neoproterozoic snowball earth simulations with a coupled climate ice sheet model ice sheets may have reached the equator in the late proterozoic era 600 800 myr ago according to geological and palaeomagnetic studies possibly resulting in a snowball earth but this period was a critical time in the evolution of multicellular animals posing the question of how early life survived under such environmental stress here we present computer simulations of this unusual climate stage with a coupled climate ice sheet model to simulate a snowball earth we use only a reduction in the solar constant compared to present day conditions and we keep atmospheric co2 concentrations near present levels we find rapid transitions into and out of full glaciation that are consistent with the geological evidence when we combine these results with a general circulation model some of the simulations result in an equatorial belt of open water that may have provided a refugium for multicellular animals

the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

gravitational collapse of orogenic crust a preliminary three dimensional finite element study although gravitational collapse has been widely accepted as a viable explanation for both synorogenic and postorogenic extension its mechanisms have not been well understood previous models of continental deformation that assumed a viscous or viscoplastic rheology are not well suited for simulating stress states and brittle deformation within the upper crust where most extensional collapse occurs here we present preliminary results of a three dimensional 3 d finite element modeling of stress and faulting patterns within the orogenic crust using a viscoelastic rheology major parameters controlling orogenic extension including topographic loading tectonic compression basal shear thermal structure and 3 d tectonic boundary conditions are systematically explored in the model for typical tectonic compression 60 100 mpa and 2 3 km elevation the model predicts synorogenic extension in high plateaus and concomitant thrusting in the lowlands near the foothills viscous relaxation within the ductile crust may amplify deviatoric stresses within the brittle crust by a few times and stress amplification is greater when the crust is hotter and therefore the brittle crust is thinner most synorogenic extension occurs in the direction orthogonal to regional compression while postorogenic extension is more likely to occur normal to the trend of mountain belts we apply the model results to the tibetan plateau and suggest that the age of the north trending grabens is not a reliable proxy for the time when the plateau had reached it present mean high elevation conversely development of the east trending south tibetan detachment system in the miocene by gravitational collapse was more difficult and may require a higher elevation of the himalayas than present or much of the tibetan plateau to have remained low till late miocene

a sedimentary prelude to marinoan glaciation cryogenian middle neoproterozoic keele formation mackenzie mountains northwestern canada abstract continental margin deposition between the sturtian and marinoan aged global glacial periods is represented in nw canada by a siliciclastic to carbonate grand cycle the keele formation a 300 600 m thick carbonate and siliciclastic deposit comprises the upper part of this interglacial cycle sedimentary facies in the lower carbonate dominated member of the keele span a broad spectrum of depositional environments from below storm wave base to peritidal to continental and are disposed across a relatively narrow paleogeographic belt lateral arrangement of facies and the gradual stratigraphic transitions between them suggest deposition in a ramp setting local slump structures and occasional debrites in deep water facies imply that the ramp was distally steepened fluvial facies are concentrated at a single locality indicating that terrigenous clastic sediment was derived from a point source or sources most strata of the carbonates are bahamian in character and similar to phanerozoic warm water carbonates in that they contain abundant calcite ooids of interpreted original aragonitic mineralogy and have well developed muddy peritidal facies nevertheless close juxtaposition of deep and shallow water facies in keele rocks indicates high amplitude sea level change during deposition on the order of 100 m suggesting accumulation in a climatic icehouse period this interpretation implies that although the globe possessed polar and high altitude glaciers at this time sea surface temperature at least at tropical latitudes warmed dramatically between neoproterozoic glacial episodes the onset of marinoan glaciation here is heralded by an abrupt shift to nearshore marine siliciclastic sedimentary rocks these deposits form the upper member of the keele formation informally called the keele clastic wedge and directly underlie ice brook formation diamictites carbonates are rare and are restricted to mudstones and flat pebble conglomerates that lack any indication of tropical conditions

the anomalous ca cycle in the ediacaran ocean evidence from ca isotopes preserved in carbonates in the three gorges area south china abstract the ediacaran cambrian was one of the most important periods for the evolution of life recent studies have provided detailed chemostratigraphies that decipher the linkages between ambient surface environmental changes and biological evolution the occurrence of skeletal fossils in ediacaran rocks suggests that the possible onset of ca biomineralization started in the latest neoproterozoic molecular clocks also predict the emergence of animals containing ca carbonate spicules in the neoproterozoic therefore it is important to estimate the transition of the ca cycle in seawater ancient ca cycles in the oceans are estimated from the calcium isotopic compositions of carbonate rocks however the lack of continuous ca isotopic data in the ediacaran leaves the ca cycle unresolved the almost continuously exposed ediacaran and cambrian strata in south china mainly comprise carbonate rocks with subordinate black shales and sandstones which contain many fossils suitable for the study of environmental and biological changes we conducted drilling from the liantuo via the nantuo doushantuo dengying and yanjiahe to the shuijingtuo formation at four sites in the three gorges area in order to obtain continuous fresh samples without surface alteration and oxidation we analyzed 44 ca 42 ca ratios in carbonate rocks with a multiple collector inductively coupled plasma mass spectrometer mc icp ms screened for diagenetic alteration the resultant 44 42 ca ratios exhibit a smooth curve the values of 44 42 ca are anomalously high relative to those in the phanerozoic and they gradually decrease towards the end of the ediacaran our new ca isotope chemostratigraphy suggests that a different ca cycle had existed during the ediacaran the long term high 44 42 ca is not simply explicable by changes in temperature differences in carbonate mineralogy or changes in the input output fluxes of ca to the ocean we propose three possible explanations for the observed high 44 42 ca ratios in the ediacaran high 44 42 ca ratios of ca inputs potential undiscovered sinks of ca with low 44 42 ca values and negligible isotope fractionation between carbonate and seawater we consider that one or all of these contributed to the high 44 42 ca ratios of these we prefer the small isotope fractionation which implies that the ca concentration in seawater was initially low in the early ediacaran and increased throughout that period

geobiology and the fossil record eukaryotes microbes and their interactions abstract geobiology attempts to understand the interactions between earth and the life which has evolved on it it is an all encompassing field that embraces both living systems on earth as well as an understanding of the history of these systems since life first evolved the fossil record and associated geological and geochemical data provide a major avenue towards understanding the evolution of geobiological systems among the many new and exciting directions of research for geobiology a particularly fertile field is represented by studies of the various interactions between eukaryotes and microbes that can be detected through examination of the fossil record for example such interactions include the various roles which microbes play in taphonomy and preservation of eukaryotes as fossils particularly cases of exceptional fossil preservation similarly recognition of microbially induced sedimentary structures miss in siliciclastic sediments has been a subject of significant recent interest documentation of miss has led to a greater understanding of the role of microbial mats in providing a distinctive structure to marine subtidal seafloors which has fostered studies of benthic eukaryote adaptations to such mat dominated environments the continued evolution of bioturbation in the cambrian led to elimination of microbial mats on shallow subtidal seafloors causing evolutionary and ecological changes in eukaryotes adapted to living on and in seafloors structured by such mats studies of these changes termed the cambrian substrate revolution have been documented for a variety of echinoderms molluscs and trace fossils and are some of the first to illuminate ecological interactions between eukaryotes and microbes through study of the fossil record this new awareness which geobiology represents has begun to produce a whole host of approaches that causes researchers in the laboratory of the molecular or microbiologist to interact with those from the laboratory of the geochemist as they are led on a field trip by the palaeontologist exploring some fundamental issue in the history of life such interactions are not only important for considerations of life on earth but provide a framework for the search for life that may have once existed on other planetary bodies such as mars data from deep time and hence the fossil record plays a central role in such research and thus geobiology opens up vast new research opportunities for palaeontology

tectonic evolution of high plateaus abstract the striking contrast in tectonic style between highlands like tibet with its active grabens and low altitude flanks like the thrusting structures in the foothills of the himalayas shows that a disorder of the tectonic stresses correlates with the topography the same is true for gravity in particular for the geoid as clearly observed in the case of the altiplano these observations can be explained physically by means of simple relationships between the dipolar mass distribution of the high topography and its compensating root this analysis leads to a proposed scenario for plateau evolution in three stages 1 1 build up with thrusting and folding 2 2 neutral state or strike slip deformation and finally 3 3 relaxation with normal faulting the basin and range represents a mature illustration of phase 3 its bouguer anomaly suggests the existence of an undulation of the lithosphere astenosphere boundary with a 200 km wavelength physical models of lithospheric boudinage are shown for one to compare with the observed topographic and gravimetric signatures

c isotopic stratification in a neoproterozoic postglacial ocean we report c isotopic data from the cap carbonate overlying the terminal proterozoic nantuo diamictites in south china the c isotopic measurements reveal a large c isotopic gradient of 3 along paleoenvironmental transect from shelf to deep basinal sedimentary facies the c isotopic stratification may attribute to the effect of biological pumping in a postglacial ocean our results confirm that the sea level rise was rapid and that atmospheric co2 was significantly high immediately following deglaciation

phosphogenesis in the immediate aftermath of the great oxidation event evidence from the turee creek group western australia abstract phosphatic peloids and pebble sized microbially bound sediment clasts have been identified within dolomitic sediment that separates centimetric columnar stromatolites in a 20 150 cm thick horizon from the 2 4 2 2 ga turee creek group in western australia petrographic examination of the phosphatic clasts reveals complex internal textures that are defined by different mixtures of fine grained apatite medium grained dolomite irregular domains of partly euhedral micro quartz and carbonaceous matter phosphatic sand sized peloids observed within microbially bound sediment clasts are similar in size texture and composition to the individual peloids in the intercolumn sediment both occurrences have finely dispersed kerogen throughout and zoned cores that are packed with both microscopic apatite granules and larger euhedral quartz crystals the percentage of apatite vs quartz vs kerogen varies between studied peloids similar sized white peloids in the intercolumn sediment and in the microbially bound sediment clasts contain illite and are lined internally by euhedral apatite crystals 20 100 m in size some peloids are kerogenous in one domain and illite bearing in another the phosphatic microbially bound sediment clasts and peloids are interpreted to represent fragments of a peritidal phosphorite redeposited in an offshore setting during high energy events peloids formed within the microbially bound sediment and were subsequently eroded out and redeposited in the intercolumn sediment white peloids are interpreted to represent peloids affected by surficial weathering during temporary exposure when the fine grained kerogenous interior was replaced by clays these weathered peloids were then re sedimented and captured both within microbially bound sediment and in the intercolumn sediment euhedral apatite crystals that line the insides of illite bearing white peloids were dated to 2104 70 ma and 2041 33 ma using u pb and th pb dating methods respectively this age is interpreted to represent the time of metamorphic fluid circulation during a period of known regional metamorphism and provides a minimum age of the reef complex

lu hf and pbsl geochronology of apatites from proterozoic terranes a first look at lu hf isotopic closure in metamorphic apatite abstract the mineral apatite is characterized by elevated and highly variable lu hf ratios that in some cases allow for single crystal dating by the lu hf isotopic system apatites from the adirondack lowlands and otter lake area in the grenville province and from the black hills south dakota yield lu hf ages that are consistently older than their respective pb step leaching ages isotopic closure for the lu hf system therefore occurs before u pb system closure in this mineral in the adirondack lowlands where h2o activity was low lu hf systematics of cm sized apatite crystals remained undisturbed during upper amphibolite facies metamorphism 700 to 675 c at 1170 1130 ma the relatively old lu hf ages of 1270 and 1230 ma observed for these apatites correlate with decreasing crystal size in contrast apatite from the fluid rich otter lake area and black hills yields unrealistically low apparent lu hf closure temperatures implying that in these apatites fluids facilitated late exchange the lu hf ages for the metamorphic apatites were thus controlled either by the prevailing temperature and grain size or by fluid activity

ediacaran δ13c chemostratigraphy of south china abstract we report new 13 c chemostratigraphic data of the ediacaran system in south china using litho and biostratigraphic markers as independent calibrators we combine new and previously published 13 c data to construct a composite 13 c profile for the ediacaran system in south china the composite curve shows in chronostratigraphic order 1 a negative 13 c excursion en1 in the doushantuo cap carbonate that overlies the ghaub age nantuo diamictite 2 a pronounced positive 13 c excursion ep1 in the lower doushantuo formation where doushantuo type acanthomorphic acritarchs red algae and animal embryos first appear 3 a mild negative 13 c excursion en2 in the middle doushantuo formation 4 a positive 13 c excursion ep2 in the upper doushantuo formation where doushantuo type acanthomorphic acritarchs micrometazoans and stem group florideophyte red algae become abundant and diverse 5 a pronounced negative 13 c excursion en3 near the uppermost doushantuo formation 6 a strongly positive 13 c excursion up to 6 3 pdb ep3 in the lower dengying formation 7 a stable 13 c plateau ca 2 5 pdb ei in the middle and upper dengying formation where abundant vendotaenid algae macroscopic ediacaran fossils biomineralized animal tubes and mostly horizontal animal traces occur and 8 a 10 negative 13 c excursion en4 immediately below the ediacaran cambrian boundary the general pattern of this composite 13 c curve is robust despite a considerable amount of variation among the 13 c profiles of different sections this composite 13 c curve provides a first order chemostratigraphic framework for the subdivision and correlation of the ediacaran system in south china

potential photosynthetic impact on phosphate stromatolite formation after the marinoan glaciation paleoceanographic implications abstract this study investigated the origin and the depositional age of phosphate stromatolites contained in the neoproterozoic salitre formation brazil the stromatolites exhibited columnar shapes and were intercalated with laminated dolostones their depositional environment was interpreted to have been an evaporitic ramp where erosion and reworking by waves prevailed laser ablation inductively coupled plasma mass spectrometry measurements of the stromatolites yielded a u pb age of 616 32 ma suggesting that they were deposited after the marinoan glaciation several types of filamentous structures were recognized in the stromatolites many of them were pseudofossil ambient inclusion trails but some were possibly microfossils to consider the general presence of photosynthetic microorganisms on the surface of shallow water stromatolites the involvement of microbial photosynthesis in phosphate stromatolite formation was expected numerical calculations determined that photosynthesis could have induced phosphate mineral precipitation at the time of deposition if the phosphorus concentration was above ca 5 m to achieve such high concentration at the shallow ocean globally elevated phosphorus concentration was considered to be the ultimate factor in addition to the local process es including upwelling and or evaporation therefore the phosphate stromatolites in the salitre formation are evidence of the development of a phosphorus rich ocean after the marinoan glaciation

late neoproterozoic paleomagnetic results from the sugetbrak formation of the aksu area tarim basin nw china and their implications to paleogeographic reconstructions and the snowball earth hypothesis abstract in order to better constrain the neoproterozoic paleogeographic reconstruction of continents and to improve the understanding of the snowball earth hypothesis paleomagnetic investigations were carried out in the aksu area of the northwestern tarim basin forty eight sites of samples were collected from the sugetbrak and chigebrak formations twenty four sites of sandstone and volcanic rock from the sugetbrak formation revealed stable characteristic remanent components chrm isolated between 500 and 680 c the computed magnetic directions from these components are consistent and significantly distinguished from those of younger ages both normal and reverse polarities have been observed and a positive fold test is revealed after bedding corrections at 95 confidence level anisotropy of magnetic susceptibility measurements shows primary sedimentary fabrics with weak anisotropy degrees a paleomagnetic pole is calculated at 19 1 n 149 7 e k 11 2 a95 9 3 with n 24 yielding a paleolatitude of 27 n for the sampling area the chemo stratigraphic correlation of this section with reference ones reveals an average age of 595 ma a new paleogeographic reconstruction has been attempted showing general low paleolatitude positions for the australia south china and tarim blocks at about 600 ma this observation provides new evidences for the snowball earth hypothesis

an integrated chemostratigraphic δ13c δ18o 87sr 86sr δ15n study of the doushantuo formation in western hubei province south china abstract high resolution isotopic analyses were conducted on core samples from the ediacaran doushantuo formation at the wangjiapeng section western hubei province in south china whereby two laterally traceable negative 13ccarb excursions en1 and en3 were recognized the magnitude and duration of these excursions permit intra basinal and inter basinal correlation which indicates that they probably represent a global change in seawater composition the occurrence of decoupled 13ccar 13corg with almost invariable 13corg values at wangjiapeng zhongling yangjiaping sections is consistent with remineralization of a dissolved organic carbon doc pool by means of sulfate reduction as recorded in en3 the synchronous presence of en3 a shift to higher 87sr 86sr and decrease of mn and fe contents and 15n values together points to a glacial influence whereby oxygenation and remineralization of reduced carbon produced 13c depleted dic glaciations cause a drop in sea level which itself leads to increased continental shelf area to be exposed to surface weathering and ultimately to enhanced delivery of radiogenic 87sr the increase of 87sr 86sr ratios sulfate and phosphate are consequences of surface runoff into oceanic environments and such perturbations induce biogeochemical changes

the evolution of deformation and topography of high elevated plateaus 1 model numerical analysis and general results a temperature dependent viscoplastic flow model of continental lithosphere is used to investigate the evolution of deformation and topography of high elevated plateaus such plateaus are products of both continent continent collision tibetan plateau and ocean continent collision the altiplano and develop in the overriding continental plate in this study we emphasize the mechanically simpler case of oceanic collision because it does not involve mass transfer between the two plates the lithosphere is deformed in response to tectonic and buoyancy forces the tectonic forces arise from subduction of an oceanic plate or underthrusting of continental lithosphere that horizontally indents and vertically shears the overriding lithosphere the buoyancy forces arise in response to horizontal density variations and tend to relax existing topography or thick crust the time evolution of the deformation and topography is investigated using a finite element technique that solves for the flow field in the overriding lithosphere the model produces dynamically supported near trench topography and inland mountain topography that is isostatically supported by a thick crust a finite region of localized deformation thick crust and high topography develops only if the model includes a horizontal thermal perturbation or an initially thick crust however only thermally perturbed lithosphere generates a plateau topography the shape and size of the calculated plateau depend on the wavelength of the thermal perturbation grashof number and density contrast between the crust and mantle the time evolution of the deformation shows a significant change in the deformation pattern as the high elevated plateau evolves during early stages compressional deformation of the crust and mantle are localized in the thermally perturbed weak zone at later stages as the crust thickens buoyancy forces of larger magnitude resist further thickening of the crust and the locus of compressional crustal deformation migrates inland this migration does not affect the location of the mantle deformation which remains in the thermally weak region but it is accompanied by a significant shear deformation in the weak lower crust the separation of the crustal locus from the mantle locus of deformation emphasizes the importance of vertically dependent deformation in the formation of high elevated plateaus this demonstrates the limitations of models that ignore changes of deformation with depth such as plane stress or thin sheet models

heat flow in the southwestern okinawa trough abstract lying to the southeast of the east china sea continental shelf the okinawa trough is believed to have been formed by crustal spreading behind the ryukyu island arc it has the highest mean heat flow among the pacific marginal basins the mean of 18 reliable measurements is3 34 2 75hfu the next highest mean heat flow for a marginal basin is2 53 1 60hfu for the fiji plateau the high mean value for the okinawa trough may be partially attributable to a sampling bias because most measurements were made in or near the well sedimented central rift or graben the reported values range from 0 36 to 10 49 hfu with low values probably resulting from hydrothermal circulation in an area of large relief the high heat flow suggests that the age of the okinawa trough may be as young as 2 ma such an estimate is in gross agreement with the inferred plio pleistocene age for the drifting in the okinawa trough the opening of the okinawa trough may continue onto taiwan but the relation between the opening of the trough and the occurrence of a nearby onshore geothermal resource area remains unsolved

evolution of lithospheric strength after thrusting modeling of an eroding thrust belt indicates that immediately after thrusting the lithosphere is about twice as strong as unthickened cratonic lithosphere but after 10 to 20 m y it becomes weaker than cratonic lithosphere reaching a minimum of 40 to 70 of cratonic strength 30 to 100 m y after thrusting this may explain why rifting of thrust belts commonly occurs several tens of million years after thrusting ceases relative weakening results from 1 elevation of the geotherm caused by burial of radioisotope enriched surface rocks and 2 replacement of strong mantle rocks by weak crustal rocks in the lithospheric column low denudation rates enhance weakening by allowing radioisotope enriched rocks to remain buried longer our modeling indicates that 1 magmatism after thrusting is favored by high concentrations of heat producing elements in the lower plate and by slow denudation rates and 2 factors that affect denudation rate such as climate and the development of external drainage can significantly influence evolution of a thrust belt

a la icp ms analysis of rare earth elements on phosphatic grains of the ediacaran doushantuo phosphorite at weng an south china implication for depositional conditions and diagenetic processes the ediacaran doushantuo formation at weng9an south china hosts well preserved phosphatized microfossils known as the weng9an biota a laser ablation icp ms analysis of rare earth element ree characteristics of the fossil bearing black phosphorite unit of the doushantuo formation at weng9an was conducted with the aim of unravelling the depositional conditions and diagenetic processes during formation of the phosphorites spherical phosphatized microfossils and phosphatic clasts were analysed and the ree data display middle ree mree enriched shale normalized ree patterns the spherical phosphatized microfossils show an increase in total ree contents ree from core to rim negative correlations between ree and the extent of mree enrichment over the other ree indicated by la n sm n yb n sm n are observed for analysed spots within individual phosphatic grains which may be due to complex diagenetic history of the phosphatic grains with fluctuations in ree sources and chemical parameters in a high energy shallow water environment being additional factors the la n yb n and la n sm n characteristics of the phosphatic grains suggest they were mostly influenced by early diagenetic alteration rather than late extensive recrystallization the negative ce anomalies in the samples suggest they formed under oxic bottom water conditions positive eu anomalies are present in all samples and are likely to reflect involvement of hydrothermal fluids rather than changes in redox conditions of porewater overall this study has major implications for phosphorites as important archives for the study of geochemical proxies the ediacaran period and also evolutionary changes

ftir microspectroscopy of ediacaran phosphatized microfossils from the doushantuo formation weng an south china abstract phosphatized microfossils from ca 580 ma from the doushantuo formation in the weng an region of south china were analyzed by fourier transform infrared ftir microspectroscopy for their chemical characterization two morpho types of phosphatized embryo like fossils megasphaera and megaclonophycus were analyzed together with algal fossils transmission ir spectra of the microfossils have absorption bands of around 2960 cm 1 and 2925 cm 1 indicating the presence of aliphatic hydrocarbon anti symmetric aliphatic ch 3 and aliphatic ch 2 and have an additional band of around 1595 cm 1 probably derived from aromatic moieties aromatic c c in addition ir microscopic mapping shows that aliphatic hydrocarbon and aromatics are distributed inside the embryo like fossils the embryo like fossils appear to show three types of ch 3 ch 2 peak height ratios r 3 2 and aromatic c c ch 2 peak height ratios r c c 2 values 1 high r 3 2 low r c c 2 type r 3 2 0 2 1 0 and r c c 2 0 2 2 low r 3 2 medium r c c 2 type r 3 2 0 2 0 6 and r c c 2 1 4 and 3 low r 3 2 high r c c 2 type r 3 2 0 2 0 6 and r c c 2 1 8 all three types are contained in both megasphaera and megaclonophycus raman spectra for the carbonaceous matter within the rock sample show a similar degree of thermal alteration indicating that the organics were embedded in situ prior to thermal maturation the ir spectroscopic differences might reflect differences in original organic compositions of microorganisms and or immediate post mortem alteration this suggests that the origins of phosphatized embryo like fossils are more diverse than was previously recognized on the basis of their morphology a comparison of r 3 2 and r c c 2 values in the embryo like fossils with those of the algal fossils and the extant microorganisms indicates the possible interpretation that some of the embryo like fossils belong to animal embryo others are algae but none of them originate from bacteria

active deformation of asia from kinematics to dynamics using estimated strain rates across the actively deforming region of asia to infer variations in viscous stress it is shown that gradients of stress point uphill toward the center of tibet where gravitational potential energy per unit surface area reaches a maximum thus the dynamics of the deformation seem to obey the equation of creeping flow which expresses a balance between gradients in stress and the gravitational body force this balance in the region of the tibetan plateau yields an estimate of 10 22 pascal second for the average viscosity of the tibetan lithosphere which is only about 10 to 100 times greater than the viscosity of the convecting upper mantle

apatite glaucony association in the ediacaran doushantuo formation south china and implications for marine redox conditions abstract major phosphogenic events took place in the late paleoproterozoic and the ediacaran both of which followed an episode of atmospheric oxygenation these apparent coincidences have led to suggestions that the precambrian phosphorus cycle was partly regulated by oceanic redox conditions although direct evidence is lacking in this paper we report the discovery of glaucony in drill cores from the phosphorite rich ediacaran doushantuo formation at weng an south china which provides new insights into redox conditions and geochemical cycles during the deposition and early diagenesis of the lower doushantuo formation the presence of flaser and lenticular bedding in the sandstones suggests deposition in a tidally influenced shallow water environment apatite is present as millimeter scale flasers in sandstones but more commonly as intraclastic grains glaucony typically occurs as interstitial aggregates cementing and replacing detrital minerals and phosphatic grains with an upsection decrease in abundance the doushantuo glauconies have consistently high k2o 7 6 9 6 wt in particular phosphorite hosted glaucony has generally higher total iron tfe2o3 than sandstone hosted glaucony consistent with al3 fe3 substitution in the octahedral site as supported by raman peaks slightly shifted towards lower wavenumbers for higher fe content a decrease in detrital input probably resulted in limited al availability during early diagenesis which in turn led to the waning of glaucony upsection the phosphate grains do not show any negative ce anomalies consistent with precipitation of authigenic apatite in suboxic environments with insignificant preferential remobilisation of ce during deposition and early diagenesis ree y features show variable diagenetic ree enrichment in phosphorites but identical concave down patterns the wide occurrence of glaucony cement further documents the presence of a generally suboxic early diagenetic environment on the seafloor which was in active exchange with bottom seawater slight fluctuations of redox states in the porewater and bottom water would have promoted the cycling of fe enrichment of phosphate and ree and extensive glaucony authigenesis therefore the glaucony bearing phosphorites in the doushantuo formation are a direct result of fe redox driven diagenesis apatite glaucony association likely forms in suboxic environments and is considered as a useful mineralogical proxy for paleoredox conditions on the precambrian ocean floors

trace elements and their isotopes in bones and teeth diet environments diagenesis and dating of archeological and paleontological samples fossil biogenic apatites display trace element te compositions that can record environmental and biological signals give insights into past water compositions or be used for dating paleontological and archeological bones and teeth processes of te incorporation into apatites of skeletal phosphatic tissues are described ranging from those active in living organisms to those active during diagenesis abiotic incorporation mechanisms have been modeled theoretically and experimentally and include crystallographic controls on te partitioning coefficients inorganic surface adsorption and adsorption mediated through chelation and diffusion adsorption processes each leading to specific fractionation patterns predictions from models and experiments have been tested against te concentration and isotopic composition data on remains of contemporary wild or raised animals and on fossils of various ages and environments in living organisms tes incorporated in apatite are separated in two categories non essential elements whose concentration is compared to that of an essential element with similar behavior e g sr ca to reconstruct trophic chains and essential elements whose isotopic ratios are used to trace metabolic activity diet etc in fossils elements are incorporated during diagenesis such as rare earth elements rees and trace diagenetic processes ranging from very early quantitative adsorption likely mediated by chelators protracted diagenesis through inorganic adsorption and fractionation through diffusion adsorption to low grade metamorphism associated with intense redistribution under crystal chemical control these different steps are also identified as steps of radionuclide incorporation only samples where the earliest steps can be deciphered are useful for determining stratigraphic and archeological ages other samples will date late diagenetic events that have obliterated the paleoenvironmental signals but may be of geological significance e g tectonic events

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

rare helical spheroidal fossils from the doushantuo lagerstatte ediacaran animal embryos come of age a small quantity of helically coiled spheroidal fossils has been recovered from acid digestion of phosphorite samples from the ediacaran doushantuo formation south china these fossils consist of an internal body enclosed in a sculptured envelope that is very similar to that of doushantuo animal eggs and blastula embryos such as megasphaera ornata a hallmark of these fossils is a three dimensional spiral structure which always consists of three clockwise coils and occurs on both the envelope and the internal body the spiral structure consists of a spiral tunnel or canal flanked by two raised levees and it is punctured by a series of holes some specimens show evidence of uncoiling invagination along the spiral structure or bipectinate furrowing on the band between canals a possible ontogenetic link between these helical spheroidal fossils and megasphaera ornata is suggested by similar size similar envelope sculptures and co occurrence we tentatively interpret these fossils as postblastula embryos related to megasphaera ornata thus they may represent the most advanced embryonic fossils so far known from the ediacaran although their adult morphologies and phylogenetic affinity remain unknown

evaluating the role of preexisting weaknesses and topographic distributions in the indo asian collision by use of a thin shell numerical model thin viscous sheet models have proved to be very useful in exploring the interaction between plate boundary and gravitational forces during continental collision however simplifications of these models e g absence of faults planar geometry of the collision zone limit their use in making specific predictions regarding tectonic evolution such as the role of eastward extrusion along major strike slip faults during the indo asian collision this deficiency is overcome by a thin shell finite element model with faults that can assess the effects of preexisting fault configurations and topographic distributions on velocity fields numerical simulations of a palinspastically restored asia at ca 50 ma suggest that preexisting lithospheric weaknesses favor north south shortening during initial collision whereas preexisting high topography in southern asia promotes eastward extrusion these results underscore the first order importance of initial topography and the distribution of preexisting faults in the outcome of geodynamic modeling

protracted diagenetic alteration of ree contents in fossil bioapatites direct evidence from lu hf isotope systematics abstract fossil bones and teeth are potentially important repository for geochemical proxy data and a target for radiometric dating the concentration of many trace elements in bones and teeth increases by orders of magnitude after death and it is this diagenetic incorporation that forms the basis for several areas of geochemical study the use of bones and teeth in this context relies on two assumptions first that target metal ions are incorporated rapidly after death reflecting a known environmental signal and second that after early incorporation the bone or tooth remains as an essentially closed system resistant to later diagenetic change a wide literature has developed exploring these assumptions but relatively little direct evidence has been used to assess the long term diagenetic stability of trace elements within bones and teeth in this study we use the lu hf isotope system to show that bones and teeth of cretaceous and triassic age from both terrestrial and marine settings experience continued long term diagenetic change most likely through gradual addition of trace elements modelling suggests that diagenetic addition after initial recrystallisation may account for 50 of the total ree content in the sampled bones the extent depending on initial uptake conditions tooth enamel and enameloid may be more resistant to late diagenetic changes but dentine is probably altered to the same extent as bone these results have significant implications for the use of bones and teeth as hosts of chronological palaeoceanographic palaeoenvironmental and taphonomic information particularly in mesozoic and palaeozoic contexts

a lead isotopic study of circum antarctic manganese nodules abstract lead isotope ratios in manganese nodules from the circum antarctic ocean display systematic geographical variations eastward in the direction of current movement from south of australia they progressively decrease in the pacific sector and into the western atlantic sector until 30 w from this longitude lead isotope ratios increase in the eastern atlantic sector and reach their highest values in the indian sector near 110 e south of western australia they abruptly decrease these two lead isotope domains correspond closely to the circum antarctic neodymium isotope domains previously recognized by albarede and goldstein 1992 however the intraocean isotopic variations are much clearer for pb than nd reflecting the shorter oceanic residence time of pb 102 y compared with nd 103 y and confirming that pb is a much more sensitive marine tracer throughout the circum antarctic ocean the lead isotopic changes follow the pattern of present day oceanic circulation the pb shows an overall continental signature which is strongest in the indian sector the most probable candidate as the source of the continental signal is north atlantic deep water nadw which enters the circum antarctic in the south atlantic as nadw mixes with circumpolar water lead isotope ratios increase eastward in the indian sector reflecting progressive loss of the pacific derived pb signal the abrupt decrease of lead isotope ratios between the indian and pacific sectors appears to be a consequence of addition of pacific seawater through the pacific lead isotope ratios gradually decrease eastward approaching values of east pacific rise metalliferous sediments ocean ridge basalts and south pacific mn nodules this trend results from an increasing influence of pacific ocean water containing a component derived from pacific volcanism the pacific signature persists into the argentine basin in the southwest atlantic ocean reflecting significant input of water from the drake passage northward into the atlantic the pacific pb component is seen in the atlantic and indian oceans near southern africa as well as in the atlantic and indian sectors of the circum antarctic lead and neodymium isotopes correlate in the atlantic indian sectors but not in the pacific sector reflecting differences in residence times and modes of water mass mixing the renewal of circum antarctic water occurs on a timescale which is short compared to the residence time of nd but long compared to pb as a result dissolved pb is subject to considerable changes over the timescale of circumpolar current movement while nd is less easily affected nadw is added locally in the south atlantic and moves eastward as a water mass as nadw mixes with circumpolar water cpw through the indian sector changes affecting nd pb in nadw and cpw occur in tandem and neodymium and lead isotopes are correlated through the pacific sector gradual addition of pacific seawater occurs over a large region and nd pb relationships are not coupled because simple isotopic mixing systematics are not conserved there neodymium and lead isotopes vary independently the consistency of lead isotope variations with present day ocean bottom water circulation is remarkable considering that the mn nodules as sampled reflect the integrated flux of pb over 105 106 y the oceanic mixing time is 103 y the residence time for pb is 102 y and the circulation time of the circumantarctic ocean is 30 y processes occurring over such different timescales can show such a relationship only if the system is stable over the longest time interval thus on average element sources and ocean circulation through the pleistocene have been similar to the present day despite significant perturbations during glacial periods

phosphate lu hf geochronology abstract igneous metamorphic and biogenic apatite contains lu and hf in proportions that make this mineral suitable for dating by the 176lu 176hf method we present a new method for separation of lu and hf from apatite that involves a single extraction column step for hf and a second exchange column step for lu this procedure allows rapid sample processing prior to analysis by mc icp ms results from the igneous gardiner skaergaard and khibina intrusions indicate that the closure temperature for the lu hf isotopic system in apatite is relatively high the lu hf isochron age of 60 ma for the skaergaard intrusion east greenland is older than the generally accepted emplacement age but does not appear to be a product of mixing a centimeter sized apatite from the otter lake area grenvillian province yields high and variable 176lu 177hf ratios showing that metamorphic apatite may in some cases allow for mono mineral dating the most important application of the method is undoubtedly its potential use for dating sedimentary formations although we find biogenic apatite in porous matrix highly susceptible to post depositional resetting of the lu hf isotopic system the preservation of a near depositional age for a shark tooth from the impermeable london clay and recent work on crystalline authigenic phosphorites earth planet sci lett 201 2002 203 show the lu hf system to be a promising tool for direct dating of sedimentary formations

nanosims u pb dating of fossil associated apatite crystals from ediacaran 570 ma doushantuo formation abstract sedimentary rocks record a wealth of information on the evolution of lithosphere hydrosphere atmosphere and biosphere however uncertainty about the age of many precambrian sequences makes it very difficult to establish the accurate temporal framework which can dramatically improve our understanding of the scale and duration of geological and biological events the neoproterozoic doushantuo formation south china preserve unique assemblages of early multicellular fossils and records valuable paleoenvironmental information the age of these formations is thus critical for understanding the important biological and climatic events that occurred towards the end of the proterozoic eon in this study we used nanosims to date samples from the doushantuo formations by apatite u pb geochronology fossil associated precipitated apatite grains were identified using optical microscope and raman spectrometer the resultant age is 305 26 ma high common pb and low u concentration in those apatite specimens limit the precision of age determination while the yielded age suggests that the doushantuo formation may have experience a post depositional hydrothermal event

isotopic evidence for geochemical decoupling between ancient epeiric seas and bordering oceans implications for secular curves isotopic analysis of conodonts and their host limestones sampled between two regionally extensive altered volcanic ash layers in eastern laurentia shows that a 454 ma epeiric sea maintained large lateral differences in nd and c isotope compositions this is consistent with inferred temperature salinity defined epicontinental water masses and restricted circulation between epicontinental and oceanic environments because the majority of old marine fossils and sedimentary rocks are known from epeiric seas some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses rather than global scale changes in the state of the earth ocean system

assessing the veracity of precambrian sponge fossils using in situ nanoscale analytical techniques abstract paleontological inferences molecular clocks and biomarker fossils indicate sponges evolved in the cryogenian but precambrian sponge fossils are rare poorly substantiated and controversial spicule like microstructures slms hosted in phosphatized fossils from the ediacaran doushantuo formation 635 551 ma at weng an of south china have been interpreted as cylindrical siliceous monaxons and their hosting fossils as the oldest demosponges in the fossil record in order to assess their veracity as the oldest spiculate demosponges we utilize a suite of in situ nanoscale analytical techniques including scanning electron microscopy synchrotron x ray fluorescence mapping x ray absorption near edge structure xanes spectroscopy focused ion beam electron microscopy and transmission electron microscopy to evaluate the ultrastructures and elemental chemical and mineralogical compositions of the slms our data decisively shows that the slms are carbonaceous in composition and rectangular in transverse sections and therefore are not cylindrical siliceous spicules instead the slms may be microbial strands axial filaments of early hexactinellids or acicular crystals molded by organic matter regardless our new data invalidate the oldest and only precambrian demosponges with mineralized spicules these results indicate that interpretations of precambrian sponge fossils should be scrutinized with compositional mineralogical and ultrastructural data collected using in situ analytical techniques in addition our conclusions affirm that no unequivocal biomineralizing sponges occur below the ediacaran cambrian boundary if hexactinellids and demosponges did diverge in the cryogenian as suggested by molecular clocks and biomarkers they either evolved biomineralization long after their divergence or their biomineralized spicules were never preserved until after the ediacaran cambrian boundary in either case the dearth of biomineralizing sponge fossils in the precambrian and their abundance in the early cambrian must reflect a geobiologically significant aspect of the precambrian phanerozoic transition

role of oblique convergence in the active deformation of the himalayas and southern tibet plateau noting similarities with subduction along curved oceanic trenches and using a simple block model we show that radial vergence evident in earthquake slip vectors along the himalayan deformation front east west extension on north trending normal faults in the himalayas and southern tibet and right lateral strike slip on the karakorum jiali fault zone can all result from basal shear caused by the indian plate sliding obliquely beneath tibet along a gently dipping arcuate plate boundary within the framework of this mechanism the normal faults in the himalayas and southern tibet are not proxies for the uplift history of tibet the distribution and style of the faults in the himalayas and southern tibet suggest that the basal drag from the underthrusting indian lithosphere extends northward beneath most of southern tibet

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

flower like apatite recording microbial processes through deep geological time and its implication to the search for mineral records of life on mars the biological cycling of phosphorus on earth could be as early as the origin of life in early archean however because of the low abundance and fine particle size phosphate related to microbial ecophysiological activities in early sedimentary rocks especially those deposited before the great oxidation event goe ca 2 45 2 32 ma is still poorly addressed it is not until recently that certain petrographic and mineralogical features of apatite in the early precambrian sedimentary rocks were found related to microbial activities in this study we report high resolution electron microscopic investigations on apatite from the neoarchean to early paleoproterozoic banded iron formations bifs mesoproterozoic to lower cambrian black shale and phosphorites and pliocene sediments apatite in bifs occurs as 4 8 m radial flowers with petals made of apatite rods their mineralogical and petrologic features are highly similar to those in the younger sedimentary rocks in which biomass have been confirmed to play an important role in the mineralization of phosphate we suggest that these sedimentary rocks or sediments have experienced similar phosphogenetic processes mediated by biomass that led to the mineralization of phosphorus the formation and preservation of phosphate apatite with conspicuous recognizable features in association with biological activities from late archean to pliocene implies its universal significance in recording microbial processes through deep geological evolution with mild dynamic processes the martian sub surface has better preservation conditions than earth and the micro structure of phosphate formed in environments mediated by microorganisms could be recognized by high resolution observations on the surface of mars or returned samples if microbial life ever developed on mars

when did the roof collapse late miocene north south extension in the high himalaya revealed by th pb monazite dating of the khula kangri granite th pb ion microprobe measurements made on 12 monazite grains from the khula kangri granite tibet bhutan frontier are interpreted to indicate that crystallization occurred at 12 5 0 4 ma the leucogranite is cut by the gonto la detachment part of the southern tibet detachment system that has allowed upper level north directed extension of the himalayan orogen significant orogen normal extension in southern tibet appears to have continued 8 10 m y later than previously recognized this is the first reported crystallization age for a leucogranite east of the yadong cross structure an apparent 70 km offset of the high himalaya and southern tibet detachment west of the yadong cross structure reliable ages for high himalaya events major main central thrust slip granite generation and emplacement attainment of critical topography and major detachment extension group between ca 24 and 19 ma we interpret the west to east change across the yadong cross structure to be due to either 1 an abrupt 10 m y younging of principal high himalayan events or 2 a deeper thus younger exposed part of the footwall of the southern tibet detachment near khula kangri the southern tibet detachment is cut by the highly oblique yadong gulu rift a manifestation of tibet plateau east west extension integrated estimates of magnitude and rate of detachment displacement suggest that the observed postcrystallization north directed extension lasted for 1 3 m y after which time the yadong gulu rift formed this interpretation is consistent with initiation of east west extension of tibet at ca 8 ma

fossil preservation through phosphatization and silicification in the ediacaran doushantuo formation south china a comparative synthesis phosphatized and silicified microfossils preserved through replication of organic templates by authigenic calcium phosphate and silica respectively form through mechanistically similar mineralization processes but under notably different conditions in order to characterize the environments and taphonomic processes associated with these preservational pathways we compare phosphatized and silicified fossil assemblages in the ediacaran doushantuo formation south china in terms of stratigraphic and paleogeographic distribution preservation and biodiversity several differences are recognized 1 phosphatization and silicification both occurred in sub tidal doushantuo sediments but phosphatized fossils generally occur in shallower sub tidal facies than silicified fossils 2 although both phosphatized and silicified fossils are preserved three dimensionally the latter are typically more compressed and 3 whereas phosphatized assemblages are numerically dominated by multicellular acritarchs putative metazoan eggs and embryos silicified assemblages are dominated by prokaryotes these differences can be explained by environmental and taphonomic distinctions phosphatized fossils occur in intraclastic phosphorites and phosphatic carbonates and were likely fossilized near the suboxic anoxic boundary in sediments under oxic bottom waters with redox and biogeochemical gradients conducive to p remineralization phosphate burial via iron pumping and calcium phosphate precipitation reworking and winnowing of phosphatized fossils preferentially concentrated certain taxa prior to final burial in contrast silicified fossils typically occur in chert nodules which formed deeper in the sediment column in ferruginous pore waters and perhaps under ferruginous bottom waters where bacterial sulfate reduction and pyrite formation created low ph conditions conducive to calcite dissolution and silica precipitation once formed chert nodules mostly remained in situ but some were reworked with turbidites and olistostromes developed in slope facies despite these distinctions phosphatized and silicified fossil assemblages in the doushantuo formation generally overlap in time and space and representatives of all major groups and many common genera of doushantuo organisms are present in both phosphorites and cherts consequently although both preservational pathways have paleoenvironmental and taphonomic biases their relative biases on taxonomic presence absence data are minimal given sufficient sampling efforts and paleobiologic and biostratigraphic information based on presence absence but not abundance data from the doushantuo formation appears to be robust

possible ree constraints on the depositional and diagenetic environment of doushantuo formation phosphorites containing the earliest metazoan fauna abstract the neoproterozoic doushantuo formation south china preserves a unique assemblage of what are probably the earliest metazoan fossils sponges cnidarians and bilaterians in the world that could contribute to a better understanding of early faunal evolution on earth the formation spans an interval of dramatic biological chemical and climatic change in weng an mine in southwest china extensive paleontological investigations show embryonic larval and adult animal fossils occur in the upper ore bed of the doushantuo phosphorite but no animal forms have been found in the lower ore bed taphonomy and experiments on fossilization suggest that no fauna lived on or in the lower ore bed as it was deposited rare earth element ree data for the doushantuo phosphorites in weng an document the paleoenvironmental condition under which the phosphorites were deposited the shale normalized ree patterns of the upper phosphorite bed upb are characterized by old vendian early cambrian phosphorite patterns the ree patterns of the lower phosphorite bed lpb are significantly different with less negative ce anomalies and less lree and hree depletion the ree characteristics suggest that the doushantuo phosphorites record seawater condition at the time of sediment deposition and are not affected by subsequent diagenesis the ree compositions of the doushantuo phosphorites show that the marine depositional environment changed from anoxic in the lower ore bed to oxic in the upper ore bed the uniquely diverse and abundant doushantuo eukaryote fossil accumulations at weng an thus may be related to a change from anoxic to oxic bottom water conditions

a composite reference section for terminal proterozoic strata of southern namibia integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal proterozoic to cambrian witvlei and nama groups of namibia coupled with precise u pb zircon age constraints these correlations make it possible to construct a composite reference section for use in calibrating terminal proterozoic chronostratigraphy the namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the proterozoic cambrian boundary the two glacial horizons may represent each of two distinct varanger age glaciations better known from the north atlantic region from the higher of the two glacial horizons up the composite stratigraphy preserves one of the thickest and most complete available records of carbon isotope variability in post varanger terminal proterozoic seawater four carbon isotope chemostratigraphic intervals are recognized 1 a postglacial negative 13 c excursion npg interval 2 a rising interval pr interval of increasing positive delta 13 c values 3 a falling interval pf interval characterized by decreasing positive 13 c and culminating in near zero or negative values and 4 an interval of moderately positive relatively invariant 13 c values i interval that extends up to the unconformity that contains the proterozoic cambrian boundary each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world by comparing sediment accumulation rate in the radiometrically calibrated namibian stratigraphy with sediment accumulation rates in correlative sections in arctic canada and oman a maximum age of 564 ma is estimated for the end of the younger varanger glaciation 25 m y younger than previous estimates

diagenetic overprint to a negative carbon isotope anomaly associated with the gaskiers glaciation of the ediacaran doushantuo formation in south china abstract ediacaran carbon isotope records contain significant and long lasting negative anomalies for which convincing explanations have not been given we recognized a diagenetic overprint contributing to the 120 m thick negative anomaly interval in the upper part of the ediacaran doushantuo formation at yangjiaping hunan province in south china coarse grained carbonate facies deposited in an outer shelf setting of the yangtze platform provided an opportunity to examine diagenetic components by measuring micro drilled sub samples scattered 13c values of bulk samples 5 to 14 in this anomaly interval are here attributed to the secondary precipitation of highly 13c depleted up to 33 calcite cement on dolomite sedimentary grains of less negative 13c this diagenetic overprint was not observed in the intervals above or below the anomaly in these intervals the 13c difference between the cements and the grains is insignificant strontium isotopic variation in the anomaly interval is identical to that of the interval with negative 13c anomaly in the middle doushantuo formation reported from the three gorges area which is presumably correlated with the gaskiers glaciation this correlation scheme indicates a long term hiatus 35 myrs between the anomaly horizon and the overlying dengying formation at yangjiaping we suggest that the anaerobic oxidization of reduced carbon organic carbon or methane by sulfate reduction was the most likely process responsible for 13c depleted secondary calcite which progressively filled intergranular porosity an increase in atmospheric oxygen during ediacaran and the gaskiers glaciation enhanced oxidative weathering and elevated seawater sulfate concentrations to a level sufficient for diagenetic alteration this co occurred with an increase in 87sr 86sr from 0 7080 to 0 7085 in middle ediacaran we also show that the 13c profile of the doushantuo formation at yangjiaping is similar to that at weng an in guizhou province another section recording an outer shelf depositional environment although further study is needed correlation between the yangjiaping unit 3 with the 13c scattered interval in weng an suggests that the embryo bearing phosphorites at weng an were formed likely before the gaskiers glaciation

in situ u pb dating of monazite xenotime and zircon from the lantian black shales time constraints on provenances deposition and fluid flow events abstract the lantian formation located in eastern south china block scb forms the lower sequence of the ediacaran period at the transition from proterozoic to paleozoic the fossils of lantian formation have been proposed to predate the avalon biota about 579 565 ma the earliest known fossil assemblages with complex morphologies based largely on stratigraphy mineral chemistry and in situ u pb dating of monazites xenotimes and zircons from the black shale in fossil rich zone are studied for the first time to provide critical age constraints for provenance early diagenesis and fluid flow events detrital zircons and monazites give age populations similar to that of the doushantuo formation indicative of the same provenances for ediacaran basins throughout the scb the low y monazites formed during early diagenesis yield a weighted mean age of 612 29 ma n 9 mswd 3 1 age spectrum of analyzed monazites demonstrated that the sedimentation started from at least around 645 ma subsequently the overprinting events could be recorded by xenotimes and zircons precipitated from fluids during late diagenesis age results from xenotimes ranging from 441 ma and 585 ma were contemporaneous with the timing of collision of scb with gondwanaland and a weighted mean age of 394 8 3 7 ma n 23 mswd 2 8 obtained from diagenetic zircons could be correlated to the post tectonic granites widespread on the scb the pre existing monazites generally remain pristine and the multi staged fluids were locally interacted with the samples to form th rich domains in monazites

age and implications of the phosphatic birmania formation rajasthan india abstract the birmania inlier in western rajasthan india contains important phosphate deposits the depositional age of which is poorly constrained here we provide the first direct age constraints for the phosphate bearing birmania formation using a combination of paleontological and detrital zircon u pb data the occurrence of the multicellular algal fossil wengania exquisita in phosphatic chert of the birmania formation suggest that it was deposited during the ediacaran detrital zircon age distributions contain prominent populations of 1 7 1 9 ga grains with subordinate younger grains that range from 650 to 980 ma the distribution broadly resembles those from neoproterozoic strata from both cratonic and himalayan india but like zircon age distributions from the marwar group lacks 1 0 1 2 ga grains which may suggest that both areas shared similar local sources the lack of zircon grains younger than 650 ma is consistent with an ediacaran depositional age because almost all cambrian or younger strata from india have yielded cambrian or latest neoproterozoic age grains these findings raise the possibility of a previously unrecognized late neoproterozoic episode of phosphogenesis on the india craton the presence of wengania exquisita further supports strong palaeogeographic affinity between the india and south china during the neoproterozoic and cambrian

petrographic analysis of new specimens of the putative microfossil vernanimalcula guizhouena doushantuo formation south china the neoproterozoic doushantuo formation contains some of the earliest evidence for animals and animal embryos including the putative bilaterian vernanimalcula guizhouena samples of the black facies of the weng an phosphate member of the doushantuo formation were disaggregated in a weak buffered formic acid solution in order to concentrate microfossils and mounted in epoxy the resulting grain mounts were made into thin sections and examined petrographically five specimens with morphology matching what was described as vernanimalcula were found in a single sample from the black facies up to five individual repeated structures are found in each specimen measurements of lengths and widths of morphological features were made on the new specimens these measurements are found to be consistently similar across the new specimens and within the range reported for the original specimens a diagenetic sequence is constructed using the available evidence this sequence consists of the early phosphatization of a biological structure a second phase of internal sedimentation compaction and dewatering and a later phase of phosphatic cementation while this study does not constitute the final word on the biogenicity of these morphologies it seems unlikely that phosphatization alone could be responsible for a large number of specimens with the same repeated form with the same dimensions the strikingly similar sizes and morphologies as well as the observed diagenetic sequence lead us to conclude that these specimens are likely biogenic in nature

silicified glendonites in the ediacaran doushantuo formation south china and their potential paleoclimatic implications the ediacaran period is punctuated by the ca 580 ma gaskiers glaciation in newfoundland however paleoclimatic data are scarce in ediacaran successions in south china where abundant geochemical and paleobiological data are shaping current understanding of ediacaran evolutionary and environmental history here we report the occurrence of silicified glendonites in the ediacaran doushantuo formation deposited in an inner shelf environment on the south china block petrographic evidence suggests that these silicified glendonites are pseudomorphs after syndepositional or early authigenic ikaites formed at near freezing temperatures the glendonite bearing stratigraphic interval is characterized by positive 13c values it predates both the negative 13c excursion en3 widely believed to be an equivalent of the shuram negative excursion and excursion en2 although alternative interpretations may be possible these glendonites may be related to and correlated with the gaskiers glaciation if confirmed this correlation suggests that the shuram event postdates the gaskiers glaciation thus having important implications for ediacaran climate changes carbon cycles and biological evolution

tectonomagmatic evolution of cenozoic extension in the north american cordillera the spatial and temporal distributions of cenozoic extension and magmatism in the cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt 100 km pre extension and followed the onset of intermediate and silicic magmatism by no more than a few million years extension began in early eocene time in southern british columbia northern washington idaho and montana farther s extension began at about the eocene oligocene boundary in the great basin and slightly later in the mojave sonora desert region the intervening area at the latitude of las vegas remained quiescent until mid miocene time compositional and isotopic characteristics of most pre miocene magmas are consistent with their containing major components of melted continental crust in mid miocene time two major changes occurred widening of the area of extension and the widespread appearance of basaltic magmas the area affected by extension from southwestern montana to the lake mead region widened to several hundred kilometres by this time extension in southern british columbia northern washington and northern montana had ceased probably before the end of the eocene and extension s of lake mead except in the gulf of california had waned regions affected by the broader belt of extension during late miocene pliocene and quaternary time experienced basaltic magmatism which began along a central rift zone in the northern part of the region and which had within a few million years spread to include most of the region later basaltic activity has tended to concentrate in restricted zones especially near the margins of the extended area we recognize a correlation between the amount of earlier crustal thickening and cenozoic extension and between the length of time after shortening but before extension and the degree to which a given region was intruded by late cretaceous plutons the localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal mechanical model developed in a companion paper sonder et al this raises the possibility that stresses inherent in the north american plate dominated over plate interaction forces as controls of the cenozoic tectonomagmatic evolution of the north american cordillera especially in its earlier stages

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

eumetazoan fossils in terminal proterozoic phosphorites phosphatic sedimentary rocks preserve a record of early animal life different from and complementary to that provided by ediacaran fossils in terminal proterozoic sandstones and shales phosphorites of the doushantuo formation south china contain eggs egg cases and stereoblastulae that document animals of unspecified phylogenetic position small fossils containing putative spicules may specifically record the presence of sponges microfossils recently interpreted as the preserved gastrulae of cnidarian and bilaterian metazoans can alternatively be interpreted as conventional algal cysts and or egg cases modified by diagenetic processes known to have had a pervasive influence on doushantuo phosphorites regardless of this interpretation evidence for doushantuo eumetazoans is provided by millimeter scale tubes that display tabulation and apical budding characteristic of some cnidaria especially the extinct tabulates like some ediacaran remains these small benthic colonial fossils may represent stem group eumetazoans or stem group cnidarians that lived in the late proterozoic ocean

a new sims zircon u pb date from the ediacaran doushantuo formation age constraint on the weng an biota as a well known phosphatized lagerstatte the ediacaran weng9an biota in central guizhou province of south china contains diverse acanthomorphic acritarchs algal thalli tubular microfossils as well as various spheroidal fossils these fossils provide crucial palaeontological evidence for the radiation of multicellular eukaryotes after the termination of the neoproterozoic global glaciation while the weng9an biota is generally considered as early ediacaran in age on the basis of phosphorite pb pb isochron ages ranging from 572 ma to 599 ma the reliability and accuracy of these age data have been questioned and some geologists have proposed that the weng9an biota may be younger than 580 ma instead here we report a sims zircon u pb age of 609 5 ma for a tuffaceous bed immediately above the upper phosphorite unit in the doushantuo formation at zhangcunping yichang south china litho bio and chemostratigraphic correlations suggest that the upper phosphorite unit at zhangcunping can be well correlated with the upper phosphorite unit at weng9an which is the main horizon of the weng9an biota we therefore conclude that the weng9an biota could be as old as 609 5 ma

is the ogcheon metamorphic belt of korea the eastward continuation of the nanhua basin of china abstract the ogcheon metamorphic belt in central korea has been interpreted to be the eastward extension of the nanhua basin aulacogen of southeast china in this paper comparisons are made between the two regions based on stratigraphic thermal tectonic and other considerations from this comparison correlation of geological events between the nanhua basin and ogcheon metamorphic belt are at best equivocal the closest similarity is the presence in both regions of two major diamictite units of glacial origin and neoproterozoic age 750 690 ma range in china but of controversial origin and uncertain age in korea volcanic rocks in both regions appear to have similar petrological and geochemical traits and are interpreted to be rift related however their ages are different mostly 795 780 ma in the nanhua basin and c 750 ma in central korea so correlation remains uncertain more isotopic data from both regions may shed light on this comparison correlation between other pre carboniferous stratigraphic units in the two regions is hampered by the uncertainty about the stratigraphic age and succession in the ogcheon metamorphic belt stemming mainly from the absence of fossils and the strong tectonic metamorphic overprint both regions appear to have undergone deformation and metamorphism during the middle triassic indosinian songrim in some uncertain way related to the collision between the north and south china plates in the ogcheon metamorphic belt there has been no confirmation of a mid palaeozoic event but in the nanhua basin that event is recorded by stratigraphic and palaeogeographic evidence in the nanhua basin there is no evidence for an early permian metamorphic event that appears on isotopic grounds to have affected the ogcheon metamorphic belt this apparent difference has been interpreted as a result of diachronous deformation during late palaeozoic early mesozoic plate collisions that took place earlier in the east in korea than in the west in china

acritarchs of las ventanas formation ediacaran uruguay implications for the timing of coeval rifting and glacial events in western gondwana abstract acritarchs and other organic walled microfossils occurring in siltstones of the las ventanas formation quebrada de viera and el perdido members are systematically described and illustrated the assemblage includes the following species leiosphaeridia tenuissima leiosphaeridia minutissima lophosphaeridium sp soldadophycus bossii soldadophycus major soldadophycus sp and vendotaenia antiqua the microflora is characterized by low diversity six species dominance of l tenuissima absence of acanthomorphic acritarchs and relatively large size of sphaeromorphs reaching 400 m in diameter a number of species are shared with acritarch assemblages preserved in the overlying arroyo del soldado group differences between assemblages include the occurrence of abundant bavlinella faveolata and small size of spheroids in the arroyo del soldado group the assemblage occurring in las ventanas formation is assigned to the ediacaran leiosphere palynoflora which spans the interval between the base of the ediacaran end of the marinoan glaciation 635 ma and the termination of the gaskiers glaciation 582 ma an early ediacaran age between 615 and 579 ma is also supported by available radiometric ages an extensional setting for las ventanas basin is suggested on the basis of the bimodal synsedimentary volcanism strong palaeorelief great thickness of alluvial fan conglomerates and the evolution from continental to open marine environments diamictites occurring in the quebrada de viera member are described for the first time including associated dropstones which suggest a glacial origin if confirmed this would be one more example of the association between rifting and glaciation in the neoproterozoic coeval with a low diversity high abundance acritarch microflora a causal relationship between these tectonic climatic and biologic events is discussed

initiation of asymmetric extension in continental lithosphere govers r and wortel m r 1993 initiation of asymmetric extension in continental lithosphere in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 x 96 the physical conditions are investigated under which the lithospheric scale style of extension is pure shear or simple shear we focus on the initial stages of continental extension to monitor how symmetric or asymmetric modes of extension evolve from specific tectonic conditions continental collision magma intrusions and interaction between the lithosphere and the underlying mantle are investigated as sources for extension we use a finite element method to model the thermo mechanical evolution of continental lithosphere experimental flow laws are used to model the elastic hutle power law creep or diffusion creep rheology of lithospheric rocks our results indicate that if in plane forces change from compressive to tensile immediately after a rapid mountain building phase initiation of a lithosphere scale detachment fault is possible we find a strong dependence of the extensional style on the distribution with depth of residual stresses from the collision phase this result is consistent with observations of gravitational collapse in regions like the aegean and the basin and range province where detachment faults have exhumed lower crustal rocks the predicted dip direction of the fault also agrees with observations in these areas intrusion of magma into continental lithosphere which is subject io in plane tensile forces will cause localization of pure shear deformation the style of deformation resulting from mantle plumes impinging to the base of the lithosphere is symmetric delamination of lithospheric mantle may initiate detachment faults if delamination occurs at the end of a collision phase when in plane forces change sign from compressive to tenstle this result also strongly depends on the assumed residual stress distribution if delamination occurs during the mountam huilding phase the style of deformation will be pure shear another interesting outcome from our modeling is that dramatic strain weakening as a result of a deformation mechanism change from dislocation creep to diffusion creep reduces the tendency to strain localization a large volume of the published work on continental extension has been concentrated on quantifying the stretching factor p of the pure shear model of mckenzie 1978a in some basins it was found that the kinematics of deformation could be more accurately described by depth dependent stretching factors royden and keen 19801 which introduced asymmetry near basin edges in the simple shear model of wemicke

the hf isotope composition of global seawater and the evolution of hf isotopes in the deep pacific ocean from fe mn crusts this paper presents a revised chemical purification method of hf for the measurement of hf isotope ratios of fe mn crusts on a new generation of double focusing multiple collector plasma source mass spectrometer mc icp ms by measuring surface scrapings of hydrogenetic fe mn crusts distributed over the three major ocean basins the present day hf isotope distribution of oceanic deep water is assessed in detail the results show an ehf provinciality correlated with those of other radiogenic isotope tracers such as nd and pb in agreement with previous studies this supports the use of hf isotopes as tracer of element source provenance and water masses fe mn crusts display the same present day hf nd isotope array as given before for mn nodules the smaller isotopic variability of hf compared with nd may either be caused by a more efficient mixing of hf than nd in the ocean due to a longer residence time or may be a consequence of a systematically more radiogenic hf than nd isotope signature delivered to the oceans by weathering of continental crust a hf isotope time series was measured on crust va13 2 to assess the hf isotope compositions of the central pacific deep water over the past 26 ma no consistency is observed between the hf and pb isotope time series in contrast hf and nd isotope time series display similar patterns which are however apparently offset by 2 ma prior to 14 ma differential diffusion of hf and nd does not explain this offset the smaller amplitude in the hf isotope variations compared with the nd isotopes rather argues for more efficient mixing of hf in the ocean we suggest that both isotope systems have responded in a similar way to the processes affecting the dissolved radiogenic isotope composition of pacific deep water during this time interval the parallel increase in ehf and end observed between 14 and 3 ma may probably be attributed to the increased inputs of hf and nd into the central pacific ocean derived from the weathering of the pacific islands arcs over the past 3 ma an increased aeolian continental input derived from asia most likely caused negative shifts in end and ehf recorded by va13 2 the fact that hf and nd isotope compositions plot along the present day array for fe mn crusts and mn nodules over the entire past 26 ma suggests that aeolian supply of hf to the pacific ocean has been a long term important feature

integrated chemostratigraphy of the doushantuo formation at the northern xiaofenghe section yangtze gorges south china and its implication for ediacaran stratigraphic correlation and ocean redox models abstract the ediacaran doushantuo formation in the yangtze gorges of south china plays an important role in our understanding of biological evolution global correlation and ocean redox conditions because of the availability of high resolution paleontological and geochemical data and numerous radiometric dates however integrated study has been focused on the jiulongwan section that was largely deposited below wave base in a restricted shelf lagoon jiang et al 2011 zhu et al 2011 studies of shallower water successions are lacking and this presents a challenge to test ediacaran stratigraphic correlation and ocean redox models to fill this knowledge gap we conducted a high resolution integrated study of the doushantuo formation at the northern xiaofenghe nxf section approximately 35 km to the northeast and paleogeographically updip of the jiulongwan section with the exception of the basal 20 m nxf sediments were deposited above normal wave base integrated biostratigraphic and chemostratigraphic data indicate that the 140 m thick nxf section correlates with the lower doushantuo formation member i and much of member ii i e the lower ca 70 m of the formation at jiulongwan geochemical data from nxf and other doushantuo sections indicate that euxinic conditions may have been limited to a shelf lagoon represented by the jiulongwan section that was restricted between the proximal inner shelf and a distal shelf margin shoal complex at least during early doushantuo time following the deposition of the doushantuo cap dolostone further integrated studies are necessary to test whether euxinic conditions existed in open marine shelves in south china and elsewhere during the ediacaran period

scenarios for the evolution of life on mars received 10 march 2005 revised 2 august 2005 accepted 7 september 2005 published 22 november 2005 1 as the environmental histories of earth and mars have diverged drastically after the first few hundred million years so would the history of any life on them while earth has had liquid water on its surface for billions of years mars most likely had long dry and cold environments interspersed with warmer and wetter periods life thrived on earth but may have been severely restricted on mars there it could be present today in liquid water in or beneath ice sheets or glaciers in subterranean aqueous reservoirs especially in regions of elevated heat flow or in protected habitats such as lava tubes caves or cracks and fissures the potential for life is enhanced in regions where elevated heat flow may occur such as in parts of the tharsis and elysium volcanic provinces possible organisms would be chemoautotrophic psychrophiles adapted to a nutrient poor environment or photoautotrophic life in selected near surface habitats alternatively life may have evolved alternating cycles between active and dormant forms in which case microbes could be present in dormant forms close to the surface and in active forms in protected environments periodic liquid water on mars could have provided opportunities for biologic activity as well as evolutionary progress at the surface during the short lived climatic perturbations ancient organisms from any of these environments may also have left a detectable fossil record

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

comparative taphonomy and phylogenetic signal of phosphatized weng an and kuanchuanpu biotas abstract the ediacaran cambrian is a time with globally enhanced production of phosphorites this includes perhaps the two most famous deposits with variable cell and soft tissue phosphatization in microfossils the ediacaran doushantuo formation which contains the weng an biota and the lower cambrian kuanchuanpu formation and equivalents found in china a wide range of variables is responsible for phosphatization during this interval including depositional and geochemical conditions and microbial activity degree of microfossil phosphatization is widely dependant upon original cell and tissue types microfossils with non diagenetic morphologies that are iconic for modern crown group metazoans have had the highest level of acceptance for their interpreted phylogenetic affinity microfossils with these iconic morphologies are rare from the weng an biota but relatively common from the kuanchuanpu biota thus metazoan affinity for doushantuo microfossils has commonly relied on relatively simple and less well known morphologic features of embryonic organisms for which there is not a concensus on the nature of their phylogenetic signal differences in the phosphatized microfossil record between the weng an and kuanchuanpu biotas are due to the combination of a primary signal reflecting the initiation of the cambrian explosion and a secondary signal driven by a multitude of taphonomic processes operating at different levels of effectiveness during deposition of these two units

temperatures heat flux and frictional stress near major thrust faults we examine the two dimensional advective and conductive transport of heat in a region of thrust faulting both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor s 1 b z vsin below what they would be with the same heat sources but in the absence of advection in this expression z is the depth to the fault v is the slip rate is the component of the dip of the fault in the direction of underthrusting is thermal diffusivity and b is a dimensionless factor that is essentially equal to one for most forms of heating initial changes in temperatures near the fault are given by t z t t z 0 12vsin t z 0 z these simple formulae are successful because of the neglible influence of lateral conduction of heat at least for slip rates of a few mm yr or more two time constants govern the transition from the initial change in temperature to steady state t1 u v where u is the distance along the fault in the direction of underthrusting from the surface to the depth in question and t2 z 2 2 where z is the depth to the fault when elapsed times exceed the sum of these two time constants temperatures differ from their steady state values by only about 10 percent the numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted putain an application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 mpa and are greater than 30 mpa calculations of temperatures appropriate for the himalaya suggest that shear stresses of 100 mpa on the main central thrust probably are required to account for the tertiary granites of the region if melting took place after slip began on the thrust similarly the cut off in seismicity at a depth of about 15 km in the himalaya if due to temperatures exceeding 350 to 450 c implies a deviatoric stress close to 100 mpa

neoproterozoic diamictite cap carbonate succession and δ13c chemostratigraphy from eastern sonora mexico abstract despite the occurrence of neoproterozoic strata throughout the southwestern u s and sonora mexico glacial units overlain by enigmatic cap carbonates have not been well documented south of death valley california here we describe in detail the first glaciogenic diamictite and cap carbonate succession from mexico found in the cerro las bolas group the diamictite is exposed near sahuaripa sonora and is overlain by a 5 m thick very finely laminated dolostone with soft sediment folds carbon isotopic chemostratigraphy of the finely laminated dolostone reveals a negative 13c anomaly down to 3 2 pdb characteristic of cap carbonates worldwide carbon isotopic values rise to 10 across 400 m of section in overlying carbonates of the mina el mezquite and monteso formations the pattern recorded here is mostly characteristic of post sturtian ca 700 ma but pre marinoan ca 635 ma time however the cerro las bolas group shares ambiguity common to most neoproterozoic successions it lacks useful radiometric age constraints and biostratigraphically useful fossils and its 13c signature is oscillatory and therefore somewhat equivocal

on the stability of isostatically compensated mountain belts the formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence the evolution of potential energy stored in an orogen is dependent on 1 the density structure 2 the thermal evolution and 3 the way convergent deformation is partitioned between crust and mantle lithosphere it is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension or collapse may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened e g england 1987 in this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the brace goetze model for the rheology of the lithosphere the reference lithosphere defined to be in potential energy and isostatic balance with the mid ocean ridges changes with different thermal parameters of the lithosphere the instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts measured relative to the reference state with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range it is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates greater than a few times 10 16 s 1 and leading to significant finite extension providing the moho temperatures exceed about 650 700 c a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust consequently processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse the calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening i e fl fc 2 where fl and fc are the lithospheric and crustal thickening factors

palaeoenvironment and carbon isotope stratigraphy of upper proterozoic carbonates of the yangtze platform the 13c values for phanerozoic marine carbonates display secular trends mainly in the range 1 to 2 1 these have been interpreted as primary features reflecting changes in the carbon isotope composition of dissolved bicarbonate in seawater brought about largely by variations in the proportions of carbon being sequestered in the oxidized and reduced carbon reservoirs1 2 carbon isotope trends are not as well established for the precambrian but there are indications that upper proterozoic sequences contain carbonates with unusually high 13c values3 9 here we present the results of the first carbon isotopic study of the upper proterozoic carbonate strata from the type locality of the sinian system in the yangtze gorges sections near yichang south china and discuss their implications for accumulation of organic matter we also compare the results for these chinese strata with 13c trends in other upper proterozoic cambrian sequences and assess the extent to which such isotopic data can be used to make stratigraphic correlations between widely spaced sequences

wide but not ubiquitous distribution of glendonite in the doushantuo formation south china implications for ediacaran climate abstract glendonites are pseudomorphs of syndepositional or early authigenic ikaite caco3 6h2o that often forms at near freezing temperatures silicified glendonite has been reported from inner shelf deposits of the lower ediacaran doushantuo formation at a single stratigraphic section in south china where they are stratigraphically associated with the positive 13c excursion ep1 but predate the negative 13c excursion en3 shuram excursion indicating a period of cool climate somewhere between 609 ma and 551 ma this interpretation predicts a wider geographic distribution of ediacaran glendonites in equivalent strata in the yangtze block of south china to test this prediction we conducted a regional survey of doushantuo formation and found that although glendonites are not universally present they occur in expected stratigraphic intervals and in association with ep1 at two new sections representing inner shelf and intrashelf basin facies the wide but not ubiquitous distribution of doushantuo glendonites indicates that ikaite precipitation and glendonite formation was controlled by both regional climatic and local geochemical factors glendonites at the new localities are stellate clusters pseudomorphed by calcite spar and sometimes are rimmed with silica the calcite spar is characterized by highly variable and mostly negative 13c values as low as 37 indicating that diagenetic transformation of precursor ikaite to calcitic glendonite may be related to anaerobic oxidation of organic matter or methane in sediment the new data suggest an early ediacaran cold period prior to en3 or the shuram excursion and facilitation of glendonite ikaite formation by both climatic and local geochemical conditions such as redox conditions and phosphate concentrations

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

chemostratigraphy of neoproterozoic cap carbonates from the volta basin west africa abstract the neoproterozoic era includes some of the most extreme ice ages in earth history the exact number of glaciations is unknown although there were at least two events of global reach and possibly an equal number of lesser advances neoproterozoic glacial deposits in west africa have proven particularly difficult to correlate with better constrained successions elsewhere in most west african successions only a single glaciogenic deposit is present generally within a stratigraphic triad of tillite carbonate and bedded chert the age of these deposits and their synchroneity across the craton remain controversial we report isotopic data from carbonates that cap tillites in the volta basin sud banboli group in the three sections measured 13 c pdb begins at 2 to 3 and drops to 5 to 8 correlated with a drop in both 18 o and mg ca this pattern both the drop in 13 c and its correlation with 18 o and lithology has been observed in neoproterozoic cap carbonates on other cratons suggesting that it reflects primary depositional variations in addition the strong positive correlation among lithology 13 c and 18 o suggests that lithological variation may have exerted a principal control on isotopic variation implying that the dolomite formed in isotopic equilibrium with seawater as a primary precipitate or very early diagenetic replacement this would require low levels of sulfate estimated to be 1 mm or less consistent with available s isotopic data the presence of barite crusts in association with some cap carbonates and the conditions of seawater chemistry hypothesized in the aftermath of a snowball earth the stratigraphic pattern of carbon isotope variation in the volta basin cap carbonates suggests that underlying tillites are likely marinoan 650 600 ma in age this interpretation is supported by distinctive lithological characters observed in volta basin cap carbonates and accepted marinoan correlatives tillites elsewhere on the craton have been assigned latest proterozoic or early cambrian ages either the basis for these proposals requires reexamination or west african tillites record more than one glacial event

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

ediacaran chemostratigraphy and paleoceanography at a shallow marine setting in northwestern hunan province south china abstract the geochemical and lithological features of the ediacaran strata at yangjiaping in northwestern hunan province south china record information about the depositional environments redox conditions and carbon cycling in a shallow water setting in this region the chemostratigraphic profile of the inorganic carbon isotope reveals three negative anomaly intervals in the ediacaran section that comprises the thick 360 m doushantuo and the thin 110 m dengying formations two of the three intervals at the base of the doushantuo formation and the pc c boundary correspond to the global trend another anomaly interval in the uppermost 120 m of the doushantuo formation appears large amplitude fluctuations of the inorganic carbon isotope as well of the oxygen isotope this interval can be correlated to one of the anomaly intervals of the three george section either at the upper or at the middle doushantuo formation the latter correlation scheme implies association with the gaskiers glaciation and demands a substantial hiatus at the doushantuo dengying boundary decoupling between organic and inorganic carbon isotopes was recognized in two intervals of the lower doushantuo formation and the upper doushantuo to the lower dengying formations the lower and upper intervals were likely attribute to unusual carbon cycling reflecting from development and diminishing of the dissolved organic carbon doc reservoir respectively shallow water sedimentary facies and high organic carbon isotopic values are characteristic of the yangjiaping section especially in the oolitic facies of the middle doushantuo formation and in the supratidal facies of the lower dengying formation in these intervals yangjiaping was in an isolated lagoonal setting where the carbon cycle was independent of that of the open ocean in the south china region

superheavy s isotopes from glacier associated sediments of the neoproterozoic of south china oceanic anoxia or sulfate limitation black shales and mn carbonates interbedded with glacial deposits from the neo proterozoic of southern china exhibit extremely heavy values of pyrite s isotopes that may reflect the peculiar environment of earth at this time 34 s averages 30 at tanganshan and 44 at xiangtan compared with typical values of 0 to 5 found in younger deposits furthermore there is no distinction between the shales and the mn carbonate ores in the neoproterozoic unlike the younger deposits which show much lighter 34 s in the shales than in the mn ores the spread is 25 most other chemical parameters are very similar to both the younger mn deposits and those from the paleoproterozoic the exception is the rare earth elements ree all neoproterozoic fe ores and most neoproterozoic mn ores lack the positive eu anomaly that characterizes archean and paleoproterozoic fe mn accumulations on the other hand neoproterozoic mn deposits have positive ce anomalies on north american shale composite nasc normalized plots in contrast to other mnco 3 ores the ree is also higher than in other mn deposits but lower than in modern deep sea crusts sulfide s values in all neoproterozoic shales tend to be exceptionally variable and to often show much heavier values than can be found in marine strata from the phanerozoic therefore the anomalous 34 s values we observed reflect peculiar conditions in the world oceans at this time rather than purely local effects times of enrichment of seawater sulfate in 34 s do not correspond to periods of glaciation so the likely cause of the s isotope patterns is not worldwide glaciation but a generally low level of dissolved sulfate s in the neoproterozoic oceans that allowed modest increases in the amounts of s removed as pyrite to drive down the oceanic s reservoir enough to produce strong rayleigh reservoir effects the abundance worldwide of sturtian age mn and fe deposits indicates an increase in fe flux to the oceans that would have been sufficient to depress so 4 2 levels severely and to result in residu al dissolved s extremely enriched in 34 s ree evidence indicates that most of this enhanced fe and mn flux came from diagenetic remobilization of detrital oxides

new chronological constraints for cryogenian to cambrian rocks in the three gorges weng an and chengjiang areas south china abstract the neoproterozoic and cambrian were two of the most dramatic periods in the history of the earth because large multi cellular animals first appeared then in the so called cambrian explosion to better understand this event many paleontological and geochemical studies now focus on rocks in south china because of the fossiliferous succession and good exposure since the recognition of the yangtze gorges and chengjiang area as type localities of the sinian ediacaran and meishucunian early cambrian systems both sections have been intensively investigated in order to decipher the relationships between the evolution of life and surface environmental changes it is necessary to understand their paleontological geochemical and geo chronological constraints this study presents new chronological constraints for the cryogenian to cambrian rocks in the three gorges weng an and chengjiang areas south china we discovered two tuff layers one at the base of the shuijingtuo fm at three gorges and the other at the bottom of the dahai member in chengjiang in addition we collected sandstones from neoproterozoic strata in the three gorges chengjiang and weng an areas for provenance analysis zircons separated from the tuff layers provide new nano sims u pb ages of 526 4 5 4 ma at the base of the shuijingtuo fm and 523 9 6 7 ma at the bottom of the dahai member the tuffaceous beds occur at an unconformity and nodules are common in the three gorges meishucun and taoying sections indicating that major and relatively wide scale volcanic and sedimentological events occurred at ca 525 ma moreover carbonate carbon isotope chemostratigraphies at morocco siberia three gorges and meishucun display different characteristics during the tommotian one possibility is that the south china ocean was separated from an outer ocean at that time detrital zircons in sandstones have age populations at ca 2 7 1 8 1 6 1 0 and from 0 9 to 0 75 ga indicating that the paleo hinterland of the nanhua and kangdian rift basins was geologically complex despite the lack of ca 1 6 ga rocks in the yangtze and cathaysia blocks these data are nevertheless interpreted to indicate derivation of the zircons from basement rocks in the yangtze craton

pb pb ages of neoproterozoic doushantuo phosphorites in south china constraints on early metazoan evolution and glaciation events the pb pb isochron ages of 576 14 ma for the upper part of the upper phosphorite bed upb this study and 599 4 ma for the lower part of the upb earth planet sci lett 201 2002 203 in the doushantuo formation in the weng an area south china suggest that the deposition of the upb lasted at least 5 ma and indicate that the observed microfossil assemblage animal remains multicellular algae and giant process bearing acritarchs in the doushantuo formation predates all well dated diverse ediacaran assemblages and represents the earlier paleontological record of eukaryotes radiation on earth the 576 14 ma age in the upper part of the upb approximates the age of a negative delta 13 c excursion near the boundary between the doushantuo and the overlying dengying formation if the doushantuo dengying boundary at the weng an area is equivalent to that in the yangtze gorges area this negative excursion might correlate to a post marinoan glaciation in south china suggesting that the ice age following doushantuo sediment deposition is younger than 590 ma the 599 4 ma age in the lower part of the upb earth planet sci lett 201 2002 203 represents the minimum age of the nantuo diamictites which are generally correlated with the marinoan glaciation c 2004 elsevier b v all rights reserved

lu hf isotope systematics of fossil biogenic apatite and their effects on geochronology abstract reliable methods for direct dating of biogenic apatite from pre pleistocene fossils are currently not available and recent attempts using the lu hf decay system yielded highly inaccurate ages for both bones and teeth the geological processes accounting for this poor accuracy of lu hf chronometry are not yet understood here we explore lu hf systematics in fossil bones and teeth in detail by applying five different sample digestion techniques that are tested on bones and composites of bone and sediment our current dataset implies that dissolution methods only slightly affect the resulting lu hf ages while clear differences between the individual digestion techniques became apparent for element concentrations by analysing the insoluble leftovers from incomplete sample dissolution four main reservoirs of hf in fossil bones were identified 1 a radiogenic end member associated with apatite 2 an unradiogenic end member represented by the authigenic minerals or the embedding sediment 3 a highly unradiogenic end member that can be attributed to detrital zircon and 4 a moderately soluble phase probably a zr hf phosphate that yielded very low lu hf but a highly radiogenic hf isotope composition at the same time this zr hf phase must have been precipitated within the fossil bone sample at a late stage of burial history thereby incorporating radiogenic 176 hf released from apatite surfaces over geological timescales a second focus of our study is the effect of different sediment matrices and of crystal size on the preservation of pristine lu hf isotope compositions in bioapatite because near depositional lu hf ages of phosphate fossils have previously been reported for the london clay england and a calcareous marl from tendaguru tanzania we herein investigate specimens fossilised in carbonate matrices calcareous marl from oker germany carbonate concretions from the santana formation brazil carbonate from the eifel germany and argillaceous matrices with low permeability oil shale of messel germany posidonienschiefer of holzmaden germany materials analysed from these localities include bones teeth conodonts as well as coproliths and diagenetic minerals siderite montgomeryite and messelite near depositional lu hf ages were obtained for a bony fish sample notelops brama encapsulated in an early diagenetic carbonate concretion from the early cretaceous santana formation brazil and for conodonts from a middle devonian carbonate from the eifel germany low 176 lu 177 hf ratios in all materials from the middle eocene messel oil shale e g bones fish scales sediment siderite result in poor age precision and an age that is near depositional due to this large analytical error in agreement with previous results all other ages determined here for both bones and teeth are by far younger than respective chronostratigraphic ages a model illustrating the behaviour of lu and hf over time with respect to the fossilisation process is presented which accounts both for the formation of a late diagenetic radiogenic zr hf phase and long term open system behaviour the continuous lu hf element exchange between the fossils and the embedding sediment is probably related to the nm scale crystal size of fossil bones dentine and also of enamel that generate large surface areas facilitating sorption desorption processes and open system behaviour

integrated stratigraphy of the upper neoproterozoic succession in yunnan province of south china re evaluation of global correlation and carbon cycle abstract this study provides a chemostratigraphic and sequence stratigraphic framework of a relatively unfossiliferous sub cambrian marine sedimentary succession in yunnan province of south china our results allow its global stratigraphic correlation and provide an improved understanding of marine carbon cycle during the late neoproterozoic the succession has a basal glacial diamictite nantuo formation and comprises two large scale depositional cycles towards the precambrian cambrian pc c boundary each of which changes from siliciclastic to carbonate sediments with environments ranging from shallow subtidal to evaporitic supratidal in the absence of biostratigraphic control attribution of the tillite has remained equivocal between the 710 ma sturtian and 635 ma marinoan glaciations a compilation of known paleomagnetic and other geochronologic constraints suggests that the nantuo glaciation in south china was at least diachronous in which the nantuo tillite in yunnan belongs to the older glaciation stratigraphic patterns of the diagenetically screened 13c variation and depositional sequences during the carbonate dominated interval show a striking similarity with those of the upper cryogenian and entire terminal proterozoic carbonates at widely separated other localities rather than the terminal proterozoic alone supporting the view that deposition of the nantuo tillite in yunnan reflects the sturtian rather than marinoan glaciation further considerations of the 13c results provide three important implications for the late neoproterozoic marine carbon cycle first comparison of different 13c profiles suggests considerable variability in the absolute values of carbonate on both intra and inter basinal scales which differs profoundly from their relative homogeneity in marine phanerozoic counterparts and may therefore reflect lateral isotopic heterogeneity in the oceanic carbonate reservoir during the late neoproterozoic second an empirical correlation between the 13c and relative sea level changes was applied to evaluate hypothesized explanations for 13c temporal variation in surface sea yielding short term inverse and long term positive relationships the short term trend marks periods of the negative 13c excursions and is compatible to most of the existing arguments for the 13c events on the other hand mechanisms causing the long term trend remain enigmatic in the absence of comparable data from other localities our observation however suggests that at least two different mechanisms operated marine carbon cycle during the late neoproterozoic third the negative 13c anomaly characterizing the period following the marinoan glaciation worldwide was not coincident between kerogen and carbonate but occurs as a lagged drop in yunnan where a 7 depletion in kerogen preceded an 3 depletion in carbonate their lagged trends likely reflect different sources as kerogen and carbonate signals are considered to monitor surface and deeper waters respectively the negative 13c event would therefore have occurred in surface water followed by a smaller 13c depletion through the entire water column due to vertical mixing between surface and deeper waters at the studied site this mechanism supports the proposed hypothesis that injection of the 13c depleted atmospheric co2 into the surface sea deriving ultimately from quantitative dissociation of methane hydrate in terrestrial permafrost during the post glacial warming contributed to the negative 13c anomaly

lu hf apatite geochronology of mafic cumulates an example from a fe ti mineralization at smalands taberg southern sweden abstract we have employed lu hf apatite chronology to determine the age of a magnetite rich melatroctolite in southern sweden known as the smalands taberg fe ti ore deposit the melatroctolite is poor in incompatible elements and minerals forming at more advanced stages of differentiation which precludes the use of minerals baddeleyite zircon titanite or apatite commonly used for dating the emplacement of igneous rocks enclaves of leucogabbro in the melatroctolite have ree patterns parallel with the host melatroctolite and identical initial hf and nd isotope compositions these characteristics are conclusive evidence for a common parental magma the leucogabbro is a slightly more evolved facies that underwent limited fractionation of olivine and titanomagnetite the two major mineral phases in the melatroctolite apatite and plagioclase separated from the leucogabbro plus a whole rock sample define a lu hf isochron with a slope corresponding to an age of 1204 3 1 8 ma this result is close to the lower age boundary previously reported for this magmatic event comprising dolerites syenites and granites the lu hf apatite chronometer merits attention as a new tool in generating accurate and precise ages for igneous silica undersaturated rocks that may be difficult to date with traditional techniques

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

acanthomorphic acritarchs from the ediacaran doushantuo formation at zhangcunping in south china with implications for the evolution of early ediacaran eukaryotes abstract microfossil assemblages preserved in the ediacaran doushantuo formation phosphorites i e the weng an biota and chert nodules in south china provide key evidence for the diversification of marine eukaryotes in the aftermath of cryogenian global glaciation two biozones i e the lower tianzhushania spinosa biozone and the upper hocosphaeridium anozos biozone have been proposed on the basis of acanthomorphic acritarchs preserved in the doushantuo formation chert nodules in the yangtze gorges area strata containing the weng an biota at weng an have been correlated either with the upper acritarch biozone based mainly on chemostratigraphic data or with the lower acritarch biozone according to the occurrence of tianzhushania spinosa in both assemblages these alternative correlations need to be resolved in order to improve our understanding of the evolutionary pattern of microscopic eukaryotes after the marinoan glaciation to address this problem we carried out an integrated litho bio and chemostratigraphic study of the doushantuo formation in the zhangcunping area where the microfossil assemblage resembles the weng an biota at weng an in both taxonomic composition and biostratigraphic ranges our study supports the litho and chemostratigraphic correlation of the doushantuo formation between the zhangcunping and yangtze gorges areas thus the doushantuo formation at zhangcunping provides a link between the weng an and yangtze gorges areas together with published paleontological and stratigraphic data the present study favors a correlation between the fossiliferous strata at zhangcunping and the upper part of the t spinosa biozone recognized in the yangtze gorges area the strata yielding the weng an biota at weng an can be at least partially correlated with the t spinosa biozone supporting the idea that the weng an microfossil assemblage is biostratigraphically transitional between the lower and upper acanthomorphic assemblages this correlation is consistent with the fact that several taxa that used to be regarded characteristic of the h anozos biozone may actually range downsection into the t spinosa biozone thus supporting an earlier diversification of doushantuo acanthomorphs these results call for more detailed investigation on the taxonomic composition and biostratigraphic ranges of acanthomorphs in the t spinosa biozone and re consideration of acanthomorphic biozonation of the doushantuo formation in the yangtze gorges area

origin of regional rooted low angle normal faults a mechanical model and its tectonic implications rooted listric low angle normal faults 20 of regional extent have been recognized widely in the past few years in the north american cordillera and elsewhere the low angle geometry of these crustal scale normal faults conflicts with anderson s 1942 classic theory of faulting in that theory the orientations of principal stresses are assumed to be vertical and horizontal the predicted dip angle of normal faults is about 60 rather than 20 or less recent geological and geophysical studies in the mid tertiary extensional terrane of southeastern california and western arizona suggest that thick mylonitic gneisses in the lower plates of low angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust directed ductile flow possibly related to the gravitational spreading of thickened lower crust may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface a simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust this model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered in particular appropriate stress conditions for the formation of regional low angle normal faults 20 can be produced by the superposition of two stress fields a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant this superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust these results may explain both puzzling parallel belts of extension and compression and the occurrence of major low angle normal faults in some orogenic systems

new lu hf and pb pb age constraints on the earliest animal fossils abstract the neoproterozoic doushantuo formation south china preserves a unique assemblage of early multicellular fossils and overlies rocks which are thought to have formed during an ice age of global extent the age of this formation is thus critical for understanding the important biological and climatic events that occurred towards the end of the proterozoic eon until now direct dating of sedimentary formations such as the doushantuo has been difficult and associated with large uncertainties here we show that dating of doushantuo phosphorites by a novel lu hf dating method and conventional pb pb geochronometry independently yield ages of 584 26 ma and 599 3 4 2 ma respectively these ages are in agreement with bio and chemostratigraphical observations and show that the doushantuo animal remains predate diverse ediacaran fossil assemblages making them the oldest unambiguous remains of metazoans currently known furthermore the pb pb age for the post glacial doushantuo rocks suggests that the neoproterozoic glaciation in china might predate glacial rocks in eastern north america commonly associated with the younger marinoan of two major neoproterozoic glaciations the combination of lu hf and pb pb dating shows considerable potential for dating other phosphorite successions and future application of these methods could therefore provide further constraints on proterozoic biological and environmental history

subduction of asian lithospheric mantle beneath tibet inferred from models of continental collision the relative northward motion of the indian subcontinent that followed the onset of continental collision with asia has produced extensive deformation of the earth s crust giving rise to the world s highest mountains in the himalayan chain and the world s largest high elevation region the tibetan plateau the formation of the broad mountain belt implies that contrary to the original tenets of plate tectonics the lithospheric plates have experienced widespread deformation far from the plate boundary1 several models have been proposed2 6 to explain the manner in which this post collisional deformation is distributed within the continental lithosphere of the indian and asian plates here we propose an alternative model in which subduction of the asian lithospheric mantle develops following the collision of india our model is supported by numerical calculations of crustal deformation and thickening and is consistent with available geological and geophysical data7 9 this picture suggests that lithospheric mantle is not deformed along with the crust and would imply that continental collision zones are more analogous to oceanic subduction zones than was previously believed