thinning factor distributions viewed through numerical models of continental extension a longstanding question surrounding rifted margins concerns how the observed fault restored extension in the upper crust is usually less than that calculated from subsidence models or from crustal thickness estimates the so called extension discrepancy here we revisit this issue drawing on recently completed numerical results we extract thinning profiles from four end member geodynamic model rifts with varying width and asymmetry and propose tectonic models that best explain those results we then relate the spatial and temporal evolution of upper to lower crustal thinning or crustal depth dependent thinning ddt and crustal thinning to mantle thinning or lithospheric ddt which are difficult to achieve in natural systems due to the lack of observations that constrain thinning at different stages between pre rift extension and lithospheric breakup our results support the hypothesis that crustal ddt cannot be the main cause of the extension discrepancy which may be overestimated because of the difficulty in recognizing distributed deformation and polyphase and detachment faulting in seismic data more importantly the results support that lithospheric ddt is likely to dominate at specific stages of rift evolution because crustal and mantle thinning distributions are not always spatially coincident and at times are not even balanced by an equal magnitude of thinning in two dimensions moreover either pure or simple shear models can apply at various points of time and space depending on the type of rift both ddt and pure simple shear variations across space and time can result in observed complex fault geometries uplift subsidence and thermal histories

half graben versus large offset low angle normal fault importance of keeping cool during normal faulting 1 we study the effect of temperature in the crust on the style of faulting during the extension of a model lithosphere two distinct fault patterns are seen depending on whether the brittle layer thickness hl is greater or less than a critical value hc for a thin layer hl hc a rift develops with multiple faults bounding an evolving set of grabens numerical results show that the value of hc depends on how the thermal structure and therefore the thickness distribution of the lithosphere evolve during extension in the region where the displacement occurs along the fault the brittle lithosphere thins and we show that this promotes development of multiple faults in models with heat transfer only by rock motion and conduction the advective thinning effect can greatly reduce hc with rift velocities greater than 1 cm yr 1 hc is reduced by a factor of 5 to unreasonably small values however inclusion of simulated hydrothermal cooling increases hc this allows large offsets to develop in model lithospheres with brittle layers of order 10 km thick extension of layers thicker than 10 km tends to produce a rift with multiple graben structures finally we compare the result of a simplified analytic model of extension to data we find that the style of faulting seen in a variety of continental rifts is broadly consistent with our model predictions

normal faulting and the topographic roughness of mid ocean ridge flanks the ocean floor on the flanks of mid ocean ridges is covered by abyssal hills topographic features elongated perpendicularly to the direction of relative plate motion these topographic features are interpreted as normal fault blocks and or volcanic constructions that originated near the ridge axis and were later rafted onto the ridge flanks by seafloor spreading the purpose of this paper is to quantify the topographic roughness of a profile perpendicular to the strike of a number of normal faults given a fault population and a mechanical model for the response of the lithosphere to faulting we obtain expressions for the variation in root mean square roughness with profile length and for the power spectral density of a profile given three parameters a fault density number of faults per unit length crossed by the profile an average squared fault scarp height and a characteristic length of flexure to keep matters simple we make a number of assumptions and approximations namely that the lithosphere behaves as an elastic plate that faults have an infinite length and a vertical dip that the response of topography to a number of faults is simply the sum of the responses to each fault and that faults have random locations and scarp heights independently chosen from some statistical distribution the theory predicts that the roughness length relationship power spectral density should follow power laws for scales wavelengths less than a characteristic scale proportional to the length scale of flexure we compare the predictions of the theory with actual measurements of mid ocean ridge flank roughness and find good first order agreement in particular we use independent estimates of fault densities average squared fault scarp heights and flexural length scales to predict the topographic roughness of the east pacific rise and we find that normal faulting can explain all the observed topographic roughness nevertheless there are some differences between predictions and observations these differences are likely to be due to processes other than faulting that create topographic relief e g volcanism and to spatial correlations of fault scarp heights despite these shortcomings the approach presented here provides a first step in understanding the topographic roughness signal by quantifying the contribution of the geological processes that generate surface relief

control of rheological stratification on rifting geometry a symmetric model resolving the upper plate paradox numerical experiments reproduce the fundamental architecture of magma poor rifted margins such as the iberian or alpine margins if the lithosphere has a weak mid crustal channel on top of strong lower crust and a horizontal thermal weakness in the rift center during model extension the upper crust undergoes distributed collapse into the rift center where the thermally weakened portion of the model tears among the features reproduced by the modeling we observe 1 an array of tilted upper crustal blocks resting directly on exhumed mantle at the distal margin 2 consistently oceanward dipping normal faults 3 a mid crustal high strain zone at the base of the crustal blocks s reflector 4 new ocean floor up against a low angle normal fault at the tip of the continent 5 shear zones consistent with continentward dipping reflectors in the mantle lithosphere 6 the mismatch frequently observed between stretching values inferred from surface extension and bulk crustal thinning at distal margins upper plate paradox rifting in the experiment is symmetric at a lithospheric scale and the above features develop on both sides of the rift center we discuss three controversial points in more detail 1 weak versus strong lower crust 2 the deformation pattern in the mantle and 3 the significance of detachment faults during continental breakup we argue that the transition from wide rifting towards narrow rifting with a pronounced polarity towards the rift center is associated with the advective growth of a thermal perturbation in the mantle lithosphere

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

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

interaction between sediments and basalt injections in young gulf of california type spreading centers during the transitional stage from stretching of continental crust to accretion of new oceanic crust the upwelhng basaltic magma in spreading troughs of narrow ocean basins and pull apart basins may form sills in soft sediment the injections of subsequent great magmatic pulses are empla ced next to or on top of the contact zone of the older ones in this manner a distinctive sill sediment complex is built up

some comments on two layer extensional models for the evolution of sedimentary basins simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves in this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform we review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks that subcrustal heat input during rifting occurs over an area much broader than the rift proper and that there may be broad regional uplift before and or during rifting to explain the uplift of the flanks during rifting and their postrift subsidence we invoke two mechanisms subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks the uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis but uplift due to both sources can in certain cases be incorrectly attributed to regional uplift or doming we present a simplified analysis of the two layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere we provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history application of this method to the central part of the pattani trough in the intracratonic gulf of thailand shows that extension and thinning of the lithosphere of a factor of 2 since late eocene can account for much of the subsidence history and the present day heat flow our analysis suggests that thinning of the crust in this area is 20 greater than thinning of the whole lithosphere assuming that none of the lithosphere is removed from the region of extension during rifting we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere such an extensional process produces significant uplift of the flanks of the rift and as a result of erosion of the uplifted areas the effective cross sectional area of the basin can be increased by as much as 10 20 depending on the rate of erosion compared to the area that would have been created by a process of uniform extension finally we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins

the continental extension discrepancy and anomalous subsidence pattern in the western qiongdongnan basin south china sea abstract passive margins are first order tectonic features on earth formed by polyphase extension and breakup of continental lithosphere it is accepted that mildly extended basins in the proximal margin form largely by uniform continental thinning but the continental extension in deepwater is much debated here we choose a line from the western qiongdongnan basin in the magma poor south china sea margin to further explore the style of continental extension and the corresponding subsidence patterns where high resolution seismic and heat flow data are available structural analysis reveals that the basin architecture is extremely asymmetric and mainly controlled by the se directed normal faults soling into a low angle detachment fault however fault timings nicely show that the faults form through polyphase rifting and from inside outwards and do not follow a simple sequential faulting like the west iberia margin where the upper and lower crustal deformation are strongly coupled perhaps the fault order here reflects a much weaker and decoupled lower crust then we divide the basin into five zones and calculate the average fault derived extension factors moreover the whole crustal extension factors are also estimated after assessing the initial crustal thickness on basis of the backstripped subsidence and the lithospheric extension factors are numerically modeled using a coupled lithosphere and basin scale model constrained by the heat flow and sedimentation data results show that along the profile the overall lithospheric extension is much less than that of the whole crust and only at the south end of the profile does lithospheric extension exceeds the whole crust showing lithospheric depth dependent thinning the whole crustal extension is roughly equal to the faults derived extension in the northern part displaying no extension discrepancy in the middle part the whole crustal extension is less than the faults derived extension denoting inverse extension discrepancy while the whole crustal extension is in far excess of the faults derived extension in the southern part showing obvious positive extension discrepancy we infer that the inverse and positive discrepancy of crustal extension was likely to be caused by landward directed lower crustal flow driven by the horizontal pressure arising from the combination of the differential extension in the faulted upper crust in the mantle lithosphere and the differential sedimentation during the syn rift period

extension discrepancy at north atlantic nonvolcanic rifted margins depth dependent stretching or unrecognized faulting rifted margins show an apparent discrepancy between the amount of extension measurable from faults and the amount of crustal or lithospheric thinning determined from wide angle data or subsidence this extension discrepancy has been interpreted in terms of depth dependent stretching in which the upper crust is extended and thinned far less than the rest of the lithosphere here i show that at those nonvolcanic margins where the velocity structure is well known the upper and lower crust appear to thin equally toward the margin implying that the discrepancy is not due to depth dependent stretching but more likely due to unrecognized polyphase and top basement faulting

quaternary low angle slip on detachment faults in death valley california detachment faults on the west flank of the black mountains nevada and california dip 29 36 and cut subhorizontal layers of the 0 77 ma bishop ash steeply dipping normal faults confined to the hanging walls of the detachments offset layers of the 0 64 ma lava creek b tephra and the base of 0 12 0 18 ma lake manly gravel these faults sole into and do not cut the low angle detachments therefore the detachments accrued any measurable slip across the kinematically linked hanging wall faults an analysis of the orientations of hundreds of the hanging wall faults shows that extension occurred at modest slip rates 1 mm yr under a steep to vertically oriented maximum principal stress the black mountain detachments are appropriately described as the basal detachments of near critical coulomb wedges we infer that the formation of late pleistocene and holocene range front fault scarps accompanied seismogenic slip on the detachments

depth dependent stretching a different approach uplift of the crust during rifting is a major problem that has been left unresolved by uniform stretching models various depth dependent stretching models have been proposed to account for this but these require decoupling of the crust and mantle and result in space problems within the mantle a significant modification of these discontinuous nonuniform or depth dependent stretching models is proposed here that accounts for synrift uplift and doming but does not result in these problems the model involves continuous nonuniform stretching within a polygonal region bounded by symmetrical outward sloping boundaries within the mantle this geometry results in thinning of the lithosphere below crustal regions that are less stretched or unstretched crustal uplift resulting from asthenospheric upwelling below these regions this in turn results in stretching factors for the mantle that are less than those for the crust a relationship opposite to relationships derived by discontinuous nonuniform stretching models corollary predictions for mantle derived heat flow within and outside regions of stretching of these two different depth dependent models may allow them to be distinguished from each other and from regions where uniform stretching predominates

structural characteristics of the ocean continent transition along the rifted continental margin offshore central labrador abstract studies of the structural deformation and the nature of the ocean continent transition zone at the rifted continental margin offshore central labrador and its conjugate off west greenland have been conducted over several decades and helped to define our understanding of the controlling factors in the development of magma poor margins globally new two dimensional seismic reflection data are now available across the labrador margin which can help to modify and refine this understanding in this paper we describe some of these new seismic data and the corresponding potential field data in relation to earlier studies including an existing deep seismic crustal velocity profile these data cross the three major structural and compositional zones below the sedimentary wedge defined in earlier deep seismic experiments as 1 extended zone of continental crust including the hyper extended region partly underlain by serpentinized mantle 2 exhumed and serpentinized continental mantle zone and 3 zone of oceanic crust we are able to extend these zones laterally over 200 km along the margin the rifted continental crust is hyper extended over tens of kilometers landward of the breakup location where many varieties of normal faults and possible low angle detachment surfaces are present some of which are near or at the base of the crust further seaward mantle was serpentinized and likely exhumed and possesses a seismic character very different from the continental crust basement in this zone may locally form prominent ridges within the oceanic zone we define a further sub division into a proto oceanic domain with thin variable oceanic crust of about 70 65 ma in age magnetic chrons 27 to 31 and a steady state seafloor spreading region further seaward thus seafloor spreading starts at about chron 31 time potential field data and models are broadly consistent with these zones a strongly reflective post rift volcanic event is present across much of the region especially over the exhumed mantle zone and parts of the oceanic crust these results are discussed and compared to earlier studies of the west greenland conjugate margin and to other well studied magma poor margins given that this rift occurred in thick cold cratonic lithosphere the results are remarkably similar to those off iberia and newfoundland suggesting that a long rift history and or mantle metasomatism may have weakened the lithosphere prior to the main rifting event

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

on the evolution of rifted continental margins comparison of models and observations for the nova scotian margin summary a model for the formation and evolution of rifted continental margins based on lithospheric extension during rifting and its thermal and mechanical consequences is proposed model predictions are then compared with geological and geophysical observations from a transect of the 185 ma old rifted margin off nova scotia through the scotian basin three kinematic models of the rifting process are discussed these are 1 the uniform extension model in which the amount of extension is uniform with depth but varies with position across the margin 2 the uniform extension and melt segregation model which has similar properties but also provides an explanation for the properties of the extended continental crust and its transition to oceanic crust by postulating that basaltic melt segregates from the asthenosphere and migrates to the crust and 3 the depth dependent extension model in which the first order consequences of rapidly changing rheological properties with depth in the lithosphere are included by decoupling the lithosphere into two zones with depth each of which undergoes differing amounts of extension these rift models predict the form of crustal and lithospheric thinning subsidence and temperature change once the amount of extension has been determined this is estimated from seismic measurements of present crustal thickness on the assumption that the crust had a uniform thickness equal to that currently measured in the adjacent continental region before rifting occurred rifting is also considered to occur instantaneously a time stepping thermo mechanical model is used to predict the cooling of rifted margin additional thermal contraction subsidence and its amplification by water and sediment loading thermal aspects are calculated using a finite difference model of time dependent conductive heat transport whereas regional isostatic response to loading is calculated by a finite element model the models are coupled because the temperature distribution is used to define a rheological lithosphere an elastic region with thickness that varies in time and space as the model evolves for flexural calculations in the mechanical model secondary coupling occurs through perturbations to the temperature field by sedimentary thermal blanketing and advection of heat during isostatic adjustment the model predictions of 1 sedimentary basin stratigraphy 2 moho position 3 free air gravity anomaly 4 age depth relations for deep exploratory wells and 5 subsidence and temperature histories agree well with observations from the scotian basin additional effects due to sedimentary and crustal radiogenic heat production lateral heat transport and the possible existence of a near surface brittle listric faulted region created during rifting are also considered it is concluded that a model in which rifting occurred by depth dependent extension and which includes a finite thickness for the rheological lithosphere radiogenic heat production in the sediments and crust and a brittle listric faulted crustal layer affords an accurate description of the first order processes that occur during rifting and evolution of this margin

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

symmetric alternative to asymmetric rifting models we describe the first numerical simulation of continental rifting that reproduces the three major structures observed at magma poor margins such as the galicia margin west of the iberian peninsula or the apulia margin in the alps 1 distal continental margins consist of fault bounded blocks separated by oceanward dipping normal faults 2 at the tip of the continent lower crust is scarcely preserved or absent and upper crust directly overlies exhumed mantle 3 the base of the rotated crustal blocks is a prominent seismic reflector and represents a high strain zone with a top to the ocean sense of shear in our model these structures do not reflect asymmetric rift geometry at a lithospheric scale instead they derive from upper crustal collapse over a mid crustal shear zone into the rift center and are present on both sides of the rift axis the model has a horizontal weak zone in the middle crust on top of strong lower crust and a localized vertical zone of thermal weakness in the rift center we hypothesize that the development of a thermal perturbation and associated strain localization in the deeper lithosphere may cause the transition from widely distributed faulting and crustal thinning to constricted faulting directed toward a well defined rift center

the eagle pass detachment southeastern arizona product of mid miocene listric normal faulting in the southern basin and range mid miocene low angle normal faulting is an integral part of the regional tectonic strain of the southern basin and range the distinguishing characteristic is the presence of detachments allochthons of moderately to steeply dipping hanging wall rocks resting in very low angle fault contact on structurally deeper generally older footwall rocks rotation of detachment strata was accomplished by imbricate listric normal faulting and coordinated tear faulting regional structural domains each thousands of kilometres in area contain detachments marked by uniform sense of detachment strata rotation the eagle pass detachment 70 km northeast of tucson arizona reveals informative structural details of parts of the system steeply southwest dipping mid miocene volcanic and sedimentary rocks several thousands of metres in thickness rest in very low angle 10 fault contact on precambrian granitic basement detachment strata were faulted into position by ne directed translation accompanied by rotation of strata to steep sw dips the detachment fault which underlies the 25 km2 detachment has the form of a large northeast trending mullion trough with relief of at least 500 m along the keel of the detachment precambrian quartz monzonite was transformed into a hydrothermally altered selvage of microbreccia the steep 80 northwest margin of the mullion trough served as a tear fault along which quartz monzonite was crushed volcanics were dragged and fanglomerate was locally sheared into flattened pebble conglomerate detachments like that at eagle pass resemble slump blocks on a grand scale they are elements of a regional system of extensional deformation which coincides with the belt of cordilleran metamorphic core complexes mid miocene detachment faulting post dates the formation of exposed mylonitic tectonites of core complex affinity but the direction of translation of the detachments is parallel to penetrative lineation in nearby and or underlying tectonite moreover in southeastern arizona examples the sense of translation of detachments matches the sense of simple shear movements disclosed by tectonite fabrics we speculate that large crustal displacements achieved during extension induced formation of tectonite produced a geometric and or dynamic condition which strongly influenced subsequent detachment faulting consequently with renewed or continued extension in mid miocene time deep upper crustal slumping was triggered and detachment rocks were translated along listric normal faults in the direction and sense of the earlier displacements which had accommodated the formation of tectonite

lithospheric strength variations as a control on new plate boundaries examples from the northern red sea region abstract the complex plate boundary between arabia and africa at the northern end of the red sea includes the gulf of suez rift and the gulf of aqaba dead sea transform geologic evidence indicates that during the earliest phase of rifting the red sea propagated nnw towards the mediterranean sea creating the gulf of suez subsequently the majority of the relative movement between the plates shifted eastward to the dead sea transform we propose that an increase in the strength of the lithosphere across the mediterranean continental margin acted as a barrier to the propagation of the rift a new plate boundary the dead sea transform formed along a zone of minimum strength we present an analysis of lithospheric strength variations across the mediterranean continental margin the main factors controlling these variations are the geotherm crustal thickness and composition and sediment thickness the analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone this strength profile also favors the creation of thin continental slivers such as the levant west of the dead sea transform and the continental promontory containing socotra island at the mouth of the gulf of aden calculations of strength variations based on changes of crustal thickness geotherm and sediment thickness can be extended to other geologic settings as well they can explain the location of rerifting events at intracratonic basins of backarc basins and of major continental strike slip zones

to see or not to see rifted margin extension a characteristic of rifted margins is the extension discrepancy i e the amount of extension estimated from fault geometries on seismic images is far too little to explain the observed crustal thinning and subsidence either the crust has been thinned in some other way or the amount of extension has been severely underestimated to investigate the latter we create a model structural section across a rifted margin by focusing extension in the center of a rift producing successive phases of crosscutting faults from one side of this section a synthetic seismic image is generated and interpreted as if it were a real profile just as for real margins apparent listric faults and eroded fault block crests are seen but these are not present in the model and instead represent intersecting fault surfaces and are thus diagnostic of polyphase faulting just as for real margins the amount of extension measured from the seismic is only a fraction of the true extension just as for real margins this extension discrepancy increases markedly oceanward demonstrably for the synthetic margin and by implication for real margins the extension discrepancy is the failure of the seismic method to image unambiguously the polyphase faulting required to accommodate increasing extension combined with a general lack of awareness of the features outlined here diagnostic of such faulting

seismic evidence for the crustal deformation and kinematic evolution of the nansha block south china sea abstract to understand the crustal deformation of the nansha block nb this study presents an interpretation of a recently acquired multichannel seismic profile that traverses the southwest subbasin swsb the nb and the nansha trough nt a total of six cenozoic unconformities and three stratigraphic sequences are identified on the basis of tectonic structural and stratigraphic features the nb is divided from northwest to southeast into two structural domains namely the hyper stretched and forebulge domains based on the interpretation of the seismic profile the fault derived and whole crustal extension factors f and c respectively for the nb are calculated the calculation results show that c is greater than f in a limited space of appr 200 km where the crust underwent depth dependent extension we suggest that the crustal extensional pattern of the nb may be a combination of distal limited depth dependent extension and proximal uniform extension the interpretation of the unconformities is also used to calculated the fault derived crustal extension at various stages the two main extension stages the syn rift and syn drift account for appr 70 and 25 of the total extension respectively a comparison with the seismic profiles of the eastern nb shows that the crustal extension along the continental margin of the nb is heterogeneous with more significant crustal deformation on the west side of the nb the heterogeneity of the crustal extension along the depth and margin may be related to the thermal condition and rheological structure of the crust or lithosphere

continental lithospheric thinning and breakup in response to upwelling divergent mantle flow application to the woodlark newfoundland and iberia margins abstract depth uniform stretching is not the dominant deformation process for thinning continental lithosphere leading to breakup it cannot explain the observed depth dependent lithosphere stretching and mantle exhumation at rifted continental margins depth dependent lithosphere thinning in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust has been observed at many non volcanic and volcanic rifted continental margins including conjugate margin pairs passive continental margins show a paucity of brittle deformation in the upper crust during continental lithosphere thinning leading to breakup and sea floor spreading initiation a new model of rifted continental margin formation has been developed that assumes that deformation and thinning of continental lithosphere leading to breakup occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere and that this deformation evolves into sea floor spreading the new model successfully predicts depth dependent stretching of continental margin lithosphere for both non volcanic and volcanic margins and mantle exhumation at non volcanic margins both of which are observed but are not explained by existing depth uniform continental lithosphere stretching models the new model provides a balance of extensional strain supplies an explanation for the paucity of synrift brittle deformation and offers a simple transition from prebreakup lithosphere thinning to sea floor spreading the observed diversity of rifted continental margin structure and width of the ocean continent transition can be explained by variability in the form of the upwelling divergent flow field the new upwelling divergent flow model of continental lithosphere thinning leading to continental breakup successfully predicts the observed bathymetry and margin geometry for the most recent segment of sea floor spreading initiation in the woodlark basin in the western pacific and the observed bathymetry and free air gravity anomaly for the newfoundland and iberian margins

lower crustal extension across the northern carnarvon basin australia evidence for an eastward dipping detachment asymmetry and strain partitioning along conjugate margins are often explained in terms of detachment faults nevertheless cogent evidence for their existence remains limited furthermore even when inferred detachment faults are imaged seismically it is difficult to demonstrate that differential displacement has occurred across these surfaces we present tectonic and stratigraphic evidence from the northern carnarvon basin northwest australia that documents the existence of an intracrustal eastward dipping detachment by integrating sequence stratigraphy with kinematic and isostatic models of basin development we conclude that the northern carnarvon basin was formed as a consequence of four extension events 1 a broadly distributed late permian event 2 a predominantly localized rhaetian event responsible for the inception of the barrow and dampier subbasins 3 a localized callovian fault reactivation within the barrow dampier subbasins and 4 a tithonian valanginian event that generated large post valanginian regional subsidence across the northern carnarvon basin with only minor accompanying brittle deformation and erosional truncation the regional distribution and amplitude of the post valanginian subsidence are not consistent with the minor amounts of tithonian valanginian brittle upper crustal extension observed on the margin large portions of the platform were emergent or at very shallow water depths prior to the tithonian valanginian extension to match the distribution and magnitude of the post valanginian thermal type subsidence requires significant lower crustal and mantle extension across the northern carnarvon basin such a distribution of extension implies the existence of an eastward dipping intracrustal detachment having a ramp flat ramp geometry that effectively thinned the lower crust and lithospheric mantle the detachment breached the surface close to the position of the continent ocean boundary west of the exmouth plateau the flat component of the detachment occurred at midcrustal depths 15 km across the plateau and ramped beneath the australian continent lower crustal ductile extension provides a viable mechanism to generate large regional subsidence with little attendant brittle deformation which may explain the paradox that both sides of many conjugate margins appear to be the upper plate

crustal stretching style variations in the northern margin of the south china sea linking deep seismic profiles with regional scale gravity inversion is a powerful tool to deduce the architecture of rifted margins and their structural evolution here we map upper and lower crustal thicknesses of the northern south china sea scs margin in order to investigate the occurrence of depth dependent crustal extension from the proximal to the distal margin by comparing upper and lower crustal stretching factors we find that the northern margin of the scs is segmented in three parts 1 sedimentary basins where upper crust is stretched more than lower crust 2 distal margin where lower crust is stretched more than upper crust 3 mostly proximal margin regions where the two layers have similar stretching factors our results suggest that sedimentary basins and distal margin prominently feature depth dependent extension however accommodated by different processes while differential thinning within sedimentary basins appears to be governed by lateral pressure variations inducing lower crustal flow we suggest the distal margin to be affected by a combination of mantle flow induced lower crustal shearing and sequential fault activity during crustal hyper extension

sequential faulting explains the asymmetry and extension discrepancy of conjugate margins rifted continental margins are the sites that record the thinning and break up of continents to form new oceans cesar ranero and marta perez gussinye use seismic imaging to accurately measure fault extension at the conjugate west iberia and newfoundland margins and compare this to crustal thinning they use these observations to create a balanced kinematic model of rifting that resolves the extension discrepancy in which crustal thinning appears to be greater than the extension caused by brittle faulting by producing both fault controlled crustal thinning which progresses from a rift basin to the asymmetric structure and extreme thinning of conjugate rifted margins these authors use seismic imaging to accurately measure fault extension at the conjugate west iberia and newfoundland margins and compare this with crustal thinning they use these observations to create a balanced kinematic model of rifting that resolves the extension discrepancy where crustal thinning seems to be greater than the extension caused by brittle faulting during early extension cold continental lithosphere thins and subsides creating rift basins if extension continues to final break up the split and greatly thinned plates subside deep below sea level to form a conjugate pair of rifted margins although basins and margins are ubiquitous structures the deformation processes leading from moderately extended basins to highly stretched margins are unclear as studies consistently report that crustal thinning is greater than extension caused by brittle faulting1 2 3 4 this extension discrepancy might arise from differential stretching of brittle and ductile crustal layers2 but that does not readily explain the typical asymmetric structure of conjugate margins5 6 in cross section one margin displays gradual thinning accompanied by large faults and the conjugate margin displays abrupt thinning but smaller scale faulting5 whole crust detachments active from early in the rifting could in theory create both thinning and asymmetry1 but are mechanically problematical furthermore the extension discrepancy occurs at both conjugate margins leading to the apparent contradiction that both seem to be upper plates to a detachment fault7 8 alternative models propose that much brittle extension is undetected because of seismic imaging limitations caused either by subseismic resolution faulting9 invisible deformation along top basement 100 km scale detachments8 or the structural complexity of cross cutting arrays of faults3 here we use depth migrated seismic images to accurately measure fault extension and compare it with crustal thinning the observations are used to create a balanced kinematic model of rifting that resolves the extension discrepancy by producing both fault controlled crustal thinning which progresses from a rift basin to the asymmetric structure and extreme thinning of conjugate rifted margins contrary to current wisdom the observations support the idea that thinning is to a first degree explained by simple andersonian faulting that is unambiguously visible in seismic data

quantitative fault studies on the east pacific rise a comparison of sonar imaging techniques a synthesis is presented of published statistics on the lengths scarp heights and spacing of normal faults on the flanks of the east pacific rise epr between 19 s and 21 n nine data sets are included which were collected at different times employing one of four different sonar mapping systems the scripps deep tow instrument seamarc i seamarc ii and gloria with widely ranging data collection capabilities the epr in this region is morphologically quite similar and in some cases the data sets were obtained from the same area of the seafloor it is assumed therefore that data collection procedure and system resolution have had a greater influence on the fault statistics observed than varying tectonic processes we try to account for these sampling effects such that quantitative comparisons of fault development at midocean ridges may then be made we discuss the problem of identifying faults in sonar images and the errors associated with measurements of the relevant parameters we also investigate the statistical properties of the fault populations and derive expressions for correcting observed values of the population means at finite resolution to obtain the corresponding values at infinite resolution the theoretical predictions compare well with the results of a decimation procedure applied to real data sets to simulate diminishing resolution we also discuss the effects that surveying geometry can have on the interpretation of fault lengths the data synthesis is augmented by an analysis of the correlation between scarp height and fault length for areas near 3 5 s and 12 n on the epr the correlation which we find to be linear is used to estimate 1 the minimum operational fault target size for each of the four mapping systems and 2 the topography due to faulting in an area where only fault lengths are known

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

mechanisms of extension at nonvolcanic margins evidence from the galicia interior basin west of iberia 1 we have studied a nonvolcanic margin the west iberia margin to understand how the mechanisms of thinning evolve with increasing extension we present a coincident prestack depth migrated seismic section and a wide angle profile across a mesozoic abandoned rift the galicia interior basin gib the data show that the basin is asymmetric with major faults dipping to the east the velocity structure at both basin flanks is different suggesting that the basin formed along a paleozoic terrain boundary the ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension at the rift flanks stretching factor 2 the ratio is fairly constant indicating that stretching of upper and lower crust was uniform toward the center of the basin 3 5 5 5 fault block size decreases as the crust thins and faults reach progressively deeper crustal levels indicating a switch from ductile to brittle behavior of the lower crust at 3 5 faults exhume lower crustal rocks to shallow levels creating an excess of lower crust within their footwalls we infer that initially extension occurred by large scale uniform pure shear but as extension increased it switched to simple shear along deep penetrating faults as most of the crust was brittle the predominant brittle deformation might have driven small scale flow 40 km of the deepest crust to accommodate fault offsets resulting in a smooth moho topography the gib might provide a type example of nonvolcanic rifting of cold and thin crust

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

thermal evolution of a hyperextended rift basin mauleon basin western pyrenees onshore and offshore geological and geophysical observations and numerical modeling have greatly improved the conceptual understanding of magma poor rifted margins however critical questions remain concerning the thermal evolution of the prerift to synrift phases of thinning ending with the formation of hyperextended crust and mantle exhumation in the western pyrenees the mauleon basin preserves the structural and stratigraphic record of cretaceous extension exhumation and sedimentation of the proximal to distal margin development pyrenean shortening uplifted basement and overlying sedimentary basins without pervasive shortening or reheating making the mauleon basin an ideal locality to study the temporal and thermal evolution of magma poor hyperextended rift systems through coupling bedrock and detrital zircon u th he thermochronometric data from transects characterizing different structural rifting domains these new data indicate the basin was heated during early rifting to 180 c with geothermal gradients of 80 100 c km the proximal margin recorded rift related exhumation cooling at ca 98 ma whereas the distal margin remained 180 c until the onset of paleocene pyrenean shortening lithospheric scale numerical modeling shows high geothermal gradients 80 c km and synrift sediments 180 c can be reached early in rift evolution via heat advection by lithospheric depth dependent thinning and blanketing caused by the lower thermal conductivity of synrift sediments mauleon basin thermochronometric data and numerical modeling illustrate that reheating of basement and synrift strata might play an important role and should be considered in the future development of conceptual and numerical models for hyperextended magma poor continental rifted margins

climate changes control offshore crustal structure at south china sea continental margin a b s t r a c t rifted continental lithosphere subsides as a consequence of combined crustal thinning and mantle lithosphere cooling yet basins on some continental margins experience anomalous subsidence events that postdate active extension deep basins on the northern margin of the south china sea notably the baiyun sag show basement subsidence accelerating after 21 ma postdating extension by several million years we combine geophysical observations and numerical forward modeling to show that loading of the offshore basins by increased sediment flux caused by faster onshore erosion following early miocene monsoon intensification is a viable trigger for ductile flow after the cessation of active extension this illustrates that offshore basin dynamics at continental margins with weak crust can be controlled by onshore surface processes in a newly recognized form of climate tectonic coupling

abrupt plate accelerations shape rifted continental margins rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved it is well known that strain rate dependent processes control rift evolution yet quantified extension histories of earth s major passive margins have become available only recently here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions we find that rifted margins feature an initial slow rift phase less than ten millimetres per year full rate and that an abrupt increase of plate divergence introduces a fast rift phase plate acceleration takes place before continental rupture and considerable margin area is created during each phase we reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions the extension models suggest that the two phase velocity behaviour is caused by a rift intrinsic strength velocity feedback which can be robustly inferred for diverse lithosphere configurations and rheologies our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force this mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes offering new insights into the balance of plate driving forces through time

formation of the steer s head geometry of sedimentary basins by differential stretching of the crust and mantle if the lithospheric mantle is stretched over a wider region than the crust postrift stratigraphic onlap will occur at basin margins giving rise to a steer s head geometry space problems are avoided by making the total amount of extension in the lithospheric mantle equal to that in the crust the predicted pattern of onlap agrees well with that observed if the lithospheric mantle is stretched over a fractionally wider region than is the crust thus extensive stratigraphic onlap can be produced easily without recourse to sea level rise or flexural rigidity

pattern of mantle thinning from subsidence and heat flow measurements in the gulf of suez evidence for the rotation of sinai and along strike flow from the red sea we have undertaken a combined analysis of tectonic subsidence heat flow and uplift data for the gulf of suez results show significant differences along the length of the rift the heat flow and subsidence in the northern gulf of suez can be fitted by close to uniform extension in the early miocene with minimal extension since the middle miocene in contrast the southern gulf of suez requires that the early miocene extension is followed by both continuous slow 1 mm yr extension and mantle thinning greatly in excess of the crustal thinning the continued extension of the southern gulf of suez is interpreted as indicating a counterclockwise rotation of the sinai peninsula that allows the southern gulf of suez to accommodate 10 15 of the motion of the dead sea transform our best estimate of the net crustal and mantle extension factors for southern suez are 1 4 1 5 and 2 2 2 6 respectively while these values are approximate they indicate a significant input of heat into the southern gulf of suez rift that is in excess of that directly advected by extension modeling indicates that while 1 2 of the extension occurred in the early miocene most of the lithospheric thinning and virtually all of the excess thinning of the mantle lithosphere developed since then in addition the gulf of suez is bordered by large and growing rift flank uplifts particularly on the sinai peninsula the geometry of the motions at the sinai triple junction region has created a large contrast in extension between the red sea and gulf of suez sinai since the middle miocene these lateral temperature gradients drove the flow of hot material that is responsible for the extra heating beneath southern suez and sinai the rift flank uplift along most of the gulf of suez is interpreted as resulting from a combination of secondary convection and flexure the much greater uplift in southern sinai is produced by the additional convective flow from the red sea

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

modes of continental extension in a crustal wedge abstract we ran numerical experiments of the extension of a crustal wedge as an approximation to extension in an orogenic belt or a continental margin we study the effects of the strength of the lower crust and of a weak mid crustal shear zone on the resulting extension styles a weak mid crustal shear zone effectively decouples upper crustal extension from lower crustal flow without the mid crustal shear zone the degree of coupling between the upper and the lower crust increases and extension of the whole crust tends to focus on the thickest part of the wedge we identify three distinct modes of extension determined by the strength of the lower crust which are characterized by 1 localized asymmetric crustal exhumation in a single massif when the lower crust is weak 2 the formation of rolling hinge normal faults and the exhumation of lower crust in multiple core complexes with an intermediate strength lower crust and 3 distributed domino faulting over the weak mid crustal shear zone when the lower crust is strong a frictionally stronger mid crustal shear zone does not change the overall model behaviors but extension occurred over multiple rolling hinges the 3 modes of extension share characteristics similar to geological models proposed to explain the formation of metamorphic core complexes 1 the crustal flow model for the weak lower crust 2 the rolling hinge and crustal flow models when the lower crust is intermediate and 3 the flexural uplift model when the lower crust is strong finally we show that the intensity of decoupling between the far field extension and lower crustal flow driven by the regional pressure gradient in the wedge control the overall style of extension in the models

fault populations and their relationship to the scaling of surface roughness the relationship between the scaling properties of faulted geological surfaces and parameters describing the underlying fault population are investigated using simulations of a dip slip faulted surface analysis of multiple simulations of sections through the surface allowed the construction of a statistical relationship between the parameters defining the fault population and the fractal dimension of the surface the results indicate a direct if complex relationship between the fault population and the scaling of the surface roughness the main determining factor is the displacement distribution with spacing and dip having only a minor contribution this relationship is tested against examples from the moray firth scotland and the central indian ocean

development of active low angle normal fault systems during orogenic collapse insight from tibet active north trending rifts in tibet vary significantly in their character as a function of extension magnitude most rifts are characterized by internally drained basins bounded by high angle normal faults with paleogene or older rocks in the footwall however the central part of the yadong gulu rift near lhasa and the newly documented lunggar rift in west central tibet are bounded by low angle normal faults detachments with mylonitic rocks and miocene granites in the footwall and exhibit active basin incision and intrabasin topographic highs in areas of inferred maximum extension we suggest that tibetan rifts initiate as high angle normal fault and half graben or graben basin systems and evolve in response to increasing extension and footwall isostatic rebound into detachment systems that are active at uppermost crustal levels and above which rift basin fill is being uplifted and eroded

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

structural comparison of archetypal atlantic rifted margins a review of observations and concepts abstract in this study we compare three pairs of conjugate rifted margins that are often referred to as archetypes of rift systems despite numerous differences a remarkable first order structural similarity appears between the magma poor iberia newfoundland the magma rich mid norway east greenland and the sediment rich angola brazil rifted margins typical is the seaward arrangement of distinct domains proximal necking distal and outer that each present specific comparable structural characteristics our study also suggests that magma rich systems may go through a stage of hyper extension indicating that extreme crustal thinning does not preclude a magmatic breakup in this contribution we clarify the definition of a number of terms introduced recently in rifted margin studies we review the major features constitutive of these key referenced systems discuss their similarities and differences and examine how the related deformation modes develop in the life cycle of a rift that goes to seafloor spreading we conclude that the distinct domains observed in the margin architecture represent distinct stages in the evolution of rifted margin independently of their later evolution into magma poor or magma rich environments

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