hydrothermal effects on organic matter high vs low temperature components abstract hydrothermal activity and sea bed mounds have been explored in guaymas basin central gulf of california by the deep sea drilling project piston coring dredging and diving with the d s r v alvin the deep seated magmatic heat flow causes extensive thermal alteration of both organic and mineral matter especially in the case of the south rift of guaymas basin this results in hydrothermal circulation with concomitant migration of organic pyrolysate to the seabed the bitumen at depth of dsdp sites 477 and 481 contained thermogenic components such as primary olefins thermodynamically stabilized molecular markers 17 h hopanes and elemental sulfur with traces of polynuclear aromatic hydrocarbons pah the major components were n alkanes with no carbon number predominance and unresolvable complex branched and cyclic hydrocarbons hump which are typical of mature petroleum and were generated by the heat from the intrusives and magma dredge samples recovered from a mound in guaymas basin contained petroleum of a thermogenic origin as confirmed by the presence of gasoline range hydrocarbons a broad distribution of n alkanes c13 c33 no carbon number predominance hump pristane and phytane olefins and pah and large amounts of aromatic naphthenic and asphaltic material similar petroleums subsequently recovered from other active mounds in the southern rift by the d s r v alvin had analogous compositions ranging from volatile to heavy oils in pyrrhotite chimneys the extractable organic matter consisted of mainly higher molecular weight pah these petroleums are derived from the immature primarily marine organic matter at depth by thermal alteration rapid quenching by hydrothermal removal and migration followed by condensation at the sea bed results in the manifestations of oil as part of the sulfide formations exterior exposed oil samples are degraded and leached whereas interior samples are essentially unaltered the pah are high temperature resynthesis aromatization products that condense in the hottest parts of the vent systems

pronounced changes in atmospheric circulation and dust source area during the mid pleistocene as indicated by the caotan loess soil sequence in north china the quaternary loess soil sequence in north china is of significant value for tracing variations in atmospheric circulation in this study we measured the anisotropy of magnetic susceptibility ams of the caotan ct loess soil sequence from the northeastern margin of the tibetan plateau tp and analyzed the surface textural features of quartz grains of representative samples the ams results indicate a primary sedimentary magnetic fabric without significant disturbance by pedogenesis the dec kmax values indicate a significant shift of the dominant surface wind direction from nearly n s to w e since 300 ka while the surface textural features of the quartz grains indicate an increase in the contribution from fluvioglacial sediments derived from the tp at the same time as there is no evidence of significant climatic changes around 300 ka we attribute the coeval changes of atmospheric circulation and dust source area to the uplift of the tp during the mid pleistocene and the resulting climatic cooling cooling of the tp may have led not only to the enhancement of the plateau monsoon but also to the significant accumulation of plateau glaciers which would have resulted in accelerated physical erosion of the underlying rocks thus increasing the quantity of fine grained fluvioglacial sediments to be transported to the study site

late oligocene miocene mid latitude aridification and wind patterns in the asian interior the asian interior has the largest mid latitude arid zone in the northern hemisphere and so has become increasingly attractive for studying the initiation and the past extent of aridification in this zone given the enormousness of the asian interior it remains unclear how old and extensive the eolian deposits might have been and what wind regimes have been responsible for the formation of the mid latitude arid zone here we report new eolian records of widespread tertiary eolian deposits in a region far from the chinese loess plateau the giant junggar inland basin of northwestern china our results demonstrate that the earliest eolian deposition initiated ca 24 ma we interpret that the tertiary eolian dust in the junggar basin was transported by westerly winds possibly from areas in kazakhstan the dust differs from the airborne dust transported by winter monsoon winds from the deserts of mongolia and northern china that accumulated on the loess plateau these results further reveal that the climate pattern similar to that of the present has prevailed at least since the latest oligocene in central asia

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

mudflat distal fan and shallow lake sedimentation upper vallesian turolian in the tianshui basin central china evidence against the late miocene eolian loess the tianshui basin in central china contains a thick sedimentary sequence 1400 m of continental deposits aragonian to villafranchian miocene pliocene in age intense himalayan movements around the paleogene miocene boundary triggered the uplift of mountain ranges around the tianshui basin providing the deposition site for continental sediments the sedimentary infill of the basin consists of four stratigraphic units i to iv this paper focuses on unit ii most of the accommodation space was occupied by unit i so during the sedimentation of unit ii the morphology of the basin was relatively flat promoting the development of wide distal fan mudflat areas and wide shallow lakes deposits include red mudstones pedogenic and groundwater calcretes reworked calcrete deposits sheet floods fluvial channels rippled sandstones siltstones ooidal peloidal packstones palustrine limestones bioturbated marls and intraclastic limestones marls the characteristics and organization of the deposits indicate the gradual transition from alluvial to lake environments within the distal fan mudflat the deposition of reworked calcrete clasts as one of the most striking facies of the basin may be attributed to substantial recycling of calcrete levels and red clays at the lake margins the presence of ooids and palustrine limestones suggests the possibility of ramplike margins within different energy settings the origin of some of the deposits of the basin s qa i section is under discussion and their consideration as eolian has been recently proposed however we believe this possibility is precluded by the characteristics of the deposits our proposal has important implications for revising the interpretation of miocene paleoclimatic conditions in central asia

rapid exhumation at 8 ma on the liupan shan thrust fault from apatite fission track thermochronology implications for growth of the northeastern tibetan plateau margin abstract the liupan shan mountain is one of the outermost ranges in northeastern tibetan plateau the onset of its uplift provides insight on whether the plateau grew sequentially outward or broad areas of the plateau deformed simultaneously the apatite fission track method can be used to date rapid cooling as the result of unroofing of rocks in response to tectonically induced vertical motion fission track data from apatite in early cretaceous sedimentary strata exposed on the hanging wall of liupan shan thrust fault in northeastern tibet suggest that a late cenozoic rapid cooling events occurred at 7 3 8 2 ma or 8 ma this age apparently dates fault activity of the liupan shan thrust in response to northeastward growth of the tibetan plateau uplift and lateral growth which continues today numerous studies of the tibetan plateau suggest that the onset of deformation in eastern and northern margin of tibetan plateau occurred in the latest miocene

the record of changing hematite and goethite accumulation over the past 22 myr on the chinese loess plateau from magnetic measurements and diffuse reflectance spectroscopy 1 high field isothermal remanence hirm and diffuse reflectance spectroscopy drs wereusedto characterize hematite and goethite in a suite of bulkand particle sized fractioned samples from the chinese loess plateau representing the main changes in magnetic mineralogy over the past 22 myr the record of drs defined hematite largely reflects variations in the pedogenic fraction 8 mm makes the main contribution to goethite concentrations in the bulk samples pedogenic goethite concentrations generally vary inversely with those of hematite the lack of consistent correlations between hirm inferred mineral content and drs measurements casts doubt on the validity of using hirm to characterize the concentrations and relative proportions of hematite and goethite in sediments representing a long time span or a wide grain size distribution the progressive decline in the coercivity of the antiferromagnetic minerals in bulk samples throughout the whole period is mainly an expression of changes in the detrital fraction reflecting changes in the source region possibly involving increased aluminum substitution the good correlation between drs defined hematite and frequency dependent magnetic susceptibility in several chronologically defined subgroups of samples is consistent with the hypothesis that maghemite forms a transitional phase in a weathering sequence from ferrihydrite to hematite the results shed new light on using hirm to characterize hematite and goethite in sediments and on the history of weathering and climate change over the past 22 myr in the dust source regions and on the loess plateau

observations of near bed velocity profiles and seabed roughness in tidal currents flowing over sandy gravels measurements of near bed velocity profiles and bottom roughness in tidal currents flowing over sandy gravels in the west solent england have shown that although the seabed in the immediate vicinity of the study area was generally level and devoid of regular bed forms the observed velocity profiles were consistently curved concave downwards this behaviour may be attributed to the presence of an internal boundary layer which was due to form drag on irregular topography upstream of the measurement position loosely packed gravels of the type found in the west solent had a geometric mean roughness length of about 0 3 cm comparison of the observed ratio of roughness length to mean particle size with laboratory data suggests an equivalent two dimensional roughness element spacing of about three particle diameters in contrast to tidal flows over rippled sand beds the roughness lengths for sandy gravels remained relatively constant throughout the tidal cycle furthermore these measurements provided no systematic evidence of the effects of flow acceleration on profilederived estimates of the friction velocity u and the roughness length z0

comparison of 14c ages of hydrothermal petroleums abstract in order to set limits on the time frame of formation of hydrothermal petroleum we have obtained 14 c ages on samples from three diverse regions gulf of california guaymas basin northeast pacific ocean escanaba trough and middle valley and the east african rift tanganyika trough the results date the source of carbon and therefore provide maximum ages for the formation and emplacement of the hydrothermal petroleums the youngest petroleum occurs iin the souther trough of guaymas basin 3200 6600 yr mean 4692 yr in the northern trough the petroleum is slightly older 7400 yr significantly older hydrothermal petroleum occurs in escanaba trough 17 000 yr and middle valley 29 000 yr a continental example from the east african rift has an age of 25 000 yr comparable to the ages observed in the oceanic samples from the northeast pacific ocean these ages affirm that hydrothermal petroleum formation is a very rapid process and took place some time between the latest pleistocene and the present in these active hydrothermal systems

an 11 ma old red clay sequence on the eastern chinese loess plateau abstract older red clay deposits on the chinese loess plateau contain information of early paleomonsoon evolution and tectonic history of the ordos platform and the tibetan plateau a well conserved eolian red clay sequence was recently discovered at shilou on the eastern chinese loess plateau paleomagnetic measurements demonstrate that polarity zones recorded in the shilou red clay profile correspond well with polarity zones between c2an 1n and c5n 2n of the gpts and the basal age of the study profile is approximately 11 0 ma field observation grain size frequency distribution and quartz grain morphology suggest a wind blown origin for this sequence this indicates that the eastern loess plateau began to accumulate dust deposits at 11 ma ago implying earlier establishment of the east asian paleomonsoon probably owing to uplifting of the tibetan plateau and ongoing global cooling it also demonstrates that the eastern loess plateau belonging to the ordos platform was slowly uplifted and suffered from erosion in the early neogene and has no conserved eolian deposits older than 11 ma as in the western chinese loess plateau

survey of hydrothermally generated petroleums from the guaymas basin spreading center abstract the guaymas basin spreading center and hydrothermal system are generating an assortment of petroleum like bitumens by pyrolysis of organic matter in the overlying uncosolidated sediments samples collected by d s v alvin from hydrothermal fields in the southern trough exhibit large variations in the quantities and character of the solvent soluble organic materials various pyrolytic regimes combined with fluctuating thermal gradients and migration velocities may impart a compositional fractionation possibly by differential gaseous solubilization and hydrodynamic alteration of the multi component fluids the data suggest that differential condensation solidification biodegradation and water washing cause selective removal of components at the seabed many hydrothermal oils are unlike normal reservoir petroleums because they contain significant concentrations of polar material due to their rapid genesis and transport and probable solubilization or advection of polar immature components the biomarker distributions confirm high temperature pyrolysis at depth with variable entrainment of less thermally mature bitumens during transport data for shallow core samples from the northern trough indicate that those locations experienced variable influx predominantly volatiles of a low temperature pyrolysate

post orogenic tectonic evolution of the qinling belt central china insights from a magnetostratigraphic study of a cretaceous intra mountain basin sedimentary succession abstract the huicheng basin straddling the west and east qinling belt in central china is a key feature linking the ne tibetan plateau and the rifted basins in east china and its sedimentary sequences provide important archives to elucidate tectonic evolution of the qinling belt and possibly the eastward expansion of the tibetan plateau the age of upper red fluvial lacustrine facies sequence known as the chengxian group has been previously assigned to neogene in spite of the lack of age definitive fossils which seems problematic in order to better constrain the age of this group and to establish a chronostratigraphic framework we conducted a magnetostratigraphic study of about 420 m thick composite section and collected 738 paleomagnetic samples progressive thermal demagnetization of these samples reveals both single and two component magnetizations characteristic remanent magnetizations chrms are mostly isolated at or above 350 and show northeasterly downward directions indicating unanimously normal polarities through the composite section positive fold and tilting tests suggest that the chrms are likely primary and that this sequence was possibly deposited during the cretaceous normal superchron instead of the previously assigned neogene by correlating with other cretaceous basins developed in west and east qinling and taking into account the polyphase tectonic overprints from structural analysis we conclude that the huicheng basin was formed during the post orogenic tectonic regime in response to far field effects caused by the northward subduction of the neo tethys ocean and the subsequent india eurasia collision

magnetostratigraphic and uranium series dating of fossiliferous cave sediments in jinyuan cave liaoning province northeast china abstract chronological sequences of quaternary terrestrial mammalian faunas can provide important information about the evolutionary history of mammals regional biostratigraphy and paleoenvironmental changes here we report the results of studies of a thick nearly continuous sedimentary sequence from jinyuan cave in liaoning province northeast china the sequences contains four mammalian faunas ranging in age from the late pliocene to the late middle pleistocene detailed paleomagnetic and u series age determinations date the upper unit of the sedimentary sequence to 2 2 0 3 ma the results suggest that cave development and infilling were closely associated with the tectonic evolution of the bohai basin including the subsidence of the basin and the uplift of the surrounding mountains such as luotuo hill together with the large mammalian faunas from both south and north china we suggest that these mammalian faunas were smaller in size and lower in species diversity during the middle pleistocene transition mpt 1 2 0 7 ma than in the earlier and later pleistocene this indicates that major climatic variations during the mpt in north china may have reduced their taxonomic abundance and diversity the extremely warm and wet interglacial climates that followed the mpt in monsoonal east asia provided an ecological niche for the evolution and migration of these mammalian faunas resulting in an increase in the numbers and diversity of mammals in east asia

neogene evolution of the north eastern tibetan plateau based on sedimentary paleoclimatic and tectonic evidence abstract in this study the development of the north eastern tibetan plateau is investigated via an integrated study of the sedimentology magnetostratigraphy palaeogeography and cross section comparisons of the neogene sedimentary basins of the tibetan plateau from 23 to 17 ma this region was characterized by widespread angular unconformities and a sedimentary hiatus indicating the comprehensive uplift of the plateau the tectonic event that created the tibetan plateau triggered the evolution of sedimentary basins during the middle miocene climatic optimum event the growing qilian shan formed a barrier leading to cooler and drier conditions on the western side of the mountain however humid and warm conditions persisted on the eastern side of the range after this event this region was influenced by a global cooling trend and enhanced tectonic activity in the north eastern tibetan plateau at approximately 10 8 ma sedimentary data which were consistent with low temperature thermochronological records indicated the inversion of the surrounding mountains and rapid exhumation of the laji shan since 5 0 3 6 ma these intermontane basins gradually shrank and disappeared due to the overall uplift of the tibetan plateau and the accelerated global cooling trend and were associated with unconformities and growth strata that formed during stepwise north eastward thrusting tectonism

provenance of cenozoic sediments in the xining basin revealed by nd and pb isotopic evidence implications for tectonic uplift of the ne tibetan plateau

hydrothermal petroleum genesis migration and deposition in guaymas basin gulf of california hydrothermal activity and seabed mounds have been explored in guaymas basin by the deep sea drilling project dsdp piston coring dredging and diving with the deep submersible research vessel dsrv alvin sedimentary organic matter derived primarily from immature degraded microbial detritus is easily converted to petroleum under the hydrothermal regime these petroleums are mature and migrate in the fluids and by diffusion to the seabed the fluid migration is aided by near critical aqueous solution and supercritical carbon dioxide and hydrocarbons petroleum compositions vary from condensates to naphthenic to waxy all with significant amounts of asphaltenes and hydrothermal products such as olefins and toxic polynuclear aromatic hydrocarbons pah the heavy ends condense at the seabed depositing mainly as a cement with the sulfides and other minerals and to a lesser extent as entrapped oil and crystalline wax in vugs and conduits of the mounds the pah are high temperature resynthesis aromatiza

acoustic sensing of particles suspended by wave bottom interactions abstract the inability to measure the temporal and spatial distributions of particles suspended by the action of combined waves and currents is a limiting factor in obtaining quantitative data to test the various theoretical or empirical models which have been hypothesized to explain the process of particle suspension in a beach estuary or continental shelf environment an experiment has been conducted in a wave tank to examine the feasibility of using a 1 mhz acoustic system to study the temporal dependence and spatial distribution of sand grains raised from the tank bottom during the passage of waves of known period and amplitude data showing the acoustic detection of fine to medium sized sand grains suspended by surface gravity waves are presented to illustrate the suitability of using acoustic systems to study the particle suspension process acoustic systems used in conjunction with instruments such as current meters and reynolds stress measuring devices will allow a more complete understanding of wave current bottom interactions and the associated suspension of sediments

magnetostratigraphy and depositional history of the miocene wushan basin on the ne tibetan plateau china implications for middle miocene tectonics of the west qinling fault zone abstract based on field mapping section measurement and magnetostratigraphy 1700 m of sedimentary rocks have accumulated in the wushan basin between 16 ma and 6 ma basin geometry sedimentation characteristics and the early syn depositional deformation along the northern margin of the basin indicate that formation of the wushan basin was related to tectonic deformation along the west qinling fault zone during the middle miocene a series of basins of similar age to the wushan basin were generated along and to the south of the west qinling fault zone while basalts also erupted in this region at this time we suggest that the middle miocene 16 ma may represent a change in kinematics and deformation style in the region along and to the south of the west qinling fault zone at this time there was a transition from nne ssw compressional deformation that dominated the region since the late paleogene to the development of wnw ese and or e w trending strike slip movement and associated transpressional and transtensional activity that continues today the miocene wushan basin may have developed in association with transpression along the west qinling fault zone whether this transition was related to the onset of strike slip along the east kunlun fault and related deformation transfer lower crustal flow or removal of mantle lithosphere the middle miocene provides direct evidence for a change in the kinematic style along the plateau margin

high paleointensities of the geomagnetic field from thermomagnetic studies on rift valley pillow basalts from the mid atlantic ridge nineteen pillow basalts dredged within the rift valley of the mid atlantic ridge at 36 8on were studied by the thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor previously reported fission track ages are 2 000 to 6 000 years for the youngest rocks mainly olivine basalts and 10 000 to 100 000 years for the others mainly plagioclase basalts and pyroxene basalts all but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates stability of the direction of nrm during its thermal demagnetization constant ratio of nrm trm natural remanent magnetization to thermoremanent magnetization over 50 or more of the original nrm intensity 80 to 94 for 11 specimens and reproducibility of low temperature partial trm ptrm however strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in curie temperature 15 to 80oc during the paleointensity experiments below 250oc notwithstanding the linearity of the nrm trm plot in this temperature interval this alteration probably due to low temperature oxidation of the specimens seems typical of young pillow basalts and may result in paleointensity estimates which are too high this result shows that excellence of remanence tests nrmtrm linearity and ptrm stability does not ensure the absence of chemical changes during the thellier experiments and therefore the validity of the paleointensity obtained the assumption that reliable paleointensities are obtained when the curie point increase is 10oc or less led to the selection of five of our specimens all from the youngest group their mean paleointensity is 64 2 20 5 xt standard deviation and the corresponding virtual axial dipole moment vadm is 11 5 3 7 x 1022 a m 2 given the variations of the vadm of the earth s magnetic field over the last 6 000 years as established from archeomagnetic studies our paleointensity results suggest that the latest eruptions on the inner floor of the rift valley at 36 8on occurred 1 500 1 000 years ago

spatial variations in paleowind direction during the last glacial period in north china reconstructed from variations in the anisotropy of magnetic susceptibility of loess deposits abstract anisotropy of magnetic susceptibility ams of chinese loess is considered to be an effective tool for determining paleowind direction however the relationship between ams and the paleowind direction is still a matter of debate this study reports the results of ams measurements of chinese loess deposited during the last glacial period on slopes of varying slope angles and orientations the sites are located on the chinese loess plateau in west qinling and on the eastern margin of qilian mountain the results show that within the same region magnetic lineations are clustered along similar orientations despite differences in slope exposure and slope angle but that different regions exhibit different directions of magnetic lineation these results suggest that the alignment of the magnetic grains during deposition of the eolian deposits was determined by air circulation rather than by water flow on the surface of the slopes and therefore that the ams of chinese loess can be used to determine paleowind directions in addition our results indicate that the ams of chinese loess is determined mainly by the patterns of regional surface wind flow that occurred during dust accumulation rather than by the uniform pattern of large scale atmospheric circulation in addition since wind direction is influenced significantly by regional topography the ams of chinese loess may have the potential to detect significant changes in past regional topography

yunnan a refuge for trilophodont proboscideans during the late miocene aridification of east asia fossil elephantimorph proboscideans are very common in miocene faunas of east asia however their evolution and migration associated with ecological pressure have been studied little especially in china in the present article analyses of the dynamics of the elephantimorph genera of northern and southern china were performed the results indicate considerable depression of elephantimorph proboscidean diversity in northern china and continuous success of this group in the yunnan region southern china during the late miocene notably all trilophodont elephantimorph proboscideans were extinct by the end of the middle miocene in northern china and did not recover until the latest miocene however trilophodont and tetralophodont elephantimorph proboscideans coexisted throughout the late miocene in the yunnan region therefore we interpret the yunnan region as a late miocene refuge for proboscideans especially trilophodonts from severe aridity in northern china caused by uplift of the tibetan plateau and retreat of the paratethys sea further analysis of the dynamics of coexisting genera indicates that the components of elephantimorph proboscidean assemblages in northern china display patterns distinct from those of yunnan southeast asia and siwalik therefore ecological barriers between northern china and siwalik and between northern china and yunnan southeast asia are hypothesized to have existed by as early as the middle miocene the latter barrier occasionally opened e g at the boundary between the middle and late miocene and thus provided a pathway for the survival of trilophodont proboscideans escaping from ecological pressures in northern china

comment on mudflat distal fan and shallow lake sedimentation upper vallesian turolian in the tianshui basin central china evidence against the late miocene eolian loess by a m alonso zarza z zhao c h song j j li j zhang a martin perez r martin garcia x x wang y zhang and m h zhang sedimentary geology 222 2009 42 51 abstract alonso zarza et al 2009 argued that the miocene sedimentary sequence near qinan qa i guo et al 2002 north to tianshui in central china represents mudflat distal fan and shallow lake sediments here we show that they 1 did not take into account extensive observational evidence supporting an eolian origin 2 erroneously correlated deposits of different origins and small lateral extent occurring in different geomorphic units 3 inferred a common origin for distal fans in the tianshui and qinan regions despite contrasting tectonic and geomorphic conditions and 4 in some cases misinterpreted features typical of eolian deposits accordingly their study is not a sufficient basis for challenging earlier interpretations of asian paleoclimates

neotectonics around the ordos block north china a review and new insights abstract the multiphase intensive intra continental deformation of the eurasian continent in the cenozoic caused by indo asian and pacific asian collisions has been studied extensively however its cenozoic intra continental deformation process and dynamics remains poorly constrained the western north china plate of the eurasian continent is characteristic of the cenozoic faulted basin system around ordos block and is a critical region to determine this deformation process here new structural data and fault kinematic analysis coupled with new geochronological results delineate a two stage cenozoic tectonic evolution in the region providing new structural evidence to decipher the intra continental deformation due to indo asian and pacific asian collisions the first stage is characterized by the formation of faulted basins in the northwestern and southeastern margins of the ordos block dominated by a pure shear mechanism this stage further comprises the nw se extension spanning the eocene to late miocene ca 10 5 ma and the subsequent basin inversion triggered by the nw se compression during the late miocene ca 10 5 9 5 ma its tectonism is associated mainly with the far field effect of the northwestward subduction of the pacific plate the second stage is characterized by the development of the shanxi and hetao basins in the eastern and northern parts of the ordos block respectively and an intensive mountain building process in the western part since the late miocene ca 9 5 ma which is connected with a simple shear mechanism this stage is furthermore divided into three alternating episodes of shortening and extension events these resulted predominantly from far field responses to the northeastward growth of the tibetan plateau and partially from the pacific subduction

intensified aridity of the asian interior recorded by the magnetism of red clay in altun shan ne tibetan plateau abstract the aridity history of monsoon dominated areas has previously been traced by high resolution studies of the well known loess and red clay sequences on the chinese loess plateau however the aridity process further west in china and the differences from the monsoon regime are unclear because no continuous aeolian sequences have been found in those areas here we describe and interpret an 88 4 m thick neogene aeolian sequence containing 40 visually defined brownish red clay and gray caliche nodule layers in the eastern xorhol basin northeast of the tibetan plateau magnetostratigraphic results show that the sequence was deposited between 13 and 2 6 ma the sequence can be divided into two parts with different climate conditions according to magnetic susceptibility ms the lower part 13 11 5 ma indicates a more changeable climate with higher ms values average 15 10 8 m3 kg 1 dominated by magnetite the upper part 10 3 2 6 ma reflects a drier climate with lower stable ms values average 9 5 10 8 m3 kg 1 mainly arising from hematite our results indicate that the intensified aridity of the asian interior started between 11 5 ma and 10 3 ma when ms decreased significantly due to outward growth of the tibetan plateau at around 12 ma together with ongoing global cooling and regression of the neotethys ocean during the cenozoic

cenozoic tectonic evolution of the south ningxia region northeastern tibetan plateau inferred from new structural investigations and fault kinematic analyses abstract the northeastward growth process of the tibetan plateau has been a key issue to be long termly debated which is well documented by the south ningxia region at the northeasternmost periphery of the tibetan plateau due to its cenozoic convex to the northeast arcuate range and basin structures here new structural measurements and fault kinematic analysis together with new geochronological results delineate a two stage cenozoic tectonic evolution in the region providing the new structural evidences to decipher the northeastward growth process of the tibetan plateau the first stage is marked by widespread sedimentary basin formation during the oligocene to late miocene ca 30 10 5 ma under nw se extensional regime and subsequent basin inversion triggered by nw se compression in the late miocene ca 10 5 9 5 ma these events were probably associated mainly with the far field effect of the northwestward subduction of the pacific plate a little related to the plateau growth the second stage largely presents an intensive mountain building process and following re deformation since the late miocene ca 9 5 ma which was predominantly a result of the far field responses to the northeastward growth of the tibetan plateau and partially to the pacific subduction the second stage further comprises three alternating episodes of shortening and extension events the earliest episode of ne sw compression caused intensive crustal shortening in a long period from the late miocene to early pleistocene ca 9 5 1 8 ma leading to a significant mountain building process in this region a gentle ne sw extension subsequently dominated here in the late pleistocene ca 18 ka as manifested by the widespread appearance of paleo lakes along the nw striking boundary faults since the end of the late pleistocene ca 18 ka a youngest tectonic regime affected this region characteristic of a tectonic transpression causing evidently strike slip activities on the primary faults and the development of small scale pull apart basins along the haiyuan fault

acoustic detection of seabed gravel movement in turbulent tidal currents abstract the measurement of sediment transport rates in the sea remains one of the most elusive goals for oceanographers sedimentologists and coastal engineers such measurements are required to test and validate sediment transport theories progress in this direction is greatly restricted by the lack of suitable field data and is dependent on progress with new instrumental techniques developments investigated in recent years have included the use of active and passive acoustic devices to detect sediment transport in this letter we describe the measurement of seabed gravel movement using a passive acoustic system for the first time using an underwater tv camera it has been possible to simultaneously observe sediment transport and take acoustic measurements thereby gaining independent assessment of the sensors performance further support has been provided by measurements of the turbulent fluctuations in the nearbed reynolds stresses

hydrocarbon geochemistry of hydrothermally generated petroleum from escanaba trough offshore californi u s a abstract in 1986 three samples of sulfide rich sediments impregnated with hydrothermally derived asphaltic petroleum were recovered in a dredge and by submersible from escanaba trough the sediment covered southern end of the gorda ridge spreading axis offshore northern california the molecular distributions of hydrocarbons in the two pyrrhotite rich samples recovered by submersible are similar and compare well the hydrocarbon composition of the first pyrrhotite rich samples containing petroleum discovered at a 1985 dredge site about 30 km to the south of the site of the submersible dive in contrast the 1986 dredge sample composed of a polymetallic assemblage of sulfides containes petroleum in which the distribution of hydrocarbons indicates a slightly higher of maturity relative to the other samples the observation that petroleum of variable composition occurs with metallic sulfides at two and probably more distinct site indicates that petroleum generation may be a common process in the hydrothermally active escanaba trough

new insights into partial oxidation model of magnetites and thermal alteration of magnetic mineralogy of the chinese loess in air summary new rock magnetic data including low and high temperature thermomagnetic runs and hysteresis loops as well as x ray diffraction measurements are presented from the chinese loess palaeosols at jiuzhoutai on the northwestern margin of the chinese loess plateau to provide sound constraints on the partial oxidation model of magnetites and variations in magnetic mineralogy grain size and oxidation states of loess samples during thermal treatments from room temperature up to 700 c the results show that the partially oxidized magnetites in samples are first reduced probably as a result of the burning of organic matter below 300 c and then reoxidized above that however even after heating to 700 c the highly diminished in size magnetite core still remains stoichiometric as revealed by the 120 k verwey transition we conclude that the diffusion of fe2 from the magnetite core to the surrounding maghemite rim is confined to the oxidation front magnetite core maghemite rim boundary and has less effect on the stoichiometry of the magnetite core the new results also reveal that the absolute number of superparamagnetic grains magnetite maghemite remains relatively stable during heating furthermore the left downward shift in the data points of day plots mainly between 25 and 150 c is caused by a decrease in the oxidation gradient in contrast the right upward mainly between 500 and 650 c shift of the day plots corresponds to a much higher degree of oxidation resulting in the increase of a new oxidation gradient between the magnetite core or the residual maghemite mantle and the newly developed haematite outer rim which is transformed from maghemite by heating these new results place strong constraints on the fidelity of palaeomagnetic results from the chinese loess constructed by thermal demagnetization and provide a more accurate interpretation of the day plot and low temperature magnetic behaviours for these partially oxidized magnetites furthermore this study provides robust lines of evidence for the core rim coupling model between different magnetic phases magnetite maghemite and haematite

intensification of the asian winter monsoon after 7 4 ma grain size evidence from the linxia basin northeastern tibetan plateau 13 1 ma to 4 3 ma abstract grain size studies of eolian deposits loess and red clay on the chinese loess plateau have produced a detailed history of the asian winter monsoon covering the last 8 ma this paper extends the gain size record back to 13 1 ma in the linxia basin at the northeast edge of the tibetan plateau by comparing the grain size distribution of the linxia basin sediment with known eolian sediments in the region we argue that the 10 70 m fraction is mostly eolian this fraction becomes increasingly dominant in the record after 7 4 ma with almost all sediments eolian in origin after 6 2 ma given that the 10 70 m fraction in the linxia basin are well sorted and have grain size spectra that are very similar to sediments transported by the asian winter monsoon we suggest that the continuous presence of this fraction throughout our record indicates that the asian winter monsoon has been present in some form since 13 1 ma if the assumption that variation in grain size spectra on the million year time scale is primarily controlled by the character of eolian input to the basin is valid the long term pattern of the eolian fraction reveals two abrupt intensifications of the asian winter monsoon at 7 4 ma and 5 3 ma these are superimposed on a gradual intensification of the asian winter monsoon beginning at 8 0 7 4 ma we note that the two abrupt intensifications may be related to arctic ice volume increase and that the gradual intensification of the asian winter monsoon after 7 4 ma is accompanied by central asia desertification and may therefore be related to increase in the height or extent of the tibetan plateau

biogenic magnetite as a primary remanence carrier in limestone deposits abstract studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at sugarloaf key fl u s a and calcareous laminated sediments from laguna figueroa baja california mexico have revealed the existence of magnetotactic bacteria and ultrafine grained single domain magnetite in both environments magnetotactic bacteria were identified by light and electron microscopy the single domain magnetite was detected by coercivity spectra analysis with a squid magnetometer and examined under the transmission electron microscope the similarity in terms of size and shape between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine grained magnetite in these two marine environments was biologically formed these results combined with the common occurrences of ultrafine grained magnetite in limestone deposits detected rock magnetically suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks several cambrian limestone samples separately collected from siberia china and kazakhstan were examined for the presence of bacterial magnetite samples from the lower cambrian sinskian formation at siberia platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component post cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them assessing the potential role of biogenic magnetite as a primary remanence carrier in other phanerozoic limestone deposits ought to be further pursued

inclination shallowing in deep sea sediments from the north atlantic paleomagnetic data from miocene to recent deep sea sediments recovered during leg 94 of the deep sea drilling project dsdp in the north atlantic exhibit a systematic shallowing of inclinations with depth at some sites this shallowing is coincident with a downhole decrease in water content and is observed only in sediments with a carbonate content consistently greater than 80 compaction due to overburden pressure is believed to be responsible for the shallow inclinations observed in these unconsolidated sediments at sites where the carbonate content of the sediments is consistently below about 80 no significant downcore decreasing trends in the water content or inclination record are observed

magnetostratigraphy of the xihe loess soil sequence and implication for late neogene deformation of the west qinling mountains summary theqinlingmountainrangeformsanimportantclimatebarrierbetweensouthernandnorthern china its western part referred to as the west qinling constitutes a unit of northeastern tibetan plateau the uplift history of qinling during the neogene is still a contentious issue magnetostratigraphic results from the 150 m thick xihe loess soil sequence nl vi located on the alluvial highlands surrounding an intermontane basin from the west qinling indicate 17 reverse and 17 normal magnetozones they are correlative with the interval from chron c5n to chron c2ar in the geomagnetic polarity timescale the age of the nl vi section can thus be palaeomagnetically constrained to a period from 10 5 to 4 2 ma the middle portion of the section has been affected intermittently by surface erosion processes as loess is sensitive to any erosion that may have been induced by substratum deformation and tectonic uplift this sequence indicates that the intermontane basin ridge pattern of the west qinling had already been formed by 10 5 ma thus providing suitable topographic conditions for aeolian deposition this timing is consistent with a significant deformation phase between 14 and 10 ma reported for the tibetan plateau meanwhile the complete preservation of the nl vi loess soil sequence precludes both intense deformation of the substrate and rapid uplift of the west qinling between 10 5 and 4 2 ma although the erosion affected portion of the nl vi section reflects some degree of tectonic instability from 8 3 to 6 9 ma the intensity must have been sufficiently weak to prevent the erosion of the aeolian deposits these further suggest that the late miocene growth of the tibetan plateau has not significantly affected the west qinling thus the studied nl vi aeolian sequence provides not only independent evidence for the uplift history of the west qinling within an accurate chronological framework but also provides new insights into the tectonic changes on the northeastern tibetan plateau

middle miocene reorganization of deformation along the northeastern tibetan plateau temporal variations in the orientation of cenozoic range growth in northeastern tibet define two modes by which india asia convergence was accommodated thermochronological age elevation transects from the hanging walls of two major thrust fault systems reveal diachronous miocene exhumation of the laji jishi shan in northeastern tibet whereas accelerated growth of the wnw trending eastern laji shan began ca 22 ma rapid growth of the adjacent north trending jishi shan did not commence until ca 13 ma this change in thrust fault orientation refl ects a middle miocene change in the kinematic style of plateau growth from long standing nne ssw contraction that mimicked the plate convergence direction to the inclusion of new structures accommodating east west motion this kinematic shift in northeastern tibet coincides with expansion of the plateau margin in southeastern tibet the onset of normal faulting in central tibet and accelerated shortening in northern tibet together these phenomena suggest a plateau wide reorganization of deformation

hydrothermal petroleum in mineralized mounds at the seabed of guaymas basin petroleum has been dredged from an active hydrothermal mound area in the southern rift of guaymas basin gulf of california this organic matter is composed of gasoline range aliphatic and aromatic hydrocarbons and predominantly residual polar asphaltic material the aliphatic hydrocarbons of two bitumen samples have very different boiling range and composition both samples contain polynuclear aromatic hydrocarbons and olefins which indicate formation at pyrolytic temperatures the overall compositional data indicate an origin from biological detritus by thermal alteration and rapid quenching by hydrothermal removal followed by condensation at the seabed

northeastward growth of the tibet plateau deduced from balanced reconstruction of two depositional areas the qaidam and hexi corridor basins china we address the problem of late cenozoic uplift erosion and growth of northeastern tibet by reconstructing from isopach maps and drill holes the volumes and masses of sediments deposited in the qaidam and hexi corridor basins since 35 myr ago the mass budget is based on simple geometrical assumptions such as regional similarity of the thickness ratios between strata of different ages in the qaidam where our record extends back to the oligocene the budget shows a huge rise of the accumulation rates after the beginning of the pliocene 5 3 ma the early pliocene seems to be the period of maximum deposition with accumulation rates in excess of 1 mm yr 1 2 7 kg m 2 yr 1 of compacted rocks throughout the basin there also seems to be a southeastward shift of the largest depocenters between the upper pliocene 3 4 1 6 ma and the quaternary in the hexi corridor sedimentation is confined to small foreland flexural depressions associated with the frontal thrusts of the qilian shan and occurs at an average rate one order of magnitude smaller than in the qaidam basin the accumulation rate is maximum in the quaternary the sedimentation history appears to support a plateau building mechanism resulting from the combination of two geologicallly common processes crustal scale thrusting and sedimentary basin infilling the time needed to completely fill the qaidam basin and make its catchment a plateau closely resembling that of the highest part of tibet qangtang is of the order of 9 myr the mechanism now at work north of the kunlun which involves rapid infilling of broad flat areas separated by relatively narrow mountain ranges has thus probably been important in producing the high smooth topography that characterizes much of central tibet

laboratory simulation of hydrothermal petroleum formation from sediment in escanaba trough offshore from northern california petroleum associated with sulfide rich sediment is present in escanaba trough at the southern end of the gorda ridge spreading axis offshore from northern california within the exclusive economic zone eez of the u s this location and occurrence are important for evaluation of the mineral and energy resource potential of the seafloor under u s jurisdiction in escanaba trough petroleum is believed to be formed by hydrothermal processes acting on mainly terrigenous organic material in quaternary river derived sediment to attempt to simulate these processes in the laboratory portions of a pleistocene gray green mud obtained from 1 5 m below the seafloor at a water depth of 3250 m in escanaba trough were heated in the presence of water in four hydrous pyrolysis experiments conducted at temperatures ranging from 250 to 350 c and at a pressure of 350 bar for 1 0 4 5 days distributions of n alkanes isoprenoid hydrocarbons triterpanes and steranes in the heated samples were compared with those in a sample of hydrothermal petroleum from the same area mud samples heated for less than 4 5 days at less than 350 c show changes in some but not all molecular marker ratios of organic compounds that are consistent with those expected during hydrothermal petroleum formation our results suggest that the organic matter in this type of sediment serves as one possible source for some of the compounds found in the hydrothermal petroleum

organic geochemistry of deep sea drilling project sediments from the gulf of calofornia hydrothermal effects on unconsolidated diatom ooze abstract the effects of intrusive thermal stress have been studied on a number of pleistocene sediment samples obtained from leg 64 of the dsdp ipod program in the gulf of california samples were selected from sites 477 478 and 481 where the organic matter was subjected to thermal stress from sill intrusions for comparison purposes samples from sites 474 and 479 were selected as representative of unaltered material the gc and gc ms data show that lipids of the thermally unaltered samples were derived from microbial and terrestrial higher plant detritus samples from sill proximities were found to contain thermally derived distillates and those adjacent to sills contained essentially no lipids curie point pyrolysis combined with gc and gc ms was used to show that kerogens from the unaltered samples reflected their predominantly autochthonous microbial origin pyrograms of the altered kerogens were much less complex than the unaltered samples reflecting the thermal effects the kerogens adjacent to the sills produce little or no pyrolysis products since these intrusions into unconsolidated wet sediments resulted in in situ pyrolysis of the organic matter examination of the kerogens by esr showed that spin density and line width pass through a maximum during the course of alteration but esr g values show no correlation with maturity stable carbon isotope 13 c values of kerogens decrease by 1 1 5 near the sills at sites 477 and 481 and the atomic n c decreases slightly with proximity to a smaller sill at site 478 differences in maturation behavior between site 477 and 481 and site 478 are attributed to dissimilarities in thermal stress and to chemical and isotopic heterogeneity of guaymas basin protokerogen

magnetostratigraphy of a late miocene pliocene loess soil sequence in the western loess plateau in china 1 eolian deposits of pliocene age have never been reported from the western loess plateau in china here a 73 7 m eolian sequence is dated using magnetostratigraphic method and micromammalian fossils the polarity zonation correlates with the geomagnetic polarity timescale gpts from chron 3br 1r to chron 2an 3n indicating an age from 7 10 to 3 52 ma bp the sequence is characterized by clear expression of eighty four pairs of loess and soil layers while individual soils are basically indefinable for the eolian deposits of the same age in the eastern loess plateau suggesting that the later has experienced other geological processes this sequence extends the upper limit of the previously reported miocene loess soil sequences at qinan into the pliocene about 3 52 ma bp

a pedogenic model for the magnetic enhancement of late miocene fluvial lacustrine sediments from the xining basin ne tibetan plateau

a comparison between shields threshold criterion and the movement of loosely packed gravel in a tidal channel observations of the threshold of movement of loosely packed gravel in a tidal current are described for gravel with equivalent spherical diameters d in the range 0 2 d 5 0cm the critical friction velocity u c corresponding to the initiation of sediment transport is given by u c 7 0 d0 2 at large values of d within the quoted range the value u c is significantly lower than would be obtained by a shields experiment u c d0 5 by comparing our values of u c with those obtained under well controlled laboratory conditions the discrepancy with shields is shown to be due to the open spacing between and exposure of individual pebbles on the seabed by comparing our results with those from upland gravel streams and flume experiments it is suggested that shields assumed an excessively large water depth to particle size ratio as a constraint within which the critical sediment entrainment number 0c is valid

an intercomparison between visual and acoustic detection of seabed gravel movement abstract an intercomparison between acoustic and visual estimates of sediment transport rates is presented specifically two hydrophones were mounted at 24 cm above a seabed composed of gravel these were used to passively monitor the movement of bedload material by measuring the acoustic intensity generated due to the collision of moving gravel particles with one another and the bed simultaneous underwater tv observations were conducted by directing a low light camera fixed at 50 cm above the bed on to the area of seabed directly below the hydrophones the tv data has been used as a coarse reference scale to which the performance of the acoustic system could be compared high temporal resolution estimates of mass transport rates were derived from the visual and acoustic data the similarity of these two measurement sets has been examined the results show that the measurement of gravel movement by acoustic monitoring is feasible

magnetostratigraphy of an early middle miocene loess soil sequence in the western loess plateau of china 1 stratigraphy and magnetic susceptibility of the quaternary eolian deposits at different sites in china can be closely correlated confirming their spatial coherence and temporal continuity the extent to which these conclusions are true for the miocene loess soil sequences remains to be addressed here a 231 9 m eolian section qa iii is geomagnetically dated yielding an age span of 21 4 11 4 ma with the portion older than 19 6 ma being partially water reworked the magnetozones litholigic structure and magnetic susceptibility of the sequence are highly correlative with the qa i and qa ii sections 30 km west of qa iii these sequences demonstrate that the miocene loess deposits are also spatially correlative in stratigraphy present a near continuous sequence of paleoenvironment changes and that magnetic susceptibility is a useful tool for stratigraphic correlation the basal age of qa iii confirms again the onset of asian inland deserts and monsoon dominate climates by the early miocene

hydrothermal pyrolysis of organic matter in guaymas basin i comparison of hydrocarbon distributions in subsurface sediments and seabed petroleums abstract the bitumens of a selection of subsurface sediments from guaymas basin gulf of california have been analyzed in order to assess the hydrothermal effects on the organic matter and to better delineate the processes influencing the generation deposition and alteration of the hydrothermal petroleums at the seabed although the thermally generated subsurface bitumens are generally rich in polar asphaltic materials as are the seabed oils the hydrocarbon transformations parallel the extent of hydrothermal alteration as reflected in the downcore mineralogy at dsdp sites 477 and 478 at site 477 a progressive change from n alkane to aromatic hydrocarbon dominated patterns in the high resolution gas chromatography resolvable fractions of the bitumens accompanies decreases in organic carbon and bitumen yields with depth these transformations are indicative of sequential hydrothermal alteration of the sediments with depth due to increasing temperature and or progressive passage of hydrothermal fluid through the sedimentary column the polycyclic aromatic hydrocarbon pah distributions undergo extensive transformations e g dealkylation consistent with hydrothermal alterations are dominant at depth and have the broadest range as thermal maturation indices the aliphatic and aromatic components of the seabed oils can be transported to the seabed independently as a result of their generation under different temperature or depth regimes and sequentially over time during pyrolysis the concentrations of the parent pah in the thermally altered subsurface bitumens are comparable to those in the seabed oils the differential dissolution of the more soluble aromatics and selective solidification condensation of the higher molecular weight pah out of the high temperature fluids exiting at the seabed chimneys result in aromatic distributions of the oils significantly skewed towards the larger components e g benzo ghi perylene relative to the subsurface bitumens comparison of the downcore hydrocarbon distributions with those in the seabed petroleums support the contention that the hydrothermal oils are mixtures of pyrolysates from different depths or thermal regimes and are significantly altered by differential solubilization during transport and at the seabed

acoustic profiling of suspended sediments in the marine bottom boundary layer an acoustic 3 mhz backscattering profilometer has been developed for study of sediment transport in the bottom boundary layer laboratory experiments indicate reasonable correlations between backscattering intensity and mass concentrations for well sorted fine to coarse quartz sands typical of those found in marine sediments departure of the calibration curves from their expected values is thought to be due to the non spherical shape of the quartz grains field experiments on the inner shelf using the profilometer combined with velocity profiles show individual profiles taken at 1 hz during erosion events are characterized by highly variable concentrations surface wave groups rather than individual waves appear to be correlated with groups of profiles with high concentrations extending 25 100 cm above bottom averaging produces smooth profiles which fit well log normal distributions predicted by steady flow suspended load theories

cenozoic exhumation and deformation of northeastern tibet and the qinling is tibetan lower crustal flow diverging around the sichuan basin apatite fission track thermochronology data elucidate the cooling exhumation history of the qinling qin mountains which contain a paleozoic mesozoic orogenic collage north of the sichuan basin and northeast of the tibetan plateau in particular we examine the extent to which the qinling were affected by the rising plateau the northern and eastern qinling show continuous cooling and slow exhumation since the cretaceous in contrast in the southwestern qinling rapid cooling initiated at 9 4 ma a few million years later than in the eastern tibetan plateau a compilation of major cenozoic faults in the eastern tibetan plateau and the qinling and their kinematic and dynamic characterization shows that deformation in the qinling has predominantly been strike slip active sinistral and dextral strike slip faults delineate an area of eastward rock flow and bound the area of rapid late cenozoic cooling outlined by apatite fission track thermochronology these data can be interpreted to indicate that lower crustal flow has been diverted around the longmen shan and beneath the southwestern qinling causing active plateau uplift in this area alternatively northeastern tibet may be growing eastward faster in the western qinling than the entire south china block is extruding to the east

acoustic imaging of sea bed geometry a high resolution remote tracking sonar hrrts ii abstract a high resolution remote tracking sonar hrrts ii is described which is capable of measuring a 2 d profile of the sea bed over a maximum distance of 5 m under conditions of significant sediment transport the backscattered signal from a pulsed 30 s 5 mhz sonar provides acoustic range measurements of the small scale topography generated by bedforms at any point in time as well as changes in this small scale topography and the average elevation of sections of the sea bed over time bedform height and spacing can be determined accurately although bedform shape is distorted by smoothing of large slope changes as a result of the finite size of the footprint of this narrow beamed 0 74 sonar the system operates on command from a shore based micro computer and topographic resolution of the order of a few millimetres is possible resolution deteriorates as wave energy increases to levels where vibration of the transducer and large concentrations of sediment in the water column result in signal attenuation and a decrease in the signal to noise ratio

measurement of bedload gravel transport the calibration of a self generated noise system self generated noise sgn is a passive acoustic technique for the measurement of bedload transport through the acoustic detection of intergranular collisions using a submerged hydrophone system sgn is of interest because of the possibilities it offers to the measurement of transport parameters at the frequencies of turbulent or oscillating current scales this paper describes the background of the technique and reports the results from laboratory experiments on the relationship between sgn and the absolute transport rate

organic geothermometry of petroleum from escanaba trough offshore northern california abstract we have measured the extent of hopane and sterane isomerization and of monoaromatic steroid hydrocarbon aromatization in a sample of hydrothermally derived petroleum from the escanaba trough a sediment covered volcanically active ridge axis the results along with kinetic parameters predict the possible time temperature history of the petrolum forming process for this sample the extent of these reactions is consistent with petroleum formation by intense heating about 300 350 c if the time period of this heating was as short as about 100 yr such a time scale is reasonable for hydrothermal discharge events associated with ridge crest volcanism

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

heat flow measurements on a hydrothermally active slow spreading ridge the escanaba trough maximum heat flow measurements at three locations in the sediment filled escanaba trough of the gorda ridge exceed 1200 mw m at other ridge crests with thick sediment cover heat flow values of this magnitude are accompanied by high temperature hydrothermal vent activity and massive sulfide deposition a dredge haul from the southernmost high heat flow location recovered pyrrhotite thereby confirming the presence of recent high temperature venting

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

chinese loess and the paleomonsoon abstract this paper presents a general review of the recent research advances of the east asian paleomonsoon based mainly on studies of the loess soil sequences in the chinese loess plateau in the last 2 6 million years the paleomonsoonal history may be divided into about 166 events on a time scale of the earth s orbital variations during the last glacial period millenial scale oscillations of the monsoon system were prominent which can be fairly well correlated with the dansgaard oeschger cycles recognized in the greenland ice cores the monsoonal rainfall belt has experienced a wide repeated advance retreat change during the glacial interglacial cycles of the pleistocene both temporal and spatial changes of the monsoon system in the quaternary could have been linked closely to global ice volume variations

major changes in east asian climate in the mid pliocene triggered by the uplift of the tibetan plateau or global cooling since the mid pliocene east asian climates have experienced significant changes one view suggests that significant uplift of the tibetan plateau during this period could have been responsible for these dramatic changes in the strength of the east asian monsoon and for asian interior acidification while some other authors attribute these changes to the ongoing global cooling and rapid growth of the arctic ice sheet up to the present which factor dominates the major changes of east asian climate in the mid pliocene is still a contentious issue this study presents an analysis of several climate proxies including grain size cao na2o mgo tio2 ratio na ka ratio and dust accumulation rates of the xifeng red clay sequence in the eastern chinese loess plateau and the xihe pliocene loess soil sequence in west qinling they reveal that aridity in the continental interior and winter monsoon circulation both intensified whereas the east asian summer monsoon showed a weakening rather than intensifying trend since the mid pliocene these changes are also supported by the other multi proxy records from various regions in east asia previous numerical modeling studies have demonstrated that uplift of the tibetan plateau would have simultaneously enhanced continental scale summer and winter monsoon strength as well as central asian aridity the mid pliocene climate changes in east asia are therefore unlikely to be a response to plateau uplift on the contrary our recent modeling results give support to the view that ongoing cooling could have intensified both the aridity of the interior and the strength of the winter monsoon but weakened the summer monsoon in east asia c 2012 elsevier ltd all rights reserved

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

estimation of the kinetics of geochemical reactions with geophysical models of sedimentary basins and applications abstract the configurational isomerisation and aromatisation reactions which affect the steroid and triterpenoid hydrocarbons present in most organic rich sedimentary rocks are good indicators of the thermal histories of these rocks the extents to which these reactions have occurred can be assessed by gas chromatography mass spectrometry analysis of the organic extracts of the sediment samples we have assumed the reactions can be described as first order and used the model of sedimentary basin formation by extension of the lithosphere to provide thermal histories for the integration of the relevant arrhenius and rate equations governing the reaction rates the activation energies and frequency factors of the reactions were derived by comparing the reaction extents predicted by the model with geochemical observations from basins northern north sea and pannonian basin formed by lithospheric extension which have not experienced major uplift these values were then used to test the thermal consequences of the lithospheric stretching model for nova scotian continental shelf to calculate the amount of uplift and erosion experienced by lower jurassic sediments in the paris and lower saxony basins and the geographical variation in heat flow in a sedimentary basin formed by lithospheric flexure in the foreland basin of alberta in all cases the figures so calculated appear geologically reasonable and receive support from other independent techniques for their estimation the calculated activation energies of all reactions and the frequency factor for aromatisation are chemically sensible the relative values of the frequency factors for the isomerisation reactions could be expected the more sterically hindered c 20 site in a sterane has a lower frequency factor than that for isomerisation at the less hindered c 22 site in hopanes however both values are very low 0 006 and 0 016 s 1 respectively for the back reactions and imply an entropy change of 282 and 271 j mol 1 c 1 respectively the transition states are therefore much more ordered than the reactants this could be the case if catalysis was important no significant variation in catalytic activity can yet be demonstrated in the rocks investigated here

environmental magnetism past present and future environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport deposition or transformation of magnetic grains is influenced by environmental processes in the atmosphere hydrosphere and lithosphere the first explicit description of environmental magnetism as a distinct field was in 1980 since that time environmental magnetism has become a broad field that is finding application in an ever increasing array of scientific disciplines in this review of the present state of environmental magnetic studies we divide the field into three broad but arbitrary categories the first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments this category includes the correlation of sediment cores using magnetic susceptibility measurements studies of geomagnetic field behavior the analysis of depositional and postdepositional mechanical processes that affect sediments and the examination of magnetic parameters that might represent proxies for paleoclimatic variation the second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment these provenance investigations include studies of changes in catchment derived sediment in lakes fluctuations in contributions from terrigenous aeolian and glaciogenic components in deep sea sediments and the origin of atmospheric particulates the final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments including pedogenesis authigenetic diagenetic formation of ferrimagnetic phases dissolution of magnetic minerals due to reductive diagenesis and contributions of biomagnetism to sedimentary magnetism because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques it is likely that the scope of environmental magnetism will continue to expand rapidly environmental magnetism is capable of providing important data for studies of global environmental change climatic processes and the impact of humans on the environment all of which are major research initiatives in the international scientific community these factors suggest that environmental magnetism has a bright and diverse future

mineral magnetic variation of the jiaodao chinese loess paleosol sequence and its bearing on long term climatic variability 1 to retrieve reliable long term paleoclimatic signals from the chinese loess paleosol sequences deposited over the past 2 6 myr multiparameter mineral magnetic investigations have been conducted on the jiaodao section on the chinese central loess plateau first the lithogenic magnetic components and the pedogenic overprinting were chemically separated by the citrate bicarbonate dithionite cbd method the magnetic properties of the least pedogenically altered loess units and post cbd loess paleosols are dominated by coarse grained lithogenic magnetite increasing degree of pedogenesis increases both the concentration and grain size of pedogenic magnetite maghemite long term changes of the pre cbd magnetic properties suggest a long term decrease in summer monsoon intensity and a long term increase in winter monsoon intensity from the late pliocene to the late pleistocene long term variations of the post cbd magnetic properties suggest an increase in both concentration and grain size of coarse grained lithogenic psd md magnetite grains and hence indicate a long term increase in winter monsoon intensity over that period the coercivity decrease of the lithogenic component for the loess unit l15 may be related to the accelerated uplift of the tibetan plateau and its adjacent regions during the period of l15 deposition from paleosol s5 to loess l1 higher values of coercivity of the lithogenic component occur in typical loess and paleosols and lower values are found at the loess paleosol transition these coercivity variations are thought to be related to significant changes in the source region as well as dust deposition in the loess plateau caused by changes in ice snow or vegetational cover these new results lead to a better understanding of the relationships between the effects of tectonic paleoclimatic and paleoenvironmental processes in northwestern china

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

hydrothermally derived petroleum examples from guaymas basin gulf of california and escanaba trough northeast pacific ocean petroleum is associated with sulfide rich sediment at two active oceanic spreading axes in the northeast pacific ocean in the guaymas basin a spreading axis in the gulf of california petroleum having a wide range of compositions forms by hydrothermal alteration of organic matter in quaternary sediment composed mainly of marine diatomaceous ooze and muddy turbidites in escanaba trough at the southern end of the gorda ridge spreading axis offshore northern california petroleum is formed by hydrothermal processes acting on mainly terrigenous organic material in quaternary turbiditic river derived sediment comparisons of the distributions of hydrocarbons n alkanes isoprenoids terpanes steranes and aromatics show that chemical differences among four petroleum samp es are such that two samples from guaymas basin can be distinguished from two samples from escanaba trough distinguishing characteristics resulting from differences in sources include n alkane distributions and certain sterane ratios distinguishing characteristics resulting from differences in thermal histories of the organic matter include hopane and sterane epimer ratios and various distributions of polycyclic aromatic hydrocarbons these oils differ from conventionally derived petroleum in that they are admixtures of products generated over a wide range of thermal regimes and their generation expulsion and migration occurred simultaneously over an instantaneous geological time period the potential economic significance of hydrothermally derived petroleum is uncertain but the act that petroleum can form at active oceanic spreading axes adds a new facet to understanding the processes of petroleum generation expulsion and migration

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

hydrothermal petroleums from yellowstone national park wyoming u s a abstract in yellowstone national park oil is actively discharging with thermal waters at several widely separated thermal areas samples from two locations were studied petroleum from calcite springs occurs as vapor condensates in steam vents detailed geochemical analyses show a complex origin for the petroleum fluid deeply buried hydrothermally altered sedimentary rocks are believed to have contributed a high temperature pyrolysate consisting largely of polynuclear aromatic hydrocarbons and heterocyclic compounds shallow sedimentary rocks which are also hydrothermally altered have contributed large amounts of aromatic and polar compounds as well as minor amounts of hydrocarbons the permian phosphoria formation is suggested as a likely source rock for the bulk of the oils autochthonous pyrolysates and lipids from thermally altered surface and near surface bacterial and land plant debris have also been introduced to the vents the latter may have been introduced by downward percolating meteoric water the oil from rainbow springs is a highly paraffinic low s crude oil depleted in light hydrocarbons unlike the calcite springs samples the oil from rainbow springs is believed to have originated from a single sedimentary source rock with little or no hydrocarbon contributions from surface biota the oil contains a unique molecular marker assemblage that suggests both gymnosperm and angiosperm input and places the age of the source unit in the tertiary consideration of the local stratigraphy and water temperatures limits the probable source zone to the eocene aycross formation an intra volcanic lacustrine mudstone calculation of deep water temperatures indicates that this unit is exposed to thermal waters in an extensive near surface aquifer such conditions are consistent with the widespread surface manifestations and biomarker kinetic models consideration of the petroleum chemistry and regional hydrodynamics indicates that the yellowstone park oils did not migrate into the park from distant sources the chemical data are also inconsistent with the oils being generated and trapped prior to tertiary volcanism at both calcite springs and rainbow springs oil generation appears to be contemporaneous and is attributed to thermal alteration of relatively shallow sedimentary rocks by hydrothermal fluids

petroleum associated with polymetallic sulfide in sediment from gorda ridge a sediment sample impregnated with asphaltic petroleum and polymetallic sulfide was dredged from the southern end of gorda ridge the escanaba trough off northern california within the offshore exclusive economic zone of the united states the molecular distributions of hydrocarbons in this petroleum show that it was probably derived from terrestrial organic matter in turbidite sediment filling the escanaba trough hydrothermal activity at the gorda ridge spreading center provided the heat for petroleum formation and was the source of fluids for sulfide mineralization

intensification of aridity in the eastern margin of the tibetan plateau since 300ka bp inferred from loess soil sequences western sichuan province southwest china abstract the climate in the eastern margin of the tibetan plateau tp is mainly controlled by the indian ocean summer monsoon and the tp monsoon widely distributed aeolian deposits in this region are one of the best terrestrial materials for studies on the evolutionary history of the indian summer monsoon and the climatic response to uplift of the tp in this study two loess palaeosol sequences from the western sichuan province in southwest china were studied in detail using geochemical and sedimentological methods carbon isotope compositions of organic matter major element geochemistry and grain size records of these two loess palaeosol sequences indicate that the extent of aridity in the eastern margin of the tp increased rapidly between 300 and 150 ka bp and stabilized at a relatively higher level after 150 ka bp marine records from the indian ocean indicate that the indian summer monsoon did not experience obvious weakening since 300 ka bp the deep sea 18 o record which reflects changes in global ice volume also lacks any obvious changes during this period therefore we interpret this palaeoclimate event to reflect the decreased influence of the indian ocean summer monsoon on this region which is related to the uplift of the tp since 300 ka bp this process of increasing aridity is a local palaeoclimate event which is not recorded in the loess palaeosol sequences in the chinese loess plateau in northern china where the climate is mainly controlled by the east asian monsoon this indicates that the climate response to the uplift of the tp differs in different monsoon regions

hydrothermal alteration of organic matter in sediments of the northeastern pacific ocean part 1 middle valley juan de fuca ridge samples from the ocean drilling program leg 169 in middle valley ne pacific ocean were analyzed to study maturation by accelerated diagenesis and catagenesis of the sedimentary organic matter to hydrothermal petroleum some data from site 858 in middle valley leg 139 are also discussed for completeness sediments at sites 858 and 1036 in middle valley were altered either by the full variety of high temperature fluids which generated hydrothermal petroleum or by accelerated thermal diagenesis of the immature organic matter in situ without specific migration of those petroleum bitumen products the hydrothermal petroleums from the deeper sediments have migrated and accumulated in discrete intervals at shallower depths within a contemporary temperature window of 60 135 c at both of these sites maturation for the n alkanes proceeded from high cpi values immature to 1 0 mature i e strong even c number preference due to their low toc contents these sediments did not generate high amounts of hydrocarbons from the kerogen and thus revealed this unique even c number preference furthermore the significant content of terrestrial organic matter yielded petroleums low in gas and volatiles biomarkers were converted from the biological natural product precursors to the geological derivatives with maturities reflecting the thermal stress high temperature alteration also yielded polynuclear aromatic hydrocarbons

direct dating of eocene reverse faulting in northeastern tibet using ar dating of fault clays and low temperature thermochronometry abstract paired together fault gouge dating and low temperature thermochronometry overcome the limitations and assumptions inherent in each independent technique here we establish timing of brittle faulting along the west qinling fault of northeastern tibet by dating several size fractions of fault gouge clay that represent variable populations of illite polytypes results show that the authigenic or fault generated component of illite formed at 50 8 ma and that the detrital component formed at 236 7 ma indicating a middle eocene age of faulting and a middle triassic age of the wall rocks comparing this dataset with published thermochronology from hanging wall rocks supports the interpretation that the west qinling fault initiated at 50 ma and continued until at least middle miocene time and that authigenic clay growth occurred at ambient temperatures of 110 c lack of overprinting of younger clay ages at this site may indicate that rocks were out of the thermal window for authigenic clay formation during later faulting episodes the potential for temperature to control illite growth has implications for interpretation of authigenic illite ages and their relationship to deformation episodes within fault zones

hydrothermal alteration of organic matter in sediments of the northeastern pacific ocean part 2 escanaba trough gorda ridge samples from the ocean drilling program leg 169 in escanaba trough gorda ridge ne pacific ocean were analyzed to study maturation by accelerated diagenesis and or by catagenesis of the sedimentary organic matter to hydrothermal petroleum at site 1038 the hydrothermal petroleums have migrated after generation to shallower horizons the n alkane maturation was indicated by the strong even c number preference i e cpi 1 0 as in the case of middle valley all samples contained admixed organic matter of terrigenous and marine components as indicated by the distributions of the biomarkers the biological precursors were catagenetically altered to their equivalent mature compounds the presence of high molecular weight pahs in some sediment sections at site 1038 reflected the high temperature alteration and reworking of organic matter into mature hydrothermal petroleum at site 1037 the reference hole in the escanaba trough the n alkanes with a strong predominance of odd c number homologs reflected immature non marine lipid components essentially throughout the hole maturation of organic matter was only observed below 450 mbsf with n alkanes showing a cpi 1 0 the strong even c number predominance in those intervals was attributed to initial maturation by high heat flow during the early rifting process

pulsed miocene range growth in northeastern tibet insights from xunhua basin magnetostratigraphy and provenance author s lease ro burbank dw hough b wang z yuan d abstract sedimentary rocks in tibetan plateau basins archive the spatiotemporal patterns of deformation erosion and associated climate change that resulted from cenozoic continental collision despite growing understanding of basin development in northeastern tibet during initial india asia collision as well as in the late miocene holocene surprisingly little is known about the intervening period a time when the plateau may have undergone fundamental tectonic changes to fill this gap we present new magnetostratigraphy from a g2300 m thick fluviolacustrine succession that spans ca 30 9 3 ma an integrated analysis of sedimentology subsidence and provenance from this section reveals the sequential pulsed erosion of multiple ranges bordering the xunhua basin emergence of the wnw trending laji shan is highlighted by a doubling of sediment accumulation rates between 24 and 21 ma and a transition to coarse allu vial facies at 20 3 ma detrital zircon u pb age spectra show that these coarse sediments came from basement terranes within the laji shan together these observations suggest accelerated growth of the laji shan and its coupled foreland basin at ca 22 ma the most rapid accumulation rates in xunhua basin occur within the finest grained strata and suggest an underfilled basin during the fastest interval of laji shan deformation growth of the laji shan occurred northward of the contemporaneous plateau margin which had been defined since ca 45 50 ma by the west qinling lying 60 km farther south hence following 20 25 m y of apparent stability the deformation front in this region jumped 60 km to the north at ca 22 ma subsequently growth of the north trending jishi shan occurred at ca 13 ma and is highlighted by an acceleration in xunhua basin accumulation rates between 12 and 9 ma as well as by a significant change in detrital zircon provenance of nearby linxia basin deposits by 11 5 ma initial growth of the wnw trending laji shan in the early miocene and subsequent growth of the north trending jishi shan 10 m y later support interpretations of a middle mio cene kinematic reorganization in northeastern tibet 2012 geological society of america

sea bed noises reveal role of turbulent bursting phenomenon in sediment transport by tidal currents in many engineering and sedimentological applications it is usual to relate sediment movement to the time averaged bed shear stress however the shear stress exerted on the sea bed by tidal currents is intermittent in nature due to the turbulent bursting phenomenon1 2 sediment movement at the sea bed also occurs in bursts3 4 however attempts to link these two processes have failed because of the poor temporal resolution of existing sediment transport measurement techniques recent experimental advances now make this possible and we describe here the first detailed observations of sediment movement and fluid turbulence in tidal currents flowing over sandy gravels these results show that high stress values are a necessary but not sufficient condition for bed load sediment movement and that form drag on individual particles may have greater dynamical significance than the stress this work also suggests that it may now be possible to describe sediment movement at a level which is deterministic in terms of the turbulence and at very much shorter timescales than have been used previously

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

stable isotope evidence for topographic growth and basin segmentation implications for the evolution of the ne tibetan plateau lithologic magnetostratigraphic and stable isotope records from the neogene xunhua and linxia basins along the tibetan plateau9s northeastern margin suggest that topography in the intervening jishi shan mountain range began to develop between 16 and 11 ma perturbations to local climate patterns resulting from the evolution of local topography are tracked through comparison of stable isotope compositions of calcareous basin fill materials across the jishi shan similarity of isotopic compositions is interpreted to reflect the presence of integrated basins whereas distinct isotopic compositions reflect unique basin hydrologies divergent isotope trends develop between ca 16 and 11 ma and are indicative of hydrologic separation in the adjacent xunhua and linxia basins and increased aridity in the leeward xunhua basin the development of aridity in the lee of the growing topography along the plateau9s northeast margin highlights the importance of evaporative enrichment in this extremely continental setting and explains the presence of anomalously positive 18 o values in modern rainfall our findings add to a growing body of evidence for deformation along the plateau9s north and northeastern margins in the middle to late miocene

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

origin of the miocene pliocene red earth formation at xifeng in northern china and implications for paleoenvironments abstract in the eastern loess plateau region of northern china the quaternary loess soil sequences of the last 2 6 ma are underlain by the hipparion red earth formation magneto stratigraphic studies suggest that it is a near continuous terrestrial record for the period from 7 to 2 6 ma bp and therefore has great potential for the study of asian paleoclimates the origin of this formation is controversial and needs further study in this work the xifeng red earth section a type section for the eastern loess plateau region was studied using optical microscopic analysis grain size measurements and both major and trace element geochemical properties to address questions of its origin and environmental implications the results show that this formation consists of three parts the lower part 6 2 ma bp is a water reworked deposit related to alluvial and slope processes the middle part from 6 2 to 3 6 ma bp was derived from in situ eolian dust deposition but was significantly affected by groundwater oscillations and the upper part from 3 6 to 2 6 ma bp is an eolian formation similar to the quaternary loess soil sequence the grain size of the upper part is significantly coarser than for the middle part but finer than for the overlying loess because the eolian dust deposits in the middle reaches of the yellow river were mainly transported from the deserts in northern china by the northwesterly winter monsoon wind and the westerlies we interpret the eolian origin of the middle and upper red earth formation beginning approximately at 6 2 ma bp as an indication of the strengthening of the aridification of the asian continent desert lands in central asia must have been formed at or by that time to provide a significant source of dust however the generally finer grain size of the red earth suggests a weaker transporting wind and or a more remote source the latter implies a smaller extent of deserts than in the quaternary the high degree of similarity between the geochemical properties of the red earth and loess samples seems to suggest a rather similar source areas and comparable dust transporting trajectories the removal of the dust deposits from groundwater influence and the increase in grain size at 3 6 ma bp are attributable to both tectonic and climatic causes the initiation of these phenomena corresponds to an intensive uplift of the tibetan plateau and also to a simultaneous uplift of the sedimentary basins in the surrounding areas it is also approximately synchronous with an increase in eolian mass accumulation rate in the north pacific both suggesting an increased continental aridity in the asian dry lands and an increasing intensity of transporting winds

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials

magnetic effects of maghemitization of oceanic crust both theoretical considerations and available experimental results indicate that magnetic effects of maghemitization are strongly dependent on the grain size of the originally unoxidized titanomagnetite maghemitization of single domain titanomagnetite results in a decrease in coercivity an increase in susceptibility and a large decrease in q ratio maghemitization of multidomain titanomagnetite results in an increase in coercivity a decrease in susceptibility and no large changes in q ratio single domain titanomagnetite is probably resistant to the development of a chemical remanent magnetization crm whereas multidomain titanomagnetite can acquire a crm during maghemitization the behavior of pseudo single domain titanomagnetite which is the main carrier of remanence in submarine extrusive rocks is investigated by comparing the magnetic properties of the french american mid ocean undersea study famous less than 0 1 m y old and the leg 37 3 5 m y old pillow basalts recovered from the mid atlantic ridge near 37 n combining electron microprobe analyses curie temperature measurements and cell edge determinations we find that the famous rocks are already oxidized z 0 38 possibly as a result of some high temperature maghemitization during cooling of the magma comparison with the more highly oxidized z 0 7 leg 37 pillow basalts indicates that low temperature maghemitization of such rocks does not result in appreciable changes of coercivity and susceptibility although the q ratio does decrease and crm seems to be acquired such a crm could account for the anomalously low magnetic inclinations observed at most of the leg 37 sites

source to sink fluctuations of asian aeolian deposits since the late oligocene abstract the source to sink fluctuations of asian aeolian deposits have been investigated over the past two decades using multiple mineralogical and geochemical tracers these source tracers have substantially improved our understanding of the processes of production transport and deposition of asian dust cycling from the asian inland basins to the north pacific ocean however due to differences in the sensitivity and interpretation of these various tracers there is controversy about which regions were the dominant dust sources and whether the dust provenances were stable at tectonic and glacial interglacial timescales here we comprehensively review the source to sink system of asian aeolian deposits on the chinese loess plateau since the late oligocene by assessing the utility of recently employed provenance tracers we conclude that integrating different tracers can effectively distinguish major primary and secondary dust sources the combination of sr nd isotopes quartz esr ci 18o data and zircon age spectra can decipher the source to sink fluctuations of asian aeolian deposits at tectonic and glacial interglacial timescales comparison of these sensitive source tracers with tectonic and climatic evidence indicates that the phased growth of the northern tibetan plateau played a key role in asian dust production and transport during the late oligocene to miocene while the onset and expansion of the northern hemisphere ice sheets had a substantial impact on the source to sink fluctuations since the late pliocene