exploration of deep magnetite deposit under thick and conductive overburden with ex component of sotem a case study in china in northern china thick loess strata with low resistivity are widely present and can shield the deep induced electromagnetic field signal and influence the accuracy of deep targets while electromagnetic prospecting is being performed to achieve deep target exploration with high resolution a new technique known as the short offset transient electromagnetic method sotem is applied in this research area this method can be used to perform near source configuration surveys but shows no obvious difference between the deep magnetite deposit and low resistivity layer in this survey area therefore it cannot explore the magnetite deposit directly using the traditional magnetic component hz of sotem the ex component which can more effectively distinguish the high resistivity surrounding rock and overlying low resistivity layer than the hz component is used for exploration first 2d forward modeling and sensitivity analysis of the high resistivity body are carried out to determine the different responses of the ex and hz components respectively moreover to better determine the interface between the surrounding rock and low resistivity layer 1d occam inversion and the generalized inverse matrix inversion method are used furthermore aiming at the static effect of the ex component in field exploration joint inversion of the ex and hz components is carried out for comprehensive interpretation finally a field test is conducted and the drilling results are used for verification the research results can provide an effective geophysical model for the exploration of contact metasomatic magnetite deposits

geographic and seasonal distributions of brunt vaisala frequency and rossby radii in the north pacific and north atlantic abstract long term mean temperature and salinity profiles computed from an edited set of historical hydrographic data have been used to calculate mean profiles of density and brunt vaisala seasonal variations in internal rossby radii are everywhere surprisingly small

seismic reflection and magnetotelluric profiles across the luobusa ophiolite evidence for the deep structure of the yarlung zangbo suture zone southern tibet abstract the luobusa chromite mine in tibet the largest chromite deposit discovered in china to date is hosted in the luobusa ophiolite in the eastern section of the yarlung zangbo suture zone a combined seismic reflection and magnetotelluric survey was carried out on the mantle peridotites in which the chromitite deposits occur in order to investigate their origin and to prospect for other deposits in the area the survey indicates that the luobusa chromitite bearing ultramafic block is a tectonic slice thrust onto the southern margin of the gangdese batholith by multiple south dipping faults the maximum thickness of the ultramafic rock mass is 4 km and it is underlain by the upper crust of the indian plate low resistivity bodies in the upper crust of the luobusa area lie at a depth of up to 5 km and strata at a depth of 7 km cross the yarlung zangbo suture zone at a relatively small angle between depths of 20 50 km there are a number of oval low resistivity bodies the lower parts of which might have been close to the moho at a depth of approximately 65 km near the yarlung zangbo river it is inferred that such low resistivity bodies arise from partially molten bodies in the middle and lower crust and are related to deep tectonic activity and mantle material upwelling in the yarlung zangbo suture zone

an audio magnetotelluric investigation of the otjiwarongo and katima mulilo regions namibia abstractas an additional opportunistic component to the southern african magnetotelluric experiment natural source audio magnetotelluric amt data were acquired during phase iv to investigate the local scale electric conductivity subsurface structure in the otjiwarongo and katima mulilo regions namibia as an aid in locating the installation points for high voltage direct current earth electrodes the study showed that the shallow subsurface of areas containing one measurement site in the otjiwarongo region and three sites in the katima mulilo region have appropriate high conductivities for the optimal placement of the earth electrodes both of the amt surveys are situated close to the edge of the orogenic neo proterozoic damara mobile belt dmb previous studies all suggest the existence of a highly conductive midcrustal zone which correlates well with the spatial location of the dmb two dimensional inverse modeling of the otjiwarongo amt data confirms the existence of the high conductive zone at m

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

integrated geological and geophysical studies for delineation of chromite deposits a case study from tangarparha orissa india in orissa india chromite deposits occur in a ne sw trending belt as discontinuous pods associated with tectonically deformed and metamorphosed ultramafic rocks geological mapping and detailed geophysical survey including gravity magnetic electrical and electromagnetic methods for exploring chromite were conducted in a 5 km2 area at tangarparha located within the belt lithologies include sheared granite quartzofeldspathic gneiss and mafic ultramafic rocks the calculated bouguer anomaly map shows a distinct positive anomaly up to 16 mgal in the northern part of the area indicating the existence of a very high density rock in the subsurface the trend surface analysis technique was applied to the gravity and magnetic data for regional residual separation the 2d and 2 5d forward modelings of the residual gravity anomaly suggest the presence of lithologies with densities higher than mafic ultramafic rocks in the subsurface chromite fragments recovered from pits within the soil cover around t

coesite and clinopyroxene exsolution lamellae in chromites in situ ultrahigh pressure evidence from podiform chromitites in the luobusa ophiolite southern tibet abstract we report in situ petrological evidence of deep mantle origin from podiform chromitites in the luobusa ophiolite southern tibet analytical transmission electron microscopy measurements reveal that chromites in podiform chromitites from the luobusa ophiolite have numerous exsolution lamellae of diopsidic clinopyroxene and coesite which indicate an ultrahigh pressure origin of over 3 gpa 100 km deep the presence of these lamellae coupled with abundant micro inclusions of clinopyroxene requires high solubility of sio2 and cao in the host chromite and suggests a precursor of chromite a cafe2o4 structured high pressure polymorph stable at pressures over 12 5 gpa 380 km deep these nano scale observations and geological occurrence indicate that the mantle peridotite under the tibetan mid ocean ridge was transported from the deep mantle at least 100 km probably more than 380 km deep by the mantle convection it implies that the root of mantle upwelling has a much deeper origin than previously believed

multiple sources of the upper triassic flysch in the eastern himalaya orogen tibet china implications to palaeogeography and palaeotectonic evolution abstract the upper triassic flysch langjiexue group tentatively named the shannan terrane of the eastern himalaya orogen has been tectonically assigned either to the tethys himalaya or the yarlung zangbo suture zone yzsz in this work geochronology of detrital zircon u pb isotope shows that the shannan terrane is characterized by the population of 260 200 ma peak ca 240 ma strongly supporting the view of no affinity to the tethys himalaya the detrital zircons dated as 400 290 ma display relatively positive ehf t values of 5 0 to 15 0 with t dm c ages of 2 6 1 3 ga for the shannan terrane whereas highly negative are of 20 0 to 5 0 for the lhasa terrane indicating that the two terranes have different devonian carboniferous sources numerous cr spinels found in the shannan terrane but not in the lhasa terrane exhibit contents in cr 2 o 3 and cr of mainly 44 100 and 48 95 in tio 2 of 0 01 1 0 and in al 2 o 3 of 5 257 respectively denoting several parent lithologies these differences suggested that the shannan terrane has multiple sources not only from the lhasa terrane but also from oceanic island arc seamount and mid ocean ridge areas as well as likely from greater india and australia considering the early cretaceous diabase dykes within the upper triassic flysch defined to the comei bunbury large igneous province we propose that the langjiexue group could have been deposited on the ocean between india and australia and would have not stopped till the lhasa terrane was separated from australia during the terminal triassic according to the cenozoic deformation and metamorphic history and palaeogeography of the langjiexue group we postulate that the shannan terrane could have been loaded onto the greater india during the middle early cretaceous and subsequently drifted northward to the collision zone of india and asia implying that it does not represent an accretionary prism within the yzsz

modification of the kinematic wave philip infiltration overland flow model in previous work a solution was developed for the kinematic wave overland flow equation with rainfall excess given by philip s infiltration equation subsequent application of this solution to field data from small plots revealed the need for an improved model the principal requirement is the incorporation of depression storage into the rising limb calculation a second requirement is a more precise approximation of the water surface profile at the end of rainfall this improves the falling limb calculations this paper shows the solutions obtained by incorporating these improvements and demonstrates their effects on predicted hydrographs and water surface profiles

compositions of chromite associated minerals and parental magmas of podiform chromite deposits the role of slab contamination of asthenospheric melts in suprasubduction zone environments abstract podiform chromitites in the mantle sections of ophiolites belong to either high cr metallurgical or high al refractory varieties their highly variable compositions are reflected by different cr s 100cr cr al and cr2o3 and al2o3 contents of the chromite falling in the boninitic and morb fields respectively parental magmas of high cr chromitites have higher sc mn co and ni and lower ti v zn and ga concentrations than morb melts their trace element patterns are similar to those of boninites except for ni and zn in contrast high al chromitites have parental magmas characterized by generally flat morb normalized patterns showing slight enrichments in v mn and co and depletion in ni and zn regardless of their compositions both types of chromitites have chondrite normalized platinum group element pge patterns showing enrichment in ipge and depletion in ppge a variety of platinum group minerals are typically present in both types occurring either as euhedral inclusions or along fractures in chromite grains these minerals have a wide span of re os isotopic compositions reflecting a variety of origins there is a diversity of unusual minerals and mineral inclusions associated with podiform chromitites the presence of these minerals suggest that grains of amphibolite plagioclase amphibole and zircon and eclogite coesite kyanite and garnet were present in the magmas from which chromite crystallized multiphase mineral inclusions demonstrate that podiform chromitites form from hydrous mafic magmas in suprasubduction zone environments ssz we propose a new model in which chromitite formation was involved in intra oceanic subduction zones initiated in closing oceanic basins continued subduction carries oceanic and possibly continental crustal materials to deep levels where they are metamorphosed under greenschist amphibolite and eclogite facies conditions the tearing and breakoff of the subducted slab possibly along the transitional contact between amphibolites and eclogites create a slab window through which the underlying asthenosphere rises and melts to generate cr rich mafic magmas these upward migrating magmas pass through the subduction zone and assimilate the subducted slab as a result of slab contamination these magmas become more siliceous more oxidized and more hydrous rapidly triggering chromite crystallization minute grains of chromite are suspended in the upward moving magmas as they migrate through the overlying metasomatized mantle wedge such chromite bearing magmas eventually deposit chromite in magma conduits in the uppermost mantle close to the moho where the upward flow changes from vertical to subhorizontal and velocity is greatly reduced highly reduced and ultrahigh pressure minerals including diamonds are reported in literature both in podiform chromitites and host peridotites of ophiolites some of these minerals in association with host peridotites may have been brought by the uprising asthenosphere at mid oceanic ridges due to the mantle convection it is also possible that some diamonds may have formed in the subducted slab below about 150 km some minerals of subducted slabs are preserved because they are encapsulated in chromite grains where they are protected from the ssz melts some of these ssz mantle wedges are emplaced on land to become podiform chromitite bearing ophiolites

geometry of ophiolites in eastern cuba from 3d inversion of aeromagnetic data constrained by surface geology this study combined geophysical geologic and topographic information to investigate the mayari baracoa ophiolitic belt in eastern cuba a recently developed interpretation technique for 3d inversion of magnetic data was employed to determine the geometry at depth of ophiolitic and other rocks based on measured susceptibilities lithologies were divided into four groups the geophysical data allowed 3d imaging of ophiolites serpentinized peridotites and gabbros as well as sedimentary and volcanic rocks the study verified that both the pinares de mayari plateau and the sagua de tanamo basin have been strongly influenced by tectonic activity the modeling showed evidence of more east west structural deformation of the ophiolite belt than had been previously reported the depth of the depocenter of the sagua de tanamo basin and its rate of subsidence were determined we identified some areas with potential for economic deposits of chromium cobalt and nickel as well as precious metals these were related to the thickness of the peridotite layer the modeling also corroborated the presence of previously mapped faults and revealed other previously unrecognized faults

correlation lengths of meltwater flow through ripe snowpacks colorado front range usa the flowpaths of meltwater through snow is known to be an inhomogeneous process the spatial distribution of meltwater flowing from the bottom of melting snowpacks is the result of horizontal and vertical flow paths within the snowpack the ability to characterize the spatial distribution of these meltwater flowpaths would be useful in developing models of snow melt runoff which could better characterize snow melt hydrographs we analyzed near infrared aerial photos of melting snow using a moving window analysis which can characterize correlation lengths in the reflectance of the snow surface near infrared is sensitive to snow grain size which in turn may indicate higher amounts of meltwater the grains grow faster if the liquid water content is higher correlation lengths were live to seven metres and the probability of finding such correlation lengths increased from 0 22 in may 1997 when the melt had just started to 0 68 by late june when melt was well established liquid water content at the snow surface was sampled with a dielectric sensor at 0 5 m intervals on 100 m 2 and 75 m 2 grids semi variograms showed a sill at five to six metres the liquid water measurements at the snow surface suggest that the correlation lengths derived from the infrared aerial photos represent surface expressions of higher flow density through the melting snowpack a circular array of 16 snow melt lysimeters each with areas of 0 2 m 2 was operated for two years at niwot ridge in the colorado front range variograms indicated that flows were correlated over a distance of five to seven metres these three independent methods all suggest a spatial organization of five to seven metres for flowpaths draining ripe snowpacks in the rocky mountains

roles of mesoscale eddies in the kuroshio paths a high resolution ocean general circulation model is developed to simulate connections between the kuroshio path variations and mesoscale eddy activities as realistically as possible the climatological mean of the modeled kuroshio takes a nearshore nonlarge meander path it is found that the model is capable of simulating two types of nonlarge meander state and a possible version of the large meander state the offshore nonlarge meander is generated through interaction between the kuroshio and an anticyclonic eddy the large meander occurs just after significant intensification of the anticyclonic kuroshio recirculation successive intrusion of anticyclonic eddies from the upstream region is responsible for this process those anticyclonic eddies are advected by the kuroshio from the region northeast of luzon island and increase the upstream kuroshio volume transport on an interannual time scale the cyclonic eddies propagating from the kuroshio extension region on the other hand weaken the kuroshio meander after the merger the kuroshio path variations south of japan thus seem to be closely related to eddy activities in the subtropical gyre system

audiomagnetotelluric characterization of range front faults snake range nevada two controlled source audio magnetotelluric csamt profiles were collected on the eastern flank of the snake range in eastern nevada across geologically complex terrain to investigate the suspected presence of faults along the range front the location of the range bounding faults is not easily determined on geologic grounds because of the presence of extensive young sedimentary cover and overall geologic complexity characterization of the presence location and structural style of the range front faults is critical to assessment of connectivity of groundwater aquifers near the mountain front and in adjacent alluvial basins a total of 48 csamt soundings were recorded along two lines that were chosen to maximize subsurface geologic information two generations of faults were interpreted based on the csamt data an older low angle fault that is cut by a younger more steeply dipping fault lack of deep boreholes in the region required that the subsurface interpretation rely on analogy from surface outcrops within and adjacent to the study area the success of the csamt method as applied in this study hinged on near ideal collection conditions the relatively high contrast in electrical resistivity provided by the rock types involved and well developed geologic conceptual models of the region

mapping chromite deposits with audio magnetotellurics in the luobusa ophiolite of southern tibet the exploration of podiform chromites in the indus yarlong zangbo suture zone of southern tibet has proved difficult in recent decades because most known deposits pinch out and then reappear in the same direction a number of ground based geophysical approaches including gravity magnetic and controlled source audio frequency magnetotelluric methods have been applied to explore for these chromite deposits but have mostly failed to delineate prospective areas we present a successful podiform chromite exploration case history that is based on audio magnetotellurics amt more than 8 000 amt stations were utilized in this study within a 10 km2 area of the ophiolite belt line separations were 80 m or 40 m and station separation was 20 m we implemented bostick conversion and non linear conjugate gradient inversions were implemented for data interpretation while two dimensional 2d resistivity sections and three dimensional 3d resistivity imaging were used to elucidate the inner structure and distribu

mapping deeply buried karst cavities using controlled source audio magnetotellurics a case history of a tunnel investigation in southwest china abstractkarst cavity mapping is attracting great interest from engineering geologists because of its relation to the dangerous geohazards faced during engineering construction ground based geophysical methods still face challenges in karst mapping and concealed karst cavities potentially pose threats to tunnel construction in southwest china given the significant contrast in electrical resistivity between karst cavities and their host rocks geoelectrical methods are widely used for mapping these cavities we have developed a successful case history of mapping karst cavities on a planned railway route using controlled source audio magnetotellurics csamt scalar csamt with frequencies ranging from 0 5 to 8192 hz was used for field data acquisition a full frequency domain apparent resistivity correction method was used for near field corrections electromagnetic array profiling emap filtering was used for topographic and static shift corrections and the bostick conversion was used for data interp

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

a model to couple overland flow and infiltration into macroporous vadose zone most vegetated land surfaces contain macropores that may have a significant effect on the rate of infiltration of water under ponded conditions on the ground surface owing to the small scale variations of the land topography microtopography only portions of the land area may get ponded during the process of overland flow as the macropores transmit water at much higher rates than the primary soil matrix higher macropore activation in ponded areas produces larger effective infiltration rates into the soil therefore overland flow and infiltration into the macroporous vadose zone are interrelated representing the microtopographic variation of the land surface by a simple sine wave function a method was developed to relate the ponding area to the average ponding depth which was determined by overland flow a numerical model coupling overland flow and infiltration into the macroporous vadose zone was developed overland flow was simulated using the st venant equations with the inertia terms neglected a single macropore model was used to simulate the infiltration into the macroporous vadose zone the interaction between overland flow and the infiltration into the macroporous vadose zone was analyzed for a hypothetical watershed the sensitivity analysis revealed that the interaction of macropore flow and overland flow is significant for the conditions tested the macropore flow and the overland flow were found to be more sensitive to the macroporosity and less sensitive to the microtopographic surface variation

on the variation of infiltration into a homogeneous soil matrix containing a population of macropores a population of channels penetrates a soil matrix that is otherwise homogeneous the channels are assumed to be vertical and of circular cross section with constant diameter but channel depths and radii are allowed to vary over the population this paper uses a model based on the green ampt assumptions to predict infiltration rates through the walls and bases of channels resulting in a mathematical description of the way in which infiltration develops in the soil and in which channels fill with water until eventually overland flow is initiated this model takes account of the fact that there may be a random distribution of channels in space expressions for the expected mean and variance of infiltration volumes are derived some example results are given and show that below the matrix wetting front the variance of the change in water content with 30 and 50 channels per square meter may be of the same order as the mean change in water content the presence of channels can lead to significant increases in predicted channel volumes

the northeastward current southeast of okinawa island observed during november 2000 to august 2001 1 in order to estimate the variability of current structure and mean geostrophic volume transport in the southeast of okinawa island the round trip acoustic travel times between sea bottom and surface at nine sites were measured by inverted echo sounder with pressure gauge pies from november 2000 to august 2001 vertical sections of geostrophic velocity are calculated by using vertical profiles of specific volume anomaly derived by the gravest empirical mode method from the pies data in the upper 500 m layer over a slope shallower than 1000 m depth the northeastward current reached up to 60 cm s 1 during the occupation of an anticyclonic eddy and decreased to 15 cm s 1 during the occupation of a cyclonic eddy around the mean of 20 cm s 1 the temporal mean geostrophic volume transport relative to the 2000 dbar level was estimated to be 6 1 sv 1 sv 106 m3 s 1 northeastward in the west of 128 85 e