architecture of miocene overbank deposits in northern pakistan abstract understanding the geometry of sediment bodies and patterns of sediment disruption in ancient fluvial overbank successions is fundamental to interpretations of overbank deposition overbank deposits of the fluvial chinji formation siwalik group can be divided into sequences 1 20 m thick defined by the alternation of stratified sediments and paleosols preservation of stratified sequences does not support a gradual vertical aggradation of overbank sediments over the extent of the floodplain but instead indicates periods of rapid sediment deposition followed by long hiatuses and soil development rapid deposition of overbank sequences is predicted by models that hypothesize 1 episodic vertical aggradation of the entire floodplain 2 deposition controlled by the growth of alluvial ridges 3 incision and filling of local valleys 4 rapid filling of localized low areas or 5 rapid deposition associated with river channel avulsion these models also have important implications for understanding fossil preservation and floodplain paleoecology paleosol bounded sequences in the chinji formation can pinch out laterally over kilometers indicating that episodes of rapid deposition were restricted to local areas on the floodplain there is no evidence that bases of sequences are erosional and they do not appear to record valley incision and filling sequences generally do not thin and fine systematically away from the terminating margins of major channel deposits suggesting they are not related to the growth of alluvial ridges along active channels lithologic variations and patterns of sediment disruption within sequences appear to reflect the rapid tilling of low areas on the floodplain both adjacent to and distal from the major river channel this infilling of floodplain topography may reflect a continuous process whereby local areas were always being filled somewhere on the floodplain by sediments from minor tributary or crevasse channels alternatively the filling of local flood plain topography may have been caused by widespread but short lived events associated with river channel avulsion

a review of magmatism and deformation history along the ne asian margin from ca 95 to 30 ma transition from the izanagi to pacific plate subduction in the early cenozoic abstract previous plate reconstruction in the paleo pacific domain indicates that the izanagi and pacific plates influenced the ne asian continental margin during the mesozoic and cenozoic respectively however it remains unclear how and when the transition from izanagi to pacific plate subduction occurred because most evidence came from the oceanic crust however the onshore geological records are poorly understood so in order to solve this problem we review recently published data and identify three magmatic episodes a magma gap and four deformation stages from ca 95 to 30 ma margin parallel izanagi pacific ridge subduction is proposed to explain the changing magmatic and deformation signatures and other regional events in the early cenozoic from ca 95 to 65 ma the n or nnw directed subduction of the izanagi plate triggered sinistral shearing along the nne striking faults arc magmas at that time incorporated older crustal materials from ca 65 to 55 ma the nww striking transform faults and ridge segments were subducted developing small slab windows and facilitating the input of juvenile magmas local extension was active above these windows during ca 55 to 45 ma the ridge was totally subducted and a margin wide slab window was opened resulting in an arc magma gap and extension massive asthenosphere upwelling caused the extensive accretion of juvenile materials from ca 45 30 ma the dextral nne trending faults resulted from the w directed subduction of the pacific plate new arc magmas were generated from juvenile materials and thus are isotopically depleted

evidence for thin oceanic crust on the extinct aegir ridge norwegian basin ne atlantic derived from satellite gravity inversion 1 satellite gravity inversion incorporating a lithosphere thermal gravity correction has been used to map crustal thickness and lithosphere thinning factor for the ne atlantic predicted oceanic crustal thicknesses in the norwegian basin are between 4 and 7 km on the extinct aegir ridge increasing to 9 14 km at the margins consistent with volcanic margin continental breakup at the end of the paleocene the observation from gravity inversion and seismic refraction studies of thin oceanic crust produced by the aegir ridge in the oligocene may have implications for the temporal evolution of asthenosphere temperature under the ne atlantic during the tertiary thin oligocene oceanic crust may imply cool normal asthenosphere temperatures during the oligocene in contrast to elevated asthenosphere temperatures in the paleocene and miocene recent as indicated by the formation of volcanic margins and iceland respectively

breakup of the newfoundland iberia rift abstract the newfoundland iberia rift is considered to be a type example of a non volcanic rift key features of the conjugate margins are transition zones tzs that lie between clearly continental crust and presumed normal penrose type oceanic crust that appears up to 150 180 km farther seaward basement ridges drilled in the iberia tz consist of exhumed serpentinized peridotite of continental affinity consistent with seismic refraction studies although the boundaries between continental crust and the tzs can be defined with relative confidence there are major questions about the position and nature of the change from rifting to normal sea floor spreading at the seaward edges of the tzs notably drilling of presumed oceanic crust in the young m series anomalies to investigate and potentially resolve these conflicts we analysed the tectonic history and deep pre cenomanian stratigraphy of the rift using seismic reflection profiles and drilling results rifting occurred in two main phases late triassic earliest jurassic and late jurassic early cretaceous the first phase formed continental rift basins without significant thinning of continental crust the second phase led to continental breakup with extension concentrated in three episodes that culminated near the end of berriasian hauterivian and aptian time the first two episodes appear to correlate with separation of continental crust in the southern and northern parts of the rift respectively suggesting that the rift opened from south to north in a two step process the third episode persisted through barremian and aptian time we suggest that during this period there was continued exhumation of subcontinental mantle lithosphere at the plate boundary and that elevated in plane tensile stress throughout the rift caused intraplate extension primarily within the exhumed mantle this rifting may have been interrupted for a time during the barremian when melt was introduced from the southern edge of the rift by plume magmatism that formed the southeast newfoundland ridge and j anomaly ridge and the conjugate madeira tore rise we propose that the rising asthenosphere breached the subcontinental mantle lithosphere in latest aptian earliest albian time initiating sea floor spreading this resulted in relaxation of in plane tensile stress i e a pulse of relative compression that caused internal plate deformation and enhanced mass wasting this aptian event produced a strong rift wide reflection that is unconformably onlapped by post rift sediments that were deposited as a stable sea floor spreading regime was established although previously considered to be a breakup unconformity associated with separation of continental crust the event instead marks the final separation of the subcontinental mantle lithosphere our analysis indicates that interpretation of tectonic events in a non volcanic rift must consider the rheology of the full thickness of the continental lithosphere in addition to spatial and temporal changes in extension that may occur from segment to segment along the rift

geochemistry geochronology and tectonic setting of early cretaceous volcanic rocks in the northern segment of the tan lu fault region northeast china abstract we present new geochemical and geochronological data for volcanic and related rocks in the regions of the jia yi and dun mi faults in order to constrain the late mesozoic tectonic evolution of the northern segment of the tan lu fault zircon u pb dating shows that rhyolite and intermediate mafic rocks along the southern part of the jia yi fault formed at 124 and 113 ma respectively whereas the volcanic rocks along the northern parts of the jia yi and dun mi faults formed at 100 ma the rhyolite has an a type granitoid affinity with high alkalis low mgo ti and p contents high rare earth element ree contents and ga al ratios enrichments in large ion lithophile liles e g rb th and u and high field strength element hfses e g nb ta zr and y and marked negative eu anomalies these features indicate that the rhyolites were derived from partial melting of crustal material in an extensional environment the basaltic rocks are enriched in light rees and liles e g rb k th and u and depleted in heavy rees hfses e g nb ta ti and p and sr these geochemical characteristics indicate that these rocks are calc alkaline basalts that formed in an intraplate extensional tectonic setting the dacite is a medium to high k calc alkaline i type granite that was derived from a mixed source involving both crustal and mantle components in a magmatic arc therefore the volcanic rocks along the jia yi and dun mi faults were formed in an extensional regime at 124 100 ma early cretaceous and these faults were extensional strike slip faults at this time

evidence for the carnian pluvial episode in gondwana new multiproxy climate records and their bearing on early dinosaur diversification abstract the triassic hothouse world experienced dynamic changes in climate and ecosystems that set the earth s climate and biotic states for much of the mesozoic era a critical time interval was the carnian stage 237 227 ma which not only saw the first appearance of dinosaurs in the fossil record but witnessed a large igneous province eruption wrangellia lip and a sudden climate shift the carnian pluvial episode cpe compelling hypotheses suggest that the wrangellia eruptions caused the cpe which in turn set the stage for the origin and initial diversification of dinosaurs unfortunately testing these hypotheses is difficult because most relevant carnian sedimentary archives possess few if any absolute age constraints and there are little detailed paleoclimatic data for the cpe outside of the tethys region there is little existing evidence for the cpe in gondwana even though this region contains by far the best fossil record of early dinosaurs and their close relatives we present new high resolution geochronologic and paleoclimatic data from carnian aged strata in the ischigualasto villa union basin of northwest argentina which also preserves fossils of the earliest known dinosaurs new ca tims u pb zircon age of 234 47 0 44 ma from the lower los rastros formation demonstrates that most of the overlying lacustrine strata within this basin was deposited during or after the cpe including large dinosauriform footprints multiproxy paleoenvironmental data sedimentology clay mineralogy c and o stable isotopes and fossils from the same strata are the first detailed paleoclimate data for the cpe in gondwana and provide evidence that the cpe interval in western gondwana was indeed warmer and more humid than before or after these data are consistent with the interpretation of the cpe as a global event but direct linkages in gondwana and the tethys with the origin and initial diversification of dinosaurs are less clear

new shrimp u pb zircon ages from the heilongjiang high pressure belt constraints on the mesozoic evolution of ne china the heilongjiang complex is a high pressure accretionary belt composed mainly of serpentinite glaucophane bearing metabasalt epidote glaucophane albite schist greenschist marble two mica schist muscovite albite schist quartz schist and quartzite chert located along the suture zone separating the jiamusi and songliao blocks in ne china shrimp u pb zircon analyses of two samples of granitic gneiss from yilan that are interleaved with the heilongjiang complex record diverse protolith ages that are consistent with derivation from regional permian granitoids 263 7 ma and the adjacent mashan complex 492 6 ma indicating tectonic incorporation of older granitic components into the accretionary complex the age of the basaltic protolith of two samples of glaucophane albite schist from yilan is 245 ma and is slightly younger than the protolith age of two samples of amphibole schist from mudanjiang located about 200 km south of yilan that define weighted mean 206pb 238u ages of 257 4 and 257 5 ma all four samples contain a few older inherited zircon grains and also show the effects of an isotopic disturbance at 200 to 190 ma most likely related to the subsequent high pressure metamorphism the timing of which has been determined in previous studies the basaltic rocks record rifting and ocean development from 260 to 210 ma based on previous studies in which both e morb and oib protolith signatures were identified detrital zircons in two samples of mica schist from mudanjiang show distinct populations at 700 ma 500 to 450 ma 350 to 213 ma peak at 255 ma 290 to 200 ma and 200 ma all except the youngest ages can be matched with rocks from the older components of the jiamusi block and with other areas in the central asia orogenic belt caob supporting the view that the jiamusi block was most likely the eastern segment of the caob prior to rifting the youngest concordant magmatic detrital grain with an age of 207 3 ma defines the maximum age of sedimentation this also limits the timing of high pressure metamorphism in the heilongjiang complex to post late triassic consistent with argon data obtained in previous studies final closure between the jiamusi and songliao blocks took place in the latest triassic early jurassic with the two blocks accreted as a result of pacific ocean subduction this suggests that the heilongjiang complex records the time when north south closure of the caob in china gave way to westward movement related to the pacific ocean subduction which has dominated the tectonics of ne china and far east russia since that time

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

the relationship between depth age and gravity in the oceans we reassess the applicability of the thermal plate cooling model to the subsidence of the north pacific atlantic and north indian ocean basins we use a new numerical plate model in which the thermophysical parameters of the lithosphere vary with temperature according to the results of laboratory experiments and the ridge temperature structure is consistent with the thickness of the oceanic crust we first attempt to exclude thickened crust from our data set and then to exclude swells and downwellings by masking regions of the data that remains that have significant gravity anomalies when there exists a clear regional correlation between intermediate wavelength gravity and topography we find that the average variation of depth with age is consistent with conventional half space models until about 90 myr thereafter the departure from the half space cooling curve is more rapid than predicted using simple conductive plate cooling models the depth age curves in the pacific and atlantic show 250 m of temporary shallowing between the ages of 90 130 myr a result consistent with the outcome of experiments on the initiation of small scale boundary layer convection the results do not change significantly if the estimated component of the gravity arising from plate cooling is subtracted prior to calculation of the correlation between gravity and topography a 90 km thick conductive plate is nevertheless a reasonable model for the average temperature structure of the oldest part of the pacific ocean lithosphere in the pacific the broad topographic undulations associated with the line island swell the hawaiian swell and surrounding basins have correlated gravity anomalies and an admittance of approximately 30 mgal km 1 and are likely to result from convective circulation in the upper mantle in the northeast atlantic the intermediate wavelength admittance over the cape verde swell is similar in the northwest atlantic over the bermuda swell it is slightly larger but not as well constrained

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

structure of the se greenland margin from seismic reflection and refraction data implications for nascent spreading center subsidence and asymmetric crustal accretion during north atlantic opening 1 seismic reflection and refraction data from the se greenland margin provide a detailed view of a volcanic rifted margin from archean continental crust to near to average oceanic crust over a spatial scale of 400 km the sigma iii transect located 600 km south of the greenland iceland ridge and the presumed track of the iceland hot spot shows that the continent ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup initially 18 3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8 10 km thick crust changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system which was close to sea level for at least 1 m y of subaerial spreading despite a reduction in productivity from 17 to 13 5 km thick crust over this time interval this could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system the thermal anomaly must be exhaustible consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial to shallow marine and finally to deep marine extrusion in 2 m y is shown by the morphological changes in addition comparison to the conjugate hatton bank shows a clear asymmetry in the early accretion history of north atlantic oceanic crust nearly double the volume of material was emplaced on the greenland margin compared to hatton bank and may indicate east directed ridge migration during initial opening

effect of thermal contrasts on gravity modeling at passive margins results from the western barents sea the western barents sea passive margin is a key locality to demonstrate the effect of the thermal structure of the lithosphere on forward gravity modeling this margin developed by shear motion between the eurasian and greenland plates during the early tertiary and it is a significant border zone between young hot oceanic lithosphere and cooler continental lithosphere we construct two dimensional gravity models of 125 km thick lithosphere based on expansion of mantle rocks determined from thermal modeling the approach has a substantial impact over traditional shallow gravity models here demonstrated on a previously published model on the basis of a 140 mgal free air anomaly the old model proposes an anomalous high density oceanic crust emplaced in a leaky transform adjacent to the continent during early margin development however the lithospheric models predict a homogeneous oceanic crust while preserving regional isostasy at base lithosphere from continent to ocean two further tests agree with this conclusion a map of bouguer corrected ers 1 satellite data reveals no residual anomalies originating from the oceanic crust at the margin admittance analysis shows a strong oceanic lithosphere and the high coherence between bathymetry and free air gravity discounts a significant subsurface load the high gravity anomalies at the margin are thus an edge effect enhanced by sedimentation onto the strong oceanic lithosphere and shaped by the effect of the lithospheric thermal field other results of this work include a new continent ocean boundary map and two crustal transects across the margin

geochronological framework of the early paleozoic bainaimiao cu mo au deposit ne china and its tectonic implications abstract the bainaimiao cu mo au deposit of ne china is an important ore deposit in the middle section of the northern margin of the north china craton the early paleozoic bainaimiao group is the main ore hosting rock the mineralization at the deposit shows features of porphyry alteration and late stage orogenesis and transformation zircon la icp ms u pb age data indicate that the ages of the third and fifth formations of the bainaimiao group are 492 7 2 9 ma mswd 0 53 and 488 9 3 1 ma mswd 0 92 respectively the age of quartz diorite that intrudes the bainaimiao group is 459 3 6 4 ma mswd 2 20 molybdenite samples from massive cu mo bearing ores and quartz veins in the southern ore belt yield a re os isochron age of 438 2 2 7 ma mswd 0 16 which is consistent with the re os isochron age of molybdenite in the northern ore belt implying that the two ore belts belong to the same mineralization system muscovite from a post magmatic cu mo bearing quartz calcite vein yields an ar ar isochron age of 422 5 3 9 ma mswd 0 64 with an initial 40ar 36ar ratio of 286 21 the well defined plateau age of the muscovite is 422 4 2 6 ma mswd 0 05 which represents the time of the post magmatic orogenic transformation event based on our new age data and previous findings we propose that the bainaimiao cu mo au deposit formed in an active continental margin setting and experienced four stages of ore mineralization 1 a late cambrian middle ordovician volcanic sedimentary stage 2 a late ordovician porphyry mineralization stage 3 a late silurian regional metamorphism stage and 4 an orogenic transformation stage subhedral and euhedral paleoproterozoic 2402 1810 ma inherited zircons indicate that the bainaimiao group has a tectonic affinity with the north china craton the central asian orogenic belt which is closely related to the complex closure of the paleo asian ocean is favorable for prospecting for paleozoic porphyry cu mo or cu au mineralization and associated epithermal au deposits

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

geochemistry and geochronology of the late permian mafic intrusions along the boundary area of jiamusi and songnen zhangguangcai range massifs and adjacent regions northeastern china petrogenesis and implications for the tectonic evolution of the mudanjiang ocean abstract this paper presents zircon u pb ages whole rock major and trace element data and hf isotope data for the metagabbros from the zhushan pluton and gabbros from the taiping pluton along the boundary area of jiamusi and songnen zhangguangcai range massifs and adjacent regions which will not only place important constraints on the rock forming ages source characteristics and tectonic setting of these gabbros but will also provide insights into understanding the permian tectonic evolution between the jiamusi massif and the songnen zhangguangcai range massif zircon u pb dating determined using laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectrometry indicates that the magmatic zircons from the zhushan and taiping plutons yield 206pb 238u ages of 256 2 ma and 259 3 ma respectively interpreted as the emplacement ages of the intrusions the metagabbros from the zhushan pluton display the geochemical characteristics of calc alkaline series rocks and are enriched in light rare earth and large ion lithophile elements and depleted in nb ta p zr and hf the ehf t values of magmatic zircons in these metagabbros vary from 5 47 to 0 74 all these geochemical features indicate that the primary magma of the zhushan pluton was derived from an enriched lithospheric mantle source that was metasomatized by subducted slab derived fluids the gabbros from the taiping pluton are also enriched in large ion lithophile elements e g rb ba and u relative to high field strength elements and have negative nb ta p anomalies with ehf t values of 4 02 to 1 70 it is inferred that they also formed from a primary magma generated by the partial melting of enriched lithospheric mantle that was metasomatized by subducted slab derived fluids the rocks from the zhushan and taiping plutons have similar petrogenetic processes but their primary magmas are likely to be derived from two distinct magma sources based on geochemical and isotopic compositions according to zircon u pb ages geochemical data hf isotope data and regional geological investigations we conclude that the generation of the metagabbros in the zhushan pluton was most likely related to westward subduction of the mudanjiang oceanic plate beneath the songnen zhangguangcai range massif and the gabbros from the taiping pluton were the result of eastward subduction of the mudanjiang oceanic plate beneath the jiamusi massif hence we suggest that the jiamusi massif and songnen zhangguangcai range massif were not amalgamated before the late permian

hydrothermal evolution and ore genesis of the laozuoshan au skarn deposit northeast china constrains from mineralogy fluid inclusion and o c s pb isotope geochemistry abstract the laozuoshan mine located in the central jiamusi block is the largest gold skarn deposit in northeast china it comprises three ore zones east middle west with complex stratigraphic and structural controls the skarn type orebodies mainly occur in the east and middle ore zones along the contact zones between hercynian granite and precambrian metasedimentary rocks the earliest stage of alteration is characterized by potassium hornfels subsequently prograde skarn formation occurred characterized by garnet in the grossular andradite series and pyroxene in the hedenbergite diopside series this was followed by a retrograde skarn overprint of hydrous silicates and ore minerals three types of garnet with distinct textures and elemental compositions are recognized which indicate fluctuating physicochemical conditions in the hydrothermal fluids the fluids associated to the prograde skarn are characterized by high temperature and high salinity which underwent fluid boiling that led to precipitation of minor pyrite and arsenopyrite the main gold and sulfide mineralization is related to the retrograde skarn overprint where cooling and strong fluid rock interactions occurred gold precipitation is estimated to have occurred at temperatures of 380 470 c based on the arsenopyrite geothermometer and fluid inclusion microthermometry the fluid inclusion and c o isotope data indicate that the ore forming fluids were dominated by magmatic water during the prograde skarn stage and the main ore forming stages mixing between meteoric and magmatic fluids occurred during the latest stages of the retrograde skarn formation both sulfur and lead isotope compositions of sulfides support a magmatic origin for the ore forming materials on the basis of the available geological and geochemical evidence we propose that the laozuoshan deposit is a reduced gold skarn deposit genetically related to the granitic pluton

hematite and calcite coatings on fossil vertebrates abstract hematite coatings are common on vertebrate fossils from paleocene eocene paleosol deposits in the bighorn basin wyoming in general hematite coatings are found only on fossils and are limited to soils exhibiting hydromorphic features and moderate maturity petrographic and isotopic evidence suggests that hematite and micritic calcite formed at nearly the same time in a pedogenic environment whereas sparry calcite formed later at greater burial depths the parent material of paleosols is rich in iron supplying an ample source of iron for hematite formation decomposition of animal tissues around bones may enhance the weathering of iron bearing minerals in soils surrounding carcasses while the bones might provide favorable sites for iron accumulation the predominance of discrete smectite together with regional geothermal history suggests that burial temperatures have not exceeded 70 c hematite coatings on fossils can serve as a substrate for geochemical analysis in continental paleoclimatic research owing to their pedogenic origin abundance and resistance to diagenetic alteration

principles of regression and transgression the nature of the interplay between accommodation and sediment supply the rate of change of accommodation at the shoreline a and the rate of sediment supply s are the primary factors controlling regression and transgression in the geological record or retreat and advance of a coastal depositional system the generally accepted notion here referred to as the a s ratio concept stating that the migration of a shoreline is controlled principally by the magnitude of the ratio of a to s is insufficient and somewhat misleading partly owing to the problem of dimensional confusion the interplay of a and s inevitably results in autoretreat of the shoreline whereby the seaward advance of any shoreline is halted and is subsequently turned to landward retreat provided there is a continuous rise of relative sea level how effective the autoretreat process is depends for a given period of relative sea level rising upon the length of potential time period for the seaward advance of shoreline and is proportional to a 2 s and a super 1 5 s super 0 5 in two and three dimensional sediment dispersal modes respectively autoretreat becomes more effective as a increases and or s decreases a functions more critically to the effectiveness than s does the a s ratio concept is only approximately applicable when autoretreat is less effective i e with very low a and or very high s in this sense the autoretreat theory and the a s ratio concept are complementary to each other the autoretreat mechanism is expected to be substantially useful not only in fluvial deltas but also in many other depositional systems e g barrier islands whose evolution has been considered in terms of the a s ratio concept

geochronology and geochemistry of the heilongjiang complex and the granitoids from the lesser xing an zhangguangcai range implications for the late paleozoic mesozoic tectonics of eastern ne china abstract an integrated study on two subduction related units the heilongjiang complex and magmatic rocks in the lesser xing an zhangguangcai range was carried out to explore the paleozoic to early mesozoic tectonic framework of the eastern northeast china with a focus on the jiamusi block the heilongjiang complex which is considered as the suture between the jiamusi and songliao blocks is composed of blueschist amphibolite and ultramafic rocks the blueschist from the yilan area shows geochemical features similar to oib and has a 206pb 238u age of 288 2 ma which is interpreted as the crystallization age of protolith formed in an ocean island setting in contrast amphibolite from the xiachengzi area shows geochemical features similar to n morb zircons in this amphibolite show magmatic character and yield a 206pb 238u age of 248 4 ma which is interpreted as the protolith age calcic amphiboles in the amphibolite give a 40ar 39ar plateau age of 195 3 ma which is interpreted as the metamorphic age of the heilongjiang complex magmatic rocks in the lesser xing an zhangguangcai range mostly belong to i type granites that were formed in an active continental margin setting zircon u pb ages of these granites range from 200 ma to 174 ma with a westward younging trend shown as two groups of emplacing ages of 200 ma in the east and 180 ma in the west our new data support an early occurrence likely since the early permian of a large ocean namely the mudanjiang ocean in the area whose subduction resulted in the metamorphic unit the heilongjiang complex and the magmatic unit the lesser xing an zhangguangcai range in the jurassic

opening and closure history of the mudanjiang ocean in the eastern central asian orogenic belt geochronological and geochemical constraints from early mesozoic intrusive rocks abstract the tectonic evolution of the ancient mudanjiang ocean within the central asian orogenic belt caob is strongly debated the ocean played an important role in the amalgamation of the songnen and jiamusi massifs however the timings of its opening and closure have remained ambiguous until now in this study we analyzed early mesozoic intrusive rocks from the eastern songnen and western jiamusi massifs in the eastern caob the new zircon u pb ages hf isotope data and whole rock major and trace element data are used to reconstruct the tectonic evolution of the mudanjiang ocean zircon u pb dating indicates that early mesozoic magmatism in the eastern songnen massif occurred in three stages early to middle triassic ca 250 ma late triassic ca 211 ma and early jurassic ca 190 ma the triassic intrusive rocks typically consist of bimodal rock suites which include gabbros hornblende gabbros and granitoids the compositional information indicates an extensional environment that was probably related to the final closure of the paleo asian ocean we integrated the results with observations from triassic a type granitoids and coeval sedimentary formations in the eastern songnen massif as well as depositional ages of metasedimentary rocks from heilongjiang complex we conclude that the opening of the mudanjiang ocean took place in the early to middle triassic the early jurassic intrusive rocks are bimodal and include olivine gabbros hornblendites hornblende gabbros gabbro diorites and granitoids the bimodal rock suite indicates a back arc style extensional environment this setting formed in relation to westward subduction of the paleo pacific plate beneath the eurasia during the early jurassic following subduction the closure of the mudanjiang ocean and subsequent amalgamation of the songnen and jiamusi massifs happened during the late early jurassic to middle jurassic this sequence of events is further supported by ages of metamorphism and deformation acquired from the heilongjiang complex based on these observations we conclude that the mudanjiang ocean existed between the middle triassic and early jurassic making it rather short lived

the relationship between gravity and bathymetry in the pacific ocean surface ship and satellite derived data have been compiled in new free air gravity anomaly bathymetry and geoid anomaly maps of the pacific ocean basin and its margin the maps are based on smoothed values of the gravity anomaly bathymetry and geoid interpolated on to a 90 x 90km grid the smoothed values of bathymetry and geoid anomaly have been corrected for age the resulting maps show the pacific ocean basin is associated with a number of ene wsw trending geoid anomaly highs with amplitudes of approx or 5m and wavelengths of approx 3000km the most prominent of these highs correlate with the magellan seamounts marshall gilbert islands magellan rise and the hess rise hawaiian ridge regions the correlation between geoid anomaly and bathymetry cannot be explained by models of static compensation but is consistent with a model in which the geoid anomaly and bathymetry are supported by some form of dynamic compensation suggests that the dynamic compensation which characterizes ocean lithosphere older than 80ma is the result of mantle convection on scales that are smaller than the lithospheric plates themselves from authors

permian tectonic evolution of the mudanjiang ocean evidence from zircon u pb hf isotopes and geochemistry of a n s trending granitoid belt in the jiamusi massif ne china abstract a combined study of la icp ms zircon u pb dating and geochemical analyses major and trace elements and hf isotopic compositions for five permian granitic plutons mingyi tuoyaozi mengjiagang hengtoushan and qingbei plutons from the jiamusi massif was carried out to determine their ages petrogenesis and tectonic evolution the studied granitic plutons are composed of syengranites monzogranites and granodiorites and they were emplaced in the early middle permian 278 263 ma these granitic plutons are mostly high k calc alkaline and weakly peraluminous and show consistent correlations of different oxides versus sio 2 they are all enriched in large ion lithophile elements e g rb th k and light rare earth elements and depleted in high field strength elements e g nb ta ti and heavy rare earth elements and they have relatively homogeneous hf isotopic compositions with ehf t values varying from 6 16 to 2 95 and two stage model ages ranging from 1681 to 1111 ma according to their emplacement ages geochemical characteristics and hf isotopic compositions we conclude that these granitoids might be originated from parental magmas with similar compositions but evolved different degrees of fractionation and their magmas were derived from the partial melting of amphibolite facies mafic lower crust these data combined with previous studies on contemporaneous magma tectonic activities in the jiamusi massif and songnen zhangguangcai range massif indicate that two paralleled n s trending permian magmatic belts are distributed in these two massifs the eastwards subduction of the mudanjiang oceanic plate beneath the jiamusi massif induced crustal melting to produce the studied permian n s trending granitoids in the jiamusi massif furthermore westwards subduction of the mudanjiang oceanic plate beneath the songnen zhangguangcai range massif gave rise to permian magmatism along eastern margin of the songnen zhangguangcai range massif taken together we suggest that the jiamusi massif and songnen zhangguangcai range massif were not collided before the permian and a double side subduction model is favored for the tectonic evolution of the mudanjiang ocean during the permian

fluvial architecture and paleoenvironmental evolution of the los colorados formation norian postrift stage of the ischigualasto villa union basin nw argentina

early jurassic mafic magmatism in the lesser xing an zhangguangcai range ne china and its tectonic implications constraints from zircon u pb chronology and geochemistry abstract la icp ms zircon u pb dating and geochemical data have been obtained from five representative mafic ultramafic intrusions in the lesser xing an zhangguangcai range ne china with the aim of improving our understanding of the mesozoic tectonic evolution in the region and in particular determining the time of initiation of the circum pacific tectonic system the selected zircons exhibit striped absorption in cathodoluminescence cl images and have high th u ratios 0 20 3 16 indicating a magmatic origin the zircon u pb dates indicate that most of these magmatic zircons other than a few relics that were captured and entrained in the magma formed in the late early jurassic 186 182 ma and not as previously supposed in the middle caledonian the five mafic ultramafic plutons are composed of olivine gabbro hornblendite gabbro hornblende gabbro and gabbro diorite the olivine gabbro and hornblendite display cumulate textures implying that fractional crystallization of olivine and plagioclase took place during magma evolution these mafic ultramafic igneous rocks have sio2 37 3 55 7 mgo 3 05 13 3 al2o3 11 8 23 8 mg 42 69 mg 100mg mg fe2 total and eu 0 88 1 32 and they display three types of rare earth element ree distribution patterns right slipped flat patterns or dome like the trace element spider diagrams show that the rocks are enriched in large ion lithophile elements liles such as ba k and sr and depleted in high field strength elements hfses such as nb ta zr and hf the zircons have ehf 186 182 ma 2 7 to 12 0 and tdm1 366 732 ma the geochemical data indicate that the early jurassic mafic magma originated in an extensional environment from the partial melting of a depleted mantle wedge that had been metasomatized by fluids released from a fossil subducted slab these data combined with information on the spatial variation of coeval igneous rocks indicate that the formation of the early jurassic mafic ultramafic rocks in the lesser xing an zhangguangcai range was related to the subduction of the paleo pacific plate beneath the eurasian continent and this event would mark the beginning of the circum pacific tectonic system

a process oriented approach to modeling the gravity signature of continental margins the gravity anomaly at a continental margin can be regarded as the result of all the processes that have shaped it through time these include rifting sedimentation and magmatic underplating this article discusses a new process oriented approach to gravity interpretation which uses seismic constraints on sediment thickness and the underplate at continental margins to estimate the gravity anomaly that is associated with rifting sedimentation and magmatic underplating by comparing the calculations to the observed free air gravity edge effect anomaly we have been able to determine the elastic thickness and crustal structure and to provide an estimate of the location of the ocean continent transition since crustal structure is a proxy for the amount of heating that occurred at the time of rifting the approach has important implications for the deep water hydrocarbons industry

insight into the nature of the ocean continent transition off west iberia from a deep multichannel seismic reflection profile a high quality multichannel seismic reflection profile across the west iberia margin confirms the existence of a 130 km wide transition zone between tilted continental fault blocks and unequivocal oceanic crust the zone is characterized by a 1 0 2 5 km thick seismically unreflective upper basement layer lying over reflective basement that contains major landward dipping reflectors that mostly truncate against the base of the upper layer and penetrate to depths of 10 19 km below the basement surface and minor seaward dipping reflectors moho reflections are absent in this zone we suggest that the unreflective layer represents tectonically exposed upper mantle that has undergone extensive serpentinization enhanced by vigorous seawater circulation about 40 km farther north the transition zone narrows to 35 km and the unreflective layer is less continuous and widespread

palynology of the late triassic ischigualasto formation argentina paleoecological and paleogeographic implications in this study we describe and compare the palynological assemblages from the carnian ischigualasto formation of westernmost gondwana to those of eastern gondwana which have been referred to the onslow and ipswich phytogeographic provinces the palynofloras studied contain many taxa common to both regions as well as several species that are widely recorded in european carnian successions and less frequently recovered in the carnian of the southern hemisphere for the first time in argentinean assemblages we recognize anapiculatisporites spiniger cadargasporites granulatus cycadopites stonei ellipsovelatisporites plicatus enzonalasporites vigens ovalipollis pseudoalatus o ovalis patinasporites densus quadraeculina anellaeformis samaropollenites speciosus staurosaccites quadrifidus rimaesporites aquilonalis vallasporites ignacii and zonalasporites cinctus the new evidence extends the distribution of the onslow palynoflora to include carnian associations formerly assigned to the ipswich province the affinities of argentinean palynofloras seem to indicate the existence of a similar flora extending from the northern australian margin west timor through the western tethys coasts to westernmost gondwana during the carnian moreover the new data reinforce the close relationship between some diagnostic species and humid environmental conditions

evolution of a permian intraoceanic arc trench system in the solonker suture zone central asian orogenic belt china and mongolia abstract the identification of a fossil arc trench system from the ophiolite decorated solonker suture zone in the southernmost central asian orogenic belt caob enables us to constrain the timing of pre subduction extension ca 299 290 ma subduction initiation ca 294 280 ma ridge trench collision ca 281 273 ma and slab break off ca 255 248 ma in the permian a fraction of proto arc crust ca 45 km long up to 8 km wide is preserved as a volcanic plutonic sequence and is juxtaposed against a wide ca 30 80 km forearc melange this proto arc crust comprises two distinct magma series island arc tholeiite iat and mid ocean ridge basalt morb both of which have strong supra subduction zone ssz geochemical signatures zircons from a gabbro and a plagiogranite yielded weighted mean 206pb 238u ages of 284 0 4 0 and 288 0 6 0 ma the forearc melange consists of numerous ophiolite fragments and continental margin derived olistoliths blocks that predate the ophiolite the olistoliths are best represented by a gabbroic block 291 8 2 3 ma that contains granite xenoliths 312 6 1 8 and 313 6 3 1 ma other dated blocks include a trondhjemite 323 9 2 7 ma a gabbro 296 6 1 7 ma and a tonalite 294 9 2 4 ma small bodies of diabase andesite and diorite in the forearc melange exhibit a wide variety of geochemical signatures we dated zircons from an n morb like diabase 274 4 2 5 ma an e morb like diabase 252 5 2 3 ma a transitional sanukitoid adakite andesite 250 2 2 4 ma a sanukitoid high mg diorite 251 8 1 1 ma and an anorthosite 252 2 1 7 ma the n morb like diabase contains ca 301 394 ma zircon xenocrysts suggesting assimilation of trench sediments when a spreading ridge intersected a trench the other dated rocks simultaneously formed near the permian triassic boundary and captured abundant zircon xenocrysts ca 269 295 ma ca 301 495 ma and ca 923 2501 ma our new formation ages constrain a magmatic episode in response to slab break off beneath a fossil forearc in a young post collisional setting and the youngest xenocryst ages ca 269 273 ma may define the maximum depositional age of trench sediments

changing lake dynamics and sequence stratigraphy of synrift lacustrine strata in a half graben an example from the triassic ischigualasto villa union basin argentina well exposed triassic rift strata from the ischigualasto villa union basin nw argentina include a 80 to ca 515 m thick lacustrine dominated package that can be correlated across a half graben using key stratigraphic surfaces sequence boundaries lacustrine flooding surfaces and forced regressive surfaces the characteristics of the synrift lacustrine fill in different parts of the half graben have been examined and the mechanisms controlling sedimentation inferred a variety of sedimentary environments are recognized including volcaniclastic floodplain mildly saline lake and playa lake offshore lacustrine delta front to fluvial dominated and wave dominated deltas distributary and fluvial channel and interdistributary bay the succession can be divided into four stratigraphic sequences ss1 to ss4 the oldest of which ss1 contains volcaniclastic fluvial and saline lake deposits it is thickest close to the western border fault zone reflecting more rapid subsidence here accommodation exceeded sediment and water input during ss1 the second and third sequences ss2 and ss3 mark the onset of widespread lacustrine sedimentation reflecting a balance between accommodation creation and water and sediment fluxes sequences ss2 and ss3 are represented by offshore meromictic lacustrine and deltaic deposits the latter mostly sourced from the flexural and southern axial margins of the half graben the presence of stacked parasequences bound by lacustrine flooding surfaces is related to climatically induced lake level fluctuations superimposed on variable rates of subsidence on the controlling rift border fault zone the youngest sequence ss4 is represented by the deposits of littoral lacustrine and shallow shelf deltas distinguished by a change in lithofacies palaeocurrents and sandstone composition suggesting a switch in sediment supply to the footwall margin to the nw the change in the sediment source is related to reduced footwall uplift the possible presence of a relay ramp and or supply from a captured antecedent drainage network during ss4 the rate of creation of accommodation was exceeded by the sediment and water discharge the stratigraphic evolution of lacustrine strata in the half graben was mainly controlled by tectonic processes including subsidence rate and the growth and evolution of the border fault zone but changing climate inducing changes in water balance and lake level and autocyclic processes delta lobe switching were also important

geochemistry and geochronology of the blueschist in the heilongjiang complex and its implications in the late paleozoic tectonics of eastern ne china abstract the paleozoic to early mesozoic tectonic framework and evolution of northeast china especially the jiamusi block and its related structural belts are highly debated in this paper geochemical geochronological and isotopic analyses were carried out on the blueschist in the heilongjiang complex to address these issues the heilongjiang complex defines the suture belt between the jiamusi block and the songliao block in ne china and the blueschist is a major composition for this complex coexisting with mafic ultramafic rocks greenschist quartzite and mica schist the blueschist has a mineral association of sodic amphibole epidote chlorite phengite albite and quartz with accessory phases of apatite titanite zircon and ilmenite together with the lithological association the major and trace element compositions present that the protoliths of the blueschist can be divided into the alkaline and tholeiitic basalts and have oib affinities formed in an ocean island setting indicated by the la yb n values of 3 57 11 54 and the la sm n values of 0 69 3 64 the high and positive e nd t values of 3 7 to 9 0 and relative enrichment in nb vs th and ta vs u show that both the alkaline and tholeiitic basalts may be derived from the asthenospheric mantle with insignificant crustal contamination magmatic zircons from the blueschist in yilan area yield a 206 pb 238 u age of 281 3 ma interpreted as its protolithic age the youngest ages of 200 ma of the detrital zircons in the associated mica schist from mudanjiang area place constraints on the timing of metamorphism for the blueschist these indicate that a big ocean existed between the jiamusi and songliao blocks at least since the early permian and the blueschist formed since the late triassic to late jurassic by the subduction of this ocean such an ocean during the permian jurassic is difficult to be interpreted by the tectonic evolution of the paleo asian ocean

sequence model for coastal plain depositional systems of the upper triassic betic cordillera southern spain abstract the deposits of the upper triassic of the subbetic zone betic cordillera offer an example of semi arid fluvial deposits passing into shallow water facies through a mud flat environment from the study of the vertical changes in facies a sequence model is proposed for the upper triassic succession relating for each systems tract the changes in sea level the accommodation space and the evolution of the facies within a coastal plain depositional system the lowstand depositional system is made up of a package of thick amalgamated sandstone strata limited below by an erosive surface with little incision these are sheet flood deposits and channel fills which form part of an extensive alluvial system in which wide but shallow water courses appear with the development of extensive sand bars these sandbodies from the lowstand phase often contain mudrock intraclasts plant remains and erosive surfaces the accommodation space being extremely limited the preservation of the floodplain or mud flat deposits and palaeosols is practically nil the result is an amalgamation of extensive sandbodies with fine intercalated claystone levels the transgressive depositional systems consist mainly of a red claystone series although the first sediments of these systems are composed of sandstone with major claystone intercalations these lower sandy levels of the transgressive phase do not exceed 50 cm in thickness and usually contain burrows and small scale sedimentary structures in the intermediate part of the red claystone series of this transgressive episode of saline mud flat facies there are sandstone levels of a little developed fluvial system with channels which migrate laterally due to the rise of the base level in addition the predominance and the great development of the red claystone facies reflect the increase in the accommodation space which permitted a greater accumulation of sediments during a transgressive phase than during the lowstand phase in the upper part of the transgressive depositional systems beds of calcrete and carniolar limestone are common indicating a decreased sedimentation rate the generation of accommodation space begins to slow down leading to the enlargement of sandbodies corresponding to terminal fan deposits friend 1978 these developed over a flat topography where the flow scattered and expanded laterally in the highstand phase the accommodation space reduced even more and the sedimentation rate is lower favouring a great development of pedogenic carbonates furthermore sand deposition was reduced or nil and the gradient of the slope was low so that marine floods over the coastal plain were more frequent permitting the precipitation of carbonates and sulphates in coastal salt pans

accretion subduction erosion and tectonic extrusion during late paleozoic to mesozoic orogenesis in ne china abstract the high pressure heilongjiang complex records a long and complicated evolution from the late paleozoic to mesozoic in ne china which is still subject to controversy in this paper we present a new detailed study including structural geochemical and geochronological analyses ar ar dating on metamorphic minerals u pb sims dating on zircons two main periods of oceanward accretionary growth are identified between 305 ma and 195 ma and around 112 101 ma these periods of accretionary growth were associated with the emplacement of voluminous magmatism slab roll back with continental back arc extension and growth of a forearc accretionary prism between these periods of accretionary growth a major phase of regional subduction erosion took place between 195 ma and 142 ma at the eastern edge of the songliao block the main effects of this subduction erosion phase were that regional jurassic early cretaceous extrusion overlapped with mesozoic accretion and erosion in ne china which disturbed the initial orogenic architecture with a review of relevant zircon ages of this accretionary orogen we unravel the complicated geodynamic history of accretion subduction erosion and tectonic extrusion that shaped ne china from the late paleozoic to mesozoic

continental lithospheric thinning and breakup in response to upwelling divergent mantle flow application to the woodlark newfoundland and iberia margins abstract depth uniform stretching is not the dominant deformation process for thinning continental lithosphere leading to breakup it cannot explain the observed depth dependent lithosphere stretching and mantle exhumation at rifted continental margins depth dependent lithosphere thinning in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust has been observed at many non volcanic and volcanic rifted continental margins including conjugate margin pairs passive continental margins show a paucity of brittle deformation in the upper crust during continental lithosphere thinning leading to breakup and sea floor spreading initiation a new model of rifted continental margin formation has been developed that assumes that deformation and thinning of continental lithosphere leading to breakup occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere and that this deformation evolves into sea floor spreading the new model successfully predicts depth dependent stretching of continental margin lithosphere for both non volcanic and volcanic margins and mantle exhumation at non volcanic margins both of which are observed but are not explained by existing depth uniform continental lithosphere stretching models the new model provides a balance of extensional strain supplies an explanation for the paucity of synrift brittle deformation and offers a simple transition from prebreakup lithosphere thinning to sea floor spreading the observed diversity of rifted continental margin structure and width of the ocean continent transition can be explained by variability in the form of the upwelling divergent flow field the new upwelling divergent flow model of continental lithosphere thinning leading to continental breakup successfully predicts the observed bathymetry and margin geometry for the most recent segment of sea floor spreading initiation in the woodlark basin in the western pacific and the observed bathymetry and free air gravity anomaly for the newfoundland and iberian margins

palaeosols as indicators of geomorphic stability in two old red sandstone alluvial suites south wales palaeosols fossil soils are abundant in the alluvial late silurian early devonian moor cliffs formation and rat island mudstone freshwater west formation of south dyfed south wales the palaeosols occur in thick mudstones deposited on floodplains under a seasonal semi arid climate and represent vertisol type soils simple single profiles are rare and most of the mudstone intervals exhibit complex overprinted profiles however a major difference exists between the palaeo vertisols in each of the two units studied the moor cliffs formation contains thick floodplain intervals with evidence of frequent erosion and reactivation of earlier soil horizons the associated coarser channel deposits mainly represent ephemeral streams the alluvial system was unstable flashy and prone to extensive stripping of stored floodplain sediment arroyos unstable ephemeral stream channels probably developed during this interval evidence for any marine influence on soil development is missing and previous speculations that the formation may represent deposition in an upper intertidal or supratidal system are not supported the rat island mudstone represents a complete contrast in that there was little or no stripping of floodplain sediments the channel deposits are typically of the fining upwards type representing more stable low sinuosity channels and with ephemeral stream floods by integrating the palaeosols a more detailed picture of the geomorphic history of these units can be formulated comparison with contemporary drainage basins indicates that a change in climate was a possible cause for the differences between the two units less frequent and more severe storms occurring during the deposition of the rat island mudstone

lower crustal extension across the northern carnarvon basin australia evidence for an eastward dipping detachment asymmetry and strain partitioning along conjugate margins are often explained in terms of detachment faults nevertheless cogent evidence for their existence remains limited furthermore even when inferred detachment faults are imaged seismically it is difficult to demonstrate that differential displacement has occurred across these surfaces we present tectonic and stratigraphic evidence from the northern carnarvon basin northwest australia that documents the existence of an intracrustal eastward dipping detachment by integrating sequence stratigraphy with kinematic and isostatic models of basin development we conclude that the northern carnarvon basin was formed as a consequence of four extension events 1 a broadly distributed late permian event 2 a predominantly localized rhaetian event responsible for the inception of the barrow and dampier subbasins 3 a localized callovian fault reactivation within the barrow dampier subbasins and 4 a tithonian valanginian event that generated large post valanginian regional subsidence across the northern carnarvon basin with only minor accompanying brittle deformation and erosional truncation the regional distribution and amplitude of the post valanginian subsidence are not consistent with the minor amounts of tithonian valanginian brittle upper crustal extension observed on the margin large portions of the platform were emergent or at very shallow water depths prior to the tithonian valanginian extension to match the distribution and magnitude of the post valanginian thermal type subsidence requires significant lower crustal and mantle extension across the northern carnarvon basin such a distribution of extension implies the existence of an eastward dipping intracrustal detachment having a ramp flat ramp geometry that effectively thinned the lower crust and lithospheric mantle the detachment breached the surface close to the position of the continent ocean boundary west of the exmouth plateau the flat component of the detachment occurred at midcrustal depths 15 km across the plateau and ramped beneath the australian continent lower crustal ductile extension provides a viable mechanism to generate large regional subsidence with little attendant brittle deformation which may explain the paradox that both sides of many conjugate margins appear to be the upper plate

a triassic jurassic westward scissor like subduction history of the mudanjiang ocean and amalgamation of the jiamusi block in ne china constraints from whole rock geochemistry and zircon u pb and lu hf isotopes of the lesser xing an zhangguangcai range granitoids abstract ne china recorded the key tectonic evolution history of the eurasian plate from the paleozoic mesozoic collisional formation of the central asian orogenic belt to the mesozoic subduction of the paleo pacific ocean to better understand this tectonic transition it is crucial to constrain the time and pattern of the initial subduction of the paleo pacific ocean recently some researchers proposed that the mudanjiang ocean existed between the songnen and jiamusi blocks was part of the paleo pacific ocean here through geochemical and geochronological studies on the widespread granitoids in the lesser xing an zhangguangcai range in the eastern songnen block we verify that these magmatic rocks show volcanic arc affinity with increased mantle contribution from east to the west of the range likely related to a flattening subduction of the mudanjiang ocean in addition a universal westward younging trend for over 70 myr can be observed for the granitoids throughout the lesser xing an zhangguangcai range indicating a long lasting subduction of the mudanjiang ocean more interestingly the oldest ages of the granitoids in the east display a northward younging trend from 275 ma to 218 ma suggesting that the subduction of the mudanjiang ocean had been initiated at latest by 275 ma in the south and then progressively expanded to the north based on these observations we proposed a new tectonic evolution model for the mudanjiang ocean i e a triassic jurassic westward scissor like subduction and closure to contribute to the understanding of the early subduction of the paleo pacific ocean

initial subduction of the paleo pacific oceanic plate in ne china constraints from whole rock geochemistry and zircon u pb and lu hf isotopes of the khanka lake granitoids abstract northeast china is located in the eastern part of the central asian orogenic belt caob and was influenced by paleo pacific subduction during the mesozoic abundant granitoids from the late paleozoic to early mesozoic in ne china record this process including the khanka lake granitoids which resulted in extensive growth of continental crust in the area however the question of how and when the paleo pacific tectonic system began to affect ne china is still highly controversial the khanka lake granitoids can be subdivided into two main components based on their geochemical characteristics namely granodiorite and syenogranite the granodiorite has a u pb age of 249 ma and is adakite like enriched in lree and liles with high mg sr la yb sr y and na 2 o k 2 o with zircon e hf t values of 0 65 to 1 61 produced by the magma mixing between melting of the lower continental crust and juvenile basaltic magma the syenogranite has zircon u pb ages of 209 to 199 ma and geochemical features of highly fractionated i type granites with high sio 2 total alkalis and low mg and mg fe cr and ni and positive zircon e hf t of 1 72 to 5 12 indicating an origin from remelting of juvenile crust the granitoids were intruded by felsic veins between 195 and 184 ma with positive zircon e hf t from 0 57 to 5 32 the e hf t values of the granitoids become more positive as the zircon u pb ages become younger suggesting continuous melting of juvenile crust during subduction it is concluded that the khanka lake granitoids record the early stage of subduction of the paleo pacific oceanic plate which commenced at least ca 250 ma ago

archean geodynamics in the central zone north china craton constraints from geochemistry of two contrasting series of granitoids in the fuping and wutai complexes abstract the archean to paleoproterozoic central zone of the north china craton is situated between the eastern and western archean continental blocks and contains two contrasting series of neoarchean granitoids the 2523 2486 ma tonalite trondhjemite granodiorite ttg gneisses in the fuping complex and the 2555 2525 ma calc alkaline granitoids tonalite granodiorite granite and monzogranite in the wutai complex the fuping ttg gneisses most likely formed from partial melting of 2 7 ga basalts at 50 km with an involvement of 3 0 ga crustal material the wutai granitoids have higher k 2 o lile and rb sr but lower sr y and la n yb n than the fuping ttg gneisses are characterized by nd t dm from 2 5 to 2 8 ga and e nd t from 0 49 to 3 34 and are derived from partial melting of a juvenile source at the geochemistry of these two contrasting series of neoarchean granitoids provides further evidence that the wutai complex originated and evolved separately from the fuping complex the wutai complex most likely formed as an oceanic island arc with volcanism and synvolcanic granitoid intrusions at 2555 2525 ma the wutai complex was subsequently accreted onto the eastern archean continental block and was probably responsible for crustal thickening and ttg magmatism at 2523 2486 ma in the fuping complex as part of the taihangshan hengshan block at the western margin of the eastern archean continental block

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

paleo pacific subduction accretion evidence from geochemical and u pb zircon dating of the nadanhada accretionary complex ne china the nadanhada terrane located along the eastern margin of eurasia contains a typical accretionary complex related to paleo pacific plate subduction accretion the yuejinshan complex is the first stage accretion complex that consists of meta clastic rocks and metamafic ultramafic rocks whereas the raohe complex forms the main parts of the terrane and consists of limestone bedded chert and mafic ultramafic rocks embedded as olistolith blocks in a weakly sheared matrix of clastic meta sedimentary rocks geochemical data indicate that the yuejinshan metabasalts have normal mid ocean ridge basalt n morb affinity whereas the raohe basaltic pillow lavas have an affinity to ocean island basalts oib sensitive high resolution ion microprobe shrimp u pb zircon analyses of gabbro in the raohe complex yield a weighted mean 206pb 238u zircon age of 216 5 ma whereas two samples of granite intruded into the complex yield weighted mean 206pb 238u zircon ages of 128 2 and 129 2 ma laser ablation inductively coupled plasma mass spectrometry la icpms u pb zircon analyses of basaltic pillow lava in the raohe complex define a weighted mean age of 167 1 ma two sandstone samples in the raohe complex record younger concordant zircon weighted mean ages of 167 17 and 137 3 ma these new data support the view that accretion of the raohe complex was between 170 and 137 ma and that final emplacement of the raohe complex took place at 137 130 ma the accretion of the yuejinshan complex probably occurred between the 210 and 180 ma suggesting that paleo pacific plate subduction was initiated in the late triassic to early jurassic

sequence stratigraphy of nonmarine jurassic cretaceous rocks central cordilleran foreland basin system an analysis of the upper jurassic lower cretaceous morrison and cedar mountain formations of utah and colorado has resulted in a general sequence stratigraphic model for nonmarine rocks in this model nonmarine deposition is governed by changes in basin accommodation development and corresponding shifts in depositional base level these fluctuations result in deposition of systematically varying facies and architectural elements that allow nonmarine depositional sequences to be recognized internally nonmarine depositional sequences comprise three systems tracts degradational transitional and aggradational which are analogous to the lowstand transgressive and highstand systems tracts of marine depositional sequences degradational systems tracts overlie sequence bounding unconformities and consist of relatively coarse grained low sinuosity fluvial deposits that are either contained within incised valleys or deposited as widespread thin sheets above shallow erosion surfaces transitional systems tracts represent an increase in basin accommodation development following degradational systems tract deposition they are marked by the transition from laterally continuous low sinuosity fluvial channel sandstones and conglomerates to lenticular and ribbon like meandering and anastomosing channel sandstones aggradational systems tracts are characterized by meandering anastomosing channel sandstones and abundant fine grained overbank and lacustrine deposits the upper jurassic lower cretaceous nonmarine rocks of the study area contain three depositional sequences the first of these the uj 1 sequence consists primarily of an aggradational systems tract overlain by a sequence bounding unconformity however the lower parts of this sequence are transitional with older marine rocks and can be considered the late stages of a marine highstand systems tract the upper morrison sequence uj 2 consists of degradational transitional and aggradational systems tracts above the uj 2 sequence are a sequence bounding unconformity and degradational and transitional systems tracts of the lk 1 sequence represented by the buckhorn conglomerate the buckhorn is overlain by a sequence bounding unconformity and transitional aggradational systems tracts of the lk 2 sequence that is composed of the upper part of the cedar mountain formation the upper jurassic lower cretaceous sequences in utah and colorado can be traced regionally and correlated with nonmarine depositional sequences in central utah and sequences that contain nonmarine transitional and marine rocks in central wyoming these sequences were deposited in the back bulge forebulge and distal foredeep depozones of the late jurassic early cretaceous foreland basin system

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

an algorithm to calculate the gravity anomaly of sedimentary basins with exponential density depth relationships we derive wavenumber domain expressions to calculate the gravity anomaly of a body with irregular bounding surfaces and an exponential density depth relationship we apply the method to sedimentary basins which commonly have this type of geometry and density distribution the mathematical formulation also allows the exponential density depth relationship to be measured from an arbitrary irregular surface rather than the top surface using this arrangement the gravity anomaly of exhumed sedimentary basins can be predicted if the amount of eroded section can be estimated the corresponding inverse algorithms are also derived examples of the use of the forward algorithms from the galicia interior basin and the central irish sea basin are used to illustrate these methods

the sequence stratigraphy of fluvial depositional systems the role of floodplain sediment storage abstract the currently used model for alluvial sequence stratigraphy has been criticised miall 1991 and an alternative model is offered here in this model alluviation mainly takes place during transgressive and not highstand phases when accommodation space allows sediment storage the high capacity for some river systems to store floodplain sediments is a critical consideration the effects of climatic and intrinsic geomorphic changes during fluvial deposition are likely to greatly complicate simple base level controlled sequence models and recognising higher order fourth and fifth order cycles will be particularly difficult

magmatism at the west iberia non volcanic rifted continental margin evidence from analyses of magnetic anomalies we discuss the magmatic development of the west iberia non volcanic rifted continental margin in the north atlantic ocean so called non volcanic rifted continental margins are characterized by a lack of syn rift magmatism and are considered to be largely amagmatic however this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously we concentrate our attention on the recently described zone of exhumed continental mantle zecm to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the zecm leading to the onset of seafloor spreading we use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide angle seismic experiments presented elsewhere forward and inverse modelling of a sea surface magnetic anomaly chart and of surface and deep towed magnetometer profiles shows that anomalies within the zecm differ in trend amplitude and source type from those in the adjacent oceanic crust and thinned continental crust the zecm anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin we interpret such bodies as syn extensional intrusions that increase in volume oceanward they eventually merge in the vicinity of a margin parallel basement peridotite ridge to give rise to a continuous crust that records reversals in the earth s magnetic field from the time of anomaly m4 n m5 r i e to mark the onset of seafloor spreading we find no evidence for anomalies formed by seafloor spreading at either slow or ultraslow rates before m5 r 128 ma

the accommodation concept in sequence stratigraphy some dimensional problems and possible redefinition abstract a debate is provoked here as to what is meant exactly by the term accommodation and how well this concept as it presently stands serves the conceptual and quantitative framework of sequence stratigraphy accommodation in terms of its original definition contains some conceptual flaws it represents a space that has three dimensional extent associated with a certain availability of sediment and thus is not really independent of sedimentation however this space is fairly unspecifiable because of the inherent difficulty of distinguishing real accommodation from the space which is very unlikely to be filled by sediment anti accommodation the accommodation concept cannot be extended to subaerial environments because the equilibrium profile of streams which is sometimes used for this purpose is difficult to recognize even if it existed a possible redefinition of accommodation is suggested as the thickness measured at a specified site and time of a space which becomes filled with sediments during a specified time interval this accommodation is not a space but the thickness of a space not something potential but an objective result not specifiable from the graded profile or geomorphic base level but from stratigraphic base level and not separated from sedimentation but partly dependent from it potential accommodation may be used to describe the maximum of possible accommodation at a specified site and time which can substantially coincide with the height of a water column at a given site and time

paleolimnology evolution in rift basins the ischigualasto villa union basin central western argentina during the triassic abstract extensional basins offer a high preservation potential for sedimentary and fossil records nonmarine rift basins contain diverse environments including fluvial and lacustrine lake deposits have long been recognized as significant archives of the paleoenvironmental record the ischigualasto villa union basin represents the only nearly continuous continental triassic record with up to 6000 m of alluvial fluvial and lacustrine deposits the deep lacustrine interval is known as the los rastros formation and consists of several lacustrine deltaic cycles a detailed facies analysis allowed the characterization of five facies associations and two types of cycles 1 a shoreline dominated progradational type i cycle containing facies association a offshore lacustrine deposits and facies association b marginal palustrine deposits along the shoreline of the lake or facies association c lacustrine siliciclastic coastal deposits and 2 a delta dominated progradational type ii cycle characterized by facies association a offshore lacustrine and prodelta deposits facies association d delta front deposits and facies association e delta plain deposits the sedimentary evolution of the chanares los rastros sequence is defined within this extensional setting as one depositional sequence with three system tracts lowstand transgressive and highstand system tract biotic abiotic and sedimentologic data allowed the interpretation of the limnologic dynamics of the los rastros paleolake as a deep freshwater lake within a half graben exhibiting meromixis and showing a change from moderate eutrophic conditions with low oxygen content to relatively oxygenated oligotrophic conditions the successive fluctuations recorded in lake level are considered in the sequence stratigraphic framework of bohacs et al to reflect the relative balance of sediment and water input versus accommodation in an overfilled lacustrine basin

principles of regression and transgression the nature of the interplay between accommodation and sediment supply perspectives abstract the rate of change of accommodation at the shoreline a and the rate of sediment supply s are the primary factors controlling regression and transgression in the geological record or retreat and advance of a coastal depositional system the generally accepted notion here referred to as the a s ratio concept stating that the migration of a shoreline is controlled principally by the magnitude of the ratio of a to s is insufficient and somewhat misleading partly owing to the problem of dimensional confusion the interplay of a and s inevitably results in autoretreat of the shoreline whereby the seaward advance of any shoreline is halted and is subsequently turned to landward retreat provided there is a continuous rise of relative sea l vel how effective the autoretreat process is depends for a given period of relative sea level rising upon the length of potential time period for the seaward advance of shoreline and is proportional to a2 s and a1 5 s0 5 in two and three dimensional sediment dispersal modes respectively autoretreat becomes more effective as a increases and or s decreases a functions more critically to the effectiveness than s does the a s ratio concept is only approximately applicable when autoretreat is less effective i e with very low a and or very high s in this sense the autoretreat theory and the a s ratio concept are complementary to each other the autoretreat mechanism is expected to be substantially useful not only in fluvial deltas but also in many other depositional systems e g barrier islands whose evolution has been considered in terms of the a s ratio concept

taphonomy of the south brazilian triassic paleoherpetofauna pattern of death transport and burial taphonomic aspects of the paleoherpetofauna of the santa maria and caturrita formations triassic of southern brazil are for the first time specifically dealt with a total of 1096 specimens from articulated skeletons to reworked and isolated bone fragments were studied the four major reptilian groups of the paleoherpetofauna cynodonts dicynodonts thecodonts and rhynchosaurs from three distinct local faunas preservation of bones and their spatial distribution were analysed allowing the establishment of four taphonomic classes i relatively articulated skeletons ii articulated bones iii isolated well preserved bones iv isolated fragmented bones the numerical sequence of classes reflects the increasing disorganization of the skeletal material the higher the class number the more complex is the taphonomic story to beinvestigated class i is composed of transported and rapidly buried carcasses under the influence of catastrophic events on the other extreme fragmented pieces from class iv represent elements exposed for a long time to the influence of weathering trampling and scavenger activity the study of selective transport voorhies s groups of bone elements from dicynodonts and rhynchosaurs the most representative groups of the triassic herpetofauna revealed that hydraulic selection by moving water was not important as a factor of disarticulation and scattering although it did occur at some sites the computation of the fossil recovery rate showed that in a general way the preservation is low and that there is a bias favouring skulls over represented in the collection and vertebrae under represented the analysis of size classes of the triassic reptilian remains taken as analogues to age classes showed a distribution very similar to that exhibited by living communities indicating that small young intermediate and large old individuals were buried together in proportions approximating the distribution pattern in living populations this is taken as an evidence of catastrophic rather than attritional death by bringing together all the stratigraphic paleontological and taphonomic data the reconstruction of the taphonomic history was possible this model postulates a multi episodic history of catastrophic death and rapid burial of complete carcasses caused by periodic floods which alternated with periods of exposure of bone elements and little or no burial indicating that large amounts of water in the depositional system alternated with periods of low water level or even dryness

multimode surface waveform tomography of the pacific ocean a closer look at the lithospheric cooling signature summary we present a regional surface waveform tomography of the pacific upper mantle obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode rayleigh waves to constrain the vsv structure down to 400 km depth we have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion we have furthermore improved path coverage in the south pacific region by including rayleigh wave observations from the french polynesian pacific lithosphere and upper mantle experiment deployment this improvement has led to imaging of vertical low velocity structures associated with hotspots within the south pacific super swell region we have produced an age dependent average cross section for the pacific ocean lithosphere and found that the increase in vsv with age is broadly compatible with a half space cooling model of oceanic lithosphere formation we cannot confirm evidence for a pacific wide reheating event our synthetic tests show that detailed interpretation of average vsv trends across the pacific ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region a condition that is difficult to achieve in such a large oceanic basin with current seismic stations

sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt ellensburg formation late miocene central washington late miocene volcaniclastics of the ellensburg formation washington northwestern usa are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a cascade range source area two facies associations are recognized the first is composed of laterally restricted well sorted polymictic conglomerates representing a gravel bedload regime during inter eruption periods the second comprises laterally extensive sheets bounded by deeply developed paleosols composed of monomictic pebbly pumiceous dacitic sandstones with intercalated debris flow and hyperconcentrated flood flow deposits these sheets aggraded in response to eruptive events that are sometimes recorded by air fall tephras at the bases of sequences debris flow and hyperconcentrated flood flow deposits occur as far as 120 km from source but are uncommon beyond 50 km hyperconcentrated flood flow deposits initially increase in abundance away from source as debris flow deposits diminish suggesting formation of hyperconcentrated flood flows by dilution of debris flows sandy facies form broad sheets dominated by scour fill bedding in proximal and medial settings and grade to narrower trough cross bedded sheets in distal settings suggesting moderation of flood discharges with distance base level changes associated with episodic sediment influx caused incision aggradation cycles in mainstream settings and episodic impoundment of tributaries to form lakes or rapidly aggrading poorly drained floodplains although volcanism was the primary control on depositional style concurrent development of the yakima fold belt produced a structure consequent drainage pattern that determined sediment dispersal and basin subsidence permitted preservation of both syn eruption and inter eruption facies detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics but dominates deposits of small tributary streams flowing toward rather than from the cascades

sensitivity of gravity and topography regressions to earth and planetary structures abstract the availability of global gravity fields and topography through calculation services like the international centre for global earth models allows easy access to gravity data greatly enlarging the spectrum of users the applications extend much farther than the classic modeling through the gravity specialist we investigate the sensitivity of the joint analysis of topography and gravity data based on linear regression analysis and clustering of the response to particular characteristics of the lithosphere structure the parameters of the regression analysis are predicted to have characteristic values which allow to distinguish continental crust from oceanic crust and signalize the presence of crustal inhomogeneity predictions are made through theoretical considerations and on synthetic models we use the south atlantic ocean and the confining south american and african continents for illustration where the regression parameters distinguish oceanic crust from the ridge up to the bathymetric inflection point from the transitional crust and the continental crust allowing to map these units the general properties of the parameters are statistically relevant since the errors on the parameters are

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

spatial temporal relationships of mesozoic volcanic rocks in ne china constraints on tectonic overprinting and transformations between multiple tectonic regimes abstract la icp ms zircon u pb ages and geochemical data are presented for the mesozoic volcanic rocks in northeast china with the aim of determining the tectonic settings of the volcanism and constraining the timing of the overprinting and transformations between the paleo asian ocean mongol okhotsk and circum pacific tectonic regimes the new ages together with other available age data from the literature indicate that mesozoic volcanism in ne china can be subdivided into six episodes late triassic 228 201 ma early middle jurassic 190 173 ma middle late jurassic 166 155 ma early early cretaceous 145 138 ma late early cretaceous 133 106 ma and late cretaceous 97 88 ma the late triassic volcanic rocks occur in the lesser xing an zhangguangcai ranges where the volcanic rocks are bimodal and in the eastern heilongjiang jilin provinces where the volcanics are a type rhyolites implying that they formed in an extensional environment after the final closure of the paleo asian ocean the early middle jurassic 190 173 ma volcanic rocks both in the erguna massif and the eastern heilongjiang jilin provinces belong chemically to the calc alkaline series implying an active continental margin setting the volcanics in the erguna massif are related to the subduction of the mongol okhotsk oceanic plate beneath the massif and those in the eastern jilin heilongjiang provinces are related to the subduction of the paleo pacific plate beneath the eurasian continent the coeval bimodal volcanic rocks in the lesser xing an zhangguangcai ranges were probably formed under an extensional environment similar to a backarc setting of double direction subduction volcanic rocks of middle late jurassic 155 166 ma and early early cretaceous 145 138 ma age only occur in the great xing an range and the northern hebei and western liaoning provinces limited to the west of the songliao basin and they belong chemically to high k calc alkaline series and a type rhyolites respectively combined with the regional unconformity and thrust structures in the northern hebei and western liaoning provinces we conclude that these volcanics formed during a collapse or delamination of a thickened continental crust related to the evolution of the mongol okhotsk suture belt the late early cretaceous volcanic rocks widely distributed in ne china belong chemically to a low to medium k calc alkaline series in the eastern heilongjiang jilin provinces i e the eurasian continental margin and to a bimodal volcanic rock association within both the songliao basin and the great xing an range the volcanics in the eastern heilongjiang jilin provinces formed in an active continental margin setting related to the subduction of the paleo pacific plate beneath the eurasian continent and the bimodal volcanics formed under an extensional environment related either to a backarc setting or to delamination of a thickened crust or both late cretaceous volcanics limited to the eastern heilongjiang jilin provinces and the eastern north china craton ncc consist of calc alkaline rocks in the eastern heilongjiang jilin provinces and alkaline basalts in the eastern ncc suggesting that the former originated during subduction of the paleo pacific plate beneath the eurasian continent whereas the latter formed in an extensional environment similar to a backarc setting taking all this into account we conclude that 1 the transformation from the paleo asian ocean regime to the circum pacific tectonic regime happened during the late triassic to early jurassic 2 the effect of the mongol okhotsk suture belt on ne china was mainly in the early jurassic middle late jurassic and early early cretaceous and 3 the late early cretaceous and late cretaceous volcanics can be attributed to the subduction of the paleo pacific plate beneath the eurasian continent

geochronology and geochemistry of the paleoproterozoic meta basalts from the jiao liao ji belt north china craton implications for petrogenesis and tectonic setting abstract geochronology geochemical and isotopic studies were carried out on the paleoproterozoic meta basalts and related rocks from the jiao liao ji belt north china craton to understand the mantle source characteristics and the geodynamic setting u pb isotopic dating using the la icpms method on zircons from the metamorphosed volcanic rocks reveals that they formed at ca 2204 2158 ma and were metamorphosed at ca 1895 1919 ma respectively the meta basalts are of calc alkali affinity and have arc like geochemical compositions they are enriched in lrees la yb n 3 5 5 1 and liles sr ba rb depleted in hfses nb ta zr hf ti and show variable high nd t values 2 4 to 4 5 the basaltic rocks experienced significant fractional crystallization and minor crustal contamination during magma evolution modeling suggests that the parent magma of the meta basalts was derived from 20 partial melting of spinel garnet lherzolites that were previously metasomatized by subduction zone fluids melts in the late archean petrological geochronological and geochemical data suggest that the paleoproterozoic volcanic rocks formed in an active continental margin setting and were subsequently deformed and metamorphosed to amphibolite facies due to the arc continental collision at ca 1 9 ga rather than in a continental rifting setting as previously thought by most workers

crustal accretion at the reykjanes ridge 61 62 n we report results of a seismic gravity and magnetic survey of the reykjanes ridge spreading center at 61 62 n about 600 km from the center of the iceland mantle plume anomalously shallow water on the ridge crest enabled us to record seismic refractions on a 2 4 km hydrophone streamer the velocity within layer 2a is 2 4 0 3 km s 1 and its mean thickness is 400 100 m the velocity at the base of layer 2a is 3 3 0 3 km s 1 on the ridge axis increasing with crustal age to 4 0 km s 1 at 1 5 ma and 4 5 km s 1 at 5 ma assuming that seismic layer 2a on the ridge axis is also the extrusive layer i e the magnetic source layer we have successfully modeled the variations in amplitude of the magnetic field the best magnetic model includes enhanced magnetization within layer 2a at the sites of recent volcanic activity as independently recognized in side scan sonar data we also present a full crustal seismic model based on wide angle seismic recordings on digital ocean bottom hydrophones and disposable sonobuoys the seismic model is complemented by gravity modeling which further suggests that the ridge crest is in isostatic equilibrium the zero age crust is 10 0 km thick while crust of age 5 ma is 7 8 km thick these crustal thicknesses are greater than those of normal oceanic crust which we attribute to the presence of anomalously hot asthenospheric mantle beneath the spreading center we suggest that the variation in thickness between 0 ma and 5 ma crust is caused by temporal variation in the plume fed asthenospheric temperature beneath the reykjanes ridge

the hatton bank continental margin iii structure from wide angle obs and multichannel seismic refraction profiles summary in two recent papers normal incidence and expanding spread profiles esps shot across the hatton bank passive continental margin showed seaward dipping reflectors in the upper crust and elevated velocities in the lower crust the reflectors were interpreted as subaerial basalt flows and the high velocities as underplated or intruded igneous material in the lower crust a recent gravity model shows high densities in the lower crust supporting the velocity model in this study we present the forward modelling of two wide angle seismic refraction datasets shot perpendicular to strike and through the esp midpoints the final velocity model across the margin is consistent with all the seismic datasets but differs in some important respects from the esp based model a high velocity volcanic pile is outlined mid way up the continental slope and the inferred velocities in the lower cnst are slightly lower the differences between the two models can be explained by the three dimensional nature of the margin the esps were modelled assuming that the sub sediment structure was invariant along strike the final velocity model is used to obtain an estimate of the total thickness of melt accreted across the margin which is calculated to thicken from zero at hatton bank to about 12 km mid way down the continental slope at the bottom of the continental slope the adjacent igneous section forming the oceanic crust is about 18 km thick and thins rapidly northwestwards to 11 km as the oceanic crust thins its seismic velocity decreases this suggests that it is less mafic and that there is a post rift decrease in the percentage of mantle melting this decrease may be explained by a decrease in asthenospheric temperature and or by a reduction in volatile content

allogenic controls on the fluvial architecture and fossil preservation of the upper triassic ischigualasto formation nw argentina abstract the upper triassic ischigualasto formation in nw argentina was deposited in a fluvial system during the synrift filling of the extensional ischigualasto villa union basin the expansive exposures of the fluvial architecture and paleosols provide a framework to reconstruct the paleoenvironmental evolution of this basin during the upper triassic using continental sequence stratigraphy the ischigualasto formation deposition can be divided into seven sequential sedimentary stages the 1 bypass stage 2 confined low accommodation stage 3 confined high accommodation stage 4 unstable accommodation stage 5 unconfined high accommodation stage 6 unconfined low accommodation stage and finally 7 unconfined high accommodation stage the sedimentary evolution of the ischigualasto formation was driven by different allogenic controls such as rises and falls in lake levels local tectonism subsidence volcanism and climate which also produced modifications of the equilibrium profile of the fluvial systems all of these factors result in different accommodations in central and flank areas of the basin which led to different architectural configurations of channels and floodplains allogenic processes affected not only the sequence stratigraphy of the basin but also the vertebrate and plant taphocenosis therefore the sequence stratigraphy can be used not only as a predictive tool related to fossil occurrence but also to understand the taphonomic history of the basin at each temporal interval

staircase 40 ar39ar spectra of fine grained white mica timing and duration of deformation and empirical constraints on argon diffusion staircase 40 ar 39 ar spectra of 2 to 6 m white micas in ductile carbonate mylonites morcles doldenhorn nappe switzerland record ages between 32 and 13 ma these ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold thrust nappe we interpret these data to record protracted white mica neocrystallization and growth during low temperature ductile deformation such that the differences between the maximum and minimum ages in the 40 ar 39 ar spectra record the time span over which mica formed and grew we have modeled the argon concentrations in these micas taking into account the thermal history of the nappe mica growth and diffusional loss of argon our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite and that the retentivity of argon in white mica is greater than generally thought

three dimensional gravity inversion for moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction summary this paper describes a method for determining moho depth lithosphere thinning factor 1 1 and the location of the ocean continent transition at rifted continental margins using 3 d gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere the lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of 100 mgal and for which a correction must be made in order to determine moho depth accurately from gravity inversion we describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present day temperature field from which we calculate the lithosphere thermal density and gravity anomalies for continental margin lithosphere the lithosphere thermal perturbation is calculated from the lithosphere thinning factor 1 1 obtained from crustal thinning determined by gravity inversion and breakup age for thermal re equilibration time for oceanic lithosphere the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere a correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading we investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the north atlantic

record of permian early triassic continental arc magmatism in the western margin of the jiamusi block ne china petrogenesis and implications for paleo pacific subduction in this paper we report zircon u pb ages hf isotopes and whole rock geochemical data for the permian to early triassic granitoids from the western margin of the jiamusi block wjb ne china the intermediate to felsic sio2 59 67 74 04 wt granitoids belong to calc alkaline series and are characterized by enrichments in light rare earth elements and large ion lithophile elements with pronounced negative nb ta and ti anomalies revealing typical continental magmatic arc geochemical signatures the zircon u pb determinations on the granodiorite monzogranite syenogranite and quartz diorite samples yielded ages between ca 275 245 ma which together with the published coeval intrusive rocks indicates that permian to early triassic continental arc magmatism occurred extensively in the wjb the low and mainly negative zircon e hf t values between 7 6 and 1 6 and the zircon hf model ages of 1 2 1 8 ga which are significantly older than their crystallization ages suggest that they were mainly derived from reworking of ancient crustal materials with a limited input of juvenile components the geochemical systematics and petrogenetic considerations indicate that the studied granitoids were generated from a zone of melting assimilation storage and homogenization i e a mashed zone at the base of paleo to mesoproterozoic continental crust where large portions of igneous rocks and minor clay poor sediments involved in the source region in combination with regional geological data we argue that the jiamusi block was unlikely the rifted segment of the songliao block and two possible geodynamical models were proposed to interpret the formation of the ca 275 245 ma granitoids in the wjb in the context of permian global plate reconstruction we suggest that paleo pacific plate subduction was initiated in the permian to early triassic beneath the jiamusi block and even whole eastern ne china