bowers swell evidence for a zone of compressive deformation concentric with bowers ridge bering sea abstract bowers swell is a newly discovered bathymetric feature which is up to 90 m high between 12 and 20 km wide and which extends arcuately about 400 km along the northern and eastern sides of bowers ridge the swell was first revealed on gloria sonographs and subsequently mapped on seismic reflection and 3 5 khz bathymetric profiles these geophysical data show that the swell caps an arcuate anticlinal ridge which is composed of deformed strata in an ancient trench on the northern and eastern sides of bowers ridge the trench fill beneath the swell is actively deforming as shown by faulting of the sea floor and by thinning of the strata across the crest of the swell thinning and faulting of the trench strata preclude an origin for the swell by simple sediment draping over an older basement high we considered several models for the origin of bowers swell including folding and uplift of the underlying trench sediment during the interaction between the pacific plate beneath the aleutian ridge and a remnant oceanic slab beneath bowers ridge however such plate motions should generate extensive seismicity beneath bowers ridge which is aseismic and refraction data do not show any remnant slab beneath bowers ridge another origin considered for bowers swell invokes sediment deformation resulting from differential loading and diapirism in the trench fill however diapirism is not evident on seismic reflection profiles across the swell we favour a model in which sediment deformation and swell formation resulted from a few tens of kilometers of low seismicity motion by intraplate crustal blocks beneath the aleutian basin this motion may result from the translation of blocks in western alaska to the south west forcing the movement of the bering sea margin west of alaska into the abyssal aleutian basin

paleomagnetic evidence for block rotations in central hokkaido south sakhalin northeast asia abstract the eocene to lower miocene strata in the bykov region of south sakhalin island far east russia show a mean paleomagnetic direction of dec 28 7 inc 55 7 95 6 7 and n 22 and a paleomagnetic pole at 66 3 n 245 7 e and a95 8 8 which shows that south sakhalin rotated 25 15 clockwise with respect to northeast asia since the early miocene this pole agrees with that of the eocene to middle miocene sedimentary rocks in central hokkaido japan it suggests that both south sakhalin and central hokkaido have rotated clockwise with respect to northeast asia since the early miocene these clockwise rotations can be interpreted as a vertical axis rotation of the blocks distributed in the north south direction more than 1000 km along the dextral hokkaido sakhalin shear zone considering the development of an en echelon structure of the basins in the shear zone we recognized five blocks each of which had rotated since the early miocene we conclude that this dextral shear is responsible not only for these block rotations but also for the opening of the kuril basin and the japan sea and that the shear motion was caused by oblique convergence between the eurasian and the okhotsk plates since the early miocene

evidence for lithospheric detachment in the central andes from local earthquake tomography abstract data from three temporary seismic networks were merged for tomographic inversion although the deployments did not coincide in time spatial overlap was achieved by re occupying existing sites travel times and t operators of about 1600 earthquakes were inverted for 3d models of p p s and p wave attenuation q p 1 all three attributes provide a consistent image of the entire subduction zone on a lithospheric scale the tomographic images reveal low velocities and high attenuation in the crust and mantle underlying the western cordillera and most of the puna plateau indicative of weak rheology and mostly asthenospheric mantle in contrast forearc and eastern foreland are characterized by high q p values corresponding to cold temperatures in accordance with thermal models in the backarc between 23 s and 24 s a high velocity high q p structure beneath the eastern cordillera and eastern puna is interpreted as detaching continental lithosphere that has been thickened in the orogenic process south of this structure the mantle is characterized by low velocities high p s ratios and low q p values here it is believed that lithosphere originally underlying andean crust has already been removed this is supported by new estimates of crustal thickness and volcanic activity

structural controls on the evolution of the kutai basin east kalimantan abstract the kutai basin formed in the middle eocene as a result of extension linked to the opening of the makassar straits and philippine sea seismic profiles across the northern margin of the kutai basin show inverted middle eocene half graben oriented nne ssw and n s field observations geophysical data and computer modelling elucidate the evolution of one such inversion fold nw se and ne sw trending fractures and vein sets in the cretaceous basement have been reactivated during the tertiary offset of middle eocene carbonate horizons and rapid syn tectonic thickening of upper oligocene sediments on seismic sections indicate late oligocene extension on nw se trending en echelon extensional faults early middle miocene n7 n8 inversion was concentrated on east facing half graben and asymmetric inversion anticlines are found on both northern and southern margins of the basin slicken fibre measurements indicate a shortening direction oriented 290 310 ne sw faults were reactivated with a dominantly dextral transpressional sense of displacement faults oriented nw se were reactivated with both sinistral and dextral senses of movement leading to the offset of fold axes above basement faults the presence of dominantly wnw vergent thrusts indicates likely compression from the ese initial extension during the middle eocene was accommodated on nne ssw n s and ne sw trending faults renewed extension on nw se trending faults during the late oligocene occurred under a different kinematic regime indicating a rotation of the extension direction by between 45 and 90 miocene collisions with the margins of northern and eastern sundaland triggered the punctuated inversion of the basin inversion was concentrated in the weak continental crust underlying both the kutai basin and various tertiary basins in sulawesi whereas the stronger oceanic crust or attenuated continental crust underlying the makassar straits acted as a passive conduit for compressional stresses

effect of three dimensional slab geometry on deformation in the mantle wedge implications for shear wave anisotropy shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench parallel fast directions three dimensional flow may give rise to trench parallel stretching and provide an explanation for these patterns of seismic anisotropy along strike variations in slab geometry produce trench parallel pressure gradients and are therefore a possible mechanism for three dimensional flow in this study we quantify the effects of variable slab dip curved slabs oblique subduction and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones temperature dynamic pressure velocity and strain are calculated with high resolution three dimensional finite element models these models include temperature and stress dependent rheology and parameterized slab and trench geometry thick layers 20 60 km with strong trench parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15 or strong curvature in the slab in these cases strong trench parallel stretching develops when flow lines have an oblique to trench normal orientation this suggests that trench parallel seismically fast directions may not indicate trench parallel flow lines in systems with large along strike variations an oblique component of stretching is confined to a 20 30 km layer above the slab in systems with oblique subduction the effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench parallel flow that extends 50 100 km into the core of the mantle wedge

tectonic evolution of the tehuantepec ridge the tehuantepec ridge is one of the most prominent lithospheric structures of the cocos plate yet its tectonic evolution has remained poorly constrained until now calculated ocean floor ages and spreading rates morphostructural analysis of the ridge and the surrounding ocean floor are used to infer the tectonic evolution and pattern of the tehuantepec ridge and associated structures the ocean floor age estimates south of the tehuantepec ridge suggest an age of 26 ma at the middle america trench a mean age difference across the tehuantepec ridge of 7 ma suggests that the tehuantepec ridge would be formed as a long transform fault on the guadalupe plate prior to 15 ma the full spreading rate estimates show that there might be a differential full spreading rate across the transform fault between 15 and 5 ma when the oceanic plate just south of clipperton fracture zone was decelerating between 15 and 10 ma then accelerating for the next 5 ma we propose a model in which between 13 and 8 ma temporarily transpressional intratransform spreading centers would have been existing as a consequence of the east pacific rise onset with an angle of 108 clockwise with respect to the ceased mathematician ridge our model estimates indicate that between 21 and 15 ma a prominent pseudofault was formed south of the tehuantepec ridge due to an unstable offset on the mathematician ridge that migrated northward the tehuantepec ridge and the pseudofault trace bound a deeper oceanic basin than the surrounding area with no corresponding anomalous depth on the western side of the east pacific rise the complete absence of a pseudofault trace and a deeper oceanic basin on the western side of the east pacific rise as well as the spreading rate changes suggest the existence of a microplate embedded into the cocos plate just south of the tehuantepec ridge also the asymmetric and sharp morphology of the tehuantepec ridge suggests that it may be the expression of a major transpressional structure along the former transform fault on the guadalupe plate 15 20 ma ago d 2005 elsevier b v all rights reserved

mantle flow at a slab edge seismic anisotropy in the kamchatka region the junction of the aleutian island and the kamchatka peninsula defines a sharp turn in the boundary of the pacific and north american plates terminating the subduction zones of the northwest pacific the regional pattern of shear wave birefringence near the junction indicates that trench parallel strain follows the seismogenic benioff zone but rotates to trench normal beyond the slab edge asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge and may influence the shallowing dip of the benioff zone at the aleutian junction

the komandorsky basin as a product of spreading behind a transform plate boundary abstract a long term study of the komandorsky basin in particular by the cruises of r vvulkanolog and r vdmitry mendeleev in the summer of 1988 made it possible to describe in detail the main structural features of this marginal basin several ne and n s trending spreading axes were detected in the basin in the rear of the transform sector of the aleutian arc spreading axes are shifted relative to each other along nw striking transform faults there is variation in style from north to south along the strike of the spreading axes and a less well defined symmetry compared to that typical of mid oceanic ridges the tectonic pattern and heat flow reflect several stages starting from the eocene in the history of the opening of the basin caused by the changes in the velocity and direction of the relative motion between the pacific and north american plates

quaternary intra arc extension in the central trans mexican volcanic belt the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc parallel faulting major intra arc basins within the central part of the belt between long 998w and 1028w are from west to east the cuitzeo and acambay grabens the aljibes half graben and the mezquital graben in this region 100 eastwest2striking 2 km long steeply dipping normal faults expressed by pronounced multi event fault scarps have been mapped of the 100 faults 65 displace rocks of known quaternary age younger than 1 6 ma and 22 cut rocks with a documented age younger than 750 ka known historical surface ruptures are limited to faults of the acambay graben overall the faults cause north south to north northwest2south southeast2oriented extension of 1 km which is distributed over a 30250 km wide zone that has 5210 faults in cross section and a relative extension of 3 quaternary vertical slip rates estimated for 13 of the faults have a mean of 0 07 mm yr they are highest at the southern margin of the cuitzeo graben the northern margin of the acambay graben and in the aljibes half graben where they measure between 0 16 and 0 18 mm yr the north south to north northwest2south southeast2oriented quaternary bulk extension rate of the system is likely to be 0 2 6 0 05 mm yr this fault system is in an initial stage of coalescence its western and eastern parts consist mostly of isolated fault segments whereas in the central part deformation is localized onto a few through going faults the longest of these are the venta de bravo fault 45 km and the acambay tixmadeje fault 34 km the short fault traces have a simple structure whereas the longer ones are commonly composed of two or more segments there is no obvious migratory pattern of quaternary fault activity which suggests that the entire region is tectonically active

multiple active spreading centres in the hot north fiji basin southwest pacific a possible model for archaean seafloor dynamics abstract the north fiji basin nfb in the southwest pacific is one of the most evolved back arc basins within the basin active spreading is taking place along several ridges some of which display the characteristics of spreading ridges observed in mature large oceans characterized by a range of spreading rates segmented accretion centres organized magnetic lineations and propagators for example however the great cumulate length of the spreading centres of the nfb relative to the basin s surface makes it an unique example among the world s oceanic basins we have reviewed a comprehensive body of geological geophysical and geochemical evidence multiple spreading centres of various trends high heat flow high geoid and slow upper mantle seismic velocities which collectively demonstrate that a hot upper mantle underlies the north fiji basin active upper mantle convection is the key process controlling the large crustal and lithospheric production in the nfb which would be thus largely independent from the evolution of even the largest structural features such as the vitiaz and new hebrides subduction zones petrological studies of the archaean greenstone belts have led to a similar picture for the entire earth during the archaean numerous short active ridges connected by unstable triple junctions would have at this time provided an efficient convective mechanism allowing for a large heat dissipation we propose that the presently active processes in the north fiji basin are modern analogues to archaean seafloor dynamics

asymmetric deformation in the backarc region of the kuril arc northwest pacific new insights from analogue modeling 1 the eocene to middle late miocene tectonic evolution of the kuril arc and backarc region has been simulated with analogue experiments the experiments simulate asymmetric deformation in the overriding plate due to anticlockwise rollback of the subducting pacific plate the results show the formation of a n s to ne sw oriented dextral shear zone near the far edge of the retreating boundary analogous to the sakhalin hokkaido dextral shear zone contemporaneously normal faults and grabens form striking parallel to the retreating boundary near the far edge but striking more oblique near the hinge point and away from the retreating boundary this is similar to extensional structures observed in the kuril basin and the sea of okhotsk furthermore the model shows that the amount of extension progressively decreases away from the retreating boundary this appears to have also happened in the kuril okhotsk region as evidenced by crustal thickness variation in the region finally the model results show that extension is increasingly accommodated by the region close to the retreating boundary with progressive deformation this can account for the eocene early miocene extension in the sea of okhotsk followed by miocene spreading in the kuril basin

the tectonics of the okhotsk sea abstract the northern and central parts of the okhotsk sea form an epimesozoic platform the hetero aged acoustic basement is represented by deformed geosynclinal rocks from cretaceous to precambrian in age the slightly deformed sedimentary cover levelled the uneven surface of the acoustic basement and this upper paleogene neogene cover filled up the system of the structural basins the general nw se and w e extensions of the taphrogenic horsts and grabens of the acoustic basement were formed due to extension and subsidence of the earth s crust during the late paleogene neogene

evidence for cenozoic crustal extension in the bering sea region geophysical and regional geologic data provide evidence that parts of the oceanic crust in the abyssal basins of the bering sea have been created or altered by crustal extension and back arc spreading these processes have occurred during and since early eocene time when the aleutian ridge developed and isolated oceanic crust within parts of the bering sea the crust in the aleutian basin previously noted as presumably early cretaceous in age m1 m13 anomalies is still uncertain some crust may be younger vitus arch a buried 100 to 200 km wide extensionally deformed zone with linear basement structures and geophysical anomalies crosses the entire west central aleutian basin we suggest that the arch and the inferred fracture zones in the aleutian basin are early cenozoic structures related to the early entrapment history of the bering sea these structures lie on trend with known early cenozoic structures near the bowers shirshov aleutian ridge junction and on the beringian continental margin with possible continuation into alaska the structures may have coeval and cogenetic histories for early cenozoic and possibly younger times cenozoic deformation within parts of the bering sea region is principally extensional although the total amount of extension is not known as examples the komandorsky basin formed by back arc seafloor spreading the aleutian ridge has been extensively sheared and extensional block faulting is common sedimentary basins of the bering shelf have formed by extension associated with wrench faulting the cenozoic deformation throughout the bering sea region probably results from the interaction of major lithospheric plates and associated regional strike slip faults we present models for the bering sea over the past 55 m y that show oceanic plate entrapment back arc faulting and spreading along vitus arch breakup of the oceanic crust in the aleutian basin at fracture zones and back arc spreading in bowers basin

structures of the oceanic lithosphere asthenosphere boundary mineral physics modeling and seismological signatures 1 we explore possible models for the seismological signature of the oceanic lithosphere asthenosphere boundary lab using the latest mineral physics observations the key features that need to be explained by any viable model include 1 a sharp 20 km width and a large 5 10 velocity drop 2 lab depth at 70 km in the old oceanic upper mantle and 3 an age dependent lab depth in the young oceanic upper mantle we examine the plausibility of both partial melt and sub solidus models because many of the lab observations in the old oceanic regions are located in areas where temperature is 1000 1200 k significant partial melting is difficult we examine a layered model and a melt accumulation model at the lab and show that both models are difficult to reconcile with seismological observations a sub solidus model assuming absorption band ab physical dispersion is inconsistent with the large velocity drop at the lab we explore a new sub solidus model originally proposed by karato 2012 that depends on grain boundary sliding in contrast to the previous model where only the ab behavior was assumed the new model predicts an age dependent lab structure including the age dependent lab depth and its sharpness strategies to test these models are presented

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

geomorphological hierarchies for complex mega dunes and their implications for mega dune evolution in the badain jaran desert abstract the evolution of mega dunes is sometimes attributed to factors other than the wind but evidence for this is lacking it is assumed that the dominance of wind in maintaining the evolution of mega dunes should be characterized by regular height spacing relationships that have been found for simple dunes or wind ripples which are dominantly formed by the wind in this context we studied the height spacing relationship for the complex reversing mega dunes in the badain jaran desert which features the tallest mega dunes in the world the complex mega dunes were divided into three hierarchical orders according to the cumulative probability plots of dune height and spacing measurements and the coefficients of variability of dune heights and spacings were in accordance with values reported for other deserts the relationship between dune spacing and height for all the three orders of dunes could be expressed reasonably well by a uniform linear function that was also applied to the height wavelength relationship for wind ripples in other deserts this relationship was found to be similar to those for several other deserts and subaqueous bedforms this implies that there should be few unique factors in maintaining the evolution of complex mega dunes in the badain jaran desert compared with the superimposed simple dunes and dunes in other deserts dune fields and subaqueous bedforms and that the tallest mega dunes on the earth can be maintained by the wind

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

two phase opening model for the okinawa trough inferred from paleomagnetic study of the ryukyu arc tertiary volcanic and sedimentary rocks were collected at 50 sites of both the central ryukyu arc and the southern ryukyu arc for a paleomagnetic and geochronological study in an attempt to understand the opening of the okinawa trough back arc basin stable primary components of remanent magnetization were isolated through both thermal and alternating field demagnetization experiments from 12 sites of three geological units of the central ryukyu arc and from 12 sites of two geological units of the southern ryukyu arc the paleomagnetic results were compared with the tertiary apparent pole wander path of eurasia in order to investigate the tectonic movement of the ryukyu arc with respect to the asian continent the comparison and the radiometric ages indicate that the southern ryukyu arc rotated clockwise through 25 between 10 ma and 6 ma in contrast the central ryukyu arc experienced little rotational motion since 17 ma we propose the following two phase opening model for the okinawa trough opening the first stage took place between 10 ma and 6 ma the southern part of the trough opened by means of a wedge mode to cause the clockwise rotation of the southern ryukyu arc in the central part the parallel opening occurred and the ryukyu arc drifted without any rotation the second phase started recently in the whole okinawa trough it is observed as the present activities in the trough

late cretaceous and cenozoic tectono stratigraphic development of the east java sea basin indonesia abstract the east java sea basin is underlain by a metamorphic basement complex subsidence of this basement during the late cretaceous resulted in accumulation of up to 3 km of marine upper cretaceous sediments megasequence 1 contraction and near peneplanation of the upper cretaceous sediments and underlying basement occurred before the middle early eocene producing a regional unconformity which defines the base of megasequence 2 the cenozoic east java sea basin started to form during the early eocene by crustal extension on both planar normal faults and extensionally reactivated cretaceous thrusts normal faulting was pulsed from the early eocene to early oligocene and affected a progressively larger area with time complex structural geometries evolved in response to local extensional reactivation of obliquely orientated pre existing structures the resultant palaeogene fault controlled sub basins were filled with fluvial coastal plain and shelf clastic and carbonate sediments recording an overall transgressive evolution regional subsidence became dominant over fault controlled subsidence during the early oligocene basin fill during this time was dominated by deep marine fine grained clastic sediments a regional unconformity of intra oligocene age is recognized and is overlain by oligocene to lowermost miocene deep water calcareous mudrocks and limestones which locally onlap erosionally truncated eocene rocks the lower eocene to lowermost miocene sediment package comprises megasequence 2 regional inversion of palaeogene sub basins commenced in the early miocene and continues to the present day the syn inversion lower miocene to recent sediments comprise megasequence 3 inversion has resulted from regional contraction and resultant reverse reactivation of palaeogene normal faults and cretaceous thrusts regional subsidence has been continuous during the inversion history resulting in a gradual reversal of depocentre location palaeogene depocentres became neogene highs whereas palaeogene platforms became neogene depocentres miocene deposition during inversion was dominated by the alternation of fine grained carbonate dominated and clastic dominated cycles subsequent pliocene sandstone deposition followed fluvial incision during an early pliocene lowstand the depositional history during the pleistocene to recent records rapid relative sea level changes progradation of clastic and carbonate sediments erosional truncation channelling and slumping the tertiary structural geometrical evolution and preserved sediment distribution can be explained by dominantly dip slip fault movement during the extensional and contractional phases of basin development and deformation basin scale cross sectional geometries similar to classical positive flower structures have evolved by the reverse reactivation of a geometrically complex extensional fault system

tertiary tectonics of the sea of okhotsk russia far field effects of the india eurasia collision a new structural map of the western sea of okhotsk based on grids of multichannel seismic data provides several new insights into the tertiary deformation of asia especially in regard to the extension of extrusion tectonics linked to the india eurasia collision into northeastern asia the sedimentary basins in this offshore region are the result of two regional shear systems in the west the north trending sakhalin hokkaido dextral fault system transects sakhalin island accessory structures closely related to its dextral shear include northeast trending normal faults as well as northwest trending en echelon folds and thrusts the accessory normal faults are predominantly eocene to lower miocene and indicate transtensional shear whereas the folds and thrusts are mostly younger and indicate late miocene and pliocene transpression in the north an east west trending sinistral shear zone enters the sea at its northwest corner this sinistral shear system links farther west to extensional faults of the early tertiary baikal rift extending northeastward from the terminus of this sinistral shear zone are a series of long predominantly northwest dipping listric normal faults that form a large lazy s shaped pull apart basin the shantar liziansky basin slb normal faults in this 500 140 km basin accommodate 15 to 20 extension extension and related sinistral shear appear to be largely eocene to oligocene in age with lesser later activity to the northeast of slb a further extension of the sinistral shear zone appears to bend northeastward transecting pustorets and penzhina basins and following the course of the older mesozoic mongol okhotsk chukotsk active margin and suture it possibly reaches as far as the arctic as suggested by previous workers the baikal rift and its associated sinistral shear zone may be interpreted as by products of the india eurasia collision in eocene time mapping results included here show that this diffuse sinistral shear system extends much farther to the northeast the sinistral shear zone is transected by the dextral sakhalin hokkaido fault zone in the kashevarov region of central slb where a fan of north to northwest striking late tertiary transpressional fault splays marks the northern termination of the dextral fault zone older slb normal faults are locally deformed and half grabens are partly inverted by the slightly younger dextral splays the two regional fault systems are essentially conjugate and the pattern of intersection closely resembles that predicted by recent modelling studies of india eurasia collision tectonics

kinematics of subduction and subduction induced flow in the upper mantle 1 results of fluid dynamical experiments are presented to model the kinematics of lithospheric subduction in the upper mantle the experiments model a dense high viscosity plate subducting lithosphere overlying a less dense low viscosity layer upper mantle the overriding lithosphere is not incorporated several important features of slab behavior were investigated including the temporal variability of hinge line migration the kinematic behavior of the slab and the subduction induced upper mantle flow both fixed and free trailing edge boundary conditions of the subducting plate were investigated results show that hinge line retreat is a natural consequence of subduction of a negatively buoyant slab the migration rate increases until the slab approaches the upper lower mantle discontinuity resulting in a decrease in migration rate followed by a renewed increase and finally approaching a steady state slab retreat results in mantle flow with material initially located underneath the slab flowing around the lateral slab edges toward the mantle wedge experimental results indicate that all rollback induced flow occurs around the lateral slab edges forcing the hinge line to attain a convex shape toward the direction of retreat no signs for poloidal flow underneath the slab tip have been detected only a small component of toroidal type flow was observed underneath slanting slab tips for a fixed trailing edge the slab does not sink vertically downward but sinks at an angle in a regressive manner for a free trailing edge slab sinking is oriented more vertically while the surface part of the subducting plate is pulled into the subduction zone

initial plate geometry shortening variations and evolution of the bolivian orocline comparisons of newly published cross sections across the bolivian andes with existing cross sections through argentina emphasize significant along strike changes in crustal shortening a sharp decrease in the magnitude of crustal shortening from 530 km to 150 km north to south occurs at 23 s a 20 40 m y difference in the ages at which deformation was initiated accompanies the abrupt decrease in the magnitude of shortening extending the western margin of south america to account for 530 km of shortening in the bolivian andes and 150 km of shortening in argentina produces a central andean salient that is perpendicular to the nazca plate shortening direction from 60 to 26 ma during this same time interval the chilean coast south of 23 s was in an orientation sufficiently oblique to oceanic convergence to allow for predominantly strike slip offset and backarc extension deformation within the andean mountain chain may be a function of plate convergence where the oblique nature of convergence south of 23 s inhibited mountain building whereas north of 23 s normal convergence to a central andean salient facilitated contractional deformation the magnitude of deformation north of 23 s is a consequence of both plate convergence direction providing a longer period of contractional deformation from ca 70 ma to the present and a thick phanerozoic sedimentary package that permitted large magnitudes of thin skinned deformation significant along strike changes in the shape of the south american margin allowing for convergence to change from compression to extension along the strike of the orogen may help explain the dramatic differences in timing amount and style of deformation in the andes

the nature of orogenic crust in the central andes 1 the central andes 16 22 s are part of an active continental margin mountain belt and the result of shortening of the weak western edge of south america between the strong lithospheres of the subducting nazca plate and the underthrusting brazilian shield we have combined receiver function and surface wave dispersion results from the banjo seda project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure the major results are as follows 1 the crust supporting the high elevations is thick and has a felsic to intermediate bulk composition 2 the relatively strong brazilian lithosphere is underthrusting as far west 65 5 w as the high elevations of the western part of the eastern cordillera ec but does not underthrust the entire altiplano 3 the subcrustal lithosphere is delaminating piecemeal under the altiplano ec boundary but is not completely removed beneath the central altiplano the altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation leading to lower crustal flow and flat topography in contrast in the high relief inland sloping regions of the ec and sub andean zone the upper crust is still strongly coupled across the basal thrust of the fold thrust belt to the underthrusting brazilian shield lithosphere subcrustal shortening between the altiplano and brazilian lithosphere appears to be accommodated by delamination near the altiplano ec boundary our study suggests that orogenic reworking may be an important part of the felsification of continental crust

extension and basin formation in the southern andes caused by increased convergence rate a mid cenozoic trigger for the andes the southern andes between 33 and 45 s latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin which itself was characterized by volcanic activity the basins are filled with thick up to 3000 m accumulations of interbedded sedimentary and volcanic strata of late oligocene early miocene age we interpret that these basins developed during a phase of moderate extension within the plate margin system triggered by an increased rate of convergence of the farallon nazca and south american plates between 28 and 26 ma this history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling although extensional basins of this age are only well described in the southern andes the convergence history and volcanic chronology are similar farther north in the central andes 18 33 s leading to the speculation that extension may have characterized the late oligocene early miocene interval regionally we hypothesize that this extension was a necessary condition to subsequent building of the modern andes mountains

trenching studies of active faults in kamchatka eastern russia palaeoseismic tectonic and hazard implications the central part of the kamchatka peninsula is characterized by a well defined depression associated with active volcanism aligned ne sw on the east the depression is bounded by a prominent system of active faults known as the east kamchatka fault zone ekfz in order to improve understanding of the behaviour and kinematic role of this fault zone a fieldwork programme including study of trenches was conducted in the north central part of this system aerial photograph analysis ground truthed indicates a westward fault dip with predominantly normal slip while lateral offsets of river terraces and stream channels demonstrate a combined dextral component over 20 excavated pits and natural exposures were examined to confirm a detailed tephra succession extending from the early holocene to recent historic eruptions this chronological framework then provided age control on five past faulting events recognised in three trenches these events took place at about 10 5 6 0 4 5 and in a two event succession within a short time span at 3 3 3 2 ka bp event clustering may be characteristic and fault length displacement values suggest earthquakes of m6 5 thus representing a significant new element in regional seismic hazard evaluations additional to events generated at the subduction interface the relatively long gap in faulting since the two most recent events may also be significant for hazard scenarios and there is a possible link between the faulting and volcanic activity in the depression overall the ekfz together with the nachiki transverse zone farther south is thought to define a regional scale block that is extending eastwards independently from the rest of kamchatka

revised age of aleutian island arc formation implies high rate of magma production radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the aleutian island arc provides insight into the timing of arc formation in the middle eocene twenty eight 40 ar 39 ar ages constrain the duration of arc magmatism to the last 46 m y basaltic lavas from the finger bay volcanics the oldest exposed rocks in the arc gave an isochron age of 37 4 0 6 ma which is 12 17 m y younger than a widely cited age of 55 50 ma three main pulses of arc wide magmatism occurred at 38 29 16 11 and 6 0 ma which coincide with periods of intense magmatism in other western pacific island arcs using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m y we calculate a time averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle

tectonics of an arc arc junction an example from kyushu island at the junction of the southwest japan arc and the ryukyu arc abstract three distinctive geologic events occurred synchronously during the last 6 m y on kyushu island lying at the junction of the southwest japan arc and the ryukyu arc these events included 1 dextral fault displacement along a 100 km section of the median tectonic line 2 formation of a rectangular volcano tectonic depression 70 km long and 40 km wide and 3 approximately 30 of counterclockwise crustal rotation we propose an integrated tectonic model that explains these three tectonic features in terms of subduction of the philippine sea plate which resumed at around 6 ma after a halt of more than 5 m y oblique subduction of the philippine sea plate beneath the southwest japan arc detached a fore arc sliver that was displaced along a dextral fault the median tectonic line north of the nankai trough a volcano tectonic depression the hohi volcanic zone was formed by the pull apart structure on the western margin of the median tectonic line by contrast normal subduction of the philippine sea plate under the ryukyu arc caused back arc spreading observed as crustal extension in the northern okinawa trough the crustal extension caused the counterclockwise rotation for the northernmost ryukyu arc including southern kyushu