continental oceanic crustal transition off southwest africa more than 1 426 mi 2 300 km of 48 trace 12 fold seismic reflection profiles were used to examine the nature of the continent ocean boundary off southwest africa south of the orange river faulted blocks which we interpret as rifted continental crust can be traced seaward to the 3 km isobath the transition to oceanic basement occurs in a zone 19 to 25 mi 30 to 40 km wide beneath the continental rise although structural details are obscured by a thick seaward dipping wedge of pre aii pre late early cretaceous overburden oceanic crust at the presumed contact is not older than magnetic anomaly m9 126 to 121 m y b p north of the orange river a pronounced hinge in continental crust correlates with magnetic anomaly g of rabinowitz 1976 seaward of the hinge a complex fault block terrane is evident oceanic basement can not be traced with any certainty landward of anomaly m4 intracontinental stretching and associated volcanism appear to have been important in the early history of the cape basin rifting and local dike intrusion may explain the presence of some lineated magnetic anomalies previously attributed to sea floor spreading related extrusives form at least part of the pre aii wedge just seaward of the hinge zone our interpretation of the seismic data suggests that the initiation of normal spreading in the cape basin postdates by 4 to 9 m y the valanginian age derived from prevailing plate tectonic reconstructions of the south atlantic

the composition and its impact on the methane sorption of lacustrine shales from the upper triassic yanchang formation ordos basin china abstract the organic geochemistry mineralogy and methane sorption of lacustrine shales of the upper triassic yanchang formation collected from the south eastern ordos basin were investigated to characterize them and clarify the effects of shale composition on their sorbed gas capacity these yanchang shales have recently been selected as a target area for shale gas exploration in typical terrestrial strata in china the two main sections of these shales containing type ii organic matters chang 7 and chang 9 have relatively high total organic carbon content toc of 2 10 the two shales also have similar mineralogies mainly comprising quartz clay minerals and feldspars both the chang 7 and chang 9 shales are generally in the oil window chang 9 is slightly more mature than chang 7 higher methane sorption capacity was observed for chang 9 than for chang 7 shales determined on a dried basis at 50 c methane sorption measurements were further performed on three samples from which the residual bitumen had been extracted and their corresponding kerogen fractions to gain insight into the effects of shale composition on methane sorption this was significantly higher in solvent extracted samples than in raw samples indicating that residual bitumen largely restricts methane sorption on such shales a positive correlation between the amount of clay minerals and methane sorption capacity of bulk rocks was evident suggesting that clay mineral content is relevant to methane sorption this result was also supported by the much higher methane sorption capacity of solvent extracted shales compared to the extracted kerogen from those shales when measured sorption data was normalized to toc values the effects of both residual bitumen and clay mineral on the methane sorption of bulk rocks have complicated the evaluation of methane sorption on organic matter in these mature shales

brazilian and african passive margins of the central segment of the south atlantic ocean kinematic constraints the thinning of passive continental margins is usually explained by models using pure stretching or simple shear these models imply hypothetical extensional structures and large horizontal movements between the two conjugate margins more than 250 km for the brazilian and angolan margins refraction reflection data together with the most recent and tightest pre opening fit using continental and oceanic geological and geophysical constraints show that the substratum of the sag basin is divided into an autochthonous part upper continental crust and an allochthonous part exhumed material the thinning process which evolves in an elevated position of the system until at least the break up seems to be depth dependent and to mainly concern the lower middle crust which we postulate is exhumed this exhumation does not explain the entire thinning of the system horizontal motions cannot alone explain the formation of the huge angolan brazilian basin similar observations are made on the whole central segment of the south atlantic ocean part of the lower continental crust is still missing and it seems most improbable that the continental crust maintains its integrity throughout the thinning process

getting over continent ocean boundaries the idea of a simple linear boundary between continental and oceanic crust at extended continental margins is widely recognized to be an oversimplification despite this such boundaries continue to be mapped because of their perceived utility in palinspastic and plate kinematic reconstructions to examine whether this perception is justified we review the data and models on which basis continent ocean boundaries are interpreted and map a set of such interpretations worldwide from more than 150 publications the maps show that the location of the continent ocean boundary is rarely consistently estimated within the 10 100 km observational uncertainty that might be expected of the geophysical data used for doing so that this is the case regardless of whether the transition zone behind the boundary is classified as magma rich or magma poor and that the geographical separation of estimates exceeds the width of single study continent ocean transition zones the average of global maximum separations across sets of three or more estimates is large 167 km and mostly a consequence of interpretations published over the last decade we interpret this to indicate an extra component of uncertainty that is related to authors understanding of the range of features that are interpretable at extended continental margins we go on to discuss the implications of this uncertainty for palinspastic and plate kinematic modelling using examples from the literature and from the south atlantic ocean we conclude that a precise continent ocean boundary concept with locational uncertainty defined from the ensembles is of limited value for palinspastic reconstructions because the reconstruction process tends to bunch the ensemble within a region that is i of similar width to the observational uncertainties associated with continent ocean boundary estimates ii narrower than the regions of uncertainty about rotated features implied by the propagation of uncertainties from plate rotation parameters and iii coincident within all the above uncertainties with the more easily mapped continental shelf gravity anomaly secondly we conclude that estimated continent ocean boundaries are of limited use in developing or testing plate kinematic reconstructions because i reconstructions built using them as markers do not within uncertainty limits defined from the ensembles differ greatly from those using more easily determined bathymetric or gravity anomaly contours and ii because it is impossible to segment and date them with useful precision to use as markers of the edges of rigid oceanic lithosphere outside of the constraints of a pre existing plate kinematic model

anoxic vs dysoxic events reflected in sediment geochemistry during the cenomanian turonian boundary event cretaceous in the umbria marche basin of central italy abstract one of the most spectacular expressions of global oceanic anoxic events oaes of the cretaceous is the livello bonarelli of central italy deposited during the cenomanian turonian boundary event ctbe or oae2 at around 93 5 ma at furlo in the umbria marche basin in italy the bonarelli consists of approximately 1 m of c org rich shales interbedded with radiolarian rich layers and contrasts sharply with the extensive over and underlying siliceous limestones several minor c org rich intervals herein termed black levels average thickness 4 5 cm are interbedded within the scaglia bianca beneath the bonarelli x ray fluorescence and icp ms analyses show that these sediments consist of different mixing systems with three end members 1 sio 2 caco 3 scaglia bianca 2 average shale caco 3 black levels and 3 average shale sio 2 bonarelli given the sharp lithologic contacts and the little geochemical mixing between the groups these are thought to represent rapid switching events between sedimentary modes major element concentrations are generally low except for si and p owing to dilution by silica up to 96 and carbonates as al normalised ratios for elements such as ti or zr taken as proxies for higher energy environments are close to average shale values excess sio 2 is most likely linked to biogenic processes rather than quartz element al ratios of ti mg k rb and zr are fairly constant pointing to a mostly homogeneous source area the black levels and bonarelli are characterised by lower mg al ratios compared to the scaglia bianca sediments suggesting variability in chlorite input which could be linked with short term sea level drops indicated by ti al ratios the bonarelli sediments are severely depleted in mn which is indicative of oxygen depleted bottom water conditions and an open marine environment while the black levels appear to have been deposited in a much more restricted setting a model involving partial physical isolation of the furlo site by a sill coupled to omz variations by extra basinal forcing mechanisms is proposed to account for these conflicting data based on re mo ratios the bonarelli sediments were deposited under varying oxygen depletion levels from suboxic to anoxic while re mo ratios in the black levels show deposition under less oxygen deficient conditions most minor elements many of them redox sensitive or sulphide residing are strongly enriched in the c org rich sediments further documenting the absence of oxygen and potential availability of hydrogen sulphide in the water column at times while high zn concentrations suggest elevated submarine hydrothermal activity during this time interval ba is also present in very high concentrations up to 1 4 with an average weighed concentration of 0 6 for the entire bonarelli and is probably indicative of high paleoproductivity in this area this is further supported by high p concentrations hinting at nutrient availability at this site

cenozoic high k alkaline magmatism and associated cu mo au mineralization in the jinping fan si pan region southeastern ailao shan red river shear zone southwestern china northwestern vietnam abstract the jinping fan si pan jfp cenozoic magmatic and cu mo au metallogenic belt in the southeastern part of the ailao shan shear zone host the tongchang chang an habo and chinh sang cu mo au deposits these deposits form an integrated epithermal porphyry regional mineralization system associated with 40 32 ma high k alkaline magmatism the magmatic rocks in the belt have relatively low tio2

the ocean study area tectonic history from magnetic anomaly data and seismic reflectivity the ocean experiment is a detailed geophysical study of a region of the cape verde basin a dense network of new magnetic and gravity profiles has enabled us to constrain the spreading rate history of the region and the location of fracture zones the main features on the gravity profiles are lineated perpendicular to the seafloor spreading magnetic anomaly lineations significant along axis variability in spreading history suggests that the mid atlantic ridge behaved as a series of loosely coupled segments within which spreading was fundamentally asymmetric such variability is associated with a minor jump in the ridge axis which changes the offset and expression of one of the fracture zones deep seismic reflection and refraction lines were oriented parallel and perpendicular to the magnetic lineations seismic reflections occur at all levels within the crust decreasing in amplitude and coherence below the level of the moho analysis of the subbasement reflectivity provides compelling evidence that at least two major sets of dipping structure are present and are imaged separately on the two perpendicular sets of seismic profiles dipping reflections on flow line dip profiles which are interpreted as faults due to their association with offsets in the basement surface appear to strike parallel to the paleoridge axis the majority of reflections that may be identified as faults dip toward the west and although basement topography suggests that east dipping faults are also present no reflections may be interpreted unambiguously as such east dipping reflections observed only in the middle to lower crust have a more obscure origin dipping reflections seen on isochron strike profiles show clear contrasts in strength lateral coherence depth and dip population a number of these strike parallel to flow lines comparing reflection and refraction data shows that both the layer 2 layer 3 boundary and the moho are marked by a change in the character of reflections and suggests that they may represent important structural as well as seismological boundaries within the oceanic lithosphere

thermal and tectonic history of the ordos basin china evidence from apatite fission track analysis vitrinite reflectance and k ar dating apatite fission track analysis vitrinite reflectance data and k ar dating of permian carboniferous and mesozoic core samples have been successfully integrated to reconstruct the thermal and tectonic history of the ordos basin china apatite fission track ages of carboniferous jurassic sedimentary rocks range between 3 and 137 ma and are significantly younger than the stratigraphic ages confined fission track lengths demonstrate exclusively mixed length distribution indicating complex thermal history the data suggest that the samples must have all experienced higher paleotemperatures in the past mean vitrinite reflectance values r0 of the triassic rocks range from 0 61 to 1 06 giving a high coalification gradient of 0 36 km and suggesting a high paleothermal gradient of 57 c km permian carboniferous rocks have ro values on the order of 1 0 3 0 and locally up to 4 0 6 0 some high r0 values coincide with positive gravity and magnetic anomalies k ar dating on permian triassi samples reveals distinct illitization at 170 160 ma during which a thermal event occurred due to subsurface magmatic intrusion related to the early yanshanian movement the petroleum source rocks of the upper triassic experienced peak temperatures ranging from 90 to 160 c corresponding to the oil window and permian carboniferous source rocks were heated to more than 150 c passing through and out of the gas window due to rapid uplift and erosion in response to the rise of the qinghai tibet plateau associated with the asia india collision and the himalayan orogeny cooling has taken place at least since approximately 23 ma the difference in the rate and amount of uplift between the eastern and western parts of the basin resulted in differential uplift and the present day s ructural pattern of the basin

cyclostratigraphy and orbital tuning of the terrestrial upper santonian lower danian in songliao basin northeastern china abstract the songke 1 north sk 1n borehole recovered a continuous 1541 66 m late santonian early danian terrestrial succession in songliao basin sb northeastern china it provides a unique record for improving our understanding of continental paleoclimate and ecological system in cretaceous greenhouse world here we use thorium th logging data as a paleoenvironmental and paleoclimatic proxy to conduct a detailed cyclostratigraphic study on the sk 1n core power spectra evolutionary fast fourier transformation and wavelet analysis all reveal significant decameter to meter scale sedimentary cycles in the nenjiang k 2 n sifangtai k 2 s and mingshui k 2 m formations the ratios of cycle wavelengths in these stratigraphic units are 20 5 2 1 and are interpreted as milankovitch cycles of 405 kyr and 100 kyr eccentricity 38 4 kyr obliquity and 20 kyr precession cycles respectively an astronomical time scale ats is established by tuning filtered 405 kyr eccentricity cycles to a target curve of the astronomical solution la2010d based on the magnetostratigraphic time framework of the sk 1n borehole this ats provides precise numerical ages for stratigraphic boundaries biozones geological and geophysical events and serves as a basis for correlation of strata and events between marine and terrestrial systems the cretaceous paleogene k pg campanian maastrichtian santonian campanian boundaries are estimated at core depths of 318 m 752 8 m and 1751 1 m respectively a 3 8 myr long hiatus between the nenjiang k 2 n and sifangtai k 2 s formations occurs from 76 1 to 79 9 million years ago the ages and durations of magnetochrons c33r to c30n are precisely estimated and provide new constraints on the late cretaceous geomagnetic polarity time scale gpts and south atlantic sea floor spreading rates

cyclostratigraphy and astronomical tuning during the oligocene in the jizhong depression bohai bay basin northeastern china abstract the dongying formation ed and the first member of the shahejie formation es1 in the central raoyang sag cover about 1705 m long sedimentary series of terrestrial lacustrine infill we employed the gamma ray gr log data as a paleoclimatic and paleoenvironmental proxy to conduct the high resolution cyclostratigraphy research in well w33 and w62 prominent sedimentary cycles in decameters and meters were revealed by multi taper mtm power spectra evolutionary fast flourier transformation fft and correlation coefficient analyses in the es1 and ed intervals the 405 kyr long eccentricity 125 kyr and 100 kyr short eccentricity 40 kyr obliquity and 23 kyr precession can be recognized with a wavelength ratio of approximately 20 5 2 1 respectively by tuning the 405 kyr filter output in well w33 to the orbital target curve of la2010d astronomical solution we established an astronomical time scale ats that provided absolute age controls during the oligocene and shed light on the event correlation among isolated depressions in the bohai bay basin sediment accumulation rates sars were derived from the extracted 405 kyr long eccentricity and were tested with an average of 17 cm kyr age boundaries of ng ed1 ed1 ed2 ed2 ed3 ed3 es1 and es1 es2 are 23 03 ma 24 34 ma 25 53 ma 29 05 ma and 33 10 ma respectively based on the ats and sars we discussed the depositional evolution within age boundaries cause of high sars and origin of formation overpressure after the largest lake transgression shallow water deltas and fluvial plains transitionally dominate the central raoyang sag with shallowing water depth thin interlayers of mudstones and sandstones originate from shallow water deltaic systems with low sand to mud ratios under high sars enhanced by mountain uplift and low sand to mud ratios formation overpressure results mainly from high porosities in under compacted mudstone intervals

ocean continent boundary in the iberia abyssal plain from multichannel seismic data abstract the ocean continent boundary of the west iberia margin is marked by a basement ridge trending n s four segments of this ridge are recognized each of them being progressively offset westward from 40 n to 43 n because the setting and seismic character of the ridge in the iberia abyssal plain are similar to those of the galicia margin ridge which is made of serpentinized peridotite we think that the southern segments of the ridge are also made of the same mantle material the segmentation of the ridge suggests that the northward propagation of the continental break up during the north atlantic opening in early cretaceous times was discontinuous each segment possibly corresponding to a propagation step east of the ridge the basement of the whole iberia abyssal plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the s reflector previously recognized off galicia by analogy with the galicia margin we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle the s reflector corresponding to the contact between crustal rocks and serpentinite

high resolution sediment accumulation rate determined by cyclostratigraphy and its impact on the organic matter abundance of the hydrocarbon source rock in the yanchang formation ordos basin china abstract the upper triassic chang 7 unit consists of more than 110 m of lacustrine sediments including mudstone and shale and it is considered to be the most important hydrocarbon source rock in the ordos basin power spectral wavelet evolutionary fast fourier transform fft and correlation coefficient analyses of the gamma ray gr and density den logging series from well y56 in the chang 7 unit reveal strong astronomical signals of long eccentricity 405 kyr short eccentricity 100 kyr and obliquity 34 kyr cycles those provide an excellent opportunity to establish floating astronomical time scales ats and to discuss how the variations in the sediment accumulation rate sar impact organic matter abundance based on the 405 kyr tuning ats results the chang 7 unit in well y56 covers a 7 26 ma period from 239 50 ma to 232 24 ma the ages of each subunit in the chang 7 unit are estimated to be 232 24 234 25 ma 234 25 236 90 ma and 236 90 239 50 ma respectively the high resolution sars of the chang 7 unit vary from 1 18 cm kyr to 2 08 cm kyr with an average of 1 57 cm kyr according to the chemical index of alteration cia and the correlation between the sar and the al2o3 sio2 or al2o3 tio2 ratio the main factor affecting the sar of the chang 7 unit is the amount of terrestrial debris input which is the result of the warm humid paleoclimate and the intense volcanic activity during phase i of the indosinian in addition the total organic carbon per unit of paleoproductivity tocpup was applied to investigate the impact of the sar on the organic matter enrichment in order to eliminate the effect of paleoproductivity on the organic matter enrichment we conclude that an sar is sufficient to protect the organic matter and prevent oxidation but not too high to dilute the organic matter which reduces the organic matter abundance of the source rock

deep seismic reflection study of a passive margin southeastern gulf of guinea a large grid of deep imaging marine seismic reflection data has been acquired in the gulf of guinea the data show that the architecture of old atlantic igneous oceanic crust and upper mantle is highly variable particularly if reflection moho is taken to be the base of the crust most abrupt changes in oceanic basement thickness and depth to moho can be correlated with fracture zone crossings but significant variations can occur between fracture zones and along flow lines especially near the ocean continent transition reflection moho is usually continuous from ocean to continent and does not display any systematic changes in character continuity or reflection time even beneath the innermost shelf areas there are several varieties of intra crustal reflectors including those that mark different levels within the oceanic gabbroic complex and events that diagonally link the top of oceanic seismic layer 2 and moho different types of sub moho dipping reflections also are observed some are associated with fracture zones some originate within continental crust and dip toward the ocean dissecting moho without offsetting it and still others originate at the oceanic moho and dip toward the continent the transition from oceanic to continental crust is generally quite sharp north of lat 1 s but the exact nature of the transition ranges from rift block geology to abrupt juxtapositions of oceanic and continental crustal rocks south of about lat 1 s the transition to continental crust is more gradual involving a progressive thickening of oceanic crust toward land this difference may relate to the occurrence of much more oblique initial rifting north of 1 s

repeat ridge jumps and microcontinent separation insights from ne arabian sea abstract microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume ridge interactions india separated from seychelles at 64 ma by asymmetric sea floor spreading initially when the spreading centre in the mascarene basin jumped towards the indian sub continent between magnetic chrons c29 and c28 the subsequent tectonics is difficult to comprehend since laxmi ridge another microcontinent formed during the later phase most of the studies considered the laxmi ridge as a sliver others considered it to be oceanic crust high resolution deep 25 km seismic data reveals that i the ridge possesses 15 km deep sea ward dipping reflector sdr packages ii normal faulted rift valleys devoid of syn rift sedimentary packages and iii axial magma chambers 5 7 km beneath the ridge top additionally from 2d forward gravity models we deduce that the ridge most possibly comprises of high density oceanic crust we conclude the laxmi ridge to be indeed composed of oceanic crust and a fossil spreading centre we thus identified the ridge jumps and their relation to the seychelles microcontinent separation previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating spreading rate at the ridge and plate ages for repeated ridge jumps the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating we find that the medium spreading rates and high magmatic heating due to the reunion plume and young plates favoured rapid and repeated ridge jumps towards the plume

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

reconstruction of redox conditions during deposition of organic rich shales of the upper triassic yanchang formation ordos basin china the concentration and distribution of organic matter phosphate nodules and framboidal pyrite preserved in shales are controlled by organic matter decomposition and the cycles of phosphorus sulfur and iron which in turn are controlled by bottom water redox conditions the organic rich shale in the 7th member of the upper triassic yanchang formation ch7 contains high organic carbon concentrations and abundant phosphate nodules and pyrite framboids providing an opportunity to better understand the redox conditions at the time of deposition framboidal pyrite in the ch7 organic rich shales are composed of octahedral pyritohedral and spherulitic microcrystals with uniform crystal size sem photographs and statistical data suggest that pyrite framboids have large mean diameters 10 0 to 18 1 m and standard deviations 2 8 to 5 9 m this may indicate that the organic rich shale was deposited under oxic suboxic bottom water conditions the corg p ratios of the organic rich shale exhibit considerable variation ranging from 7 2 to 216 but the majority of them 83 2 belong to the lower intermediate range 130 suggesting that the redox conditions were predominantly oxic suboxic and intermittently anoxic the corg p ratios also show periodic vertical variation which suggests that redox conditions fluctuated between oxic suboxic and anoxic additional support for the trends based on corg p ratios is provided by dopt values which show similar vertical variation and redox patterns all evidence shows that the redox conditions during deposition of the ch7 organic rich shale was predominantly oxic suboxic accompanied by intermittent anoxic environments the result of this study will be helpful in better understanding the formation mechanism of the ch7 organic rich shale

geochemical characteristics of crude oil in fluvial deposits from maling oilfield of ordos basin china abstract maling oilfield is one of the earliest discovered oilfields in the ordos basin it is also a typical representative of oil reservoirs in fluvial deposits however due to the deficiency of geochemical study on the crude oils and source rocks the interpretation of the oil origin is largely based on the geological situation maling crude oils 21 and source rocks 14 from the basin have been systematically analyzed the distribution of the biomarkers indicates that the crude oils mainly originated from source rocks deposited under a weakly reducing to sub oxic freshwater lacustrine environment besides algae higher land plants have also made an important contribution to the source organic matter comparison of the distributions of the biomarkers suggests that there is a correlation between the maling oilfield oils and the source rocks of the yanchang formation

neocomian to early aptian syn rift evolution of the normal to oblique rifted north gabon margin interior and n komi basins abstract the north gabon coastal rift basins consist of a set of 130 150 long segment asymmetrically tilted half grabens interior basin and 000 020 short segment en echelon half grabens n komi basin separated by 040 060 major transverse faults tectono sedimentary analysis of field and subsurface data reveals the control exerted by extensional tectonism over continental sedimentation during berriasian to early barremian times uniform uniaxial 040 060 extension was responsible for the stretching of the brittle upper crust over a 100 km wide domain during late barremian early aptian times the main locus of extension stepped westward resulting in severe end rift uplift and erosion of the failed interior and n komi rift basins early cretaceous coastal rifts in north gabon display a wide range of styles from oblique rifting n komi basin normal rifting interior basin to transform rifting the pre existing precambrian tectonic fabric exerts a strong control over the mode and over the 100 300 km scale segmentation of the rifting

quantification of deep time orbital forcing by average spectral misfit quantification of milankovitch orbital cyclicity within ancient strata has become a principal tool for refinement of the geologic time scale however accurate characterization of the orbital signal in deep time paleoclimate records is commonly challenged by inadequate radiometric time constraints for calibration of the spatial rhythms to temporal periods this problem can potentially introduce large errors into derivative orbital timescales in this study we develop a new method for the identification and calibration of orbital cyclicity in cyclostratigraphic records the method average spectral misfit or asm yields an objective estimate of the optimal sedimentation rate for a stratigraphic interval that preserves a record of orbital forcing the technique also provides a formal statistical test for rejecting the null hypothesis no orbital signal application of the method to assess orbital cyclicity in the upper bridge creek limestone member turonian of the western interior basin highlights the utility of this new cyclostratigraphic tool and provides a means to independently evaluate conflicting interpretations of the lithologic cycles importantly asm offers a new consistent standard by which orbital timescales may be compared hence the quality of an orbital timescale can be formally qualified by reporting its average spectral misfit and null hypothesis significance level this technique will permit improvement of mesozoic cenozoic orbital timescales and extension of orbital time scale development into the paleozoic as the method is not dependent upon well constrained radiometric age data

direct dating of left lateral deformation along the red river shear zone china and vietnam 1 exposures of high grade midcrustal rocks within the red river shear zone rrsz which separates the indochina and south china blocks exhibit clear evidence of left lateral ductile deformation assuming that the south china sea represents a pull apart basin formed at the southeastern termination of the rrsz it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the rrsz between 32 and 17 ma at a rate of 4 cm yr while 40ar 39ar thermochronometry indicates that left lateral slip occurred along the rrsz between 25 and 17 ma the timing of earlier high temperature deformation has not been directly constrained in situ th pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite grade metamorphism and synchronous left lateral shearing results from northern segments of the rrsz in yunnan china indicate that synkinematic garnet growth occurred between 34 and 21 ma and are the first to document late oligocene metamorphism and left lateral shearing data from the southern rrsz within vietnam are complicated by tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the indosinian orogeny 220 ma the period during which sinistral deformation is now constrained to have occurred along the rrsz i e 34 17 ma is essentially coincident with spreading of the south china seafloor 32 17 ma this temporal and kinematic link between left lateral shearing along the rrsz and opening of the south china sea supports the view that indochina was extruded from asia as a block along lithospheric scale strike slip faults

the origin of the world class tin polymetallic deposits in the gejiu district sw china constraints from metal zoning characteristics and 40ar 39ar geochronology the gejiu tin polymetallic deposits in the western cathaysia block of south china comprise the world s largest primary tin district with a total resource of approximately 300 million metric ton ores at an average grade of 1 wt percent sn tin polymetallic mineralization occurs in five deposits and has four ore types i e greisen skarn stratabound cassiterite sulfide mostly oxidized and vein type ore in each deposit the orebodies typically occur in an extensive hydrothermal system centered on a shallow late cretaceous granitoid cupola metal zoning is well developed both vertically and horizontally over the entire district from w be bi mo sn ores inside granite intrusions to sn cu dominated ores at intrusion margins and farther out to pb zn deposits in the surrounding host carbonate this zoning pattern is similar to that of other hydrothermal deposits in other parts of the world indicating a close genetic relationship between magmatism and mineralization in this paper we dated thirteen mica samples from all types of mineralization and from the five deposits in the gejiu district the ages range from 77 4 0 6 ma to 95 3 0 7 ma and are similar to the existing zircon u pb age of the granitic intrusions 77 4 2 5 85 8 0 6 indicating a genetic relationship between the mineralization and the intrusions geological characteristics metal zoning patterns and new geochronological data all indicate that the tin polymetallic ores in the gejiu district are hydrothermal in origin and are genetically related to the nearby granitic intrusions it is unlikely that the deposits are syngenetic as has been proposed in recent years

seismic reflection character of the cameroon volcanic line evidence for uplifted oceanic crust deep imaging multifold seismic fines across submarine parts of the cameroon volcanic line west africa gulf of guinea show asymmetric uplift of oceanic crust associated with extensive magmatism the main pulse of uplift occurred after creation of a regional sequence boundary believed to be miocene in age the apparent synchroneity of uplift argues against the cameroon line being a simple hotspot trace as previously inferred one plausible theory of origin for the seaward part of the cameroon volcanic line and its asymmetric uplift geometry combines regional asthenospheric upwelling with restriction of magmatic egress to regularly spaced weak spots corresponding to fracture zone crossings horizontal motion and buckling also may have occurred along the cameroon volcanic line

the association of continental crust rocks with ophiolites in the northern apennines italy implications for the continent ocean transition in the western tethys abstract the late cretaceous sedimentary melanges from the external liguride units of the northern apennines include large slide blocks of ophiolites and lower and upper continental crust rocks representative of a continent ocean transition between the internal liguride oceanic domain and the thinned continental margin of the adria plate the slide blocks preserve a record of the long lived history of rifting which led to opening of the jurassic western tethys basin the external liguride ophiolites consist of 1 undepleted spinel peridoties partly re equilibrated under plagioclase facies conditions which were interpreted as unroofed subcontinental mantle 2 rare gabbroic rocks mainly troctolite to olivine bearing gabbro probably crystallised from n morb magmas and 3 basalts with n to t morb affinity covered by late callovian early oxfordian radiolarian cherts both gabbroic rocks and basalts locally intrude the mantle peridotites and postdate their re equilibration to plagioclase facies conditions the slide blocks of lower continental crust are composed of gabbro derived mafic granulites and felsic granulites the latter include quartzo feldspathic granulites and rare quartz poor to quartz free charnockitic rocks in both mafic and felsic granulites granulite facies re equilibration was followed by a retrograde metamorphic evolution to amphibolite greenschist and subgreenschist facies conditions retrogression is commonly accompanied by deformations progressively changing from plastic to brittle the upper crustal rocks occurring as slide blocks consist of hercynian granitoids with orogenic affinity mainly biotite bearing granodiorites and peraluminous two mica leucogranites locally the granitoids are intruded by basaltic dykes or capped by basaltic flows and radiolarian cherts the granitoids underwent polyphase brittle deformations under subgreenschist facies conditions which predated the basalt emplacement the tectono metamorphic evolution recorded by the slide blocks of the external liguride units started in the late carboniferous early permian about 290 ma with the emplacement at deep crustal levels of the gabbroic protoliths for the mafic granulites the associated felsic granulites likely represent the remnants of the lower continental crust intruded by the gabbro derived granulites mafic and felsic granulites subsequently underwent tectonic exhumation in permo triassic times as testified by the development of granulite to amphibolite facies ductile shear zones the granulites were finally exhumed to shallow levels probably in association with the subcontinental mantle in late triassic middle jurassic times the latter period was most likely characterized by extensive brittle faulting at shallow crustal levels thus giving rise to extensional allochthons formed by stretched slices of granitoids the western tethys opening is finally testified by the basalt intrusion and effusion in the late jurassic followed by deep sea pelagic sedimentation the external liguride crustal stratigraphy can be regarded as a fossil example of the transitional realm at the continent ocean boundary this reconstruction fits well with the available data on the present day continental margins derived from passive lithosphere stretching

the linkage of nitrogen isotopic composition and depositional environment of black mudstones in the upper triassic yanchang formation ordos basin northern china abstract in this study we report new analyses of stable nitrogen isotopic compositions 15n of the black mudstones from the chang 7 member of the upper triassic yanchang formation in the ordos basin china to examine the possible linkage of such isotopic compositions with the depositional environment these mudstones were deposited mainly under suboxic bottom water column conditions the core descriptions and trace element concentrations differ between samples obtained from the chang 73 and chang 71 2 members situated at the bottom and top of the chang 7 member respectively such that the depositional condition of the former implies a more reduced water regime the bulk rock nitrogen isotope 15nbulk values of the samples obtained from the chang 73 member are higher than those of the chang 71 2 member no significant differences were identified in the organic carbon isotopes 13corg total organic carbon toc or total nitrogen tn values between the two units the results of 15nbulk cross plotting with the toc tn and carbon to nitrogen atomic ratio c n show an absence of significant modification of the primary nitrogen isotopic compositions from post depositional processes given the depositional history of these units we suggest that the sedimentary redox environment can have a significant influence on 15nbulk denitrification which occurs mainly in suboxic conditions plays an important role in such an environment resulting in 15n enrichment during the original deposition thus owing to its sensitivity to sedimentary redox environments the 15nbulk index can be an effective tool for evaluating depositional redox conditions particularly for further subdivision of suboxic environments

mineralogical and geochemical anomalies of late permian coals from the fusui coalfield guangxi province southern china influences of terrigenous materials and hydrothermal fluids abstract the late permian coal in the fusui coalfield of southern china is characterized by high fe sulfide and organic sulfur contents 2 60 and 2 94 respectively trace elements including zr 354 g g on average hf 9 18 g g y and rare earth elements rey 302 g g li 97 9 g g and cs 7 02 g g are significantly enriched in these coals in addition to hg and se enrichment in the roof and floor of the coal fluorine mo and u are enriched in the roof the floor is rich in cl s fe pb and cd compared to the upper continental crust rey in the parting and coal bench samples are characterized by heavy rey and light rey enrichment respectively the coals partings and host rocks roof and floor have negative eu anomalies the coal benches have higher ratios of u th yb la nb ta and zr hf and more abundant heavy rare earth elements than their adjacent partings these geochemical anomalies are attributed to the composition of terrigenous materials derived from the yunkai upland multi stage syngenetic and epigenetic hydrothermal fluid activities and intensive leaching and re distribution of lithophile elements from partings to the underlying coal benches both the organic and sulfide sulfur are also derived mainly from the hydrothermal fluids rather than the marine influence the minerals in the samples studied including kaolinite quartz and ree bearing minerals are of both terrigenous and hydrothermal origin al oxyhydroxides crystalline feso 4 oh water bearing fe oxysulfate and goyazite were derived from the hydrothermal activity kaolinite quartz ree bearing minerals and apatite from the sediment source region were also subjected to destruction by hydrothermal fluid leaching

uranium enrichment in lacustrine oil source rocks of the chang 7 member of the yanchang formation erdos basin china abstract the oil source rocks of the chang 7 member of the yanchang formation in the erdos basin were deposited during maximum lake extension during the late triassic and show a remarkable positive uranium anomaly with an average uranium content as high as 51 1 g g uranium is enriched together with organic matter and elements such as fe s cu v and mo in the rocks the detailed biological markers determined in the chang 7 member indicate that the lake water column was oxidizing during deposition of the chang 7 member however redox indicators for sediments such as s 2 content v sc and v v ni ratios demonstrate that it was a typical anoxic diagenetic setting the contrasted redox conditions between the water column and the sediment with a very high content of organic matter provided favorable physical and chemical conditions for syngenetic uranium enrichment in the oil source rocks of the chang 7 member possible uranium sources may be the extensive u rich volcanic ash that resulted from contemporaneous volcanic eruption and uranium material transported by hydrothermal conduits into the basin the uranium from terrestrial clastics was unlike because uranium concentration was not higher in the margin area of basin where the terrestrial material input was high as indicated by correlative analysis the oil source rocks of the chang 7 member show high gamma ray values for radioactive well log data that reflect a positive uranium anomaly and are characterized by high resistance low electric potential and low density as a result well log data can be used to identify positive uranium anomalies and spatial distribution of the oil source rocks in the erdos basin the estimation of the total uranium reserves in the chang 7 member attain 0 8 10 8 t

late triassic tuff intervals in the ordos basin central china their depositional petrographic geochemical characteristics and regional implications abstract tuff intervals of upper triassic yanchang formation are laterally widespread in the ordos basin central china this paper focuses on magmatic origins and potential source regions of these tuff intervals through detail depositional petrographic and geochemical analyses most of the tuff intervals are well documented at the bottom of the chang7 oil reservoir unit and can be correlated laterally and certain tuff beds are reworked by turbidity current or seismic activity petrographic studies of the chang7 tuffs indicate that they are composed of crystal shards lithic shards and altered glass shards and the crystal shards include plagioclase quartz and biotite alteration of the chang7 tuffs is ubiquitous thus most of these tuffs transformed into illite smectite i s mixed layers which are identified by scanning electron microscopy sem and x ray diffraction xrd less common minerals are also detected in the chang7 tuffs such as zircon hematite siderite anatase major elements are determined by the x ray fluorescence xrf analysis the results indicate that the chang7 tuffs are enriched in k2o average 4 21 the ratio of sio2 al2o3 ranges from 1 73 to 2 85 average 2 17 and the ratio of tio2 al2o3 varies between 0 006 and 0 032 average 0 017 which imply that the chang7 tuffs originated from a felsic parental magma trace elements are determined by inductively coupled plasma mass spectrometry icp ms indicating the total rare earth element ree concentrations are variable and range from 117 46 to 466 83 ppm average 251 88 ppm ree distribution pattern of the chang7 tuffs presents a lree rightward incline with flat hree curve the value of eu ranges from 0 151 to 0 837 average 0 492 suggesting a strong to weak negative eu anomaly the chang7 tuffs show positive anomalies in rb th and u and negative anomalies in nb sr and eu on a primitive mantle normalized spidergram a preliminary analysis of the geochemical composition of the chang7 tuffs suggests a parental magma origin of rhyodacite dacite which came from volcanic arc related setting along an active continental margin combined with the chronology and geochemical studies of the synchronous tianshui rhyolite in the west qinling mountains we propose that the west qinling mountains is one of the potential source regions of these tuffs and the middle late triassic terminal closure of eastern tethys provided the arc related magma

environmental response to volcanic activity and its effect on organic matter enrichment in the permian lucaogou formation of the malang sag santanghu basin northwest china abstract lacustrine organic rich fine grained rocks are widespread in the second member of the permian lucaogou formation p2l2 in the malang sag santanghu basin with complex lithology and significant changes of lithofacies due to frequent volcanic activity in this study the mineralogical and petrological characteristics types of organic matter om and effect of volcanism on the sedimentary environment and om accumulation were analyzed using organic carbon and oxygen isotopes as well as major and trace element data the results revealed that the p2l2 was characterized by mixed deposition of volcanic ash and carbonate minerals with four main rock types tuff dolomitic tuff tuffaceous dolomite and dolomite the fine grained rocks dominantly contain type i and ii1 kerogens which were in a low mature stage and exhibited excellent original hydrocarbon generative potential when volcanic activity was strong large amounts of nutrients were released by the volcanic ash which then entered the lake in turn promoting the blooming of phytoplankton and causing the 13c values in the tuff to decrease in addition the paleotemperature decreased for a short time and the suboxic euxinic environment was conducive to the preservation of om resulting in high total organic carbon toc content in the tuff in contrast during intermittent or weak periods of volcanic activity the climate was hotter and drier and evaporation resulted in a sustained increase of salinity consequently the growth of algae was restricted which brought about an increase in the 13c values and relatively low toc content in the dolomite the relationships among primary productivity redox conditions clastic influx proxies and toc content illustrate that primary productivity was the most vital control on the accumulation of om high primary productivity was mainly driven by volcanic ash falls meanwhile high salinity and suboxic euxinic conditions were beneficial to the preservation of om giving rise to a high degree of om enrichment

anatomy of late orogenic extension the northern apennines case abstract late orogenic extensional structures overprint the compressional structures of the northern apennines fold and thrust belt the tyrrhenian sea is the result of major continental extension and rifting which started in the thickened zone of the apulian corso sardinian collision it is believed to have commenced as early as in the late oligocene or early miocene in the corsica basin extension post dates the collision by approximately 20 ma the extensional stress field observed in the internal part of the northern apennines co exists with compression in the foreland part of the belt field borehole and refraction and reflection data were used to investigate the geometry of extension and how it affected the compressional abric in the northern apennines analysis of the collected data has revealed a complex geometry for the extension across the belt a geological cross section from the island of elba to the west of the italian peninsula to ancona in the east as well as a geodynamic model for the evolution of the northern apennines is presented and discussed from the early miocene stretching of the collision thickened crust created a n s rift which evolved into the present northern tyrrhenian sea typically two sets of extensional faults a low angle e dipping and a w dipping one are observed across the chain usually the w dipping faults have similar attitudes to those of the older reverse faults reactivating some of the thrusts as normal faults the e dipping normal faults however cut down section through the pre existing thrusts and in the internal part of the belt accommodate most of the displacement associated with extension a change in fault polarity occurs in the tuscan region and major faults dip towards the west a type subduction of the apulian crust under the corso sardinian block occurred after collision leading to the delamination of the apulian lithospheric mantle away from the crust collision induced delamination of the mantle lithosphere away from the zone of collision resulted in mechanical thinning of the lithosphere beneath the orogenic belt this process induced late orogenic crustal stretching in the internal zone of the northern apennines as the convergence between the apulian and corso sardinian blocks continued the extensional and compressional fronts accompanied by the delamination migrated from the hinterland towards the foreland of the orogen

biogenic flux of al to sediment in the central equatorial pacific ocean evidence for increased productivity during glacial periods we examined the flux of al to sediment accumulating beneath the zone of elevated productivity in the central equatorial pacific ocean along a surface sediment transect at 135 w as well as downcore for a 650 kyr record at 1 3 n 133 6 w across the surface transect a pronounced broadly equatorially symmetric increase in al accumulation is observed relative to ti with al ti ratios reaching values 3 4 times that of potential detrital sources the profile parallels biogenic accumulation and the modeled flux of particulate 234th suggesting rapid and preferential adsorptive removal of al from seawater by settling biogenic particles normative calculations confirm that most al is unsupported by the terrigenous fraction the observed distributions are consistent with previous observations of the relative and absolute behavior of al and ti in seawater and we can construct a reasonable mass balance between the amount of seawater sourced al retained in the sediment and the amount of seawater al available in the overlying column the close tie between al ti and biogenic accumulation as opposed to concentration emphasizes that biogenic sedimentary al ti responds to removal transport phenomena and not bulk sediment composition thus in these sediments dominated by the biogenic component the bulk al ti ratio reflects biogenic particle flux and by extension productivity of the overlying seawater the downcore profile of al ti at 1 3 n displays marked increases during glacial episodes similar to that observed across the surface transect from a background value near al ti of average upper crust the excursions in al ti are stratigraphically coincident with maxima in both bulk and caco3 accumulation and the excess al appears to not be preferentially affiliated with opaline or organic phases consistent with the similar behavioral removal of al and 234th the latter of which responds to the total particle flux the al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits these results collectively indicate that 1 al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater therefore the common tenet that al is dominantly associated with terrestrial particulate matter and the subsequent use of al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment needs to be reevaluated 2 the al ti ratio in biogenic sediment can be used to trace the productivity of the overlying water providing a powerful new paleochemical tool to investigate oceanic response to climatic variation 3 the close correlation between the al ti productivity signal and carbonate maxima downcore at 1 3 n suggests that the sedimentary carbonate maxima in the central equatorial pacific ocean record increased productivity during glacial episodes

major trace and platinum group element geochemistry of the upper triassic nonmarine hot shales in the ordos basin central china abstract the upper triassic chang 7 non marine hot shales have been proven to contribute to mesozoic oil reservoirs in the ordos basin central china these shales have been studied extensively in sedimentology petrology and organic geochemistry however the factors controlling the development of the shales are still ambiguously constrained this paper presents the major trace and platinum group elements pges data to address these issues the chemical index of alteration cia chemical index of weathering ciw c values and clay mineral compositions indicate that the chang 7 hot shales were developed in a humid paleoclimate the indicators of redox conditions eu eu ce ce ce anom u u th v v ni and v cr suggest that the chang 7 hot shales were developed under anoxic environment uranium enrichment average 46 10 ppm is an important distinguished signature of the chang 7 hot shales which was mainly related to synchronous volcanic ash sediments and or possibly hydrothermal fluids the pge concentrations of the chang 7 hot shales were reported for the first time here and can be subdivided into two groups the total abundances of the group one samples are relatively lower and derived mainly from detrital minerals while the group two samples are relatively higher and are also likely linked to the synchronous volcanic ash sediments and or possibly hydrothermal fluids combined with the regional geological setting we tentatively propose that the tectonics is the major control on the development of the chang 7 hot shales

an early miocene transition in deformation regime within the red river fault zone yunnan and its significance for indo asian tectonics recent work has tended to support the view that lateral extrusion of indochina along the red river fault zone was a significant factor in accommodating the convergence of india with asia to better understand the nature and timing of this feature 40ar 39 ar age spectrum analyses were undertaken on minerals from the ailao shan red river metamorphic belt in southern yunnan china interpreted using diffusion domain theory these results yield thermal histories in the temperature range 550 150 c discontinuities in cooling rates observed from these results suggest a transition in deformation style in the shear zone during the early miocene at least in the vicinity of our samples left lateral strike slip ductile deformation appears to have ceased by about 20 ma the mylonitic gneisses in the shear zone were subsequently obliquely unroofed probably due to a component of normal faulting along the eastern edge of the shear zone causing a brief locally 100 c m y cooling episode this transition may have resulted from a component of northeast southwest extension attributable to greater clockwise rotation of indochina relative to south china after 20 ma than that consistent with pure strike slip along the fault k ar ages on clay minerals from gouge within the normal faults bounding the ailao shan and diancang shan yield a range of ages between 20 and 185 ma but show a significant clustering at 20 25 ma these results suggest that early miocene movement on a precursor to the range front normal fault in the ailao shan caused oblique unroofing of the shear zone this early miocene event corresponds in time with a transition from low to very high denudation rates throughout the himalaya and southern tibet this coincidence may reflect that once left lateral strike slip motion on the red river fault zone slowed or was terminated accommodation of much of the continued northward convergence of india occurred on the main central thrust resulting in rapid uplift in the tethyan himalaya and gangdese belt

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

stability of the astronomical frequencies over the earth s history for paleoclimate studies the expected changes over the past 500 million years in the principal astronomical frequencies influencing the earth s climate may be strong enough to be detectable in the geological records and such effects have been inferred in several cases calculations suggest that the shortening of the earth moon distance and of the length of the day back in time induced a shortening of the fundamental periods for the obliquity and climatic precession from 54 to 35 41 to 29 23 to 19 and 19 to 16 thousand years over the last half billion years at the same time the precessional constant increased from 50 to 61 arc seconds per year the changes in the frequencies of the planetary system due to its chaotic motion are much smaller their influence on the changes of the periods of climatic precession obliquity and eccentricity of the earth s orbit around the sun can be neglected eccentricity periods used for quaternary climate studies may therefore be considered to have been more or less constant for pre quaternary times

petrogenesis and geodynamic implications of the gejiu igneous complex in the western cathaysia block south china abstract the gejiu tin district in western cathaysia block comprises a series of igneous rocks including equigranular and porphyritic granites gabbro and nepheline syenite systematic shrimp or la icp ms zircon u pb analyses of 15 representative samples from various phases of the gejiu complex yielded late cretaceous ages of 78 85 ma based on their mineralogical geochemical and sr nd hf isotope characteristics these rocks are categorized into three groups felsic rocks alkaline rocks and mafic rocks the felsic rock group includes the equigranular and porphyritic granites geochemical characteristics include high sio 2 contents enrichment in rb th u nb ta nd and hf and depletion in ba k sr p eu and ti compared to primitive mantle ree patterns feature slight lree enrichment with pronounced negative eu anomalies geochemical data and sr nd and hf isotopic compositions indicate that the felsic rocks were probably generated by partial melting of crustal source rocks with a minor input from mantle materials the mafic rocks gabbro and mafic microgranular enclaves have distinct geochemical and isotopic features consistent with derivation from an enriched mantle source with variable degrees of mixing with crustal derived magmas strontium nd and hf isotopic compositions of the alkaline rocks are similar with those of the mafic rocks suggesting that they have a similar source nevertheless petrological and geochemical characteristics of these rocks indicate that they experienced extensive crystal fractionation and limited crustal contamination based on the emplacement of the gabbro mafic microgranular enclaves syenite granites in the gejiu district together with contemporaneous geological events in other parts of the western cathaysia block we suggest that a widespread extension related magmatic episode affected the entire region in the late cretaceous possibly as a result of lithospheric thinning basaltic underplating and associated crustal melting

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

gravity profile interpretation using the r parameter imaging technique with application to ore exploration abstract this paper describes a fast imaging technique the so called here the r parameter imaging technique for the interpretation of gravity data measured along profile the technique calculates the r parameter a statistical parameter called the correlation coefficient between the analytic signal of the measured gravity field and that of the synthetic response of an assumed interpretive model in the restricted class of spheres and cylinders the characteristic source parameters depth shape origin point of the gravity profile and amplitude factor of the model correspond to the maximum r parameter value the method has been verified on noise free numerical examples following that the accuracy of the method has been carefully assessed on both noisy numerical examples and numerical examples with interference effects finally the technique has been successfully applied to five field data examples from canada sweden and cuba for mineral exploration and it is found that the obtained results are in good agreement with those reported in the published literature

obliquity forced climate during the early triassic hothouse in china the start of the mesozoic era is marked by roughly 5 m y of earth system upheavals including unstable biotic recovery repeated global warming ocean anoxia and perturbations in the global carbon cycle intervals between crises were comparably hospitable to life the causes of these upheavals are unknown but are thought to be linked to recurrent siberian volcanism here two marine sedimentary successions at chaohu and daxiakou south china are evaluated for paleoclimate change from astronomical forcing in these sections gamma ray variations indicative of terrestrial weathering reveal enhanced obliquity cycling over prolonged intervals characterized by a 32 8 k y periodicity with strong 1 2 m y modulations these suggest a 22 h length of day and 1 2 m y interaction between the orbital inclinations of earth and mars comparing the 1 2 m y obliquity modulation cycles in these sections with early triassic records of global sea level temperature redox and biotic evolution suggests that long term astronomical forcing was involved in the repeated climatic and biotic upheavals that took place throughout the early triassic

margin segmentation and volcano tectonic architecture along the volcanic margin off argentina uruguay south atlantic abstract some 25 000 kilometers of regional multichannel seismic data acquired by bgr along the continental margins off argentina and uruguay document that the early cretaceous south atlantic continental breakup and initial sea floor spreading were accompanied by large scale transient volcanism emplacing voluminous extrusives manifested in the seismic data by huge wedges of seaward dipping reflectors sdrs the emplacement of the deeply buried 60 120 km wide sdrs was probably episodic as documented by at least three superimposed sdrs units distinct along margin variations in architecture volume and width of the sdrs wedges are probably related to margin segmentation it is suggested that the margin can be divided at least in four compartments bounded by the falkland fracture zone falkland transfer the colorado transfer the ventana transfer and the salado transfer the individual margin segments reflect in the distribution and thickness of the post rift sediments the individual transfer zones may have acted as rift propagation barriers selectively directing rift segments in left stepping patterns along the western south atlantic margin although we found extensive variations in the architecture style and extent of the seaward dipping reflector sequences a general trend is that the largest volumes are emplaced close to the proposed transfer zones and the width of the sdrs wedges decreases northward within the individual margin segments we suggest that mainly adiabatic decompression and melt generation from shallow sources can explain distinct along margin variations in the volcano tectonic architecture and volumes of extruded magmas

influence of volcanic and hydrothermal activity on organic matter enrichment in the upper triassic yanchang formation southern ordos basin central china abstract based on the field investigation of oil shales in the southern ordos basin detailed observations and systematic sample analyses of core samples from a well that penetrated an oil shale the influence of volcanic and hydrothermal activity on the chang 8 chang 7 members of the yanchang formation on organic matter om enrichment was geochemically and petrologically analyzed according to maceral analyses and rock pyrolysis the kerogen types of the samples in the study area are widely distributed those of the oil shale with the highest om abundance are mostly type ii i and are immature or in a stage of low maturity six layers of tuff and tuffaceous mudstone were identified by core observation and element characteristics they are characterized by higher total rare earth element ree and ce values and th contents and lower la yb n and eu values mineral assemblages of anhydrite pyrite marcasite and relevant diagrams of the elemental geochemistry confirmed the existence of hydrothermal deposits the high yn hon and u th ratios and total sulfur ts contents indicate that the chang 7 member experienced four episodes of intense hydrothermal activity early in the chang 73 member volcanic ash and hydrothermal fluids brought in a large amount of nutrients which not only increased primary productivity but also increased the salinity of the water column enhanced anoxia in the sedimentary environment of the lake bottom promoted the preservation of om and finally formed the oil shale layer with the richest om however the strong volcanic eruption in the late chang 73 member led to changes in the regional climate causing it to dry and become colder and resulting in a decrease of primary productivity earthquakes triggered by volcanism resulted in turbidite deposits which destroyed the preservation and diluted the enrichment of om this study reveals in detail the influence of volcanic and hydrothermal activity on om enrichment during the evolution of the basin and thereby deepens our theoretical understanding of om enrichment

nature of the transition from continental to oceanic crust and the meaning of reflection moho the evolution from continental to oceanic crust in the gulf of guinea involves the foundering of highly stretched and tilted slivers of continental crust the emplacement of mafic volcanic material above and between these slivers and the progressive lateral replacement of continental fault blocks with oceanic igneous rocks this oceanization of continental fault blocks appears to be a gradual process and may occur over flow line distances of 150 km or more it is hypothesized that the reflection moho is a dynamic surface that may mark the abrupt transition to a more ductile regime partial melt if so the reflection moho may have little paleostructural memory except possibly in the vicinity of fracture zones this casts uncertainty on the meaning of traditional measurements of crustal thickness and the applicability of extension computations that utilize the moho as a datum

fractures in sandstone reservoirs with ultra low permeability a case study of the upper triassic yanchang formation in the ordos basin china the upper triassic yanchang formation in the ordos basin central china is a typical sandstone reservoir with an ultra low permeability high angle tectonic fractures and diagenetic fractures such as near horizontal bedding fractures intragranular fractures and grain boundary fractures are abundant fractures are major pathways and enhance fluid flow in sandstone reservoirs with ultra low permeability because of their weak lateral continuity and their small apertures under lithostatic pressure bedding fractures make a relatively small contribution to the overall permeability of reservoirs as they are both of small size and low permeability intragranular fractures and grain boundary fractures mainly improve the connectivity of reservoirs by connecting the matrix pores high angle tectonic fractures control the fluid movement in the ultra low permeability reservoirs under the effect of different successive tectonic stress fields four assemblages of high angle tectonic fractures developed in the sandstone reservoirs under the present day stress differently oriented fractures have different connectivities apertures and permeabilities the northeast southwest fractures parallel to the maximum principal stress have a good connectivity large apertures and a high permeability forming the dominant flow paths knowledge of these paths can be used for optimizing well placement because of their sensitivity to pressure fractures in different directions will show varying apertures as the formation pressure decreases therefore the permeability of the fractures of different orientations and its impact on reservoirs vary at different developmental stages

the floating astronomical time scale for the terrestrial late cretaceous qingshankou formation from the songliao basin of northeast china and its stratigraphic and paleoclimate implications abstract the upper cretaceous qingshankou formation k 2 qn in the songliao basin slb of northeast china consists of up to 550 m thick lacustrine mudstone and shale that constitute one of the most important source rocks of the daqing oil field a high resolution cyclostratigraphic analysis of the natural gamma ray logging from 10 wells of the qingshankou formation k 2 qn reveals orbital cycles of precession 20 ka obliquity 40 ka and eccentricity 100 ka and 405 ka providing strong evidence for astronomically driven climate changes in the late cretaceous terrestrial environments floating astronomical time scales ats are established for all sections which demonstrate variable durations of k 2 qn across the basin 1 09 ma 5 20 ma and strong diachroneity of the lacustrine strata four periods of high depositional rates can be identified in the central parts of the basin possibly recording deposition during times of sustained wet climate and high chemical weathering an ats established from well m206 in the central depression zone of the basin where the most complete and stable milankovitch cycles are present suggests that the maximum duration of the k 2 qn is 5 20 ma from 94 27 ma to 89 07 ma late cenomanian to early coniacian the lacustrine anoxic event 1 lae1 at the cenomanian turonian boundary lasted 210 310 ka during which the most prolific petroleum source rocks in slb were deposited the onset 94 21 94 18 ma and duration 210 310 ka of lae1 in slb are comparable to those of the oceanic anoxic event 2 oae2 onset at 94 21 ma and duration of 320 900 ka suggesting that the same trigger mechanism such as increased atmospheric co 2 from large scale igneous activity may have initiated high primary productivity and organic carbon burial in both marine and terrestrial systems

cyclostratigraphy and its revolutionizing applications in the earth and planetary sciences over the past 25 yr the science of stratigraphy has evolved to include time correlative data from vastly disparate components of the earth system not least of these is the global signal afforded by cyclostratigraphy which has recorded the evolution of earth s astronomical milankovitch forcing of insolation and the paleoclimate system fossil astronomical signals are collected from cyclic sedimentary sequences by detailed sampling and study of facies geochemistry mineralogy rock magnetism color etc in step with the documentation of astronomically forced paleoclimate from ever older older geologic times innovations in computational science have provided ever longer high accuracy astronomical model targets that can be used for time scale calibration the earth s orbit is affected by motions of other planets notably the orbital perihelia of venus and jupiter which impose a dominant 405 k y eccentricity cycle on earth s orbital evolution the large mass of jupiter stabilizes this cycle over hundreds of millions of years the cyclostratigraphic record of 405 k y cycles is therefore often used to correct chronologies affected by variable sedimentation earth s shape and rotation rate are influenced by tidal dissipation and climate friction these effects affect earth s precession rate through time thus a record of earth moon evolution is also embedded in cyclostratigraphy the geochronologic value of cyclostratigraphy has been affirmed through intercalibration with high precision radioisotope dating which today has the potential to define the ages of stratigraphic horizons with 2 uncertainties at the scale of a precession cycle precession index phasing relative to that of the obliquity elucidates the seasonal nature of astronomical forcing of the paleoclimate system cyclostratigraphy contributes to our knowledge of planetary dynamics for times prior to the current ca 50 ma limit of accurate astronomical solutions and it will guide our future understanding of solar system evolution and the evidence for chaotic diffusion astronomical modeling is undergoing its own revolution with development of new numerical integrators to extend accuracy further back in time finally space exploration has revealed prominent sedimentary bedding and ice stratigraphy on the surface of mars with patterns suggestive of astronomical forcing analogous to earth

continental break up mechanism lessons from intermediate and fast extension settings continental break up mechanisms vary systematically between slow and fast extension systems slow extension break up has been established from studies of the central atlantic european and adria margins this study focuses on the intermediate and fast cases from gabon and east india and draws from the interpretation of reflection seismic gravimetric and magnetic data interpretation indicates continental break up via continental mantle unroofing in all systems with modifications produced by magmatism in faster extension systems break up of the intermediate extension gabon system involves partial upper continental crustal decoupling from continental mantle whereas in the fast east coast india system decoupled and lower crustal regimes underwent upwarping in soggy zones in the footwalls of major normal faults usually upper crustal break up is affected by pre existing anisotropies which form systems of constraining rails for extending continental crust this modifies the local stress regimes they regain a regional character as the function of constraining rails vanishes during progressive unroofing of the upper mantle different regions attain different amounts of upper crustal stretching prior to the break up the break up location is then controlled by the upper crustal energy balance principle of wound linkage by which the minimum physical work is performed for linking upper crustal wounds leading to successful upper crustal break up

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

structure of the gabon margin from integrated seismic reflection and gravity data in the south gabon basin deep multi channel seismic reflection and gravity modeling analysis have shed light on key features of the structure of the margin the thinned continental crust beneath the gabon margin appears to be composed of two distinct layers separated by a clear strong and more or less continuous reflector running in the 7 10 s twt window the lower crust is characterized by a higher density intermediate between the lower values of the upper crust and the denser values of the mantle the lower crust is irregularly shaped and presents lateral thickness variations along the direction of thinning and along the coast in the offshore thinned continental domain the lower and upper crust form a 20 25 km thick body crustal thicknesses point to a relatively sharp and narrow transition along a few tens of kilometers between the unthinned and the thinned continental crust the high density layer identified offshore gabon presents similar characteristics in density geometry and spatial distribution as the underplated magmatic bodies observed along volcanic margins e g along the south atlantic namibia margin or the north atlantic voring margin although this lower crustal body could possibly represent ultra mafic serpentinized rocks or high grade metamorphic crustal rocks we suggest that it could be composed of mafic rocks magmas resulting from partial melting during rifting may underplate the crust and or be intruded in the lower crust through a system of dykes and sills in this view the present day crustal thicknesses along rifted margins characterized by magmatic underplating and or intrusion are not representative of the thinning that the crust experienced during rifting results of this study point to relatively shallow sedimentary basins along the south gabon margin the deepest offshore depocenters located under the westernmost side of the continental platform appear to be associated with the deepest syn rift basins these basins seem to extend along 20 to 40 km in the ne sw direction with a present day average thickness of 7 3 km offshore gabon whereas the crustal thinning appears significant the syn rift deposit are not thick we suggest that the area was anomalously uplifted during the rifting phase due to an elevated thermal lithospheric gradient the conclusions derived from our seismic and gravity analysis are consistent with the implications such a thermal anomaly would have on the tectonic evolution of a rifted margin with 1 an underplated high density lower crustal layer 2 shallow depth syn rift basins associated with a relatively thin crust and subsequently 3 elevated recorded subsidence rates in the initial post rift stages research highlights the gabon margin appears characterized by an underplated high density lower crustal layer shallow depth syn rift basins are associated with a relatively thin crust seismic and gravity analysis points to an anomalous uplift of the gabon margin during the rifting phase crustal structure syn rift infill and post rift subsidence are consistent an uplift of the gabon margin during the rifting phase the uplift of the gabon margin is possibly due to an elevated thermal lithospheric gradient

astronomical tuning of the end permian extinction and the early triassic epoch of south china and germany abstract the timing of the end permian mass extinction and subsequent prolonged recovery during the early triassic epoch can be established from astronomically controlled climate cycles recorded in continuous marine sedimentary sections astronomical cycle tuning of spectral gamma ray logs from biostratigraphically constrained cyclic stratigraphy through marine sections at meishan chaohu daxiakou and guandao in south china yields an integrated time scale for the early triassic which is consistent with scaling of magnetostratigraphy from climatic cycles in continental deposits of the germanic basin the main marine mass extinction interval at meishan is constrained to less than 40 of a 100 kyr kilo year cycle i e c 13 is estimated to have lasted c 13 from 2 to 4 across the smithian spathian boundary at chaohu was completed in 50 kyr the earliest marine reptiles in the mesozoic at chaohu that are considered to represent a significant recovery of marine ecosystems did not appear until 4 7 myr million years after the end permian extinction the durations of the griesbachian dienerian smithian and spathian substages including the uncertainty in placement of widely used conodont biostratigraphic datums for their boundaries are 1 4 0 1 0 6 0 1 1 7 0 1 and 1 4 0 1 myr implying a total span for the early triassic of 5 1 0 1 myr therefore relative to an assigned 251 902 0 024 ma for the permian triassic boundary from the meishan gssp the ages for these substage boundaries are 250 5 0 1 ma for base dienerian 249 9 0 1 ma for base smithian base of olenekian stage 248 2 0 1 ma for base spathian and 246 8 0 1 ma for the base of the anisian stage this astronomical calibrated timescale provides rates for the recurrent carbon isotope excursions and for trends in sedimentation accumulation through the early triassic of studied sections in south china

mechanism for the enrichment of organic matter in the liushagang formation of the weixinan sag beibuwan basin china abstract eocene lacustrine organic rich shales mudstones and shales in the middle member of liushagang formation designated as ls2 are the most important source rocks in the beibuwan basin located in the northwest of the south china sea in this study we explored the mechanisms responsible for the accumulation of organic matter in source rocks from ls2 of weixinan sag by investigating the paleoclimate paleoredox paleoproductivity and lithological characteristics of the parent rocks using biomarkers and elemental geochemistry data based on our analytical data the deposition of the source rocks from ls2 occurred in a warm humid paleoclimate with moderate chemical weathering as indicated by cia indexes and sr cu ratios the sr ba ratios and gammacerane indexes indicated that the source rocks from ls2 were deposited in a freshwater paleolake the corg p ratios and redox sensitive elements rses toc bivariate diagrams demonstrated that the organic rich shales and mudstones shales deposited under anoxic and oxic suboxic conditions respectively the levels of toc copper cu molybdenum mo and c30 4 methylsterance index in the organic rich shales were higher than those of the mudstones and shales indicating greater algae bloom and paleoproductivity during the deposition of the organic rich shales compared to during the sedimentation of the mudstones and shales combined with la yb ree sio2 tio2 al2o3 tio2 nb ta and cr co ratios it is considered that the parent rocks of the organic rich shales were granite of relatively intermediate lithological compositions with greater levels of nutrients such as phosphorus p mo cu compared to those of the mudstones and shales large scale decomposition of dead algae led to oxygen depletion which created favorable conditions for the preservation of organic matter and the release of p from the sediments to water the increased content of p in water subsequently aggravated algal blooms such a positive feedback loop was conducive to the formation of organic rich shales the inter relationships of algal blooms controlled by p paleoproductivity and paleoredox conditions play an important role in the accumulation of organic matter in organic rich shales from ls2 of weixinan sag beibuwan basin

constraints for plate reconstruction using gravity data implications for source and reservoir distribution in brazilian and west african margin basins abstract evaluation of tertiary sequences of west africa and brazil typically requires large budgets and staffs to identify drilling targets since these tertiary sedimentary systems extend for hundreds of kilometers proc petrol geol deepwater depositional syst 2001 2 marine petrol geol 1990 94 122 well beyond the limits of individual 3d and many 2d seismic surveys additional information is necessary to interpret the most favourable locations for detailed exploration potential field data are powerful but often underutilized assets in building an exploration framework for reducing costs and interpretation risks although drilling locations are normally based on 3d seismic interpretations reservoir distribution is controlled by features mappable with other methods such as potential fields other factors crucial to petroleum maturation migration and trap formation relate to deep seated controls that are well imaged and sometimes exclusively imaged on potential field data our observations from potential field data help identify reservoir distribution source pod locations source maturity possible migration pathways and potential traps this paper illustrates the power of combining both regional and basinal scale interpretations based primarily on gravity analyses with extensive knowledge of the underlying geology we review data sets and methods employed and describe results of a regional overview based on special plate tectonic reconstructions of the south atlantic examples from basin scale work offshore west africa and brazil are used to illustrate factors important to hydrocarbon exploration we conclude with a look ahead to improvements in methodology and application

three common source rock evaluation errors made by geologists during prospect or play appraisals geologists are frequently called on to evaluate the source rocks associated with their exploration prospects or plays the three most common questions asked and answered about the source rock during project reviews are what s the total organic carbon toc what kerogen type does rock eval indicate and what maturity level does the vitrinite reflectance data point to the answers to these seemingly innocuous questions may in fact be providing a false sense of security about the source rock in question understanding how this line of questioning can lead you astray and make you the victim of the toc myth if i have high toc i have a good source rock the rock eval fallacy the rock eval data tell me what kind of kerogen is in my source rock and the vitrinite reflectance deficiency vitrinite reflectance will tell me if my source rock is generating is important some of the solutions to these problems include fully integrating toc and rock eval data supplementing rock eval data with pyrolysis gas chromatography and using burial history diagrams to help interpret vitrinite reflectance harry dembicki jr is a senior geological advisor for geochemistry in the geological technology group at anadarko petroleum corporation where he provides technical support to both exploration and development teams he holds a b s degree 1973 in geology from the state university college of new york at new paltz and a ph d 1977 in geology with emphasis in organic geochemistry from indiana university he previously worked as an organic geochemist for conoco and marathon

the jinshajiang ailaoshan suture zone china tectonostratigraphy age and evolution abstract the jinshajiang suture zone is important for enhancing our understanding of the evolution of the paleo tethys and its age tectonic setting and relationship to the ailaoshan suture zone have long been controversial based on integrated tectonic biostratigraphic chemostratigraphic and isotope geochronological studies four tectono stratigraphic units can be recognized in the jinshajiang suture zone the eaqing complex the jinshajiang ophiolitic melange the gajinxueshan group and the zhongxinrong group isotope geochronology indicates that the redefined eaqing complex composed of high grade metamorphic rocks might represent the metamorphic basement of the jinshajiang area or a remnant micro continental fragment eaqing complex protolith rocks are pre devonian and probably of early middle proterozoic age and are correlated with those of the ailaoshan complex two zircon u pb ages of 340 3 and 294 3 ma separately dated from the shusong and xuitui plagiogranites within the ophiolitic assemblage indicate that the jinshajiang oceanic lithosphere formed in latest devonian to earliest carboniferous times the oceanic lithosphere was formed in association with the opening and spreading of the jinshajiang oceanic basin and was contiguous and equivalent to the ailaoshan oceanic lithosphere preserved in the shuanggou ophiolitic melange in the ailaoshan suture zone the latter yielded a u pb age of 362 41 ma from plagiogranite the re defined gajinxueshan and zhongxinrong groups are dated as carboniferous to permian and latest permian to middle triassic respectively on the basis of fossils and u pb dating of basic volcanic interbeds the gajinxueshan group formed in bathyal slope to neritic shelf environments and the zhongxinrong group as bathyal to abyssal turbidites in the jinshajiang ailaoshan back arc basin latest permian earliest middle triassic synorogenic granitoids with ages of 238 18 and 227 5 255 8 ma respectively and an upper triassic overlap molasse sequence indicate a middle triassic age for the jinshajiang ailaoshan suture formed by collision of the changdu simao block with south china

crustal structure inferred from gravity anomalies in south china abstract the bouguer gravity anomaly map of the mainland of south china and the south china sea compiled from the most recent 1 500 000 and 1 1 000 000 gravity data is presented the distribution of faults including the secondary faults which can be located by a method based on cross correlation between the theoretical anomaly due to a vertical step and the second vertical derivative of the bouguer anomalies large strike slip faults without vertical displacement or with a small throw which are difficult to identify on the bouguer gravity map and can be inferred from the distribution of the secondary faults and major faults inferred from the zones of gravity gradients on the bouguer gravity map is presented also comparison of the regional gravity anomalies with known density interfaces deduced from seismic data and geological maps shows that the main geological factors causing gravity anomalies are granites low velocity layers in the upper crust the moho discontinuity and the upper crustal thickness variations the broad trend of gravity anomalies depends mainly on the moho and the upper crustal thickness variations we have attempted to isolate approximately the anomalies due to the upper crustal thickness variations by subtracting the anomalies due to the moho from the regional anomalies and then the upper crustal thickness in south china can be estimated by inversion of the gravity anomalies

paleoproductivity of the chang 7 unit in the ordos basin north china and its controlling factors abstract primary paleoproductivity provides basic organic matter for the formation of high quality hydrocarbon source rock a progressive method to quantitatively calculate paleoproductivity on the basis of total organic carbon toc has been proposed to evaluate the contribution of primary paleoproductivity to a hydrocarbon source rock with high organic matter abundance in this study pyrite contents organic maceral compositions and high resolution sedimentation rates srs were used to recover the amount of organic matter oxidation and to exclude the effect of terrestrial clastic input during the deposition of the chang 7 sediment multiple calculation methods were used to quantitatively reconstruct paleoproductivity and the results suggest that the ordos basin was a eutrophic lacustrine basin approximately 2 104 mg c cm 2 kyr 1 to 10 104 mg c cm 2 kyr 1 during the chang 7 stage compared with modern lake productivity the paleoclimatic analysis indicates that a warm humid paleoclimate was beneficial for increasing the weathering intensity which could have provided more nutrient inputs to the depositional environment with incoming terrestrial debris in addition the plate collision between the north china block and yangtze block generated moderate volcanic ash that spread to the study area and provided large amounts of essential nutrients for algae blooms moreover a brackish water condition promoted upward mixing of deep water which inhibited water mass stratification and then facilitated primary paleoproductivity in general the chang 7 high quality hydrocarbon source rock is a result of increased paleoproductivity resulting from suitable paleoclimate conditions brackish water masses and abundant essential nutrient inputs

deep structure of the west african continental margin congo zaire angola between 5 s and 8 s from reflection refraction seismics and gravity data summary the deep structure of the west african continental margin between 5 s and 8 s was investigated using vertical reflection and wide angle reflection refraction techniques during the zaiango project a joint programme conducted in 2000 april by ifremer and totalfinaelf to penetrate below the salt layer a non conventional low frequency seismic source was used in the single bubble mode together with ocean bottom instruments hydrophones and seismometers and a 4 5 km long streamer that recorded multichannel seismic reflection mcs the data show that the continental crust thins abruptly over a lateral distance of less than 50 km from 30 km thick below the continental platform based on gravity data to less than 4 km thick below the lower congo basin that formed prior to the aptian salt deposition this subsalt sedimentary basin 180 km wide 4 km thick with velocities varying from 4 7 km s 1 to 5 8 km s 1 at the bottom is located between the foot of the continental slope and the oceanic domain it is underlain by crust of an intermediary or transitional type between continental crust and what can be recognized as oceanic crust in the transitional zone a crustal upper layer is present below the pre salt sedimentary basin 3 to 7 km thick with velocities increasing from 5 8 km s 1 at the top to 6 8 km s 1 at the bottom of the layer this layer appears to thin regularly from 6 7 km thick below the depocentre of the pre salt sedimentary basin to 3 4 km thick below the western termination of the basin below this upper crustal layer an anomalous velocity layer 7 2 to 7 8 km s 1 is documented below the eastern side of the basin where the crustal thinning is at a maximum the origin of this layer is unknown several arguments like rifting duration between 15 ma and 30 ma or the absence of seaward dipping reflectors precludes the hypothesis of underplated mantle material but other hypotheses such as serpentinized material or high grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks are plausible near the ocean termination of the basin the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles 7 11 and 14 having a dense ocean bottom seismometer hydrophone obs obh spacing on these profiles an anomalous velocity layer is present in the westernmost part of the transitional zone below the basement ridge and in the oceanic domain this layer absent on profile 3 may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process

yanshanian late mesozoic ore deposits in china an introduction to the special issue abstract the late jurassic cretaceous yanshanian orogeny or yenshan movement one of the most important tectonothermal events is first recognized in china especially eastern china this late mesozoic orogeny which was initiated most likely by a mesozoic tectonic switch strongly reworked or destructed the older continental lithospheres or cratonic keels that are manifested by alternating compressive and extensional deformation voluminous igneous rocks and a variety of characteristic magmatic hydrothermal mineral systems despite its first discovery and definition in yenshan yinshan area of north china craton the yanshanian orogeny probably is of global tectonic magmatic and metallogenic significance however there have been hot debates on the precise starting time accurate duration or time interval detailed processes and evolution linked to deep lithospheres tectonic nature and geodynamic mechanism s of the yanshanian orogeny which inevitably have hindered the understanding of the genesis mineralizing processes and geodynamic mechanism of the late mesozoic magmatic hydrothermal mineral systems this special issue captures some of the latest research results on the yanshanian ore deposits that are involved into a few main mesozoic metallogenic belts or provinces from northeast to southwest china including 1 the jiaodong peninsula metallogenic province in the north china craton 2 the middle lower yangtze river valley metallogenic belt in the central eastern china 3 the jiangnan and 4 the nanling metallogenic belts in the south china block 5 the southeastern china coast metallogenic belt and 6 the sanjiang metallogenic belt in southwest china through a multidisciplinary study this special issue re investigated and re evaluated the relationship between the late mesozoic magmatic hydrothermal mineral systems and the yanshanian tectonothermal events in the studied metallogenic belts or provinces a few important contributions to the topic in this special issue yanshanian metallogeny are summarized as followings 1 a new ore deposit type i e the intracontinental reactivation type has been suggested to interpret the genesis of those au polymetallic deposits that are hosted within older metamorphic rocks and related to the late mesozoic basin and range extensional settings 2 late mesozoic re activation of the preexisting structures by the yanshanian tectono thermal event s might be an important mechanism controlling the yanshanian large scale mineralization 3 a type granites formed by partial melting of the mesoproterozoic crust but with inputs from mantle derived melt are also favorable for sn mineralization in addition to s type and i type granites as previously recognized 4 calculated oxygen fugacities o 2 of granitic magmas based on chemical compositions of primary biotite have been confirmed to be effective proxy for distinguishing cu au mo w sn pb zn mineralized granites from barren granites 5 a significant epoch of w sn magmatic hydrothermal ore system at ca 145 135 ma has been identified in the southeastern china coast metallogenic belt and 6 in addition to traditional structural geology mineralogy petrology geochemistry and geochronology new analytical techniques e g cu isotopes and data treatment method e g bi dimensional empirical mode decomposition can be used to provide more constraints for deep exploration

the role of break up localization in microcontinent separation along a strike slip margin the east india elan bank case study the elan bank microcontinent was separated from east india during the early cretaceous break up the crustal architecture and rifting geometry of east india and the elan bank margins document that the early break up between india and antarctica was initiated in the eastern portions of the cauvery and krishna godavari rift zones and in the southern portion of elan bank however the westwards break up propagation along the krishna godavari rift zone continued even after the break up in the overstepping portion of the cauvery rift zone eventually the western propagating end of the krishna godavari rift zone became hard linked with the failed western portion of the cauvery rift zone by the dextral coromandel transfer fault zone consequently the break up location between india and antarctica shifted from its initial to its final location along the northern portion of the elan bank formed by the western krishna godavari rift zone the competition between the two rift zones to capture continental break up and asymmetric ridge propagation resulted in a ridge jump and the elan bank microcontinent release

long period eccentricity control on sedimentary sequences in the continental madrid basin middle miocene spain article i nfo the middle miocene valdearenas muduex section in the internally drained continental madrid basin central spain is dated bio magnetostratigraphically between 15 2 ma and 11 5 ma the section contains two formation scale sedimentary sequences that both consist of a siliciclastic lower part and a calcareous upper part siliciclastic sedimentation took place in distal floodplain and fluvial environments while limestones resulted from carbonate precipitation in calcic soil profiles and in ephemeral lacustrine water bodies spectral analysis of the l colour time series points to the influence of the 405 kyr and 0 97 myr eccentricity cycles while the bases of the two calcareous intervals correlate to successive minima of the 2 4 myr eccentricity cycle the 405 kyr cycle lags maximum eccentricity whereas the 0 97 and 2 4 myr cycleslag minimum eccentricity each by approximately a quarter of a cycle no obliquity forcing isdetected the observed orbital configuration of 2 4 myr minima at the base of limestone dominated intervals is similar to a previously documented late miocene shift in the teruel basin of northeast spain our results indicate that long period eccentricity clim ate forcing may well be as ignificant player on long tectonic time scales in continental basin fill

comment on a new scheme for the opening of the south atlantic ocean and the dissection of an aptian salt basin by trond h torsvik sonia rousse cinthia labails and mark a smethurst p torsvik et al recently presented a revised model for the opening of the south atlantic ocean according to these authors this new plate tectonic model is internally consistent and consistent with globally balanced plate motion solutions and takes into account realistic intraplate deformation however this model shows a number of kinematic and geological problems that we underline here together with some comparisons with the new kinematic model recently proposed by moulin et al

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

application of bi dimensional empirical mode decomposition bemd modeling for extracting gravity anomaly indicating the ore controlling geological architectures and granites in the gejiu tin copper polymetallic ore field southwestern china the gejiu tin copper polymetallic ore field located at the westernmost end of the cathaysia block in south china is one of the largest tin polymetallic ore fields in the world it is associated with magmatic hydrothermal ore forming processes triggered by deeply seated structures and concealed granites the bi dimensional empirical mode decomposition bemd was used to decompose gravity data covering an area including the gejiu tin copper polymetallic ore field in the west and the bozhushan silver lead zinc polymetallic ore field in the east which yielded three two dimensional intrinsic mode function bimfs images and one residue res m n image that depict four layers of geological architectures at different wavelengths within the study area the high pass filtered gravity component image bimf1 is interpreted to depict the shallow geological architecture which indicates that the skarn alteration and tin copper mineralization with positive gravity anomalies are distributed around the granites characterized by negative gravity anomalies the band pass filtered gravity component image bimf2 is interpreted to depict the middle shallow geological architecture which indicates that the outcropped granites in the western gejiu ore field bounded by the gejiu fault may extend to the eastern gejiu ore field to form one integrated granitic complex in the subsurface the other band pass filtered gravity component image bimf3 is interpreted to depict the middle lower geological architecture which indicates that there may be an ew trending granite zone with negative gravity anomaly at middle lower depth connecting the gejiu tin copper polymetallic field to the bozhushan silver lead zinc polymetallic field displaying the existence of an ew trending regional polymetallic ore forming zone within the study area the low pass filtered gravity component image res m n is interpreted to depict the lowest geological architecture within the study area which reflects that there may be a pair of the nw trending uplift zone of the mantle and or the basement with positive gravity anomaly and the depression zone of the mantle and or the basement with negative gravity anomaly the gejiu tin copper polymetallic deposits and the related granites are located at the transitional zone between the uplift and depression whereas the bozhushan silver lead zinc polymetallic deposits and the related granites are situated within the depression zone which implies that the diversity of the late yanshanian granites and the related polymetallic deposits in the study area may be controlled by the complexity of the crust mantle interaction at depth

batholith stock scale exploration targeting based on multi source geological and geophysical datasets in the luanchuan mo polymetallic district china abstract the luanchuan mo polymetallic district which hosts world class porphyry skarn type mo w deposits is located in the qinling orogenic belt of the north china craton the mining depth of the porphyry skarn type mo w deposit in the luanchuan district is close to 550 m and most exploration drillholes extend to more than 1000 m the upper crustal architecture is important for understanding the geological setting and metallogenic model for mineral potential targeting on a district scale in this study subsurface intrusions and structures were defined using multi level gravity data processing i e bi dimensional empirical mode decomposition and geophysical inversion including gravity magnetotellurics and controlled source audio frequency magnetotellurics the results of our integrated geological and geophysical studies suggest that the late jurassic granite stocks are controlled by ne nee and nw nww striking faults the porphyry skarn type mo w deposits are distributed around the ore controlling late jurassic granite stocks which are connected to the granite batholith at depth the volume of this batholith is about 96 96 km3 based on geophysical interpretations and geochemical signatures of biotites from the granite stocks the metallogenic potential of the granite stocks at nannihu shangfanggou and huangbeiling is considered to be greater than that of the granite stocks at shibaogou yuku

inorganic geochemistry of albian sediments from the lower saxony basin nw germany palaeoenvironmental constraints and orbital cycles abstract within the international albicore project upper albian sediments from a 245 m core kirchrode i drilled in hannover were studied by inorganic geochemical methods more than 650 samples minimum sample spacing 0 5 m were analysed for major and minor element concentrations this chemical database was used to reconstruct the depositional environment and to investigate the influence of high frequency milankovitch cycles on the chemical composition of the sediments the chemical composition of the sedimentary sequence is primarily controlled by variations in carbonate content corresponding to ca al ratios variations in both ti al and si al ratios are interpreted as fluctuations in detrital composition resulting from increases in smectite and quartz content in the upper section of the core high amounts of reactive iron but low pyrite concentrations the lack of enrichment of redox sensitive trace elements v cr and high mn concentrations indicate oxic bottom water conditions and the absence of an oxygen minimum zone for the upper albian at the kirchrode site the nutrient related trace elements ba p zn suggest a generally oligotrophic setting with slightly elevated productivity only in the uppermost upper albian only one layer was identified as containing volcanogenic material at 87 20 m this layer is characterised by elevated montmorillonite zr and nb mass balance calculations for the entire core suggest that the high mn values cannot be explained by riverine input alone therefore a coastal oxygen minimum zone south of the kirchrode site is postulated as an additional mn source cyclic variations in both carbonate content and al normalised concentrations of predominantly detrital elements like si and ti are seen in a 60 m interval 40 100 m where sedimentological palaeontological and geochemical data suggest stable depositional conditions although carbonate cyclicity cyclicity in detrital input and nutrient related element abundances do not correlate with each other three strong peaks in the results of spectral analysis of these chemical data for this interval appear to correspond to milankovitch frequencies in the eccentricity obliquity and precession bands

integrated geophysical imaging of a concealed mineral deposit a case study of the world class pebble porphyry deposit in southwestern alaska abstractwe combined aeromagnetic induced polarization magnetotelluric and gravity surveys as well as drillhole geologic alteration magnetic susceptibility and density data for exploration and characterization of the cu au mo pebble porphyry deposit this undeveloped deposit is almost completely concealed by postmineralization sedimentary and volcanic rocks presenting an exploration challenge individual geophysical methods primarily assist regional characterization positive chargeability and conductivity anomalies are observed over a broad region surrounding the deposit likely representing sulfide minerals that accumulated during multiple stages of hydrothermal alteration the mineralized area occupies only a small part of the chargeability anomaly because sulfide precipitation was not unique to the deposit and mafic rocks also exhibit strong chargeability conductivity anomalies similarly reflect widespread sulfides as well as water saturated glacial sediments mineralogical and magnetic suscep

ground based measurements of spatial and temporal variability of snow accumulation in east antarctica the east antarctic ice sheet is the largest highest coldest driest and windiest ice sheet on earth understanding of the surface mass balance smb of antarctica is necessary to determine the present state of the ice sheet to make predictions of its potential contribution to sea level rise and to determine its past history for paleoclimatic reconstructions however smb values are poorly known because of logistic constraints in extreme polar environments and they represent one of the biggest challenges of antarctic science snow accumulation is the most important parameter for the smb of ice sheets smb varies on a number of scales from small scale features sastrugi to ice sheet scale smb patterns determined mainly by temperature elevation distance from the coast and wind driven processes in situ measurements of smb are performed at single points by stakes ultrasonic sounders snow pits and firn and ice cores and laterally by continuous measurements using ground penetrating radar smb for large regions can only be achieved practically by using remote sensing and or numerical climate modeling however these techniques rely on ground truthing to improve the resolution and accuracy the separation of spatial and temporal variations of smb in transient regimes is necessary for accurate interpretation of ice core records in this review we provide an overview of the various measurement techniques related difficulties and limitations of data interpretation describe spatial characteristics of east antarctic smb and issues related to the spatial and temporal representativity of measurements and provide recommendations on how to perform in situ measurements

geology and genesis of kafang cu sn deposit gejiu district sw china kafang is one of the main ore deposits in the world class gejiu polymetallic tin district sw china there are three main mineralization types in the kafang deposit i e skarn cu sn ores stratiform cu ores hosted by basalt and stratiform cu sn ores hosted by carbonate the skarn mainly consists of garnet and pyroxene and retrograde altered rocks these retrograde altered rocks are superimposed on the skarn and are composed of actinolite chlorite epidote and phlogopite major ore minerals are chalcopyrite pyrrhotite cassiterite pyrite and scheelite sulfur and pb isotopic components hint that the sources of different types of mineralization are distinctive and indicate that the skarn ore mainly originated from granitic magma whereas the basalt hosted cu ores mainly derived from basalt microthermometry results of fluid inclusions display a gradual change during the ore forming process the homogenization temperature of different types of inclusions continuously decreases from early to late mineralization stages the salinities and freezing temperatures exhibit similar evolutionary tendencies with the t homogenizaion while the densities of the different types keep constant the majority being less than 1 oxygen and hydrogen isotopic values o and d of the hydrothermal fluids fall within ranges of 3 1 to 7 7 with an average of 6 15 calculated at the corresponding homogenization temperature and 73 and 98 with an average of 86 5 respectively microthermometry data and h o isotopes indicate that the ore forming fluid of the kafang deposit is mainly derived from magma in the early stage and a mixture of meteoric and magmatic water in late stage molybdenite re os age of the skarn type mineralization is 83 4 2 1 ma and the stratiform ores hosted by basalt is 84 2 7 3 ma which are consistent with the la icp ms zircon age of the xinshan granite intrusion 83 1 0 4 ma the evidence listed above reflects the fact that different ore styles in the kafang deposit belong to the same mineralization system

new tectono geochronological constraints on timing of shearing along the ailao shan red river shear zone implications for genesis of ailao shan gold mineralization abstract several world class gold deposits are located along the ailao shan als belt in eastern tibet china the genesis of gold mineralization along the belt however has been the subject of debates in the last decades which highlights the importance of dating shearing magmatism and mineralization along the ailao shan red river shear zone asrr through detailed field observations and microscopic analysis a group of leucocratic intrusions from within and outside the shear zone along the als belt are investigated in the present paper pre syn and post shearing intrusions are grouped based on structural and microstructural analysis la icp ms and sims dating of the intrusions revealed the existence of two age populations a group of ages older than 30 ma and the other younger than 28 ma the former are distributed both within and outside the shear zone and the latter in contrast occur only within the shear zone our new results show that the ductile shearing along the asrr shear zone initiated since ca 30 ma ago the dating results place major constraints on timing of shearing along the asrr shear zone and have profound implications on the genesis of gold mineralization along the als belt the present study reveals that ductile shearing along the asrr shear zone was resulted from extrusion of the indochina block late during the indian eurasian plate collision meanwhile we conclude that the gold mineralization took place prior to the shearing but in close relation to an early magmatism 30 ma ascribed to post collisional extension collapse involving mantle processes early during the plate collision

thrusting in oceanic crust during continental drift offshore niger delta equatorial africa 1 two and three dimensional 3 d seismic reflection data acquired over oceanic crust in the deepwater west niger delta reveal convincing evidence for compressional tectonics during oceanic crustal spreading using the 3 d seismic data set we describe numerous inclined seismic reflections that dissect the entire oceanic crust from the top of the crust to the level of the moho that are interpreted as thrusts thrust propagation results in the development of associated hanging wall anticlines and footwall synclines these structures are orthogonal to and clearly postdate normal faults that formed during the accretion of oceanic crust during continental drift and strike at right angles to them the charcot ridge is located 140 km south of these thrusts and is a significantly larger structure it is a triangular shaped uplifted region of oceanic crust measuring 80 by 150 km and is located along the ne sw oriented charcot fracture zone two interpretations are possible for the role of the fracture zone in the development of the charcot ridge 1 a thin skinned model whereby the oceanic crust west of the fracture zone has been thrust southeastward with detachment occurring close to the level of the moho the ridge forms as a result of translation and folding above a crustal scale ramp flat thrust geometry 2 a thick skinned model where there is no detachment close to the moho with the thrust fault being much steeper penetrating the crust and probably the mantle lithosphere in this interpretation the structure formed owing to the compressional reactivation of the fracture zone approximate dating of onlapping reflections on either side of the ridge constrains the timing of its formation as between 25 and 120 ma ago the charcot ridge represents one of the largest thrust structures to be identified in a passive margin setting many other compressional folds with the same orientation formed to the northeast in the benue trough probably during the santonian as a result of a change in the spreading direction during south atlantic rifting we speculate that the same causal mechanism applies for the formation of the charcot ridge

geochemical study of crude oils from the xifeng oilfield of the ordos basin china abstract the xifeng oilfield is the largest newly discovered oilfield in the ordos basin in order to determine the possible source crude oils collected systematically from the oilfield and an adjacent oilfield have been examined isotopically and molecularly the predominance of long chain n alkanes high abundance of c29 sterane lower ratios of c25 c26 tricyclic terpane and c25 tricyclic terpane c24 tetracyclic terpane and high c24 tetracyclic terpane c24 tetracyclic terpane c26 tricyclic terpanes ratio in the studied oils suggest generation from a source with mixed terrigenous and algal bacterial organic matter the presence of diterpenoid hydrocarbon with abietane skeletons is characteristic of the main contribution of higher land plants to the oils the biomarker distributions in the oils show that they were formed under a weakly reducing freshwater environment molecular maturity parameters indicate that the crude oils are mature the oil source rock correlation and oil migration investigation suggest that the oils in the xifeng oilfield originated from the source rocks of the yanchang formation deposited in a shallow to deep freshwater lacustrine environment especially chang 7 source rocks the data from the distribution of pyrrolic nitrogen compounds indicate that the charging direction of the chang 8 crude oils is mainly from the zhuang 12 well northeast of the oilfield toward the southwest this direction of oil migration is consistent with that indicated by regional geological data

metallogenic model of the wulong gold district china and associated assessment of exploration criteria based on multi scale geoscience datasets abstract the combined use of geological and geophysical modeling is effective for delineating subsurface exploration criteria and conducting quantitative analyses to determine potential mineral targets in this paper total horizontal derivative multi scale edge detection worms and bi dimensional empirical mode decomposition bemd of district scale gravity and magnetic data were applied to extract geophysical information reflecting deep seated structures and concealed intrusions in addition deposit scale 3d modeling and controlled source audio frequency magnetotelluric data were used to identify potential ore controlling structures the results of our integrated multi scale geological and geophysical studies suggest that the early cretaceous intrusions and gold deposits in the wulong gold district which is located on the liaodong peninsula china at the ne margin of the north china craton ncc are spatially and genetically associated with a network of ne and nw striking faults gravity and magnetic modeling effectively identified buried early cretaceous intrusions and nw striking faults at depths from zero to five kilometers in summary the combined use of multiple source and multi scale geophysical information and multi scale 3d geological modeling facilitated delineation of the metallogenic model and provided exploration criteria for determining potential exploration targets in the study area

further evidence for 1 85 ga metamorphism in the central zone of the north china craton shrimp u pb dating of zircon from metamorphic rocks in the lushan area henan province abstract this paper reports shrimp zircon u pb dating of precambrian supracrustal and granitic rocks from the lushan area henan province in the southern portion of the central zone also referred to as the trans north china orogen of the north china craton a graphite garnet sillimanite gneiss sample tw0006 1 of the shangtaihua group gives a range of inherited zircon ages from 2 73 to 2 26 ga and a metamorphic zircon age of 1 84 0 07 ga a garnet bearing gneissic granitoid sample twj358 1 which is considered to intrude the shangtaihua group gives a magmatic zircon age of 2 14 0 02 ga and a metamorphic zircon age of 1 87 ga the metamorphic zircon ages of 1 87 1 84 ga obtained in this study indicate that an important tectonothermal event occurred at the end of the paleoproterozoic in the lushan area this supports the southern continuation of a central zone in the north china craton that workers have recently considered to result from continent continent collision it is also evident that the shangtaihua group was formed during the paleoproterozoic between 2 26 and 2 14 ga and not during the archean as previously considered