bending thin lithosphere causes localized snapping and not distributed crunching implications for abyssal hill formation horizontal loads i e pushes or pulls on the lithosphere are thought to produce significant localized dip slip faults but vertical loads are generally considered to produce broadly distributed bending or flexure the same processes of weakening or yielding that lead to localization of deformation on faults during lithospheric stretching or shortening can produce highly localized faulting during lithospheric bending localized bending or snapping should occur when the bending moment begins to decrease with increasing plate curvature if yielding causes a reduction in cohesion then thin brittle layers may respond to bending in the snapping mode while thicker layers respond in a more distributed crunching mode for a mohr coulomb layer floating on an invicid substrate it is the ratio of the cohesion to the average shear stress needed to overcome friction that controls the mode of bending the average horizontal stress on the layer also affects the mode of bending for estimated rock properties a brittle layer has to be less than a few up to perhaps 10 km thick to break in a localized way lithosphere at many mid ocean ridges is thought to be only a few kilometers thick and also may be a site of vertical loading the localized snapping mode of bending may produce abyssal hills at fast spreading mid ocean ridges

axial high topography and partial melt in the crust and mantle beneath the western galapagos spreading center 1 the hot spot influenced western galapagos spreading center gsc has an axial topographic high that reaches heights of 700 m relative to seafloor depth 25 km from the axis we investigate the cause of the unusual size of the axial high using a model that determines the flexural response to loads resulting from the thermal and magmatic structure of the lithosphere the thermal structure simulated is appropriate for large amounts of cooling by hydrothermal circulation which tends to minimize the amount of partial melt needed to explain the axial topography nonetheless results reveal that the large axial high near 92 w requires that either the crust below the magma lens contains 35 partial melt or that 20 melt is present in the lower crust and at least 3 in the mantle within a narrow column 35 in the crust are considered unreasonable it is likely that much of the axial high region of the gsc is underlain by a narrow region of partially molten mantle of widths approaching those imaged seismically beneath the east pacific rise a narrow zone of mantle upwelling and melting driven largely by melt buoyancy is a plausible explanation

the role of magmatic loads and rift jumps in generating seaward dipping reflectors on volcanic rifted margins abstract the largest volcanic constructs on earth are the seismically imaged seaward dipping reflector sdr units found offshore of many rifted continental margins including most that border the atlantic ocean whether their formation requires large magnitude i e 10 s of km of normal fault slip or results from the deflection of the lithosphere by the weight of volcanic flows is controversial though there is evidence for faulting associated with some sdrs this paper considers the range of structures that can be produced by magmatic and volcanic loading alone to do this an idealized mechanical model for the construction of rift related volcanic flow structures is developed dikes open as plates move away from the center of a model rift and volcanic flows fill the depression produced by the load caused by dike solidification the thin elastic plate flexure approximation allows a closed form description of the shape of both the contacts between flows and between the flows and underlying dikes the model depends on two independent parameters the flexure parameter and the maximum isostatically supported extrusive layer thickness w 0 for reasonable values of these parameters the model reproduces the observed down dip thickening of flows and the range of reflector dip angles a numerical scheme using the analytic results allows simulation of the effect of temporal changes in the locus of magmatic spreading as well as changes in the amount of volcanic infill either jumps in the location of the center of diking or periods with no volcanism result in separate units or packages of model sdrs in which the flow dike contact dips landward consistent with observations previously attributed only to listric normal fault offset when jumps in the spreading center are small i e less than they result in thicker narrower volcanic units on one side of a rift compared to those on the other side this is similar to the asymmetric distributions of volcanic packages seen across some conjugate margins

thermal models of oceanic crustal accretion linking geophysical geological and petrological observations 1 models of oceanic crustal accretion should be able to reproduce both petrological and geological observations from ophiolites and geophysical observations from active spreading ridges we have developed thermal models of crustal accretion at fast spreading ridges that incorporate petrological variation and allow for a variable vertical distribution of crystallization including petrological variation has a major effect on the distribution of melt within the crust and the consequent interpretation of model results in terms of geophysical observables in particular the narrow melt lens imaged by seismic reflection surveys can be produced by the presence of large scale compositional stratification within the crust such as that observed in the oman ophiolite controlling the vertical distribution of crystallization allows a range of crustal accretion mechanisms to be tested including the many sills and gabbro glacier models both gabbro glacier and many sills models can broadly match the geophysical observations but the preferred model is a hybrid where the uppermost 25 or more of the lower crust forms in a gabbro glacier and the lowermost 25 or more of the lower crust forms by crystallization in situ the success of all models is dependent on the strength and distribution of hydrothermal cooling

the uplift history of the haiyuan liupan shan region northeast of the present tibetan plateau integrated constraint from stratigraphy and thermochronology abstractthe haiyuan liupan shan delimits the northeastern margin of the tibetan plateau we present the integrated results of new stratigraphic investigations apatite fission track analyses and related thermal history modeling tied to the structural fabric of this region these constraints provide insight into the timing of northward propagation of the plateau with 1 local relief creation and enhanced erosion in the latest jurassic to earliest cretaceous representing the local element of a complex regional far field deformation response to the lhasa qiangtang collision with the south asian margin 2 evidence for likely late cretaceous exhumation but this is uncertain given the regional sedimentary gap that exists across this interval 3 eocene relief generation as a first likely response to far field effects of the india asia collision and related to wider regional events recognized in this part of northwestern china and 4 the latest deformation episode propagating strongly into the haiyuan

tectonics of the east pacific rise near 12 50 n a submersible study abstract using the diving saucer cyana a portion of the east pacific rise between 12 38 and 12 54 n has been studied and mapped four main zones are defined 1 an active volcanic zone located in the central graben the graben displays a variable relief along strike from less than 10 m up to 55 m and has an en echelon pattern in plan view 2 an active tectonic zone comprising the central graben and extending up to about 2 km from either side of the rise axis 3 an inactive tectonic zone starting 2 km away from the axis and formed by discontinuous horst and graben type of structures and 4 off axis volcanoes seamounts located less than 20 km on either side of the rise axis at 12 50 n the active tectonic zone is extremely narrow and the mode of deformation is through fissuring in strong contrast with that observed in slow or intermediate rate ridges the presence or absence of a magma reservoir at shallow depth explains the difference in the width of the active tectonic zone the state of isostatic equilibrium or lack of equilibrium explains the different modes of deformation

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

constraints from sea beam bathymetry on the development of normal faults on the east pacific rise we address the question of the horizontal extent of active upper crustal extension near a fast spreading ridge by applying seafloor slope criteria for normal fault identification to twelve sea beam swaths that cross the axis of the east pacific rise between 9 10 and 9 50 n both the density of and the total throw on identified fault system increase with axial distance to at least 30 km from the rise indicating that the half width of the zone of active normal faulting is 30 km or more the densities of inward and outward facing normal fault scarps in this distance range appear to be similar the distance from the rise to the nearest large throw fault zone is approximately axisymmetric to the north of 9 23 n but the mid point between nearest large throw fault zones is offset 2 3 km to the west of the bathymetric axis south of 9 23 n and thus tracks the center of the axial magma chamber reflector and presumably the axis of most recent magmatic emplacement

neogene extension and volcanism in the kunlun fault zone northern tibet new constraints on the age of the kunlun fault 1 the geology of the jingyu basin north tibet provides constraints on the relationship between localized e w extension in the qiangtang block and strike slip motion along the kunlun fault eocene to oligocene sw ne compression formed topographic relief in the present western kunlun shan pull apart basins started forming in the western kunlun ranges around 15 ma possibly coincident with the initiation of subduction of the tarim qaidam lithosphere under the kunlun shan extensional tectonism is still active and the jingyu basin is extending as a normal fault connected with the kunlun fault was recently active localized extension allows tertiary and quaternary shoshonitic magmas to reach the surface from crustal depths of 50 to 60 km miocene strike slip faulting indicates that the initiation of the kunlun strike slip fault is early neogene in age and that the total amount of horizontal movement that occurred along the fault has to be reconsidered

a revised chronology for tertiary sedimentation in the sikouzi basin implications for the tectonic evolution of the northeastern corner of the tibetan plateau abstract the space time pattern of development of high topography associated with the tibetan plateau remains incomplete due in large part to sparse proxy data that allow estimates of the growth of individual mountain ranges here we present the results of a stratigraphic investigation of a 3100 m section of tertiary fluvio lacustrine strata in the sikouzi basin located at the northeastern most corner of the tibetan plateau results of magnetostratigraphic analysis coupled with a newly discovered miocene in situ fossil assemblage suggest that the section preserves a nearly complete record extending from initial basin formation at 29 myr until 0 5 myr sediment accumulation rates increase from relatively slow accumulation during the oligocene miocene 9 cm kyr to rapid accumulation in the pliocene 22 cm kyr apparently occurring in two sustained pulses at 10 5 ma and 5 ma respectively the increase in accumulation rates starting at 10 5 ma is interpreted as heralding the initial uplift of the liupan shan consistent with cooling and exhumation of the range zheng et al 2006 the pliocene increase in accumulation rate is accompanied by a distinct change in lithofacies the appearance of coarse detritus with provenance from the madong shan and up section changes in bedding orientation that are interpreted as growth strata collectively our data suggest that the generation of relief in the madong shan began at 5 4 ma moreover analysis of paleomagnetic declinations from the sikouzi section reveals a clockwise rotation of the sikouzi basin during the time period 24 5 4 ma that was followed by a subsequent counterclockwise rotation 5 4 0 5 ma our study suggests that activity along the liupan shan haiyuan fault system probably initiated at 10 5 ma but that the present day configuration of the fault system and associated ranges near the fault tip dates to pliocene time

the configuration of equilibrium beach profile in south china abstract to understand the evolution and stability of beaches prediction of the equilibrium beach profile ebp is theoretically and practically important in the present paper a new equation h ae bx c is developed to predict the change in beach profile for sections above the water level and the adjacent nearshore portions moreover fractal analysis is applied to predict types of ebp for the first time using the field data collected from liao zuikou and nanwan beaches south china three types of ebp termed upward concave ebp u ebp downward concave ebp d ebp and medium characteristic ebp m ebp are given for the studied region of south china

longshore current and sediment transport on composite beach profiles abstract natural beaches tend to exhibit an equilibrium profile that is planar nearshore and nonplanar concave up offshore the longshore current on this type of beach profile depends on the horizontal distance to the location of the intersection between the planar and nonplanar profiles as the width of the planar beach face decreases the location of the maximum longshore current moves closer to the shore the dependency of the corresponding longshore sediment transport rate on the location of the intersection between the two profiles is demonstrated for two energetics based sediment transport models again a narrower beach face results in the maximum sediment transport being closer to the shore total sediment transport rates are also a function of the planar beach face width this suggests that longshore transport rates are modulated by the tidal elevation

continental collision slowing due to viscous mantle lithosphere rather than topography the rate of india s convergence with eurasia has decreased exponentially since their collision which suggests that the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved the forces that control plate motion are fundamental to plate tectonic theory it has generally been thought that deforming and thickening continental crust is responsible for slowing down the convergence between two continental plates marin clark now shows however that the rate of penetration of the indian subcontinent into eurasia has decreased exponentially since the plates collided which suggests that contractional strain across tibet has been constant throughout the collision she infers that this reflects deformation of the mantle lithosphere and that under conditions of constant stress and strength the deforming continental lithosphere creates viscous resistance that affects plate motion irrespective of how topography evolved because the inertia of tectonic plates is negligible plate velocities result from the balance of forces acting at plate margins and along their base1 observations of past plate motion derived from marine magnetic anomalies provide evidence of how continental deformation may contribute to plate driving forces2 3 4 5 6 7 8 a decrease in convergence rate at the inception of continental collision is expected because of the greater buoyancy of continental than oceanic lithosphere2 3 but post collisional rates are less well understood slowing of convergence has generally been attributed to the development of high topography that further resists convergent motion7 8 9 10 however the role of deforming continental mantle lithosphere on plate motions has not previously been considered here i show that the rate of india s penetration into eurasia has decreased exponentially since their collision the exponential decrease in convergence rate suggests that contractional strain across tibet has been constant throughout the collision at a rate of 7 03 10 16 s 1 which matches the current rate a constant bulk strain rate of the orogen suggests that convergent motion is resisted by constant average stress constant force applied to a relatively uniform layer or interface at depth this finding follows new evidence that the mantle lithosphere beneath tibet is intact11 which supports the interpretation that the long term strain history of tibet reflects deformation of the mantle lithosphere under conditions of constant stress and strength the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved

cenozoic tectonic evolution of the elashan range and its surroundings northern tibetan plateau as constrained by paleomagnetism and apatite fission track analyses abstract magnetostratigraphic studies of sediments in the western gonghe and eastern qaidam basins together with apatite fission track aft analyses in the elashan range suggest that the elashan range and its surroundings experienced a two stage evolution a protracted uplift from the cretaceous to the early tertiary which explains the absence of the paleogene strata in the region and an accelerated growth of the elashan range since the latest oligocene caused by extensively distributed thrust faults in the region this later phase of uplift resulted in the subsequent thick sediments within the both easternmost qaidam and gonghe basins basal ages of 21 22 ma in the wulan section of 20 ma in the southern gonghe section and of l2 ma in the chaka section the two phases of thrust motion and range growth mean that the qaidam and gonghe basins were never united during cenozoic times the strike slip is likely to have occurred at l0 ma as shown by the paleomagnetic declination data which show a rapid transition in rotation sense from counterclockwise to clockwise at that time this time is also similar to the 9 ma age of initiation of the wenquan fault determined by backward calculation based on its late quaternary slip rate eventually the qinghai nan shan thrust belt began to develop at 6 ma

plate bending stresses at axial highs and implications for faulting behavior abstract we develop flexural models of axial high topography which include the effects of plastic yielding and use the results to examine the contribution of plate bending stresses and strains to fault patterns at the east pacific rise epr the lithosphere responds to buoyancy near the ridge axis created by the presence of elevated temperatures and melt within the crust parameterizing these quantities we find topography at wide axial highs can be fit assuming the crust cools to lithosphere temperatures to 3 4 km depth within 2 km from the axis and the presence of 20 30 melt in the lower crust a trade off in buoyancy implies the greater the amount of crustal cooling the less melt is required significant crustal cooling leads to lithospheric strengthening but plastic bending effects allow the plate to deflect over the distance needed to fit the observed axial relief the plate develops at the axis with non zero curvature and unbends as it moves away from the axis placing a section of the upper surface or seafloor into extension seafloor stresses are extensional beginning a few kilometers from the axis and continuing up to distances of 35 km from the axis for models that fit wide axial high topography a compilation of previous faulting studies at the epr reveals that distance from the axis over which fault slip remains active appears to depend on the width of the axial high for narrow highs this region is confined to at most 15 km from the axis but at wider axial highs it reaches distances of 20 45 km comparable to regions of extension predicted by the flexural models the model strain can be as large as 1 9 amounting to half the observed strain assuming a fault slip angle of 45 but most strain assuming a slip angle of 60 these results strongly suggest that bending stresses contribute significantly to normal faulting on the flanks of fast spreading ridges and may have a strong influence on faulting patterns there

tracking stress and hydrothermal activity along the eastern lau spreading center using seismic anisotropy abstract using p wave refraction data from the l scan controlled source seismic experiment the three dimensional anisotropy structure of the upper crust is determined along the eastern lau spreading center elsc the tomographic image of anisotropy magnitude and orientation is constructed from 197 000 travel time measurements of p wave arrivals recorded on 83 ocean bottom seismometers providing the most detailed and extensive view of upper crustal spreading center anisotropy to date p wave speeds vary with the azimuth of the seismic ray path a type of anisotropy produced by stress aligned lithospheric cracks and microcracks across the study area the fast axes of anisotropy are generally oriented parallel to the trend of the spreading center as expected for ridge parallel cracks that form in association with seafloor spreading the seismic study encompassed four individual spreading segments of the elsc separated by three overlapping spreading centers osc the two larger oscs exhibit high anisotropy that penetrates deep into the upper crust surrounding these oscs and in the wake of the largest are regions where the fast axes of anisotropy are misaligned relative to the general trend in these areas the observations agree with numerical models and seafloor morphology that suggest tensile stress concentrations and a high degree of brittle crack formation the tomographic images indicate that the location and dynamics of persistent ridge offsets can be tracked back through time via their anomalous anisotropy along the spreading centers and away from ridge tips the anisotropy is greater where the melt supply is inferred to be greater and smaller where the melt supply is inferred to be smaller this is opposite to the trend expected if simple tectonic stress models govern anisotropy alternatively hydrothermal activity is expected to increase with magma supply and can explain higher anisotropy via hydrofracturing this study provides the first evidence that seismic anisotropy tracks variations in hydrologic activity along the crests of oceanic spreading centers

constraints on a buoyant model for the formation of the axial topographic high on the east pacific rise bathymetry and gravity data from four geophysical surveys along the 15 19 s superfast spreading section of the east pacific rise are used to study formation of the axial topographic high after removing a best fitting thermal model the averaged residual axial topography is 265 m high and 15 km wide the residual gravity shows no evidence for shallow isostatic compensation of this topography suggesting deep rooted compensation or dynamic support previous models postulate that the axial high is compensated by low densities in a zone of partial melting within the upper mantle we model the physical properties of the lithosphere and asthenosphere required to match the residual gravity and bathymetry using a nonlinear inversion assuming the high is uplifted by buoyancy forces from low density mantle deep rooted buoyancy forces cannot create a narrow axial high unless they occur in a low viscosity conduit surrounded by high viscosity mantle this high viscosity region could be created by depletion and extraction of all water during the melting process we find that the viscosity of the asthenosphere beneath the melt conduit must also be lower than the viscosity of the residual mantle surrounding the conduit our most reasonable model has a viscosity of 1020 pa s in the residual mantle surrounding a melt conduit which extends 50 km below the seafloor and a viscosity of 1016 6 pa s in the asthenosphere below the melt conduit the viscosity of the melt conduit must be about 1015 pa s or about 105 times lower than the surrounding residual mantle with a density contrast equivalent to the retention of a small fraction of melt 5 because higher melt concentrations are probably required to create even small changes in viscosity and the viscosity of the undepleted mantle is probably higher than 1016 6 pa s we conclude that it is unlikely that the axial topographic high is formed by buoyancy forces in the crust and mantle

the rise and fall of axial highs during ridge jumps 1 we simulate jumps of ocean spreading centers with axial high topography using elastoplastic thin plate flexure models processes considered include ridge abandonment the breaking of a stressed plate on the ridge flank and renewed spreading at the site of this break we compare model results to topography at the east pacific rise between 15 25 n and 16 n where there is strong evidence of a recent ridge jump at an apparently abandoned ridge gravity data do not suggest buoyant support of topography model deflections during cooling and melt solidification stages of ridge abandonment are of small vertical amplitude because of plate strengthening resulting in the preservation of a frozen fossil high the present day high is bounded by slopes with up to a 40 grade a scenario very difficult to achieve flexurally given generally accepted constraints on lithospheric strength we model these slopes by assuming that the height at which magma is accreted increases rapidly after the ridge jumps this increase is attributed to high overburden pressure on melt that resided in an initially deep magma chamber followed by a rapid increase in temperature and melt supply to the region shortly after spreading began the high is widest at the segment center suggesting that magmatic activity began near the center of the segment propagated south and then north the mantle bouguer anomaly exhibits a bull s eye pattern centered at the widest part of the high but the depth of the axis is nearly constant along the length of the segment we reconcile these observations by assigning different cross axis widths to a low density zone within the crust

longshore sediment transport on non planar beaches abstract for a concave up 2 3 power bruun beach profile the following two energetics based sediment transport models are developed 1 a bagnold type model and 2 a combined wave current stress model the stress model is calibrated with the bagnold model using observed transport rates on planar beaches the sediment transport profiles for the two models are in agreement within the surf zone for the planar beach case but the stress model is also applied seaward of the breaker line where the bagnold model is not a mean swash transport of sand is predicted by the bagnold model for a 1 2 power least squares approximation to total depth including setup setdown on a bruun beach profile the total longshore transport of sand is determined for each transport model as a function of the turbulent lateral mixing strength the total sand transport is found to be less on a concave up beach profile than for the corresponding planar beach case

apatite fission track analysis apatite fission track aft analysis has been widely used during the past 30 years to constrain the low temperature thermal histories of many igneous metamorphic and sedimentary rocks in a wide range of geological settings applicable geological settings include orogenic belts rifted margins faults sedimentary basins cratons and mineral deposits the types of geologic problems that can be addressed include the timing and rates of tectonic events sedimentary basin evolution the timing of hydrocarbon generation and ore mineralization the absolute age of volcanic deposits the effects of major climatic changes on the near surface geothermal gradient and long term landscape evolution early work by naeser 1967 and wagner 1968 1969 first established the basic procedures that enabled the fission track dating method to be applied routinely to these geologic problems fleischer et al 1975 summarized the early studies of the broader discipline of nuclear track detection in different solid state materials more recent comprehensive overviews of fission track applications have been provided by naeser and mcculloh 1989 wagner and van den haute 1992 gallagher et al 1998 van den haute and de corte 1998 dumitru 2000 and gleadow et al 2002 etched natural fission tracks in several apatite grains are shown in figure 1 successful aft analysis is limited by the following 1 the availability of apatite from which useful aft data can be obtained often due to a lack of apatite of sufficient grain size and quality within available rock types or alternatively the lack of available rock samples due to minimal outcrop exposure or other reasons 2 economic considerations in terms of the time and money required to obtain sufficient aft data 3 the inherent limitations of aft data to resolve geological thermal history information often related to limited numbers of accumulated spontaneous fission tracks due to

volcanic growth faults and the origin of pacific abyssal hills the topographic features known as abyssal hills characterize 30 of the ocean floor and yet their origin has been the subject of vigorous debate for over 40 years submersible based investigations show that pacific abyssal hills are created on the flanks of the east pacific rise as horsts and grabens which lengthen with time hills are bounded on one side by ridge facing scarps produced by normal faulting and on the other by more gentle slopes produced by volcanic growth faulting

wave setup and setdown generated by obliquely incident waves an analytical theory is developed for the wave setup and setdown induced by obliquely incident waves on an impermeable swell built beach profile the wave setup and setdown are found to decrease as wave obliquity increases the incorporation of wave obliquity in wave setup and setdown formulation offers the physical reality in engineering applications the general solutions presented in this paper yield the limiting case of normal wave incidence and the result is consistent with the classical theories published the present theory is primarily applicable to the spilling and plunging breaker across the surf zone within which wave amplitude is assumed to be linearly related to the local water depth experiments were conducted in a large scale wave basin to compare with theoretical results and especially to investigate the applicability of this assumption to the case of obliquely incident waves the dimensionless setup versus the distance offshore within the surf zone is found to depend on wave breaking angle and the shape of the beach profile and it has a non zero value at the original shoreline position this implies that the original shoreline will advance landwards and that the extent of this movement can be related to wave angle at breaking and the beach profile under consideration the results of the present theory are in good agreement with experimental data and field measurements available

faulting patterns near 19 30 s on the east pacific rise fault formation and growth at a superfast spreading center 1 structural mapping from high resolution sonar and bathymetric data collected near 19 30 s on the southern east pacific rise epr reveals multiple packets of small closely spaced outward dipping faults that form antithetic to larger inward dipping faults the mean spacing and height of inward dipping scarps are 1 6 and 1 9 times greater than those of outward dipping faults and the median spacing and height of inward dipping scarps are 2 0 and 1 7 times greater respectively accordingly inward dipping faults are less abundant 35 45 but accommodate more 50 55 of the roughly 4 0 strain although previous studies have shown that epr fault populations exhibit exponential size frequency distributions one dimensional throw and spacing data from the 19 30 s region are not described well by this model the first major 40 m scarp height fault is observed within 1 4 km of the ridge crest and invariably dips inward further off axis a transition from greater inward dipping to greater outward dipping fault abundance constrains the minimum half width of the active plate boundary zone to be 5 15 km inspection of additional data sets from 16 and 18 30 s indicates a pattern of large inward dipping faults with associated hanging wall deformation similar to that observed in the 19 30 s region this pattern is distinct from the horst and graben morphology observed along the fast spreading northern epr the pervasive hanging wall deformation observed on the flanks of the southern epr suggests master fault geometries that are more sharply curved or kinked at depth relative to their abyssal hill equivalents on the northern epr boundary element modeling indicates the pattern and geometry of faulting observed along the southern epr is consistent with fault nucleation above a shallow sill like magma chamber with internal overpressure since the influence of the sill on fault growth is strongly dependent on the depth of the sill the different patterns of faulting observed along the northern and southern epr may reflect the small difference in axial magma chamber depth between these two regions with a slightly shallower chamber beneath the southern epr

wave setup setdown and longshore current on non planar beaches abstract wave induced setup setdown and longshore currents are examined theoretically for non planar concave up beaches a beach profile in which the still water depth is proportional to the horizontal distance offshore raised to the 2 3 power is examined in detail the total mean water depth which includes the sum of the still water depth for this 2 3 power beach profile plus the wave induced setup setdown may be approximated shoreward of the breaker line in a best least squares sense by a profile that is proportional to the horizontal distance offshore raised to the 1 2 power the longshore current profile for this non planar beach is found to differ significantly from that predicted analytically for a planar beach the longshore current velocity does not vanish at the shoreline as in the planar beach case in addition the peak velocities and the total longshore transport of water are found to be less than for the corresponding planar beach cases for a given concave up beach profile the influence of the lateral mixing increases as the wave height decreases

long term continental deformation associated with transpressive plate motion the san andreas fault a synthesis of transpressive mountain building as evidenced by rock uplift and topography along the entire san andreas fault reveals a complex crustal response to oblique plate motion convergent deformation increases toward the fault but does not correlate with the angle of plate motion obliquity the shortening estimated from rock uplift is also insufficient to account for the fault normal motion based on relative plate velocity this suggests that near field convergence is influenced by local structural complexity and is not purely driven by regional transpression and that the fault normal component of plate motion is partly accommodated elsewhere heterogeneity in deformation and degree of slip partitioning highlight the importance of other factors in shaping transpressive continental deformation including surface processes material anisotropy and strain weakening

flexure and thickening of the lithosphere at the east pacific rise we have formulated a new class of plate bending equations which simulates the flexure of the lithosphere at mid ocean ridges the stress accumulation effect in a growing moving and thickening lithosphere significantly changes the flexural behavior it is shown that the growing plate flexes more easily than the conventional static plate due to the fact that the bending stress vanishes on the base of the plate we apply this new plate model to the east pacific rise compared with madsen et al 1984 the fits to the gravity and topography observations are improved assuming the thickness varies in proportion to the square root of distance from the ridge crest the best fitting thickening rate for the growing plate model is 0 7 km1 2 or 5 5 km m y 1 2 which further implies that the 600 or 700 c isotherm marks the bottom boundary of the effective elastic thickness of the lithosphere

a global and regional stochastic analysis of near ridge abyssal hill morphology this paper presents the results of a global and regional stochastic analysis of near ridge abyssal hill morphology the analysis includes the use of sea beam data for the estimation of stochastic parameters up to order 4 these parameters provide important quantitative physical information regarding abyssal hills including their rms height azimuthal orientation characteristic width aspect ratio hausdorff dimension skewness tilt and peakiness the global data set consists of 64 sea beam swaths near the rivera cocos and nazca spreading sections of the east pacific rise the mid atlantic ridge and the central indian ridge in one form of analysis the parameters are averaged among spreading rate bins each of the spreading rate subsets can be identified as unique from the others in at least one aspect the slowest spreading rate subset mid atlantic data exhibit the largest scales rms height and characteristic width and length of abyssal hills these parameters generally decrease as spreading rate increases up to the fast spreading rate data pacific cocos but increase going from fast to very fast pacific nazca spreading rate data this indicates some complexity in the relationship between spreading rate and abyssal hill morphology the plan view aspect ratio is nearly twice as large for the fast spreading rate data than for any of the other subsets and is smallest for the very fast spreading rate data the fractal dimension is nearly identical for all spreading rate subsets the vertical skewness is positive for the slow and medium spreading rate data indicating larger peaks than troughs and negative for the fast spreading rate data indicating larger troughs than peaks the kurtosis or peakiness is everywhere larger than the gaussian value of 3 and tends to be larger in the atlantic than the pacific the tilting parameter provides substantial evidence indicating steeper inward facing slopes in the medium and fast spreading rate data but only marginal evidence for it in the slow spreading rate data from an analysis of correlations among parameters it is found that subsets sometimes behave differently from the entire data set in particular while over the global data set the characteristic width exhibits a well resolved positive trend when plotted versus rms height these parameters exhibit a more gradual positive trend in the mid atlantic data and a negative trend in the pacific cocos data in addition the plan view aspect ratio while generally uncorrelated with rms height for the global data set is positively correlated with rms height for the pacific cocos data set these results emphasize a strong uniqueness of the pacific cocos data relative to the rest of the data global set the pacific cocos data consist of 27 swaths concentrated between the siquieros and orozco fracture zones these data provide very good abyssal hill coverage of this well mapped and well studied region and form the basis of a regional analysis of the correlation between ridge morphology and stochastic abyssal hill parameters in this analysis it is found that abyssal hill parameters are correlated with the depth of the adjacent ridge axis indicating that the relative abundance in magma supply which likely controls the ridge axis depth may also be an important influence on the formation of abyssal hills

variation in cross sectional area of the axial ridge along the east pacific rise evidence for the magmatic budget of a fast spreading center along the fast and ultrafast spreading east pacific rise the cross sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation using an automated method we measure the ridge area volume per kilometer along axis where there is complete bathymetric coverage along 3500 km of the northern and southern east pacific rise our two study areas encompass numerous transform faults large overlapping spreading centers small overlapping spreading centers and smaller discontinuities first second third and fourth order discontinuities the cross sectional area variation mimics the undulation of the ridge crest depth local area maxima occur toward the middle of segments and the axial area decreases by 40 or more at first and second order discontinuities third order discontinuities are generally marked by smaller disruptions in the ridge area and fourth order offsets do not systematically correspond with features of the cross sectional area profile the correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross sectional area represents a longer term average of the ridge s magmatic state than axial depth a correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low density crust and mantle also a correlation between larger area and higher mgo content of axial basalts suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone the cross sectional area of the axial ridge appears to correlate with the width of the axis centered low velocity zone in the crust these observations as well as the absence of large relict axial ridges off axis indicate that the axial ridge originates from buoyancy due to thermal expansion and the presence of melt in the crust and mantle within about 10 km of the rise axis portions of the northern east pacific rise underlain by a magma chamber reflector generally occur where the ridge cross sectional area is greatest this supports the connection between processes which inflate the axial ridge and those which heat the crust and upper mantle and produce melt thus while the axial high on fast spreading centers resembles a constructional volcano in cross section it is more like a long narrow balloon whose cross sectional area is a sensitive indicator of magma supply using this relationship we predict with 75 confidence that at least 45 of the unsurveyed northern east pacific rise 18 n to 13 n and 9 n to 5 n and at least 60 of the southern east pacific rise 4 s to 23 s is underlain by a magma chamber reflector

contribution of volcanism and tectonism to axial and flank morphology of the southern east pacific rise 17 10 17 40 s from a study of layer 2a geometry multichannel seismic images from the east pacific rise 17 10 17 40 s are used to study the geometry of seismic layer 2a in the near axis region wave equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2a event processed stacks are compared with seafloor morphology and tectonic fabric imaged in side scan sonar data assuming layer 2a corresponds with the extrusive crust these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge crest and ridge flank morphology we find variations in axial thickening of layer 2a which imply twofold variation in the width of the extrusive layer accumulation zone as well as systematic changes in the pattern of accumulation of this layer on the ridge flanks the layer 2a horizon mimics abyssal hill relief consistent with a horst and graben origin for this topography short wavelength variations in the thickness of layer 2a are superimposed on this relief which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone the integrated off axis thickness of layer 2a is correlated with ridge cross sectional area and indicates persistent 100 000 years spatial gradients in extrusive layer thickness systematic changes in the cross axis shape of the ridge observed over distances of 5 10 km cannot be attributed to the extrusive layer and it appears that axial structure beneath the volcanics governs the cross axis morphology of the ridge

an alternative dynamic model of the axial topographic high at fast spreading ridges recent models that attempt to explain the formation of an axial topographic high at fast spreading ridges invoke the presence of a narrow low density low viscosity melt conduit to buoyantly support the high mantle up welling beneath mid ocean ridges must be highly focused to create the narrow melt zone required by these buoyancy models we show that an alternative model in which extensional stresses in the upper crust are relieved by diking predicts uplift at the ridge and can match the shape and magnitude of the axial high in this model the extensional stresses deepen toward the ridge axis creating dynamic moments within the crust that are balanced by the axial topography along axis variations in the height and width of the axial high may be caused by changes in the depth and width of the axial magma chamber or flexural rigidity of the lithosphere this model is similar to dynamic models of median valley formation at slow spreading ridges in that the axial topography is uncompensated and supported by extensional stresses in the lithosphere and asthenosphere in addition to being able to explain the axial topography observed at fast spreading ridges this model is consistent with either broad passive or narrow dynamic upwelling at mid ocean ridges the change from median valley to axial high is caused by differences in strength of the axial lithosphere and rate of thickening of the plate

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

interplay between faults and lava flows in construction of the upper oceanic crust the east pacific rise crest 9 25 9 58 n the distribution of faults and fault characteristics along the east pacific rise epr crest between 9 25 n and 9 58 n were studied using high resolution side scan sonar data and near bottom bathymetric profiles the resulting analysis shows important variations in the density of deformational features and tectonic strain estimates at young seafloor relative to older sediment covered seafloor of the same spreading age we estimate that the expression of tectonic deformation and associated strain on old seafloor is 5 times greater than that on young seafloor owing to the frequent fault burial by recent lava flows thus the unseen volcanically overprinted tectonic deformation may contribute from 30 to 100 of the 300 m of subsidence required to fully build up the extrusive pile layer 2a many longer lava flows greater than 1 km dam against inward facing fault scarps this limits their length at distances of 1 2 km which are coincident with where the extrusive layer acquires its full thickness more than 2 of plate separation at the epr is accommodated by brittle deformation which consists mainly of inward facing faults 70 faulting at the epr crest occurs within the narrow 4 km wide upper crust that behaves as a brittle lid overlying the axial magma chamber deformation at greater distances off axis up to 40 km is accommodated by flexure of the lithosphere due to thermal subsidence resulting in 50 inward facing faults accommodating 50 of the strain on the basis of observed burial of faults by lava flows and damming of flows by fault scarps we find that the development of layer 2a is strongly controlled by low relief growth faults that form at the ridge crest and its upper flanks in turn those faults have a profound impact on how lava flows are distributed along and across the ridge crest

the tectonic evolution of the southern alps new zealand insights from fully thermally coupled dynamical modelling summary we present here the results of numerical models of the southern alps in which the thermal and dynamical developments of the orogen are fully coupled to one another this interlinkage allows the use of a wide range of thermal and physical features of the orogen as constraints on the applicability of our models in particular the thermal aspect of this method enables the prediction of the distribution of thermochronological ages at the surface of the model perturbation of the geothermal structure due to rapid uplift and exhumation causes an intrinsic weakening and concentration of strain along the equivalent of the alpine fault zone in these models this thermal weakening of the crust also produces a zone of high strain antithetic to the alpine fault in the upper crust that is comparable to the main divide fault zone within the southern alps the introduction of orographic rainfall and erosional processes into the model leads to the development of high asymmetric topography comparable to that of the southern alps this topographic profile results from the capture of available precipitation by the windward side of the orogen and the resultant rain shadow on the leeward side due to the time required to accumulate suycient topography for this rain capture evect to become significant the establishment of this high asymmetric topography lags the initiation of surface uplift by several million years comparison of the observed thermal history of the southern alps with that seen in models based on divering hypotheses of the tectonic evolution of the south island shows that the present tectonic regime of the orogen most probably developed in a single rapid reorganization of plate motions at approximately 5 ma with relative stability of the tectonic regime since that time the variation in isotopic ages along the southern alps is consistent with that expected from variation in accumulated uplift and exhumation along the orogen arising from the obliquity of convergence of the australian and pacific plates

the regional tectonic fabric of the east pacific rise from 12 50 n to 15 10 n seamarc ii backscatter data for the east pacific rise are used to create structural maps of ridge parallel fabric on the crest and flanks of the rise from 12 50 n to 15 10 n the structural data are statistically analyzed to describe the spacing density azimuth facing direction and length of faults results of the statistical studies are compared with predictions for the width of the zone of active fault formation models for the generation of abyssal hills plate kinematic predictions models of along strike segmentation and observations of the asymmetric subsidence of oceanic crust on the cocos and pacific plates comparisons of the number of faults with distance from the rise crest and examination of the stratigraphic relationship between seamounts and the ridge parallel tectonic fabric illustrate that the zone where new faults are created is located within a few kilometers of the ridge crest fault density data reveal that fault distributions do not resemble periodic processes examination of fault density and spacing data along strike indicates that there is a high probability that the mean number of faults per crustal block differs significantly from north to south changes in along strike statistics correlate well with the occurrence of three overlapping spreading centers located within the survey area additionally fault spacing s determined from seamarc i and seamarc ii backscatter imagery demonstrate that quantitative analyses of seafloor fabric are dependent upon the instrument used to collect the data analysis of inward and outward facing faults indicates that especially in the northern portion of the survey area half graben models are better predictors of abyssal hill morphology than full graben models within the survey area inward facing faults are more abundant and affect more of the oceanic crust than outward facing faults although the cocos and pacific plates subside at different rates this asymmetry is not reflected in the tectonic component of morphology fault azimuth is observed to vary as a function of crustal age although the overall trend of the change in azimuthal values agrees with the trend predicted by relative poles of opening for the pacific and cocos plates the variability in azimuthal data suggests that other processes contribute to the orientation of lineations formed near the axis of the east pacific rise finally the statistical analyses demonstrate that all fault parameters are better correlated about the ridge axis than along strike of the axis the differences between the northern and southern portions of the survey area are reflected in the seamarc ii bathymetric data which depict a continuous narrow ridge crest in the southern region and an irregularly shaped ridge crest in the northern region that shoals and deepens every 30 to 40 km this along strike variability observed over distances of less than 100 km suggests that large scale plate stresses are not the only processes responsible for generating the tectonic fabric observed on the flanks of the east pacific rise

flexing is not stretching an analogue study of flexure induced fault populations abstract flexure induced fractures are predicted to form along the axis of maximum tensile stress within a bending brittle plate the mechanics of this process differ from extensional fault growth in response to lithosphere stretching where a distributed set of simultaneously growing fractures evolves through elastic interaction to simulate extensional fault growth during lithospheric flexure partially solidified plaster layers resting on a foam rubber substrate were depressed by a linear load and fractured in analogue models the length and spacing frequency distributions of the resulting crack populations were analyzed for a series of nine thin 5 mm and ten thick 15 mm layer experiments previous analogue stretching models predict power law length frequency distributions and clustered spacings c v 1 at low strains c v

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

gravity anomalies isostasy and mantle flow at the east pacific rise crest bathymetry and gravity data obtained during a detailed hydrosweep survey of the southern east pacific rise from 7 s to 9 s are used to investigate isostasy at the axis of a fast spreading mid ocean ridge in particular we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160 km long 7 12 s 8 38 s ridge segment the crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km the summit is broad and flat and an axial summit caldera is present for the entire length of the ridge segment however the width and cross sectional shape of the crestal high vary systematically along the axis it is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment the ridge crest is marked by a free air gravity anomaly high about 15 20 km wide with an amplitude of 10 15 mgal relative to the ridge flanks mantle bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment the axial mantle bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross sectional area of the crestal bathymetric high the effects of cooling and subsidence away from the axis were removed from the bathymetry and free air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest the residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis the lithosphere was treated as a broken plate either with a constant flexural rigidity or an effective elastic thickness te which grows at a rate proportional to the square root of distance from the axis the best fitting values of te for the constant rigidity case are in the range of 0 3 0 6 km for the growing plate model te increases at a rate of 0 2 0 3 km the gravity constrains the mass deficiency to extend to a depth of 20 30 km for both lithospheric models we interpret this low density material as a region of partial melt feeding magma to the ridge axis the best fitting density anomalies imply that a 4 9 melt fraction is present beneath the crestal high upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis the mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment it thus appears that differences between the along axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three dimensional focused upwelling at slow spreading ridges to two dimensional sheetlike upwelling at fast spreading ridges rather the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along axis magma distribution

thermal stresses in the oceanic lithosphere evidence from geoid anomalies at fracture zones models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate however there has so far been little direct evidence for the magnitude and distribution of thermal stresses we present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones specifically our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones including the effect of varying thermal bending moments thin plate theory predicts vertical nonisostatic displacements of the lithosphere by plate flexure the predicted amplitude of the resulting geoid anomaly is large enough to be observed in seasat altimeter profiles furthermore the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable the anomaly due to thermal stresses has been clearly identified in geoid profiles across the clarion and the udintsev fracture zones the amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 c consistent with the maximum depth of seismicity in the oceanic lithosphere the distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

an analytic model for the prediction of wave setup longshore currents and sediment transport on beaches with seawalls an analytic model is developed to predict the time and depth averaged cross shore and longshore hydrodynamics as well as the longshore sediment transport on a planar beach backed by a seawall similar to the classical no seawall derivations of the problem the model assumes shallow water small angle of wave incidence and spilling breakers the reflected wave energy flux is conserved while the increased energy in the surf zone due to the reflected wave is assumed to be dissipated in the incident wave a partial standing wave develops in front of the seawall causing modulations of the radiation stresses the bottom stress and the resulting wave setup longshore current and longshore sediment transport the influence of these modulations increases as the beach slope decreases due to a greater number of standing waves that develop across the wider surf zone the total water depth slope including wave setup is steeper on a beach with a seawall than on a non armored beach both the longshore current and the longshore sediment transport are strongly influenced by the cross shore location of the seawall on steep beaches the total sediment transport is less than that for a natural beach for milder sloped beaches the modulations due to partial standing waves become significant and the transport may be either more or less than that for a beach with no wall depending on the location of the seawall these mixed results help to explain some of the confusion and contradictions found in the available seawall literature and may aid in focusing future research on the seawall problem

a new isostatic model for the east pacific rise crest we have developed a new isostatic model for the east pacific rise epr that explains the origin of both the axial topographic high and the rise crest gravity anomaly our model involves a thin elastic plate with a constant thickness crust that is broken at the ridge crest we assume that a buoyant force beneath the plate bends the free edge of the plate upward at the rise axis forming the topographic high and the rise crest gravity anomaly we have used this model and two conventional isostatic models to examine the isostasy of three segments of the epr at 9 l4 n 6 11 s and 16 21 s rather than modeling individual profiles we have stacked summed the gravity and bathymetry data in each area and computed a single 200 km long mean mirrored profile for each different ridge segment these stacked profiles are a remarkable improvement over even the best individual profiles and clearly show both the axial topographic high and its associated free air gravity anomaly in all three areas the best fitting model parameters are similar and give average compensation depths of about 6 7 km below the seafloor near the base of the oceanic crust and an upper limit on the flexural rigidity of the plate that increases from about 1018 n m at the ridge axis to about 3 1019 n m 5 km from the axis and 1020 n m 25 km or more from the rise crest these results indicate that the epr crest is compensated at much shallower depths than suggested by previous investigators our results are consistent with the presence of a small low density magma chamber in the lower crust although at least part of the compensating root for the axial high must extend into the upper mantle we interpret the low upper mantle densities below the rise crest as a zone of partial melt which accumulates at the base of the oceanic crust between episodic rifting events when the overlying crustal magma chamber is replenished

stable isotopic constraints on the tectonic topographic and climatic evolution of the northern margin of the tibetan plateau abstract this study presents oxygen and carbon isotopic records from lacustrine paleosol alluvial and fluvial carbonate sampled in thirteen cenozoic sedimentary sections that span the northern margin of the tibetan plateau isotopic analyses of 1475 carbonate samples yield 18 o values that range from 13 1 to 38 9 smow and 13 c values range from 11 5 to 3 7 pdb based on these analyses we see two isotopic trends regionally we interpret a paleogene decrease in average oxygen isotope values to result from an increase in the mean elevation of the study region s drainages as they tapped waters from a growing tibetan plateau to the south of the study area we interpret a neogene increase in both carbon and oxygen isotope values to result from the initiation of local mountain building in the study area and the withdrawal of marine waters which increased basin isolation aridity and temperatures the timing of inferred initiation of uplift in this region is consistent with structural thermochronological and sedimentological evidence for a tectonic transition in the early and middle miocene from accommodation of india eurasia convergence through extrusion along the altyn tagh fault to accommodation of convergence through active shortening and mountain building in this region this study also supports tectonic models for the differential uplift of the tibetan plateau in that mountain building in this study region significantly postdates estimates for the attainment of high elevation from isotopic studies of the central tibetan plateau

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

low temperature thermochronometry along the kunlun and haiyuan faults ne tibetan plateau evidence for kinematic change during late stage orogenesis the tibetan plateau is a prime example of a collisional orogen with widespread strike slip faults whose age and tectonic significance remain controversial we present new low temperature thermochronometry to date periods of exhumation associated with kunlun and haiyuan faulting two major strike slip faults within the northeastern margin of tibet apatite and zircon u th he and apatite fission track ages which record exhumation from 2 to 6 km crustal depths provide minimum bounds on fault timing results from kunlun samples show increased exhumation rates along the western fault segment at circa 12 8 ma with a possible earlier phase of motion from 30 20 ma along the central fault segment at circa 20 15 ma and along the eastern fault segment at circa 8 5 ma combined with previous studies our results suggest that motion along the haiyuan fault may have occurred as early as 15 ma along the western central fault segment before initiating at least by 10 8 ma along the eastern fault tip we relate an 250 km wide zone of transpressional shear to synchronous kunlun and haiyuan fault motion and suggest that the present day configuration of active faults along the northeastern margin of tibet was likely established since middle miocene time we interpret the onset of transpression to relate to the progressive confinement of tibet against rigid crustal blocks to the north and expansion of crustal thickening to the east during the later stages of orogen development

holocene left slip rate determined by cosmogenic surface dating on the xidatan segment of the kunlun fault qinghai china cosmogenic dating using in situ sup 26 al and sup 10 be in quartz pebbles from alluvial terrace surfaces constrains the late holocene slip rate on the xidatan segment of the kunlun fault in northeastern tibet two terrace risers offset by 24 3 and 33 4 m having respective ages of 1799 388 and 2914 471 yr imply a slip rate of 12 1 2 6 mm yr the full range of ages obtained le 22 8 k y most of them between 6 7 and 1 4 k y confirm that terrace deposition and incision hence landform evolution are modulated by post glacial climate change coupled with minimum offsets of 9 12 m this slip rate implies that great earthquakes m approximately 8 with a recurrence time of 800 1000 yr rupture the kunlun fault near 94 e

causes for axial high topography at mid ocean ridges and the role of crustal thermal structure mid ocean ridge topography is modeled as the flexural response to loads using a thin plate approximation and setting thermal structure of the lithosphere to allow but not require a region of rapid cooling near the axis loads on the lithosphere arise from the presence of low density melt densification due to cooling with distance from the ridge axis and thermal contraction stresses we find two end member classes of temperature and melt structure that can produce axial high topography and gravity observed at the east pacific rise epr one class is very similar to previous models requiring a narrow column of melt extending to at least 30 km depth within the mantle and lithosphere which cools and thickens very gradually with distance from the ridge axis the other is a new class predicting lithosphere which cools rapidly within a few kilometers of the axis and then slowly farther from the axis with melt which is contained primarily within the crust the latter solution is consistent with tomography and compliance studies at the epr which predict rapid crustal cooling within a few kilometers of the axis that is attributed to hydrothermal circulation this solution also allows the melt region to be coupled to crustal thermal structure and requires no melt anomaly within the mantle model fits predict 0 30 melt in the lower crust depending on how temperatures are distributed within the lithosphere and the degree to which thermal contraction stresses are assumed to contribute to topography the model generally predicts a wider axial high for lithosphere which is thin over a wider region near the axis this is consistent with previous correlations between large cross sectional area of the high and indicators of higher melt presence or a warmer crustal thermal regime for a slightly slower rate of lithospheric cooling at distances more than 5 km from the axis the model predicts a trough at the base of the axial high such troughs have been previously observed at the base of the high on the western flank of the southern epr where subsidence rates are anomalously low finally thick axial lithosphere reduces the amplitude of the high making it sometimes difficult to distinguish from long wavelength subsidence this morphology is comparable to that of some intermediate spreading ridges where topography is relatively flat suggesting a transition from fast to intermediate style morphology

microearthquakes on and near the east pacific rise 9 10 n records from a seismic network deployed as part of an active tomography experiment at 9 30 n on the east pacific rise epr provide an opportunity to characterize local microearthquake activity over an 8 day period with the exception of the region around the 9 03 overlapping spreading center osc no events were located on the rise axis one microearthquake located from p and s wave arrival times lies adjacent to the network 18 km to the west of the rise axis five more events located from p wave and t phase data are to the south of the network three cluster around the western arm of the 9 03 osc while two are located 25 30 km to the east of the osc the seismic moments of the microearthquakes range from 5 1019 to 4 1020 dyn cm several orders of magnitude larger than those reported in other studies of shallow events along the axis of the epr these results suggest that overlapping spreading centers may be the loci of substantial microearthquake activity and that at this location the normal faults that form on the young flanks of the epr are active off axis to distances of at least 20 km

far field lithospheric deformation in tibet during continental collision 1 crustal deformation along the present day northern margin of the tibetan plateau has occurred since mid to late eocene time soon after india collided with eurasia assuming that on these distance and time scales the lithosphere can be approximated using a thin viscous sheet we show that far field lithospheric deformation caused by an indenting boundary is expected to start shortly after collision begins when a strong region analogous to cratonic lithosphere like that of the tarim basin is included in the calculations shear and vertical strain rates concentrate along the southern edge of the strong region resulting in enhanced deformation there in calculations scaled to the dimensions of india and the tibetan plateau shear and vertical strain rates on the northern edge of the plateau increase soon after collision begins crustal thickening accelerates later in the calculations we propose that the apparent increased exhumation and perhaps surface uplift around 10 ma may signal the arrival of the thickening front

accretional curvature of lithosphere at magmatic spreading centers and the flexural support of axial highs fluid magma should tend to rise up to the level of local isostatic equilibrium at an axis of plate spreading the magma and underlying partial melt accretes to the side of the separating plates and gets denser as it cools and freezes this load bends down the lithosphere and additional accretion forms a plate with concave upward curvature the topography resulting from a curved plate passively moving off axis can be described analytically for accretion on the vertical side of uniform lithosphere when its flexural response is that of a thin elastic plate model predictions are consistent with the shape of the axial highs at fast spreading centers typically 400m high and 20 km wide as well as related gravity anomalies this model requires at least several kilometers of lithospheric thickening within a few kilometers of the ridge axis seismic data appear consistent with this lithospheric structure the magnitude and lateral distribution of faulting on the flanks of axial highs is crudely consistent with formation during plate unbending as lithosphere moves away from the axis according to this model abandoned fast spreading centers should retain an elastically supported axial high while for previous models abandoned highs should collapse

late cenozoic reorganization of the arabia eurasia collision and the comparison of short term and long term deformation rates the arabia eurasia collision deforms an area of 3 000 000 km2 of continental crust making it one of the largest regions of convergent deformation on earth there are now estimates for the active slip rates total convergence and timing of collision related deformation of regions from western turkey to eastern iran this paper shows that extrapolating the present day slip rates of many active fault systems for 3 7 million years accounts for their total displacement this result means that the present kinematics of the arabia eurasia collision are unlikely be the same as at its start which was probably in the early miocene 16 23 ma or earlier in some but not all active fault systems short term 10 year and long term 5 million year average deformation rates are consistent there is little active thickening across the turkish iranian plateau and possibly the interior of the greater caucasus these are two areas where present shortening rates would need more than 7 million years to account for the total crustal thickening and where there are structural and or stratigraphic data for pre late miocene deformation we suggest that once thick crust up to 60 km built up in the turkish iranian plateau and the greater caucasus convergence took place more easily by crustal shortening in less elevated regions such as the zagros simple folded zone the south caspian region and foothills of the greater caucasus or in other ways such as westward transport of turkey between the north and east anatolian faults the time and duration of this changeover are not known for certain and are likely be diachronous although deformation started or intensified in many of the currently active fault systems at 5 2 ma

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

active tectonics of the turkish iranian plateau 1 this paper examines how active faulting in the turkey iran caucasus region accommodates the arabia eurasia collision and the velocity field observed by gps the overall shortening across the zone is in general spatially separated partitioned into right lateral strike slip in the turkish iranian plateau and thrusting in the greater caucasus a band of counterclockwise rotating nw se right lateral strike slip faults accommodates a nw se gradient in ne directed velocity relative to eurasia between the black and caspian seas a nnw sse band of previously unrecognized oblique normal faults is present on the turkey iran border we estimate the offsets on faults from geomorphological features and show that these offsets can be achieved in 5 2 ma at present rates this implies a reorganization of deformation in the collision zone at that time after the initial collision at 12 ma probably in response to mantle induced dynamic uplift

distributed deformation around the eastern tip of the kunlun fault whether active strain within the indo asian collision zone is primarily localized along major strike slip fault systems or is distributed throughout the intervening crust between faults remains uncertain despite refined estimates of slip rates along many of the major fault zones relatively little is known about how displacement along these structures is accommodated at fault terminations here we show that a systematic decrease in left lateral slip rates along the eastern 200 km of the kunlun fault from 10 mm year to 1 mm year is coincident with high topography in the anyemaqen shan and with a broad zone of distributed shear and clockwise vorticity within the tibetan plateau geomorphic analysis of river longitudinal profiles coupled with inventories of cosmogenic radionuclides in fluvial sediment reveal correlated variations in fluvial relief and erosion rate across the anyemaqen shan that reflect ongoing differential rock uplift across the range our results imply that the termination of the kunlun fault system is accommodated by a combination of distributed crustal thickening and by clockwise rotation of the eastern fault segments

bending as a mechanism for triggering off axis volcanism on the east pacific rise we propose that bending stresses play a key role in triggering volcanism on the flanks of the east pacific rise by simultaneously opening tensile cracks near the surface and increasing the pore pressure of melt bodies trapped in the lower crust in addition crack tip stress intensities remain high during tensile opening such that bending cracks propagate downward efficiently and thus have the potential to tap overpressured melt bodies trapped in the lower crust we find that bending increases the vulnerability of the lithosphere to magmatic penetration out to distances of 20 km from the rise axis corresponding to the region of abyssal hill formation and isolated seamount generation

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

geometry formation and development of tectonic fractures on the reykjanes peninsula southwest iceland abstract a study was made of 903 fractures most of which are of postglacial age on the reykjanes peninsula some 120 fractures belonging to the vogar fissure swarm he completely within the same postglacial lava flow and were studied in great detail for the vogar fractures 75 are pure tension fractures and 25 are normal faults the average length of the vogar fractures is 611 m the average width is 0 6 m the average spacing measured perpendicular to the average strike n54e of fractures is 456 m the estimated average depth is of the order of several hundreds of meters and the average throw of the faults is 2 3 m all major fractures on the peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress the fractures group into four fissure swarms each being a part of a volcanic system which it is proposed is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula it is suggested that a slight magmatic pressure increase in these reservoirs and resulting uplift and tensile stress at the surface is the main cause of postglacial fracture formation

late quaternary and present day rates of slip along the altyn tagh fault northern margin of the tibetan plateau 1 both global positioning system gps measurements and studies of late quaternary faulting are consistent with a slip rate of 10 mm yr along the central segment of the altyn tagh fault and a systematic decrease in that rate toward the eastern end of the fault dates of terraces above and below laterally offset terrace risers yield bounds on quaternary slip rates that range from those that agree with gps measurements to values as much as three times faster we argue that offset terrace risers that are protected by topography upstream of them are more closely dated by the age of the upper terrace than by that of the lower terrace in some cases valleys upstream of the fault have been incised into bedrock and few if any terrace risers can be seen within the valleys such streams debouch onto alluviated floodplains or fans that become incised presumably during climate changes to create terrace risers the terrace risers are then displaced so that they lie downslope from bedrock ridges on the upstream side of the fault and thus the risers become protected from further incision in such cases dates of upper terraces should more closely approximate the ages of the risers than those of lower terraces such dates yield slip rates of 10 mm yr in the central segment of the fault and decreasing rates eastward although we cannot with certainty rule out the higher slip rates along the altyn tagh fault our analysis does show that viable interpretations consistent with gps measurements are more likely at least along some segments of the fault not only do these rates support the view that the tibetan plateau deforms internally by slip on a distributed network of faults in the shallow brittle crust and hence behaves as a continuum at depth but the gradual decrease toward the east also shows that the altyn tagh fault does not separate two effectively rigid lithospheric plates correspondingly the relatively low slip rate and the eastward decrease in slip rate suggest that the altyn tagh fault does not transfer a significant portion of the convergence between india and asia into northeastward extrusion of the tibetan plateau thus large scale extrusion of crustal material in india s path into eurasia seems to be limited largely to the confines of the tibetan plateau