the 1978 brooks peninsula vancouver island earthquakes a pair of mw 5 5 earthquakes occurred beneath the brooks peninsula on the northwest coast of vancouver island on 2 june and 25 july 1978 these are the largest and best recorded earthquakes to date in the vicinity of northern vancouver island and the adjacent coast margin a detailed study of these earthquakes was undertaken to examine the contemporary tectonics of this region specifically the poorly understood interaction between the explorer plate and the north american plate at the northern end of the cascadia subduction zone both earthquakes were followed by well defined aftershock sequences a four station temporary seismograph array deployed on the brooks peninsula following the 2 june mainshock allowed for accurate aftershock locations this earthquake was located at 50 15 n 127 84 w based on the center of a 9 km diameter circular region of aftershocks the 25 july earthquake was located 4 to 7 km to the northeast of the june epicenter based on waveform comparison of the two events both earthquakes occurred at 15 to 16 km depth the focal mechanisms as determined from body wave modeling are nearly identical and show left lateral strike slip motion along a shallow north dipping east west striking fault the focal mechanism and depth of these two earthquakes indicates that they were not megathrust events on the explorer north america plate boundary but rather that they occurred within the north american plate 5 to 10 km above the megathrust the northeast directed pressure axes for these earthquakes suggests coupling across the explorer north america segment of the cascadia subduction zone consistent with contemporary convergence of the explorer plate with the north american plate in a northeast southwest direction

eocene onset and late miocene acceleration of cenozoic intracontinental extension in the north qinling range weihe graben insights from apatite fission track thermochronology abstract intracontinental extension in central china is accommodated by cenozoic grabens flanking the ordos block such as the weihe graben which accommodate 5 6 mm yr of extension to define the onset and evolution of major cenozoic extension we examine the cooling history of exhumed footwall rocks in the north qinling range adjacent to the weihe graben we present results from 47 new apatite fission track aft samples primarily from two vertical transects of 1 5 2 5 km relief located on the northern flank of the north qinling footwall aft ages that get progressively older with increasing distance from the active range bounding normal fault suggest that cenozoic uplift and southward tilting was a response to range parallel extension correlations between aft ages and both elevation and track lengths combined with thermal modeling of representative samples reveal that the north qinling experienced two major stages of cenozoic exhumation 1 relatively slow exhumation in response to a small magnitude of extension that initiated at 50 ma 2 relatively rapid exhumation in response to a large magnitude of accelerated extension that initiated at 10 ma in addition small changes in cooling rate at 35 ma and 25 ma may reflect minor changes in faulting rate we interpret initial extensional at 50 ma to be a far field effect of initial india asia continental collision in contrast accelerated extension after 10 ma along the qinling is likely linked to the upward and outward growth of the tibetan plateau alternatively lower crustal flow beneath the qinling may have progressively accelerated exhumation in late miocene time

dust supply from river floodplains the case of the lower huang he yellow river recorded in a loess palaeosol sequence from the mangshan plateau the mangshan plateau is located on the south bank of the huang he yellow river just west of the city of zhengzhou well outside the loess plateau in central china mixing models of the grain size data indicate that the loess deposits are mixtures of three loess components comparison of the mixing model with existing models established for a series of loess palaeosol sequences from the loess plateau indicates that the mangshan loess has been supplied from a proximal dust source the huang he floodplain during major dust outbreaks the high accumulation rates the composition of the loess components and especially the high proportions of a sandy loess component support this owing to the exceptionally high accumulation rates the mangshan grain size magnetic susceptibility and carbonate records provide a high resolution archive of environmental and climate change copyright 2008 john wiley sons ltd

the winona basin structure and tectonics a compilation of published and new geophysical data from the winona basin off northern vancouver island has allowed a detailed interpretation of the sedimentary and tectonic history of the region to be made the basin is forming as a result of the asymmetric subsidence of a recently isolated lithospheric block that is slowly converging with the continental margin the crust beneath the basin is young 1 5 ma increasing in age from southeast to northwest and of normal oceanic thickness it is virtually non magnetic however probably because of its having been rapidly buried by turbidite sedimentation subsidence of the basin and uplift of the paul revere ridge began in the early pleistocene ca 1 8 ma and since that time up to 8 km of turbidite sediments has accumulated in the basin the nature of the fanning of the deposits suggests that the basin has been kept full throughout its history the minimum average supply rate necessary to accomplish this is about 70 106 mg year 1 this pleistocene ave

the river orontes in syria and turkey downstream variation of fluvial archives in different crustal blocks the geomorphology and quaternary history of the river orontes in western syria and south central turkey have been studied using a combination of methods field survey differential gps satellite imagery analysis of sediments to determine provenance flow direction and fluvial environment incorporation of evidence from fossils for both palaeoenvironments and biostratigraphy uranium series dating of calcrete cement reconciliation of palaeolithic archaeological contents and uplift modelling based on terrace height distribution the results underline the contrasting nature of different reaches of the orontes in part reflecting different crustal blocks with different histories of landscape evolution upstream from homs the orontes has a system of calcreted terraces that form a staircase extending to 200 m above the river new u series dating provides an age constraint within the lower part of the sequence that suggests underestimation of terrace ages in previous reviews this upper valley is separated from another terraced reach in the middle orontes by a gorge cut through the late miocene early pliocene homs basalt the middle orontes terraces have long been recognized as a source of mammalian fossils and palaeolithic artefacts particularly from latamneh near the downstream end of the reach this terraced section of the valley ends at a fault scarp marking the edge of the subsiding ghab basin a segment of the dead sea fault zone which has been filled to a depth of 1 km by dominantly lacustrine sediments of pliocene quaternary age review of the fauna from latamneh suggests that its age is 1 2 0 9 ma significantly older than previously supposed and commensurate with less uplift in this reach than both the upper and lower orontes two localities near the downstream end of the ghab have provided molluscan and ostracod assemblages that record somewhat saline environments perhaps caused by desiccation within the former lacustrine basin although they include fluvial elements the ghab is separated from another subsiding and formerly lacustrine depocentre the amik basin of hatay province turkey by a second gorge implicit of uplift this time cut through palaeogene limestone the ne sw oriented lowermost reach of the orontes is again terraced with a third and most dramatic gorge through the northern edge of the ziyaret dagi mountains which are known to have experienced rapid uplift probably again enhanced by movement on an active fault indeed a conclusion of the research in which these various reaches are compared is that the crust in the hatay region is significantly more dynamic than that further upstream where uplift has been less rapid and less continuous

continental signature of a ridge trench triple junction northern vancouver island south of the juan de fuca pacific north america triple junction located off northern vancouver island there is subduction and to the north there is oblique convergence along the queen charlotte transform fault in the mid tertiary periods of extension including crustal thinning and volcanism formed queen charlotte basin heat flow on northern vancouver island 67 mw m2 is the same as in queen charlotte basin to the north but it decreases abruptly to the south over a distance of 70 km to an average value of 46 mw m2 compared to the rest of the island the northern island has more subdued topography a lower mean elevation slightly higher bouguer gravity values and lower seismicity late tertiary volcanics which occur only on the northern part of the island geochemically resemble the tertiary masset formation which formed during periods of extension in queen charlotte basin these data are consistent with a tertiary history including periods of extension for northern vancouver island similar to queen charlotte basin and distinguish it from the rest of vancouver island the southern boundary of this northern block extends northeastward from brooks peninsula approximately above the inferred northern edge of the subducted juan de fuca plate and parallel to a series of linear magnetic anomalies associated with a northeasterly trending fault zone the abrupt transition in observed heat flow is most likely associated with the edge of the subducted plate which produces large temperature differences at depths of 30 40 km the time constant for these thermal effects to reach the surface conductively indicates that this boundary has been stable over tens of millions of years and therefore the location of the subducting plate s northern edge must have been stationary or moving very slowly over this period consequently it is likely that the triple junction has been located on average for the last 40 myr off brooks peninsula

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

a sequence stratigraphic model for reservoir sand body distribution in the lower permian shanxi formation in the ordos basin northern china abstract the lower permian shanxi formation is the main gas bearing stratigraphic unit in the northeastern ordos basin china based on an integrated investigation of well logs seismic data cores and outcrops the lower member of the shanxi formation has been subdivided into three third order sequences from base to top namely sq1 sq2 and sq3 the three sequences consist primarily of stacked lowstand systems tracts with little deposition of transgressive systems tracts and or highstand systems tracts detailed sedimentological and stratigraphic analyses indicated that the entire sequence is characterized by a regional transgression with incised valleys and channel fills marking the bases of each retro gradational sequence and swamp or inter channel bay mudstone and coal deposits capping each sequence the three sequences retracted source ward sequentially from the sandstone dominated sq1 to the mudstone dominated sq3 the spatial and temporal distributions of the reservoir sandstones was primarily controlled by retro gradational stacking patterns channel sand bodies formed during the lowstand systems tracts of sq1 sq2 and sq3 are excellent reservoirs which form excellent reservoir cap rock assemblages with the overlying fine grained sediments the unique sequence stacking patterns control the accumulation of hydrocarbons the reservoir quality of the sq1 sq2 and sq3 sequences become worse gradually from base to top within each sequence channel or incised valley fill of the lowstand systems tracts remain potential good expiration targets

controls on the erosion of cenozoic asia and the flux of clastic sediment to the ocean rates of continental erosion may be reconstructed from variations in the rate of accumulation of clastic sediment most of which lies offshore global rates of marine sedimentation are usually considered to have reached a maximum after 3 4 ma driven by enhanced erosion in a variable glacial interglacial climate however a new compilation of seismic data from the marginal seas of asia now shows that only the red river reached its historic peak after 4 ma sediment flux from asia first peaked in the early middle miocene 24 11 ma well before the initiation of a glacial climate indicating that rock uplift and especially precipitation are the key controls on erosion at least over long periods of geologic time reconstructions of weathering in east asia show that faster erosion correlates with more humid warm climates in the early middle miocene changing to less erosive drier climates after 14 ma when antarctic glaciation begins average rates of sedimentation on most east asian continental margins since 1 8 ma are 5 6 times less than the modern fluvial flux implying that the flux to the oceans varies sharply on short timescales and is not always buffered over timescales of 10 4 yr by storage in flood plains

loess stratigraphy in central china abstract loess sequences in central china provide a detailed record of climate changes during the last 2 5 million years at least 44 major shifts from glacial to interglacial conditions occurred during this time in china prolonged loess deposition lasting more than 40 millennia occurred eleven times with only four of these intervals in brunhes chron the climate oscillations took place with increased frequency between 2 4 and 0 5 ma compared to the last half million years major shifts toward cooler and drier climates occurred at about 2 4 ma 1 2 ma and 0 5 ma while less pronounced shifts are dated to 1 65 ma 0 8 ma and 0 2 ma three of the last four reversals of the earth s magnetic field whose lithostratigraphic position has been detail are recorded in a glacial deposit the low field magnetic susceptibility of loess and soils which reflects the concentration of ultrafine magnetite is related to the degree of pedogenesis and serves as a proxy measure of climate the accumulation rate of the magnetite was fairly constant throughout the last 1 2 million years and can serve as a measure of time susceptibility records from localities near xifeng and luochuan located 160 km apart are compared and used to define informal lithostratigraphic units in the loess plateau of china

the feasibility of thermal and compositional convection in earth s inner core inner core convection and the corresponding variations in grain size and alignment has been proposed to explain the complex seismic structure of the inner core including its anisotropy lateral variations and the f layer at the base of the outer core we develop a parametrized convection model to investigate the possibility of convection in the inner core focusing on the dominance of the plume mode of convection versus the translation mode we investigate thermal and compositional convection separately so as to study the end members of the system in the thermal case the dominant mode of convection is strongly dependent on the viscosity of the inner core the magnitude of which is poorly constrained furthermore recent estimates of a large core thermal conductivity result in stable thermal stratification hindering convection however an unstable density stratification may arise due to the pressure dependant partition coefficient of certain light elements we show that this unstable stratification leads to compositionally driven convection and that inner core translation is likely to be the dominant convective mode due to the low compositional diffusivity the style of convection resulting from a combination of both thermal and compositional effects is not easy to understand for reasonable parameter estimates the stabilizing thermal buoyancy is greater than the destabilizing compositional buoyancy however we anticipate complex double diffusive processes to occur given the very different thermal and compositional diffusivities

preliminary magnetostratigraphy of a thick eolian red clay loess sequence at lingtai the chinese loess plateau magnetic remanence and bulk susceptibility measurements are reported for a 305 m eolian loess red clay sequence at lingtai the chinese loess plateau polarity zonation defined by about 680 remanence data yields a basal age of about 7 05 ma for the section which represents the oldest wind blown dust deposited continuously in north central china during the late cenozoic the resultant magnetostratigraphic scheme implies a nearly linear accumulation rate about 2 98 cm ka of atmospheric dust on the loess plateau during the period of 7 05 to 2 58 ma comparison of the bulk susceptibility record with the pedogenic characteristics of the red clay observed in the field suggests that magnetic susceptibility in the red clay is not as useful as in the loess soil sequence to indicate summer monsoon variations

stacked 2 6 ma grain size record from the chinese loess based on five sections and correlation with the deep sea δ18o record 1 this study aims to establish a stacked climate record of the quaternary period from the chinese loess sequence and to address the forcing mechanisms for the regional climate history of the loess plateau by correlating the stacked record with a composite 18o record in deep sea sediments a total of 18 352 samples were obtained from five loess sections located at baoji lingtai jingchuan puxian and pingliang in the southern and middle loess plateau these yielded high resolution grain size records between section correlation of these shows that although small depositional hiatuses are present in places within a single section most parts of the sections display near continuous dust deposition throughout the quaternary the grain size records were tuned simultaneously to the theoretical variations in obliquity and precession of the earth s orbit the grain size records plotted on their orbital timescales were then averaged to form a stacked loess grain size time series termed the chiloparts record this resolves most of the orbital timescale paleoclimate events buried in the loess soil sequences of the southern and middle loess plateau and can be used as a regional archive of the pleistocene climate history in the loess plateau comparison of the chiloparts record with a composite marine 18o record shows that for the past 1 8 ma the loess paleosol record can be correlated almost cycle by cycle with the marine record several discrepancies in the climatic events between the two records have also been identified implying that regional forcing mechanisms may have played a part in the climatic evolution of the chinese loess plateau

geological manifestations of ridge collision evidence from the golfo de penas taitao basin southern chile recent geological and geophysical studies in the southern andes adjacent to the intersection of the chile rise with the peru chile trench ant naz sam triple junction have revealed a number of features and a neogene geologic history that are unique along the pacific margin of south america this history includes 1 development of a tertiary quaternary marine basin with up to 3 km of sediment infill golfo de penas taitao basin gtb 2 disruption of the region by a series of faults with both normal and strike slip movements and 3 localization of silicic near trench volcanism and epizonal plutonism and related hydrothermal activity the northern portion of the gtb began to subside in the late miocene possibly earlier and has subsequently been deformed uplifted and exposed gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today golfo de penas subsidence and uplift have thus occurred diachronously in the region although it is unclear when subsidence began in the golfo de penas tectonic disruption of the region is likely related to the liquine ofqui fault lof a major ns trending crustal shear zone that curves westward and terminates in the golfo de penas the lof has both down to the west and right lateral offset and separates the main andean cordillera on the east from a large crustal block the chiloe block on the west we hypothesize that the gtb has developed as a pull apart basin in response to northward movement of the chiloe block along the lof we propose a dynamic model whereby a stress gradient that decreases longitudinally away from the chile rise peru chile trench intersection is set up because the youngest most buoyant oceanic lithosphere is being subducted at the triple junction the chile rise is viewed as a type of indenter which is acting to drive the chiloe block northward in front of the northward migrating triple junction this model explains the unique set of geologic features found in the region and suggests that ridge trench interactions may be an important factor in orogenesis at active continental margins

the environmental effects of the uplift of the qinghai xizang plateau the intense upheaval of the qinghai xizang plateau took place 2 ma bp the terraces of the yellow river in the lanzhou region and its upper sections provide a record of the uplift of the plateau the interaction of glaciation cycles resulting from climatic changes and pulsating tectonic upheavals are both involved in the record of environmental change across the plateau three tectonic climatic events which are termed respectively the qinghai xizang movement the huanghe yellow river movement and the gonghe movement occurred 2 1 5 1 0 and 0 15 ma bp the upheaval of the qinghai xizang plateau was the trigger mechanism of the asian monsoon the loess plateau was formed in the monsoon triangle of east asia human origins and human evolution may have been stimulated by the uplift of the qinghai xizang plateau

formation of the yellow river inferred from loess palaeosol sequence in mangshan and lacustrine sediments in sanmen gorge china abstract the formation of the yellow river was a significant geological and hydrological event in the history of landscape evolution in northern china resulting from interactions between various earth system processes earlier concepts about the ages of the river s formation are quite controversial this paper investigates the age of the samen gorge the last gorge along the course of yellow river before it reaches the sea the main research materials presented here are a 172 1 m thick loess palaeosol sequence in mangshan located in the south side of the yellow river b lacustrine sediments of the former sanmen lake the chronology is based on the stratigraphical correlations with the well dated loess palaeosol sequence in luochuan of central loess plateau palaeomagnetism and optically stimulated luminescence osl and thermoluminescence tl datings an abrupt increase in the rate of sedimentation is recognized in the stratigraphical layers l 2 and above of the mangshan sequence the loess unit l 1 and palaeosol unit s 1 of mangshan are much thicker than those of other sections in loess plateau as well indicating a new and unique source only for the mangshan region the aeolian deposition of the loess is found to have started at ca 150 ka in the bottom of the former large lake west of the sanmen gorge and suggests desiccation of the former sanmen lake at this point in time the yellow river could bring large amount of fluvial sediment east of the sanmen gorge after the gorge was downcut the fluvial sediment is suggested to be the new source of loess identified in the mangshan sequence therefore it is concluded that the downcutting of the sanmen gorge started at ca 150 ka and indicates the full development of the yellow river

geophysical observations of the northern juan de fuca ridge system lessons in sea floor spreading by virtue of its proximity to the coastline of north america and to numerous oceanographic institutions the juan de fuca ridge has been the focus of a large number of marine geological geochemical and geophysical investigations systematic studies began in the early 1960 s with the geophysical survey of a d raff and r g mason which provided much of the foundation for the development of the extraordinarily successful paradigms of sea floor spreading and plate tectonics subsequent systematic and detailed studies of the plates and plate boundaries of the area by investigators from many academic industrial and government agencies including the geological survey of canada have provided the basis for much of the fundamental understanding we now have of global plate motions and the processes that are involved in the creation of new oceanic crust at sea floor spreading centres much of the success of these studies can be attributed to the geological diversity found along the juan de fuca ridge clear

the cape mendocino california earthquakes of april 1992 subduction at the triple junction the 25 april 1992 magnitude 7 1 cape mendocino thrust earthquake demonstrated that the north america gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the cascadia region the shock occurred just north of the mendocino triple junction and caused strong ground motion and moderate damage in the immediate area rupture initiated onshore at a depth of 10 5 kilometers and propagated up dip and seaward slip on steep faults in the gorda plate generated two magnitude 6 6 aftershocks on 26 april the main shock did not produce surface rupture on land but caused coastal uplift and a tsunami the emerging picture of seismicity and faulting at the triple junction suggests that the region is likely to continue experiencing significant seismicity

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

pleistocene climates in china dated by magnetic susceptibility the magnetic susceptibility of loess and interbedded soils in central china varies with the degree of pedogenesis and serves as a proxy measure of climate the concentration of magnetic minerals in the sediment is inversely proportional to sedimentation rates throughout the brunhes epoch susceptibility measurements combined with the reversal stratigraphy provide a time scale that is independent of astronomic chronology on this scale the susceptibility record closely parallels the oxygen isotope fluctuations in deep sea sediments suggesting a close interdependence of the chinese dust falls the volume of land based ice and global climate

p wave velocity structure of the earth s crust beneath vancouver island refraction data were recorded from three shot points out to a maximum distance of 330 km as part of the 1980 vancouver island seismic project visp80 these vertical component data are partially reversed and so can be interpreted in terms of two dimensional structures by iterative modeling of p wave travel times and amplitudes the structure of the upper crust is the best constrained part of the model it consists generally of a gradually increasing velocity from 5 3 km s at the surface to 6 4 km s at 2 km depth to 6 75 km s at 15 5 km depth where the velocity increases sharply to 7 km s below 20 km depth the model becomes speculative because the data provide only indirect constraints on velocities at these depths an interpretation that fits the observed times and amplitudes has a low velocity zone in the lower crust and a moho at 37 km depth the only significant departure from this general structure is beneath the central part of vancouver island where the 15 5 km boundary in the model atta

recent large earthquakes near cape mendocino and in the gorda plate broadband source time functions fault orientations and rupture complexities the northward migration of the mendocino triple junction is associated with complex faulting within the gorda plate and in the convergent zone between the gorda and north american plates this region has experienced substantial recent large earthquake activity and quantification of these faulting processes is essential for understanding the evolution of the triple junction using an empirical green function deconvolution method for teleseismic and regional surface waves and body waves we obtain relative source time functions for the april 25 1992 cape mendocino thrust earthquake m w 7 2 its two large strike slip aftershocks on april 26 1992 m w 6 5 6 6 and two large strike slip events in the gorda plate that occurred july 13 1991 m w 6 8 and august 17 1991 m w 7 1 the removal of propagation effects using empirical green functions provides unusually detailed source rupture information and indicates that all the ruptures are less than 16 s in duration analyzing the directivity effects observed in the source time functions we resolve rupture directions and corresponding fault orientations for the four largest events and place some constraints on the smaller aftershock significant differences in the rupture duration and stress drop are observed between the cape mendocino mainshock and its two largest aftershocks the mainshock which ruptured westward on a shallow dipping plane either on or paralleling the interplate contact between the gorda and north american plates had a smooth 9 s long rupture with a 0 9 4 6 mpa static stress drop the two largest aftershocks occurred within the uppermost mantle of the gorda plate and have more complex ruptures with total durations of 14 15 s and static stress drops of less than 0 3 mpa perhaps reflecting a difference between mantle and crustal earthquakes in this region the aftershocks appear to have ruptured conjugate strike slip faults accounting for differences in their damage patterns the august 17 1991 gorda plate event ruptured a southwest striking fault while the july 13 1991 event ruptured a southeast striking fault with both events relieving north south compression within the deforming gorda plate the existence of multiple active faults in the region constitutes a significant earthquake hazard associated with the complex stress environment of the migrating triple junction

time constraints for the yellow river traversing the sanmen gorge 1 the yellow river is the second longest river in china a total course of 5464 km the yellow river traverses 30 gorges within its main course and the last one is the sanmen gorge the timing of cut through the sanmen gorge indicates the final integration of the yellow river previous geological surveys identify a paleolake in the weihe basin located west of the sanmen gorge the lake was drained when the gorge was cut through using cosmogenic nuclide burial dating we obtain ages of transition from lake to fluvial environments in the weihe basin and formation of fluvial terraces within the sanmen gorge the timing of cut through the sanmen gorge is constrained to 1 3 1 5 ma zircon u pb age distributions further suggest that the weihe the largest tributary of the yellow river running from west to east along the northern section of the qinling mountains primarily flowed over the sanmen gorge followed by the upper and middle yellow river traversing 1 3 1 4 ma ago it appears that formation and integration of the yellow river occurred in a short duration in the early pleistocene probably related to global climate changes

a 900 k y record of strath terrace formation during glacial interglacial transitions in northwest china the timing of the development of strath terraces with respect to climatic variability remains equivocal previous studies attribute strath terrace formation to glacial or interglacial climates or to variations in sediment and water fluxes that cause lateral erosion followed by vertical incision a chronology of strath terrace formation spanning similar to900 k y has been generated on the basis of loess paleosol couplets and paleomagnetic thermoluminescence and radiocarbon dating of strath terraces in the qilian shan of northeastern tibet repetitive stratigraphic and geomorphic patterns on each terrace indicate that they formed during glacial interglacial transitions long term bedrock incision rates and inferred rock uplift rates appear steady and unrelated to strath formation over the past 900 k y

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

extension in the graben systems around the ordos china and its contribution to the extrusion tectonics of south china with respect to gobi mongolia the graben basins around the ordos block are major tectonic features of late cenozoic extension in north china this paper gives a synthetic view of the basic characteristics of the graben basins and of the kinematics of the graben bounding faults this view is based on the available geological data spot imagery analysis field observations and focal mechanisms of earthquakes from this region these graben basins have been grouped into two major systems the yinchuan hetao graben system along the northern and northwestern margins of the ordos and the weihe shanxi graben system along the southern and eastern margins of the ordos crustal deformation along these graben systems involves normal slip on ne sw striking graben bounding faults normal right lateral and normal left lateral slip on nne ssw and wnw ese striking faults respectively this fault pattern is consistent with nw se trending extension the amount and rate of deformation resulting from this extension have been estimated from geological data using a cross sectional area balance and the extensional factor method deduced from the tilt of faulted blocks the results of these estimates show that the uppermost pliocene quaternary extension rate in a nw se direction is in the order of 1 6 mm yr 1 across the weihe graben 0 5 mm yr 1 across the central shanxi graben on the southeastern margin of the ordos block and 3 1 mm yr 1 across the linhe graben on the northwestern margin of the ordos block the extension across these graben systems is accompanied with counterclockwise rotation of continental blocks in north china ordos taihang shan north china plain a kinematic model is proposed in which the late cenozoic extension and block rotation in north china are interpreted as the effect of the eastward termination of the left lateral slip haiyuan fault and kunlun taibai fault system the ordos block rotates counterclockwise as a result of the push of tibet through the left lateral strike slip haiyuan fault and the liupan shan thrust and fold belt the extension along the weihe shanxi graben system may represent the effect of the left lateral slip on the kunlun taibai qinling fault system the rough estimates of the extensional deformation around the ordos block indicate that this contributes to about 5 2 mm yr 1 of left lateral movement in a n125 e direction between south china and gobi mongolia this motion adds to the left lateral slip on the qinling fault zone about 7 2 mm yr 1 in a n90 e direction between the qinling shan and the hua shan north china thus the eastward motion of south china with respect to gobi mongolia is partitioned between normal faulting around the ordos and strike slip faulting in the qinling shan it is estimated to be in the order of 12 4 mm yr 1 in a n105 e direction during the quaternary

plio pleistocene drainage development in an inverted sedimentary basin vera basin betic cordillera se spain abstract the vera basin is one of a series of interconnected neogene quaternary sedimentary basins located within the internal zone of the betic cordillera southeast spain since the pliocene the vera basin has been subjected to low uplift rates 11 21 m ma 1 and inverted via compressive tectonics that are related to the ongoing oblique collision between the african and iberian plates within this paper the sedimentary and geomorphic response to basin inversion is explored sedimentary processes and environments are established for key stratigraphic units of the pliocene plio pleistocene basin fill and pleistocene dissectional landscape these data are subsequently utilised to reconstruct an evolving basin palaeogeography fault and uplift data are employed to discuss the role of tectonically driven basin inversion for controlling the resultant palaeogeographic changes and associated patterns of drainage development during the early mid pliocene the vera basin was characterised by shallow marine shelf conditions cuevas formation a major palaeogeographic reorganisation occurred during the mid late pliocene strike slip movement along the eastern basin margin coupled with uplift and basin emergence created a protected partially enclosed marine embayment that was conducive for gilbert type fan delta sedimentation from fluvial inputs along the northern and eastern basin margins espiritu santo formation the vera basin then became fully continental and internally drained through the development of a consequent drainage network that formed following the withdrawal of marine conditions during the late pliocene to early pleistocene alluvial fans developed along the northern and western basin margins grading to a bajada and terminating in a playa lake in central basin areas salmeron formation during the early mid pleistocene a switch from basin infilling to dissection took place recorded by alluvial fan incision a switch to braided river sedimentation and fluvial incision into the underlying basin fill sediments and basin margin mountainous topography fluvial incision headwards erosion expansion and modification of the consequent drainage network is documented within a series of up to four major inset river terrace levels and associated landforms fluvial incision and drainage network expansion are attributed to differential uplift and the creation of regional gradients between adjacent basins the relatively low plio pleistocene uplift rate of the vera basin 11 21 m ma 1 in comparison to adjacent basins sorbas 80 160 m ma 1 huercal overa 50 m ma 1 resulted in a switch from internal to external basin drainage ancestral forms of the principal drainage systems within the vera basin the rios almanzora aguas and antas captured basins and mountain catchment areas to the north huercal overa basin southwest sorbas basin and west sierra de los filabres range the switch from basin infilling to fluvial dissection is coincident with a phase of early mid pleistocene compressional tectonics expressed by extensional faulting this deformation is probably linked to accelerated strike slip movement along the palomares fault zone the faulting is superimposed onto the longer term pattern of plio pleistocene uplift and basin inversion

case for very low coupling stress on the cascadia ssubduction fault a fundamental problem in plate tectonics is the shear strength of major plate boundary faults this translates to the question whether the generally observed small earthquake stress drops of 3 10 mpa on major faults release most of the accumulated stress or only a small fraction of it there is strong evidence that the san andreas fault a major transform plate boundary is weak 20 mpa shear resistance it is not yet clear whether subduction thrust faults are also weak we present two types of evidence from the northern cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault comparable to that estimated for the san andreas fault first the well defined surface heat flow and heat generation allow negligible frictional heating on the plate interface the average shear stress on the fault must thus be very low over a time scale of a few million years second focal mechanism solutions for small crustal earthquakes in the southern vancouver island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress this inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low one explanation for the weakness of the fault is the presence of near lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence the conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked accumulating strain energy toward a future great earthquake the surface deformation responds to the small 20 mpa temporal changes of the stress field associated with the subduction earthquake cycle this transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin the weakness of the cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards for example a subduction earthquake may induce large strike slip earthquakes in the forearc that affect a large area

an assessment of the megathrust earthquake potential of the cascadia subduction zone the active tectonic setting of the southwest coast of canada and the pacific northwest coast of the united states is dominated by the cascadia subduction zone the zone can be divided into four segments where oceanic lithosphere is converging independently with the north american plate the winona and the explorer segments in the north the larger juan de fuca segment that extends into both canada and the united states and the gorda segment in the south the oceanic lithosphere entering the cascadia subduction zone in all segments is extremely young less than 10 ma of the other six zones around the pacific where young 20 ma lithosphere is being subducted five have had major thrust earthquakes megathrust events on the subduction interface in historic time an estimation based on potential area of rupture gives maximum possible earthquake magnitudes along the cascadia subducting margin of 8 2 for the winona segment 8 5 for the explorer segment 9 1 for the juan de fuca segment and 8 3 for the so

long term stability of global erosion rates and weathering during late cenozoic cooling reports that the geological record of erosion indicates a fourfold increase in global sedimentation rates during the past 5 million years merited a global explanation explanations offered include an increased rate of mountain formation and global cooling now jane willenbring and friedhelm von blanckenburg reanalyse original data sets and use ocean records of beryllium isotope ratios to obtain an indirect measure of past erosion rates and rock weathering they find no clear evidence either for an increase in global erosion or for a pulse of weathered material in the oceans during the past 12 million years they suggest that cenozoic global cooling must have some other cause and the mountain building that did occur had little effect on erosion or weathering flux globally accelerated uplift and global cooling have been suggested as possible causes for a fourfold increase in global sedimentation rates and by inference in erosion rates or weathering fluxes during the past 5 million years here proxy evidence is provided that indicates stable weathering fluxes in the late cenozoic era it is proposed that processes different from an increase in denudation caused cenozoic global cooling over geologic timescales co2 is emitted from the earth s interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial this balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions1 changes in this balance have been attributed to changes in topographic relief where varying rates of continental rock weathering and erosion1 2 are superimposed on fluctuations in organic carbon burial3 geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation1 2 4 widespread observations of a recent 0 5 myr fourfold increase in global sedimentation rates require a global mechanism to explain them4 5 6 accelerated uplift and global cooling have been given as possible causes2 4 6 7 but because of the links between rates of erosion and the correlated rate of weathering8 9 an increase in the drawdown of co2 that is predicted to follow may be the cause of global climate change instead2 however globally rates of uplift cannot increase everywhere in the way that apparent sedimentation rates do4 10 moreover proxy records of past atmospheric co2 provide no evidence for this large reduction in recent co2 concentrations11 12 here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record13 as evidence we recast the ocean dissolved 10be 9be isotope system as a weathering proxy spanning the past 12 myr ref 14 this proxy indicates stable weathering fluxes during the late cenozoic era the sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean we conclude that processes different from an increase in denudation caused cenozoic global cooling and that global cooling had no profound effect on spatially and temporally averaged weathering rates

late cenozoic eolian deposition in the north pacific asian drying tibetan uplift and cooling of the northern hemisphere a newly constructed record of eolian dust accumulation from the central north pacific shows that dust deposition increased by an order of magnitude quite rapidly at 3 6 ma we associate this sudden drying with the uplift of at least the northern portion of the tibetan plateau shutting off the indian ocean moisture source to central and western china this ten fold increase in atmospheric dust loading appears to be associated with the 1 m y long shift toward heavy 18o values that occurred at 3 6 2 6 ma the dust grain size record of wind intensity begins its late cenozoic coarsening a million years before the drying event at 4 5 ma the northern hemisphere cooling that results in intensification of the subpolar westerly winds may have as its ultimate cause the drawdown of atmospheric co2 in the latest miocene and the early pliocene closing of the panamanian seaway

planation surfaces in northern ethiopia abstract planation surfaces are an old fashioned topic in geomorphology but they are nevertheless important where they make up much of the landscape northern ethiopia is largely a stepped topography caused by differential erosion exhumation of old planation surfaces that were preserved under sedimentary or volcanic cover is an important process in landscape evolution the oldest planation surface is of early palaeozoic age ps1 the second is late triassic ps2 and the third is of early cretaceous age ps3 the oligocene trap volcanics buried a surface ps4 of early tertiary age which is now widely exposed by erosion as a surface that where flat enough is an exhumed planation surface the surfaces do not relate to the supposed africa wide pediplain sequence of king king l c 1975 planation surfaces upon highlands z geomorph nf 20 2 133 148 either in mode of formation and age although the region is tropical there is scarce evidence of deep weathering and few indications that the surfaces could be regarded as etchplains these surfaces indicate that eastern africa underwent long episodes of tectonic quiescence during which erosion processes were able to planate the surface at altitudes not too far from sea level only after the onset of rifting processes uplift became active and transformed a vast lowland plain into the present ethiopian highlands largely exceeding 2500 m a s l some hypotheses and speculations on the genesis of these surfaces are considered here

astronomical calibration of loess paleosol deposits at luochuan central chinese loess plateau abstract the 140 m loess paleosol profile at luochuan in the central chinese loess plateau was sampled at 5 cm intervals in loess units and at 3 cm in paleosol units in order to obtain a high resolution climatic record covering the past 2 5 million years all samples were measured for magnetic susceptibility which is regarded as a good proxy index of the east asian summer monsoon strength on the basis of the astronomical theory of pleistocene climatic change an age model of the luochuan loess paleosol sequence was developed by tuning the magnetic susceptibility record to time series of insolation changes the results show that the ages of the boundaries between the malan and lishi and lishi and wucheng loess formations are 71 and 1320 kyr bp respectively the onset of loess accumulation is at 2470 kyr bp our age model was tested by comparing the orbitally derived ages with absolute age determinations of magnetic reversals and cross spectrum analyzing with solar radiation variations for summer at 65 n these indicate that the calibration provides a reliable time scale for the luochuan loess paleosol deposit

seismicity of the gorda plate structure of the continental margin and an eastward jump of the mendocino triple junction analysis of 9 years of data from the humboldt bay seismic network sheds new light on the structure and evolution of the gorda plate and mendocino triple junction significant findings include the buttressing effect of the pacific plate which demonstrates that there is no underthrusting of the gorda plate along the mendocino fault and the pattern of left lateral northeast trending faults which demonstrates how the gorda plate accommodates n s shortening without such underthrusting focal mechanisms are consistent with northward compression of the gorda plate by the pacific plate until the gorda plate passes the triple junction beyond which the n s compressive stress is effectively removed and the focal mechanisms show a change from strike slip to normal faulting with downslab tension another significant feature is the shallow benioff zone 10 40 km which shows a double seismogenic layer unlike most other double seismic layers in deep subduction zones this one appears to be due to underplating with a new subduction zone being developed east of the former one a corner of continental margin material has been partially subducted we attribute this double seismic zone to reactivation of both the old and new subduction boundaries under n s compression produced by pacific gorda plate interaction at the triple junction because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the san andreas fault has migrated eastward we have constructed a model that accounts for both the eastward migration of the san andreas fault and the doubling of the seismic zone in the gorda plate this can be done by assuming that at the time of the last eastward jump of the fault the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the san andreas fault other scenarios for the evolution of the mendocino triple junction do not require that the subduction zone migrate eastward however the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place this is consistent with other evidence for an eastward jump of the san andreas fault

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

seismotectonics of the explorer region 1 the explorer region offshore western canada is a tectonically complex area surrounded by the pacific north america and juan de fuca plates existing tectonic models for the region differ fundamentally proposed plate configurations range from multiple independent plate fragments to an explorer plate now fused to north america along the continental margin and cut by pacific north america transform faults in the west we present new seismological data constraining the region s current tectonics we use three component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4 combined with 34 harvard centroid moment tensor solutions they represent the region s largest earthquake source parameter data set obtained by robust waveform modeling techniques in addition we perform joint epicenter determination to relocate larger earthquakes recorded since 1918 the source parameters and improved locations provide a consistent tectonic picture earthquake slip vector azimuths along the pacific plate boundary change smoothly and are significantly less northerly oriented than the pacific north america plate motion direction requiring an independent explorer plate the present day pacific explorer boundary is formed by transform faults subparallel to the revere dellwood wilson fault plate motion vectors indicate that the winona block is part of the explorer plate current explorer motion is more northerly than indicated by magnetic anomalies prior to 2 ma implying a recent change possibly coinciding with a northwestward ridge jump near explorer plate s northern end transferring the winona block from the pacific to the explorer plate in response to these plate motion changes the region north of the western sovanco fracture zone was assimilated into the pacific plate the region around the eastern sovanco fracture zone characterized by broadly distributed seismicity is composed of well defined sets of conjugate faults bounding rotating crustal blocks earthquake fault strikes agree with the dominant northwest southeast fault sets however the conjugate sets must be also active to fully accommodate present day explorer plate motion the sw portion of the strike slip nootka fault zone the explorer juan de fuca plate boundary is well defined by focused seismicity however its full extent under nootka island remains unresolved the explorer north america boundary shows sporadic low magnitude seismicity our explorer north america rotation pole predicts convergence varying from negligible at the boundary s northwest end to 2 cm yr at the se end this convergence can be accommodated either by subduction or by crustal thickening extending to the north american continent we favor subduction based on low deformation rates observed by onshore gps sites the present explorer plate system configuration is a result of stepwise reorientation of the explorer ridge system each step successively reducing the subduction rate relative to north america

natural change and human impact in a present day fluvial catchment the geul river southern netherlands abstract the geul river has a long history of channel straightening and bank protection but also has several natural meandering parts in addition some formerly fixed or protected stretches of the river have been allowed to meander freely since 1988 therefore it represents an ideal case for evaluating the contribution of human interference to the intrinsic evolution of the river in this paper we compare the response of selected not protected reaches of the present day river to different human modifications the river banks were labelled as stable unstable or erosive as might be expected stretches with a high sinuosity have a high percentage of erosive and unstable bank length while low sinuosity stretches experience hardly any erosion the amount of lateral erosion and sedimentation in natural situations is compared with that in situations with different bank protection and stabilization types erosion rates may locally attain up to 2 m per year detailed grain size analysis of point bar sections enable distinction of successive fining up sequences vertical aggradation rates within the point bars are up to 0 15 m yr 1 the highest rates are found in the youngest point bars collectively the results of this study show that when bank protection is removed the freely meandering river creates its own specific ecosystems and retention capacity increases such a return to natural environmental and ecological conditions is in accordance with the objectives of the european water framework directive

inversion of regional surface wave spectra for source parameters of aftershocks from the 1992 petrolia earthquake sequence abstract regional distance surface waves are used to study the source parameters for moderate size aftershocks of the 25 april 1992 petrolia earthquake sequence the cascadia subduction zone had been relatively seismically inactive until the onset of the mainshock m s 7 1 this underthrusting event establishes that the southern end of the north america gorda plate boundary is seismogenic it was followed by two separate and distinct large aftershocks m s 6 6 for both occurring at 07 41 and 11 41 on 26 april as well as thousands of other small aftershocks many of the aftershocks following the second large aftershock had magnitudes in the range of 4 0 to 5 5 using intermediate period surface wave spectra we estimate focal mechanisms and depths for one foreshock and six of the larger aftershocks m d 4 0 to 5 5 these seven events can be separated into two groups based on temporal spatial and principal stress orientation characteristics within two days of the mainshock four aftershocks m d 4 to 5 occurred within 4 hr of each other that were located offshore and along the mendocino fault these four aftershocks comprise one group they are shallow thrust events with northeast trending p axes we interpret these aftershocks to represent internal compression within the north american accretionary prism as a result of gorda plate subduction the other three events compose the second group the shallow strike slip mechanism determined for the 8 march foreshock m d 5 3 may reflect the right lateral strike slip motion associated with the interaction between the northern terminus of the san andreas fault system and the eastern terminus of the mendocino fault the 10 may aftershock m d 4 1 located on the coast and north of the mendocino triple junction has a thrust fault focal mechanism this event is shallow and probably occurred within the accretionary wedge on an imbricate thrust a normal fault focal mechanism is obtained for the 5 june aftershock m d 4 8 located offshore and just north of the mendocino fault this event exhibits a large component of normal motion representing internal failure within a rebounding accretionary wedge these two aftershocks and the foreshock have dissimilar locations in space and time but they do share a north northwest oriented p axis

differential impact of small scaled tectonic movements on fluvial morphology and sedimentology the huang shui catchment ne tibet plateau abstract the aim of this paper is to investigate the morphological implications of the interplay between tectonic movements at different rates timescales and spatial extent in a catchment of intermediate size the huang shui river a main tributary of the yellow river in the ne tibet plateau river incision started from a peneplain like surface that developed in post miocene pliocene times at the end of the tertiary a general tectonic uplift of the tibetan plateau initiated river incision this general uplift also caused local fragmentation of the huang shui catchment into blocks kilometers or maximally a few tens of kilometers of local extent that have subsided and or uplifted relative to one another fluvial deposition of 30 m in the subsiding blocks contrasts with erosion and formation of gorges in the uplifted blocks incision into the former peneplain was not continuous but a staircase of terraces developed under climatic influences terrace deposits are sometimes capped by interglacial soils or soil derived material apparently terrace incision occurred at the transition to the next cold period the different rates of uplift and subsidence of the individual blocks resulted in the simultaneous development of erosion and accumulation terraces of different sizes within the same catchment even within the same tectonic block this makes it impossible to connect the terraces of the different blocks except for the three youngest terraces representing the last 300 000 years thus illustrating the uniform tectonic history of the catchment since that time

the linking of the upper middle and lower reaches of the yellow river as a result of fluvial entrenchment abstract the upper middle yellow river flows through the fenwei graben a structure resulting from extensional tectonism that was formed and repeatedly extended during the cenozoic the drainage system within this graben was formerly isolated from the lower reaches of the yellow river system by the xiaoshan mountains an actively growing nw se trending range the modern course of the yellow river takes it through this range along the sanmen gorge the formation of which was of great significance in that it initiated through going drainage between the upper middle and lower reaches of the system the timing of this event which was clearly a critical point in the evolution of the yellow river can be established by dating the terraces in the gorge intermittent deepening of this gorge by the yellow river from a high level planation surface capping the mountain range has resulted in the formation of five terraces magnetostratigraphic records from aeolian deposits accumulated on these surfaces provide a geochronological sequence for this geomorphic archive in which the ages of the planation surface and of terraces t5 t4 t3 t2 and t1 have been determined as 3 63 ma 1 24 ma 0 86 ma 0 62 ma 129 ka and 12 ka respectively under the constraint of this chronological framework a model for landscape evolution is proposed here uplift of the inner fenwei graben and of the surrounding mountain ranges led to dissection of the 3 63 ma old planation surface in conjunction with the formation of the sanmen gorge drainage of the lake previously occupying the basin would have promoted incision into the fluvio lacustrine graben sediments indeed gorge formation through the xiaoshan may have been initiated or intensified by lake overflow the ages obtained for the planation surface and uppermost terrace suggest that the formation of the sanmen gorge and the initiation of the through going eastward drainage of the yellow river occurred between 3 63 and 1 24 ma before the start of gorge entrenchment the products of erosion in the modern upper catchment of the yellow river were unable to reach the sea the dramatic increase in deposition rates in the bohai gulf at the mouth of the modern yellow river in the east china sea 1 0 ma ago thus resulted from the initiation of an integral enlarged yellow river catchment drainage through the sanmen gorge it does not imply an increase in erosion rates at that time

incision rate of the yellow river in northeastern tibet constrained by 10be and 26al cosmogenic isotope dating of fluvial terraces implications for catchment evolution and plateau building unlike other large rivers flowing out of tibet the yellow river escapes from the plateau towards the ne crossing no less than five nw se striking actively growing ranges and intervening basins thick plio quaternary deposits and fluvial terraces testify to a phase of aggra dation and sediment infill up to the average surface elevation 3200 3250 m a s l of the gonghe guide and qinghai lake basins a set of seven main terraces across the gonghe basin suggests progressive down cutting of the yellow river carving the 500 m deep longyang gorge at the basin exit 10be and 26al concentrations in quartz of surface and sub surface samples of four terraces constrain the timing of incision by determining the burial age of the deposit and the exposure age of its surface modelling the depth dependence of the 10be concentration and the 26al 10be ratio allows us to constrain the onset of the ongoing phase of incision to 120 250 ka these ages suggest long term incision rates between 2 6 mm a together with the present morphology of the yellow river terraces across the gonghe basin and the longyang gorge our results imply rapid river catchment evolution and interaction between river dynamics tectonic and climate in northeastern tibet

mineral magnetic record of the chinese loess and paleosols variations in the magnetic susceptiblity of the chinese loess and its interbedded paleosols correlate strikingly well with the deep sea oxygen isotope record low susceptibility values are found for the loess layers and high values are found for the soil horizons two interpretative models have been advanced to account for these magnetic variations both of these models discount any significant in situ formation of magnetite during soil forming periods instead they infer relative concentration of detrital or atmospheric magnetite at these times we critically examine the assumptions made in these models and on the basis of some new mineral magnetic data and fromalized magnetic flux calculations offer an alternative explanation of the loess magnetic record our model identifies pedogenic formation of magnetite as the major contributor to the high magnetic susceptibility of the paleosols the formation and preservation of this pedogenic magnetite is dependent on soil froming conditions and hence reflects the regional climate

paleocurrent and fabric analyses of the imbricated fluvial gravel deposits in huangshui valley the northeastern tibetan plateau china abstract gravel deposits on fluvial terraces contain a wealth of information about the paleofluvial system in this study flow direction and provenance were determined by systematic counts of more than 2000 clasts of imbricated gravel deposits in the xining region northeastern tibetan plateau china these gravel deposits range in age from the modern huangshui riverbed to miocene aged deposits overlain by eolian sediments our major objectives were not only to collect first hand field data on the fluvial gravel sediments of the xining region but also to the reconstruct the evolution of the fluvial system these data may offer valuable information about uplift of the northeastern tibetan plateau during the late cenozoic era reconstructed flow directions of the higher and lower gravel deposits imply that the river underwent a flow reversal of approximately 130 180 in addition the lithological compositions in the higher gravel deposits differ significantly from the lower terraces suggesting that the source areas changed at the same time eolian stratigraphy overlying the gravel deposits and paleomagnetic age determination indicate that this change occurred sometime between 1 55 ma and 1 2 ma we suggest that tectonic activity could explain the dramatic changes in flow direction and lithological composition during this time period therefore this study provides a new scenario of fluvial response to tectonic uplift a reversal of flow direction in addition field observation and statistical analyses reveal a strong relationship between rock type size and roundness of clasts

sedimentary history of the western bohai coastal plain since the late pliocene implications on tectonic climatic and sea level changes abstract thick cenozoic deposits from the western bohai coastal plain a tectonic subsiding region provide the potential to study the relations between sedimentary environments and tectonic climatic and sea level changes however sedimentary history of this region extending to the whole quaternary as well as their links to tectonic climatic and sea level changes are still poorly understood mainly because of the lack of long term records with well constrained chronology in this study we present an integrated record based on sedimentology and proxies grain size and color reflectance of a 203 6 m core recovered from the western bohai coastal plain near tianjin the core was chronologically well constrained using paleomagnetic and optically stimulated luminescence dating methods the results show that from the late pliocene 3 3 ma to the late pleistocene 0 10 ma the study region was mainly dominated by fluvial setting and the extensive incursion of sea water into this region began during the last interglacial period 0 10 ma the sedimentology and the color index suggest that tectonic subsidence of the bohai basin during the plio quaternary must have played a significant role in controlling the sedimentary environments in this region the changes in base level because of sea level fluctuations during the quaternary influenced the fluvial development greatly and led to the alternations of coarse crevasse splay channel and finer floodplain deposits in the core sequence in addition climatic changes since the late pliocene also have had significant effects on the sedimentary settings in the bohai coastal plain by influencing the fluvial process with a series of mechanisms

tectonic history of the juan de fuca ridge over the last 40 million years interpretation of a new compilation of magnetic anomaly data for north of the mendocino fracture zone and younger than chron 15 39 ma indicates a transition from a pattern of stable transform faults to a pattern of persistent ridge propagation reorganizations of the propagation pattern occurred at 30 ma in response to a counterclockwise change in direction of relative motion and at 19 ma in response to a clock wise change in addition there are significant changes in the gradient of the spreading rate apparently without major changes in motion direction at 27 and 24 ma the change at 30 ma is apparently the result of the separation of the juan de fuca plate from the rest of the farallon plate along the pioneer fracture zone the pattern of absolute motion after 30 ma does not generally support menard s pivoting subduction model and suggests that juan de fuca plate motion is being significantly affected by forces transmitted across the mendocino transform fault

the oroville earthquakes a study of source characteristics and site effects half space modeling in the time domain of broadband seismic records from the oroville aftershock sequence demonstrates that these events have simple sources systematic scaling between the far field source duration and seismic moment for the aftershocks studied here and other california earthquakes confirms brune s 1970 source model and shows that the stress drops for california earthquakes range between 10 and 100 bars these events also show no trend in stress drop as a function of size for 3 0 or m sub l or 6 4 analysis of the static deformation associated with the main shock shows that the slip occurred on a fault 81 km sup 2 in area corresponding to the zone defined by the first 3 days of aftershocks teleseismic studies indicate that the seismic source area was 50 km sup 2 corresponding to a zone completely free of aftershocks in the middle of the aftershock zone both results imply a stress drop of 29 39 bars and seismic moment between 5 7 and 8 7 x 10 sup 24 dyn cm the evidence suggests that faulting occurred seismically on the central portion of the fault and propagated out slowly enlarging the fault plane complicated waveforms recorded for the same events at nearby sites on alluvium indicatemore the critical role that propagation through complex structure plays in determining the character of the observed seismic signal at close in stations modeling of these records show that dipping structure may account for the observed waveform complexities less

tectonic evolution of the explorer northern juan de fuca region from 8 ma to the present new magnetic anomaly data in conjunction with sea beam and seamarc ii data has been used to schematically model the tectonic development of the explorer northern juan de fuca region with a series of five ridge offset propagators during the last 8 m y the sovanco fracture zone played a major role in the evolution of this area since its origin as a spreading center offset between the explorer and juan de fuca ridges approximately 7 4 ma between this time and 2 5 ma the transform migrated southwards due to southward propagation of explorer ridge and lengthened by the addition of three propagating ridge offsets as well as by asymmetric spreading to the east on explorer ridge since the independence of explorer plate at 4 ma there has been clockwise rotation of the explorer spreading center relative to the juan de fuca ridge in response to forces acting at the convergent margin the sovanco fracture zone also underwent clockwise reorientation and in the process developed into a broad zone of shear the tectonic deformation associated with this rotation distorted the previously existing n s magnetic anomaly pattern explorer seamount a major geological feature in the region is modeled as a presently extinct southern spreading segment of the explorer ridge system which was isolated by the northward migration of the west end of the sovanco fracture zone the recent 1 ma tectonic evolution of explorer ridge involved both northward and southward propagation of southern explorer ridge as well as an eastward ridge jump during the brunhes ih the northern portion of the study area the axis of spreading jumped 40 km to the nw around 0 3 ma from the eastern explorer deep to the western explorer rift

an explanation for the double seismic layers north of the mendocino triple junction we propose that the gently eastward dipping double planed seismic zone observed at 15 25 km depths in the southern cascadia subduction zone just north of the mendocino triple junction is a direct consequence of the thermally controlled rheology as the oceanic lithosphere subducts to a depth of about 15 km the temperature regime causes a brittle plastic transition to occur within the oceanic crust thus a ductile layer forms in the lower oceanic crust sandwiched between the brittle upper crust and brittle upper mantle the very high strain rates near the triple junction caused by the northward push of the pacific plate on the gorda plate increase the seismicity and thus accentuate the double seismic zone in this region this model explains the focal mechanisms observed in the seismic zone and their spatial change the double seismic layers clearly define the position of the subducting gorda plate previously uncertain in the cape mendocino region

tropical response to the 8200 yr b p cold event speleothem isotopes indicate a weakened early holocene monsoon in costa rica a 1 8 o monsoon rainfall proxy record from a u th dated costa rican stalagmite 8840 4920 yr b p documents an early holocene dry period correlative with the high latitude 8200 yr b p cold event high 1 8 o values between ca 8300 and 8000 yr b p demonstrate reduced rainfall and a weaker monsoon in central america a relatively wetter and more stable monsoon was established ca 7600 yr b p the early holocene dry event suggests a tropical extratropical teleconnection to the 8200 yr b p cold event and a possible association of isthmian rainfall anomalies with high latitude climate changes the likely source of such a tropical anomaly is a decrease in atlantic thermohaline circulation and atmospheric perturbations associated with drainage of proglacial lakes and freshwater discharge into the north atlantic a weaker monsoon at 8200 yr b p may be linked to wetland contraction and a decrease in methane observed in greenland ice cores

plio pleistocene vegetation changes in the north china plain magnetostratigraphy oxygen and carbon isotopic composition of pedogenic carbonates abstract the plio pleistocene history of c4 plant biomass in northwestern china has been documented from the loess soil sequences of the loess plateau region however how c4 plants evolved in the warmer and low elevation eastern china monsoon zone is still poorly known mainly because of the unavailability of well dated geological records in this study a 203 6 m core of floodplain deposits was recovered from the north china plain near tianjin and dated magnetostratigraphically the results define a chronosequence for the last 3 3 ma the late quaternary portion of the core consists of fluvio marine sediments while the rest of the section 3 3 0 6 ma contains abundant paleosols formed on the floodplain as confirmed by soil micromorphological evidence the stable carbon and oxygen isotopic composition of pedogenic carbonates was measured to document vegetation and climate changes the results reveal mixed c3 and c4 vegetation with an estimated c4 abundance of 40 60 from 3 1 to 2 2 ma and a subsequent gradual decline to 25 until 0 6 ma this trend is consistent with the data from the loess soil sequences further west on the loess plateau suggesting they are regionally significant changes the lowering of growing season temperature and or drier conditions induced by global cooling would explain this overall decline

the huanghe yellow river and changjiang yangtze river deltas a review on their characteristics evolution and sediment discharge during the holocene abstract today asian rivers play an important role in delivering sediment from the land to the oceans however past sediment discharge before human activities modified the landscape is not well known and is estimated to have been roughly one third to one half of the present level chinese rivers have been strongly influenced by human activities and their sediment discharge has changed over the last 6000 years for the study of the long term human impact on rivers and catchment basins chinese rivers provide excellent examples because continuous historical records are available here the characteristics evolution and sediment discharge during the holocene of the deltas of two large chinese rivers huanghe yellow river and changjiang yangtze river are summarized the huanghe and changjiang deltas are different with respect to coastal environments water and sediment discharge the pattern of development of the delta lobe thickness of holocene sediments and basal topography of the incised valleys formed during the last glacial period the huanghe delta is characterized by a high concentration of sediment huge sediment discharge thin holocene deltaic sediments a lateral delta lobe shift and a steep longitudinal profile in its lower reaches the changjiang delta is characterized by a large water discharge a large interseasonal water level change a tide dominated coastal environment continuous seaward progradation with isolated river mouth sand bodies thick holocene sediments and a deep incised valley formed during the last glacial period based on the sediment volume of prograding deltaic sediments over the last 6000 years the sediment discharge of huanghe 1000 years ago is estimated to have been 1 10 8 tons year 1 10 of the present level an abrupt increase occurred at ca 1000 years ago as a consequence of human activities on the loess plateau especially involving cultivation and deforestation moreover the estimated sediment load of huanghe at sanmenxia station prior to 1000 years was 2 7 4 0 10 8 tons year or about 20 15 25 of the present level our recent study estimated that the sediment discharge of changjiang before 2000 years bp was ca 2 4 10 8 tons year total sediment discharge of both rivers before 2000 years bp is estimated to have been 3 4 10 8 tons year or only about 20 of the present level moreover the total sediment load to the lower reaches is estimated to have been 5 6 10 8 tons year or one fourth the present level

ultra high rates of loess sedimentation at zhengzhou since stage 7 implication for the yellow river erosion of the sanmen gorge abstract the mangshan loess plateau is located 25 km to the west of zhengzhou on the south bank of the yellow river here the river flows out through the sanmen gorge releasing most of its suspended load following a dramatic decrease in gradient the stratigraphy of the mangshan loess deposits consisting of a number of loess and palaeosol sequences was established following magnetostratigraphic studies and measurements of magnetic susceptibility and grain size distribution the bruhnes matuyama boundary was found at the depth of about 130 m indicating that this sequence at mangshan resembles what is observed elsewhere in the loess plateau the upper part of the mangshan loess displays extremely high sedimentation rates 50 m 3 per 1000 years lower susceptibility values and coarser grain size distribution unlike the lower part of the profile and other sections in the loess plateau this striking change indicates that the upper mangshan loess had a different sediment source different from the deserts that act as a common source for most of the loess deposits in central china this sediment source is believed to be the proximal yellow river floodplain and the ancient alluvial fan lying at the eastern end of the sanmen gorge the age estimation of the formation of the alluvial fan based on mangshan loess suggests that the yellow river may have eroded the sanmen gorge at approximately mis 7

crustal structure of the cascadia subduction zone southwestern british columbia from potential field and seismic studies the northern cascadia subduction zone is a region of convergence between the oceanic explorer and northern juan de fuca plates and the continental north american plate potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2 5 and 3 dimensional interpretations the lateral extent and thickness of the accreted wedge vary significantly along the zone the narrow metasedimentary pacific rim terrane lies immediately west of wrangellia and extends the length of vancouver island consistent with its emplacement by strike slip faulting following the accretion of wrangellia the ophiolitic crescent terrane is a narrow slice lying seaward of the pacific rim terrane but not extending northward beyond the juan de fuca plate in this region the crescent terrane was emplaced in a strike slip or obliquely convergent style

the null hypothesis globally steady rates of erosion weathering fluxes and shelf sediment accumulation during late cenozoic mountain uplift and glaciation at the largest time and space scales the pace of erosion and chemical weathering is determined by tectonic uplift rates deviations from equilibrium arise from the transient response of landscape denudation to climatic and tectonic perturbations we posit that the constraint of mass balance however makes it unlikely that such disequilibrium persists at the global scale over millions of years as has been proposed for late cenozoic erosion we synthesize weathering fluxes global sedimentation rates sediment yields and tectonic motions to show a remarkable constancy in the pace of earth surface evolution over the last 10 ma and support the null hypothesis that global rates of landscape change have remained constant over this time period despite global climate change and mountain building events this work undermines the hypothesis that increased weathering due to mountain building or climate change was the primary agent for a decrease in global temperatures

evaluating the role of climate and tectonics during non steady incision of the yellow river evidence from a 1 24 ma terrace record near lanzhou china abstract the competing roles of bedrock uplift and climatic change in the formation of fluvial terraces remain uncertain most of recent studies have attributed terrace formation to climatic changes and held that even in tectonically active settings climate variations control cycles of terrace planation and abandonment based on field investigations of loess paleosol sequences magnetostratigraphy and optically stimulated luminescence osl dating we develop a new chronology for a spectacular flight of terraces along the yellow river near lanzhou china over past 1 24 ma all the terraces are strikingly similar in that they have several meters of paleosol developed directly above fluvial deposits on the terrace treads suggesting that the abandonment of each terrace due to river incision occurs during the transition from glacial to interglacial climates however the ages of terraces cluster in two relatively short time periods 1 24 0 86 ma and 0 13 ma present during the intervening time between 0 86 ma and 0 13 ma terraces either did not form or were not preserved we suggest that this record indicates that rock uplift rates varied through time and influenced terrace formation preservation thus our results demonstrate the utility of deep chronologic records from fluvial terraces for deconvolving the effects of tectonics and climate on fluvial incision