paleoclimatic and paleomagnetic constraints on the paleozoic reconstructions of south china north china and tarim abstract paleomagnetic and paleoclimatic data provide the most useful latitudinal constraints for plate reconstructions distributions through the paleozoic of five types of climatically sensitive sediments coals evaporites reefs dolomites and limestones for south china north china and tarim are shown on 15 maps that include 1578 reliable data points these paleoclimatic data agree reasonably well with available paleomagnetic directions although significant divergence between the two exists for the early paleozoic these data indicate the following 1 1 south china was in low latitudes during the entire paleozoic with a subtropical position in the cambrian 2 2 north china also remained near the equator in the early and late paleozoic except for the ordovian and the late permian when extensive evaporites suggest slightly higher latitudinal positions while its middle paleozoic position is uncertain due to the missing stratigraphie record 3 3 in south china local tectonics appears to have played a dominant role in determining paleogeography and therefore marine sedimentation especially after the late ordovician early silurian because the areal coverage of marine sediments through time is distinctly different from what would be expected from published global sea level curves 4 4 paleoclimatic and paleomagnetic data are compatible with biogeographic data which suggest that south china was part of eastern gondwana in the early paleozoic but was widely separated from gondwana in the late paleozoic and the split between the two probably happened in the devonian giving rise to a major break up unconformity in central south china

statistical modeling and characterization of induced seismicity within the western canada sedimentary basin in western canada there has been an increase in seismic activity linked to anthropogenic energy related operations including conventional hydrocarbon production wastewater fluid injection and mor

paleomagnetism of cretaceous rocks in the jiaodong peninsula eastern china insight into block rotations and neotectonic deformation in eastern asia 1 although first order agreement of paleomagnetic directions between south china and siberia shows that terranes between them were sutured by cretaceous times deformation by ongoing tectonism has continued and been especially focused near the continental margin of eastern asia to help evaluate differential rotations and relate them to regional strike slip and extensional tectonic regimes we have conducted paleomagnetic study on cretaceous sediments and volcanic rocks from the jiaodong peninsula south of the bohai sea in eastern china bordering the tan lu fold zone tlfz paleomagnetic results from 73 sites in the early cretaceous laiyang and qingshan groups and late cretaceous wangshi group yield magnetizations of primary or synfolding mid cretaceous origins large differential block rotations of early cretaceous age are identified near the muping jimo fracture zone mjfz dissecting the peninsula although late cretaceous declinations are regionally systematic and show post late cretaceous counterclockwise rotation of 14 in the jiaolai basin relative to the reference north and south china blocks block boundaries are identified as the tlfz to the west and the mjfz in the southeast the eastern jiaodong peninsula is rotated clockwise relative to the north and south china blocks by a comparable amount 20 to the southern korean peninsula to the east and to the liaodong peninsula and yanji basin to the north although these blocks are unlikely to be tightly coupled their collective clockwise rotation mirrors counterclockwise rotations north of the japan sea and probably resulted from regional extension focused above uprise of asthenosphere beneath eastern asia during cretaceous times

the alashan terrane did not amalgamate with north china block by the late permian evidence from carboniferous and permian paleomagnetic results abstract rock magnetic and paleomagnetic studies have been carried out on the early carboniferous limestones and the late permian purple sandstones sampled in the eastern alashan terrane alt northwest of china two components were isolated from the early carboniferous limestone by thermal progressive demagnetisation a low unblocking temperature component ltc of recent origin a pre folding medium temperature component mtc the paleomagnetic pole is 13 1 n 11 0 e a 95 7 0 that is probably the result of the hydrothermal fluids from the qilian orgenic belt acquired during the late carboniferous early permian also two components were separated from the late permian purple sandstone by thermal progressive demagnetisation the ltc with the recent viscous remanent magnetisation and the higher temperature component htc revealed from three sections which has passed a regional fold test at the 95 probability level and reversal test suggesting a primary characteristic magnetisation the corresponding paleomagnetic pole is 27 2 n 18 8 e a 95 12 0 the apparent polar wander path including early carboniferous late carboniferous early permian late permian and early middle triassic poles of the alt is significantly different with those of the ncb comparison of the apwps between the alt and ncb shows a strong similarity if the apwp of hexi corridor alashan rotated counterclockwise around an euler pole at 44 n 84 e by 32 then the coeval apw path of the alt overlaps to that of the ncb this result indicates that the alt migrated to the ncb after the early middle triassic along a tectonic boundary located between helanshan mountain and zhuozishan mountain and finally amalgamated to the ncb before the early cretaceous

kinematics and geochronology of multistage ductile deformation along the eastern alxa block nw china new constraints on the relationship between the north china plate and the alxa block abstract the ne trending bayanwula shan lang shan is an important tectonic belt lying between the north china plate ncp to the east and the alxa block to the west an understanding of its nature and the timing of deformation are essential to understand the relationship between the ncp and the alxa block two phases of ductile deformation have been observed in this belt large scale top to the west ductile thrusting characterized the early deformation in the bayanwula shan lang shan nearly east west trending quartz stretching lineations and lineations formed by amphibole and biotite are well developed different types of sheath and oblique folds with east west trending fold hinges are also developed in the region the shear strain of this ductile thrust is up to 17 the ductile deformation may have resulted from the top to the west thrusting of the northern part of the ncp over the alxa block and may have occurred ca 351 ma biotite 40 ar 39 ar age later ductile deformation was expressed as ne trending sinistral shear along the entire bayanwula shan lang shan and likely occurred ca 250 ma biotite and muscovite 40 ar 39 ar ages this shear may have resulted from the collision between the yangtze and north china plates to the south during the triassic combined with recently obtained detrital zircon u pb ages for the area the ductile deformation events in the eastern alxa block indicate that the block may not have been part of the ncp at least before the end of the devonian both blocks were located in the paleo asian ocean during the paleozoic and collided or amalgamated with each other at the end of the devonian

new silurian and devonian palaeomagnetic results from the hexi corridor terrane northwest china and their tectonic implications summary a total of 239 orientated drill core samples from 23 sites were collected for palaeomagnetic study from silurian and devonian red beds marlaceous sandstone and limestone rocks in the eastern part of the hexi corridor southwest ningxia north china the characteristic high temperature component resides in both haematite and magnetite it clusters around a northwesterly and shallow to moderate downward direction and its antipode after tilt correction the primary origin of this characteristic remanent magnetization chrm is ascertained by positive fold and reversal tests at the 95 per cent confidence level the corresponding palaeopoles at 339 0 e 60 1 n with a 95 11 2 silurian and 336 0 e 56 0 n with a 95 9 2 devonian imply that the north china block ncb had a low palaeolatitude of around 15 n in the northern hemisphere during the silurian devonian period comparison with the early middle ordovician palaeopole of the ncb suggests that the ncb moved rapidly northwards by 30 8 10 9 to cross the palaeo equator during the early middle ordovician to silurian in combination with the palaeobiogeographical data from ningxia our palaeomagnetic results suggest that the ncb was located close to australia during the late devonian

location performance and detection threshold of the spanish national seismic network spain is a low to moderate seismicity area with relatively low seismic hazard however several strong shallow earthquakes have shaken the country causing casualties and extensive damage regional seismicity is monitored and surveyed by means of the spanish national seismic network maintenance and control of which are entrusted to the instituto geografico nacional this array currently comprises 120 seismic stations distributed throughout spanish territory mainland and islands basically we are interested in checking the noise conditions reliability and seismic detection capability of the spanish network by analyzing the background noise level affecting the array stations errors in hypocentral location and detection threshold which provides knowledge about network performance it also enables testing of the suitability of the velocity model used in the routine process of earthquake location to perform this study we use a method that relies on p and s wave travel times which are computed by simulation of seismic rays from virtual seismic sources placed at the nodes of a regular grid covering the study area given the characteristics of the seismicity of spain we drew maps for m l magnitudes 2 0 2 5 and 3 0 at a focal depth of 10 km and a confidence level 95 the results relate to the number of stations involved in the hypocentral location process how these stations are distributed spatially and the uncertainties of focal data errors in origin time longitude latitude and depth to assess the extent to which principal seismogenic areas are well monitored by the network we estimated the average error in the location of a seismic source from the semiaxes of the ellipsoid of confidence by calculating the radius of the equivalent sphere finally the detection threshold was determined as the magnitude of the smallest seismic event detected at least by four stations the northwest of the peninsula the pyrenees especially the westernmost segment the betic cordillera and tenerife island are the best monitored zones origin time and focal depth are data that are far from being constrained by regional events the two iberian areas with moderate seismicity and the highest seismic hazard the pyrenees and betic cordillera and the northwestern quadrant of the peninsula are the areas wherein the focus of an earthquake is determined with an approximate error of 3 km for m l 2 5 and m l 3 0 this error is common for almost the whole peninsula and the canary islands in general errors in epicenter latitude and longitude are small for near surface earthquakes increasing gradually as the depth increases but remaining close to 5 km even at a depth of 60 km the hypocentral depth seems to be well constrained to a depth of 40 km beneath the zones with the highest density of stations with an error of less than 5 km the m l magnitude detection threshold of the network is approximately 2 0 for most of spain and still less almost 1 0 for the western sector of the pyrenean region and the canary islands

the fold test in palaeomagnetism summary the fold test suggested by mcelhinny is shown to be inappropriate more correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb details of the application of this test are given with examples it is shown that the test indicates significance under far less severe folding than does the test suggested by mcelhinny

seismicity and detection location threshold in the southern great basin seismic network a spatially varying model of the detection location capabilities of the southern great basin seismic network sgbsn has been derived that is based on simple empirical relations and statistics this permits use of almost all the catalog data gathered instead of ignoring data that are below the threshold of completeness a spatially varying threshold model is developed so that subregions having lower completeness levels than the network as a whole can be outlined and the completeness level of each sub region determined such a model is required to unambiguously identify regions that are aseismic due to natural processes rather than to limited detection and or location capabilities accounting for spatial variations in detection location threshold is also important for studies in which magnitude frequency distributions are interpreted in terms of source scaling properties the characteristics of the spatial distribution of earthquakes where earthquake clusters and aseismic regions locate appear to be stable at all magnitude levels so that inferences about where strain is being accommodated will be the same whether numbers of earthquakes or strain estimated from seismic moments are examined for the southern great basin region these principal characteristics include clusters at the northern end of the furnace creek fault and in the pahranagat shear zone and a relatively large number of earthquakes in the northern and southeastern portions of the nevada test site these clusters cover regions much larger than the surface projections of any of the mapped faults the extent to which seismicity is induced by nuclear testing is unclear the predominantly aseismic regions include the area west of the death valley furnace creek fault system and an almost complete absence of events at yucca mountain finally a considerable number of isolated events in the sgbsn catalog cannot be correlated with mapped faults

montana seismic network performance an evaluation through the snes method in this paper we analyze the location performance of the montana regional seismic network mrsn using the seismic network evaluation through simulation snes method montana has a high level of seismicity that includes approximately 1500 locatable earthquakes annually the mrsn comprises 38 stations deployed over an area of approximately 50 000 km2 the application of the snes method permits us to evaluate the background noise levels of the network stations and estimate an empirical law that links the variance of p and s travel time residuals to hypocentral distance this in turn permits us to assess the appropriateness of the velocity model used by the mrsn in the location routine we constructed snes maps for m l 1 4 1 6 1 8 2 0 and 2 2 fixing the hypocentral depth at 10 km and at the 95 confidence level through application of the snes method we show that mrsn provides the best monitoring coverage in the flathead valley of northwestern montana with errors for m l 2 that are less than 2 and 6 km for epicenter and hypocentral depth respectively at magnitude 2 2 this seismic network is capable of locating earthquakes as deep as 150 km and provides a threshold of completeness down to magnitude 1 5 for most of western montana we delineate some seismogenic areas of western montana including the central portion of the centennial tectonic belt in extreme southwestern montana that are not adequately covered by the mrsn alone

a new cordaitean pollen cone and pollen grains in situ from the early permian of hexi corridor and its geotectonic significance a new species of cordaitean pollen cone is described and diagnosed from early permian strata in gansu province northwestern china the gross morphology and well preserved features of the species named cordaianthus yongchangensis z x wang et b n sun sp nov have been studied in detail the current fossil has helically arranged bracts and secondary fertile shoots the bract longer and wider than in other known fossils diverges at approximately 90 from the axis the secondary fertile shoots are obovate with scales on individual shoots and fertile scales concentrated near the apex of the secondary fertile shoots more than five pollen sacs are clustered on the fertile scales which number 8 13 as the current species differs from any known species in the fossil records it is considered as a new species of cordaianthus the species with its microstructure of bracts and scales in situ pollen sacs and pollen grains provides important clues on the evolution of cordaitopsida based on the known reproductive organs of cordaitean plants we discovered that the current fossil with its wide bracts that protect the helically arranged secondary fertile shoots is similar to modern conifer cones suggesting that the current fossil is an evolutionally advanced species in comparisons with cordaitean fossils the leaves of cordaitean plants from the alt alashan terrane during the early permian presented mixed leaf features of specimens from the ncb north china block and scb south china block in conclusion the alt was already located between the ncb and scb in the early permian c 2016 elsevier b v all rights reserved

an evaluation of seismic decoupling and underground nuclear test monitoring using high frequency seismic data an effective solution to the problem of the detection and identification of low yield coupled and fully decoupled underground nuclear explosions appears available via use of high frequency seismic data ranging up to 30 or 40 hz in order to evaluate detection identification capabilities when using such data it is necessary to estimate 1 spectral characteristics and relative amplitudes of both p and s waves from explosions and earthquakes over the frequency band from 5 to 40 hz 2 signal transmission characteristics over this band through pertinent types of earth structure and 3 recording system and ground noise characteristics over this frequency band in this study each of these topics is considered in turn as they relate to detection and discrimination of the signals from low yield coupled and decoupled explosions in the regional and teleseismic distance ranges estimates of the capabilities of specific hypothetical networks to detect and identify insofar as signal to noise ratio is an important factor in identification explosions within the ussr are then considered these estimates of signal detection capability provide the central focus for the study as they serve to translate diverse and rather complex sets of observational data and theory into concrete predictions of monitoring capability following the assessment of detection capabilities the problem of identification of small events is considered with particular emphasis on discrimination at regional distances where the network is calculated to provide signals of high signal to noise ratio the principal results and conclusions of this study are as follows 1 seismic system noise can be suppressed to levels well below ground noise at quiet sites up to frequencies at least as high as 30 40 hz when using presently available hardware 2 average amplitudes of high frequency noise in a variety of geological environments are very low and change little with time or season 3 transmission of high frequency p and s wave signals in the regional distance range in stable continental areas and shields is nearly as efficient as at 1 hz with effective q factors in shield areas being about 9000 and 4000 for high frequency pn and sn respectively while the effective q for pn waves in tectonic areas is about 1000 4 a properly designed and deployed network of 25 simple three component nonarray stations internal to the ussr and 15 similar stations surrounding the ussr is predicted to be capable of multistation detection at high signal to noise ratio of fully decoupled 1 kt explosions located at all potential decoupling sites within the ussr 5 by inference from the quantitative agreement of empirical observations and theoretical predictions when using lower frequency data over a great range of explosion yields we conclude that procedures based on the use of both detectable p and s waves will serve to identify explosion generated seismic signals at least as small as those expected from a fully decoupled 1 kt explosion

paleomagnetic constraints on the tectonic relationship between the alashan hexi corridor terrane and the north china block we report paleomagnetic results for early middle ordovician limestone rocks from the eastern part of the hexi corridor northwest china the characteristic high temperature component b isolated from 74 out of 114 samples exhibited dual polarity and passed the fold and reversal tests at the 95 confidence level the corresponding paleopole at 326 5 e 31 8 n with a95 9 5 is in agreement with the coeval poles from the stable north china block ncb it implies that the alashan hexi corridor ahc terrane was part of the ncb since the early middle ordovician which is consistent with geological evidence the silurian and devonian paleomagnetic results from the ahc terrane may therefore be used to discuss the middle paleozoic tectonic displacement and paleogeographic reconstruction of the ncb

global seismic event detection using a matched filter on long period seismograms an image derived by stacking long period seismograms is used as an empirical matched filter to detect and locate earthquakes records from 564 events mb 6 recorded at very long periods t 60 s by the 15 to 20 stations of the international deployment of accelerometers ida network are stacked using a method that emphasizes the surface wave arrivals the first 3 hours of this time versus range image are used to construct a matched filter for continuous application to the ida data the output of this filter contains spatial and temporal peaks that define the location and origin time of probable seismic events implementation of this technique to 11 years of ida data from 1981 to 1991 identifies 4061 events these include 65 of cataloged events of mb 5 5 earthquakes which appear anomalously strong relative to their surface wave magnitudes are mainly located on oceanic transform faults and probably represent unusually slow ruptures the method successfully detects two eruptions of the el chichon volcano in southern mexico from the anomalous low frequency energy radiated during the eruptions in addition 32 earthquakes are detected which are not in the standard global catalogs these events appear to be about ms 5 and are mainly located in the southern oceans where there are gaps in the coverage of the high frequency networks although these earthquakes are probably slow since they occur mostly on oceanic transform faults they are not silent as they also can be observed in higher frequency data

processing of regional seismic events using data from small aperture arrays abstract a regional on line array processing package ronapp has been developed and tested on data from a small aperture seismic array noress at norsar consisting of 12 sensors spaced with distances up to about 2 km the processing package automatically detects and locates regional seismic events in the distance range from a few kilometers to the maximum propagation range of regional s type waves sn or lg the package contains a conventional sta lta detector which works on a set of filtered beams deployed at typical phase velocities of pn and lg sn 8 0 and 4 5 km sec when a detection is found data within a certain time window around the detection time are used for estimates of refined arrival time and signal frequency and a wavenumber analysis is then performed following a proper prefiltering this gives in addition to arrival time and frequency estimates of phase velocity azimuth and signal to noise ratio for each detection following each processed detection an event location based on azimuth and s p travel times is performed if the last detection is identified as an s type wave phase velocity below 6 km sec and a preceding p wave phase velocity above 6 km sec has been detected with about the same direction of approach appropriate procedures are developed to allow for several detections within the p and s coda to keep track of used and unused detections and to identify epicenter locations that override preliminary ones as the analysis progresses through the s wave train of the event the processing package has been tested off line and on line on a number of data intervals for various experimental noress array configurations the automatic processing package provides detection results for regional events that compare favorably with those from the full norsar array and location results that deviate from locations by the fennoscandian network of seismological stations by typically 30 km which is of the order of the location uncertainty of the network

silurian and devonian paleomagnetic poles from north china and implications for gondwana abstract we have obtained paleomagnetic results from silurian and devonian rocks on the western margin of the north china block ncb ages which previously were thought to be unrepresented throughout north china high unblocking temperature components of magnetization isolated from both the silurian and devonian strata pass fold and reversal tests suggesting that they are primary remanence the corresponding paleopoles at 228 4 e 26 2 n silurian and 228 7 e 34 2 n devonian imply low paleolatitude positions of north china during the middle paleozoic probably adjacent to east gondwana

fish from the upper devonian of the shan thai terrane indicate proximity to east gondwana and south china terranes we report the first discovery of abundant devonian vertebrates from thailand an unusual form of coronodontid shark tooth with six equal sized cusps occurs only in the upper devonian of thailand shan thai terrane and south china and a new species of phoebodus is reported that occurs elsewhere only in south china and australia east gondwana terrane the occurrence of the chondrichthyan harpagodens thrinacodus ferox turner 1982 in the late devonian of thailand australia and south china predates the appearance of this species in euramerica early carboniferous these data suggest late devonian proximity of the shan thai south china and east gondwana terranes and is in accord with recent paleomagnetic data

a new method for the estimation of the completeness magnitude abstract the estimation of the magnitude of completeness m c have strong consequences in any statistical analysis of seismic catalogue and in the evaluation of the seismic hazard here a new method for its estimation is presented the goodness of the method has been tested using 10 4 simulated catalogues then the method has been applied to five experimental seismic catalogues greece italy japan northern california and southern california

grain size dependence of the magnetic behavior of pyrrhotite during its low temperature transition at 34 k the grain size dependent behavior of pyrrhotite during a magnetic lt transition at 34 k is suitable for granulometric purposes cycling down to liquid helium temperature suppresses soft md jrs more than hard sd jrs so that it may be effective in removing soft secondary nrm components the domain structure of large pyrrhotite grains changes from md at room temperature to sd or psd below the lt transition

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present paleomagnetic study of china and its environs has been the center of a major international effort for the last 10 years in this paper we critically review all available paleomagnetic poles of upper permian to tertiary age from the main blocks of china with the goal of placing constraints on models of the formation and the subsequent deformation of the region after selecting reliable poles by applying objective criteria we divide our analysis into first order motions of blocks and second order deformation within blocks for first order analysis apparent polar wander paths are constructed for the major blocks we discuss the compatibilities and contradictions between the geological and paleomagnetic records a sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented in general the major blocks were probably in contact throughout the permian and triassic but the jurassic was the key age during which most of the movement toward china s present configuration took place our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events during the cretaceous chinese poles agree with poles from other continents transferred onto eurasia at the second order we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region indicating that much of china has been affected by small 20 differential rotations this we interpret as deformation caused in part by the extrusion of the chinese blocks away from the indian collision the complete annotated list of poles is given as an appendix

the implications of regional microseismic activities a lesson from 2008 ms 8 0 wenchuan earthquake abstract the ms 8 0 wenchuan earthquake has greatly perturbed the crustal stress in longmenshan fault zone lfz and its neighboring regions and hence significantly changed the regional seismicity pattern convectional coulomb stress change model based on half space dislocation source of layered elastic or viscoelastic crust can only provide brief reference to estimate the after shock sequences roughly in this study we apply the z test statistical method to examine the earthquake catalogue five years before and after this event to detect the changes of regional seismicity intensity and their statistical significance in lfz and its neighboring regions we analyze changes of the z values with respect to time and seismic magnitude in each region our results reveal that after the wenchuan earthquake seismicity intensity has significantly increased in eastern kunlun fault southern longmenshan fault longquanshan fault and huayingshan fault in sichuan basin but has significantly decreased in northern and central longmenshan fault vicinity of the songpan minjiang fault and fubianhe fault in bayankala block respectively seismicity rate change in northern and central anninghe zemuhe fault zone is not correlated with the wenchuan earthquake these results remain within 99 confidence level of the z test statistical method we compare our statistical analysis results with calculations of coseismic coulomb failure stress change due to the wenchuan earthquake from previous studies in most of the regions and fault zones the sign of the seismicity rate change agrees with the sign of the coulomb stress change

palaeomagnetism of neoproterozoic glacial rocks of the huabei shield the north china block in gondwana abstract a palaeomagnetic study is reported of reddened facies fengtai formation of neoproterozoic glacial rocks which underlie early cambrian rocks with disconformity in the huabei north china sino korean block the diamictite preferred age 620 600 ma carries a dual polarity remanence residing in hematite of the red matrix the mean direction derived from 62 samples is d i 205 9 32 4 95 3 9 yielding a pole position at 233 e 62 n tests on the matrix deformed beneath dropstones suggest that remanence was fixed before full compaction although clasts near the base of the formation are largely overprinted overlying early cambrian sediments of the houjiashan and yutaishan formations have a similar remanence also of dual polarity d i 205 5 32 1 95 3 9 32 samples the magnetisation in the diamictite is therefore interpreted to have been acquired during loading by the ice sheet and or the overlying cambrian succession red shales from the liulaobei formation 890 840 ma near the base of the neoproterozoic succession in the huabei block yield a contrasting remanence of d i 59 4 75 3 95 7 1 19 samples equivalent to a pole position at 150 e 43 n pole positions from this study accord with lower middle cambrian poles from the australian and south china blocks with north china sited adjacent to northeastern australia in accordance with recent biogeographic and palaeogeographic models although the palaeolatitude derived from the fengtai diamictite 17 is cambrian in age and probably later than the glaciation the correlation with australia implies that glaciation in north china took place in low palaeolatitudes 20 and reinforces the view that late neoproterozoic glaciation at least at this perimeter of gondwana occurred at low latitudes neoproterozoic cambrian poles from north china south china and australia fail to conform to the popular rodinia reconstruction with the latter blocks adjacent to western north america and also fail to support the view that gondwana formed by the fusion of widely separated eastern and western segments during pan african orogeny instead they accord to poles from africa and south america in a precursor of gondwana protogondwana the subsequent palaeozoic derivation of this supercontinent was probably achieved by intracontinental strike slip motion during pan african deformation following extinction of the afro arabian arc

closure time in the east qilian ocean and early paleozoic ocean continent configuration in the helan mountains and adjacent regions nw china abstract lithofacies and faunal assemblage affinities of the helan mountains and its adjacent blocks are explored using field investigations and analysis of previous results the tectonic affinity between the alxa block and the north china block is analyzed by means of la icp ms detrital u pb zircon age spectrums the early paleozoic stratigraphic framework is constructed and stratigraphic contact relationships between the study area and its adjacent regions are assessed the ocean continent configuration during the caledonian period is discussed and the closure time in the east of north qilian ocean is determined finally the basin type and its evolutionary process during the early paleozoic are analyzed the carbonate platform facies of the alax block and the abyssal flysch deposition on the margin of the central qilian block established the coexistence of ocean and continent configuration during the cambrian during the early ordovician the stable shallow water platform shelf facies was distributed over all the study area and the biofacies was also unified the east of the north qilian ocean closed at that time the north qilian ocean and the north qinling ocean did not affect the study area but the north china sea was connected intermittently to the research area the east central qilian block the alax block the north china block and the qinling microblock collided together to form a unified megablock which resulted in the formation of a complex foreland depression during the early ordovician the eastern boundary of the basin is the aselang chedao transform fault which lies between the alax block and the north china block the southern boundary of the basin is marked by the guyuan qingtongxia fault which is located between the alax block and the central qilian block the western boundary of the basin is an overlapping boundary toward the alax block during the late ordovician when the yangtze block approached toward the north china block the shangdan ocean began to influence the study area and the turbidite fans were formed because of the large scale transgression thus the basin type is a complicated foreland depression influenced by transgression during the late ordovician

paleogeographic orientation of the sino korean plate based on evidence for a prevailing silurian wind field abstract the regional geology of southern inner mongolia records relationships between an upper silurian coastline and a small island on what was formerly the continental shelf of the sino korean plate north china block in both cases rocky shorelines are defined by unconformities between strata of ludlow age in the xibiehe formation and underlying igneous rocks of ordovician age the paleoisland occurs near bater obo about 50 km south of the chinese mongolian border the diorite core of this geological inlier is oriented on an axis n 55 e a simple dichotomy of exposed windward versus sheltered leeward environments accounts for contrasting carbonate facies on the flanks of the paleoisland wider corroboration of a prevailing silurian wind field is supported by evidence from other unconformities about 30 km to the southwest where a linear paleoshore extends for 15 km on an average bearing of n 60 e siliciclastic sandstone derived from a granodiorite source in the gushan area now sits in close proxi

continuous seismic threshold monitoring summary continuous threshold monitoring is a technique for using a seismic network to monitor a geographical area continuously in time the method provides at a given confidence level a continuous assessment of the upper magnitude limit of possible seismic events that might have occurred in the target area two approaches are presented in this paper site specific threshold monitoring by focusing a seismic network on a specific target site continuous threshold monitoring of that site is achieved we optimize the monitoring capability by tuning the frequency filters and array beams to known characteristics from previously recorded events at the site we define the threshold trace for the network as the continuous time trace of computed upper magnitude limits of seismic events in the target area at a 90 per cent confidence level as an example we have conducted a one week monitoring experiment of the northern novaya zemlya nuclear test site using the fennoscandian regional array network noress arcess finesa we find that the threshold trace is below mb 2 5 more than 99 per cent of the time 34 peaks exceed mb 2 5 all of these peaks correspond to interfering seismic events that have been independently located by a teleseismic or regional network during the entire one week time period the threshold trace exceeded mb 2 5 only for 43 min regional threshold monitoring this involves conducting site specific monitoring of a dense grid of geographical aiming points and requires the development of generic phase attenuation relationships for covering an extended geographical region using again the fennoscandian regional array network we illustrate the regional threshold monitoring approach by maps with colour contour displays we demonstrate that the network thresholds in fennoscandia and adjacent regions show strong regional dependence the thresholds are below mb 0 5 close to each array 300 km distance and range from mb 2 0 to 2 5 in parts of the norwegian sea and barents sea the thresholds also vary significantly under different background noise conditions and an increase of about 1 0 mb units is observed during a large teleseismic earthquake these regional threshold maps have advantages over standard network capability maps in being more accurate during time intervals when interfering seismic events occur they can also more easily reflect special conditions such as particularly favourable source station propagation paths and have the advantage of not being tied to specific event detection criteria the paper concludes that continuous threshold monitoring offers a valuable supplement to traditional seismic techniques used in nuclear test ban monitoring the method may also be useful for monitoring earthquake activity at low magnitudes for sites of special interest as well as for monitoring earthquake aftershock sequences

the dynamic evolution of the palaeozoic geography of eastern asia abstract new palaeogeographical reconstructions are presented for eleven time intervals through the palaeozoic of the eastern asia region from the middle cambrian at 510 ma to the end of the permian at 250 ma they centre on the continental blocks of north china south china and annamia indochina and their relationships with northeastern gondwana which was united to form part of pangea from the late carboniferous onwards also shown is the continent of tarim during the lower palaeozoic as well as the hutag uul songliao and khanka jiamasu bureya terranes both of which straddle the russian mongolian and chinese borders today from silurian times onwards we conclude that annamia and south china were united as a single continent throughout the lower palaeozoic and early devonian and were translocated by major strike slip faulting along the northeastern gondwana margin during that period from off afghanistan to outboard of the sibumasu and australian sectors of the superterrane they left the gondwana marginal area together during the lower devonian opening of the palaeotethys ocean but very shortly afterwards they themselves divided into the two separate continental blocks that we recognise today not to reunite until the triassic the various cambrian to permian rocks found in japan largely represent active volcanic arcs which originally lay to the southeast of south china although the carboniferous was more quiescent there the neotethys ocean opened during the permian dividing sibumasu and the tibetan terranes from gondwana and the palaeotethys ocean started to close progressively in the upper palaeozoic as most of the east asian continents and smaller terranes moved towards siberia the positions of the various continents and terranes have been deduced from a mixture of palaeomagnetic and faunal data the positions of large igneous provinces and kimberlites and the need to provide kinematic continuity between maps of successive ages however many uncertainties remain

evaluation of location performance and magnitude of completeness of the alaska regional seismic network by the snes method abstract we apply the seismic network evaluation through simulation snes method to evaluate the performance of the alaska regional seismic network arsn in locating regional earthquakes in terms of validity of the velocity models hypocentral errors and magnitude of completeness we find that background noise levels at the sites are highest in the aleutian region and lowest in mainland alaska we demonstrate that the arsn provides the best monitoring in the south central region with horizontal and vertical errors of less than 1 5 and 3 km respectively at the completeness level of m l 2 0 at the completeness level of m l 3 0 the network is capable of locating earthquakes for the whole region down to depths of about 250 km with satisfactory errors the lowest magnitude completeness levels of about 1 4 and even less are achieved with satisfactory location errors in limited areas of the south central region that have the densest station coverage the network does not monitor the seismogenic areas in northern western southeastern and offshore alaska at an adequate level in terms of earthquake location errors network upgrades in these areas could be optimized using the snes results

influence of paleo trade winds on facies patterns of the cambrian shanganning carbonate platform north china abstract the shanganning carbonate platform scp of the north china craton ncc was located in the trade winds belt during the cambrian which influenced its facies development in recent years numerous drillcores have been recovered from these units but the depositional environments and facies architecture of this large 2 0 105 km2 carbonate platform system have not yet been reported in detail in this study we integrate sedimentological and magnetic fabric data collected from outcrops and drillcores in order to document variation in microfacies and facies associations within the scp to infer paleowind directions and constrain the paleogeographic location of the scp and to evaluate eolian controls on platform development this analysis documented numerous carbonate microfacies belonging to six fabric types mudstone wackestone packstone grainstone framestone and dolostone along with rarer siliciclastic sediments these microfacies belong to facies associations representing tidal flat open platform platform margin slope and deep basinal depositional environments anisotropy of magnetic susceptibility ams measurements yielded mean paleocurrent directions for the early middle and late cambrian of 114 63 and 83 30 from paleo north respectively that were relatively consistent for 12 out of 13 platform margin sites all vectors being corrected for a post cambrian 150 clockwise rotation of the ncc these wind orientations are consistent with the scp shifting from south of the paleo equator in the early cambrian to north of it by the middle cambrian paleo trade winds influenced facies patterns across the platform producing a distinct windward leeward zonation the windward margin is characterized by well sorted oolitic grainstone as well as favositids coral and dasycladacean algal sponge framestone and the leeward margin by poorly sorted oolitic wackestone grainstone and fine bioclastic sediments the present study is innovative in 1 integrating microfacies and ams analyses in order to establish paleowind patterns and influences on an ancient carbonate platform 2 documenting windward leeward variation in quantifiable sediment properties across the scp and 3 constraining the paleo hemispheric location of the ncc during the cambrian

mesozoic cenozoic rift drift sequence of asian fragments from gondwanaland abstract the university of cambridge atlas map plotting computer program has been used to plot a reconstruction of the dispersal of continental blocks from eastern gondwanaland from the late jurassic 160 ma until the late miocene 10 ma the geological basis for the reconstruction rests on the concept of south tibet burma western thailand malaya and sumatra having been part of the eastern gondwana continent during the late palaeozoic and early mesozoic the reconstruction is also based on the view that these asian continental blocks were rifted from eastern gondwanaland in the late jurassic this rifting episode gave rise to the continental margin of central new guinea and northern australia it was associated with the sea floor spreading anomalies preserved in the floor of the northeastern indian ocean the model considers the indication of crustal uplift in the middle jurassic followed by profound subsidence in the cretaceous and palaeogene exposed in the islands of the outer banda arc from seram to timor and the similar jurassic cretaceous facies in eastern sulawesi implies that these areas formed part of this rifted northern margin of australia in the jurassic subsequent tectonic collision and associated major strike slip movements have deformed and disrupted these rock sequences during the cainozoic the date of the rifting that removed south tibet burma western thailand and malaya from gondwanaland is controversial the indications in favour of the rifting episode having occurred in the jurassic rather than in the permo triassic are stratigraphie structural observations on the northwestern australian shelf in the banda arc and in central new guinea the identification of the continental blocks removed by this rifting is based partly on stratigraphie correlation with the banda arc and new guinea and partly on indications that the age of collision of tibet burma western thailand and malaya with asia was a late cretaceous event and not triassic jurassic as is generally held there are very few reliable land based palaeomagnetic measurements to constrain the movements of these dispersed continental fragments from gondwanaland the computer generated map reconstructions put forward here fit the available indian ocean sea floor spreading data although they are not unique solutions much of the critical ocean floor data associated with the rifting and dispersal of these continental fragments has been destroyed in the tectonic collision processes that occurred at the asian continental margin

studies of high frequency seismic noise in eastern kazakhstan abstract three seismographic stations have been established in the region of the principal underground nuclear weapons test site in the republic of kazakhstan in the soviet union instrumentation includes both surface and borehole seismometers with a bandwidth of 100 hz and a system dynamic range of 180 db power spectra of the regional ambient ground acceleration peak between 4 and 6 hz and are on average frequency independent above 10 hz where levels are approximately equal to 10 15 to 10 16 m sec 2 2 hz 1 kazakh noise levels are roughly equivalent to those observed by the regional seismic test network between 1 and 20 hz and about 10 to 20 db higher than the very low noise model for lajitas texas in the same band noise observed by the borehole sensors is about 10 db less than that observed by the surface instruments above about 20 hz under low wind conditions at two of the three kazakh sites borehole improvement is greater under high winds the third kazakh site karasu is characterized by much higher surface noise levels 15 to 20 db higher than the borehole levels from 4 to 40 hz under both low or high wind conditions which is attributed to amplification by local site conditions and the exposed terrain power spectra of individual runs of ambient vertical ground acceleration measured down the borehole show that high frequency slopes between 10 and 80 hz vary between f 0 60 and f 0 22 depending on wind and site conditions

late paleozoic tectonics of the solonker zone in the wuliji area inner mongolia china insights from stratigraphic sequence chronology and sandstone geochemistry abstract the geology in the wuliji area including the enger us and quagan qulu areas is important for understanding the late paleozoic tectonics of the solonker zone ultramafic mafic rocks in the enger us area previously interpreted as an ophiolitic suture are actually lava flows and sills in a permian turbiditic sequence and a small body of fault breccia containing serpentinite subduction zone features such as accretionary complexes magmatic arc volcanics or lp hp metamorphism are absent early permian n morb mafic rocks and late permian radiolarian cherts accompanied by turbidites and tuffeous rocks indicate a deep water setting in the quagan qulu area outcrops of the late carboniferous to permian amushan formation are composed of volcano sedimenary rocks and guyot like reef limestone along with a late permian volcano sedimentary unit a dacite lava in the late permian volcano sedimentary unit yields a zircon u pb age of 254 ma the gabbros in the quagan qulu area are intruded into the amushan formation and caused contact metamorphism of country rocks sandstones in the upper member of the amushan formation contain detrital clasts of volcanic fragments and mineral clasts of crystalline basement rocks i e biotite muscovite and garnet geochemical analysis of volcaniclastic sandstones shows a magmatic affinity to both continental island arc cia and active continental margin acm tectonic settings a late permian incipient rift setting is suggested by analyzing the lithostratigraphic sequence and related magmatism in the wuliji area the volcano sedimentary rocks in the wuliji area experienced a nearly n s shortening that was probably related to the early mesozoic nearly n s compression well developed in other areas close to the wuliji area

paleomagnetism of ca 1 35ga sills in northern north china craton and implications for paleogeographic reconstruction of the mesoproterozoic supercontinent abstract we report paleomagnetic data on precisely dated mesoproterozoic sills in zircon and baddeleyite 207pb 206pb ages of 1345 12 and 1353 14 ma respectively intruding the xiamaling and wumishan formations in north china with an aim of evaluating how the north china craton ncc was involved in the evolution of the columbia supercontinent from ca 1 78 to 1 35 ga after systematic thermal demagnetization we isolate a high temperature characteristic remanence from 18 sites with a tilt corrected site mean direction of d 294 4 i 31 7 and 95 4 3 corresponding to a mean paleomagnetic pole at 5 9 n 359 6 e n 18 with angular standard deviation of 10 0 and a95 4 3 rock magnetic experiments light microscopy and scanning electron microscopy all indicate that the main magnetic minerals in the sampled sill rocks are medium sized titanomagnetites the positive fold test and fresh titanomagnetite grains suggest that the characteristic remanence is likely to be a primary acquired at ca 1 35 ga the preliminary paleogeography reconstruction based upon the well matched apparent polar wander paths between the laurentia baltica siberia united block and the ncc supports the interpretation that the ncc may have drifted together with this united block before ca 1 35 ga during the late history of the columbia supercontinent we argue that the ncc was located in low latitudes and kept tight connections with the siberian and indian cratons during the interval between ca 1 8 and 1 35 ga and that the ca 1 35 ga sills widely developed in the mesoproterozoic xiamaling and wumishan formations as well as the ca 1 3 1 2 ga magma events well developed in siberia and northern margin of the ncc may represent the complete fragmentation of the ncc with the neighboring cratons

modeling some empirical vertical component lg relations abstract higher mode surface wave theory is used to model the vertical component lg wave observed in eastern north america at regional distances tests of the model are made to determine whether it is capable of describing empirical spectral scaling laws spatial attenuation and peak time domain lg amplitudes it is found that a simple crustal model and a rough estimate of crustal qb are all that are required to accomplish this good results are obtained if the average crustal qp is equated to the coda q of the same frequency introduction the lg wave is a wave observed on short period seismograms of local and regional seismic events which travels with a group velocity of approximately 3 5 km sec in the central united states it is usually the largest arrival on the seismogram which makes it suitable for defining a magnitude scale nuttli 1973 herrmann and nuttli 1982 as part of developing the magnitude scale there have been many studies on the spatial attenuation of the lg wave nuttli 1973 street 1976 bollinger 1979 singh and herrmann 1982 the lg wave is usually thought of as a superposition of higher mode surface waves propagating within the crustal wave guide herrmann and nuttli 1975a b attempted to deterministically model observed lg amplitudes in terms of focal mechanism focal depth and seismic moment using surface wave techniques they were limited by their inability to compute surface wave eigenfunctions at frequencies greater than 0 5 hz however their results were encouraging harkrider 1979 drawing upon the work of apsel 1979 discovered a way to compute surface wave energy integrals of the higher modes very precisely bache

minimum magnitude of completeness in earthquake catalogs examples from alaska the western united states and japan we mapped the minimum magnitude of complete reporting m c for alaska the western united states and for the junec earthquake catalog of japan mc was estimated based on its departure from the linear frequency magnitude relation of the 250 closest earthquakes to grid nodes spaced 10 km apart in all catalogs studied mc was strongly heterogeneous in offshore areas the mc was typically one unit of magnitude higher than onshore on land also mc can vary by one order of magnitude over distance less than 50 km we recommend that seismicity studies that depend on complete sets of small earthquakes should be limited to areas with similar mc or the minimum magnitude for the analysis has to be raised to the highest com mon value of mc we believe that the data quality as reflected by the mc level should be used to define the spatial extent of seismicity studies where mc plays a role the method we use calculates the goodness of fit between a power law fit to the data and the observed frequency magnitude distribution as a function of a lower cutoff of the magnitude data mc is defined as the magnitude at which 90 of the data can be modeled by a power law fit mc in the 1990s is approximately 1 2 0 4 in most parts of california 1 8 0 4 in most of alaska aleutians and panhandle excluded and at a higher level in the junec catalog for japan various sources such as ex plosions and earthquake families beneath volcanoes can lead to distributions that cannot be fit well by power laws for the hokkaido region we demonstrate how neglecting the spatial variability of m c can lead to erroneous assumptions about deviations from self similarity of earthquake scaling

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

petrography and zircon u pb isotopic study of the bayanwulashan complex constrains on the paleoproterozoic evolution of the alxa block westernmost north china craton abstract the bayanwulashan metamorphic complex bmc exposes along the eastern margin of the alxa block the westernmost part of the north china craton ncc bmc is principally composed of metamorphic rocks with amphibole plagiogneiss biotite plagioclase gneiss and granitic gneiss our research has been focused on the petrography and zircon u pb geochronology of the bmc to better understand the evolution of the alxa block and its relationship with the ncc evidences from field geology petrography and mineral chemistry indicate that two distinct metamorphic assemblages the amphibolite and greenschist facies had overprinted the preexisting granitic gneiss and suggest that the bmc experienced retrograde metamorphic episodes the la icp ms zircon u pb ages reveal that the primary magmatic activities of bmc were at ca 2 30 2 24 ga and the two metamorphic events were at ca 1 95 1 91 ga and ca 1 88 1 85 ga respectively these ages indicate that bmc initially intruded during paleoproterozoic not as previously suggested at archean period the early paleoproterozoic metamorphic records and the magmatic thermochronological data in bmc exhibit different evolution paths between the alxa block and the ncc the alxa block was most likely an independent early paleoproterozoic terrain following different amalgamation processes the alxa block combined with western block at ca 1 95 ga and then united with ncc at ca 1 85 ga

robust and reliable epicenter determinations envelope processing of local network data local event recordings are complex in the sense that relevant p and s phases vary in an unpredictable manner even between closely spaced stations thus manual analysis of such records is still commonplace our approach to solving this long standing problem of observational seismology is to bandpass filter 3 to 6 hz to ensure good signal to noise ratio snr and then form envelopes to ensure simple signals across a seismograph network the physical basis is that pg and lg are crustal wave guide phases reflecting p and s energy propagation extensive tests on envelope analysis of local records from different areas found that arrival times of the maxima of pg and lg envelopes increase very consistently with distance even in different tectonic regimes typical velocities being 6 1 and 3 5 km sec respectively these arrival time parameters are easy to extract in a semi automatic manner and are highly suitable for local epicenter determinations extensive tests on locating mining explosions were conducted and on average the envelope location errors relative to true locations were similar to those in bulletins that are based on conventional phase pickings occasionally the pg pn envelope may be very weak but can be replaced by the easily pickable non envelope pn phase additional advantages with envelope locations are transportability not overly sensitive to details of crustal structure and that envelope amplitudes can be directly converted to ground motion and magnitudes for modern stations envelopes can be formed in situ with low sampling rates of 1 to 2 hz thus greatly reducing transmission costs

age geochemistry and tectonic setting of buqingshan ophiolites north qinghai tibet plateau china abstract the buqingshan ophiolite complex is a sector of the a nyemaqen ophiolite belt in the east kunlun southern marginal suture zone in the northern part of the qinghai tibet plateau the wnw trending ophiolite complex consists of metaperidotite gabbro diabase pillow basalt massive basalt and pelagic sedimentary rocks including radiolarian chert we have determined that early paleozoic and early carboniferous early permian ophiolites are present in this ophiolite complex that were previously thought of permian middle triassic age a zircon u pb age of 467 2 0 9 ma for gabbro and a rb sr isochron age of 481 130 ma for diabase and gabbro have been obtained from the early paleozoic ophiolite slice and middle late ordovician acritarchs have been found in the melange matrix that envelopes the sliver we interpret the first two ages to represent the formation age of the ophiolite granodiorite tonalite plutons with a zircon u pb age of 402 24 ma intrude the slice early carboniferous early permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle most mafic rocks in the early paleozoic ophiolite are of n morb type with small amount of t morb and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination the basalts in the early carboniferous early permian ophiolite have similar characteristics to the mafic rocks and also show dupal anomaly and a mixed source of dmm and em ii the original formation environments of the two stage ophiolites are both mid ocean ridges these lines of evidence suggest that a mature early paleozoic kunlun qilian qinling ocean basin and a mature paleotethyan ocean basin once existed in the study area

provenance of ediacaran sinian sediments in the helanshan area north china craton constraints from u pb geochronology and hf isotopes of detrital zircons abstract the tectonic relationship between the alxa block and the north china craton has long been controversial the helanshan area lies at the western margin of the ordos block and to the east of the alxa block and it contains eidacaran zhengmuguan formation and tuerkeng formation the zhengmuguan formation is made up of tillites limestone and dolomine and the tuerkeng formation consists of silty slate a parallel unconformity marks the boundary between the tuerkeng formation and the early cambrian suyukou formation which is composed mainly of phosphatic pebbly sandstone and sandstone u pb dating ages of detrital zircons in the zhengmuguan and suyukou formations reveal two neoproterozoic age peaks of 818 4 ma n 88 and 905 8 ma n 20 neoproterozoic magmatic events have yet been reported in the basement of the ordos block while some neoproterozoic igneous rocks have been found in the basement of the alxa block moreover the two neoproterozoic age peaks correspond well with the ages of neoproterozoic igneous rocks in alxa block the hf isotopic characteristics of a part of neoproterozoic zircons from the ediacaran zhengmuguan formation in the helanshan area ehf t 7 812 to 3 274 tdmc 2211 1578 ma n 10 are similar to those neoproterozoic igneous zircons from the langshan area ehf t 1 105 to 5 928 tdmc 1 75 1 38 ga n 23 and bayinnuoergong area in the alxa block the parental magmas had a long residency time in the crust it seems therefore that alxa block provenance provided neoproterozoic clastics for helanshan area in ediacaran and alxa block had already been a part of the north china craton by the late neoproterozoic even earlier

paleomagnetism of carboniferous sediments in the hexi corridor its origin and tectonic implications we report paleomagnetic results for carboniferous rocks from the eastern part of the hexi corridor which we have demonstrated to be the western extension of the north china block ncb the characteristic high temperature remanence component b component exhibits predominantly reverse polarity and is generally separable by thermal demagnetization temperature steps between 350 and 580 c this b component can also be isolated by alternating fields afs cleaning at a peak value of 20 30 mt using a hybrid thermal af demagnetization method the b component shows a positive fold test at the 95 confidence level indicating that it was acquired before late triassic to early jurassic folding we interpret the b component to be of primary origin acquired during the carboniferous the corresponding pole positions are at 14 0 e 10 5 n 95 6 2 for the early middle carboniferous late visean to namurian and at 10 2 e 33 3 n 95 16 7 for the late carboniferous stephanian the preliminary late carboniferous pole is in agreement with that for the ncb supporting earlier claims that the hexi corridor has been the western extension of the ncb since the ordovician on the other hand these two carboniferous paleopoles are distinct from the coeval ones for inner mongolia outer mongolia south china tarim and stable europe at greater than 95 confidence suggesting that these continental blocks could not have been connected in their present configuration during carboniferous time the significant difference in paleolatitude between the ncb and australian part of gondwanaland according to the two new hexi corridor poles indicates that they had drifted apart by the late visean

low temperature magnetic behavior of titanomagnetites titanium substitution has a significant effect on the temperature dependence 5 300 k of low temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite fe3 xtixo4 0 x 0 4 and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range these features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples moreover the apparent similarities between the thermal decay of low temperature remanence of titanomagnetite with pyrrhotite pure magnetite and superparamagnetic phases can lead to complications in interpreting low temperature remanence data when titanomagnetite is present

paleomagnetic insights into the cambrian biogeographic conundrum did the north china craton link laurentia and east gondwana

tertiary paleomagnetism of north and south china and a reappraisal of late mesozoic paleomagnetic data from eurasia implications for the cenozoic tectonic history of asia abstract to contribute to the understanding of the cenozoic tectonic history of china and eastern eurasia in general we collected paleomagnetic samples from tertiary rocks in both north and south china our miocene paleomagnetic data for northern china are from four widely separated basalt sections in the inner mongolia autonomous region and shandong province both normal and reversed polarities of the characteristic magnetization are present in the stratigraphie sequence and the mean normal and reversed directions are closely antiparallel to each other the fact that the same characteristic direction was carried by both magnetite and hematite suggests that the hematite was probably formed by auto oxidation processes during primary cooling of the basalts the paleomagnetic data from these four areas also pass a regional generalized fold test the corresponding mean miocene paleomagnetic pole for northern china is longitude 261 6 e latitude 88 3 n a 95 6 4 our early tertiary paleomagnetic results for south china are from two sandstone sections around the city of nanning 22 7 n 108 3 e in guangxi zhuang autonomous region high temperature characteristic directions isolated in 31 samples from five sites pass both tilt and reversal tests the paleomagnetic pole position determined from the five sites lies at longitude 228 0 e latitude 83 6 n a 95 8 5 the tertiary paleomagnetic poles for north and south china are statistically indistinguishable from each other and thus confirm that north and south china were fully assembled at least by the tertiary our new tertiary paleomagnetic data are consistent with the hypothesis that the major continental blocks of north and south china were in the same position as today during the tertiary a critical evaluation and selection of paleomagnetic results from the late jurassic onward for siberia europe and north and south china further corroborate this hypothesis most previously existing jurassic and cretaceous paleomagnetic poles from europe siberia are of uncertain reliability and should not be used for tectonic reconstruction using poles derived from transferring reliable data from other continents we show that the previous paleomagnetic inferences of large eastward displacements between these chinese blocks during the cenozoic are no longer warranted the available paleomagnetic data that meet our acceptance criteria are also of interest concerning the boundary conditions of the propagating extrusion model for asian tectonics additional mesozoic and cenozoic paleomagnetic data from siberia and north and south china are still needed to resolve the smaller second order displacements that appear to characterize the cenozoic tectonic history of this part of asia

paleomagnetic constraints on the paleogeography of the east asian blocks during late paleozoic and early mesozoic times abstract although the axial and dominant geocentric character of the earth s magnetic field means that paleolongitude is indeterminate paleomagnetism is otherwise the only truly quantitative method available to the earth scientist for constraining paleogeography and reconstructing the kinematic evolution of continental blocks during the past half century numerous paleomagnetic results have provided substantial quantitative constraints on the origin and tectonic evolution of the major tectonic divisions now comprising eastern asian in this paper we first assess the available early paleozoic to early cretaceous paleomagnetic results from the south china north china and tarim blocks using internationally recognized reliability criteria we then construct a running mean through a 20 ma window by weighting the poles according to their q factors and fit a spherical spline with smoothing factor of 300 to derive apparent polar wander apw paths for these three major blocks during this early paleozoic to late mesozoic time interval together with late paleozoic to early mesozoic paleomagnetic poles from the qaidam qiangtang lhasa sibumasu indochina and some other smaller blocks terranes of east asia we produce a series of paleogeographic reconstructions for these major blocks and lesser terranes of east asia between mid ordovician and late jurassic times 460 160 ma which although based primarily upon paleomagnetic evidence aim to embrace the geological constraints finally we discuss the current evidence for closure times of the paleo asian mianlue and east paleo tethyan oceans which incorporate fundamental issues concerning the formation of the east asian continental collage and collision with the northern main body of the pangean supercontinent we use the collective evidence to argue that these major paleo oceans had closed by the late triassic and that the east asian sector of the supercontinent had united to become an integral part of pangea by that time 220 ma

palaeomagnetic investigation on early middle triassic sediments of the north china block a new early triassic palaeopole and its tectonic implications summary palaeomagnetic and rock magnetic study is reported of early middle triassic terrestrial sediments from the jiaocheng and yushe areas shanxi province thermomagnetic analysis indicates that magnetite and haematite are the main magnetic carriers thermal demagnetization shows that the majority of early triassic specimens from the jiaocheng area contain an intermediate temperature component itc between 500 250 c and a high temperature component htc between 500 680 c the itc has a negative fold test and might be a cretaceous or recent overprint the htc contains both normal and reversed polarities with positive fold and c level reversal tests we interpret the htc as primary magnetization unfortunately we failed to isolate a meaningful mean direction from middle triassic rocks of the yushe area new early triassic palaeopole from the jiaocheng area confirms an 10 counter clockwise displacement of the taihang early triassic palaeopoles relative to their ordos counterparts this study thus suggests that the north china block ncb has generally behaved as rigid block since the early triassic but there might be an 9 counter clockwise rotation of the taihang terrane with respect to stable ncb in the late triassic the slight rotation of the taihang terrane does not bear significantly on what has been previously concluded concerning the mesozoic history of the ncb south china block scb collision however triassic palaeomagnetic data from stable ncb indicate that the east end of the ncb and scb did not undergo large convergence or significant opposite latitudinal displacement after the initial contact at the end of the late permian

hydrochemical characteristics of groundwater and surface water for domestic and irrigation purposes in vea catchment northern ghana the vea catchment mainly underlain by crystalline basement rocks is located in northern ghana hydrogeochemical studies were carried out in this area with the objective of identifying the geochemical processes influencing water quality and suitability of surface and groundwater for agricultural and domestic uses sixty one groundwater and four surface water samples were collected from boreholes dams and rivers and analysed for ca2 mg2 na k hco3 cl and so4 2 fetot po4 3 mntot nh4 no3 no2 in addition ph total dissolved solids electrical conductivity total hardness turbidity colour salinity and dissolved oxygen were analysed chloro alkaline indices 1 and 2 and characterization of weathering processes suggest that the chemistry of groundwater is dominated by the interaction between water and rocks cation exchange and silicate weathering are the dominant processes controlling the chemical composition of the groundwater in the area studied mineral saturation indices indicate the presence of at least three groups of groundwater in the vea catchment with respect to residence time the meteoric genesis index suggests that 86 of the water samples belong to the shallow meteoric water percolation type the findings further suggest that the groundwater and surface water in the basin studied are mainly ca mg hco3 water type regardless of the geology compared to the water quality guidelines of who the study results on sodium absorption ratio sodium percentage magnesium hazard permeability index and residual sodium carbonate indicate that groundwater and surface water in the vea catchment are generally suitable for drinking and irrigation purposes

new cambrian and ordovician paleomagnetic poles for the north china block and their paleogeographic implications multicomponent magnetization has been revealed in several cambrian and ordovician sedimentary units from localities in hebei and shandong provinces of the north china block ncb a well defined component with lower unblocking temperature is characterized by northerly declination and moderate to steep inclination with both polarities and postdates the late cretaceous folding the pole position derived from the overprinted directions is close to both the cretaceous pole for the ncb as well as the present north pole the overprint is not a simple thermal remanent magnetization but is probably a chemical remanent magnetization imposed at moderately elevated temperature during the major episode of regional deformation in the late mesozoic the higher unblocking temperature characteristic components pass reversal baked contact and fold tests and are internally consistent with rocks of different lithologies from widely distributed localities moreover the data exhibit an overall similarity in the magnetic polarity pattern in the stratigraphic sequence at sampled localities hence the high unblocking temperature characteristic component was probably acquired at or near the time of deposition the cambrian and ordovician paleopoles are somewhat different from the other coeval poles for the ncb which are based on much less data and do not pass as many reliability tests but all imply that the ncb was in the equatorial zone in the early paleozoic

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

early paleozoic paleomagnetic poles from the western part of the north china block and their implications abstract we collected 917 oriented drill core samples 124 sites from 22 units spanning the early cambrian to middle ordovician formations of the north china block ncb to construct an apparent polar wander path apwp of the early paleozoic for the ncb sampling areas are located in the helanshan tongchuan hancheng and yuncheng luliang regions along the margin of the ordos basin characteristic remanent magnetizations for five periods between the cambrian and ordovician are isolated from 281 samples by careful demagnetization directions cluster around northwesterly direction with upward shallow inclination and its antipode after tilt correction reliability of these five directions is ascertained through positive fold and reversal tests the five cambrian and ordovician paleopoles plot in the present atlantic ocean which is consistent with previously reported paleomagnetic poles a newly constructed apwp between the cambrian and ordovician indicates that the ncb was located in the southern hemisphere around 15 s during cambro ordovician time motion of the ncb between the early and middle cambrian comprised a counterclockwise rotation of 22 3 7 0 and a northward latitudinal displacement of 7 2 6 8 combining the paleobiogeographic and paleoclimatic data with the paleomagnetic implies that the ncb was probably located in east gondwana in the early cambrian and may have separated from gondwana in post early cambrian time the ncb is also inferred to have been located close to siberia and north america during the middle ordovician

spectral characteristics of regional phases recorded at noress abstract the spectral characteristics of pn and lg are studied for regional events recorded at the noress array in norway the emphasis is on the potential value of spectral ratios for identifying events as earthquake chemical explosions and nuclear explosions the events studied include a suite of explosions from the titania mine in southwest norway a suite of events of unknown source type from a site offshore about 90 km from this mine suites of explosions from several mines in the soviet union about 1000 km from noress events from several locations along the 90 azimuth from noress an apparent earthquake in the north atlantic and a nuclear explosion pne at a range of 1560 km the event identification issues addressed are as follows 1 can earthquakes and explosions be identified based on the ratio of high and low frequency energy in their signal spectra 2 do spectral ratios separate mine blasts from earthquakes for all source areas 3 will spectral ratio discriminants be effective for identifying decoupled underground nuclear explosions we conclude that spectral ratios can sometimes separate events an example is separation of the titania mine blasts from the events at the nearby offshore locations although we cannot be sure how much this is due to path differences however in general spectral ratios vary as much within classes of events as they do among different classes the pne in our data set allows the simulation of spectra from a decoupled nuclear explosion by simply applying the frequency dependent decoupling factor to the observed pn spectrum for this event after applying a distance correction the spectrum for this simulated decoupled nuclear explosion is similar to those for the soviet union mine blasts in our data set

a mesoproterozoic paleomagnetic pole from the yangzhuang formation north china and its tectonics implications abstract a new paleomagnetic study has been carried out on mesoproterozoic sediments in the jixian area north china detailed stepwise thermal demagnetization allows us to isolate three components from 141 oriented drill core samples 14 sites a low temperature component component a identified from most of samples falls close to the present local earth field direction a middle temperature component component b is found only in 21 samples from four sites the component b directions after bedding correction yield a paleopole position at 336 0 e 10 2 n with a 95 17 1 which is close to the lower cambrian pole of the north china block characteristic remanent directions with dual polarities obtained from high temperature component c pass fold test the directions yield a pole position at 190 4 e 2 4 n with a 95 11 9 this result indicates that the north china block was located in low latitude at ca 1 35 ga recently several studies suggested that laurentia baltica and siberia were also situated at low latitudes from well dated paleopoles between ca 1 5 and 1 25 ga this would imply that the north china block laurentia and baltica might drift together or close in the mesoproterozoic

cambrian ordovician paleogeography of baltica recent paleomagnetic data and existing paleontological evidence show that baltica occupied temperate southern latitudes during cambrian and early ordovician time but was inverted with reference to its present orientation hence the currently opposed margins of the baltic and laurentian shield were not conterminous in the early paleozoic laurentian and baltic late precambrian rifting episodes may therefore have occurred along unrelated continental margins closure of the tornquist sea was accompanied by the counterclockwise rotation of baltica relative to avalonia in early to middle ordovician time this rotation initiated in late cambrian early ordovician time was coeval with the finnmarkian orogeny in the northern scandinavian caledonides

seismic detection capability at noress and implications for the detection threshold of a hypothetical network in the soviet union the detection threshold of a network including noress type arrays in the soviet union is estimated using the frequency dependent attenuation of pn and lg observed at noress to normalize the threshold of individual stations the normalization begins with a parameterization of the magnitude and range dependence of the noress pn and lg spectra in terms of seismic moment and apparent attenuation a relationship between these spectral amplitudes and the time domain amplitudes used in signal detection at noress is then determined this allows an estimate of noress detection capability from the spectral parameterization and a prediction of the pn detection capability for regions with different wave propagation characteristics a comparable prediction is not possible for lg because of dispersion and the non stationarity of pre lg noise instead an empirical relationship for lg detectability based on observed temporal amplitudes is determined to validate the results for noress predictions based on our model are compared with observed detection statistics and with results obtained by ringdal 1986 who compared detections at noress to bulletins produced by local seismic networks the simulations represent the detection capability of a network of noress type arrays 20 within and 13 outside the soviet union the 90 ml threshold for detecting three pn phases for events in the soviet union is estimated to be between 2 4 and 2 7 if the frequency dependent attenuation and noise throughout the network are the same as those observed at noress this threshold is reduced by 0 1 ml if pn q is 50 higher in the soviet union than estimated for scandinavia and is increased by 0 2 0 3 ml if pn q is 50 lower the ml thresholds are reduced by 0 2 0 3 if the detection criteria include lg as well as pn phases while these simulations are carefully normalized by the actual detection capability of a prototype station for test ban treaty monitoring observations of attenuation and noise inside the soviet union are needed to determine how they actually differ from those at noress also it must be noted that the noress detection capability estimates and therefore the simulations are for average conditions and there are regular variations that must be considered when estimating treaty monitoring capability

upper cambrian to middle ordovician magnetostratigraphy from the kulumbe river section northwestern siberia abstract we present the magnetostratigraphy of an upper cambrian to middle ordovician sedimentary section sampled along the kulumbe river in northwestern siberia thermal demagnetization yields the determination of two magnetic components a component of steep inclination is interpreted as a remagnetization acquired during the emplacement of the siberian traps at the permian triassic boundary another component with a moderate inclination essentially carried by hematite and sometimes by magnetite likely represents a syndepositional or nearly syndepositional magnetization of the studied deposits a sequence of 17 magnetic intervals is observed between the upper part of the upper cambrian and the tremadoc three of them being poorly defined by single samples we also find that the llanvirn magnetostratigraphic sequence contains only one reversed polarity interval together with magnetostratigraphic results previously obtained from the moyero river section from siberia our data suggest the occurrence of a 25 30 myr long period during the lower and the middle ordovician characterized by a very low magnetic reversal frequency possibly containing a superchron and dominated by reversed polarity this geomagnetic feature would be compatible with a time constant of about 150 myr in the reversal process through the whole phanerozoic

teleseismic event detection using the noress array with special reference to low yield semipalatinsk explosions abstract the small aperture noress array in norway has been designed for improving signal to noise ratios at high frequencies 1 to 15 hz while the main motivation for this has been to enhance the capabilities for detecting and characterizing weak seismic events at regional distances it has been found that the array is also very effective in the teleseismic distance range in particular for eurasia this results from a combination of two factors 1 due to high q paths eurasian earthquake and explosion recordings generally show high dominant frequencies typically in the range of 1 5 to 4 0 hz where the noise level at the site is low and 2 the array provides a particularly high signal to noise ratio gain of 12 to 14 db 0 6 to 0 7 m b units at these frequencies analysis of noress on line detection performance shows that there are large regional variations even within small epicentral areas this is interpreted as resulting mainly from signal focusing effects underneath the source areas in analogy to the receiver focusing effects earlier observed across large arrays a detailed study of noress recordings of underground nuclear explosions at the semipalatinsk test site has shown that relative to worldwide m b noress has a significant positive m b bias it is highest 1 0 m b units for the eastern part shagan river and somewhat lower 0 4 m b units for the western parts degelen konystan although there is a fair amount of scatter within each area analyzing observed signal to noise ratios at the noress site for a set of low yield nuclear explosions at semipalatinsk with published yields available in the soviet literature it is estimated that fully coupled explosions of yields as low as 0 1 kt would be detectable by noress under normal noise conditions however to give a precise threshold is difficult because of the low number of reference events available and the significant amount of extrapolation involved it is emphasized that this detection level will not be achieved in cases when the noise level is abnormally high e g in the coda of a large earthquake or if coupling conditions are not optimal e g in the case of full or partial cavity decoupling it must also be noted that the event identification threshold is necessarily higher than the signal detection threshold

magnetostratigraphic constraints on the gondwanan origin of north china cambrian ordovician boundary results summary a significant gap in the middle palaeozoic apparent polar wander apw path precludes polarity definition of the early palaeozoic palaeopole for north china this problem can in part be resolved by the intercontinental correlation of magnetic polarity patterns across small time intervals a magnetostratigraphic study was carried out on upper cambrian to lower ordovician sediments near zhaogezhuang long 118 5 e lat 39 7 n north china after stepwise thermal or thermal and alternating field demagnetizations a characteristic magnetic component with normal and antipodal reversed directions was identified these data drawn from 49 samples yield a north palaeopole at long 294 6 e lat 32 9 n dp 3 0 dm 5 3 a concordant magnetic polarity pattern around the cambrian ordovician boundary and lowest ordovician obtained from different continents favours a southern hemisphere origin 17 formation site using the cambrian ordovician apw paths between north china and gondwana we suggest that the north china block ncb was part of gondwana during the cambro lowest ordovician and started breaking away from gondwana in the lower ordovician this finding is contrary to some palaeomagnetic models where the ncb was quite separate from gondwana in the late proterozoic and was attached to the pacific side of antarctica

preliminary phanerozoic polar wander paths for the north and south china blocks polar wander paths for the north and south china blocks suggest that 1 both were parts of gondwana in the palaeozoic 2 the north china block accreted to siberia in the late permian and 3 the south china block accreted to the north china block in the middle triassic to the early jurassic comparison of the polar wander path for the south china block with that for northern eurasia suggests that relative motion of over 4 000 km has occurred between them

low temperature behavior of single domain through multidomain magnetite abstract we have investigated the low temperature behavior of a suite of grown synthetic and natural magnetites that span single domain sd and multidomain md behavior synthetic samples had been grown in the laboratory either in an aqueous medium or in glass natural samples included sd magnetites occurring in plagioclase and truly md magnetites in the form of large octahedra in all experiments a sample was first given a saturation remanence at room temperature next moment was measured continuously during cooling and warming between 230 k and 60 k similar to results reported earlier by other workers magnetic memory is large in sd samples whereas truly md samples are almost completely demagnetized by cycling between room temperature and 60 k pseudo single domain samples exhibit behavior that is intermediate with respect to that of the sd and truly md states when data from this study are combined with data obtained by hartstra 10 from sized natural magnetites it is found that the percentage of total remanence that survives cycling between room temperature and 60 k decreases linearly with the logarithm of grain size and thus with increasing number of domains this relation suggests that memory can provide a reasonable estimate of grain size in those magnetite bearing rocks for which these samples provide good analogues remarkably some of the large natural octahedra provide a magnified view of md response to low temperatures and thus reveal two surprising and intriguing types of behavior first below approximately 180 k these octahedra demagnetize through a series of large barkhausen jumps second near 117 k these same octahedra exhibit a wild zone where magnetic moment executes large random excursions we interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and possibly the broadening of domain walls as temperatures drop toward the isotropic point one implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by md grains even without passage through the isotropic point of magnetite

mesozoic litho and magneto stratigraphic evidence from the central tibetan plateau for megamonsoon evolution and potential evaporites the megamonsoon was a striking event that profoundly impacted the climatic environment and related mineral sources salts coals and oil gases in the mesozoic how this event impacted asia is unknown here we firstly reported a mesozoic stratigraphic sequence in the northern qiangtang basin in the central tibetan plateau based on lithofacies and chronologies of paleontology and magnetostratigraphy hpw the planetary and megamonsoon circulations controlled the asian climate with time has been recorded using the basic principles of physical geography present analogs and a newly developed model the evolution of the stratigraphic sequence was interpreted to demonstrate that the qiangtang basin has been subjected to a megamonsoon climate with heavy precipitation during its northward movement since the latest permian but has been subjected to a dry environment due to moving into the northern hemisphere subtropic high zone in the middle triassic and monsoonal retreat in the early late jurassic early oxfordian approximately 161 ma the coupling of the hot dry climate and the close of the meso tethys along with sea retreat in the late jurassic ensured a large potential time window for evaporite potash formation in the qiangtang basin while the tropic megamonsoon rain forest in the late permian to the early triassic and in the late triassic to middle jurassic favored the formation of coal and hydrocarbon source rocks c 2016 international association for gondwana research published by elsevier b v all rights reserved

new paleomagnetic results for ordovician and silurian rocks of the tarim block northwest china and their paleogeographic implications abstract the kinematics of the tarim block is poorly understood prior to the late paleozoic and this study aims to clarify motions during ordovician and silurian times we report the paleomagnetic record from 12 sites in ordovician marine sediments and 21 sites in silurian fine grained sandstones and mudstones from the aksu and kalpin areas of northwestern tarim together with our previously reported preliminary results the ordovician sediments isolate a sporadically preserved pre cenozoic characteristic remanence with dual polarity although a positive fold test can only define the remanence as pre cenozoic the corresponding paleomagnetic pole 33 7 s 185 0 e dp dm 2 7 4 0 is removed from post silurian paleopoles from tarim and combined with evidence from magnetic fabrics indicating a quasi primary origin a characteristic remanence isolated at 17 sites in silurian sediments from the aksu and kalpin areas includes 7 sites with suspected contamination from regional permian igneous activity the 10 remaining sites all from kalpin yield a positive fold test and a paleopole at 19 1 n 172 9 e a95 5 5 significantly removed from younger paleopoles from tarim indicating a late llandovery to wenlock remanence age the new mid late ordovician paleopole places the tarim block at 32 s probably in the vicinity of the arabian margin of gondwana rapid apw between 460 and 433 ma records large differential dextral motions of tarim together with other continental blocks along the paleo asian ocean margin which brought the china continental blocks into proximity with australia by mid silurian times in low 9 n northerly paleolatitudes

tectonic and paleogeographic controls on development of the early middle ordovician shanganning carbonate platform ordos basin north china lower middle ordovician platform carbonates are important oil and gas reservoir rocks in the ordos basin of the north china craton ncc in recent years numerous cores have been recovered from these units but the depositional environments and facies architecture of this large carbonate platform system have not yet been reported in detail this study integrates sedimentological and magnetic fabric data collected from outcrops and cores to document variation in microfacies and facies associations within the shanganning carbonate platform scp infer paleowind directions and the paleogeographic location of the scp and evaluate tectonic and paleogeographic controls on platform development this analysis documented 25 microfacies belonging to 7 microfacies fabric and sediment types carbonate mudstone wackestone packstone grainstone evaporite bindstone framestone siliciclastics and dolostone microfacies can be grouped into facies associations that represent tidal flat open platform restricted platform evaporite platform platform margin slope and deep basinal depositional environments these environments are mainly distributed zonally around the l shaped qingyang paleo uplift on the western and southern margins of the scp anisotropy of magnetic susceptibility measurements at five platform margin locales yielded paleocurrent directions of all approximately 318 348 51 81 42 72 80 110 and 54 84 from paleo north after correction for a 120 150 clockwise rotation of the ncc since the ordovician the depositional architecture of the scp depended on a combination of tectonic and paleogeographic wind influences local tectonic movements controlled the spatial distribution of facies associations around the qingyang paleo uplift which was characterized by open restricted and evaporite platform facies in the platform interior and mainly slope and basinal facies on the outer platform margin to the west and south of the qingyang paleo uplift vertical tectonic movements resulted in cyclic submergence and emergence of the platform over its approximately 20 m y long formation history paleo trade winds influenced aspects of facies zonation across the platform including growth of metazoan reefs on the windward platform margin and formation of oolitic grainstone and microbial reefs on the leeward platform margin the facies model for the scp developed in this study should provide a useful reference framework for future carbonate oil and gas exploration in lower middle ordovician units of the ordos basin furthermore this study makes innovative use of magnetic anisotropy data in combination with standard petrographic core and field data to discern facies patterns and controls on a paleo carbonate platform which is an integrated methodology that may serve as a model for future studies of other carbonate platforms

seismic network evaluation through simulation an application to the italian national seismic network abstract a properly organized seismic network is a valuable tool for monitoring seismic zones and assessing seismic hazards in this paper we propose a new method seismic network evaluation through simulation snes to evaluate the performance of hypocenter location of a seismic network the snes method gives as a function of magnitude hypocentral depth and confidence level the spatial distribution of the number of active stations in the location procedure and their relative azimuthal gaps along with confidence intervals in hypocentral parameters the application of the snes method also permits evaluation of the magnitude of completeness m c the background noise levels at the stations and assessment of the appropriateness of the velocity model used in location routine italy sits on a tectonically active plate boundary at the convergence of the eurasian and african lithospheric plates and has a high level of seismicity in this paper we apply the snes method to the italian national seismic network rete sismica nazionale centralizzata dell istituto nazionale di geofisica e vulcanologia rsnc ingv which has monitored italian seismicity since the early 1980s following the destructive 1980 irpinia earthquake in recent years the rsnc ingv has grown significantly in fact in february 2010 it received signals from 305 seismic stations 258 with wideband three component sensors we constructed snes maps for magnitudes of 1 5 2 2 5 and 3 fixing the hypocentral depth at 10 km and the confidence level at 95 through the application of the snes method we show that the rsnc ingv provides the best monitoring coverage in the apennine mountains with errors that for m 2 are less than 2 and 4 km for epicenter and hypocentral depth respectively at m 2 5 this seismic network is capable of constraining earthquake hypocenters to depths of about 150 km for most of the italian peninsula this seismic network provides a threshold of completeness down to m 2 for almost the entire italian territory

true polar wander during the middle paleozoic true polar wander would be recognized paleomagnetically as identical apparent polar wander paths for all surface elements the apparent polar wander paths for the late ordovician late devonian interval for laurentia baltica and gondwana have nearly identical looping shapes that can be brought into superposition the paths of these continents are well documented but even the less well known paths of south china and siberia reveal similar lengths the resulting reconstruction places the northern andean margin of south america opposite the appalachian margin of laurentia with baltica and laurentia adjoined in the fit of bullard and colleagues siberia and south china would be to the north of africa if their paleopoles are taken at face value for middle paleozoic time there is of course no information about oceanic domains butnit is interesting that all continental elements insofar as is known appear to have similar apparent polar wander tracks that can be plausibly superimposed without causing overlap in the positions of the continents albeit speculatively because of the lack of information about the oceanic elements it is suggested in this study that true polar wander may have occurred with a cumulative magnitude of about 75 during a 75 ma interval and may have been of greater magnitude than the apparent polar wander due to relative motions during the middle paleozoic this middle paleozoic rate of true polar wander appears to have been an order of magnitude greater than the average rate during the late mesozoic and tertiary

microstructure and significance of cordaitean reproductive organs from the lower permian of gansu northwest china abstract the late paleozoic strata are well exposed in yongchang county gansu province northwest china the strata contain numerous plant fossils hence yongchang county is a good location to study paleozoic plants and plate movement using well preserved cordaitean cones and seeds from the lower permian taiyuan formation of yongchang county two new species of fossil plants are described one attributed to the new organ genus hexianthus h shenii sp nov and the other to the seed genus samaropsis s shenii sp nov the micro and macro scale features of the cones including the epidermal features of the bracts and scales are analyzed in the laboratory numerous cordaitean cones with helically arranged bracts and scales h shenii sp nov present in the taiyuan formation of yongchang county share similar characteristics with the cordaitean cones which are endemic to the cathaysian flora this suggests that the study region was characterized by the cathaysian flora in the cisuralian similar to the hexi corridor to the south of the study region based on the study of fossil cuticles and their associated plants the study region of yongchang county probably had a warm and humid climate during the cisuralian in addition according to the associations between different fossil plants from several localities in gansu province from the cathaysian flora in the lower permian to the cathaysian angaran mixed flora in the upper permian a possible boundary between the cathaysian and angaran floras in gansu province can be placed along the hexi corridor which provides important information for the study of floral associations in northwest china during the late paleozoic

field and temperature behavior of remanence in synthetic goethite paleomagnetic implications synthetic powders of pure goethite exhibit an antiferromagnetic behavior and negligible isothermal remanent magnetization up to 4 tesla however thermoremanence up to 0 3 am kg 1 can be imparted to the samples high field measurements show median acquisition field of 9 13 t and suggest saturation fields of the order of 10 t large increase in remanence at low temperature is an intrinsic feature of goethite the weak ferromagnetism observed is linked to a bulk origin for spin imbalance rather than to an effect of very fine grain size or surface defects the results are compared with properties of rocks bearing goethite and hematite

magnetostratigraphy of the moyero river section north western siberia constraints on geomagnetic reversal frequency during the early palaeozoic summary we present the magnetostratigraphy of the early palaeozoic moyero river section in north western siberia the section investigated which consists mainly of reddish limestones marls and greyish limestones extends from the upper cambrian to the lower silurian palaeomagnetic analyses reveal two magnetic components one is interpreted as a remagnetization acquired during siberian trap emplacement the other is believed to be early diagenetic consistent with the established lower palaeozoic apparent polar wander path for siberia reversed polarity dominates from the upper cambrian to the middle ordovician while normal polarity dominates from the upper ordovician to the lower silurian only a few reversals which are poorly defined are observed through the section in particular during the llandeilo these data are in agreement with previous magnetostratigraphic data from the baltic platform and siberia and indicate that magnetic reversal frequency was low during the ordovician because of the time delayed process of acquisition of the magnetization carried by haematite we cannot clearly ascertain if a reversed superchron occurred during the lower to middle ordovician however our results suggest that a 15 myr long reversed interval existed during the arenig

multiple phases of deformation in the southern helanshan tectonic belt northern china abstract the southern helanshan tectonic belt shtb is surrounded by the alxa massif the ordos basin and the tibetan plateau and its phanerozoic structures records crucial phanerozoic deformation characteristics for studying the multi phased intracontinental deformation processes among these neighboring units combined with regional geology our structural analysis reveals that the shtb has experienced five phases of shortening deformation during the phanerozoic the first phase d1 is represented by e w trending overturned folds in the cambrian and ordovician strata these folds resulted from two episodes of n s shortening during the late cambrian early ordovician and late ordovician early devonian respectively which were attributed to the closure of the qilian ocean the second phase d2 is characterized by e w trending open folds involving the upper devonian strata which was the products of n s shortening caused by the assembly of the qilian ocean and the north china craton ncc at the end of devonian the third phase d3 is indicated by some wnw trending jura type folds in the carboniferous strata formed by nne ssw shortening during the late triassic early jurassic related to the collision between the ncc and the south china block the fourth phase d4 is inferred by nne trending shortening structures in the jurassic and its underlying strata generated by wnw ese shortening in the late jurassic echoing the paleo pacific subduction the fifth stage d5 is characterized by the ne sw shortening in the late miocene leading to the formation of many nw se striking thrust faults in the miocene and its underlying strata because the outward expansion of the tibetan plateau

earthquake detection probability within a seismically quiet area application to the bruchsal geothermal field in applications such as oil and gas production deep geothermal energy production underground storage and mining it is common practice to implement local seismic networks to monitor and to mitigate induced seismicity for this purpose it is crucial to determine the capability of the network to detect a seismic event of predefined magnitude in the target area the determination of the magnitude of completeness of a network is particularly required to properly interpret seismic monitoring results we propose a method to compute the detection probability for existing local seismic networks which i strictly follows the applied detection sequence ii estimates the detection capability where seismicity has not yet occurred and iii delivers the results in terms of probabilities the procedure includes a calibration of a local magnitude scale using regional earthquakes recorded by the network and located outside the monitored area it involves pre processing of the seismograms recorded at each station as performed during the triggering sequence which is assumed based on amplitude thresholds then the calibrated magnitude distance amplitude relations are extrapolated at short distances and combined to reproduce the network detection sequence this generates a probability to detect a seismic event of a given magnitude at a specified location this observation based approach is an alternative to a fully theoretical detection capability modelling and includes field conditions seismic wave attenuation by geometrical spreading and intrinsic attenuation site effect and instrumental responses are partly accounted for by the calibration we apply this procedure on the seismic network deployed in the bruchsal geothermal field germany although the system was in good working order no induced seismicity was identified in the area between june 2010 when monitoring started and november 2012 the recording of distant seismicity during this time period however allowed the application of the proposed procedure according to the applied network detection parameters the results indicate that the absence of seismicity can be interpreted as a 95 probability that no seismic event with ml 0 7 occurred below the network at 2 4 km depth i e in the geothermal reservoir

a preliminary study of regional seismic discrimination in central asia with emphasis on western china abstract in support of a comprehensive test ban treaty we have started to evaluate regional seismic event discrimination capabilities for central asia emphasizing western china we have measured noise and signal levels of over 260 earthquakes and 26 underground nuclear explosions recorded at the broadband digital station wmq in western china and over 120 earthquakes and five nuclear explosions at station aak in kyrgyzstan the explosions are from the kazakh test site kts and lop nor china the earthquakes are mostly from northwest china event magnitudes measured as either m b or m l range between 2 5 and 6 5 and maximum event station distance is about 1700 km we formed phase spectral cross spectral short period long period and long period ratios to test many possible event discriminants all ratios were corrected for distance effects before forming ratio versus magnitude discrimination plots the most consistent discriminants for separating earthquakes from explosions are p s phase ratios for frequencies above 3 to 4 hz p wave f 1 hz to rayleigh wave 0 03 hz f p s ratios separation between earthquakes and explosions increases with frequency and for the p r ratios separation increases with increasing p frequency the p 3 to 6 hz s 0 75 to 1 5 hz cross spectral ratios also separate earthquakes and explosions these cross spectral ratios may prove quite useful because due to weak high frequency l g signals from explosions the high frequency p l g ratio sometimes cannot be evaluated but the p 3 to 6 hz l g 0 75 to 1 5 hz can be evaluated successfully p spectral ratios 3 to 6 hz 0 75 to 1 5 hz also separate earthquakes and explosions but not quite as consistently as the p s ratios however p spectral ratios may prove useful in cases where path effects block or highly attenuate s n and l g waves rendering the p s ratios unavailable in contrast to nevada test site nts events the l g 3 to 6 hz 0 75 to 1 5 hz spectral ratio does not separate earthquakes and nuclear explosions in central asia this observation may be a source effect as asian explosions are thought to be detonated in highly lithified rocks below the water table while most of the smaller m b for station aak the short period spectral and cross spectral ratios identify a few earthquakes not recorded at wmq from the north pamir region that plot with the lop nor explosions the waveforms from these earthquakes lack surface waves and a distinct l g phase these events which we interpret as having occurred in a subducted slab do separate from the explosions on high frequency f 4 hz p s discrimination plots for station wmq long period r l rayleigh wave love wave ratios separate kts explosions and asian earthquakes however r l ratios fail to identify the large lop nor explosions recorded at station aak because the love waves are as large as the rayleigh waves suggesting that strong tectonic release occurs with these explosions because we have examined only one path at a single station for these explosions source radiation pattern may be influencing this observation

hellenic unified seismological network an evaluation of its performance through snes method summary in this paper we analyse the location performance of the hellenic greek unified seismological network husn by seismic network evaluation through simulation method snes this method gives as a function of magnitude hypocentral depth and confidence level the spatial distribution of the number of active stations in the location procedure and their relative azimuthal gaps and confidence intervals in hypocentral parameters regarding both the geometry of the seismic network and the use of an inadequate velocity model greece is located on a tectonically active plate boundary at the convergence of the eurasian and african lithospheric plates and exhibits a high level of seismicity the husn monitors the seismicity in greek territory from 2007 at present it is composed by 88 seismic stations appropriately distribute in the area of greece the application of the snes method permitted us to evaluate the background noise levels recorded by the network stations and estimate an empirical law that links the variance of p and s traveltime residuals to hypocentral distance the statistical analysis of the p and s traveltime residuals allowed us to assess the appropriateness of the velocity model used by the husn in the location routine process we constructed snes maps for magnitudes of 2 2 5 and 3 fixing the hypocentral depth to 10 km and the confidence level to 95 per cent we also investigated by two different vertical sections the behaviour of the errors in hypocentral parameters estimates as function of depth finally we also evaluated fixing the hypocentral depth to 10 km and the confidence level to 95 per cent the magnitude of completeness through the application of the snes method we demonstrate that the husn provides the best monitoring coverage in western greece with errors that for 2 5 are less than 2 and 5 km for epicentre and hypocentral depth respectively at magnitude 2 5 this seismic network is capable of constraining earthquake hypocentres to depths of about 160 km and more and provides a threshold of completeness down to magnitude 2 for most of greek territory we delineate some seismogenic areas of southern greece that probably are not adequately covered by husn use of the snes technique could optimize upgrades of the network in these areas

frequency dependent attenuation in eastern kazakhstan and implications for seismic detection thresholds in the soviet union abstract the frequency dependent attenuation of regional seismic phases recorded by three stations near the nuclear explosion test site in eastern kazakhstan is estimated by inverting spectra from 21 events with magnitudes between 2 3 and 4 6 at distances between 200 and 1300 km the pn spectra are inverted between 1 and 10 hz and the lg spectra are inverted between 0 5 and 2 5 hz the motivation for this study is that previous estimates of detection capability in the soviet union are based on data recorded in other regions eastern north america and scandinavia and therefore have large uncertainty the data recently recorded in eastern kazakhstan provide an excellent opportunity to compare regional wave propagation and noise characteristics at these sites to conditions assumed in previous detection capability simulations it is found that attenuation in eastern kazakhstan is not much different from attenuation in scandinavia but it is greater than attenuation in eastern north america this implies that estimates of detection thresholds that assume attenuation like that observed in eastern north america will be lower than estimates of detection thresholds that assume attenuation like that observed in eastern kazakhstan or scandinavia however it is not known how well data recorded in eastern kazakhstan represent conditions in other areas of the soviet union

hydrochemical characterization of groundwater in the accra plains of ghana chemical and isotopic analyses are used to characterize and identify the relevant water rock interactions which are responsible for the poor groundwater quality in the accra plains four main water types are identified processes that singly or in combination influence the chemical composition of each water type include halite dissolution carbonate dissolution and precipitation seawater intrusion cation exchange evaporative concentration of solutes and aluminosilicates dissolution these processes contribute considerably to the concentration of major ions in the groundwater stable isotope contents of the groundwater suggest mainly direct integrative recharge a few samples plot along the meteoric seawater mix line which is coincidentally the evaporative line the cl br ratios of some of these are close to 300 confirming marine origin others probably concentrated by evaporation have their cl br ratios significantly lower than 300 groundwater is qualitatively good for drinking purposes only along the foothills of the akwapim togoland ranges