cenozoic lithospheric evolution of the bohai bay basin eastern north china craton constraint from tectono thermal modeling abstract it is well established that the lithosphere beneath the eastern north china craton ncc had been thinned before the cenozoic a 2d multi phase extension model in which the initial crustal and lithospheric thicknesses are variable is presented to reconstruct the initial thicknesses of the crust and lithosphere in the early cenozoic and to further investigate the lithospheric evolution beneath the eastern ncc through the cenozoic we conduct thermal modeling along three profiles from east to west in the bohai bay basin which is the center of the lithospheric destruction and thinning of the ncc using multiple constraints such as tectonic subsidence the present day heat flow and the moho depth we determine the initial crustal and lithospheric thicknesses of the bohai bay basin before the cenozoic rift to be 33 36 km and 80 105 km respectively the model results show that the most rapid lithospheric thinning during the cenozoic occurred in the middle eocene for most depressions and the thinning activity ceased at the end of the oligocene reaching a minimum lithospheric thickness of 53 74 km followed by a thermal relaxation phase combined with previous studies we infer that the lithosphere beneath the eastern ncc experienced two stages of alternating thinning and thickening notable thinning in the early cretaceous and paleogene and thickening in the late cretaceous and late cenozoic we believe that thermo chemical erosion together with extension was probably the major mechanism of the significant lithospheric removal during the mesozoic whereas the cenozoic lithospheric thinning was mainly dominated by tectonic extension in the eastern ncc lithospheric thickening was generally a result of thermal cooling

cenozoic tectonic evolution of the bohai bay basin and its coupling relationship with pacific plate subduction abstract the bohai bay basin is a mesozoic cenozoic rift basin in eastern china based mainly on a balanced section analysis this study compares the spatio temporal differences of tectonic evolution in relation to strike slip faults among different depressions within the basin in combination with the analysis of subsidence characteristics the study also attempts to clarify the cenozoic tectonic evolution of the basin and its coupling relationship with the subduction of the pacific plate it was found that 1 the strike slip faults were activated generally from south to north and from west to east during the cenozoic 2 there is a negative correlation between the intensity of tectonic activity in the bohai bay basin and subduction rate of the pacific plate and 3 the migration direction of the basin depocenters is consistent with the direction of pacific plate subduction

integrated computer aided basin modeling applied to analysis of hydrocarbon generation history in a northern italian oil field abstract the application of reaction kinetics to the analysis of hydrocarbon hc generation processes requires a detailed temperature history of potential source rocks in order to obtain optimum results the necessary accuracy can be obtained by employing integrated computer aided basin modeling techniques to simulate the depositional history resulting in a continuous record of temperatures throughout the study area the technique has been applied to the analysis of an on off structure hc generation problem in a northern italian oil field the simulation of the geologic processes shows that low sedimentation rates and more importantly the high thermal conductivities of the carbonate rocks resulted in low temperatures throughout most of the geologic history of the study area which were followed by a sudden rapid temperature increase in the off structure area during the last 6 5 ma the calculated temperature history enables an accurate determination of the hc generation history using a kinetic approach which shows how hc generation is closely controlled by the sedimentation history and structural development and which also describes its relation to trap formation it also shows why hcs are generated at very low vitrinite reflectance levels i e peak oil generation at approximately 0 6 r o determination of the hc generation rate gives additional critical information as even source rocks located in apparently good potential zones at present i e in the on structure area can be of minor importance for a trap due to slow generation rates and consequent low expulsion efficiencies

compositions of large metal nodules in mesosiderites links to iron meteorite group iiiab and the origin of mesosiderite subgroups large mesosiderite metal nodules were studied petrographically and by neutron activation their large sizes minimized the effects of silicate contamination and dilution by fe reduced from feo the remarkable mixture of subequal amounts of metal and basaltic and pyroxenitic silicates in mesosiderites can be explained by the accretion of a core like metal mass from one asteroid onto the surface of another asteroid our data show that the accreted mass had a composition roughly similar to that of the iiiab iron meteorite core consistent with other evidence of close genetic links between these and other related groups the siderophile compositional pattern the very limited compositional range the inferred large projectile target ratio and the electrical interconnectedness of mesosiderite metal indicate that the bulk of the metal was molten when accreted mesosiderites have been divided into plagioclase and tridymiterich subgroup a and orthopyroxene rich subgroup b on the basis of modal silicate mineralogy roughly the same subgroups are defined by metal compositions au as ni and cu are generally higher and w lower in subgroup a than in subgroup b in our sample set a widmanstatten pattern is only observed in nodules from subgroup a a plausible model is that subgroup a best preserves the compositions of the target regolithic silicates and accreted core subgroup b is comprised of these materials together with a minor amount of coremantle materials consisting of olivine and refractory ni poor metal the independent covariation of ga re and ir may in part reflect fractionations during impact heating events the anomalous mesosiderite reckling peak a79015 is strongly depleted in refractory siderophiles and slightly enriched in volatile siderophiles such evolved metal requires a formation distinct from common mesosiderite metal

seismic structure of the northeastern japan convergent margin a synthesis many studies recently made on the basis of seismic observations have revealed a detailed structure of the crust and the upper mantle beneath the northeastern japan arc and its relationship to seismic and volcanic activity spatial distributions of the depths to the conrad and the moho discontinuities estimated from shallow earthquake data and seismic explosion data show that both discontinuities are deep in the middle of the land area and shallow toward the coastlines of the japan sea and the pacific ocean the pn velocity has a lateral variation it is as low as 7 5 km s beneath the land area while that beneath the japan sea and the pacific ocean is 8 0 8 2 km s it changes abruptly at the transition zones which are located along the coastlines precise structure and location of the subducted pacific plate beneath the land area is inferred from converted or reflected seismic waves at the top or bottom of the plate the pacific plate is composed of a thin 5 km low velocity upper layer and a thick high velocity lower layer its total thickness being 80 90 km the upper plane seismicity of the double seismic zone is confined to the thin low velocity upper layer which probably corresponds to the subducted former oceanic crust the lower plane seismicity lies at the middle of the high velocity lower layer and the lower half of the plate below it is incapable of generating earthquakes the shallower portion of the upper surface of the plate beneath the pacific ocean along which major seismicity with low angle thrust faultings is actually occurring is also located by seismic observations on land and in the sea the pacific plate subducts at an extremely low angle of 5 for the first 25 km depth and then the dip steepens rather abruptly to 30 normal fault type events at the top of the plate have not been detected in the portion where the downward bending is the largest but have been ditected near the trench axis where it is rather small tomographic inversions for seismic velocity structure clearly delineate the inclined high velocity pacific plate with a thickness of 80 90 km and low velocity zones in the crust and the mantle wedge beneath active volcanoes seismic attenuation tomography also shows similar zones of low q value beneath active volcanoes although its spatial resolution is much lower the low velocity zones with 2 6 velocity lows are continuously distributed from the upper crust just beneath active volcanoes to a depth of 100 150 km in the mantle wedge and are approximately parallel to the dip of the underlying pacific plate these low velocity zones probably reflect the pathway of magma ascent from a depth in the mantle wedge to the earth s surface corresponding to a portion of the subduction induced secondary mantle wedge flow

deep shallow coupling response of the cenozoic bohai bay basin to plate interactions around the eurasian plate abstract the bohai bay basin bbb is a meso cenozoic intracontinental petroliferous basin where four pre existing continental scale fault zones intersect in east china the bbb has experienced complex deformational and evolutionary history that records significant information about how the lithosphere there has responded to the coeval plate interactions around the eurasian plate and or their resulting deep crust mantle processes in the cenozoic now both deep and shallow deformational and evolutionary processes of the bbb have been well studied but many geodynamic mechanisms previously proposed for those processes are too general to explain them well based on various data and principles we first summarized both deep and shallow responses and processes of the bbb in the cenozoic and then matched reasonable geodynamic mechanisms for each of them the results show as follows the cenozoic differential reactivation of pre existing continental scale weak zones controlled the coeval deep shallow differential coupling evolution of the bbb the paleogene extension in the bbb was mainly caused by crust mantle processes triggered by the stagnant izanagi slab the cenozoic strike slip faulting was influenced by various plate interactions in which the oblique subduction and anticlockwise rotation of young pacific plate resulted in the complex pre oligocene strike slip history of the tan lu fault zone to the south of the zhang peng fault zone while the india eurasia collision has caused the intense and synchronous strike slip faulting of various faults there since 35 ma which was then weakened by the oblique subduction and clockwise rotation of young philippine sea plate in the neogene the compression resulting from both the india eurasia collision and the philippine sea plate s subduction contributed to the latest paleogene tectonic inversion in the bbb the arrival of mantle flow triggered by both the india eurasia collision and the stagnant slab of the pacific plate resulted in the tectonic reactivation there in the late neogene

composition of metal in aubrites constraints on core formation abstract aubrites are differentiated stone meteorites with small amounts of metal study of eight aubrites reveals that fe ni grains occur in a variety of textures from irregular m sized particles to rounded nodules of up to 1 5 cm in diameter the pattern of siderophile element abundances in the silicates is characteristic of igneous metal segregation the more siderophilic elements are more depleted despite large uncertainties in metal silicate distribution coefficients the calculated minimum metal content of the aubrite precursor is not unlike that of enstatite chondrites and suggests that metal segregation was an extensive process in the early magmatic evolution of the aubrite parent body however the lack of appreciable fractionation in the trace element signature of the metal suggests that the metal now observed in aubrites did not undergo fractional crystallization in a core we argue instead that the analyzed metal nodules represent a fraction of the iron nickel that during partial melting was not completely segregated from the silicates

formation of mesosiderites by low velocity impacts as a natural consequence of planet formation mesosiderites are an enigmatic class of stony iron meteorites composed of roughly equal amounts of pyroxenitic and basaltic silicates and metallic iron nickel olivine is rare we suggest here that during the period of planet formation mesosiderites originated by the low velocity collisions of large metallic core fragments with the surface of a differentiated asteroid size body relative velocities of the asteroids native to this region were low 1 km s 1 because it was distant from massive protoplanets however a few differentiated asteroids were destroyed by high velocity collisions with interlopers perturbed by protoplanets into highly eccentric orbits these collisions reduced mantles and crusts to small silicate fragments but left cores in the form of large durable metal fragments mesosiderite like pyroxenite basalt metal mixtures were formed when large core fragments accreted at low velocities to the regolith of an intact asteroid the metal contents of other regolithic regions remained low most of the olivine in mesosiderite and howardite breccias we attribute to the mantles of the disrupted parent bodies the low 12 20 mg g 1 olivine contents indicate that the amount of debris accreted from disrupted asteroids was small relative to the volumes of the regoliths

crustal strains in the japanese islands as deduced from dense gps array data from the nationwide continuous gps network operated by the geographical survey institute of japan were used to estimate crustal strains in the japanese islands we used horizontal velocity vectors at 610 sites for the period april 1996 to march 1997 and employed the least squares prediction method to interpolate the velocities to a uniform 7km 7km grid over japan the gridded velocities were then differentiated in space to calculate principal components of strains results clearly portray tectonic strainings of the japanese islands in one year 1 dilatational strains show that the japanese islands are under a compressive strain regime induced by the subducting oceanic plates 2 distribution of high maximum shear strains coincides with the regions of crustal activities and 3 principal axes of the strains indicate influences of e w and nw se compression due to the converging oceanic plates north south extension in kyushu is also visible

full interseismic locking of the nankai and japan west kurile subduction zones an analysis of uniform elastic strain accumulation in japan constrained by permanent gps we analyze permanent global positioning system gps data obtained over japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the pacific and philippine sea plates below the japanese islands we first derive the gps strain rate fields that characterize the crustal deformation of southern and northern japan and invert them to determine the effective subduction velocity along the central nankai trough on one side and the japan west kurile trench on the other these reference free velocities are close to those predicted by plate motion models with respect to eurasia we conclude that the eurasian reference frame gives a good approximation to the subduction motion and that to first order both subduction zones were fully locked during the period of measurements we then test whether the coupling ratio shows local variations within the seismogenic zones to do this we divide the subduction interface into 35 km 30 km elements that we model by point source groups and we invert the gps velocity field referenced to eurasia to derive the coupling ratio between 0 and 1 on each fault element the results are coherent over the 3 years and confirm that both the central nankai and the japan west kurile seismogenic zones are homogeneously fully locked most of the coupling ratios are close to 1 and a few are close to 0 intermediate values are rare the zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 sanriku oki interplate earthquake japan trench or to a small overestimation of the actual lower limit of the locked zone we conclude that within the resolution of the gps data and the model 1 partial coupling did not exist during these 3 years along the nankai and japan west kurile trenches 2 the small seismic coupling ratio previously derived from earthquakes analysis for the japan and kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip and 3 the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading finally we discuss the nonelastic residual deformation within the frame of the long term deformation of the japanese islands

subsidence transition during the post rift stage of the dongpu sag bohai bay basin ne china a new geodynamic model abstract the dongpu sag located in the bohai bay basin ne china is a cenozoic continental rift basin the post rift evolution of the dongpu sag is associated with the development of petroleum reservoirs and has implications for neogene quaternary basin evolution along the eastern margin of eurasia to determine the nature and origin of post rift subsidence in the dongpu sag we apply backstripping modified strain rate inversion and revised finite extension modelling techniques using data from 14 real and synthetic wells that are intersected by three seismic lines our results reveal discrepancies by subsidence based on backstripping of well data the observed subsidence minus that predicted by modified strain rate inversion and revised finite extension modelling the predicted subsidence during the miocene the observed subsidence was smaller than the predicted subsidence leaving negative discrepancies referred to here as insufficient subsidence ranging from 343 to 96 m in contrast during the pliocene quaternary the observed subsidence was greater than the predicted subsidence by 123 to 407 m which left positive discrepancies referred to as over sufficient subsidence therefore we infer a transition from insufficient to over sufficient subsidence during the post rift stage normal faulting that started at ca 5 3 ma is estimated to have produced only 20 of the over sufficient subsidence therefore the remaining over sufficient subsidence as well as the preceding insufficient subsidence and the transition between the two were likely controlled by lithosphere processes we propose a new tectonic model in which variations in the conditions e g rate direction and angle associated with subduction of the pacific plate resulted in a change of heat flow decreasing from a linear to a curvilinear pattern leading to a transition from insufficient to over sufficient subsidence

an anomalous eucrite dhofar 007 and a possible genetic relationship with mesosiderites we studied the texture mineralogy and bulk chemical composition of dhofar 007 a basaltic achondrite dhofar 007 is a polymict breccia that is mostly composed of coarse grained granular cg clasts with a minor amount of xenolithic components such as a fragment of mg rich pyroxene the coarse grained relict gabbroic texture mineral chemistry and bulk chemical data of the coarse grained clast indicate that the cg clasts were originally a cumulate rock crystallized in a crust of the parent body however in contrast to monomict eucrites the siderophile elements are highly enriched and could have been introduced by impact events dhofar 007 appears to have experienced a two stage postcrystallization thermal history rapid cooling at high temperatures and slow cooling at lower temperatures the presence of pigeonite with closely spaced fine augite lamellae suggests that this rock was cooled rapidly from higher temperatures 0 5 c yr at 1000 c than typical cumulate eucrites however the presence of the cloudy zone in taenite and the ni profile across the kamacite taenite boundaries indicates that the cooling rate was very slow at lower temperatures 1 10 c myr at 600 700 c the slow cooling rate is comparable to those in mesosiderites and pallasites the two stage thermal history and the relative abundance of siderophile elements similar to those for metallic portions in mesosiderites suggest that dhofar 007 is a large inclusion of mesosiderite however we cannot rule out a possibility that dhofar 007 is an anomalous eucrite

cenozoic structural deformation and dynamic processes of the bohai bay basin province china abstract cenozoic structures in the bohai bay basin province can be subdivided into eleven extensional systems and three strike slip systems the extensional systems consist of normal faults and transfer faults the normal faults predominantly trend nne and ne and their attitudes vary in different tectonic settings paleogene rifting sub basins were developed in the hanging walls of the normal faults that were most likely growth faults neogene quaternary sequences were deposited in both the rifting sub basins and horsts to form a unified basin province the extensional systems were overprinted by three nne trending right lateral strike slip systems fault zones although the principal displacement zones pdz of the strike slip fault zones are developed only in the basement and lower basin sequences in some cross sections the structural deformation characteristics of the upper basin sequences also indicate that they are basement involved right lateral strike slip fault zones according to the relationships between faults and sedimentary sequences the extensional systems were mainly developed from the middle paleocene to the late oligocene whereas the strike slip systems were mainly developed from the oligocene to the miocene strike slip deformation was intensified as extensional deformation was weakened extensional deformation was derived from horizontal tension induced by upwelling of hot mantle material whereas strike slip deformation was probably related to a regional stress field induced by plate movement

architecture and evolution of the cenozoic offshore bohai bay basin eastern china abstract the cenozoic offshore bohai bay basin has similar graben horst configurations as in many rift basins however different areas of it have different stratigraphic fill resulted from jointly effects of basement fabrics rifting and strike slipping within the liaodongwan subbasin the liaodong rise is a transpressional structural belt bounded by the tanlu strike slip fault the bodong rise is similar to the liaodong rise the eastern bozhong subbasin has a graben horst configuration caused by extensional stress the southern and northwestern parts of the bozhong subbasin are characterized by large scale horsts that are limited by south dipping listric normal faults the bozhong subbasin shows many similarities with the faulted sag basin the southern bohai bay basin is characterized by ew oriented half grabens and horsts resulted from s n extensional stress the cenozoic evolution of the basin began with paleogene rifting followed by the neogene quaternary thermal subsidence stage the main rifting episode caused by the upwelling of the mantle is characterized by strong differential block faulting during the deposition of the eocene sha 3 member during the deposition of the oligocene dongying formation previously isolated and scattered sags connected with each other and became a broad and unified lake basin in addition the dextral movement of the tanlu fault zone initiated the development of several transpressional structural belts in fault bends in the eastern basin extensional systems were also superimposed by strike slip systems during thermal subsidence neotectonic movement produced many secondary faults in shallower strata and the dextral movement of the tanlu fault zone also altered the eastern basin architecture

petrology and geochemistry of a silicate clast from the mount padbury mesosiderite implications for metal silicate mixing events of mesosiderite petrological and bulk geochemical studies were performed on a large silicate clast from the mount padbury mesosiderite the silicate clast is composed mainly of pyroxene and plagioclase with minor amounts of ilmenite spinel and other accessory minerals and it shows subophitic texture pyroxenes in the clast are similar to those in type 5 eucrites and could have experienced prolonged thermal metamorphism after rapid crystallization from a near surface melt ilmenite and spinel vary chemically indicating growth under disequilibrium conditions the clast seems to have experienced an episode of rapid reheating and cooling possibly as a result of metal silicate mixing abundances of siderophile elements are obviously higher than in eucrites although the clast is also extremely depleted in highly siderophile elements the fractionated pattern can be explained by injection of fe fes melts generated by partial melting of metallic portions during metal silicate mixing the silicate clast had a complex petrogenesis that could have included 1 rapid crystallization from magma in a lava flow or a shallow intrusion 2 prolonged thermal metamorphism to equilibrate the mineral compositions of pyroxene and plagioclase after primary crystallization 3 metal silicate mixing probably caused by the impact of solid metal bodies on the surface of the mesosiderite parent body and 4 partial melting of metal and sulfide portions and silicate in some cases caused by the collisional heating which produced fe fes melts with highly fractionated siderophile elements that were injected into silicate portions along cracks and fractures

slip history of the 1994 sanriku haruka oki japan earthquake deduced from strong motion data abstract we analyzed the seismic waves of the 1994 sanriku haruka oki earthquake m w 7 7 which occurred in the aftershock area of the 1968 tokachi oki earthquake m w 8 2 applying a multiple time window inversion scheme to near source strong motion data we obtained a detailed spatiotemporal rupture process and compared it with that of the tokachi oki earthquake the fault geometry is constructed based on the aftershock distribution the obtained rupture model is consistent with the cmt solution even for a non double couple component the total seismic moment is 4 0 10 20 n m large slips are concentrated in three asperities the first asperity centers about 40 km south and 50 km west from the hypocenter with a maximum slip of 4 4 m the second one centers about 60 km west from the hypocenter with a maximum slip of 2 2 m and the third one lies about 110 km west from the hypocenter with a maximum slip of 2 6 m the obtained moment rate and the duration on the first and second asperities are lower and much longer than those on the third asperity respectively the first asperity does not overlap with an area of large slip during the tokachi oki earthquake but the second or third seem to overlap with or be adjacent to the asperity of the tokachi oki earthquake our inversion result also shows an abrupt change of the rupture velocity from 1 8 to 3 0 km sec at the central part of the fault plane a difference of the seismic coupling between the oceanic and the continental lithospheres at the trenchward side and at the landward side of the 143 e meridian seems to affect the rupture process of this earthquake

petrogenesis of mesosiderites i origin of mafic lithologies and comparison with basaltic achondrites abstract new petrologic and trace element data on basaltic and gabbroic clasts in mesosiderites and basaltic achondrites combined with existing petrologic and trace element data have served as a basis for interpretation of the petrogenesis of mesosiderite clasts compared with the basaltic achondrites the mesosiderite basaltic and gabbroic clasts contain more abundant modal tridymite and merrillite and more commonly contain augite as a late magmatic phase their pyroxenes tend to be more mgo rich and have lower fe mn ratios which are positively correlated with the fe mg ratio some of the mesosiderite basaltic and gabbroic clasts contain xenocrystic plagioclase the basaltic clasts commonly show super chondritic eu sm ratios and slight lree depletions the cumulate gabbro clasts are extremely depleted in lil have very high eu sm ratios and high lu sm ratios all of these features can be explained by a model in which the mafic lithologies of mesosiderites were formed by remelting of a mixed basalt cumulate gabbro metal source region near the parent body surface reducing agents p c s from the metallic fraction reduce feo out of the silicate melt and result in the positive correlation of fe mn with fe mg observed in pyroxene analyses and the modal abundances of tridymite merrillite and augite xenocrystic plagioclase is a natural consequence of remelting a cumulate gabbro rich source as are the unique ree patterns observed

early cenozoic two phase extension and late cenozoic thermal subsidence and inversion of the bohai basin northern china abstract the bohai basin is one of a family of early cenozoic extensional basins that lie along the eastern margin of asia from russia to vietnam initial extension was probably triggered by subduction roll back of the oceanic pacific plate from the asian continent there were two phases in the bohai basin s rift history the earlier paleocene early eocene phase resulted in the deposition of the kongdian formation and the fourth lowest member of the shahejie formation in a series of elongate half grabens these half grabens have master faults with a nne ssw orientation secondary normal faults are typically clockwise oblique to the master faults indicating a component of dextral transtension deposition was focused in the west and south of the present basin these rocks are mainly alluvial fan and fluvial red beds the architecture of the basin underwent an important change at ca 43 45 ma middle eocene beginning with the deposition of the third member of the shahejie formation in part these sediments were deposited in the same half grabens as the kongdian formation but the bozhong depression in the central part of the basin originated at this time and became the major depocentre the bozhong depression superficially resembles a pull apart basin it formed when continued transtension of the earlier tertiary fault systems to the east and west created an extensional overlap between them during this phase the basin as a whole had a geometry with elements typical of both pull apart and transtensional basins regional extension in many east asian basins ended at the end of the oligocene probably because of the onset of transpression within eastern asia caused by the collision of australia with the philippine sea plate dextral transpression caused minor inversion of some of the earlier normal faults in bohai but as a whole the basin began to subside in a post rift phase of thermal subsidence that has lasted until the present day

a unique high mn fe microgabbro in the parnallee ll3 ordinary chondrite nebular mixture or planetary differentiate from a previously unrecognized planetary body the study of planetary materials in chondritic meteorites constrains the compositional diversity of materials in different nebular environments and provides information on the degree of differentiation of early planetary bodies we studied a unique microgabbro fragment from the parnallee ll3 unequilibrated ordinary chondrite the fragment which was originally identified by its ophitic to sub ophitic texture exhibits features characteristic of lunar and terrestrial tholeiitic basalts extreme compositional zoning in clinopyroxene wo 10 en 65 fs 25 to wo 15 en 2fs 83 a multiply saturated major element composition similar to mid ocean ridge basalt with 3 1 wt na 2o and 0 15 wt k 2o and uniformly enriched rare earth elements c 10 c1 a high bulkmno feo ratio 0 064 distinguishes the microgabbro from other basaltic rocks and suggests the precursor material formed or reached equilibrium in a reducing environment however the absence of fe metal and the extreme enrichment of feo up to 40 wt in late crystallizing pyroxferroite requires the last crystallization event to have occurred in a relatively oxidizing environment we suggest the microgabbro formed by partial melting in a planetary body after removal of metallic fe examination of possible planetary source materials such as alkali rich eucritic material a volatile depleted c1 carbonaceous chondrite or h group chondrite shows that multiple stage fractionation is required to produce a melt with the feo mgo ratio of the microgabbro from these materials the increasing number of planetary igneous fragments observed in unequilibrated ordinary chondrites uoc suggests the picture of uoc as primitive assemblages of unprocessed material is overly simplistic

39ar 40ar ages and thermal history of mesosiderites determinations of 39ar 40ar ages for plagioclase separates of three clasts from the vaca muerta and mount padbury mesosiderites give similar results ages increase monotonically by 0 2 0 3 ga when plotted against increasing 39ar release from the samples and give average values of 3 95 ga when combined with revised ages for mesosiderite analyses previously reported nineteen analyses of thirteen mesosiderites give an average ar ar age of 3 94 0 10 ga the average age at 90 39ar release is 4 13 0 14 ga and the ages at 10 39ar release range over 3 32 3 97 ga it seems unlikely that the 39ar 40ar ages were produced by direct impact heating but they probably are independent evidence for slow cooling deep within the mesosiderite parent body we conclude that the sloped age plateaus and the significant range in ar ar closure temperatures calculated for these mesosiderites 75 340 c are due to significant differences in ar diffusion properties among samples from the ar diffusion data we estimate an approximate mesosiderite cooling rate of 0 2 c ma which is similar to the preferred cooling rate of 0 1 0 5 c ma derived by others from ni diffusion and metal textures the 39ar 40ar ages seem incompatible with very slow cooling rates of 0 2 c ma unless the closure temperatures for ar diffusion and formation of metal cooling textures on the parent body substantially overlap any possible role of fragmentation of the mesosiderite parent body in producing the younger ar ar ages and the very slow cooling rates is unclear

cretaceous anomalous subsidence and its response to dynamic topography in the songliao basin northeast china to understand the formation mechanisms and evolution of the songliao basin we applied backstripping and strain rate inversion methods to 20 wells and reinterpret two seismic sections in the songliao basin the obtained data were used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of this area the predicted post rift subsidence based on the uniform stretching model that followed earlier lithospheric thinning events was much lower than the subsidence provided by backstripping the anomalous subsidence during the post rift stage was between 200 m and 800 m in the songliao basin and the depocenter of anomalous subsidence moved eastward between ca 110 and 80 ma before returning to the west between ca 80 and 65 ma regional variations in the subsidence rates suggest a possible deficit in the negative buoyancy mantle loading that is induced by the sinking slab beneath the songliao basin in addition the anomalous post rift subsidence was primarily driven by downward drag pressure from the subducting western izanagi slab and asthenospheric mantle flow beneath the songliao basin the basement rifting in the songliao basin during the early cretaceous and the normal faulting that occurred during the post rift stage resulted in a secondary contribution to the anomalous post rift subsidence unlike the songliao basin several early cretaceous rift basins south of the yanshan belt were only filled during the syn rift subsidence and the anomalous post rift uplift of the yanshan belt was driven by deep mantle flow

development of the north china craton during the late archaean and its final amalgamation at 1 8 ga some speculations on its position within a global palaeoproterozoic supercontinent abstract new data obtained over the past few years indicate that by the end of the archaean the north china craton consisted of two major crustal entities identified as the eastern and western blocks the formation of island and magmatic arcs along the western margin of the eastern block record the latest archaean events so far identified the location of the western block at this time is unknown however the occurrence of palaeoproterozoic passive margin sediments similar in age to those occurring in the eastern block indicates essentially similar conditions and possible separation by an ocean of unknown dimensions re initiation of east dipping subduction beneath the western margin of the eastern block resulted in closure of this palaeoproterozoic ocean bringing the western block into close proximity continued subduction resulted in a major continental continental collision and the western margin of the eastern block was again reactivated leading to extensive thrusting high pressure metamorphism and the generation of crustal melts this occurred between 1 88 and 1 79 ga ago resulting in the formation of the trans north china orogen and final amalgamation of the north china craton although information is sparse to non existent we have reviewed the possible location of the north china craton at the time of collision and note a similarity in terms of age lithology and distribution of major crustal components between the north china craton and baltica although speculative we present this model in the hope that it will stimulate debate and lead to further research in these areas

geochemistry and origin of metal olivine clasts and matrix in the dong ujimqin qi mesosiderite the dong ujimqin qi mesosiderite is the first recorded fall of a stony iron meteorite in china according to silicate textures and metal composition this meteorite is classified as a member of subgroup ib instrumental neutron activation analyses inaa of metals show that the matrix metal has lower concentrations of os ir re and pt but higher concentrations of ni and au than the 7 5 cm metal nodule present in the meteorite we attribute these compositional differences to fractional crystallization of molten metal studies of olivine clasts show that feo contents are uniform in individual olivine crystals but are variable for different olivine clasts although concentrations of rare earth elements rees change within olivine clasts they all exhibit a vee shaped pattern relative to ci chondrites the relatively high concentrations of rees in olivine and the shape of ree patterns require a liquid high in rees and especially in light rees as such a liquid was absent from the region where basaltic and gabbroic clasts formed mesosiderite olivine must have formed in a part of the differentiated asteroid that is different from the location where other mesosiderite silicate clasts formed

thermal and shock history of mesosiderites and their large parent asteroid to elucidate the geological evolution of the mesosiderite stony iron meteorites and their 3 4 3 8 gy 40ar 39ar ages we have investigated their shock and thermal histories we have studied shock metamorphism in sixteen mesosiderites and find that none have been shocked to more than 10 gpa shock stage s1 s2 three mesosiderites contain a tiny fraction 0 1 overall of mineral fragments that were shocked to shock stage s3 s6 levels the uniformity of shock features within all fragments shows that these fragments were not shocked in situ our shock data for mesosiderites the absence of evidence of 3 6 3 9 gy impact melt the shock history of impact heated ordinary chondrites and the difficulty in quantitatively removing ar in an impact event all suggest that the mesosiderite parent body did not suffer a major impact event 3 6 3 9 gy ago metallographic cooling rates of 0 03 c my at 400 c were estimated from taenite lamellae in four mesosiderites using the latest diffusion coefficients and fe ni p phase diagram cooling rates of 0 01 c my at 425 325 c were estimated from published compositional data for kamacite grains in four mesosiderites these two techniques and four other semiquantitative metallographic cooling rate indicators show that the mesosiderites cooled slower than any iron meteorite we infer that cooling rates at 400 c were 0 02 0 03 c my and certainly less than 0 5 c my the inferred cooling rate is too slow to allow ar closure before 4 gy all of the shock thermal and age data for mesosiderites are consistent with slow cooling at depth 1 my after metal and silicate were mixed around 4 4 gy ago thermal models indicate that the mesosiderites probably cooled in an asteroid some 200 400 km in radius

compositional differences between basaltic and gabbroic clasts in mesosiderites two gabbroic pebbles and one pebble with basaltic textural affinities from the vaca muerta mesosiderite were analyzed by instrumental neutron activation analysis and electron microprobe these analyses plus literature data reveal remarkable compositional differences between basaltic and gabbroic clasts in mesosiderites on average basaltic clasts have small 15 depletions in light relative to heavy rare earth elements rees and moderate positive eu anomalies i e h chondrite normalizedla lu 0 85 eu sm 1 30 the meanla lu andeu sm abundance ratios of gabbroic clasts are at least 3 3 smaller and 28 greater respectively intense meteoroid bombardment probably persisted until after the mesosiderite parent body had differentiated near surface mixtures of major amounts of basalt and lesser amounts of cumulate eucritic rocks were essentially totally melted and rapidly cooled accounting for the moderately fractionated ree patterns of most basaltic clasts at greater depths impact heating partially melted cumulate gabbros incompatible element rich eutectic liquids drained away creating large ree fractionations in the residual rocks these rocks cooled slowly and developed gabbroic textures

the estherville mesosiderite upb rbsr and smnd isotopic study of a polymict breccia abstract a systematic u pb sm nd and rb sr isotopic study shows that the estherville mesosiderite was formed between 4 56 and 4 43 ga observed isotopic heterogeneity is in agreement with multiple generations of meteoritic impacts described in other mesosiderites at least part of the estherville silicate fraction was formed early in solar system history as indicated by the pb pb 4555 35 ma u pb 4560 31 ma rb sr 4542 203 ma and sm nd 4533 94 ma ages mesosiderites therefore present not only petrological but also geochronological similarities with eucrites the pb isotopic composition of the metal phase plots on the same isochron as the silicates indicating formation and subsequent mixing with silicates early in the history of the solar system this is consistent with previous observations indicating that iron was reduced during the silicate magmatic stage most likely a consequence of mixing with metal in addition to these more ancient portions of the estherville breccia other parts were formed later as suggested by the pb pb 4422 50 ma and u pb 4437 11 ma ages observed in a second group of leaches and residues this age is similar to some cumulate eucrite ages and may represent the formation of a second mesosiderite component the sm nd and the rb sr ages obtained on estherville show large errors that may be a consequence of the mixing between the 4 56 and 4 43 ga endmembers estherville like most mesosiderites was affected by a major heating event around 3 5 3 7 ga as shown by the ar ar ages bogard et al 1990 this heating event partially disturbed the rb sr isotopic system rb sr metamorphic ages range between 3 81 and 4 08 ga

petrogenesis of the eet 92023 achondrite and implications for early impact events we report petrology and geochemistry of an achondrite eet 92023 and compare it with normal and anomalous eucrites eet 92023 is an unbrecciated achondrite and shows a granular texture mainly composed of low ca pyroxene and plagioclase petrologically similar to normal cumulate eucrites such as moore county however this rock contains a significant amount of kamacite and taenite not common in unbrecciated crystalline eucrites eet 92023 contains a significant amount of platinum group elements pges ca 10 of ci several orders of magnitude higher than those of monomict eucrites we suggest that the metallic phases carrying pges were incorporated by a projectile during or before igneous crystallization and thermal metamorphism the projectile was likely to be an iron meteorite rather than chondritic materials as indicated by the lack of olivine and the presence of free silica therefore the oxygen isotopic signature is indigenous rather than due to contamination of the projectile material with different oxygen isotopic compositions a significant thermal event involving partial melting and metamorphism after the impact event indicates that eet 92023 records early impact events which took place shortly after the crust formation on a differentiated protoplanet when the crust was still hot

formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid we propose that mesosiderites formed when a 200 400 km diameter asteroid with a molten core was disrupted by a 50 150 km diameter projectile to test whether impacts can excavate core iron and mix it with crustal material we used a low resolution smoothed particle hydrodynamics computer simulation for 50 300 km diameter differentiated targets we found that significant proportions of scrambled core material and hence potential mesosiderite metal material could be generated for near catastrophic impacts that reduce the target to 80 of its original diameter and about half of its original mass the proportion of scrambled core material would be about 5 vol equivalent to 10 vol of mesosiderite like material the paucity of olivine in mesosiderites and the lack of metal poor or troilite rich meteorites from the mesosiderite body probably reflect biased sampling mesosiderites may be olivine poor because mantle material was preferentially excluded from the metal rich regions of the reaccreted body molten metal globules probably crystallized around small cool fragments of crust hindering migration of metal to the core if mantle fragments were much hotter and larger than crustal fragments little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally the rapid cooling rates of mesosiderites above 850 c can be attributed to local thermal equilibration between hot and cold ejecta very slow cooling below 400 c probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris we infer that our model is more plausible than an earlier model that invoked an impact at 1 km s to mix projectile metal with target silicates if large impacts cannot effectively strip mantles from asteroidal cores as we infer we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material this may help to explain why relatively few olivine rich a type and metal rich asteroids m type are known some s type asteroids may be scrambled differentiated bodies

spatial variations of current tectonic stress field and its relationship to the structure and rheology of lithosphere around the bohai sea north china abstract the tectonic stress field in the middle upper crust is closely related to the structure and rheology of the lithosphere to determine the stress field in the deep crust we inversed the focal mechanism solutions fmss of 62 earthquakes that occurred between 2009 and 2015 in the bohai sea and its surrounding areas using broadband seismic waveforms collected from 140 stations we then derived the tectonic stress field using the software satsi spatial and temporal stress inversion based on the damped linear inversion method the inversion results show that both the maximum 1 and minimum 3 principle stress axes throughout the entire region are nearly horizontal except in the tangshan and haicheng areas suggesting that the study area is predominantly under a strike slip faulting stress regime the 1 and 3 axes are found to be oriented in the nee sww or nearly e w and nnw sse or nearly s n directions respectively these results indicate that the stress field in the north china craton is controlled by the combined effects of the pacific plate westward subduction and the india eurasia plate collision however localized normal faulting stress regimes where the vertical stress v 1 are observed in the tangshan and haicheng areas where low viscosity bodies lvbs were identified using geophysical data based on the analysis of focal mechanism solutions active faults and lithosphere rheology characteristics in the tangshan and haicheng areas we speculate that the anomalous stress regime is caused by the local extension resulting from the movement of strike slip faults under the action of the regional stress field the existence of lvb may indicate weakness in the crust that favors the accumulation of tectonic stress and triggers large earthquakes

39ar 40ar dating of mesosiderites evidence for major parent body disruption 4 ga ago 39ar 40ar ages were measured on 17 samples of 14 different mesosiderites and indicate major degassing of ar by one or more thermal events less than 4 0 ga ago three samples gave release patterns of constant age whereas the other samples typically showed 200 400 ma increases in 39ar 40ar age with increasing gas extraction temperature these stepwise releases indicate ages of 3 4 3 8 ga 1 ga 109 a averaged ages for individual meteorites are 3 45 3 82 ga no substantial evidence exists in the temperature releases for 39ar 40ar ages older than 4 0 ga and the ar ages appear to be in contradiction to 244pu fission track ages previously reported for a few mesosiderites the ar ages probably date the event that caused disturbance in previously reported rb sr sm nd and pb pb ages of estherville and other mesosiderites data on release of ar as a function of extraction temperature were used in arrhenius plots to estimate activation energies for ar diffusion of 18 52 kcal mol these values were then used to calculate closure temperatures against ar loss of 2 340 c and 180 600 c for assumed cooling rates of 1 c ma and 1 c a respectively the ar data suggest that for certain assumed models of mesosiderite formation the 39ar 40ar ages may date the time of formation of those metal textures in mesosiderites which have led others to calculate cooling rates of 1 c ma models which call for metal silicate mixing 4 5 ga ago followed by cooling at 1 c ma from temperatures 1150 c seem unlikely single event models which produce the silicate metal mixture at high temperatures rapidly cool the objects to temperatures of 500 c followed by cooling of 1 c ma and are not consistent with the ar data unless the thermal event occurred 500 c and left them deeply buried to undergo slow cooling

age of the subducting pacific slab beneath east asia and its geodynamic implications abstract we study the age of the subducting pacific slab beneath east asia using a high resolution model of p wave tomography and paleo age data of ancient seafloor our results show that the lithosphere age of the subducting slab becomes younger from the japan trench 130 ma to the slab s western edge 90 ma beneath east china and the flat stagnant slab in the mantle transition zone mtz is the subducted pacific plate rather than the proposed izanagi plate which should have already collapsed into the lower mantle the flat pacific slab has been in the mtz for no more than 10 20 million years considerably less than the age of the big mantle wedge beneath east asia 110 million years hence the present flat pacific slab in the mtz has contributed to the cenozoic destruction of the east asian continental lithosphere with extensive intraplate volcanism and back arc spreading whereas the destruction of the north china craton during the early cretaceous 140 110 ma was caused by the subduction of the izanagi or the paleo pacific plate

east asia seismotectonics magmatism and mantle dynamics abstract in this article we review the significant recent results of geophysical studies and discuss their implications on seismotectonics magmatism and mantle dynamics in east asia high resolution geophysical imaging revealed structural heterogeneities in the source areas of large crustal earthquakes which may reflect magma and fluids that affected the rupture nucleation of large earthquakes in subduction zone regions the crustal fluids originate from the dehydration of the subducting slab magmatism in arc and back arc areas is caused by the corner flow in the mantle wedge and dehydration of the subducting slab the intraplate magmatism has different origins the continental volcanoes in northeast asia such as changbai and wudalianchi seem to be caused by the corner flow in the big mantle wedge bmw above the stagnant slab in the mantle transition zone and the deep dehydration of the stagnant slab as well the tengchong volcano in southwest china is possibly caused by a similar process in bmw above the subducting burma microplate or indian plate the hainan volcano in southernmost china seems to be a hotspot fed by a lower mantle plume associated with the pacific and philippine sea slabs deep subduction in the east and the indian slab s deep subduction in the west down to the lower mantle the occurrence of deep earthquakes under the japan sea and the east asia margin may be related to a metastable olivine wedge in the subducting pacific slab the stagnant slab finally collapses down to the bottom of the mantle which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and cause the slab plume interactions some of these issues such as the origin of intraplate magmatism are still controversial and so further detailed studies are needed from now

anomalous cenozoic subsidence along the passive continental margin from ireland to mid norway two dimensional flexural backstripping and thermal modelling assuming uniform stretching and cooling is applied to four interpreted depth converted seismic profiles across the rockall faroe shetland and voring basins along 1600 km of the atlantic continental margin of nw europe the results reveal a significant discrepancy between the modelled palaeo depths for the base of the cenozoic succession and those proven by geological evidence at control points subaerial conditions or depositional depth ranges in wells the discrepancy is of rm scale much larger than the possible range of parameter error determined by sensitivity tests up to 0 5 km assuming a cretaceous rift episode 100 ma the discrepancy is at least 1 7 km in the rockall basin up to 2 1 km in the faroe shetland basin and at least 1 km in the voring basin which also contains evidence of kilometre scale uplift of the inner margin assuming unproven a second rift in the early cenozoic 60 ma the discrepancy remains of kilometre scale in the rockall and faroe shetland basins the restorations also provide evidence of uplift both above compressive structures and across the modelled profiles as seaward rotations of palaeo bathymetric records the palaeo bathymetric discrepancy corresponds to an anomaly in subsidence that is the cumulative product of all the tectonic episodes that have affected the nw european margin and may incorporate both permanent effects of the last episode of lithospheric extension and transient responses to the interaction of the margin with mantle convective flow any explanation must accommodate both the large magnitude of anomalous subsidence along the margin and evidence of its episodic character

the polymict eucrites the polymict eucrites are a suite of about 14 polymict achondritic meteorites that contain at least 90 vol of eucritic material and are found both in antarctica and elsewhere they are part of a continuum of polymict breccias that includes howardites and polymict diogenites rare gas data suggest that some are gas rich regolith breccias the clasts contained in polymict eucrites sample a variety of lithologies from their parent body and include eucritic clasts alkali enriched eucrites pyroxene cumulate eucrites two pyroxene mafic rocks possibly feldspar cumulate eucrites diogenites fayalitic lithologies and a variety of other mineral clasts the eucritic clasts are probably samples of near surface lithologies on the parent body but the presence of two pyroxene mafic rocks is considered to be evidence of magmatic activity at pressures in excess of 1 kbar in the parent body partial melt models may explain some of the diversity among the clasts but cumulus processes are believed to have been important if pressures greater than 1 kbar are confirmed then the basaltic achondrite parent body bap must have a diameter in excess of 200 km the variety of lithologies suggests that not all basaltic achondrites are samples of igneous rocks formed by single stage melting of a chondritic asteroid but are instead samples of an evolved and diverse crust produced by serial magmatism

differentiation history of the mesosiderite parent body constraints from trace elements and manganese chromium isotope systematics in vaca muerta silicate clasts abstract we report here the results of a study of trace element microdistributions and 53mn 53cr systematics in several basaltic and orthopyroxenitic clasts from the vaca muerta mesosiderite ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that following igneous crystallization these clasts underwent extensive metamorphic equilibration that resulted in intra and inter grain redistribution of elements there is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents such as phosphorus derived from the mesosiderite metal at the time of metal silicate mixing furthermore our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal silicate mixing event after taking into account the effects of these postmagmatic secondary processes comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites respectively suggests that the primary igneous petrogenesis including parent magma and source compositions of vaca muerta silicates were similar to those of achondritic meteorites of the howardite eucrite diogenite hed association internal 53mn 53cr isochrons obtained for two basaltic pebble 16 and 4679 and two orthopyroxenitic 4659 and 4670 clasts show that chromium isotopes are equilibrated within each clast nevertheless just as for noncumulate eucrites and diogenites 53cr excesses in whole rock samples of the basaltic clasts 1 01 e in pebble 16 1 07 e in 4679 are significantly higher than in the orthopyroxene rich clasts 0 62 e in 4659 0 53 e in 4670 as in the case of the hed parent body this suggests that mn cr fractionation in the parent body of the vaca muerta silicate clasts occurred very early in the history of the solar system when 53mn was still extant however the slope of the 53mn 53cr isochron defined by the whole rock samples of vaca muerta clasts corresponding to a 53mn 55mn ratio of 3 3 0 6 10 6 is distinctly lower than that defined by the hed whole rock samples corresponding to a 53mn 55mn ratio of 4 7 0 5 10 6 indicating that the global mn cr fractionation event that established mantle source reservoirs on the parent body of the vaca muerta silicate clasts occurred 2 ma after a similar event on the hed parent body

mechanisms for oil depletion and enrichment on the shijiutuo uplift bohai bay basin china the shijiutuo uplift is a major uplift to the north of the bozhong depression the largest generative kitchen in the bozhong subbasin bohai bay basin although the n35 2 trap on this uplift contains a medium size oil accumulation and the q32 6 trap contains china s third largest offshore oil accumulation the q31 1 trap between the n35 2 and q32 6 traps with very similar evolution history was confirmed to be dry biomarker associations of crude oil and source rock samples were analyzed and three dimensional migration pathway modeling was conducted to investigate the origin of oils and mechanisms for oil enrichment and depletion on the uplift multiple parameter oil source correlation and hierarchical cluster analysis using 10 selected biomarker parameters allowed the identification of four source related oil classes almost all oils from the shijiutuo uplift are derived from the eocene shahejie formation whereas oils found between the shijiutuo uplift and the bozhong depression either are derived from or have important contributions from the oligocene dongying formation variations in oil classes and biomarker parameters suggest sequential migration of oil generated from the shahejie and then dongying formations in the bozhong depression which is reasonably supported by petroleum migration pathway modeling oil charge from two oil prone source rock intervals and more importantly focusing of oil originating from a large area of the bozhong generative kitchen into the same trap accounted for oil enrichment and formation of china s third largest offshore oil field in the q32 6 structure the complexity and primary control of the sealing surface top surface of the carrier bed morphology on the positions of migration pathways caused the q31 1 trap to be shielded from migration of oil originating from the bozhong depression resulting in oil depletion in this trap shadows to petroleum migration may occur because of the three dimensional behavior of petroleum migration and two dimensional migration modeling may be misleading in predicting petroleum occurrences

cenozoic tectonic model of the bohai bay basin in china modelling results and seismic interpretation illustrate that the cenozoic evolution of the bohai bay basin bbb can be divided into different stages a transtensional phase during paleocene early oligocene time created ne trending strike slip faults and e w trending normal faults which were driven roughly by n s extension making an angle of 25 with the strike slip faults seismic data interpretation yields evidence that inversion phases occurred within the ne xialiaohe depression of the greater bohai bay basin this inversion phase is attributed to rotation and partial inversion that occurred during late oligocene time leading to formation of inversion structures along the ne part of tanlu fault this episode is attributed to an anticlockwise rotation of the eastern part of the bbb driven by the convergence between the pacific and eurasian plates the tectonic scenario described was simulated in scaled analogue models which were extended by pulling two basement plates away from each other partial inversion was simulated by rotation of one of the plates relative to the other model results show many of the features observed in the bbb our model results are used to argue that unlike the two episode extension and whole basin inversion models previously proposed for the bbb a single n s aligned extension followed by anticlockwise rotation accounts for the cenozoic evolution of the bbb and produces some of the structural complexities observed in the basin

asteroids and meteorites origin of stony iron meteorites at mantle core boundaries abstract stony iron meteorites formed at the core mantle interfaces of small asteroidal parents the mesosiderites formed when the thick crust of a largely molten parent body 100 200 km in diameter foundered and sank through the mantle to the core pallasites formed in smaller parent bodies 50 100 km in which olivine crystals from the partially molten mantle sank to the core mantle interface and rafted there subsequent collisions stripped away the rocky mantles of both kinds of parent bodies exposing the stony iron surfaces of their cores to direct impacts which continue to knock off meteorite fragments

81kr kr age and multiple cosmic ray exposure history of the vaca muerta mesosiderite abstract noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate metal mixture from the vaca muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods trapped noble gases were degassed completely by a high temperature thermal event probably at the formation of the vaca muerta parent body vmpb the presence of fissiogenic xe isotopes from extinct 244pu in the bulk samples might be a result of rapid cooling from an early high temperature metamorphism high concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic kr and xe spallogenic ne from na and unique ar isotopic compositions were observed the 81kr kr exposure age of 168 8 myr for the silicate pebble is distinctly longer than the age of 139 8 myr for the bulk samples the precursor of the pebble had been irradiated on the surface of the vmpb for more than 60 myr first stage irradiation with subsequent incorporation into bulk materials approximately 4 gyr ago the vaca muerta meteorite was excavated from the vmpb 140 myr ago second stage irradiation relative diffusion rates among the cosmogenic ar kr and xe based on data obtained by stepwise heating indicate that kr and xe can be partially retained in pyroxene and plagioclase under the condition that resets the k ar system this result supports the presence of fission xe and of excess concentration of cosmogenic kr which could have survived the thermal event approximately 3 8 gyr ago

neogene quaternary postrift tectonic reactivation of the bohai bay basin eastern china bohai bay basin located in eastern china is considered a cenozoic rifted basin the basin is atypical in terms of its neogene quaternary postrift subsidence history in that it experienced intensive tectonic reactivation rather than the relative tectonic quiescence experienced during this stage by most rift basins this neogene quaternary tectonic reactivation arose principally in response to two tectonic events 1 activity on a dense array of shallow faults and 2 accelerated tectonic subsidence that occurred during the postrift stage these two events were neither strictly temporally nor spatially equivalent the dense array of shallow faults form a northwest southeast trending belt in the central part of the basin with displacement induced by the reactivation of older northeast and northwest trending basement faults and an associated substantial component of strike slip displacement occurring after 5 3 ma the intensive reactivation of these faults contributed to the atypically accelerated rate of postrift tectonic subsidence of the basin that commenced ca however this was not the sole cause of this accelerated tectonic subsidence a combination of geological activity deep within the crust led to the buildup of intraplate stresses and this combined with ongoing thermal subsidence acted as an additional contributory factor that drove unusually high rates of subsidence for this basin this episode of accelerated postrift tectonic reactivation resulted in conditions favorable for hydrocarbon accumulation

recovery of strain rate variation from inversion of subsidence data despite considerable advances over the past 20 years in our understanding of continental lithospheric deformation see for example refs 1 and 2 we know little about the detailed spatial and temporal variations in strain rate at geologically relevant scales 102 104 km and 1 100 million years such measurements are needed to constrain dynamic models of lithospheric deformation3 although it is not yet obvious how detailed strain rate information can be obtained from mountain belts subsidence data from sedimentary basins should contain information about at least the vertical component of the strain rate tensor here i show that inverse theory can be used to calculate the temporal strain rate variation needed to fit observed subsidence patterns in extensional sedimentary basins no a priori information concerning either the duration of rifting or the total strain is required inverse calculations produce good fits to synthetic data and to subsidence data from the dolomites of northern italy calculated strain rates are not constant but vary between 10 18 and 10 15 s l error analysis shows that these results are significant and well resolved all of the stratigraphic sections studied exhibit a minimum in strain rate during extension a feature predicted by models of lithosphere deformation by power law creep

seismic evidence for significant lateral variations in lithospheric thickness beneath the central and western north china craton abstract we have migrated teleseismic s receiver functions to construct detailed lithospheric structure images that cover the three constituent parts of the north china craton ncc our images show that in contrast to the eastern ncc where significantly thinned lithosphere 60 100 km is widespread the central and western ncc are characterized by the coexistence of both preserved thick and dramatically thinned lithosphere the thick lithosphere 200 km is present beneath the stable ordos plateau and the thinned lithosphere up to 80 km is found in the late cenozoic yinchuan hetao and shaanxi shanxi rift areas with sharp changes occurring over a lateral distance of

petrogenesis of acapulcoites and lodranites a shock melting model abstract acapulcoites are modeled as having formed by shock melting cr like carbonaceous chondrite precursors the degree of melting of some acapulcoites was low enough to allow the preservation of 3 6 vol relict chondrules shock effects in acapulcoites include veins of metallic fe ni and troilite polycrystalline kamacite fine grained metal troilite assemblages metallic cu and irregularly shaped troilite grains within metallic fe ni while at elevated temperatures acapulcoites experienced appreciable reduction because graphite is present in some acapulcoites and lodranites it seems likely that carbon was the principal reducing agent reduction is responsible for the low contents of olivine fa 4 14 mol and low ca pyroxene fs 3 13 mol in the acapulcoites the observation that in more than two thirds of the acapulcoites the fa value is lower than the fs value in contrast to the case for equilibrated ordinary chondrites the low feo mno ratios in acapulcoite olivine 16 18 compared to 32 38 in equilibrated h chondrites the relatively high modal orthopyroxene olivine ratios e g 1 7 in monument draw compared to 0 74 in h chondrites and reverse zoning in some mafic silicate grains lodranites formed in a similar manner to acapulcoites but suffered more extensive heating loss of plagioclase and loss of an fe ni s melt acapulcoites and lodranites experienced moderate post shock annealing presumably resulting from burial beneath material of low thermal diffusivity the annealing process repaired damaged olivine crystal lattices lending acapulcoites and lodranites the appearance of unshocked i e shock stage s1 rocks any high pressure phases that may have formed during initial shock reverted to their low pressure polymorphs during annealing some samples were subsequently shocked again several acapulcoites reached shock stage s2 levels alh 84190 reached s3 and the lodranite mac 88177 reached s5

classification of mafic clasts from mesosiderites implications for endogenous igneous processes we have analyzed thirteen igneous pebbles from the vaca muerta eet87500 and bondoc mesosiderites by electron microprobe and instrumental neutron activation and combined these data with literature data for forty three analyzed mesosiderite clasts we classify these well characterized clasts into the following five principal groups 1 1 polygenic and monogenic cumulates 39 are coarse grained gabbros that are highly depleted in incompatible elements relative to h chondrites they formed at moderate depth either as residues of low degree partial melting of pre existing cumulate eucrites or as cumulates from parent melts similar to cumulate eucrites 2 2 polygenic basalts 30 are finer grained rocks with positive europium anomalies la lu ratios 1 and lower rare earth element abundances than basaltic eucrites it seems likely that these rocks were formed near their parent body surface by remelting mixtures of major amounts of basaltic eucrites and lesser amounts of cumulate eucrites 3 3 quench textured rocks comprise two compositional groups a those which resemble basaltic eucrites 5 and b those which resemble cumulate eucrites 2 the quench textured rocks are probably monogenic they formed most likely when small scale impacts at their parent body surface totally melted small amounts of basaltic or cumulate eucrite material 4 4 monogenic basalts 11 resemble basaltic eucrites and formed by endogenous igneous processes on the mesosiderite parent body mpb the monogenic basalts and those quench textured rocks that resemble basaltic eucrites in composition plot subparallel to the nuevo laredo trend eucrites on diagrams of incompatible element concentration vs mg previous work has shown that the nuevo laredo trend eucrites were probably produced by fractional crystallization it seems likely that the monogenic basalts from the mpb had a similar origin 5 5 ultramafic rocks are cumulates consisting mainly of large crystals of orthopyroxene 9 or olivine 4 orthopyroxenite clasts closely resemble diogenites and were formed most likely by endogenous igneous processes olivine crystals vary significantly in major element composition some may be endogenous but others may have been derived from the disrupted mantle of a different parent body

neogene residual subsidence and its response to a sinking slab in the deep mantle of eastern china abstract mantle convection could have a significant effect on basin evolution however research quantifying this relationship is controversial to understand the formation mechanism and evolution of the cenozoic rift basins in eastern china we applied the back stripping technique and strain rate inversion modeling to 119 wells from sangjiang basin bohai bay basin north yellow sea basin south yellow sea basin east china sea shelf basin and four basins within the northern south china sea margin the modeled results can be used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of eastern china residual subsidence is defined as the difference between the theoretical and observed tectonic subsidences our results show that the residual subsidence since 20 ma in eastern china generally increases from 100 to 300 m in terrestrial areas to 1 2 1 8 km on the continental shelf our observed residual subsidence is generally consistent with the present day dynamic topography that is predicted from mantle flow models and it is associated with a stagnant slab within the mantle that has been observed by seismic tomography the migration pathway of the residual subsidence since 20 ma is consistent with the movement direction of the pacific and philippine plates therefore we suggest that the residual subsidence might be a dynamic subsidence induced by a negative buoyancy of the sinking slab in the deep mantle beneath eastern china

the galim ll eh polymict breccia evidence for impact induced exchange between reduced and oxidized meteoritic material galim is a polymict breccia consisting of a heavily shocked shock stage s6 ll6 chondrite galim a and an impact melted eh chondrite galim b relict chondrules in galim b served as nucleation sites for euhedral enstatite grains crystallizing from the impact melt many of the reduced phases typical of eh chondrites e g si bearing metallic fe ni ti bearing troilite are absent galim b was probably shock melted while in contact with a more oxidized source namely galim a during this event si was oxidized from the metal and ti was oxidized from troilite galim a contains shock veins and recrystallized unzoned olivine the absence of evidence for reduction in galim a may indicate that the amount of ll material greatly exceeded that of eh material shock metamorphism may have taken place on the ll parent body shock induced redox reactions such as those inferred for the galim breccia appear to be restricted mainly to asteroids because the low end tail of their relative velocity distribution permits mixing of intact disparate materials including accretion of projectiles of different oxidation states whereas the peak of the distribution leads to high equilibration shock pressures allowing impact induced exchange between previously accreted disequilibrated materials galim probably formed by a two stage process 1 accretion to the ll parent body of an intact eh projectile at low relative velocities and 2 shock metamorphism of the assemblage by the subsequent impact of another projectile at significantly higher relative velocities

basement structures underneath the northern south yellow sea basin east china implications for the collision between the north china and south china blocks abstract the south yellow sea basin sysb as the eastern part of the yangtze block has undergone multiple tectonic events since the formation of its basement the north china south china collision affected intensely the structures of the basin the relationship between the deep structures in the basement and collisional orogeny has long been a matter of debate in this paper 2d seismic magmatic gravity and drilling data were integrated to unravel the basement structures in the interpreted seismic sections the basement can be subdivided into three seismic facies the seismic facies i and iii correspond to two basement layers the seismic facies ii is interpreted as shear zones that were formed during the mesozoic collision and exhumation the influence of the north china south china collision propagated from north to south in the sysb according to observations from drill holes and seismic sections the northern part of the basin the yantai depression uplifted during the earlier stage of the collision resulting in the erosion of the upper paleozoic and triassic strata in the middle and southern parts the laoshan uplift and qingdao depression the compression was obviously after that in the yantai depression the exhumation of the qianliyan basement metamorphic complex of the yangtze block occurred gradually the shear zones were initially developed during the collision during early mesozoic the exhumation process of metamorphic rocks lasted till late mesozoic and the jurassic cretaceous strata were deposited in the hanging wall of the northern boundary fault

an inverse method for determining lithospheric strain rate variation on geological timescales abstract inverse theory is used to calculate the strain rate variation as a function of time needed to fit observed subsidence patterns within extensional sedimentary basins no a priori information concerning either the duration of rifting or the total strain is required the method is one dimensional but can be generalised for use with two and three dimensional data conventional kinematic modelling of such basins normally assumes that lithospheric stretching is instantaneous or that the strain rate during a rifting period of known duration is constant the scheme is tested on a variety of synthetic data and applied to subsidence data from the dolomites northern italy linear error analysis combined with random and systematic noise tests show that variations in strain rate are significant and well resolved in the dolomites inverse calculations produce good fits to the data yielding total strains and durations of lithospheric stretching that are consistent with geological constraints calculated strain rates are not constant but vary between 10 18 10 15 s 1 information concerning spatial and temporal strain rate variation will provide important constraints for dynamic models of lithospheric deformation

the interplate coupling and stress accumulation process of large earthquakes along the nankai trough southwest japan derived from geodetic and seismic data abstract the interplate coupling and stress accumulation process associated with the subduction of the philippine sea plate beneath the eurasian plate along the nankai trough where large earthquakes have occurred periodically have been investigated in detail taking into account the three dimensional complicated structure of the philippine sea plate and its motion relative to the eurasian plate for this purpose we mainly used extensive geodetic data in southwest japan such as levelling tide gauge and trilateration which presumably are effective means of revealing coupling properties and applied a three dimensional finite element method here two seismic periods associated with the 1944 tonankai and 1946 nankaido earthquakes were considered interseismic and pre seismic stages comparing the coupled regions obtained for the two cases a possible explanation of its temporal change was proposed it might gradually spread from a shallower to deeper portion with transition from the interseismic to pre seismic stage a comparison of the coupling properties between southwest japan and northeast japan reveals the following properties in southwest japan the coupling is generally strong and its region is limited in a relatively shallow portion the pre seismic coupled regions coincide with the co seismic and post seismic fault zone suggesting that the accumulated stress is released at the time of a large earthquake in northeast japan on the other hand the coupling is weak and its extent appears to reach a depth of about 100 km and the co seismic and post seismic regions are limited to the upper half of the coupled region the weakly coupled region in southwest japan seems to correspond well to the region with high seismic activity a high dip angle subducting slab and the possible existence of seamounts in contrast to the strongly coupled region correlated with low seismicity low dip angle subduction and no seamounts on the western coupled region off shikoku an anomalously strongly coupled region corresponding to a convergence rate of 7 8 cm year 1 was identified this may be caused by geomorphological properties such as the low dip angle of the subducting plate and smooth plate boundary resulting in a large amount of stress concentration there the stress field associated with the interplate coupling was also calculated there is fair agreement between the observations and calculations near the plate boundary beneath the southern kii peninsula and shikoku although the effects of the interplate coupling may have some influence on strike slip faulting with the n s compressive axes beneath shikoku other earthquake generating stress such as a two layered double buckling would be dominant there

composition and petrology of hed polymict breccias the regolith of 4 vesta the polymict breccias of the howardite eucrite and diogenite hed clan of meteorites preserve records of regolith processes that occur on vesta their putative home world these breccias howardites polymict eucrites and polymict diogenites are impact engendered mixtures of diogenites and eucrites the compositions of polymict breccias can be used to constrain the lithologic diversity of the vestan crust and the excavation depths of these materials we have done petrological and compositional studies of multiple samples of 5 polymict eucrites and 28 howardites to investigate these issues older analyses were done on samples of approx 0 5 gram mass by inaa newer analyses on samples of approx 5 gram mass by xrf and icp ms we estimate the percentage of eucritic material poem of polymict breccias by comparing their al and or ca contents to those of average basaltic eucrite and diogenite our samples have poem ranging from 28 to 98 adding two polymict diogenites from extends the range to poem 10 one hypothesis is that ancient well mixed vestan regolith has poem approx 67 and has a higher content of admixed impactor material several of our howardites have poem of 59 74 al and or ca contents 10 of poem 67 about a third have ni contents 300 micro g g suggesting they contain 2 chondritic material cm and or cr these may be regolithic howardites only one lew 85313 contains ne dominated by a solar wind sw component pca 02066 is dominated by impact melt material of polymict parentage and petrologically appears to be a mature regolith breccia yet it does not contain sw ne gro 95602 falls within the poem window contains sw ne yet has a ni content of 193 micro g g its petrologic characteristics suggest it was formed from immature regolith no polymict breccia clasts no glass trace element characteristics of the polymict breccias demonstrate the dominance of main group eucrites as the basaltic component mixing diagrams of zr nb ba hf and ta with al show no evidence for a significant contribution from stannern trend eucrites an exception is polymict eucrite lew 86001 poem 92 which is dominated by stannern trend basaltic debris howardite lap 04838 poem 84 has higher incompatible trace concentrations than other polymict breccias excluding lew 86001 and either contains a stannern trend basaltic component or has a significant contributions from evolved eucrites like nuevo laredo

olivine coronas metamorphism and the thermal history of the morristown and emery mesosiderites abstract coronas are present on all millimeter sized mineral clasts of olivine in the emery and morristown mesosiderites and are a manifestation of high temperature t 850 1100 c metamorphism these coronas formed by reaction and diffusion between olivine and a mesosiderite like matrix assemblage the bulk composition of the coronas can be approximated by a mixture of 10 25 wt olivine and as 90 75 wt metal free matrix except for p and cr which are significantly enriched in coronas phosphorus and cr diffused relatively rapidly to coronas and were derived from a large volume of matrix most likely from metal that was originally enriched in these elements prior to metamorphism the coronas in both meteorites show a similar zone sequence but are systematically thicker in emery 800 m wide than in morristown 350 m wide suggesting that emery experienced more grain growth and more intensive metamorphism than morristown textural relationships suggest that corona formation and high temperature metamorphism occurred largely after intensive millimeter scale brecciation and after or during metal silicate mixing a local equilibrium model can explain many features of the coronas but chemical equilibrium was maintained only on a very small scale overgrowths are present on plagioclase in the coronas of both mesosiderites and probably formed during high temperature metamorphism the compositional interface between core and overgrowth plagioclase is extremely sharp suggesting that cooling rates were 0 1 c y at the peak temperature of metamorphism consistent with high temperature metamorphism occurring in a near surface region of the parent body

the antarctic achondrite alha 76005 a polymict eucrite abstract alha 76005 is a basaltic achondrite containing few if any orthopyroxenes its bulk major and trace element composition is like that of a non cumulate eucrite and unlike that of a howardite it contains a variety of igneous clasts which differ in their textures pyroxene plagioclase ratios and pyroxene and plagioclase compositions one clast no 4 was found to have the ree pattern of a cumulate eucrite and an oxygen isotopic composition different from that of the bulk meteorite both the chemical and oxygen isotopic composition of clast no 4 suggest that it was derived from a source different from its host these observations lead to the conclusion that alha 76005 is a polymict eucrite

diverse eucritic pebbles in the vaca muerta mesosiderite abstract seven 5 cm basaltic pebbles from the vaca muerta mesosiderite were analyzed by instrumental neutron activation analysis and electron microprobe three additional pebbles were only examined petrographically two pebbles are impact melt breccias four are similar texturally to main group eucrites or some clasts in polymict eucrites three are similar to cumulate eucrites and one is a fragmental breccia similar to certain polymict eucrites cumulate pebbles have extremely low ree concentrations and high eu sm ratios reflecting the virtual absence of intercumulus liquid siderophile interelement ratios in the pebbles are similar to those in vaca muerta metal except for anomalously low ir higher silica and merrillite in the pebbles relative to eucrites may reflect reduction of feo in the silicates by p from mesosiderite metal pebbles were annealed following incorporation into the vaca muerta host impacts were the dominant heat source the occurrence of 20 impact melt breccias among the pebbles and 35 40 melt breccias among mesosiderite whole rocks indicate that mesosiderites were more extensively impact melted than howardites we suggest three alternative models to account for this 1 mesosiderites were derived from a separate parent body that experienced a higher meteoroid fluence than the basaltic achondrite parent body 2 the low velocity accretion of large metallic core fragments from disrupted differentiated asteroids led to metal enrichment of a few regions of the regolith and heat retention in these mesosideritic regions and 3 mesosideritic regolith regions were less likely to be reduced to fine particles by extensive shock than howarditic regions

basin evaluation using burial history calculations an overview burial history calculations available to the petroleum geologist include plots of burial history and geohistory porosity and conductivity as a function of depth and lithology as well as the results obtained from algorithms to handle unconformities and backstripped crustal subsidence calculations that predict the breakdown of kerogen to hydrocarbons in terms of time and temperature and methods for determining heat flow from geologic models and vitrinite reflectance data inputs required for these calculations include depths of formation tops ages of formation tops lithologies of formations depths of water at deposition of the different formations porosities of the formations as a function of depth bottom hole temperatures and or formation temperatures and any ke ogen content and vitrinite reflectance values as functions of depth sources of many of these data may be well logs well reports geologic papers and or seismic sections the paper considers how parameters can be extracted from burial history calculations to construct maps that can be compared to the current locations of oil and gas fields and so used to locate and rank prospective acreage these parameters include the potential source rock maturity of a formation plotted as a function of time and depth the thermal history of any hydrocarbons associated with the potential source rock the rate of sediment accumulation of a formation as a function of time and subsidence rate of a formation as a function of time subsidence rate includes total subsidence compaction induced subsidence isostatic response to sediment load and backstripped crustal subsidence total subsidence minus the effects of compaction and isostatic response

initiation and origin of dextral deformation at mid eocene in the western bohai bay basin east china abstract the cenozoic back arc rift basins along the continental margin of east asia are featured by both extensional and dextral deformation systems here we investigate the timing and interaction of these two types of systems based on seismic data stratigraphic isopach maps and drilling data in the jizhong depression in the northwestern bohai bay basin bbb east china the results show the rifting was initiated as early as the eocene while the dextral deformation did not occur until ca 42 ma the jizhong depression changed from a simple rift basin with boundary faults to the west to a geometrically more complex dextral transtensional basin at ca 42 ma this event was roughly concordant with the initial dextral slip on the tan lu fault lying at the eastern bbb and together they transformed the bbb into a complex pull apart rift basin the tectonic reorganization of the bbb was probably the response to the change from an orthogonal to an oblique subduction of the pacific plate beneath east asia at that time

the july 12 1993 hokkaido nansei oki japan earthquake coseismic slip pattern from strong motion and teleseismic recordings we employ a finite fault inversion scheme to infer the distribution of coseismic slip for the july 12 1993 hokkaido nansei oki earthquake using strong ground motions recorded by the japan meteorological agency within 400 km of the epicenter and vertical p waveforms recorded by the global digital seismograph network at teleseismic distances the assumed fault geometry is based on the location of the aftershock zone and comprises two fault segments with different orientations a northern segment striking at n20 e with a 30 dip to the west and a southern segment with an n20 w strike for the southern segment we use both westerly and easterly dip directions to test thrust orientations previously proposed for this portion of the fault the variance reduction is greater using a shallow west dipping segment suggesting that the direction of dip did not change as the rupture propagated south from the hypocenter this indicates that the earthquake resulted from the shallow underthrusting of hokkaido beneath the sea of japan static vertical movements predicted by the corresponding distribution of fault slip are consistent with the general pattern of surface deformation observed following the earthquake fault rupture in the northern segment accounts for about 60 of the total p wave seismic moment of 3 4 1020 n m and includes a large circular slip zone 4 m peak near the earthquake hypocenter at depths between 10 and 25 km slip in the southern segment is also predominantly shallower than 25 km but the maximum coseismic displacements 2 0 2 5 m are observed at a depth of about 5 km this significant shallow slip in the southern portion of the rupture zone may have been responsible for the large tsunami that devastated the small offshore island of okushiri localized shallow faulting near the island however may require a steep westerly dip to reconcile the measured values of ground subsidence

mineralogy petrology chemistry and 39ar 40ar and exposure ages of the caddo county iab iron evidence for early partial melt segregation of a gabbro area rich in plagioclase diopside we describe coarse grained gabbroic material rich in plagioclase and diopside in the caddo county iab iron meteorite the gabbro is not a clast within a breccia but rather is an area with transitional margins located mainly at silicate metal boundaries only a few cm away from fine grained ultramafic silicates similar to winonaites medium grained orthopyroxene and olivine are found in transitional areas and show no shock disturbance of their textures a vein like region starting in an area of fine grained ultramafic silicate extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside these textures and mineralogies suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts from chondritic source materials compositional data on two mineralogically distinct samples of the gabbro rich portion of the inclusion were obtained by inaa compared to an average of iab silicate inclusions or winonaites the caddo county gabbro is enriched in the incompatible lithophile elements na ca sc ree and hf consistent with a melt origin for the gabbro the cosmogenic space exposure age of caddo county 5 1 ma is significantly younger than exposure ages of some other iab meteorites an 39ar 40ar age determination of the gabbroic material indicates a series of upward steps in age from 4 516 ga to 4 523 ga with a few high temperature ages up to 4 54 ga the older age could approximate the primary recrystallization age of silicates the stepped ar age spectrum may indicate differences in ar closure temperatures during slow cooling of 2 20 c myr in the parent body alternatively the younger ar ar ages may date a shock event which occurred while caddo county was hot and which also created textures such as a linear metal vein with rounded zigzag walls in a diopside crystal and slightly misoriented rounded plagioclase domain

distinct lateral variation of lithospheric thickness in the northeastern north china craton abstract a detailed knowledge of the thickness of the lithosphere in the northeastern north china craton ncc is important for understanding the significant tectonic reactivation of the craton in the mesozoic and cenozoic time we achieve this goal by applying the newly proposed wave equation based migration technique to the s receiver functions recently collected in the region distinct negative signals are identified below the moho in all the s receiver function migrated images and stacks which we interpret as representing the s to p conversions from the lithosphere asthenosphere boundary lab the imaged lab is as shallow as 60 70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior these observations indicate widespread lithospheric thinning in the study region in comparison with the 180 km lithospheric thicknesses typical of most cratonic regions the revealed topography of the lab generally agrees with the lateral variation in upper mantle seismic anisotropy previously measured through sks splitting analysis in particular a sharp lab step of 40 km is detected at the triple junction of the basin and mountains at almost the same place where an abrupt change from nw se to ne sw in fast polarization direction of shear waves was found these findings suggest a close correlation between the seismic anisotropy and hence deformation of the upper mantle the lithospheric thickness and the surface tectonics of the northeastern ncc while the thinned lithosphere and the nw se fast shear wave polarizations in the east areas probably are related to the dominant nw se tectonic extension in the late mesozoic cenozoic time the thicker lithosphere and the ne sw fast polarization direction in the west mountain ranges may reflect earlier contractional deformations of the region synthetic tests indicate that the lab beneath the northeastern ncc is a well defined zone 10 20 km thick combined with seismic tomography results and geochemical and petrological data this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton

interplate coupling along the nankai trough off southwest japan derived from gps measurements we investigate interplate coupling between the philippine sea and the eurasian plates at the nankai trough off southwest japan during the interseismic period plate coupling is modeled by imaginary backward normal slip superposed on the steady state subduction we divide the plate interface into several segments and estimate coupling on each segment from gps horizontal velocity data estimated coupling is in general agreement with the rate of steady state subduction predicted from a plate motion model this implies that current coupling at the nankai trough is so strong that much of the relative plate motion is taken up by shortening of the leading edge of the overriding plate it seems that stronger interseismic coupling is realized on the segment where larger coseismic dislocation occurred and vice versa time fluctuation of plate coupling from 1995 to 1996 is found on a small segment

a detailed receiver function image of the sedimentary structure in the bohai bay basin abstract if the site at which receiver functions are constructed is filled by sediments then the waveforms from these receiver functions are dominantly controlled by the sedimentary structures within the first few seconds after the direct p arrival based on this observation waveform data collected at 44 temporary seismic stations have been used to image the sedimentary structure of the bohai bay basin a major continental petroliferous basin in eastern china an adapted hybrid global waveform inversion method was applied to the receiver functions to extract structural information beneath each of the stations the derived s velocity structure provides for the first time a basin scale seismic image of detailed sedimentary stratification the sedimentary cover of the basin is about 2 12 km thick consisting of cenozoic mesozoic and paleozoic strata from top to bottom the structural features presented in the s velocity image coincide quite well with the depression uplift type of tectonic system in the bohai bay basin the reconstructed morphology of the sedimentary layers provides seismological evidence for the two stage evolution of the intracontinental basin that were caused by an intensive tectonic regime transition in late mesozoic immediately following the lithospheric reforming of the eastern china continent

magnesian basalt clasts from the eet 92014 and kapoeta howardites and a discussion of alleged primary magnesian hed basalts we have performed petrologic and geochemical studies on pyroxene phyric magnesian basalt clasts from the howardites eet 92014 and kapoeta pyroxenes in the eet 92014 clast are strongly zoned from wo4 5en68 9fs26 6 cores to wo22 2en18 5fs59 2 rims the pyroxene has also suffered later feo metasomatism along healed fractures that has resulted in compositions with atypically high feo mno ratios metasomatized pyroxenes reach compositions as ferroan as wo3 2en37 0fs59 8 pyroxenes in the kapoeta clast have cores that are more magnesian wo1 8en80 2fs18 0 and are less strongly zoned to wo3 6en51 0fs45 4 atypically high feo mno in the most ferroan pyroxenes in this clast coupled with late ferroan olivine up to fa64 reflect late stage addition of feo to the melt rather than igneous fractionation the eet 92014 clast is enriched in the refractory incompatible lithophile elements and shows a refractory lithophile element pattern similar to stannem and bouvante the clast is enriched in cr2o3 and mgo and depleted in al2o3 and cao compared to main group or stannern trend eucrites based on bulk major and trace lithophile element composition this pyroxene phyric clast cannot be directly related to basaltic eucrites this clast is either an impact melt of a trace element rich polymict eucrite or howardite target or a primary melt of a more magnesian source region than that which produced basaltic eucrites we favor the former interpretation low siderophile element abundances within the ranges for monomict eucrites indicate 0 5 chondritic contamination in the clast however the kapoeta clast has a major element composition within the range of howardites and likely is an impact melt of such a target lithology comparison with other reported magnesian basalt clasts from howardites indicates that in all likelihood all heretofore alleged primary magnesian basalt clasts are impact melts of polymict targets two magnesian eucrites alha81001 and pomozdino and magnesian eucrite clasts from the polymict eucrite petersburg may possibly be primary melts of a magnesian source region not directly related to basaltic eucrites

geothermal evidence of meso cenozoic lithosphere thinning in the jiyang sub basin bohai bay basin eastern north china craton abstract the bohai bay basin is at the center of lithosphere destruction and thinning in the eastern north china craton ncc in this paper the thermal lithospheric thicknesses of the meso cenozoic jiyang sub basin in the bohai bay basin were calculated by reconstructing the thermal history of the sedimentary basin using apatite fission track and vitrinite reflectance data the results show that the jiyang sub basin experienced two heat flow peaks in the late early cretaceous and in the middle to late paleogene with heat flow values of 84 88 mw m 2 and 85 88 mw m 2 respectively the thermal lithosphere thicknesses of the jiyang sub basin calculated from the modeled thermal histories experienced two thinning stages in the cretaceous and in the paleogene the lithosphere began with an initial thickness of 150 km in the early mesozoic and reached a thinning peak of 51 km in the late early cretaceous the thickness then increased to approximately 80 km at the end of the cretaceous a second thinning peak occurred in the middle and late paleogene with a thickness of only 48 km which corresponds to a rift phase in the bohai bay basin the lithosphere thickened thereafter and is 78 km at present the north china craton now has a thin lithosphere our research provides new geothermal evidence for the lithospheric thinning of the ncc

a mesosiderite rock from northern siberia russia not a meteorite a possible mesosiderite meteorite was found in the area of the putorana plateau noril sk district siberia russia although this rock resembles a mesosiderite in its hand sample aspect and in having ni bearing iron metal it is not a meteorite this inference is based on the lack of a fusion crust the lack of cosmogenic nuclides oxygen with terrestrial isotope ratios and several mineral chemical criteria most likely the rock is from the iron metal bearing basalts of the siberian trap basalt sequence which are mined for their base and platinum group metals mesosiderite imposters like this may be recognized by 1 the presence of cu metal in hand sample or as microscopic blebs in the low ni metal kamacite 2 the absence of high ni metal taenite and 3 the presence of iron carbide cohenite enclosing the kamacite even if these macroscopic tests are inconclusive isotopic and mineral chemical tests will also distinguish rocks like this from mesosiderites

interseismic deformation at the nankai trough japan subduction zone uplift along the coast of southwestern japan following the 1944 tonankai ms 8 0 and 1946 nankaido ms 8 2 earthquakes has been inferred from the 1950 1985 tide gage records although uplift rates vary with geographic position the temporal dependence at each site can be described as the superposition of an exponentially decaying time constant 5 years transient significant only in the first decade postseismic and a steady interseismic rate the steady interseismic uplift rate is defined only over the 1956 1985 interval but the linearity in time ofthat uplift record is such that uplift over the remainder of the interseismic cycle 80 years might reasonably be expected to occur at the same rate moreover the spatial pattern and magnitude of the steady uplift rate are reasonably close to those predicted by a simple dislocation model of subduction however the currently observed steady uplift rate is not consistent with the uplift rate inferred from geodetic leveling for the final half of the previous earthquake cycle thus either the hypothesis of a steady interseismic uplift rate after the first decade postseismic is wrong or the strain accumulation pattern for one earthquake need not be the same as for the preceding earthquake on the same segment

quest for dynamic topography observations from southeast asia transient dynamic topography is maintained by time varying density anomalies deep within earth9s mantle there is considerable interest in predicting and measuring dynamic topography because its spatial and temporal variation will help to track changes in the pattern of mantle convection despite the availability of increasingly elaborate predictive models there is little agreement about the amplitude wavelength and rate of change of dynamic topography here we analyze a hybrid data set of subsidence observations from continental and oceanic basins in southeast asia where published dynamic models predict 1 2 km of regional subsidence residual subsidence was isolated by removing the well defined effects of lithospheric extension and or cooling at long wavelengths 10 3 km the amplitude of residual subsidence i e maximum permissible dynamic subsidence has an upper bound of only 300 m the spatial distribution of permissible dynamic topography has no simple spatial or temporal relationship to the pattern of subduction over the past 65 m y anomalous subsidence commenced 5 10 m y ago accelerating to a present day rate of 100 m m y our results show that density anomalies within the lower mantle have little apparent influence at earth9s surface instead the distribution of residual subsidence reported here can be accounted for by minor variations in lithospheric thickness or by transient density variations beneath the lithospheric plate the absence of measurable dynamic topography is surprising and indicates that mantle mass anomalies are supported elsewhere presumably at internal boundaries within earth our hybrid data set can thus be used as an independent constraint on the viscosity structure of the mantle