friction of mineralogically controlled serpentinites and implications for fault weakness serpentines are common minerals in several major tectonic faults in a variety of geodynamic settings and have variable frictional strength and complex deformation processes here we present friction experiments carried out on a suite of serpentine samples that include veins of antigorite lizardite and fibrous serpentine chrysotile and polygonal serpentine together with massive samples of retrograde lizardite and chrysotile rich and prograde antigorite rich serpentinites these samples were characterized from the hand specimen down to the nanoscale to precisely constrain their mineralogical composition and are interpreted to represent typical fault rocks and host rocks in serpentine bearing shear zones respectively experiments were performed at effective normal stress from 5 to 120 mpa at temperatures of 25 c and 170 c and water saturated i e under the faulting conditions of the brittle upper lithosphere friction of antigorite samples either massive or vein is relatively high 0 53 retrograde massive serpentinites constituted primarily of lizardite and fibrous serpentines are frictionally weak 0 30 end members lizardite and fibrous serpentines are even weaker 0 15 0 19 and this weakness is unchanged at high temperature we document deformation of lizardite and fibrous serpentines occurring predominantly via mode ii cracking crystal fiber folding and frictional sliding which account for the observed mechanical weakness when combined with frictional reactivation analysis our data provide mechanical evidence for fault weakness inferred from earthquake dip distributions at oceanic outer rises and low angle normal faults beneath rifted continental margins and at slow ultraslow spreading mid ocean ridges

subsurface fault geometries and crustal extension in the eastern basin and range province western u s abstract we provide the first synthesis of seismic reflection data and active present day crustal deformation for the greater wasatch fault zone we analyzed a number of previously unpublished seismic reflection lines horizontal and vertical crustal velocities from continuous gps and surface geology to investigate the relationships between interseismic strain accumulation subsurface fault geometry and geologic slip rates on seismogenic faults across the eastern third of the northern basin and range province the seismic reflection data show recent activity along high angle normal faults that become listric with depth and appear to sole into preexisting decollements possibly reactivating them we interpret these listric normal faults as reactivated sevier age structures that are connected at depth with a regionally extensive detachment horizon these observations of subsurface structure are consistent with the mapped geology in areas that have experienced significant extension we modeled the crustal deformation data using a buried dislocation source in a homogeneous elastic half space the estimated model results include a low angle dislocation 8 20 at a locking depth of 7 10 km and slipping at 3 2 0 2 mm yr despite the model s relative simplicity we find that the predicted location of the dislocation is consistent with the interpreted seismic reflection data and suggests an active regionally extensive sub horizontal surface in the eastern basin and range this result may imply that this surface represents aseismic creep across a reactivated low angle fault plane or the onset of ductile flow in the lower crust at or beneath the brittle ductile transition zone under the present day basin and range extensional regime this result may also have implications for crustal rheology and suggests that geodesy might under some circumstances serve as an appropriate tool for inferring deeper crustal structure

galicia bank ocean continent transition zone new seismic reflection constraints abstract the west iberia continental margin is a type locale for magma poor rifting and studies there have been instrumental in changing the classical view of the ocean continent transition oct from a discrete boundary juxtaposing continental and oceanic crust into a more complicated zone of varying width that can include exhumed mantle this study examines two new seismic lines in the galicia bank area extending west of the peridotite ridge showing high resolution images of five new ridges these ridges could be hyperextended continental crust exhumed continental mantle or rough ultra slow spreading oceanic crust there are no tilted fault blocks with pre syn rift stratigraphy that would indicate continental crust there are also no faults indicating mid ocean spreading with seismic layer stratigraphy indicating normal oceanic crust the ridges have no coherent internal seismic structure and some resemble the topographic profile of the peridotite ridge therefore it is likely the western ridges are also mainly composed of serpentinized mantle these western ridges are also similar to small oceanic core complexes observed along the active part of the mid atlantic ridge which also contain exhumed serpentinized mantle this implies that there is a gradual transition within our study area from continental extension to seafloor spreading exhumation of continental mantle results in the formation of peridotite ridges then transitions to episodic volcanism which produces local thin basaltic crust and exhumation of oceanic core complexes asymmetric processes during initial rifting and spreading result in contrasting structures on the two resulting margins

structural style formation of low angle normal fault and its controls on the evolution of baiyun rift northern margin of the south china sea abstract recent studies of high resolution seismic reflection profiles in the northern rifted margin of the south china sea revealed that low angle normal faults dominated the structural evolution of deepwater basins rather than high angle normal faults solely as was previously assumed however none of the studies proposed kinematic restorations of the fault system and the relationship to continental margin formation the baiyun rift is located in the slope of the northern margin of the south china sea underlain by extremely thinned continental crust it is filled with a relatively complete sequence of cenozoic sediments up to 10 km and has been considered the best documented deepwater basin basin wide 3d seismic data together with the available deepwater well data have allowed the structural framework to be built and tectono stratigraphic history of the baiyun rift to be traced in detailed from the rift stage a quantitative study of the primary geometry and kinematics of the major faults can be performed consequently results show the inception of the rift is characterized by faults slipped at high angles the later stage mid eocene of intensive extension is dominated by a fault system with large horizontal displacements 10 km and initial low dips

pore pressure stress increase and fault weakening in low angle normal faulting low angle dip 30 normal faults accommodate much extension of the continental crust they apparently move under low resolved shear stress and are anomalously weak characteristics that they share with the san andreas fault structural textural and geochemical arguments suggest that low angle normal faults are weak in both the ductile and brittle regimes partly or totally due to elevated pore fluid pressure high pore pressure in detachment zones may be contained by upper plate strata mineral precipitation in their hanging walls formation of low permeability microbreccia layers threshold pressure gradients and low permeability mylonites below chlorite breccia mechanical analysis shows that fault weakening may preclude equality of the regional and fault zone stress tensors and predicts reorientation and increase of principal stresses in weak fault zones these changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks fault weakening focuses extension in the upper crust onto low angle normal ductile brittle shear zones in the midcrust promoting propagation of low angle brittle normal faults into the upper crust

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

movement along a low angle normal fault the s reflector west of spain 1 the existence of normal faults that moved at low angles less than 20 has long been debated one possible low angle fault is the s detachment at the west galicia spain margin and thought to occur at the top of serpentinized mantle it is unlikely that s was a large submarine slide as it was probably active over several million years without the development of any compressional features such as toe thrusts it appears to have rooted beneath the conjugate flemish cap margin and it is similar to structures elsewhere that also appear to be rooted detachments here we analyze depth images to identify synrift sediment packages above s and use the geometry of these synrift packages to constrain the angle at which s both formed and remained active we find that s must have remained active at angles below 15 too low to be explained simply by the low friction coefficient of partially serpentinized peridotites instead we suggest that transient high fluid pressures must have developed within the serpentinites and propose a model in which anastomosing fault strands are alternately active and sealed enabling moderately high fluid pressures to develop

analogies to oceanic behaviour in the continental breakup of the western woodlark basin the woodlark basin d entrecasteaux island region off northeast papua new guinea fig 1 offers a rare glimpse of the propagation of active rifting and continental breakup1 into orogenically thickened and subsequently extended lithosphere2 10 rifting continental breakup and the subsequent formation of oceanic lithosphere by sea floor spreading often involve rupture controlled by propagation of a rift tip11 20 we recognize several tectonic elements in the evolving western woodlark intracontinental system that are close geometric analogues of wholly oceanic propagators and a kinematic development that can be viewed as an extrapolation of oceanic tectonics the response to deformational stresses in the final stages of breakup in a thick relatively hot and hence weak continental lithosphere appears akin to that of oceanic lithosphere

evidence for seismogenic normal faults at shallow dips in continental rifts several recent observations indicate that normal faulting earthquakes occasionally occur on faults dipping 35 dips often considered shallow most of these occur in the woodlark and aegean rifts the woodlark and aegean rifts are found to generate significantly more earthquakes than others and are the most rapidly extending so display the widest variety of fault behavior even within the woodlark rift system extension rates vary along strike with the shallowest dipping faults confined to the most rapidly rifting segment here several events mw 6 0 6 8 feature nodal planes dipping 23 35 these planes are subparallel to shear zones bounding nearby metamorphic core complexes including one imaged to 8 9 km depth by seismic reflection profiling in the western gulf of corinth at least one large event mw 6 4 occurred on a fault dipping 33 as the woodlark example this rift segment exhibits a high opening rate 10 20 mm yr several other cases elsewhere based on older historic data microseismicity or geologic inference suggest seismic slip at similar or shallower dips still no documented large earthquake exhibits seismic slip on sub horizontal surfaces dip 10 15 stress rotation may explain the 23 35 dips but thus far no realistic mechanism has been found more likely these faults represent surfaces somewhat weaker than surrounding rock through some combination of modest cohesion of the surrounding rock and slightly lower frictional coefficients on the fault such weakening may be a consequence of high slip rates which rapidly generate large offsets and mature fault systems

obsidian hydration a new paleothermometer the natural hydration of obsidian was first proposed as a dating technique for young geological and archaeological specimens by friedman and smith 1960 who noted that the thickness of the hydrated layer on obsidian artifacts increases with time this approach is however sensitive to temperature and humidity under earth surface conditions this has made obsidian hydration dating more difficult but potentially provides a unique tool for paleoclimatic reconstructions in this paper we present the first successful application of this approach based on combining laboratory based experimental calibrations with archaeological samples from the chalco site in the basin of mexico dated using stratigraphically correlated 14 c results and measuring hydration depths by secondary ion mass spectrometry the resultant data suggest first that this approach is viable even given the existing uncertainties and that a cooling trend occurred in the basin of mexico over the past 1450 yr a result corroborated by other paleoclimatic data

continental subduction intracrustal shortening and coeval upper crustal extension p t evolution of subducted south iberian paleomargin metapelites betics se spain abstract the nevado filabride complex represents the basement and permo triassic cover of the south iberian passive margin metamorphosed during miocene continental subduction under the alboran crustal domain it forms a nappe stack with the thick calar alto unit 6 km thick sandwiched between the ragua unit below and the bedar macael unit above the nappe stacking occurred by ductile flow along basal syn metamorphic shear zones structural analysis of the calar alto unit together with thermobarometric data of samples from both the bottom and top of the unit served us to understand the tectonic significance of this episode of intracrustal shortening in the context of a strongly extended orogen multiequilibrium thermobarometry shows that the calar alto schist underwent a prograde p t path at mantle depths from 1 6 gpa at 450 500 c to 1 0 gpa at 550 570 c during the growth of the schistosity s1 parallel to the compositional layering after detachment from the downgoing slab and underplating the unit underwent nearly isobaric cooling by up to 250 c from peak metamorphic conditions the main crenulation cleavage s2 and the mylonitic foliation sm developed within the crustal nappe stack during decompression and heating sm at the base of the unit grew between 0 8 gpa at 400 c and 0 4 gpa at 500 c while s2 at the top of the unit developed between 0 6 gpa at 300 c and 0 2 gpa at 400 c these shortening structures developed simultaneously to the activity of overlying middle miocene extensional detachments intracrustal shortening and coeval upper crustal extension resulted in a mode of exhumation where the material rise entailed an important horizontal component of displacement at rates of approximately 15 mm year being exhumed along an s shaped p t path after peak metamorphic conditions and its detachment from the subducting slab

reconstructing thermal properties of firn at summit greenland from a temperature profile time series we have constrained the value for thermal diffusivity of near surface snow and firn at summit station greenland using a fourier type analysis applied to hourly temperature measurements collected from eight thermistors in a closed off air filled borehole between may 2004 and july 2008 an implicit finite difference method suggests that a bulk diffusivity of 25 3m2 a 1 is the most reasonable for representing macroscale heat transport in the top 30 m of firn and snow this value represents an average diffusivity and in a conduction only model generates temperature series whose phase shifts with depth most closely match those of the summit borehole data rms difference between measurements and model output is 6 days this bulk value derived numerically and corroborated analytically is useful over large tracts of the greenland ice sheet where density and microstructure are unknown

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

compressional tectonic inversion of the algero balearic basin latemost miocene to present oblique convergence at the palomares margin western mediterranean interpretation of new multichannel seismic reflection profiles indicates that the palomares margin was formed by crustal scale extension and coeval magmatic accretion during middle to late miocene opening of the algero balearic basin the margin formed at the transition between thinned continental crust intruded by arc volcanism and back arc oceanic crust deformation produced during the later positive inversion of the margin offshore and onshore is partitioned between n50 e striking reverse faults and associated folds like the sierra cabrera and abubacer anticlines and n10 20 e sinistral strike slip faults like palomares and terreros faults parametric subbottom profiles and multibeam bathymetry offshore structural analysis available gps geodetic displacement data and earthquake focal mechanisms jointly indicate that tectonic inversion of the palomares margin is currently active the palomares margin shows a structural pattern comparable to the north maghrebian margins where africa eurasia plate convergence is accommodated by ne sw reverse faults nnw sse sinistral faults and wnw ese dextral ones contractive structures at this margin contribute to the general inversion of the western mediterranean since 7 ma coeval to inversion at the algerian margin shortening at the alboran ridge and al idrisi faults occurred later since 5 ma indicating a westward propagation of the compressional inversion of the western mediterranean

polyphase rifting and break up of the central mozambique margin abstract the break up of the gondwana supercontinent resulted in the formation of the central mozambique passive margin as africa and antarctica were separated during the mid jurassic period although plate kinematics during the oceanic spreading phase are well constrained the initial fit of africa and antarctica their earliest relative movements and margin architectures remain active areas of interest this study uses high quality multi channel seismic reflection profiles to identify the major crustal domains in the angoche and beira regions of the central mozambique margin our results show that the central mozambique passive margin is characterised by intense but localised magmatic activity evidenced by the existence of seaward dipping reflectors sdrs in the angoche region and magmatic sills and volcanoclastic material marking the beira high the angoche and beira regions possess faulted upper continental crusts with possible exhumation of lower crustal material forming an extended ocean continent transition oct the beira high segment reveals an offshore continental fragment which is overlain by a faulted pre rift sedimentary unit likely to belong to the karoo group the combination of our seismic interpretation with existing geophysical and geological data has allowed us to propose a break up model which supports the idea that the central mozambique margin was affected by polyphase rifting the beira high basement is formed by a strike slip deformation along a proposed lithospheric weakness the lurio pebane shear zone northwestern southeastern oriented extension follows and results in continental break up and oceanic spreading our results suggest a segmentation of the central mozambique margin with oceanisation first occurring in the angoche segment the formation of the first oceanic crust in the beira segment followed likely delayed by the formation and failure of the northern beira high rift

waveform inversion of the s reflector west of spain fine structure of a detachment fault the s reflection west of iberia has been interpreted as a low angle detachment fault separating crustal fault blocks from partially serpentinized mantle we apply full waveform inversion to investigate the fine structure of s our results confirm that s is largely a step increase in velocity and density probably from crustal rocks to partially serpentinized mantle peridotites a 50 m thick low velocity zone above s might represent a main fault zone of highly serpentinized peridotites or hydrofractured and altered crustal rocks above the main fault zone both interpretations imply focused fluid flow along s raising the possibility that low angle movement along s was aided by the development of local transient high fluid pressures

seismic slip on a low angle normal fault in the gulf of corinth evidence from high resolution cluster analysis of microearthquakes the gulf of corinth in western greece is one of the most active extensional zones in the aegean region it is still an open question whether extension can be actively accommodated on low angle faulting or if those faults as seen in geological records have been rotated whilst numerous fault plane solutions obtained from a dense temporary network deployed in the western part of the gulf in july august of 1991 showed one of the nodal planes as a subhorizomal plane slip on the high angle conjugate plane is equally probable from the focal mechanism data rigo et al 1996 since part of the activity occurred in spatial clusters with similar focal mechanisms we used a high resolution cluster analysis to determine the most likely active plane exploiting the waveform similarity of these events relative onset times of p and s waves could be determined at subsample accuracy less than 0 01 s the cluster analyzed here contains 12 evems among these 8 have a well con strained normal faulting fault plane solution with a shal low 12 20 o north dipping plane and a steeply south dipping plane a master evem relocation shows that the relocated 12 hypocenter cemroids are aligned along the low angle plane showing clear evidence for active low angle normal faulting

is the sevier desert reflection of west central utah a normal fault a prominent west dipping reflection that can be traced in seismic reflection profiles over an area of 7000 km 2 beneath the sevier desert basin of west central utah is generally referred to as the sevier desert detacbment and is widely regarded as one of tbe best examples of an upper crustal low angle normal fault the absence of evidence for fault related deformation in drill cuttings and core from two industry boreholes that intersect this feature casts doubt on the fault interpretation the existing interpretation is based mainly on the observation that high angle normal faulting is restricted largely to tertiary sedimentary and volcanic rocks above the reflection an alternative explanation is tbat the bigh angle faults are related to the withdrawal of early deposited lacustrine salt which even today is as much as 2 km thick reevaluation of the seismic data suggests that the sevier desert reflection consists of two spatially and genetically distinct segments a shallow segment here interpreted as an unconformity between paleozoic and tertiary strata and a fortuitously aligned deeper segment that is traceable to mid and lower crustal levels and that appears to represent a thrust fault related to the cretaceous sevier orogeny

amount of extension on small faults an example from the viking graben extension estimates based on palinspastic restoration of only the largest faults in a deformed terrane may be significantly deficient due to neglect of small faults i e unsampled faults most of which have smaller displacements than do sampled faults small faults individually account for little extension but there may be large numbers of them the cumulative extension on small faults can be calculated from the fractal size distributions of fault displacements independent extension estimates support the inference from fault population statistics that small faults account for significant amounts of extension for example restorations of mesozoic faults interpreted from seismic reflection profiles across the viking graben in the north sea yield extension estimates that are only 40 75 of independent estimates the fractal size distribution of fault displacements in the viking graben is consistent with the hypothesis that the remaining 25 60 of extension resulted from displacements on small faults

assessment of crustal velocity models using seismic refraction and reflection tomography two tomographic methods for assessing velocity models obtained from wide angle seismic traveltime data are presented through four case studies the modelling inversion of wide angle traveltimes usually involves some aspects that are quite subjective for example 1 identifying and including later phases that are often difficult to pick within the seismic coda 2 assigning specific layers to arrivals 3 incorporating pre conceived structure not specifically required by the data and 4 selecting a model parametrization these steps are applied to maximize model constraint and minimize model non uniqueness however these steps may cause the overall approach to appear ad hoc and thereby diminish the credibility of the final model the effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide angle data set the first arrival times for data sets with moho reflections the tomographic velocity model can be used to invert the pmp times for a minimum structure moho in this way crustal velocity and moho models can be obtained that require the least amount of subjective input and the model structure that is required by the wide angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data the tomographic models are not intended to supersede the preferred models since the latter model is typically better resolved and more interpretable this form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above this was followed by conventional first arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth minimum structure models the case studies are from the se canadian cordillera across the rocky mountain trench central india across the narmada son lineament the iberia margin across the galicia bank and the central chilean margin across the valparaiso basin and a subducting seamount these case studies span the range of modern wide angle experiments and data sets in terms of shot receiver spacing marine and land acquisition lateral heterogeneity of the study area and availability of wide angle reflections and coincident near vertical reflection data the results are surprising given the amount of structure in the smooth tomographically derived models that is consistent with the more subjectively derived models the results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide angle traveltime data

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

crustal strain dependent serpentinisation in the porcupine basin offshore ireland abstract mantle hydration serpentinisation at magma poor rifted margins is thought to play a key role in controlling the kinematics of low angle faults and thus hyperextension and crustal breakup however because geophysical data principally provide observations of the final structure of a margin little is known about the evolution of serpentinisation and how this governs tectonics during hyperextension here we present new observational evidence on how crustal strain dependent serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of the porcupine basin offshore ireland we present three new p wave seismic velocity models that show the seismic structure of the uppermost lithosphere and the geometry of the moho across and along the basin axis we use neighbouring seismic reflection lines to our tomographic models to estimate crustal stretching c of 2 5 in the north at 52 5 n and 10 in the south at 51 7 n these values suggest that no crustal embrittlement occurred in the northernmost region and that rifting may have progressed to crustal breakup in the southern part of the study area we observed a decrease in mantle velocities across the basin axis from east to west these variations occur in a region where c is within the range at which crustal embrittlement and serpentinisation are possible c 3 4 across the basin axis the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting indicating fault controlled mantle hydration mantle velocities also suggest that the degree of serpentinisation together with the amount of crustal faulting increases southwards along the basin axis seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is 15 this observation is consistent with laboratory measurements that show that at this degree of serpentinisation mantle rocks are sufficiently weak to allow low angle normal faulting based on these results we propose two alternative formation models for the porcupine basin the first involves a northward propagation of the hyperextension processes while the second model suggests higher extension rates in the centre of the basin than in the north both scenarios postulate that the amount of crustal strain determines the extent and degree of serpentinisation which eventually controls the development of detachments faults with advanced stretching

evidence for fault weakness and fluid flow within an active low angle normal fault determining the composition and physical properties of shallow dipping active normal faults dips 35 with respect to the horizontal is important for understanding how such faults slip under low resolved shear stress and accommodate significant extension of the crust and lithosphere seismic reflection images1 and earthquake source parameters2 show that a magnitude 6 2 earthquake occurred at about 5 km depth on or close to a normal fault with a dip of 25 30 located ahead of a propagating spreading centre in the woodlark basin here we present results from a genetic algorithm inversion of seismic reflection data which shows that the fault at 4 5 km depth contains a 33 m thick layer with seismic velocities of about 4 3 km s 1 which we interpret to be composed of serpentinite fault gouge isolated zones exhibit velocities as low as 1 7 km s 1 with high porosities which we suggest are maintained by high fluid pressures we propose that hydrothermal fluid flow possibly driven by a deep magmatic heat source and high extensional stresses ahead of the ridge tip have created conditions for fault weakness and strain localization on the low angle normal fault

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

continental hyperextension mantle exhumation and thin oceanic crust at the continent ocean transition west iberia new insights from wide angle seismic hyperextension of continental crust at the deep galicia rifted margin in the north atlantic has been accommodated by the rotation of continental fault blocks which are underlain by the s reflector an interpreted detachment fault along which exhumed and serpentinized mantle peridotite is observed west of these features the enigmatic peridotite ridge has been inferred to delimit the western extent of the continent ocean transition an outstanding question at this margin is where oceanic crust begins with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies here we present results from a 160 km long wide angle seismic profile western extension 1 travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the s reflector the s reflector correlates with the 6 0 7 0 km s 1 velocity contours corresponding to peridotite serpentinization of 60 30 respectively west of the peridotite ridge shallow and sparse moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer which increases in thickness from 0 5 km at 20 km west of the peridotite ridge to 1 5 km 35 km further west p wave velocities increase smoothly and rapidly below top basement to a depth of 2 8 3 5 km with an average velocity gradient of 1 0 s 1 below this velocities slowly increase toward typical mantle velocities such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth

the structure and evolution of deepwater basins in the distal margin of the northern south china sea and their implications for the formation of the continental margin abstract although it is generally accepted that the south china sea margin can be classified as a magma poor rifted margin little is known about the structural style of the distal margin and how the distal deep basins evolved observations and data from the northern south china sea margin suggest that the deepwater basins distal margin domain are commonly underlain by a highly extended continental crust and detachment fault systems have been interpreted in places we have interpreted a large seismic data set covering deepwater basins in the northern south china sea the interpreted moho on reflection seismic profiles indicates that the crustal thickness varies along sections perpendicular to the coast which is the first order criterion to divide the whole margin into a proximal domain necking zone and distal domain furthermore the underlying extremely stretched crust

three dimensional architecture structural evolution and role of inheritance controlling detachment faulting at a hyperextended distal margin the example of the err detachment system se switzerland

evolution of deep water rifted margins testing depth dependent extensional models 1 a general understanding of rifted margins which form by thinning of the continental lithosphere exists nevertheless the exact form of thinning is unclear this debate has been stimulated by acquisition of dense seismic wide angle and deep reflection surveys from atlantic ocean margins a central issue concerns the way in which thinning changes with depth we have tackled this issue by developing a generalized inverse model this model attempts to fit subsidence and crustal thinning observations by varying strain rate as a function of time and space depth dependent thinning is permitted but we do not prescribe its existence or form here the algorithm is applied to six margins including two of the most contentious conjugate margins newfoundland iberia and brazil angola calculated strain rate histories predict thinning estimates which broadly match estimates inferred from normal faulting the eastern indian and beaufort sea margins formed by largely uniform lithospheric thinning in contrast the newfoundland iberian conjugate margins formed by a pattern of strongly depth dependent strain rate to account for the paucity of syn rift decompression melting of the underlying asthenosphere the lithospheric mantle close to oceanic continent transition must thin more slowly than the overlying crust this form of depth dependency is not common for example the brazil angolan conjugate margin could have formed by uniform lithospheric thinning provided thick layers of salt were deposited in a preexisting 400 m deep topographic depression depth dependent thinning is not required to account for rapid subsidence of presalt strata

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

kinematic evolution of the southern north atlantic implications for the formation of hyperextended rift systems we focus on the southern north atlantic rifted margins to investigate the partitioning and propagation of deformation in hyperextended rift systems using plate kinematic modeling the kinematic evolution of this area is well determined by oceanic magnetic anomalies after the cretaceous normal polarity superchron however the rift and early seafloor spreading evolution 200 83 ma remains highly disputed due to contentious interpretations of the j magnetic anomaly on the iberia newfoundland conjugate margins recent studies highlight that the j anomaly is probably polygenic related to polyphased magmatic events and therefore does not correspond to an isochron we present a new palinspastic restoration without using the j magnetic anomaly as the chron m0 we combine 3 d gravity inversion results with local structural stratigraphic and geochronological constraints on the rift deformation history the restoration of the southern north atlantic itself is not the primary aim of the study but rather is used as a method to investigate the spatiotemporal evolution of hyperextended rift systems we include continental microblocks that enable the partitioning of the deformation between different rift segments which is of particular importance for the evolution of the iberia eurasia plate boundary our modeling highlights the following 1 the segmentation of the iberia newfoundland rift system during continental crust thinning 2 the northward v shape propagation of mantle exhumation and seafloor spreading 3 the complex partitioning of deformation along the iberia eurasia plate boundary and 4 a three plate propagation model which implies transtension

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

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

seasonal and interannual variations of firn densification and ice sheet surface elevation at the greenland summit seasonal and interannual variations in surface elevation at the greenland summit are modeled using a new temperature dependent formulation of firn densification and are compared with elevations from european remote sensing satellite ers 1 2 radar altimetry the rate constant and activation energy usually set as constants in the arrhenius type relation are strongly temperature dependent based on measurements of crystal growth rates a multiplicative factor in the densification rate accounts for differences between densification and grain growth rates and is chosen to match the modeled and measured density profiles from 0 to 40 m the stronger temperature dependence produces a significant seasonal cycle in the densification rate in the upper firn much of the densification and consequent surface lowering occur within 3 months in late spring early summer followed by a build up from accumulation modeled elevation changes using automatic weather station measurements of temperature and accumulation and modeled precipitation agree well with observations the respective seasonal amplitudes are 18 and 25 cm peak to peak with minima in mid summer the minimum elevation in 1995 is driven mainly by a temporary accumulation decrease and secondarily by warmer temperatures increased compaction driven by a summer warming trend decreases the modeled elevation 1992 99 by 20 cm but accumulation increases in latter years dominate the overall 4 2 cm a 1 trend

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

application of the critical coulomb wedge theory to hyper extended magma poor rifted margins the critical coulomb wedge theory ccwt has been extensively used in compressional tectonics to resolve the shape of orogenic or accretionary prisms while it is less applied to extensional and gravitational wedges despite the fact that it can be described by the same equation in particular the hyper extended domain at magma poor rifted margins forming the oceanward termination of extended continental crust satisfies the three main requirements of the ccwt 1 it presents a wedge shape 2 the rocks forming the wedge are completely brittle frictional and 3 the base of the wedge corresponds to a low friction decollement however hyper extended margins present a fully frictional behaviour only for a very thin crust therefore this study is limited to the termination of hyper extended continental crust which deforms in the latest stage of continental rifting in this paper we define a method to measure the surface slope and the basal deep of this wedge that we apply to 17 hyper extended magma poor rifted margins in order to compare the results to the values predicted by the ccwt because conjugate pairs of hyper extended magma poor rifted margins are commonly asymmetric due to detachment faulting the wedges in the upper and lower plate margins corresponding respectively to the hanging wall and footwall of the detachment system are different while the stress field in the upper plate wedge corresponds to a tectonic extensional wedge the one in the lower plate matches that of a gravity extensional wedge using typical frictional properties of phyllosilicates e g clays and serpentine the shape of the hyper extended wedges can be resolved by the ccwt using consistent fluid overpressures our results show that all lower plate margins are gravitationally stable and therefore have a close to critical shape whereas the tectonic extensional wedges at upper plate margins are critical sub or sup critical due to the detachment initial angle and the duration of the tectonic activity in this paper we discuss the geometry and structural evolution of the most distal parts of hyper extended continental margins and the formation of extensional allochthons during hyper extension using the ccwt theory

physical and structural properties of the greenland ice sheet project 2 ice core a review substantial data sets have been collected on the relaxation characteristics density grain size c axis fabrics and ultrasonic velocities of the greenland ice sheet project 2 gisp2 core to its contact with bedrock at 3053 4 m changes in all these properties paralleled closely those found in cores from byrd station antarctica and dye 3 greenland density increased progressively with depth to a maximum of 0 921 mg m3 at about 1400 m at which depth the ice became bubble free below about 2000 m in situ densities began to decrease in response to increasing ice sheet temperatures since drilling much of the ice core has undergone significant volume expansion relaxation due to microcracking and the exsolving of enclathratized gases especially in the brittle ice zone between 650 and 1400 m grain size increased linearly to about 1000 m thereafter remaining fairly constant until the younger dryas event at 1678 m where a twofold to threefold decrease in grain size occurred these grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical including a small increase in c axis concentration across the younger dryas holocene boundary increased dust levels in the wisconsin ice have contributed to the maintenance of a fine grained texture which with its strong vertical c axis fabric persisted to nearly 3000 m however beginning at about 2800 m layers of coarse grained ice intermixed with the much finer grained matrix ice are observed below 3000 m the ice became very coarse grained this change attributed to annealing recrystallization at elevated temperatures in the ice sheet was accompanied by a dispersed or ring like redistribution of the c axes about the vertical ultrasonic measurements of vertical and horizontal p wave velocities made at 10 m intervals along the entire length of the gisp2 core fully confirmed the results of the crystallo optical observations a return to fine grained ice coincided with the first appearance of brown silty ice 13 m above bedrock bedrock material consisted of 48 cm of till including boulders and cobbles overlying gray biotite granite comprising the true bedrock there is evidence that disturbed structure in the gisp2 cores begins little more than 70 of the way through the ice sheet this disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10 of the ice sheet

the hydrothermal plumbing of a serpentinite hosted detachment evidence from the west iberia non volcanic rifted continental margin this study documents the critical role of structurally induced fluid flow during the evolution of the footwall succession to a major low angle normal detachment fault drilled by the ocean drilling program leg 173 site 1068 beneath the southern iberia abyssal plain the fault zone comprises carbonate altered rodingitized and albitized metabasite rich sedimentary breccias and serpentinized mantle peridotites the brittle infrastructure of the detachment consists of mineralized high dilation breccias and meshes of mineralized extensional and shear veins that root into chlorite and serpentine cataclasite and gouge the fault rocks are underlain by cohesive serpentinite that shows kernel textures indicative of volume expansion accompanying serpentinization of peridotite the texture is disrupted and offset by small scale fractures and faults the distribution of serpentine polytypes carbonates fe ni alloys sulfides oxides and other silicate phases varies across the fault zone in patterns consistent with mineralization and replacement from solutions derived from two end member components seawater and ch4 bearing calcium hydroxide enriched hydrothermal solutions the latter form when heated seawater reacts with peridotite to form serpentinite at low water to rock ratios serpentine mineral chemistries indicate that fracture controlled serpentinite recrystallization and replacement occurred at various fo2 asio2 and ca2 conditions in places this also involved mild prograde thermal events the serpentinite also hosts tochilinite valleriite group minerals and aragonite both are interpreted as indicators of sea water incursions into the upper reaches of the detachment to account for the evidence of coeval hydrothermal mineralization and displacements across the detachment we relate hydrothermal discharge to the buffering of high pore fluid pressures by fault slip localized sources of high fluid pressures at depth are attributed to serpentinization of peridotite around the fault that promotes changes to solution mass density exothermic reactions and swelling pressures sealing of the fault between the serpentinization front and the top of the detachment results from hydrothermal mineralization alteration and serpentine gouges hydrothermal discharges from the detachment accompanying shear failure allow for variable mixing between the hydrothermal solutions and seawater and post slip convective draw down of seawater into the detachment it is suggested that the latter may have been limited in duration by ongoing mineralization leading to the restoration of the fault seal concomitant serpentinization around the detachment at depth provides scope for cyclic hydrothermal discharges and fault slip

stress state and fluid pressure level along the whipple detachment fault california brittle structures in the upper 200 m of the footwall of the whipple detachment fault are used to characterize the state of stress under which the detachment slipped they are younger than or coeval with chloride brecciation and those that record shear typically are either moderately northeast dipping normal structures or steep northwest striking southwest down structures that we interpret as conjugate shears from this we infer that the maximum principal stress was oriented 55 80 from the detachment while they were active the angle between the conjugate faults averages about 30 indicating failure in the transtensional regime during conjugate faulting the effective least principal stress was nearly equal to the tensile strength the whipple fault probably had a normal coefficient of static friction and it slipped under hydrostatic to only moderately elevated fluid pressure rapid mineralization in the surroundings of major fault zones allows transtensional failure there and permits differential stress to be higher than if cohesionless friction limits strength this causes resolved shear stress sufficient for slip on faults severely misoriented relative to the maximum principal stress

snowmelt on the greenland ice sheet as derived from passive microwave satellite data abstract the melt extent of the snow on the greenland ice sheet is of considerable importance to the ice sheet s mass and energy balance as well as arctic and global climates by comparing passive microwave satellite data to field observations variations in melt extent have been detected by establishing melt thresholds in the cross polarized gradient ratio xpgr the xpgr defined as the normalized difference between the 19 ghz horizontal channel and the 37 ghz vertical channel of the special sensor microwave imager ssm i exploits the different effects of snow wetness on different frequencies and polarizations and establishes a distinct melt signal using this xpgr melt signal seasonal and interannual variations in snowmelt extent of the ice sheet are studied the melt is found to be most extensive on the western side of the ice sheet and peaks in late july moreover there is a notable increasing trend in melt area between the years 1979 and 1991 of 4 4 per year which came to an abrupt halt in 1

distinguishing between rooted and rootless detachments a case study from the mormon mountains of southeastern nevada abstract rooted detachment faults and detachments beneath rootless slide blocks exhibit many similar structural characteristics however while rooted detachments are thought to penetrate into the midcrust and to accommodate significant crustal extension rootless detachments break to the surface downdip and are not directly involved in such extension distinguishing between these two mechanically different kinds of structure is central to the assessment of extension magnitude here we examine deformation along the mormon peak detachment a feature that has been cited as an example of both a rooted and a rootless structure located in the mormon mountains of southeastern nevada this detachment has been interpreted as one of three low angle normal faults of regional scale that together are thought to have accommodated more than 50 km of basin and range extension for the most part however the mormon peak detachment is expressed as a series of isolated exposures where paleozoic rocks are in brittle fault

ice sheet elevation changes caused by variations of the firn compaction rate induced by satellite observed temperature variations 1982 2003 changes in the surface elevation of the greenland and antarctic ice sheets and ice shelves caused by variations in the rate of firn compaction are calculated with a time dependent firn densification model driven by two decades 1982 2003 of satellite observed monthly surface temperatures the model includes the effects of melting and refreezing both the direct changes in density and the subsequent effects on the densification rate as previously shown the temperature dependent rate of densification is largest in summer but changes in winter temperatures also have a significant effect over the last decade climate warming has enhanced the rate of compaction and lowered the average surface elevation of greenland by 1 8 cma 1 and most of west antarctica by 1 9 cma 1 in east antarctica a small cooling raised the average surface elevation by 0 14 cma 1

finite extension and low angle normal faulting andersonian theory for infinitesimal strains predicts that normal faults will form at high angles to the surface in extensional terranes in contrast low angle normal faults are sometimes observed on which tens of kilometers of slip has occurred and moderate angle faults with slip of 5 to 15 km are common a simple extension of the theory to account for the stresses required to drive finite deformation shows that low angles are preferred for large extension on individual faults the formation of low angle faults may require inhomogeneities in stress or material properties but once slip begins on a low angle fault it is likely to continue whereas slip on high angle faults is likely to end after modest slip to be replaced by the formation of new high angle faults

polyphase faulting during the development of the west galicia rifted margin rifted margins are commonly characterised by an extension discrepancy the amount of extension measurable from the observed faulting is far less than that required to explain the crustal thinning it is shown here that polyphase faulting may provide a simple explanation for this paradox but can be very hard to recognise on seismic sections however at the west galicia rifted margin the galicia interior basin between the galicia bank and the mainland and the deep galicia margin to the west of the galicia bank a combination of high quality depth images seismic velocity information and stratigraphic control through odp drilling and submersible sampling provide complementary evidence for polyphase faulting berriasian hauterivian rifting in the galicia interior basin occurred along two sets of faults the first unroofed deep crustal rocks evidenced by high seismic velocities close to top basement the second cut and dismembered these early faults further rifting up until the aptian then focussed west of the galicia bank where two further phases of faulting can be inferred from the diachronous nature of seismostratigraphic units tilted within fault blocks removal of the latest phase of faulting aligns discontinuous reflections within the fault blocks into an anastomosing network of earlier faults restoration along these brings the crust back to its early hauterivian state similar to the present structure of the interior basin

strength of chrysotile serpentinite gouge under hydrothermal conditions can it explain a weak san andreas fault chrysotile bearing serpentinite is a constituent of the san andreas fault zone in central and northern california at room temperature chrysotile gouge has a very low coefficient of friction 0 2 raising the possibility that under hydrothermal conditions might be reduced sufficiently to 0 1 to explain the apparent weakness of the fault to test this hypothesis we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 c and axial shortening velocities as low as 0 001 m s as temperature increases to 100 c the strength of the chrysotile gouge decreases slightly at low velocities but at temperatures 200 c it is substantially stronger and essentially independent of velocity at the lowest velocities tested we estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 mpa thus on the sole basis of its strength chrysotile cannot be the cause of a weak san andreas fault however chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures

successive detachment faults and mantle unroofing at magma poor rifted margins seismic depth images show that magma poor rifted margins between iberia and newfoundland are characterized by two different types of detachment that led to the unroofing of a broad expanse of mantle low angle detachments develop in serpentinites at the base of pre thinned crust and control further crustal thinning these detachments are cut by large offset faults rooting at a steep angle but with an exhumed slip surface and footwall flexurally rotated to a low angle during unroofing successive generations of this second type of detachment lead to the roughly symmetric unroofing of a broad expanse of mantle as new detachments repeatedly cut through the footwall of the preceding detachment leaving the abandoned root zone as landward dipping reflectors within exhumed mantle on both sides of the developing rift

continental breakup and the onset of ultraslow seafloor spreading off flemish cap on the newfoundland rifted margin prestack depth migrated seismic reflection data collected off flemish cap on the newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system within continental crust there is no clear evidence for detachment surfaces analogous to the s reflection off the conjugate galicia bank margin demonstrating a first order asymmetry in final rift development anomalously thin 3 4 km magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized this indicates that initial accretion of the oceanic crust was in a magma limited setting similar to present day ultraslow spreading environments seaward oceanic crust thins to 1 3 km and exhibits an unusual highly reflective layering we propose that a period of magma starvation led to exhumation of mantle in an oceanic core complex that was subsequently buried by deep marine sheet flows to form this layering subsequent seafloor spreading formed normal 6 km thick oceanic crust this interpretation implies large fluctuations in the available melt supply during the early stages of seafloor spreading before a more typical slow spreading system was established

extreme crustal thinning in the south porcupine basin and the nature of the porcupine median high implications for the formation of non volcanic rifted margins the southern porcupine basin is characterized by axial stretching factors that are greater than six and typical of rifted margins as such the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup a bright reflection here named p cuts down to the west from the base of the sedimentary section is overlain by small fault blocks and appears to represent a detachment fault p may in part follow the top of partially serpentinized mantle this interpretation is consistent with gravity modelling and with numerical models of crustal embrittlement and mantle serpentinization during extension furthermore p closely resembles the s reflection west of iberia where such serpentinites are well documented although overall the basin remains symmetrical the consistent westward structural dip of the detachment implies that at high stretching factors extension became asymmetric farther south the porcupine median high appearing lens shaped in cross section overlies the tilted fault blocks and is onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it develops above the line of intersection of the crust mantle boundary with the p detachment and hence may be related to the spatial limit of serpentinization the median high may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

stress field rotation and rooted detachment faults a coulomb failure analysis several well known mechanical models have shown that unusual boundary or loading conditions can alter principal stress orientations into configurations consistent with low angle normal faulting such models however have not demonstrated whether magnitudes of reoriented stresses are sufficient to initiate and promote slip on low angle surfaces we present the results of simple coulomb failure analyses to determine whether and where such models predict frictional slip assuming geologically plausible boundary stresses pore pressures and rock strengths models that invoke a sizable shear traction at the base of the upper crust or spatially varying loads on the upper crust reorient principal stresses and failure planes but do not produce frictional failure on crustal scale detachments either in the absence of pore fluids or at hydrostatic pore fluid pressures models that reorient stresses by midcrustal dike intrusion produce slip on low angle surfaces at relatively deep crustal levels but only in the area of the dike tip the low angle failure surfaces curve into a high angle orientation a short distance from the dike all of these models also imply unsustainably high absolute tensile stresses in the upper 5 km of the crust and suggest that in any system in which stresses are allowed to evolve over time failure and stress release will occur on high angle faults before low angle ones have developed these assertions are true even when near lithostatic pore pressures are assumed unless there is an inhomogeneous extraordinarily fortuitous distribution of pore pressures and rock strengths at the time of initiation of a new detachment fault one model we tested for example required pore pressures exceeding 0 96 times lithostatic in the area of the hypothesized low angle normal fault with lower pore pressures both above and below the detachment to prevent slip and stress release on high angle normal faults in the upper part of the modeled region and on low angle thrust faults in the lower part

the mechanical feasibility of low angle normal faulting abstract low angle normal faulting is precluded by elementary rock mechanics arguments however the existence of many low angle normal faults in regions such as western north america has long suggested that this familiar theory is incomplete initiation of a low angle normal fault requires the stress tensor within the brittle upper crust to have inclined principal axes which necessitates a substantial shear stress within the vertical plane nonetheless no one has previously identified any specific extensional stress field which is suitably oriented to allow shear failure of previously unfractured rock at a low angle dip of 30 in preference to the steeper dip of 50 60 predicted by standard theory despite previous claims to the contrary this study shows for the first time that appropriately oriented stress fields can exist however they require the shear stress to locally reach 100 mpa near the base of a 10 km thick brittle layer this is only possible under extreme conditions as it requires dramatic lateral variations in the state of stress across the extending region it is suggested that a shear stress of this order can develop due to the combined effects of lower crustal flow which imparts a horizontal shear traction at the base of the brittle layer and loading which imparts shear traction in the vertical plane associated with the isostatic response to changes in heat flow caused by changes to the geometry of subducting slabs beneath an extending region regional patterns of low angle normal faulting in western north america are thus interpreted in terms of changes to the geometry of subduction of the farallon plate

the role of tectonic inheritance in the morphostructural evolution of the galicia continental margin and adjacent abyssal plains from digital bathymetric model dbm analysis nw spain the identification of recent major tectonic structures in the galicia continental margin and adjacent abyssal plains was carried out by means of a quantitative analysis of the linear structures having bathymetric expression on the seabed it was possible to identify about 5800 lineaments throughout the entire study area of approximately 271 500 km2 most lineaments are located in the charcot and coruna highs in the western sector of the galicia bank in the area of the marginal platforms and in the northern sector of the margin analysis of the lineament orientations shows a predominant ne sw direction and three relative maximum directions nw se e w and n s the total length of the lineaments identified is over 44 000 km with a mode around 5000 m and an average length of about 7800 m in light of different tectonic studies undertaken in the northwestern margin of the iberian peninsula we establish that the lineaments obtained from analysis of the digital bathymetric model of the galicia continental margin and adjacent abyssal plains would correspond to fracture systems in general the orientation of lineaments corresponds to main faults tectonic structures following the directions of ancient faults that resulted from late stages of the variscan orogeny and mesozoic extension phases related to triassic rifting and upper jurassic to early cretaceous opening of the north atlantic ocean the n s convergence between eurasian and african plates since palaeogene times until the miocene and nw se convergence from neogene to present reactivated the variscan and mesozoic fault systems and related physiography

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

3d development of detachment faulting during continental breakup abstract the developing asymmetry of rifting and continental breakup to form rifted margins has been much debated as has the formation mechanics and role of extensional detachments bespoke 3d seismic reflection data across the galicia margin west of spain image in unprecedented detail an asymmetric detachment the s reflector mapping s in 3d reveals its surface is corrugated proving that the overlying crustal blocks slipped on s surface during the rifting crucially the 3d data show that the corrugations on s perfectly match the corrugations observed on the present day block bounding faults demonstrating that s is a composite surface comprising the juxtaposed rotated roots of block bounding faults as in a rolling hinge system with each new fault propagation moving rifting oceanward changes in the orientation of the corrugations record the same oceanward migration however in contrast to previous rolling hinge models the slip of the crustal blocks on s occurred at angles as low as 20 requiring that s was unusually weak consistent with the hydration of the underlying mantle by seawater ingress following the embrittlement of the entire crust as the crust only becomes entirely brittle once thinned to 10 km the asymmetric s detachment and the hyper extension of the continental crust only developed late in the rifting process which is consistent with the observed development of asymmetry between conjugate magma poor margin pairs the 3d volume allows analysis of the heaves and along strike architecture of the normal faults whose planes laterally die or spatially link together implying overlaps in faults activity during hyper extension our results thus reveal for the first time the 3d mechanics and timing of detachment faulting growth the relationship between the detachment and the network of block bounding faults above it and the key processes controlling the asymmetrical development of conjugate rifted margins

interannual variations of shallow firn temperature at greenland summit firn temperature profiles are calculated in a thermal model using continuous surface temperatures derived from automatic weather station aws data and passive microwave data in the greenland summit region during the period 1987 1999 the results show that significant interannual variations of mean summer june to august and annual temperatures occur in the top 15 m in addition to the normal seasonal cycle of firn temperature at 5 m depth the seasonal cycle is damped to 13 of the surface seasonal amplitude but even at 15 m about 1 or 0 6 c of the seasonal cycle persists both summer and mean annual temperatures decrease from 1987 to 1992 followed by a general increasing trend interannual variability is 5 c at the surface but only is only dampened to 3 2 c at 10 m depth and 0 7 c at 15 m depth dampening of the interannual variability with depth is slower than dampening of the seasonal cycle because of the longer time constant of the interannual variation the warmer spring and summer temperatures experienced in the top 5 m due to both the seasonal cycle and interannual variations affect the rate of firn densification which is non linearly dependent on temperature during the 12 year period 1987 1999 the mean annual surface temperature is 29 2 c and the mean annual 15 m temperature is 30 1 c which is more than 1 c warmer than a 15 m borehole temperature representing the period of about 1959 and warmer than the best fit temperature history by alley and koci back to 1500 a d

a low angle detachment fault revealed three dimensional images of the s reflector fault zone along the galicia passive margin abstract a new 3 d seismic reflection volume over the galicia margin continent ocean transition zone provides an unprecedented view of the prominent s reflector detachment fault that underlies the outer part of the margin this volume images the fault s structure from breakaway to termination the filtered time structure map of the s reflector shows coherent corrugations parallel to the expected paleo extension directions with an average azimuth of 107 these corrugations maintain their orientations wavelengths and amplitudes where overlying faults sole into the s reflector suggesting that the parts of the detachment fault containing multiple crustal blocks may have slipped as discrete units during its late stages another interface above the s reflector here named s is identified and interpreted as the upper boundary of the fault zone associated with the detachment fault this layer named the s interval thickens by tens of meters from se to nw in the direction of transport localized thick accumulations also occur near overlying fault intersections suggesting either non uniform fault rock production or redistribution of fault rock during slip these observations have important implications for understanding how detachment faults form and evolve over time 3 d seismic reflection imaging has enabled unique insights into fault slip history fault rock production and redistribution

global surface temperatures over the past two millennia 1 we present reconstructions of northern and southern hemisphere mean surface temperature over the past two millennia based on high resolution proxy temperature data which retain millennial scale variability these reconstructions indicate that late 20th century warmth is unprecedented for at least roughly the past two millennia for the northern hemisphere conclusions for the southern hemisphere and global mean temperature are limited by the sparseness of available proxy data in the southern hemisphere at present

the baiyun and liwan sags two supradetachment basins on the passive continental margin of the northern south china sea abstract detachment faults have been reported on the northern and southern margins of the south china sea scs but have not been recognized in the paleocene eocene baiyun and liwan sags of the pearl river mouth basin prmb this is despite the well known very thin crust beneath the two sags in this study we use 2d and 3d seismic data to interpret the sequence stratigraphy as well as identify faults and other important structural features in the baiyun and liwan sags we show that high amplitude seismic reflectors corresponding to detachment faults in the two sags occur in multistep ramp flat geometries associated with important structures including 1 alternating sub sags and sub uplifts developing on the hanging walls 2 secondary faults 3 dome structures and 4 extensional allochthons the detachment faults dip landward and seaward in the baiyun and liwan sags respectively indicating paired detachment systems hence integrated with thin crust and subsidence centers offset at great distances from the fault bounded margins the baiyun and liwan sags are demonstrated to be supradetachment basins seated on the distal domain of the northern scs s passive continental margin the well correlated sequence stratigraphic framework in all sub sags shows that the detachment faults fit the standard detachment model in which faults are continuously active furthermore two detachment basins are inferred to have developed over hot ductile crust resulting from plate subduction in the mesozoic since the upper crust stretches far less than the middle lower crust the fault controlled extension is proposed to have had a smaller contribution to the thinned crust than a ductile extension did within the middle lower crust in the study area the recognition of detachment faults and supradetachment basins is a significant contribution towards understanding the formation of the northern passive continental margin of the scs

modeled seasonal variations of firn density induced by steady state surface air temperature cycle seasonal variations of firn density in ice sheet firn layers have been attributed to variations in deposition processes or other processes within the upper firn a recent high resolution mm scale density profile measured along a 181 m core from antarctica showed small scale density variations with a clear seasonal cycle that apparently was not related to seasonal variations in deposition or known near surface processes gerland and others 1999 a recent model of surface elevation changes zwally and li submitted produced a seasonal variation in firn densification and explained the seasonal surface elevation changes observed by satellite radar altimeters in this study we apply our 1 d time dependent numerical model of firn densification that includes a temperature dependent formulation of firn densification based on laboratory measurements of grain growth the model is driven by a steady state seasonal surface temperature and a constant accumulation rate appropriate for the measured antarctic ice core the modeled seasonal variations in firn density show that the layers of snow deposited during spring to mid summer with the highest temperature history compress to the highest density and the layers deposited during later summer to autumn with the lowest temperature history compress to the lowest density the initial amplitude of the seasonal difference of about 0 13 reduces to about 0 09 in five years and asymptotically to 0 92 at depth which is consistent with the core measurements

depositional processes and growth patterns of isolated oceanic basins the protector and pirie basins of the southern scotia sea antarctica abstract sedimentary processes in small isolated oceanic basins that form adjacent to continental margins but detached from continents remain poorly understood this work describes two such basins located in the southern scotia sea the protector and pirie basins we analysed multichannel seismic profiles to interpret morphostructural features and stratigraphy of these basins sedimentary stacking patterns and depocentre distribution illustrate basin development patterns basal units infill basement depressions formed by the submerged banks of thinned continental crust that abut the basin plains these lower and middle deposits of the sedimentary record are interpreted as pre and syn rift deposits the laterally extensive upper deposits are interpreted as post rift deposits these include five discrete units evident in seismic profiles a prominent regional reflection referred to as reflector c separates in these upper deposits two sets of seismic units that have recorded major shifts in the dominant sedimentary processes stacking patterns and paleo environmental conditions the most important processes controlling deposition of the older units those beneath reflector c include down slope gravity processes that infill depressions created by crustal thinning and seafloor spreading these occurred under the coeval influence of circumpolar deep water circulation the major processes influencing younger units those above reflector c include bottom water circulation of the circumpolar deep water and weddell sea deep water water masses which coursed along bathymetric contours of the seafloor the reflector c discontinuity developed concurrently with middle miocene tectonic changes which led to the opening and deepening of deep gateways in the south scotia ridge these facilitated overflow of weddell sea deep water from the weddell gyre into the scotia sea this overflow in turn forced the circumpolar deep water northwards analysis of the protector and pirie basins shows that their tectonic evolution influenced regional deep water circulation patterns in an area that makes a significant cold water contribution to the global conveyor belt system as a long term factor controlling basin evolution and sedimentary processes tectonics events in this region therefore influenced the present day climate system these results further clarify our understanding of deep isolated oceanic basins in terms their sedimentologic climatologic and oceanographic significance