of the extended zone varies from 240 to 770 km in the north atlantic and labrador basins the integrated extension varies from 60 km for the break between northern labrador and greenland to over 600 km for the break between newfoundland and europe the original width of the extended zones in these basins varies from 100 km to over 1460 km in general continental breakup involves two to three times less extension where it follows the preexisting structural grain than where it crosses the preexisting grain as a result seafloor spreading tends to begin first along segments which follow the structural grain extension tends to be symmetrically distributed between conjugate margins where the break follows the grain and asymmetrically distributed where the break crosses the grain failure parallel to the structural grain exhibits greater variability from margin to margin than failure across the grain this phenomenon appears to result from differences in the nature of preexisting weaknesses within the structural grain we conclude that the distribution of the continental extension along passive margins is to a large extent controlled by the orientation of the break relative to the structural grain and the nature of the preexisting weaknesses within that structural grain

along strike segmentation of the south china sea margin imposed by inherited pre rift basement structures abstract multibeam bathymetric seismic and borehole data are used to investigate a large scale strike slip structure the baiyun liwan fault zone in the northern south china sea this fault zone comprises nw to ne striking faults and negative flower structures that were generated by oblique extensional displacement notably the interpreted data reveals that the baiyun liwan fault zone was active during the cenozoic recording intense magmatism and accommodating significant intraplate deformation during progressive continental rifting and ocean spreading it bounds two distinct crustal segments and played a significant role in segmenting the northern margin of the south china sea the geometry of faults and strata within the baiyun liwan fault zone also controlled local sediment routing and depocentre evolution during the cenozoic as basement and syn rift structures change markedly across the baiyun liwan fault zone we propose this structure to be inherited from a lithospheric scale fault zone separating the mesozoic arc from forearc related terrains we therefore stress the importance of pre existing structures in the development of rifted margins with the example provided by the baiyun liwan fault zone having profound implications for palaeogeographic reconstructions in the south china sea at present the baiyun liwan fault zone is incised by the pearl river canyon and eroded by recurrent submarine landslides forming a major area of sediment bypass towards the abyssal plain

asymmetric rifts due to asymmetric mohos an experimental approach large scale asymmetries in structures and in the uplift width and volcanism of pairs of conjugate passive margins have been documented in various parts of the world these asymmetries reflect the dominant mode of extension preceding the break up of the continental lithosphere of the many parameters involved the pre rift crustal structure has been shown to play a fundamental role in controlling the process of continental rifting in this work we have used analogue models to show how initial asymmetric moho configurations are prone to produce asymmetric rifting with prominent asymmetries in the amount of extension accommodated by boundary faults in the patterns of lithospheric thinning asthenospheric upwelling and in the trajectories of magma migration after continental break up these deformative patterns are expected to lead to the development of a pair of asymmetric passive margins with one narrow strongly volcanic margin and a wider less volcanic or with areas of concentrated volcanic activity counterpart

the origin of along rift variations in faulting and magmatism in the ethiopian rift the geological record at rifts and margins worldwide often reveals considerable along strike variations in volumes of extruded and intruded igneous rocks these variations may be the result of asthenospheric heterogeneity variations in rate and timing of extension alternatively pre existing plate architecture and or the evolving kinematics of extension during breakup may exert first order control on magmatism the main ethiopian rift mer in east africa provides an excellent opportunity to address this dichotomy it exposes along strike several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north here we perform studies of volcanic cone density and rift obliquity along strike in the mer by synthesizing these new data in light of existing geophysical geochemical and petrological constraints on magma generation and emplacement we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin the timing of rift sector development the three dimensional focusing of melt and the ponding of plume material where the rift dramatically narrows each influence igneous intrusion and volcanism along the mer however rifting obliquity plays an important role in localizing intrusion into the crust beneath en echelon volcanic segments along strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting as well as the physical state of the convecting mantle at the time of breakup

building of the deepest crust at a fossil slow spreading centre pineto gabbroic sequence alpine jurassic ophiolites this work presents new field and petrological data on a poorly known lower crustal section from the alpine jurassic ophiolites the pineto gabbroic sequence from corsica france the pineto gabbroic sequence is estimated to be 1 5 km thick and mainly consists of clinopyroxene rich gabbros to gabbronorites near its stratigraphic top and of troctolites and minor olivine gab bros in its deeper sector the sequence also encloses olivine rich troctolite and mantle peridotite bodies at dif ferent stratigraphic heights the composition and the lith ological variability of the pineto gabbroic sequence recall those of the lower crustal sections at slow and ultra slow spreading ridges the gabbroic sequence considered in this study is distinct in the high proportion of troctolites and olivine gabbros which approximately constitute 2 3 of the section in particular the lower sector of the pineto gab broic sequence shows the existence of large scale frag ments of the deepest oceanic crust displaying a highly primitive bulk composition the mineral chemical varia tions document that the origin and the evolution of the pineto gabbroic rocks were mostly constrained by a process of fractional crystallisation the clinopyroxenes from the olivine gabbros and the olivine rich troctolites also record the infiltration of olivine dissolving cr2o3 rich melts that presumably formed within the mantle into replacive dunite bodies cooling rates of the troctolites and the olivine gabbros were evaluated using the ca in olivine geospee dometer we obtained high and nearly constant values of 2 2 to 1 7 c year log units which were correlated with the building of the pineto gabbroic sequence through multiple gabbroic intrusions intruded into a cold litho spheric mantle