shear velocity structure of the northern california lithosphere the authors have determined the regional shear velocity structure of the lithosphere beneath the coast ranges and the great valley in northern california from inversion of three fundamental mode rayleigh wave phase velocity curves the dispersion measurements were made along three different paths crossing the coast ranges and great valley roughly perpendicular to the north american pacific plate boundary the three dispersion curves diverge at periods greater than about 20 s phase velocities are systematically higher from the northwest to the southeast along the transform margin inverting the phase velocities for crustal and upper mantle structure shows that this divergence is indicative of a 3 5 increase in the upper mantle shear velocity from the napa great valley region to the diablo range great valley region crustal shear velocities are consistent with the lithologies expected in the middle and lower crust the increase in mantle shear velocity from northwest to southeast is anticipated by a tectonic model for the development of the california transform margin in which asthenospheric material is emplaced at the base of the north american crust in the slab gap south of the mendocino triple junction adjacent to the plate boundary this process creates a subcrustal corridor of cooling asthenosphere whichmore is gradually incorporated in the lithosphere lid they suggest that this causes the observed increase in upper mantle shear velocity away from the triple junction finite difference simulations of rayleigh wave propagation across asthenosphere corridor models produce synthetic phase velocity curves which are similar to the field observations less

a time scale for granitoid emplacement in the archean murchison province western australia by single zircon geochronology abstract in the late archean murchison province of western australia four major granitoid suites intruded into two stratigraphically distinct supracrustal sequences the luke creek and mount farmer groups single zircon u pb ages from five granitoid samples representing three of the four suites were measured by ion microprobe the earliest of the suites the pegmatite banded gneiss intruded luke creek group greenstones at 2919 12 ma 1 the luke creek group and the pegmatite banded gneiss protolith were subsequently deformed and then overlain by mount farmer group greenstones large volumes of monzogranite then intruded greenstones throughout the province two samples from this suite gave crystallization ages of 2704 51 and 2681 6 ma this package of greenstones gneiss and monzogranite was folded and metamorphosed and then intruded by two suites of post folding granitoids two samples from the more southern of these suites gave ages of 2641 5 and 2602 14 ma two additional samples were collected from a tonalite pluton previously assigned to the northern post folding granitoid suite they gave crystallization ages of 2760 8 and 2784 22 ma 1 this tonalite is now interpreted as a subvolcanic pluton associated with the andesite and dacite volcanism which produced the nearby mount farmer group greenstones no xenocrystic zircons with ages significantly in excess of 3 0 ga were found in any of the seven samples implying that little or no mid archean or older rocks are present in the region lower concordia intercept ages and 207 pb 206 pb ages from individual analyses indicate the presence of mid to late proterozoic activity throughout the murchison province concordia intercept ages were calculated using a least squares fit of a plane in three dimensions this approach obviates both the need to assign a common lead isotopic composition and the assumption that no pre recent events have disturbed the population s u pb system a further refinement of the data assessment process involved the use of the internal scan by scan scatter for defining the precision of the individual pb pb isotopic ratios compared with the observed scatter ion counting error estimates were found to frequently underestimate the uncertainties in the 207 pb 206 pb and 204 pb 206 pb ratios

crustal structure of the diablo and gabilan ranges central california a reinterpretation of existing data the compressional wave velocity structure of the crust of the coast ranges of central california was modeled from seismic refraction data reported previously for both the diablo range east of the san andreas fault and the gabilan range west of the fault two alternative velocity models were derived for each range by iterative two dimensional ray tracing each pair of models shows a sedimentary layer and an underlying crust composed of three or four layers for the diablo range the velocities in the sediments range from 2 9 to 4 8 km sec and the depth to basement varies from 0 2 km within the central diablo range to a maximum of 3 4 km north of the livermore valley in the gabilan range the velocities in the sediments overlying the granitic basement are higher north of the gabilan range 3 6 to 4 6 km sec than to the south 2 2 to 3 8 km sec and the depth to basement both north and south of the range is less than 2 km below the sediments and fractured near surface material a resolvable difference in the crustal velocity structure on opposite sides of the san andreas fault indicates compositional differences at depth the upper crust has an average velocity of 5 7 km sec at depths between 3 and 16 3 km in the diablo range and an average velocity of 6 1 km sec at depths between 2 and 9 1 km in the gabilan range the lower crust has an average velocity of 6 9 km sec at depths between 16 3 and 30 km in the diablo range and an average velocity of 6 5 km sec at depths between 10 and 24 km in the gabilan range the models also show that the depth to moho differs by several kilometers between the two ranges in the diablo range models the crust mantle boundary becomes shallower from south to north rising from a depth of 30 to 26 km in the gabilan range models the moho is at a depth of 24 to 26 km depending on the velocity assumed at the base of the crust laboratory measurements of rock velocities and the mapped surface geology allow us to interpret the velocity models in terms of crustal composition we conclude that the diablo range probably consists largely of metagraywacke to a depth of 16 3 km and gabbroic material below this depth the crust of the gabilan range probably consists of granitic material to a depth of 9 1 km and gneissic material below this depth franciscan rocks are not regionally present in the crust of the gabilan range if the gneissic lower crust consists of the same rocks as are found at the surface in the gabilan range then the presumed large scale lateral motion of the salinian block has taken place at or below the crust mantle boundary rather than along a mid crustal slip plane

dipping structure under dourbes belgium determined by receiver function modeling and inversion abstract teleseismic broadband p and s waves recorded at the nars station ne06 dourbes belgium are shown to exhibit strong anomalous particle motion not attributable to instrument miscalibration or malfunction azimuthally varying radial and tangential components have been observed on 38 recordings after vector rotation of horizontal p waves into the ray direction the tangenital p waves attain amplitudes comparable to the radial components from the east with negative polarity and west with positive polarity but tend to be zero in the north and south suggesting major discontinuities in the crust dipping southward the sh wave from the east contains a large spmp phase an s to p conversion at the free surface and then reflected back to the surface from the moho the polarity of this spmp phase presents further evidence for a southward dipping moho we employ ray theory for three dimensionally dipping interfaces to compute the p wave response linear inverse theory with smoothness constraints is applied to the simultaneous inversions of p wave receiver functions for four different backazimuths through the progressive change of interface strike and dip and the inversion of layer shear wave velocities a dipping crustal model that is consistent with both the observed waveforms and results of previous local geophysical surveys has been determined the results suggest a large velocity contrast in the shallow structure near the surface another major interface at a depth of 12 km with dip of 10 and a seismically transparent unit below the interface the interface at a depth of 12 km reportedly emerges at the midi fault 50 km north of the station ne06

crustal structure of the eastern borborema province ne brazil from the joint inversion of receiver functions and surface wave dispersion implications for plateau uplift we investigate the crustal structure of the borborema province of ne brazil by developing 44 s wave velocity depth profiles from the joint inversion of receiver functions and fundamental mode rayleigh wave group velocities the borborema province is located in the northeasternmost corner of the south american continent and represents a portion of a larger neoproterozoic mobile belt that formed during the brasiliano pan african orogeny extensional processes in the mesozoic eventually leading to the separation of africa and south america left a number of aborted rift basins in the continental interiors and episodes of diffuse intraplate volcanism and uplift marked the evolution of the province after continental breakup our velocity depth profiles reveal the existence of two crustal types in the province i the thin crustal type which consists of 30 32 5 km thick crust with an upper layer of 3 4 3 6 km s overlying a lower layer of 3 7 3 8 km s and ii the thick crustal type which consists of a 35 37 5 km thick crust with velocities between 3 5 and 3 9 km s down to 30 km depth and a gradational increase in velocity vs 4 0 km s down to upper mantle depths the crustal types correlate well with topography with the thick crustal type being mainly found in the high standing southern borborema plateau and the thin crustal type being mostly found in the low lying sertaneja depression and coastal cuestas interestingly the thin crustal type is also observed under the elevated topography of the northern plateau we argue that the thick crustal type is rheologically strong and not necessarily related to postbreakup mantle processes as it is commonly believed we propose that extensional processes in the mesozoic stretched portions of the brasiliano crust and formed the thin crustal type that is now observed in the regions of low lying topography leaving the rheologically strong thick crust of the southern plateau at higher elevations the crust making the northern plateau would have thinned and subsided during mesozoic extension as part of a greater sertaneja depression to then experience uplift in the cenozoic and achieve its present elevation

late archean mantle composition and crustal growth in the western superior province of canada neodymium and lead isotopic evidence from the wawa quetico and wabigoon subprovinces abstract neodymium and lead isotopes are presented for late archean rocks from the wawa quetico and wabigoon subprovinces sp in the western superior province ontario canada the isotopic compositions were determined on greenstone volcanic sequences pre tectonic 2 73 2 69 ga trondhjemite tonalite granodiorite ttg suites metasedimentary rocks and post tectonic 2 69 2 67 ga dioritic to granitic plutons in order to characterize the mantle and crustal reservoirs involved in the evolution of the southern part of the western superior province although derived from a depleted mantle almost all the greenstone volcanism and pre tectonic ttg suites record contamination by crustal material as old as 3 2 ga neodymium and lead isotopic compositions of the pre tectonic ttg can be modeled as products of the melting of amphibolitic crust e g subducted basalts contaminated by 1 10 of subducted sediments moreover lead isotopes of ttg suites provide evidence that the isotopic compositions of these subducted sediments varied as a result of the addition of new ca 2 7 ga juvenile material in agreement with neodymium isotopic differences recorded by sedimentary units of the quetico sp 50 50 mix of young old crusts and wawa sp 75 25 young old neodymium and lead isotopes of the post tectonic ree enriched archean sanukitoid suites define an isotopic trend which is distinct from that of the ttg suites and which is interpreted to reflect the importance of crustal assimilation in the evolution of these suites furthemore such plutons found in the wawa sp have more juvenile neodymium and lead isotopic compositions and higher nd contents than those intruded into the quetico and wabigoon sp which have more crustal isotopic signatures and lower nd concentrations this geographical difference could be due either to differences in nd contents of initial archean sanukitoid magmas lesser assimilation in the wawa sp or more probably because the wawa crust is on average more juvenile than the quetico and wabigoon crusts in agreement with the fact that the geographical difference mirrors that recorded by metasedimentary rocks deposited in quetico and wawa sp

delineation of lithosphere structure and characterization of the moho geometry under the himalaya karakoram tibet collision zone using surface wave tomography

pb isotopic composition of late archaean granites and the extent of recycling early archaean crust in the slave province northwest canada initial pb isotopic compositions have been determined for potassium feldspar from ca 2 58 to 2 62 ga plutonic rocks in the southern and central slave province of northwestern canada to evaluate the extent of recycling of ancient crust within the province large differences in initial pb compositions were measured which correlate with geographical areas of the province plutons in the east central part of the province have initial compositions only slightly more radiogenic than estimated mantle values 207pb 204pb 14 8 14 9 and were dominantly deruved from juvenile crustal sources in contrast plutons in the point lake and western contwoyto lake areas of the western slave province have radiogenic compositions 207pb 204pb 15 1 15 2 and indicate significant recycling of pre 3 5 ga crust the pb data support previous interpretations based on nd isotopes for a major isotopic boundary in the central part of the province granites from the southern part of the province near yellowknife have intermediate compositions which indicate 1 the age of the protolith to the granitoids in the yellowknife area is younger than at point lake but older than in the eastern slave or 2 the granitoids in the yellowknife area contain a mixture of an older point lake type component and younger crust the absence of pre 3 2 ga crust in the yellowknife area and lack of evidence for pre 2 8 ga inherited zircons in the yellowknife granitoids favour the former possibility evidence for recycling of ancient crustal sources such as the acasta gneiss is limited to a relatively small area of the west central part of the province suggesting that acasta aged or derived crust is not widespread in the province the marked regionality of isotopic composition may reflect a basement in the western part of the province which is itself a collage of crust of different age being younger ca 3 2 2 9 in the south relative to the point lake region 3 9 3 2 ga

structure of the crust beneath the southeastern tibetan plateau from teleseismic receiver functions abstract southeastern tibet marks the site of presumed clockwise rotation of the crust due to the india eurasian collision and abutment against the stable sichuan basin and south china block knowing the structure of the crust is a key to better understanding of crustal deformation and seismicity in this region here we analyze recordings of teleseismic earthquakes from 25 temporary broadband seismic stations and one permanent station using the receiver function method we find that the crustal thickness decreases gradually from the tibetan plateau proper to the sichuan basin and yangtze platform but that significant intra crustal heterogeneity exists on shorter lateral scales 0 30 are detected beneath the southeastern margin of tibet but in the sichuan basin and southeastern yangtze platform the values are close to the global average combined with high regional heat flow and independent evidence for mid crustal layers of high electric conductivity the large intra crustal s wave velocity reduction 12 19 and the intermediate to high average crustal poisson s ratios are consistent with partial melt in the crust beneath parts of southeastern tibet these results could be used in support of deformation models involving intra crustal flow with the caveat that significant lateral variation in location and strength of this flow may occur

crustal velocities near coalinga california modeled from a combined earthquake explosion refraction profile crustal velocity structure for the region near coalinga california has been derived from both earthquake and explosion seismic phase data recorded along a nw se seismic refraction profile on the western flank of the great valley east of the diablo range comparison of the two data sets reveals p wave phases in common which can be correlated with changes in the velocity structure below the earthquake hypocenters in addition the earthquake records reveal secondary phases at station ranges of less than 20 km that could be the result of s to p wave conversions at velocity interfaces above the earthquake hypocenters two dimensional ray trace modeling of the p wave travel times resulted in a p wave velocity model for the western flank of the great valley comprised of a a 7 to 9 km thick section of sedimentary strata with velocities similar to those found elsewhere in the great valley 1 6 to 5 2 km sec b a middle crust extending to about 14 km depth with velocities comparable to those reported for the franciscan assemblage in the diablo range 5 6 to 5 9 km sec and c a 13 to 14 km thick lower crust with velocities similar to those reported beneath the diablo range and the great valley 6 5 to 7 30 km sec this lower crust may have been derived from subducted oceanic crust that was thickened by accretionary underplating or crustal shortening

seismic tomography and deformation modeling of the junction of the san andreas and calaveras faults local earthquake p traveltime data is inverted to obtain a three dimensional tomographic image of the region centered on the junction of the san andreas and calaveras faults the resulting velocity model is then used to relocate more than 17 000 earthquakes and to produce a model of fault structure in the region these faults serve as the basis for modeling the topography using elastic dislocation methods the region is of interest because active faults join it marks the transition zone from creeping to locked fault behavior on the san andreas fault it exhibits young topography and it has a good spatial distribution of seismicity the tomographic data set is extensive consisting of 1445 events 96 stations and nearly 95 000 travel time readings tomographic images are resolvable to depths of 12 km and show significant velocity contrasts across the san andreas and calaveras faulls a low velocity zone associated with the creeping section of the san andreas fault and shallow low velocity sediments in the southern santa clara valley and nonhem salinas valley relocated earthquakes only occur where vp s kmls and indicate that portions of the san andreas and calaveras faults are non vertical although we cannot complctcly cxclude the possibility hat all or parr of this rcsults from ray uacing problems the new dips are more consistent with geological observations that dipping faults intcrscct thc surfacc where surface traces have been mapped the topographic modeling predicts extensive subsidence in regions characterized by shallow low velocity material presumably the result of recent sedimentation some details of the topography at the junction of the san andreas and calaveras faults are not consisknl with lhe modeling results suggesting that the current position or this triple junction has changed with time the model also predicts those parts of the fault subject to contraction or extension perpendicular to the fault strike and hence the sense of any dip slip component in each locality the relative vertical motion accross the fault is consistent with the fault dips found with the new hypocentral locations

the crustal structure of the axis of the great valley california from seismic refraction measurements abstract in 1982 the u s geological survey collected six seismic refraction profiles in the great valley of california three axial profiles with a maximum shot to receiver offset of 160 km and three shorter profiles perpendicular to the valley axis this paper presents the results of two dimensional raytracing and synthetic seismogram modeling of the central axial profile the crust of the central great valley is laterally heterogeneous along its axis but generally consists of a sedimentary section overlying distinct upper middle and lower crustal units the sedimentary rocks are 3 5 km thick along the profile with velocities increasing with depth from 1 6 to 4 0 km s the basement upper crust consists of four units 1 1 a 1 0 1 5 km thick layer of velocity 5 4 5 8 km s 2 2 a 3 4 km thick layer of velocity 6 0 6 3 km s 3 3 a 1 5 3 0 km thick layer of velocity 6 5 6 6 km s and 4 4 a laterally discontinuous 1 5 km thick layer of velocity 6 8 7 0 km s the mid crust lies at 11 14 km depth is 5 8 km thick and has a velocity of 6 6 6 7 km s on the northwest side of our profile the mid crust is a low velocity zone beneath the 6 8 7 0 km s lid the lower crust lies at 16 19 km depth is 7 13 km thick and has a velocity of 6 9 7 2 km s crustal thickness increases from 26 to 29 km from nw to se in the model although an unequivocal determination of crustal composition is not possible from p wave velocities alone our model has several geological and tectonic implications we interpret the upper 7 km of basement on the northwest side of the profile as an ophiolitic fragment since its thickness and velocity structure are consistent with that of oceanic crust this fragment which is not present 10 15 km to the west of the refraction profile is probably at least partially responsible for the great valley gravity and magnetic anomalies whose peaks lie about 10 km east of our profile the middle and lower crust are probably gabbroic and the product of magmatic or tectonic underplating or both the crustal structure of the great valley is dissimilar to that of the adjacent diablo range suggesting the existence of a fault or suture zone throughout the crust between these provinces

response of the ganges dispersal system to climate change a source to sink view since the last interstade the forcing of sediment dispersal systems by climate is widely accepted but there remains disagreement on the general responses to climate change such as how resulting signals propagate downstream and ultimately how they are preserved in the stratigraphic record towards a better understanding of these issues this paper presents a comprehensive overview of ganges dispersal system behavior since the last interstade which reveals major coeval responses to changes in the south asian monsoon at 3000 km from its mountain headwaters to deep sea fan the ganges dispersal system is immense and yet shows tight coupling between source area catchment basin and coastal and marine depocenters furthermore system wide responses and downstream transfer of the signal appear to occur at least as fast as current temporal resolution of 1 2 ky this tight linkage of source to sink components is considered a function of the southwest summer monsoon s overwhelming control on regional hydrology about 80 of the ganges discharge and 95 of its sediment load are delivered to the margin during only 4 months making the system extremely sensitive to this seasonal forcing in addition the regional scale and distribution of the monsoon weather system means that changes in atmospheric circulation affect the entire drainage basin rather than local subcomponents finally despite its present intensity strength of the summer monsoon has varied significantly over the past 150 ky under orbitally driven changes in insolation and global glacial boundary conditions the resulting changes in precipitation both well above and below modern values have forced system wide responses ranging from glacier advance retreat to sediment fluxes to the margin and deep sea fan the overall conclusions are 1 that this immense dispersal system responds to multimillennial scale 104 years climate change in a system wide and largely contemporaneous manner and 2 that major sedimentary signals can be transferred rapidly from source to sink with little apparent attenuation furthermore these acute responses to climate change have produced sedimentary stratigraphic features that diverge from traditional sequence models in their nature and timing

moho orientation beneath central california from regional earthquake travel times table al is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 011 2 50 payment must accompany order analysis of relative pn travel times recorded by 238 stations of the u s geological survey central california seismic network calnet from 77 regional earthquakes and explosions shows that arrivals are progressively delayed to the northeast in both the coast ranges and sierra nevada foothills by dividing the calnet into 10 subarrays and assuming laterally uniform velocities for a crust over an upper mantle half space the orientation of a planar moho beneath each subarray is calculated by inversion of relative pn arrival times the results show that the moho dips to the northeast between 3 and 6 beneath the coast ranges and 8 17 beneath the great valley and is horizontal beneath the foothills pn residuals west of the san andreas fault are relatively early by 0 35 s but are interpreted as resulting from a lateral velocity contrast across the fault rather than variations in crustal thickness unexplained pn travel time variance improves by approximately 60 after correction for the travel time due to dip and mean pn residual at each station the pn data corrected for the travel time due to a dipping moho indicate an isotropic upper mantle velocity of 7 97 0 05 km s beneath the coast ranges great valley and foothills evidence of upper mantle velocity anisotropy in the azimuthal variation in pn apparent velocity corrected for dip is only marginally significant at the 5 level however large azimuthal variations in corrected apparent velocity do exist and may be attributed to velocity heterogeneities in the crust unmodeled higher order mantle structure and gradients in upper mantle velocity arrival times of cape mendocino earthquakes recorded in the coast ranges south of san francisco bay indicate lateral variations in upper mantle velocity the 7 4 km s apparent velocity estimate from travel times of the oroville earthquake recorded in the coast ranges out to distances greater than 400 km is anomalous in that pn is normally observed at distances greater than 200 km these arrivals may originate from updip propagation along a 6 9 km s layer in the lower crust of the coast ranges subparallel to the moho

soil development in till of various ages in northeastern pennsylvania abstract eleven well drained soils formed in till parent materials of varying ages in northeastern pennsylvania were studied to determine changes in the soils with time four profiles three lackawanna and one bath were formed in woodfordian till 15 000 yr b p and two leck kill were formed in altonian till 28 000 75 000 yr b p in these soils the extractable iron oxide extractable aluminum oxide and kaolinite contents increase with age as do the total clay and fine total clay ratio with increasing age the maximum accumulation of these constituents is found deeper in the profile the extractable silicon oxide distribution is constant with depth but it decreases in overall amount with time gibbsite is found only in small amounts in the a horizon of altonian soils but occurs throughout the profile of pre wisconsinan soils although only in small amounts in general differences were found in these soils which separated them into three groups representing varying degrees of soil development a regression equation was derived to predict the age of soils formed from the altonian till based on a clay accumulation index value for soils of known woodfordian and holocene ages the equation log y 1 80 0 992 log x best fit the data with an r 2 value of 0 913 using this equation a mean age of 41 000 yr was calculated for the altonian soils this date was used to derive a second equation to predict ages for pre wisconsinan soils the equation with the highest r 2 value 0 934 was log y 1 81 0 998 log x dates for soils developed in the white deer till and the laurelton till of the pre wisconsinan stage were calculated to be 86 000 and 91 000 yr b p respectively these dates fall within ages estimated for the sangamon interglaciation and thus would appear to be too young for pre sangamonian materials the probable reason for the too young age is that the c horizon material of the pre wisconsinan soils was weathered and did not provide an accurate estimate of clay accumulation for the prediction equation

ophiolitic basement to a forearc basin and implications for continental growth the coast range great valley ophiolite california we present a compilation of 18 published models from the length of the great valley forearc basin california based on seismic reflection borehole seismic refraction gravity and aeromagnetic data to address long standing questions about the nature of the basement to the great valley its origin its tectonic history and it s mechanism of incorporation into the north american continental margin the geophysical models permit a 700 km long 70 km wide complete ophiolite sequence beneath the entire great valley in the northern great valley the ophiolite is overlain by ophiolitic breccia the ophiolite crust is 7 8 km thick and the mantle section is mostly unserpentinized beneath the southern great valley there is no ophiolitic breccia the crust may be up to 10 12 km thick and the mantle section if present at all is serpentinized to such a degree that it cannot be distinguished from sierran basement or mafic ophiolite members on the basis of velocity or density data geochemical petrological and paleomagnetic data support suprasubduction zone ophiolite formation at north american paleolatitudes and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc in the north this was apparently followed by obduction of back arc crust onto older continental basement during the late jurassic nevadan orogeny in the south the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the nevadan orogeny but were not obducted onto it instead the arc and back arc docked with the continental margin leaving the arc itself to become the basement to the great valley basin cretaceous sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc irrespective of the detailed accretionary history our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km km 1 in less than 10 myr

group velocity dispersion characteristics and one dimensional regional shear velocity structure of the eastern indian craton abstract in the past three years a semi permanent network of fifteen 3 component broadband seismographs has become operational in the eastern indian shield region occupying the archean 2 5 3 6 ga singhbhum odisha craton soc and the proterozoic 1 0 2 5 ga chotanagpur granitic gneissic terrane cggt the reliable and accurate broadband data for the recent 2015 nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one dimensional regional shear velocity structure of the region first we measure fundamental mode rayleigh and love wave group velocity dispersion curves in the period range of 7 70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern indian craton eic we observe that group velocities of rayleigh and love waves in soc are relatively high in comparison to those of cggt this could be attributed to a relatively mafic rich crust mantle structure in soc resulting from two episodes of magmatism associated with the 1 6 ga dalma and 117 ma rajmahal volcanisms the best model for the eic from the present study is found to be a two layered crust with a 14 km thick upper crust uc of average shear velocity vs of 3 0 km s and a 26 km thick lower crust lc of average vs of 3 6 km s the present study detects a sharp drop in vs 2 to 3 at 120 260 km depths underlying the eic representing the probable seismic lithosphere asthenosphere boundary lab at 120 km depth such sharp fall in vs below the lab indicates a partially molten layer further a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88 103 km depths suggesting a 88 103 km thick thermal lithosphere below the eic this could also signal the presence of small amounts of partial melts thus this 2 3 drop in vs could be attributed to the presence of partial melts in the upper mantle related to the earlier volcanic episodes viz back arc volcanism associated with the archean proterozoic subduction 1 6 ga dalma volcanism and 117 ma rajmahal volcanism the main result of our modeling provides evidences for the absence of keel or thick lithosphere below the eic

a micromorphological record of contemporary and relict pedogenic processes in soils of the indo gangetic plains implications for mineral weathering provenance and climatic changes

relationships between topography and spatial variations in groundwater and soil morphology within the taoyuan hukou tableland northwestern taiwan this study examines the relationships between topography and spatial variations in groundwater and soil morphology within the multi step taoyuan hukou tableland in northwest taiwan several different geomorphological and pedological approaches were jointly undertaken including analyses of a digital elevation model dem a reclassified soil map field survey results engineering borehole data and a hydrological simulation five major geomorphic surfaces were identified based on terrace scarps observed in the dem minor surfaces separated by small fluvial scarps or tectonic scarps were also mapped the simplified soil map and field study of soil morphology reveal a number of soil distribution patterns associated with specific topographic settings soil morphology changes from homogeneous red soils in the fore edge of surfaces to mottled soils and finally to gley soils in back edge regions such a pattern is more pronounced in the proximal part of a surface while toward the distal zone the patterns tend to be obscured by increasing amounts of mottled and gley soils the proportion of homogeneous soils increases with narrowing of the surface for fan shaped surfaces soil types appear to be dominated by mottled soils except for at terrace edges where homogeneous red soils occur while homogeneous soils dominate tectonically elevated surfaces a toposequence of homogeneousmottled gley soils occurs in the surrounded lowlands borehole data and hydrological models indicate that these soil distribution patterns are strongly related to groundwater occurrence and fluvial processes both of which are generally governed by topography despite the observed pedodiversity in this area the common feature of these toposequences i e the distribution of homogeneous red soils along the surface fore edge may serve as a basis for establishing a reliable soil chronosequence for the area 2007 elsevier b v all rights reserved

shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains 1 the nature of the lower crust across the southern african shield has been investigated by jointly inverting receiver functions and rayleigh wave group velocities for 89 broadband seismic stations located in botswana south africa and zimbabwe for large parts of both archaean and proterozoic terrains the velocity models obtained from the inversions show shear wave velocities 4 0 km s below 20 30 km depth indicating a predominantly mafic lower crust however for much of the kimberley terrain and adjacent parts of the kheis province and witwatersrand terrain in south africa as well as for the western part of the tokwe terrain in zimbabwe shear wave velocities of 3 9 km s are found below 20 30 km depth indicating an intermediate to felsic lower crust the areas of intermediate to felsic lower crust in south africa coincide with regions where ventersdorp rocks have been preserved suggesting that the more evolved composition of the lower crust may have resulted from crustal reworking and extension during the ventersdorp tectonomagmatic event at c 2 7 ga citation kgaswane e m a a nyblade j julia p h g m dirks r j durrheim and m e pasyanos 2009 shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains j geophys res 114 b12304 doi 10 1029 2008jb006217

lead isotope study of the late archean lac des iles palladium deposit canada enrichment of platinum group elements by ponded sulfide melt lead isotopic compositions of mineral separates are presented from the mineralized zones roby twilight and high grade zones of the lac des iles palladium deposit approximately 2 69 ga and tonalitic country rocks approximately 2 727 to 2 775 ga in the wabigoon subprovince of the western superior province of canada plagioclase separates show pb isotope ratios similar to those of the late archean depleted mantle sulfide minerals coexisting with plagioclase have a more radiogenic pb isotope composition but very low concentrations of u 0 2 ppb u and th 0 01 ppb compared to pb 50 ppb the isotope data are consistent with a model involving the incorporation of radiogenic pb from country rocks to sulfide melt near the base of the magma chamber or along the conduit of the parental magmas the proposed interpretation is also supported by decreasing cu pd ratios from earlier barren to late fertile gabbro intrusions in the complex the ponding sulfide melt became enriched in platinum group elements as it was interacting with a large magma volume during the evolution of the lac des iles igneous complex

reappraisal of hypothetical franciscan thrust wedging at coalinga implications for tectonic relations along the great valley flank of the california coast ranges 1 correlation of the character and orientation of seismic reflectors imaged on profiles across the coalinga anticline with known surface geology indicates that local franciscan thrust wedging favored for past geodynamic interpretations of the 1983 coalinga earthquake is an untenable hypothesis the inference that a subhorizontal detachment surface of simple shear is present beneath the anticline is consequently unnecessary and unlikely in view of the coherent areal pattern of wrench folding within the southern diablo range elsewhere along the structural front of the california coast ranges at the western flank of the great valley interpretations of similar thrust wedging based in part on analogy with supposed structural relations at coalinga are equally questionable in a wider context the seismogenic behavior of blind thrust and reverse faults in california cannot be interpreted with the invalid concept of thrust wedging at coalinga as a guiding template for geotectonic interpretations

archaean hydrothermal zircon in the abitibi greenstone belt constraints on the timing of gold mineralisation abstract hydrothermal zircons have been found in archaean mesothermal au veins and altered wallrock selvages at val d or in the abitibi greenstone belt of canada the zircons are paragenetically associated with vein quartz tourmaline mica carbonate scheelite pyrite and gold zirconium mobility and the consequent occurrence of hydrothermal zircon may be associated with the intense tourmalinisation characteristic of archaean gold deposits in this district the shrimp ion microprobe has been used to analyse hydrothermal zircons from four separate mines spatially associated with the bourlamaque batholith and has yielded ages constraining formation of the au bearing vein systems to within 20 ma of emplacement of the pluton the ion microprobe data reveal multiple stages of hydrothermal zircon growth in the vein systems contemporaneous with the regional metamorphic peak and late kinematic activity along regional structures younger including proterozoic ages previously obtained for the veins using other minerals and isotopic schemes must reflect either alteration or renewed mineral growth during much later reactivation of fluids along the same structures up to 400 ma after initial formation of the veins

fluid temperature and salinity characteristics of the matagami volcanogenic massive sulfide district quebec fluid inclusions hosted within various lithologic units of the 40 million metric ton mt matagami district abitibi greenstone belt preserve samples of archean volcanogenic massive sulfide vms and postvolcanogenic massive sulfide hydrothermal fluid microthermometric measurements on ore hosted primary two phase liquid vapor inclusions from matagami s south limb deposits indicate that the vms hydrothermal fluid was highly saline 16 2 4 7 wt nacl cacl2 equiv 1 n 230 and of moderate temperature trapping temperature tt 208 32 c 1 n 230 a fluid with these characteristics is capable of transporting 5 10 4 m 30 ppm zn as zncl3 and zncl4 2 chloride complexes however the low temperature of this fluid precluded efficient cu transport 3 ppm which may in part explain the relatively cu poor nature of the matagami deposits calculated densities of this ore fluid as high as 1 10 g cm3 are consistent with a bottom hugging brine model however a subset of the data indicate a fluid less dense than ambient seawater suggesting that buoyant hydrothermal plumes were also present a microthermometrically determined high cacl2 content xnacl 0 55 molar na ca 2 3 1 for the vms ore hosted primary fluid is consistent with an archean hydrothermal fluid more ca rich than modern day seawater quartz epidote veins located in the hydrothermal cracking zone of the bell river complex host primary liquid vapor halite inclusions these inclusions are interpreted to be samples of the deep seated equivalent to the vms ore hosted hydrothermal fluid described above microthermometry indicates that these inclusions trapped a high temperature brine tt 373 44 c 1 n 92 38 2 1 9 wt nacl equiv 1 n 92 we interpret this brine to be a phase separated product of modified model seawater 3 2 wt nacl an exsolved magmatic fluid or a combination thereof deep within the hydrothermal system at 650 to 670 c and a near lithostatic pressure of 90 mpa phase separation and subsequent convection lowered the temperature of the brine prior to its entrapment within the hydrothermal cracking zone the occurrence of high temperature brine overlain by lower temperature salinity fluid suggests a two cell convection model consistent with metal mass balance calculations for the south limb the high salinity of the ore hosted fluid inclusions indicates two possibilities 1 a significant amount of brine was incorporated into the upper cell and mixed with heated seawater during convection 2 archean seawater itself was very saline and of variable salinity with the cooling of the bell river complex lower temperature fluids dominantly of seawater origin circulated deep within the hydrothermal system modified by water rock interaction yet not phase separated these fluids sealed off the remnant permeability of the fracture network of the hydrothermal cracking zone and were locally trapped as secondary liquid vapor fluid inclusions homogenization temperature th 242 17 c 9 1 1 6 wt nacl equiv 1 n 14 hosted within the bell river complex quartz epidote vein material post and or waning stage vms hydrothermal activity is evident from the presence of quartz epidote veins crosscutting wabassee group hanging wall rocks microthermometry on quartz hosted primary liquid vapor fluid inclusions suggests that this activity occurred at relatively low temperatures th 76 177 c n 212 over a wide range of salinity 6 0 32 4 wt nacl cacl2 equiv n 212 and with a high apparent cacl2 content xnacl 0 06 these fluid inclusion data illustrate the importance of subsea floor chemical and physical processes directly related to metal transport and deposition in vms systems in particular phase separation deep within the hydrothermal system is interpreted as a key process for generating saline brines capable of forming significant ore deposits

crustal velocity anomalies in costa rica from ambient noise tomography we derive group velocity maps for crustal rayleigh waves across costa rica and corresponding 3 d shear wave velocity structure from ambient noise cross correlations between 56 seismic broadband stations the daily inter station cross correlations for the period 2010 2015 of 56 seismic broadband stations are stacked and analysed to warrant a robust extraction of empirical green s functions which then are used to measure fundamental mode rayleigh wave group velocities rayleigh wave dispersion curves show consistent patterns within the different geological domains in costa rica dispersion curves were evaluated in the microseism band from 5 to 17 s period and inverted for group velocity maps using iterative nonlinear travel time tomography the group velocities at each grid point were inverted for 1 d profiles using a non linear simulated annealing approach and transformed into the 3 d velocity structure the final tomographic model shows clearly the main velocity anomalies associated with tectonic and volcanic activity in costa rica three localized negative velocity anomalies are seen at all periods 5 17 s consistent with deep routed crustal scale magmatic systems located beneath the rincon de la vieja miravalles arenal poas and turrialba irazu volcanic systems that showed activity over the last 100 years high velocities can be found beneath the talamanca arc in southeastern costa rica where active volcanism stopped in the late miocene significant along strike variations in the morphology of the subducting cocos plate are imaged consistently by velocity variations in the forearc

three dimensional modeling of intraplate stresses in the epicentral zone of the 21 may 1997 jabalpur earthquake of mw 5 8 central india abstract three dimensional intraplate stresses induced by topography and crustal density heterogeneities have been modeled for a region in central india latitude 21 26 n longitude 77 82 e that was affected by the 1927 son valley m w 6 4 and the 1997 jabalpur m w 5 8 earthquakes the modeled stresses show that these two events occurred in the zones of significant increase in gradients of the principal stress difference 1 3 contours the highest value of these differential stresses 90 mpa is obtained at 5 km depth beneath the epicentral zone of 1927 son valley earthquake whereas a value of 35 mpa is obtained at the focal depth 35 km of the 1997 jabalpur mainshock further depth sections of estimated differential stresses delineate a south dipping high stress body penetrating the whole crust probably a mafic intrusive intruded during the deccan volcanism episode beneath a zone bounded by narmada north fault in the north and narmada south fault in the south within which all aftershocks including mainshocks of the 1997 jabalpur and the 1927 son valley have occurred the modeled direction of principal stresses as well as inferred mode of faulting at 5 20 35 and 40 km depths correlates strongly with the long period 20 50 s centroid moment tensor solutions for the broadband recordings of the m w 5 8 jabalpur mainshock results suggest that estimated large intraplate stresses associated with an inferred mafic intrusive in the presence of high pore pressure resulted from the dehydration of the serpentinite minerals and deep brittle ductile transition may contribute to the failure of rock along the pre existing weak narmada south fault leading to the earthquakes in the deep crust it is proposed that the causal association of large intraplate stresses and the pre existing deep crustal faults would control the future locales of earthquakes in the narmada son lineament

active tectonics and earthquake potential of the myanmar region this paper describes geomorphologic evidence for the principal neotectonic features of myanmar and its immediate surroundings we combine this evidence with published structural geodetic and seismic data to present an overview of the active tectonic architecture of the region and its seismic potential three tectonic systems accommodate oblique collision of the indian plate with southeast asia and extrusion of asian territory around the eastern syntaxis of the himalayan mountain range subduction and collision associated with the sunda megathrust beneath and within the indoburman range and naga hills accommodate most of the shortening across the transpressional plate boundary the sagaing fault system is the predominant locus of dextral motion associated with the northward translation of india left lateral faults of the northern shan plateau northern laos thailand and southern china facilitate extrusion of rocks around the eastern syntaxis of the himalaya all of these systems have produced major earthquakes within recorded history and continue to present major seismic hazards in the region

a seismic refraction profile across the san andreas sargent and calaveras faults west central california in 1981 the united states geological survey recorded a seismic refraction profile across the southern santa cruz mountains in west central california to examine the shallow velocity structure of this seismogenic region this 40 km long profile which consisted of three shotpoints extended northeastward from near watsonville california to coyote lake crossing the san andreas sargent and calaveras faults this entire region is characterized by a highly heterogeneous upper crust west of watsonville 1 km of alluvium with a velocity of 2 12 km sec overlies a basement with a velocity 5 45 km sec the abrupt deepening of basement by 1 5 km just east of watsonville at a subsurface fault suggests that the zayante fault to the north and the vergeles fault to the south are connected the tertiary sediment at the san andreas fault is 2 5 km thick and has a velocity of 3 34 km sec the san andreas fault is not marked by any distinctive seismic velocity features whereas a 1 km wide low velocity zone is found at both the sargent and calaveras faults east of the sargent fault the basement surface forms a broad anticlinal structure with velocities ranging from 5 4 to 6 0 km sec from the anticlinal crest basement dips to the east beneath the santa clara valley and reaches a maximum depth of 1 km on the east side the overlying alluvium has a velocity of 2 7 km sec at the crest of the basement anticlinal structure a vertical low velocity zone coincident with exposed serpentine provides strong evidence of faulting

lead lead age of komatiitic lavas and limitations on the structure and evolution of the precambrian mantle abstract using lead isotopes we have studied komatiites and associated sulfides from three complexes at barberton the lead lead isochron gives an age of 3 46 0 07gy in good agreement with the sm nd age at munro township three distinct komatiitic flows yield significantly different ages pyke hill is 2 72 0 02gy theo s flow is 2 47 0 13gy and fred s flow is 2 58 0 02gy old 2 72 gy is the emplacement age while the alignment obtained on theo s flow and fred s flow clearly points out the effect of a metamorphism posterior to 2 7 gy at cape smith the age obtained by the lead lead method is 1 6 0 13gy this age is younger than that obtained by other methods in contrast the isochron obtained on sulfide gives an age of 1 88 0 17gy in agreement with sm nd methods these results together with others from the literature are used to describe the evolution of pb in the mantle the important result inferred is that the mantle 2 70 gy ago was very different from the primitive mantle according to both th u and u pb values indeed the latter are lower than the planetary values this evolution is the result of the formation of the continental crust

a seismic refraction study of the santa clara valley and southern santa cruz mountains west central california abstract two seismic refraction profiles were recorded in the santa clara valley region of central california to determine the upper crustal seismic velocity structure a reversed 8 km long profile was recorded across the valley near gilroy and an unreversed 38 km long profile was recorded in the southern santa cruz mountains the data indicate that the valley is wedge shaped in cross section with the basement dipping 10 to the east the alluvial fill of the valley has a maximum thickness of 1 5 km and has an average compressional wave velocity of 2 3 km sec the top of the basement has a velocity of 4 3 km sec the velocity reaches 6 0 km sec at a depth of 2 km the calaveras fault zone located due east of the valley was crossed by a short unreversed profile it appears to be characterized by a low velocity v p 2 7 km sec to a depth of 2 5 km the southern santa cruz mountain profile indicates a 4 6 km sec near surface refractor and a layer with a velocity of 6 1 km sec at 2 km depth thus considered together the similarity in velocity structure determined from the two profiles indicates that the franciscan rocks of the santa cruz mountains continue beneath the valley the presence of high compressional wave velocities 6 0 km sec at 2 km depth in this franciscan terrain can best be explained by an abundance of metabasalts greenstones at that depth these seismic observations indicate that an area of at least 120 km 2 is underlain by this high velocity basaltic material therefore the upper crustal structure in the study area differs dramatically from the franciscan terrain of the diablo range where greywackies and metagreywackies v p 5 4 km sec dominate in the uppermost crust

soils of the indo gangetic plains a pedogenic response to landscape stability climatic variability and anthropogenic activity during the holocene abstract bound by the himalaya in the north and the craton in the south the indo gangetic plains igp is one of the largest fluvial plains of the world the igp is monotonously flat with a spread of surface soils in hot arid conditions of rajasthan in west to per humid conditions in west bengal the soil geomorphology of the igp detailed during the last few decades is useful in determining the interrelationship among the pedogenesis the climate and the landscape evolution during the holocene these studies demonstrate that the igp soils developed on five geomorphic surfaces with varying degree of development soils occurring on older geomorphic surfaces 2 5 ka are polygenetic with a distinct record of climatic changes and neotectonics the present synthesis is based on recent developments in pedology achieved through macro and micromorphology clay mineralogy pedogenic calcrete and polygenetic pedogenic features of the igp soils a critical evaluation of the igp soils has helped to comprehend the subtle nuances of the pedogenic processes that were also influenced by anthropogenic activities and cultivation over this vast agricultural tract during the holocene we provide state of the art information on the pedology polygenesis and soil degradation natural and anthropogenic over the last 10 ka the review has potential as a reference for critical assessment of the pedosphere for health and quality in different parts of the world in addition it facilitates developing a suitable management practices for the food security in the 21st century

two dimensional velocity structure along the synclinal axis of the great valley california abstract in 1981 and 1982 the u s geological survey conducted a seismic refraction study of the great valley central california in order to obtian a better understanding of the deep crustal structure and tectonic evolution of the region we present the interpretation of a 147 km long seismic refraction profile recorded along the synclinal axis of the valley the northern 98 km of the profile are reversed whereas the remaining southern portion is unreversed observed travel times and amplitudes of primary and secondary arrivals from three shot points have been modeled using two dimensional ray tracing techniques the resultant p wave velocity cross section consists of a 27 km thick crust whose isovelocity contours generally rise gently to the southeast the crust is described in six layers with average velocities average layer thickness in parentheses of 2 8 km sec 3 km 4 25 km sec 3 km 5 5 km sec 2 km 6 1 km sec 6 km 6 75 km sec 6 km and 7 2 km sec 7 km the second through fifth layers are separated by 1 km thick high gradient transition zone the lower crustal boundary 6 75 7 2 km sec and the moho 7 2 8 1 km sec are modeled with first order discontinuities the first layer cenozoic sedimentary deposits is characterized by a velocity gradient of 0 7 km sec km starting with a velocity of 1 7 km sec at the surface the second layer 4 25 km sec consists of the cretaceous great valley sequence and shows a very modest velocity gradient of 0 2 km sec km the third layer 5 5 km sec basement may consist of metasedimentary rocks of the franciscan assemblage fractured crystalline rocks or possibly serpentinized ophiolite the fourth layer 6 1 km sec apparently has no velocity equivalent 30 km to the west in the diablo range it and the layer beneath it 6 75 km sec may be correlated with the metamorphic and igneous rocks of the eastern foothills of the sierra nevada the basal crustal layer 7 2 km sec is evidently mafic igneous rock an independent interpretation of this profile using the reflectivity method produced a similar model an interpretation of a parallel profile 10 to 15 km east of our line shows a structure similar to our model below 15 km but a thinner sedimentary section and velocities as much as 0 9 km sec faster 6 to 15 km deep the crustal velocity structure of the central great valley is atypical of either continental or oceanic crust but is consistent with the geologic hypotheses that the valley is a thickened marginal basin formed in either a forearc or backarc setting located west of the mesozoic sierra nevadan arc

paleoclimatic data from 74kl and guliya cores new insights we analyze multi proxy climatic records from the 74kl marine core and compare it with the delta 18 o record from the guliya ice core these records have important implications for the evolution of the southwest monsoon system we observe three distinct climatic events viz 19 ka to 13 5 ka 13 5 ka to 10 ka and 3 4 ka to present from the last glacial phase to the present even though the first two events are well documented in the literature our combined analysis of data from both the cores suggests alternative mechanisms which could have complemented and strengthened these events the third most recent event occurring between 3 4 ka to present does not seem to have been well documented earlier we propose a possible mechanism to explain this event we identify representative elements which better capture the above events as compared to more commonly used elements

crustal structure beneath the indochina peninsula from teleseismic receiver functions 1 we analyze teleseismic receiver functions to determine the crustal structure beneath the indochina peninsula which is located immediately south east of the eastern himalayan syntaxis basin we found that the indochina peninsula is characterized by a thin 31 km crust with a low vp vs ratio 1 68 the intra lower crustal low velocity zone lvz is observed beneath the northwestern part of our study region we hypothesize that it is an extension of the lower crustal lvz observed beneath the southeastern tibet and the south china block and that it terminates at the dien bien phu fault dbpf a lvz observed in the upper crust beneath southeast of the dbpf indicates that the crust is ductile and earthquakes are rare citation bai l x tian and j ritsema 2010 crustal structure beneath the indochina peninsula from teleseismic receiver functions geophys res lett 37 l24308 doi 10 1029 2010gl044874

estimation of earthquake source parameters in the kachchh seismic zone gujarat india from strong motion network data using a generalized inversion technique the generalized inversion of s wave spectral amplitude data from 18 strong motion sites in the kachchh seismic zone gujarat india has been carried out to estimate source parameters of 38 aftershocks m w 2 93 5 32 from the 2001 bhuj earthquake m w 7 7 the result shows that the seismic moment m and source radius r of aftershocks are between 3 1 1013 and graphic 1 and 226 889 m respectively while stress drops vary from 0 11 to 7 44 mpa the regression analysis between m and r shows a break in linear scaling at graphic 2 the estimated stress drop values scatter significantly with m graphic 3 for larger aftershocks graphic 4 but vary in proportion with m graphic 5 in case of smaller magnitude events this can be due to the complex rupture process associated with relatively larger aftershocks in comparison to circular rupture for smaller events the high stress drop values are observed for events located on the north wagad fault at 15 30 km depths and gedi fault at 0 5 km depths this could be attributed to the large stress develop at the hypocentral depth as a result of high pore fluid pressure beneath these two fault zones the regression between m and corner frequency f c shows m is proportional to graphic 6 which is consistent for earthquakes with short rupture lengths and high stress drop values high stress drop values commonly associated with the events from kachchh and adjacent narmada son lineament rift zones indicate strong potential for generating high peak ground accelerations and hence high seismic hazard 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif 4 embed inline graphic 4 gif 5 embed inline graphic 5 gif 6 embed inline graphic 6 gif

oolitic beach barriers of the last glacial sea level lowstand at the outer bengal shelf abstract during rv sonne cruise 93 the bengal shelf was surveyed using sediment echosounding side scan sonar and sediment coring techniques a sequence of shallow ridges was discovered each between 100 m and 260 m wide and extending for kilometres parallel to the 120 130 m isobaths the ridges mark the top of a barely covered regressive sediment sequence and consist of oolitic limestone of late pleistocene age they are interpreted as contouring fossil shorelines of sea level lowstands of the last glacial maximum their lithology indicates that fine grained sediment supplied by the ganges brahmaputra river system did not pollute the nearshore environment during glacial times this argues for sediment bypassing of the bengal shelf and a direct supply to the bengal deep sea fan

lithospheric structure across the central tien shan constrained by gravity anomalies and joint inversions of receiver function and rayleigh wave dispersion abstract shear wave velocity structure across the central tien shan orogeny was generated by jointly inverting rayleigh wave phase and group velocity with teleseismic p wave receiver functions at 40 broadband seismic stations the inferred seismic structure was validated by forward modeling of the complete bouguer anomaly data the joint inversion result reveals larger crust thicknesses beneath the kokshaal 68 72 km and kyrgyz ranges 62 64 km while other units have crustal thicknesses between 48 and 58 km a fast velocity layer vs 3 6 3 9 km s in the upper crust is found in some seismic stations within the kazakh shield our models show the presence of high velocity and density layers in the lowermost crust throughout the region consistent with the presence of mafic ultramafic lithologies the large crustal thickness is associated with a thickened mafic layer in the lower crust indicating that the thickened crust may be partly caused by magmatic underplating the low velocity and density anomaly in the middle crust and low upper mantle velocity observed in our model beneath the middle tien shan reflect the presence of partial melt in the crust due to the intrusion of hot mantle material the lack of correlation between moho depth and topography together with the gravity results suggests that the topographic compensation in the central tien shan is not confined to the crust this requires significant support from the mantle to account for the relative high elevation of the middle tien shan

a special issue devoted to abitibi ore deposits in a modern context preface the abitibi greenstone belt which is part of the larger abitibi subprovince in the superior province of the canadian shield e g card and ciesielski 1986 is the largest known archcan greenstonegranite terrane 300 km n s x 400 450 km mineral production and reserves are significant particularly in relation to volcanogenic massive sulfide deposits and structurally controlled archcan goldbearing quartz vein systems the term au quartz vein being used in the sense of lindgren 1933 e g the gold quartz veins of ontario p 662669 volcanogenie massive sulfide production and reserves to 1989 1990 total 424 million metric tons

synmetamorphic lode gold deposits in high grade archean settings lode gold deposits that formed under conditions of the middle to upper amphibolite and granulite facies have been identified in the yilgarn block of western australia these deposits which form a continuum with lower metamorphic grade mesothermal gold depos its show that at least some of the fluids responsible for mesothermal gold deposits originated in or below the middle crust studies of 18 o suggest that fluid compositions were controlled by the wall rocks at least for some elements these more proximal gold deposits provide more information about deep fluid sources than do investigations of higher level mineralization deposited from fluids strongly modified by wall rock interaction during the early part of their ascent

crustal structure of the khartoum basin sudan abstract the crustal structure of the northern part of the khartoum basin has been investigated using data from 3 permanent seismic stations within 40 km of khartoum and two modeling methods h k stacking of receiver functions and a joint inversion of receiver functions and rayleigh wave group velocities the khartoum basin is one of several mesozoic rift basins in sudan associated with the central african rift system results from the h k stacking indicate that crustal thickness beneath the khartoum basin ranges between 33 and 37 km with an average of 35 km and that the crustal vp vs ratio ranges from 1 74 to 1 81 with an average of 1 78 from the joint inversion of receiver functions and rayleigh wave group velocities we obtained similar results for moho depth as well as an average shear wave velocity of 3 7 km s for the crust these results provide the first seismic estimates of moho depth for a basin in sudan when compared to average crustal thickness for unrifted proterozoic crust in eastern africa our results indicate that at most only a few km of crustal thinning may have occurred beneath the khartoum basin this finding is consistent with estimates of effective elastic plate thickness which indicate little modification of the proterozoic lithosphere beneath the basin and suggests that there may be insufficient topography on the lithosphere asthenosphere boundary beneath the sudanese basins to channel plume material westward from ethiopia

crustal architecture of the borborema province ne brazil from receiver function ccp stacks implications for mesozoic stretching and cenozoic uplift abstract we have investigated the crustal architecture of the borborema province of ne brazil by constructing common conversion point ccp receiver function stacks from teleseismic p waveforms recorded at 64 seismic stations in the region the borborema province represents the western portion of a larger neoproterozoic mobile belt that experienced extension in the mesozoic leading to the formation of a number of intra continental rift basins and eventually continental breakup after continental breakup episodes of uplift in the province perhaps related to coeval episodes of cenozoic volcanism helped shape the high topographies of the borborema plateau our receiver function ccp stacks image clear p to s conversions at the crust mantle boundary and confirm independent evidence for a 36 38 km thick crust under the southern portion of the plateau and a thinner 30 32 km thick crust in the surrounding regions including the northern plateau the cross sections also reveal the presence of an intra crustal discontinuity at 9 18 km depth under the regions of thin crust that fades away under the thick southern plateau we argue that the thin crust in the borborema province is the result of mesozoic crustal stretching and that the intra crustal discontinuity represents a low angle detachment zone that helped accommodate extension in the crust the thick crust under the southern plateau would then represent a rheologically stronger portion of the borborema province that resisted deformation by mesozoic extension while the thin crust under the northern plateau would be a portion of formerly depressed thin crust that was uplifted during the cenozoic

about the lithospheric structure of central tibet based on seismic data from the indepth iii profile signals from 11 shots and 8 earthquakes and numerous teleseismic events were recorded along the 400 km seismic line indepth iii in central tibet and interpreted together with previous seismic and tectonic data the abnormal behavior of various mantle phases reveals a complex moho transition zone especially in the northern part of the line in the changtang block where the lower crust and the mantle show unusually low velocities a shingled appearance of pn and no low velocity layer in the upper crust the strong east west anisotropy in the changtang block is related to an easterly escape movement of the whole lithosphere facilitated by the warm and weak layers in the lower crust and the upper mantle bounded apparently by two prominent west east running fault zones

postmagmatic thermal activity in the abitibi greenstone belt noranda and matagami districts evidence from whole rock pb isotope data the pb isotope compositions of the footwall sequences of the noranda and matagami volcanogenic massive sulfide deposits in the abitibi greenstone belt have been examined at noranda whole rock pb isotope data for the flavrian northwest rusty ridge and amulet formations and the subvolcanic flavrian intrusion scatter about a line that corresponds to a pb pb age of 2638 or 14 ma pb pb ages calculated for the individual units are all within error of this age but are significantly younger than the 2700 or 2 ma u pb zircon age for the flavrian intrusion the pb pb array passes above a tightly constrained field for the galena pb from ore deposits within the mine sequence the younger pb pb ages and the lack of collinearity with the sulfide ores reflect rehomogenization of the pb isotope compositions in these rocks approximately 60 m y after their formation at matagami whole rock pb isotope data fall along two arrays the first which includes the porphyritic metarhyolite of the watson lake group from the footwall and the key tuffite corresponds to a pb pb age of 2584 super 19 sub 20 ma the second array includes the bell river igneous complex and the overlying wabassee group basalts and corresponds to an age of 2647 or 14 ma these pb pb ages are approximately 75 and 140 m y younger than corresponding zircon u pb ages of 2725 or 2 and 2725 super 3 sub 2 ma both arrays pass above a tightly constrained field for galena pb from several matagami ore deposits and are probably the result of differences in the initial pb isotope compositions of the respective rocks the apparent initial pb values most likely represent heterogeneities in the mantle source regions for these rocks the younger pb pb ages for the two arrays indicate pb isotope homogenization in these rocks approximately 100 m y after their formation the cause of the different ages for the two arrays remains controversial but may be the result of either incomplete isotopic homogenization of pb or an earlier closure of the u pb systems in the rocks constituting the older array the pb homogenization implied by the young pb pb ages may be the result of the pervasive circulation of hydrothermal solutions that may have been derived from the dewatering of the lower parts of a newly formed thickened archean crust such large scale processes apparently occurred 50 to 150 m y after this crust was formed and assembled

sedimentation and basin fill history of the neogene clastic succession exposed in the southeastern fold belt of the bengal basin bangladesh a high resolution sequence stratigraphic approach abstract the tertiary basin fill history of the bengal basin suffers from oversimplification the interpretation of the sedimentary history of the basin should be consistent with the evolution of its three geo tectonic provinces namely western northeastern and eastern each province has its own basin generation and sediment fill history related mainly to the indo burmese and subordinately to the indo tibetan plate convergence this paper is mainly concerned with facies and facies sequence analysis of the neogene clastic succession within the subduction related active margin setting oblique convergence in the southeastern fold belt of the bengal basin detailed fieldwork was carried out in the sitapahar anticline of the rangamati area and the mirinja anticline of the lama area the study shows that the exposed neogene succession represents an overall basinward progradation from deep marine through shallow marine to continental fluvial environments based on regionally correlatable erosion surfaces the entire succession 3000 m thick has been grouped into three composite sequences c b and a from oldest to youngest composite sequence c begins with deep water base of slope clastics overlain by thick slope mud that passes upward into shallow marine and nearshore clastics composite sequence b characteristically depicts tide dominated open marine to coastal depositional systems with evidence of cyclic marine regression and transgression repetitive occurrence of incised channel tidal inlet tidal ridge shoal tidal flat and other tidal deposits is separated by shelfal mudstone most of the sandbodies contain a full spectrum of tide generated structures e g herringbone cross bedding bundle structure mud couplet bipolar cross lamination with reactivation surfaces tidal bedding storm activities appear to have played a subordinate role in the mid and inner shelf region rizocorallium rosselia planolites and zoophycos are the dominant ichnofacies within the shelfal mudstone this paralic sedimentation of neogene succession in the study area can serve as a good point of reference for tide dominated regressive shelf depositional systems the top of the composite sequence b is marked by a pronounced erosion surface indicating the final phase of marine regression followed by the gradual establishment of continental fluvial depositional systems represented by composite sequence a in this composite sequence stacked channel bars of low sinuosity braided rivers gradually pass upsequence into high sinuosity meandering river deposits a sequence stratigraphic approach has been adopted to interpret the basin fill history with respect to relative sea level changes and to subdivide the rock record into several sequences and units systems tracts and parasequences based on identified bounding discontinuities such as transgressive erosion surface tes regressive erosion surface res marine flooding surface mfs and incised valley floor ivf this approach provides new insight for both exploration and exploitation strategy for hydrocarbon plays that may prove vital to the oil companies engaged in exploration activities in the bengal basin it is strongly recommended here that the traditional lithostratigraphic classification of this part of the basin which is based on the assam stratigraphy be abandoned or at least revised a tentative allostratigraphic scheme is presented and it is suggested that to formalize the scheme further study both surface and subsurface is needed

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

crustal and uppermost mantle velocity structure along a profile across the pamir and southern tien shan as derived from project tipage wide angle seismic data summary utilizing seismic refraction wide angle reflection data from 11 approximately in line earthquakes 2 d p and s velocity models and a poisson s ratio model of the crust and uppermost mantle beneath the southern tien shan and the pamir have been derived along the 400 km long main profile of the tipage tien shan pamir geodynamic program project these models show that the crustal thickness varies from about 65 5 km close to the southern end of the profile beneath the south pamir through about 73 6 km under lake karakul in the north pamir to about 57 7 km 50 km south of the northern end of the profile in the southern tien shan average crustal p velocities are low with respect to the global average varying from 6 26 to 6 30 km s 1 the average crustal s velocity varies from 3 54 to 3 70 km s 1 along the profile and thus average crustal poisson s ratio varies from 0 23 beneath the central pamir in the south central part of the profile to 0 265 towards the northern end of the profile beneath the southern tien shan the main layer of the upper crust extending from about 2 km below the earth s surface to 27 km depth below sea level b s l has average p velocities of about 6 05 6 1 km s 1 except beneath the south central part of the profile where they decrease to around 5 95 km s 1 this is in contrast to the s velocities which range from 3 4 to 3 6 km s 1 and exhibit the highest values of 3 55 3 6 km s 1 where the p velocity is lowest thus for the main layer of the upper crust is 0 26 beneath the profile except beneath the south central part of the profile where it decreases to 0 22 the low value of 0 22 for under the central pamir the along strike equivalent of the qiangtang terrane in tibet is similar to that within the corresponding layer beneath the northern lhasa and southern qiangtang terranes in central tibet and is indicative of felsic rocks rich in quartz in the state the lower crust below 27 km b s l has p velocities ranging from 6 1 km s 1 at the top to 7 1 km s 1 at the base further for this layer is 0 27 0 28 towards the northern end of the profile but is low at about 0 24 beneath the central and southern parts of the profile which is similar to the situation found in the northeast tibetan plateau the low values can be explained by felsic schists and gneisses in the upper part of the lower crust transitioning to granulite facies and possibly also eclogite facies metapelites in the lower part within the uppermost mantle the average p velocity is about 8 10 8 15 km s 1 and is about 0 26 assuming an isotropic situation then a relatively cool 700 800 c uppermost mantle beneath the profile is indicated this would in turn indicate an intact mantle lid beneath the profile an upper mantle reflector dipping from 104 km b s l 120 km from the southern end of the profile to 86 km b s l 155 km from the northern end of the profile has also been identified the preferred model presented here for the crustal and lithospheric mantle structure beneath the pamir calls for nearly horizontal underthrusting of relatively cool indian mantle lithosphere the leading edge of which is outlined by the pamir seismic zone this cool indian mantle lithosphere is overlain by significantly shortening warm asian crust the moho trough that is a feature seen beneath some other orogenic belts for example the alps and the urals beneath the northern pamir may mark the southern tip of the actively underthrusting tien shan crust along the main pamir thrust

cenozoic volcanism in the california coast ranges numerical solutions we present the results of numerical simulations of cenozoic volcanism in the coast ranges of california associated with the northward migration of the mendocino triple junction mtj three aspects of the thermal evolution and magmatism in the wake of the mtj are investigated 1 mantle flow and thermal perturbation in the slabless window 2 pressure release partial melting in the upwelling asthenosphere and 3 thermally induced crustal anatexis numerical results show that upwelling of asthenosphere in the slabless window causes a drastic change in the thermal structure of the lithosphere and results in significant pressure release partial melting in the upwelling asthenosphere up to 4 5 km of basaltic magma may be produced most of this mafic magma is likely stored near the base of the crust thermally induced crustal anatexis mainly occurs in the deep crust below 20 km where a high degree 50 of partial melting may be predicted most of the crustal melt is probably generated within 1 m y after the passage of the mtj this work also suggests that the spatial and temporal distribution of cenozoic volcanism in the california coastal ranges is directly related to the variation of the relative velocity between the north america and pacific plates in the last 30 m y the voluminous volcanic rocks in the northern california coast ranges correlate to the relatively high velocity between these two plates in the last 10 m y the reduced volcanism in the central coast ranges may be related to the slow migration of the mtj between 10 and 20 ma

postcrystallization disturbance in the neodymium and lead isotope systems of metabasalts from the ramagiri schist belt southern india archean metabasaltic rocks from three separate blocks that constitute the north south trending ramagiri schist belt of the eastern dharwar craton have large variations in nd2750 even samples of the same rock type from one outcrop that are less than 50 cm apart from each other show a range of up to 5 5 nd2750 units the trends of the fields defined by the three suites in smnd vs nd2750 space intersect at smnd 0 1 to 0 2 and nd2750 8 to 12 suggesting that these rocks have lost lrees and have interacted with and exchanged at least nd with a common extraneous phase the isotopic composition of this phase suggest derivation of its neodymium from sources which had a long term history of lree enrichment this phase may have derived its isotopic composition from older continental crust one of the unaltered dikes cutting the three blocks in the belt and the surrounding gneisses gives a 147sm144nd 143nd144nd age of 2454 100 ma a minimum for the time of block assembly and alteration of rees including nd while a set of samples from the central block lie along an isochron with a pb pb age of 2746 64 ma the samples from the eastern and western blocks show large uncorrelated variations in 207pb204pb with 206pb204pb residues after acid leaching of sample powders have lead isotope ratios that are distinct for samples from each of the block and are compatible with their derivation from archean mantle the lead isotope composition of the acid leaches suggest disturbance of the metabasalts by an isotopically extraneous phase that had variable and high 207pb204pb and 208pb204pb ratios for a given 206ph204pb relative to expected archean mantle values both lead and neodymium isotope disturbances are best explained as a result of alteration of these rocks by an extraneous phase which had derived its isotopic composition from ca 3 5 ga continental crust

calculation of the composition of fractionated solid as deduced from chemical profiles in tholeiitic lava the composition of solid precipitated from cooling magma is calculated from chemical differentiation profiles and an incompatible element monitor of fractionation in a period of cooling magma evolves to a derivative liquid and a solid liquid compositions are obtained from the chemical profiles and the fractionation interval via the incompatible element and the rayleigh distillation equation the composition of the solid the only unknown can be calculated for both equilibrium and continuous fractionation models a sequence of low k tholeiitic laves basalt to rhyolite from an archean greenstone belt in northwestern quebec are used to describe the procedure chemical trends in the lava exhibit strong iron enrichment followed by depletion and evolution to rhyolite trends in the calculated solid are amplified relative to lava and bulk distribution coefficients normative minerals mg number and other parameters also show significant divergence oxidation state and amount of trapped liquid have important effects on solid composition solids have notably more basic chemical compositions than coexisting liquids and this produces a lag in differentiation indices between the two the high values of bulk distribution coefficients calculated for fe ti and p d 3 to 4 in late stages of fractionation make it possible to produce the extremely fe ti p rich solid fractions in layered igneous complexes from quartz normative liquids

s wave velocity structure below central mexico using high resolution surface wave tomography shear wave velocity of the crust below central mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense 500 km long linear seismic network vertical components of regional records from 90 well located earthquakes were used to compute rayleigh wave group velocity dispersion curves a tomographic inversion with high resolution in a zone close to the array obtained for periods between 5 and 50 s reveals significant differences relative to a reference model especially at larger periods 30 s a 2 d s wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions the results show large differences especially in the lower crust among back arc volcanic arc and fore arc regions they also show a well resolved low velocity zone just below the active part of the trans mexican volcanic belt tmvb suggesting the presence of a mantle wedge low densities in the back arc inferred from the low shear wave velocities can provide isostatic support for the tmvb

soil landform development of a part of the fold belt along the eastern coast of bangladesh abstract the evolution of landforms and soils from the jaldi and maiskhali anticlines and adjoining areas in a part of the coastal region of the north south trending fold belt of bangladesh during the late quaternary period has been investigated based on the degree of soil development and luminescence dating eight soil geomorphic units have been deciphered and grouped into four members i iv of a morphostratigraphic sequence for the study area various soil geomorphic units included in different member sub members are member i river floodplains and active tidal flats 15 ka member iv is further subdivided into sub member iva island hillocks 15 18 ka sub member ivb mainland lower hillocks 23 25 ka and sub member ivc mainland higher hillocks 30 35 ka the youngest and poorly developed soils of member i show features related to hydromorphism moderately developed soils of members ii and iii show a fersiallitisation stage of pedogenesis member iv includes strongly developed soils with a ferrugination stage of pedogenesis these soils also exhibit degradation and poor birefringence of argillans and ferriargillans indicating a significant change in conditions of pedogenesis probably related to a paleoclimatic change from a subhumid to semiarid phase 40 ka to about 16 ka to a hot humid to subhumid phase 16 ka present parent material composition and physiography also have affected the pedogenesis in the area based on ages and heights above the mean sea level for the five terraces recognized in the study area the overall base level rise rates calculated are about 3 6 mm year 18 ka present for the maiskhali island and 2 86 mm year 35 ka present for the mainland jaldi anticline these base level changes represent combined effects of eustatic sea level and tectonic uplift due to folding

comprehensive crustal structure and seismological evidence for lower crustal flow in the southeastern margin of tibet revealed by receiver functions abstract the lower crustal flow model has become an interesting proposal to describe the growth and expansion of eastern tibet to date however the geophysical results are not definitive due to the non uniqueness of the solutions found through data inversion and the resolution limitations in this study the polarity of p to s converted phases at intra crustal interfaces are analyzed in order to reveal evidence for low seismic velocity and lower crustal flow in southeastern tibet we are interested in a joint study that is both a mapping of the crustal structure moho topography and poisson s ratio and an attempt to constrain an iclvz we have determined the crustal thickness and poisson s ratio from p prfs and s srfs receiver functions acquired in 108 permanent seismic stations deployed in the sichuan and yunnan areas also we have obtained 98 measurements of pms phase splitting parameters at the array stations the crustal thickness provided by prfs is consistent with that given by srfs it varies from 60 km in the songpan ganzi sg fold system and the northern part of the sichuan yunnan diamond shaped block sydsb near the eastern himalayan syntaxis to 33 km in southern yunnan the poisson s ratio varies mostly from 0 24 to 0 32 the highest values 0 28 0 32 are found along the axis formed by the longmenshan fault the lijiang jinhe fault and in the tengchong volcano area the value of 0 28 observed in sydsb and the sg fold system is attributed to the presence of a low velocity zone in the lower crust and partly to the great sediment thickness in sichuan basin the crust is clearly anisotropic in the southeastern margin of tibet the fast wave polarization direction exhibits a nw se regional bias while the time delay varies between 0 07 s and 1 27 s being the average 0 54 s 0 12 s implying the existence a strong shear zone in the lower crust based on the negative polarity observed after the direct p wave in teleseismic receiver functions we have detected an intra crustal low velocity zone iclvz in a wide region including east and southeast of tibet and southern yunnan so both the shear zone and iclvz suggest the existence of lower crustal flow the reverberation ppps and psps ppss phases at the moho are clearly weaker in sydsb and the sg fold system than in the indochina block and eastern yunnan implying that sydsb has a structure and tectonic history different from indochina on its west side and eastern yunnan on its east side the above results provide seismological evidence for a lower crustal flow spreading from the eastern tibet and that in its advance southward has invaded sydsb widely crossed the lijiang jinhe fault and reached southern yunnan

u pb zircon geochronology of the selbaie cu zn ag au mine abitibi subprovince canada the selbaie mine is a large tonnage low grade cu zn ag au deposit with both stratiform and epithermal characteristics mineralization comprises lower grade disseminated zn ag ore and pyritic siliceous sinter of the a1 zone at the surface and higher grade cu zn ag au quartz sulfide lenses of the a2 zone below and transitional with the a1 zone and of the b zone 1 km to the west the copper rich quartz sulfide lenses comprise networks of subvertical quartz sulfide veins vein arrays and breccia they have irregular locally gradational boundaries and variable attitudes with dips from 25 o to 90os the mineralization is within a volcanic stratigraphic succession of heterolithic breccias and felsic and intermediate tuffs and flows which are cut by mafic and felsic intrusions and felsite dikes the felsic volcanic rocks and a tonalitic marginal phase from the adjacent brouillan pluton are calc alkalic and have geochemical signatures that support a cogenetic relationship the mineralization and mine stratigraphic succession are truncated by the brouillan faults a series of subparallel west directed predominantly brittle thrusts with the brouillan pluton on the upper plate above the mine stratigraphic succession galenas from stratiform and vein ore have uniform lead isotope compositions of 2o6pb 2o4pb 13 184 to 13 211 2o7pb 2o4pb 14s351 to 14 369 and 2ospb 2o4pb 33s039 to 33 100 with model values of 8 1 8 2 and model k values of 4 1 to 4 2 n 5 among the most primitive of abitibi volcanic associated massive sulfide depositss they have model ages that range from 2 71 to 2 68 ga a u pb zircon age of 2729 ma for a sericite altered calc alkalic quartz porphyry flow 150 m up section from stratiform mineralization closely approximates the age for this mineralization type a u pb zircon age for a felsite dike that cuts postore east west foliation is 2726 3 ma slightly younger but overlapping at the 20 level with the age for the quartz feldspar porphyry flow indicating that the copper rich quartz sulfide lenses are essentially coeval with the lower grade stratiform mineralization a sample of sheared chloritic granodiorite taken from one of the brouillan faults near the margin of the brouillan pluton has a complex zircon opulation six concordant anal ses of abraded multigrain fractions individual grains or grain tips 20 p 206 s

geomorphic and pedogenic evolution in coastal sediments central california abstract studies of soil chronosequences on marine terraces facilitate the use of terraces for tectonic and paleoclimatic interpretation however many areas on the california coast have received substantial eolian deposits after pedogenesis began so do not qualify as chronosequences these areas are worthy of study because they are widespread coastal landscape features and they enable us to interpret pedogenic and geomorphic processes in sandy regolith the objective of this study was to use soil and deep regolith morphology and chemistry to interpret pedogenic and geomorphic evolution on a sequence of four marine terraces san luis obispo county california the terraces are numbered 1 2 3 and 4 from oldest to youngest the eroded terrace platforms have been tentatively dated at 560 420 or 480 320 and 120 ka respectively the sand deposits on each platform may be considerably younger the amount of land surface dissection and the depth of stream incision increase with increasing terrace age and elevation morphological features suggest that the soils xeropsamments on terrace 4 haploxerolls on terrace 3 on the two youngest terraces are well drained and their morphological development is typical of soils in eolian sand deposits morphological and chemical features of the basal regolith contrast with those of the soils clay fe oxides and opaline silica were deposited by groundwater flow above the bedrock platform above the shoreline angle where the deep regolith receives additional groundwater from higher terraces redoximorphic features have developed gray mottles are larger more common and more distinct in the basal regolith of progressively older terraces erosion has removed much of the overburden on terraces 2 and 1 and soils epiaqualfs on terrace 2 epiaqualfs on terrace 1 have developed in what was once deep regolith the landscape soils and deep regolith show an evolution of processes whereby the path and direction of water movement through the regolith is controlled first by terrace morphology and stratigraphy then by the development of pedogenic features in the soil and deep regolith and by terrace dissection

crustal structure beneath the eastern part of the coast ranges diablo range of central california from explosion seismic and near earthquake data abstract seismic refraction and near earthquake data of the u s geological survey for central california have been compiled into record sections along profiles and interpreted in terms of crustal structure the profiles are located northeast of the san andreas fault of central california and run parallel to the general structures for the explosion seismic line through the centre of the diablo range an uppermost layer franciscan formation with p velocities of 3 6 5 0 km s 1 decreases in thickness towards the northwest the lower boundaries of layers with constant velocities of 5 75 and 6 8 km s 1 are found at almost constant depths of 12 and 21 km respectively between 21 and 26 km depth a well defined low velocity zone appears whose velocity is estimated as 5 3 km s 1 with the aid of a hedgehog inversion and the calculation of amplitudes this zone is underlain by a layer 3 5 km thick with a velocity of 7 6 km s 1 the upper mantle velocity beneath the moho at 29 30 km depth is 8 2 km s 1 the near earthquake profiles located 20 km southwest and parallel to the explosion seismic line follow more or less the hayward and calaveras fault systems the velocity depth distribution derived for the earthquake data is very similar to that found beneath the diablo range however the low velocity zone at 21 26 km depth does not seem to exist everywhere along the line the moho is not disturbed beneath the calaveras hayward and silver creek faults it rises slightly from the diablo range towards the southwest

crustal structure of the southern calaveras fault zone central california from seismic refraction investigations a magnitude 5 7 earthquake on 6 august 1979 within the calaveras fault zone near coyote lake of west central california motivated a seismic refraction investigation in this area a northwest southeast profile along the fault as well as two fan profiles across the fault were recorded to examine the velocity structure of this region the analysis of the data reveals a complicated upper crustal velocity structure with strong lateral variations in all directions the near surface layers consist of recent sediments with seismic p wave velocities of 2 6 to 3 2 km sec these are underlain by rocks of the great valley sequence which have an average velocity of 4 5 km sec the great valley sequence is present along the whole profile depths range from 4 3 to 4 8 km in the northwest near anderson lake and in the southeast of the profile line near hollister in the middle of the profile near coyote lake however this layer thins and we find a laterally limited higher velocity layer 5 1 km sec between depths of 2 8 to 4 8 km the high velocity zone which coincides with a gravity high also correlates spatially with the hypocentral area of the 6 august earthquake and its aftershocks and may therefore represent an asperity on the fault velocities within the fault zone were determined from the fan profiles near anderson lake a pronounced delay of first arrivals on the fan records indicates a vertical 1 to 2 km wide near surface low velocity zone along the fault near coyote lake the delays observed in the fan records correlate with two subsurface en echelon fault planes which have been previously identified from lineations in the seismicity pattern the structure of the lower crust is similar to the neighboring diablo range a 8 to 9 km thick upper crustal layer with a seismic velocity of 5 7 to 6 3 km sec is underlain by a 3 km thick layer with velocity 6 8 km sec in accordance with previous studies of the diablo range there are indications of a pronounced lower crustal low velocity zone between a depth of 17 and 23 km the presence of this low velocity zone suggests that higher velocity igneous rocks of the middle crust have been thrust over sedimentary rocks

the northern limit of the subducted juan de fuca plate system analysis of data recorded at an array of three component broadband seismograph stations deployed on northern vancouver island and the adjacent british columbia mainland at the northern end of the cascadia subduction zone provides the first constraints on the s wave velocity structure of this region and permits us to define the northern limit of the subducted juan de fuca plate system during a 2 year period more than 80 teleseisms were recorded at our five stations the method of receiver function analysis was used to constrain the s velocity structure to upper mantle depths beneath the northern three stations a relatively simple continental crust is interpreted with a well defined moho near 37 39 km depth an upper crustal s velocity discontinuity at these stations is interpreted as the top of the high velocity rocks of the wrangellia terrane in contrast more complicated structure dominated by pronounced low velocity zones dipping to the ne are interpreted beneath our southern two stations the shallower low velocity zone is 6 8 km thick has an s velocity contrast of 0 6 1 1 km s and lies within the continental crust this feature is similar to a pronounced low velocity layer the e zone imaged beneath southern vancouver island the deeper low velocity zone is interpreted as the subducted oceanic crust we interpret the pronounced change in s velocity structure that we observe as the northern limit of the subducted oceanic plate beneath vancouver island this change coincides with significant changes in topography heat flow gravity and geochemistry

crustal structure below the valley of mexico estimated from receiver functions abstract receiver functions rfs from two permanent stations cuig and zaig of the mexican broadband network were calculated to infer the crustal structure at both sites the data were stacked to allow uncertainty estimations and inverted using two global optimization algorithms simulated annealing and genetic algorithms strong and systematic azimuthal dependence was observed in the stacked functions at cuig site which is located in the mexican volcanic belt mvb assuming a one dimensional five layer model per azimuthal stack these variations were explained with the same upper crustal structure including a superficial 2 km thick low velocity layer associated with the mvb and a well defined conrad interface around 15 km depth different depths for the moho discontinuity and variations in both the s wave velocity and poisson s ratio for the upper mantle our inversion of observed rfs at cuig and theoretical considerations suggest an eastward crustal thickening of at least 5 km below the valley of mexico we tested the sensitivity of the modeling procedure by applying it to the recordings at zaig station in this case no azimuthal dependence was noticed in the stacked functions after a simultaneous inversion of three different azimuthal stacks a single four layer model satisfactorily explained all data thus revealing a flat one dimensional crustal model below the mexican mesa central where zaig station is located these results support the resolution and robustness of the applied method at both sites

holocene evolution of deep circulation in the northern north atlantic traced by sm nd and pb isotopes and bulk sediment mineralogy 1 bulk mineralogy sm nd and pb elemental and isotopic compositions of the clay size fraction of holocene sediments were analyzed in three deep north atlantic cores to trace the particle provenance the aims of the present paper are to identify the origin of the particles driven by deep currents and to reconstruct deep circulation changes over the holocene in the north atlantic the three cores are retrieved in fracture zones two of them are located in the island basin along the gyre of north atlantic deep water and the third core is located off the present deep circulation gyre in the labrador sea whereas sedimentary supplies in the labrador sea were constantly derived from proximal sources the geochemical mixing trends in the iceland basin samples indicate pronounced changes in the relative contribution of continental margin inputs over the past 6 kyr supplies from western european margin that sharply increased at 6 kyr were progressively diluted by a larger contribution of scandinavian margins over the last 3 kyr changes in composition of the particles imply significant reorganization of paleocirculation of the deep north atlantic components in the eastern basins mainly reorganizations for both iceland scotland overflow water and norwegian sea overflow water moreover the unusual pb isotopic composition of the oldest sediments from the southern iceland basin indicates that distal supplies from greenland margin were driven into the iceland basin supporting a deep connection between labrador sea and iceland basin through the charlie gibbs fracture zone prior the holocene transition period

a transect across the mesozoic accretionary margin of central california subductive accretion formed along the pacific margin of north america in the mesozoic we are exploring deep structure across this accreted margin using coordinated reflection and detailed refraction profiling gravity and magnetic interpretation together with surface and well geology although none of these techniques alone is sufficient to define the critical elements of structure in complement they place important constraints on geologic interpretation for both the foothill rocks and particularly the franciscan assemblage we find evidence of obductive emplacement overthrusting from the west in contrast to the eastward subduction of most previous views these surprises emphasize the importance of such geophysical studies to geologic analysis

lithospheric structure of the arabian shield from the joint inversion of receiver functions and surface wave group velocities abstract we estimate lithospheric velocity structure for the arabian shield by jointly modeling receiver functions and fundamental mode group velocities from events recorded by the 1995 1997 saudi arabian portable broadband deployment receiver functions are primarily sensitive to shear wave velocity contrasts and vertical travel times and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages so that their combination bridge resolution gaps associated with each individual data set our resulting models correlate well with the observed surface geology the asir terrane to the west consists of a 10 km thick upper crust of 3 3 km s overlying a lower crust of 3 7 3 8 km s in the afif terrane to the east the upper crust is 20 km thick and has an average velocity of 3 6 km s and the lower crust is about 3 8 km s separating the terranes the nabitah mobile belt is made of a gradational upper crust up to 3 6 km s at 15 km overlying an also gradational lower crust up to 4 0 km s the crust mantle transition is found to be sharp in terranes of continental affinity east and gradual in terranes of oceanic affinity west the upper mantle shear velocities range from 4 3 to 4 6 km s temperatures around 1000 c are obtained from our velocity models for a thin upper mantle lid observed beneath station taif and suggest that the lithosphere could be as thin as 50 60 km under this station

the role of fluids during formation and evolution of the southern superior province lithosphere an overview models for fluid flow and hydrothermal alteration for the abitibi greenstone belt are reviewed in the light of lithoprobe results in the region in the abitibi greenstone belt eruption of volcanic sequences over 2750 2700 ma was accompanied by pervasive low temperature hydrothermal alteration at high water rock ratios giving systematic 18o enrichment archean ambient ocean water bottom temperatures were likely ca 30 c and 18o 0 1 chert iron formations precipitated from low temperature hydrothermal discharge base metal massive sulphide deposits formed at or near the seafloor from focussed discharge of high temperature 300 400 c fluids in convective cells sited above subvolcanic intrusions the ore fluids were evolved seawater that had undergone compositional and isotopic evolution by high temperature low water rock exchange with the volcanic pile to nacl 3 7 wt or cacl2 nacl up to 30 wt brines of 18o 0 8 these volcanic associated hydrothermal deposits are associated with greenst

the sequence of magmatic and mineralization events in the abitibi greenstone belt isotopic evidence from taylor township timmins area northern ontario canada samples from the porphyry deposit and the shoot zone prospect of st andrew goldfields ltd in taylor township near matheson ontario have been dated by several different techniques and utilized as a test of the use of pb isotope measurements in determining the time of mineralization in gold deposits of the abitibi greenstone belt clear and abraded zircons from an altered sulfidic porphyry unit yield a well defined age of 2697 3 1 3 ma indicating that the original intrusive rock unit containing these zircons was either latest synvolcanic or earliest syntectonic larger bulk samples of zircon from the same unit contain many altered and cracked grains and yield an age of 2682 4 ma close to the peak of syntectonic igneous activity pb pb isochrons determined from sulfide samples in mineralized material from the taylor porphyry zone yield a two stage model age of 2663 17 ma and suggest that mineralization postdates the syntectonic granitoids these pb isotope data are compared with isotope r