a garnet two mica granite coyote mountains southern arizona geologic setting uranium lead isotopic systematics of zircon and nature of the granite source region the 58 m y old pan tak granite considered representative of the widespread early tertiary garnet two mica granites of south central arizona consists of biotite and muscovite biotite granite intruded by garnet muscovite and garnet biotite granite and associated pegmatite textural evidence indicates that muscovite is a primary magmatic mineral the granite was intruded into jurassic dioritic to granitic rocks and paleozoic metasedimentary rocks during a latest cretaceous and early tertiary regional metamorphic episode part of the pluton was subsequently mylonitized during an early or middle tertiary deformational episode of metamorphic core complex aspect the fabrics produced by these two metamorphic episodes differ in distribution style orientation and probably age the zircon population of the pan tak granite consists of a mixture of small euhedral crystals and larger rounded crystals with or without overgrowths uranium lead isotopic ratios of five size fractions of discordant zircon define a chord with a lower concordia intercept age of 58 2 m y and a late precambrian upper intercept 207 pb 206 pb ages range from 214 m y for the finest fraction to 434 m y for the coarsest fraction the pan tak discordia is interpreted as a mixing line between 58 m y old magmatic zircon and an older zircon component largely or entirely of late precambrian age geologic and isotopic evidence indicates that this older zircon component was not assimilated from the country rocks but rather was inherited from the source material of the granite the presence of this inherited zircon indicates that the pan tak magma was produced by melting of crustal rocks containing late precambrian zircon this source material was probably the 1 7 to 1 4 b y old crystalline basement of south central arizona generation of the pan tak and related granites by crustal anatexis was one aspect of a latest cretaceous to early tertiary orogenic episode in south central arizona

de glauconiarum origine the glauconitic facies is widespread on present day continental shelves from 50 s to 65 n and at water depths between 50 and 500 m and is in particularly great abundance on the upper slope and outer shelf between 200 and 300 m it is also common in many ancient rocks of post late precambrian age it occurs as sand to pebble sized essentially green particles granular facies but also as a surface coating on particles and hardgrounds and as a diffuse impregnation film and diffuse facies we suggest the replacement of the term glauconite which has been interchangeably used to designate a morphological form and a specific mineral by glaucony facies and glauconitic smectite and glauconitic mica as end members of the glauconitic mineral family the widely accepted model of burst and hower for glauconitization requires a degraded micaceous 2 1 layer lattice structure parent clay mineral however detailed analysis of numerous samples of recent glaucony reveals that such a parent substrate is exceptional the model therefore requires modification generally the parent material is carbonate particles argillaceous kaolinitic faecal pellets infillings of foraminiferal tests various mineral grains and rock fragments that pass gradually into the commonly occurring green grains we show that the process of glauconitization is achieved by de novo authigenic growth of automorphous crystallites in the pores of the substrate accompanied by progressive alteration and replacement of the substrate it is this two fold evolution that causes the verdissement of granular substrates macrofossils and hardgrounds the authigenic mineral is an iron rich and potassium poor glauconitic smectite while new smectites are growing into the remaining pore space the earlier smectites are modified by incorporation of potassium producing decreasingly expandable minerals with a non expandable glauconitic mica as the end member this mineralogical diversity of the glauconitic mineral family explains the highly variable physical and chemical properties of glaucony four categories nascent little evolved evolved and highly evolved glaucony are distinguished glauconitization appears to be controlled by a delicate balance between degree of physical confinement of a particle and the amount of ionic exchange between the micro environment and ambient open marine sea water the optimum conditions for glauconitization are those of semi confinement as a result the interior of a grain is more glauconitized than its less confined periphery similarly for identical substrate types large grains 500 m provide more favourable substrates for glauconitization than lesser confined small grains on a larger scale the formation of glaucony is governed by the availability of iron and potassium and the balance between detrital influx and winnowing low accumulation rates expose grains to the open marine environment for sufficiently long times 105 106 years for highly evolved glaucony

detailed rupture imaging of the 25 april 2015 nepal earthquake using teleseismic p waves we analyze the rupture process of the 25 april 2015 nepal earthquake with globally recorded teleseismic p waves the rupture propagated east southeast from the hypocenter for about 160 km with a duration of 55 s backprojection of both high frequency hf 0 2 to 3 hz and low frequency lf 0 05 to 0 2 hz p waves suggest a multistage rupture process from the low frequency images we resolve an initial slow downdip northward rupture near the nucleation area for the first 20 s stage 1 followed by two faster updip ruptures 20 to 40 s for stage 2 and 40 to 55 s for stage 3 which released most of the radiated energy northeast of kathmandu the centroid rupture power from lf backprojection agrees well with the global centroid moment tensor solution the spatial resolution of the backprojection images is validated by applying similar analysis to nearby aftershocks the overall rupture pattern agrees well with the aftershock distribution a multiple asperity model could explain the observed multistage rupture and aftershock distribution

upper crustal stress and seismotectonics of the garhwal himalaya using small to moderate earthquakes implications to the local structures and free fluids abstract the work presents new focal mechanism data of small to moderate 3 0 ml 5 0 upper crustal earthquakes for the garhwal himalaya from a local seismic network installed in july 2007 majority of the epicenters of these earthquakes are located close to the main central thrust mct zone we retrieved moment tensor mt solutions of 26 earthquakes by waveform inversion the mt results and 11 small to moderate earthquakes from the published records are used for stress inversions the mt solutions reveal dominatingly thrust mechanisms with few strike slip earthquakes near chamoli the seismic cross sections illustrate that these earthquakes are located around the mid crustal ramp mcr in the detachment the optimally oriented faults from stress inversions suggest that the seismogenic fault in this region is similar to a fault plane having dip angle between 12 and 25 degrees which is compatible with the dip angle of the mcr 16 in this region p axes and the maximum horizontal compressive stress are ne sw oriented the direction of the relative motion of indian plate with respect to the eurasian plate the friction coefficient estimated from stress inversions show that the chamoli region having low friction in comparison to the overall values the free fluids trapped beneath the detachment are penetrating into the local faults hence decreasing the frictional strength and altering the prevailing stress conditions of the surroundings the present study reveals that the mcr structure is seismogenically active and producing the small moderate earthquakes in the region while the mct is probably dormant at present

provenance of synorogenic foreland basin strata in southwestern montana requires revision of existing models for laramide tectonism north american cordillera

damped regional scale stress inversions methodology and examples for southern california and the coalinga aftershock sequence 1 we present a new focal mechanism stress inversion technique to produce regional scale models of stress orientation containing the minimum complexity necessary to fit the data current practice is to divide a region into small subareas and to independently fit a stress tensor to the focal mechanisms of each subarea this procedure may lead to apparent spatial variability that is actually an artifact of overfitting noisy data or nonuniquely fitting data that does not completely constrain the stress tensor to remove these artifacts while retaining any stress variations that are strongly required by the data we devise a damped inversion method to simultaneously invert for stress in all subareas while minimizing the difference in stress between adjacent subareas this method is conceptually similar to other geophysical inverse techniques that incorporate damping such as seismic tomography in checkerboard tests the damped inversion removes the stress rotation artifacts exhibited by an undamped inversion while resolving sharper true stress rotations than a simple smoothed model or a moving window inversion we show an example of a spatially damped stress field for southern california the methodology can also be used to study temporal stress changes and an example for the coalinga california aftershock sequence is shown we recommend use of the damped inversion technique for any study examining spatial or temporal variations in the stress field

authigenic greigite in mud from the continental shelf of the yellow sea off the southwest korean peninsula abstract this is the first reported occurrence of greigite in korea which was found in semi consolidated near surface mud on the continental shelf of the southeastern yellow sea the seafloor 70 m of water in the sampling site is composed of alternating massive sand and homogeneous mud which are presumably part of the holocene transgressive deposits greigite grains 0 1 2 mm in size are disseminated or form a single lamina in the mud the mineral is strongly magnetic and black with metallic luster some grains have a reddish brown tarnish caused by oxidation in reflected light greigite occurs as yellowish white to pale creamy tint and isotropic globular aggregates the mineral greigite has a chemical composition of fe 54 4 s 42 4 as 0 14 si 2 3 wt and the empirical formula on the basis of s 4 is fe2 95s4 cell dimensions cubic are measured as a 9 8635 ra v 959 6 a 3 and d 4 094 g cm3 it probably formed in a reducing micro environment around decaying organic matter

mg rich ferric illite in marine transgressive and highstand systems tracts examples from the paleoproterozoic semri group central india this paper focusses upon two glauconitic sandstones in the paleoproterozoic deoland formation and chorhat sandstone both belonging to the semri group central india in both the cases glauconitic minerals occur in sandstones deposited in the marine realm within a transgressive systems tract tst for the former unit and within a highstand systems tract hst for the latter the proportion of glauconitic minerals increases in the paleo offshore direction petrography reveals selective early glauconitization of detrital k feldspars along their fringes cleavages and the fractures created by volume expansion during progressive alteration leading to the generation of peloids with small relics of the precursors xrd and mineral chemistry reveal a structure typical of glauconite and more akin to mg rich ferric illite the mineral chemistry of the glauconitic mineral phases remains the same whether the glauconitization process was incipient or at an advanced

seismicity patterns in the himalayan plate boundary and identification of the areas of high seismic potential abstract space time distribution of earthquakes along the himalayan plate boundary has been investigated with a view to discerning fluctuations in seismicity rates associated with great earthquakes the possibilities of epicentre migration and gaps in seismic activity data from 1800 to 1976 were considered this study shows that all great earthquakes m 8 0 were preceded by seismically quiescent periods of at least 19 years furthermore earthquake quiescence has also been recognised to follow the great earthquakes three possible trends of migration of epicentres of great earthquakes have been recognised 1 1 the first trend starts from the 1905 kangra earthquake the epicentres of subsequent earthquakes migrating eastwards along the plate boundary 2 2 the second trend of migration starts from the great nepal earthquake of 1833 associated with eastward migration 3 3 the third trend commences from the great assam earthquake of 1897 and shows a westward migration of earthquake epicentres the middle section of the himalayan convergent plate boundary 80 e to 90 e in general displays a relatively low level of seismicity as compared to the adjacent sections a remarkable decrease in seismicity following the year 1970 has been observed along a section of the himalaya which lies to the east of the rupture zone of the great 1905 kangra earthquake since 1970 the seismicity rate has also dropped in the region lying in between the rupture zones of the great assam earthquakes of 1897 and 1950 as has earlier been reported by khattri and wyss 1978 such decreases in the level of seismicity along active plate boundaries have been demonstrated to be indicative of building up of stresses leading to future great earthquakes mogi 1969 mccann et al 1979

uplift and convergence along the himalayan frontal thrust of india along the himalayan thrust front in northwestern india terrace deposits exposed 20 to 30 m above modern stream level are interpreted to have been uplifted by displacement on the underlying himalayan frontal thrust a radiocarbon age limits the age of the terrace to 1665 215 calendar bc 3663 215 radiocarbon years before present yielding a vertical uplift rate of 6 9 1 8 mm yr in combination with published studies constraining the dip of the himalayan frontal thrust fault to about 30 in the study area the observed uplift rate equates to horizontal shortening across the himalayan frontal thrust of 11 9 3 1 mm yr and the slip rate of the himalayan frontal thrust of 13 8 3 6 mm yr this is similar to previously reported rate estimates along the himalayan arc based on displacement of older plio miocene age rocks or the much shorter records of geodesy and historical seismicity the similarity is consistent with the idea that convergence across the himalayan front has occurred at a relatively steady rate through time the seismic expression of this deformation includes several great m 8 historical earthquakes which due to lack of surface rupture during those events have been attributed to their occurrence on blind thrusts yet the occurrence of a possible fault scarp in the field area indicates that past earthquakes have been sufficiently large to rupture to the surface and produce coseismic scarps these observations suggest a potential for earthquakes along the himalayan frontal thrust larger than those observed historically

ore genesis constraints on the idaho cobalt belt from fluid inclusion gas noble gas isotope and ion ratio analyses a reply the idaho cobalt belt is a 60 km long alignment of deposits composed of cobaltite co pyrite chalcopyrite and gold with anomalous nb y be and rare earth elements rees in a quartz biotite tourmaline gangue hosted in mesoproterozoic metasedimentary rocks of the lemhi group it is the largest cobalt resource in the united states with historic production from the blackbird mine all of the deposits were deformed and metamorphosed to upper greenschist lower amphibolite grade in the cretaceous they occur near a 1377 ma anorogenic bimodal plutonic complex the enhanced solubility of fe co cu and au as chloride complexes together with gangue biotite rich in fe and cl and gangue quartz containing hypersaline inclusions allows that hot saline fluids were involved the isotopes of b in gangue tourmaline are suggestive of a marine source whereas those of pb in ore suggest a u th enriched source the ore and gangue minerals in this belt may have trapped components in fluid inclusions that are distinct from those in post ore minerals and metamorphic minerals such components can potentially be identified and distinguished by their relative abundances in contrasting samples therefore we obtained samples of co and cu sulfides gangue quartz biotite and tourmaline and post ore quartz veins as well as cretaceous metamorphic garnet and determined the gas noble gas isotope and ion ratios of fluid inclusion extracts by mass spectrometry and ion chromatography the most abundant gases present in extracts from each sample type are biased toward the gas rich population of inclusions trapped during maximum burial and metamorphism all have co 2 ch 4 and n 2 ar ratios of evolved crustal fluids and many yield a range of h 2 ch 4 co 2 h 2 s equilibration temperatures consistent with the metamorphic grade cretaceous garnet and post ore minerals have high r h and r s values suggestive of reduced sulfidic conditions most extracts have anomalous 4 he produced by decay of u and th and 38 ar produced by nucleogenic production from 41 k in contrast some ore and gangue minerals yield significant so 2 and have low r h and r s values of a more oxidized fluid three extracts from gangue quartz have high helium r r a values indicative of a mantle source and neon isotope compositions that require nucleogenic production of 22 ne in fluorite from u th decay two extracts from gangue quartz have estimated 40 k 40 ar that permit a precambrian age extracts from gangue quartz in three different ore zones are biased toward the hypersaline population of inclusions and have a tight range of ion ratios na k nh 4 cl br f suggestive of a single fluid their na cl br ratios suggest this fluid was a mixture of magmatic and basinal brine na k ca temperatures 279 347 c are similar to homogenization temperatures of hypersaline inclusions the high k na of the brine may be due to albitization of k silicate minerals in country rocks influx of k rich brines is consistent with the k metasomatism necessary to form gangue biotite with high cl an extract from a post ore quartz vein is distinct and has na cl br ratios that resemble metamorphic fluids in cretaceous silver veins of the coeur d alene district in the belt basin the results show that in some samples for certain components it is possible to see through the cretaceous metamorphic overprint of great import for genetic models the volatiles trapped in gangue quartz have 3 he derived from a mantle source and 22 ne derived from fluorite both of which may be attributed to nearby 1377 ma basalt rhyolite magmatism the brine trapped in gangue quartz is a mixture of magmatic fluid and evaporated seawater the former requires a granitic intrusion that is present in the bimodal intrusive complex and the latter equatorial paleolatitudes that existed in the mesoproterozoic the results permit genetic models involving heat and fluids from the neighboring bimodal plutonic complex and convection of basinal brine in the lemhi group while the inferred fluid sources in the idaho cobalt belt are similar in many respects to those in iron oxide copper gold deposits the fluids were more reduced such that iron was fixed in biotite and tourmaline instead of iron oxides

u th pb and rb sr geochronology of middle proterozoic granite and augen gneiss salmon river mountains east central idaho coarsely porphyritic granite and augen gneiss in east central idaho have been dated by the u th pb zircon method when plotted on concordia diagrams the results give nonlinear data arrays that indicate the presence of at least one component of inherited radiogenic lead as well as postcrystallization probably recent episodic lead loss nevertheless a systematic relationship between zircon size and convergence of the 207 pb 206 pb ages for the 100 and 100 150 mesh fractions from several samples are interpreted to provide a valid age of about 1 370 m y for these plutons a single sample which is free of inheritance for all size fractions confirms this conclusion rb sr whole rock samples are highly enriched in radiogenic strontium but do not define an isochron probably because the system did not remain closed following crystallization of the plutons intrusive contacts of the granitic rocks indicate that deformation and regional metamorphism of the host yellowjacket formation occurred prior to 1370 ma rather than prior to 1500 ma as previously suggested by others this refinement of the geochronology 1 removes the necessity that the yellowjacket formation and overlying proterozoic metasedimentary rocks of central and east central idaho be pre belt in age 2 permits the traditional age correlation of the east central idaho metasedimentary sequence yellowjacket through lawson creek formations with the belt supergroup and 3 suggests that a re evaluation is in order for paleogeographic models of central idaho especially the proposed topographic high known as the salmon river arch

inversion for parameters of tensile earthquakes the tensile source model generalizes the shear source model by assuming that the slip vector can be arbitrarily oriented with respect to the fault and is not constrained to lie within the fault plane the proposed inversion for the parameters of tensile sources is based on the evaluation of the isotropic iso compensated linear vector dipole clvd and double couple dc components in seismic moment tensors the most significant parameters inverted are the ratio at the fault denoted as the parameter and the inclination of the slip vector from the fault the parameter is significant for discriminating noisy moment tensors of shear earthquakes from those of tensile earthquakes the inclination can be accurately determined from the dc component in the moment tensor because the dc component rapidly decreases with increasing for example the inclination of 20 causes dc being 50 60 only the inversion is applied to earthquakes which occurred in january 1997 in west bohemia czech republic it is shown that some of these earthquakes display tensile faulting the parameter is 0 1 the inclination of the slip from the fault attains values of up to 20 this inclination is a result of tensile traction and reduced shear traction along the fault and high fluid pressure in the region

recent earthquake swarms in garhwal himalaya a precursor to moderate to great earthquakes in the region abstract himalayas one of the most seismically active belts have been formed due to continent continent collision and are probable locations for future moderate to great earthquakes it is of paramount importance to recognize precursory swarm activity considered to be long predictors of main events recently two earthquake swarms occurred across main central thrust mct in garhwal himalayan region in a time interval of nearly one month the first swarm of april 2009 had epicentral locations lying north of mct on hanging wall side which has shown less seismic activity in recent times while the second swarm occurred in may 2009 near gopeshwar having epicenters on active seismic zone located south of mct the approximate distance between the two swarms is about 50 km in present study preparation zone of 100 100 km 2 has been demarcated recognizing these swarms as precursors to moderate to great earthquake in the garhwal himalaya

geology and u pb geochronology of archean basement and proterozoic cover in the priest river complex northwestern united states and their implications for cordilleran structure and precambrian continent reconstructions precambrian basement rocks exposed within tectonic windows in the north american cordillera help to define the precambrian crustal structure of western north america and possible reconstructions of the late proterozoic supercontinent rodinia new geologic mapping and u pb dating in the infrastructure of the priest river metamorphic complex northern idaho documents the first archean basement 2651 20 ma north of the snake river plain in the north american cordillera the archean rocks are exposed in the core of an antiform and mantled by a metaquartzite that may represent the nonconformity between basement and the overlying hauser lake gneiss which is correlated with the prichard formation of the belt supergroup a structurally higher sheet of augen gneiss interleaved with the hauser lake gneiss yields a u pb zircon crystallization age somewhat greater than 1577 ma the slivers of augen gneiss were tectonically interleaved with the surrounding hauser lake gneiss near the base of the spokane dome mylo

reconnaissance lead isotope characteristics of the blackbird deposit implications for the age and origin of cobalt copper mineralization in the idaho cobalt belt united states strata bound cobalt copper deposits occur in the metasedimentary rocks of the apple creek formation mesoproterozoic in a linear belt called the idaho cobalt belt near salmon idaho we report the first set of lead isotope measurements on silicate rocks and sulfide ores and new sulfur isotope data on sulfide ores the pb isotope compositions of cobalt copper co cu ores 206pb 204pb 30 8 40 4 207pb 204pb 16 8 17 6 208pb 204pb 49 7 63 9 are very radiogenic much more radiogenic than any known sedimentary exhalative sedex deposits the data plot well beyond the crustal growth model curves a feature shared by the mississippi valley type mvt deposits which is characteristic of an upper crustal source it is unlikely that the ore lead could have come from any mafic igneous source of mantle origin proterozoic moyie sill and its equivalent or the tertiary challis volcanic group the cretaceous felsic igneous rocks of the region the idaho batholith and related rocks also have much lower pb isotope ratios than the co cu ores the pb isotope ratios of proterozoic crystalline rocks partially overlap pb isotope ratios of the apple creek formation but would have been much less radiogenic than the ores at an assumed mineralization age of about 1400 ma only the host apple creek formation is known to have appropriate pb isotope compositions 206pb 204pb 26 8 86 7 207pb 204pb 16 3 21 1 208pb 204pb 47 9 64 8 to be the source of co cu ores leaching of metals from the host sedimentary sequence by hydrothermal fluids and subsequent deposition of ores at chemically and structurally favorable sites could have resulted in the formation of co cu deposits in the idaho cobalt belt

age and origin of albian glauconites and associated clay minerals inferred from a detailed geochemical analysis abstract glauconitic grains are abundant in sandstones and siltstones from albian and cenomanian transition between lower and upper cretaceous formations of southeastern france morphological and crystal chemical data show that the green grains can be considered as glauconite s s with minute amounts of pyrite dolomite calcite and other clay minerals as grain coatings or crack fillings glauconite grains formed early during sedimentation generally from detrital chlorite biotite or clay particles and were found scattered together with other detrital mineral phases muscovite quartz in the silt in which porosity was considerably reduced afterwards by four stages of carbonate calcite dolomite cementation geochemical data especially the fe u and ce contents of the glauconitic grains strongly suggest increasing oxidizing conditions with time in the sedimentation environment the detailed geochemical analyses also allow the pristine glauconite grains to be distinguished from the other clay minerals occurring in the sequence this enabled k ar and rb sr dating of the glauconitic material which yields identical results of 97 9 0 4 and 97 9 3 5 ma respectively these dates point to a synsedimentary origin for the glauconites during the late albian about 97 98 ma the k ar data also indicate that the albian cenomanian boundary should be placed at 96 0 1 9 ma the similarity between the k ar and rb sr ages precludes any further significant pervasive recrystallization of these minerals during burial of the sequence either by heat flux or fluid flow the k ar study of the clay matrix from the same siltstones suggests synsedimentary precipitation of additional clay minerals probably together with the glauconite grains such as mixed layer illite smectite identified in the clay size fraction of the sedimentary succession

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

seismotectonics and crustal stress field in the kumaon garhwal himalaya abstract we present fault plane solutions of 94 well located small to moderate sized 1 5 ml 5 4 earthquakes which occurred in the kumaon garhwal himalaya during 2005 2008 using p wave polarity and body wave amplitudes these earthquakes show a mixture of thrust normal and strike slip type mechanism with a majority of thrust type most of the thrust earthquakes occur at a depth of 8 22 km in the main central thrust mct zone and the lower himalaya the spatial distribution of these earthquakes suggest that the strain resulting from the ongoing collision of the indian plate with the eurasian plate is being consumed by thrust fault movement mainly on the north dipping munsiari thrust and south dipping tons thrust the strike slip earthquakes are mainly observed in the lower himalaya as well as around the munsiari region in the mct zone the normal earthquakes are also observed in different parts of the kumaon garhwal himalaya and the gangetic plain their occurrence is attributed to the local structure s as well as the flexure of the indian plate stress tensor inversion of the calculated fault plane solutions indicates that the maximum compressive stress in the gangetic plain is n s directed and near vertical whereas in the kumaon garhwal himalaya it is near horizontal and nne ssw directed and correlating with the prevailing stress condition due to northward movement of indian plate

seismotectonics of the 20 october 1991 uttarkashi earthquake in garhwal himalaya north india the 20 october 1991 uttarkashi earthquake killed over a thousand people and caused extensive damage to property in the garhwal himalaya region the body wave magnitude of the earthquake was 6 6 and the fault plane solution indicates reverse faulting the hypocenrre was located at a focal depth of about 12 km between the chail and jutogh thrusts but movement propagated southward along the jamak gangori fault jgf and dunda fault df which are developed as blind faults related to the growing uttarkashi antiform

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

the motion and active deformation of india measurements of surface displacements using gps constrain the motion and deformation of india and india eurasia plate boundary deformation along the himalaya the gps velocities of plate interior sites constrain the pole of the angular velocity vector of india with respect to eurasia to lie at 25 6 1 0 n 11 1 9 0 e approximately 6 west of the nuvel 1a pole of 3 ma plate motion the angular rotation rate of 0 44 0 03 myr 1 is 14 slower than the long term rate of 0 51 myr 1 insignificant velocities between plate interior sites indicate that the exposed indian plate is stable to within 7 10 9 yr 1 the observed contraction vector across the himalaya 20 mm yr veers from n20 e in the northwest himalaya to n25 w in east nepal consistent with east west extension of southern tibet

new timing constraints for the onset of laramide deformation in southwest montana challenge our understanding of the development of a thick skinned structural style during flat slab subduction

aftershocks of the march 1999 chamoli earthquake and seismotectonic structure of the garhwal himalaya the aftershocks of the chamoli earthquake m b 6 3 of 28 march 1999 are analyzed to understand the seismotectonic structure of the garhwal himalaya the permanent as well as temporary microearthquake network data are used the mainshock occurred on the basement thrust by thrust faulting the aftershocks on the other hand are triggered at the seismogenic faults to the south of the main central thrust mct by thrust faulting as well as by strike slip faulting and all the events occurred above the plane of detachment basement thrust the mct is not the seismogenic fault

iron and chlorine as guides to stratiform cu co au deposits idaho cobalt belt usa the cu co au deposits of the idaho cobalt belt are in lithostratigraphic zones of the middle proterozoic yellowjacket formation characterized by distinctive chemical and mineralogical compositions including high concentrations of fe 15 30 wt percent fe2o3 cl 0 1 1 10 wt percent and magnetite or biotite 50 vol percent the cu co au deposits of the blackbird mine are stratabound in fe silicate facies rocks that are rich in biotite fe and cl but stratigraphically equivalent rocks farther than 10 km from ore deposits have similar compositions a lower lithostratigraphic zone containing magnetite and small cu co au deposits extends for more than 40 km the fe rich strata are probably exhalative units related to mafic volcanism and submarine hot springs but the origin of the high cl concentrations is less clear former chlorine rich pore fluids are suggested by the presence of supersaline fluid inclusions by cl rich biotite and scapolite as much as 1 87 percent cl in fe rich biotite and by high cl concentrations in rock samples chlorine is enriched in specific strata and in zones characterized by soft sediment deformation thus probably was introduced during sedimentation or diagenesis unlike some metasedimentary rocks containing scapolite and high cl the yellowjacket formation lacks evidence for evaporitic strata that could have been a source of cl more likely the cl reflects a submarine brine that carried fe k and base metals strata containing anomalous fe k cl are considered to be a guide to sub basins favorable for the occurrence of stratiform base metal deposits

great earthquakes seismicity gaps and potential for earthquake disaster along the himalaya plate boundary abstract analysis of the space time patterns of seismicity in the himalaya plate boundary has established the existence of three seismic gaps 1 1 the kashmir gap lying west of the 1905 kangra earthquake 2 2 the central gap situated between the 1905 kangra and the 1934 bihar earthquakes 3 3 the assam gap between the 1897 and 1950 assam earthquakes this study has shown that the above great earthquakes were preceded as well as followed by long periods 19 years of decreased levels of seismic activity in the epicentral regions remarkable decrease in the seismicity following the year 1970 has been observed in the western half of the central gap as well as in the assam gap local seismic investigation in the assam gap confirms this feature and the seismicity suggests the existence there of an asperity the local seismic investigations in garhwal himalaya have shown that the small earthquakes are confined to the upper 6 8 km of the crust and may have strike slip motions these earthquakes occur in a region where teleseismically recorded events were few

the status of central seismic gap a perspective based on the spatial and temporal aspects of the large himalayan earthquakes abstract the central himalaya considered as a prominent seismic gap is generally believed to be the most vulnerable segment due for a great plate boundary earthquake m 8 two significant historical earthquakes are known to have occurred in this region in ad 1505 and 1803 interpreted by some workers as great earthquakes occurrences of these events question the validity of central himalaya being a seismic gap here we analyze the macroseismal data from the central himalaya and the gangetic plains and suggest that neither of these earthquakes can be qualified as a plate boundary type event and the size of at least the 1803 does not exceed mw 7 7 our studies further indicate that the central himalayan frontal thrusts may be undergoing a quiescence of 1000 years in terms of generation of great earthquakes assuming a steady elastic strain accumulation due to plate convergence and its simple translation as periodic great earthquakes such long quiescence may appear intriguing we suggest that the long term quiescence and the spatial and temporal clustering that characterize the distribution of large earthquakes in the himalaya is a manifestation of the tectonic deformation associated with a highly evolved fold and thrust belt our analyses also indicate that the large moderate earthquakes are mostly concentrated on higher level thrusts these structures probably accommodate most of the seismogenic slip in the himalaya and the rest of the strain could possibly be expended on passive folding consequently great earthquakes m 8 are relatively infrequent as in the case of central himalaya apparently leading to long periods of quiescence

southern limits of major earthquake ruptures along the himalaya between longitudes 75 and 90 e the ruptures responsible for major earthquakes along the himalayan convergent plate margin hcpm occur in a strikewise oriented zone of frictional failure and relative slip in a buried detachment along the upper surface of the indian shield rocks subducting under the himalaya the southern limit of the rupture zone is a geotectonic lineament whose geographic location is important in assessing risk due to earthquakes a major part of this article is taken up in arguing that the available macroseismic and instrumental evidence for the 1934 bihar nepal earthquake is consistent with the view that the rupture which caused it occurred in the detachment mostly under the lesser himalaya northward from the vicinity of the surface trace of the main boundary thrust mbt since a similar location has been inferred by others for the 1905 kangra earthquake rupture a basis arises for postulating that over more than half of the length of the hcpm between 75 and 90 e longitudes the ruptures responsible for major earthquakes lie in the detachment with their southern limits geographically similarly close to the surface trace of the mbt this includes the nearly 700 km long seismic gap between the 1905 and 1934 ruptures

diagenetic evolution of the eocene ramp carbonates a paradigm from the nile valley egypt petrographical and geochemical attributes abstract carbonate rocks of east nile valley in egypt have been investigated as a paradigm for revealing the diagenetic evolution of the eocene ramp carbonates the integration of petrographical elemental geochemical and stable isotopic data revealed that the studied carbonates have been influenced by a sequence of diagenetic processes which are included in three diagenetic episodes i e eogenesis mesogenesis and telogenesis and are related to four diagenetic environments i e marine phreatic meteoric phreatic burial and uplifting environments the paragenetic sequence of the studied carbonates commences with the eodiagenetic episode which includes the marine and meteoric phreatic diagenetic environments the marine phreatic diagenetic environment comprises micritization glauconitization pyritization marine cementation fibrous calcite and syntaxial overgrowth rim cements early dolomitization formation of fine crystalline matrix replacive dolomites and early stages of mechanical compaction before significant burial the meteoric phreatic diagenetic environment includes the neomorphism dissolution meteoric cementation earlier phases of sparry calcite cements and precipitation of megaquartz in the void spaces of the peritidal carbonates during the mesogenesis episode the studied carbonates were exposed to burial diagenesis the shallow burial diagenesis resulted in the creation of point tangenetial contacts between the allochemical constituents and the precipitation of a medium coarse crystalline dolomite cement filling the dissolved fossils and dissolution pores the deep burial diagenetic processes embody the chemical compaction condensed fitted interpeneteration and pressure dissolution fabrics fracturing and burial cementation the telodiagenesis represents the last episode in the diagenetic history of the studied carbonates it witnessed the fracturing meteoric vadose cementation and hematitization processes

sulfur carbon hydrogen and oxygen isotope geochemistry of the idaho cobalt belt cobalt copper gold deposits of the idaho cobalt belt including the deposits of the blackbird district have been analyzed for their sulfur carbon hydrogen and oxygen isotope compositions to improve the understanding of ore formation previous genetic hypotheses have ranged widely linking the ores to the sedimentary or diagenetic history of the host mesoproterozoic sedimentary rocks to mesoproterozoic or cretaceous magmatism or to metamorphic shearing the 34 s values are nearly uniform throughout the blackbird district with a mean value for cobaltite coass the main cobalt mineral of 8 0 0 4 n 19 the data suggest that 1 sulfur was derived at least partly from sedimentary sources 2 redox reactions involving sulfur were probably unimportant for ore deposition and 3 the sulfur was probably transported to sites of ore formation as h 2 s hydrogen and oxygen isotope compositions of the ore forming fluid which are calculated from analyses of biotite rich wall rocks and tourmaline do not uniquely identify the source of the fluid plausible sources include formation waters metamorphic waters and mixtures of magmatic and isotopically heavy meteoric waters the calculated compositions are a poor match for the modified seawaters that form volcanogenic massive sulfide vms deposits carbon and oxygen isotope compositions of siderite a mineral that is widespread although sparse at blackbird suggest formation from mixtures of sedimentary organic carbon and magmatic metamorphic carbon the isotopic compositions of calcite in alkaline dike rocks of uncertain age are consistent with a magmatic origin several lines of evidence suggest that siderite postdated the emplacement of cobalt and copper so its significance for the ore forming event is uncertain from the stable isotope perspective the mineral deposits of the idaho cobalt belt contrast with typical vms and sedimentary exhalative deposits they show characteristics of deposit types that form in deeper environments and could be related to metamorphic processes or magmatic processes although the isotopic evidence for magmatic components is relatively weak

estimates of interseismic deformation in northeast india from gps measurements abstract estimates of interseismic deformation in northeastern india based on gps measurements at eight permanent stations 2003 2006 and six campaign sites 1997 2006 are presented here the euler pole of rotation of indian tectonic plate in itrf2000 determined from the present data set is located at 51 7 0 5 n 15 1 1 5 e with angular velocity of 0 469 0 01 myr 1 the results show that there is a statistically insignificant present day active deformation within the shillong plateau and in the foreland spur north of the plateau in the brahmaputra valley convergence rate of the northeastern gps sites with respect to the igs station lhasa along baselines that are normal to the himalayan arc in this region is 16 0 5 mm yr this represents the arc normal indo eurasian convergence rate across the northeastern boundary similar to arc normal convergence rates determined in central nepal along the himalayan arc however unlike central nepal in the arunachal himalaya the 16 mm yr shortening is distributed between the lesser as well as higher and tethyan himalayas baselines between sites on the indo burmese fold and thrust belt ibftb and shillong plateau indicate variations in the shortening rate from 1 5 mm yr on the tripura mizoram salient trs south of the plateau to 6 mm yr in the imphal recess ir to the east and 8 mm yr in naga salient ns to the northeast this suggests that the deformation in the ibftb is segmented into n s blocks along e w transverse zones exhibiting dextral slip between ns ir and sinistral slip between ir and trs baselines between the ibftb sites also show 10 0 6 mm yr convergence pointing to the existence of an active transverse zone between aizawl and imphal

stress tensor inversion using detailed microearthquake information and stability constraints application to olfus in southwest iceland using detailed microearthquake data we present a stress tensor inversion scheme with new methods for selecting the fault planes and allowing for errors in the focal mechanisms the nonuniqueness of earthquake focal mechanisms is accounted for in our inversion scheme through the introduction into the inversion of a range of well fitting focal mechanisms for each event the range of focal mechanisms significantly improves the quality of the estimated stress tensor relative localization of clusters of microearthquakes is used to obtain information about which nodal plane could be correct fault plane the clusters frequently fall on a common fault plane and if there are acceptable focal mechanisms where one nodal plane has orientation similar to the common plane we assume this is the correct fault plane for the event if there is no predefined fault plane we utilize a simple mohr coloumb failure criterion to obtain a physical choice of fault plane between the two nodal planes in the focal mechanism the nodal plane with highest relative instability is chosen as the fault plane differences between the instability and the standard slip angle criterion are investigated the new inversion scheme has been applied to microearthquake data from the olfus area in the vicinity of the southwest iceland triple junction we estimate an oblique strike slip state of stress maximum horizontal stress at n30 e and minimum horizontal stress at n60 w with significant normal faulting influence the instability fault selection criterion predicts very well the orientation of faults mapped by relative localization

paleoseismic evidence of great surface rupture earthquakes along the indian himalaya 1 toward understanding the relationship between strain accumulation and strain release in the context of the mechanics of the earthquake and mountain building process and quantifying the seismic hazard associated with the globes largest continental thrust system we describe the late quaternary expression and paleoseismic evidence for great surface rupture earthquakes at six sites along the himalayan frontal thrust hft system of india our observations span a distance of 250 km along strike of the hft uplifted and truncated fluvial terrace deposits resulting from the holocene displacements on the hft are preserved along canyons of the ghaggar markanda shajahanpur and kosi rivers dividing the elevation of the bedrock straths at each site by their ages yields estimates of the vertical uplift rate of 4 6 mm yr which when assumed to be the result of slip on an underlying thrust dipping at 20 45 are equivalent to fault slip rates of 6 18 mm yr or shortening rates of 4 16 mm yr trench exposures reveal the hft to fold and break late holocene surface sediments near the cities and villages of chandigarh kala amb rampur ganda lal dhang and ramnagar radiocarbon ages of samples obtained from the displaced sediments indicate surface rupture at each site took place after a d 1200 and before a d 1700 uncertainties attendant to the radiocarbon dating currently do not allow an unambiguous definition of the capping bound on the age of the displacement at each site and hence whether or not the displacements at all sites were contemporaneous trench exposures and vertical separations measured across scarps at rampur ganda lal dhang and ramnagar are interpreted to indicate single event displacements of 11 38 m dividing the observed single event vertical components of displacement by the estimated longer term uplift rates indicates 1330 3250 or more years should be required to accumulate the slip sufficient to produce similar sized displacements surface rupture appears to not have occurred during the historical 1905 kangra mw 7 7 1934 bihar nepal mw 8 1 and 1950 assam mw 8 4 earthquakes which also occurred along the himalayan front yet we observe clear evidence of fault scarps and displacements in young alluvium and progressive and continued offset of fluvial terrace deposits along the hft we suggest on this basis and the size and possible synchroneity of displacements recorded in the trenches that there exists the potential for earthquakes larger than recorded in the historical record and with the potential to rupture lengths of the hft greater than the 250 km we have studied

strata bound fe co cu au bi y ree deposits of the idaho cobalt belt multistage hydrothermal mineralization in a magmatic related iron oxide copper gold system mineralogical and geochemical studies of strata bound fe co cu au bi y rare earth element ree deposits of the idaho cobalt belt in east central idaho provide evidence of multistage epigenetic mineralization by magmatic hydrothermal processes in an iron oxide copper gold iocg system deposits of the idaho cobalt belt comprise three types 1 strata bound sulfide lenses in the blackbird district which are cobaltite and less commonly chalcopyrite rich with locally abundant gold native bismuth bismuthinite xenotime allanite monazite and the be rich silicate gadolinite y with sparse uraninite stannite and bi tellurides in a gangue of quartz chlorite biotite muscovite garnet tourmaline chloritoid and or siderite with locally abundant fluorapatite or magnetite 2 discordant tourmalinized breccias in the blackbird district that in places have concentrations of cobaltite chalcopyrite gold and xenotime and 3 strata bound magnetite rich lenses in the iron creek area which contain cobaltiferous pyrite and locally sparse chalcopyrite or xenotime most sulfide rich deposits in the blackbird district are enclosed by strata bound lenses composed mainly of cl rich fe biotite some deposits have quartz rich envelopes whole rock analyses of 48 co and or cu rich samples show high concentrations of au up to 26 8 ppm bi up to 9 16 wt y up to 0 83 wt rees up to 2 56 wt ni up to 6 780 ppm and be up to 1 180 ppm with locally elevated u up to 124 ppm and sn up to 133 ppm zn and pb contents are uniformly low 821 and 61 ppm respectively varimax factor analysis of bulk compositions of these samples reveals geochemically distinct element groupings that reflect statistical associations of monazite allanite and xenotime biotite and gold detrital minerals chalcopyrite and sparse stannite quartz and cobaltite with sparse selenides and tellurides significantly cu is statistically separate from co and as consistent with the general lack of abundant chalcopyrite in cobaltite rich samples paragenetic relations determined by scanning electron microscopy indicate that the earliest y ree be mineralization preceded deposition of co cu au and bi allanite xenotime and gadolinite y commonly occur as intergrowths with and inclusions in cobaltite the opposite texture is rare monazite in contrast forms a poikiloblastic matrix to cobaltite and thin rims on allanite and xenotime reflecting a later metamorphic paragenesis allanite and xenotime also show evidence of late dissolution and reprecipitation forming discordant rims on older anhedral allanite and xenotime and separate euhedral crystals of each mineral textural data suggest extensive deformation of the deposits by folding and shearing and by pervasive recrystallization all during cretaceous metamorphism sensitive high resolution ion microprobe u pb geochronology by aleinikoff et al 2012 supports these paragenetic interpretations documenting contemporaneous mesoproterozoic growth of early xenotime and crystallization of megacrystic a type granite on the northern border of the district these ages are used together with mineralogical and geochemical data from the present study to support an epigenetic iocg model for fe co cu au bi y ree deposits of the idaho cobalt belt a sulfide facies variant of iocg deposits is proposed for the blackbird district in which reducing hydrothermal conditions favored deposition of sulfide minerals over iron oxides this new model includes mesoproterozoic vein mineralization and related fe cl metasomatism that formed the biotite rich lenses a predominantly felsic magmatic source for metals in the deposits given their local abundance of y rees and be and a major sedimentary component in the hydrothermal fluids based on independent sulfur isotope and boron isotope data for sulfides and ore related tourmaline respectively

himalayan seismic hazard the devastating earthquake in bhuj on 26 january 2001 highlighted the high earthquake risk in the indian region of gujarat in their perspective bilham et al warn that major earthquakes may be expected in another part of india the himalayan arc an analysis of historic earthquakes suggests that several major earthquakes are overdue in this region but that recent low activity has focused attention away from the himalayan arc projections from earlier earthquakes suggest that just one of the possibly overdue himalayan earthquakes may lead to 200 000 predictable fatalities

insights into the metamorphic evolution of the belt purcell basin evidence from lu hf garnet geochronology we have determined lu hf garnet ages from spatially separated garnet bearing localities in northern idaho the lu hf ages are diverse and reflect a progression of mesoproterozoic metamorphic events the oldest lu hf garnet age determined in this study is 1463 24 ma for garnet within a kyanite schist exposed in the core zone of the boehls butte metamorphic complex a garnet schist from the priest river complex yields a well defined age of 1379 8 ma a garnet staurolite schist a garnet mica schist and a gem grade idaho star garnet sample all from the general vicinity of clarkia idaho yield ages of 1064 10 1081 20 and 1078 17 ma respectively a garnet amphibolite also collected from the boehls butte metamorphic complex yields an age of 1151 41 ma for garnet porphyroblasts and 137 25 ma for diffuse overgrowths the hf in all of the proterozoic garnets is extraordinarily radiogenic with ehf values ranging from 1210 to 10546 which attest to both their high lu hf ratios and their great

structural control on along strike variation in the seismicity of the northwest himalaya an overview of seismicity along the northwest himalaya exhibit along strike segmentation intricately controlled by the subsurface topographic ridges on the underthrusting indian plate as well by rift and nappe structures in the overriding wedge of the himalaya the segmentation exists for thrust dominated large magnitude earthquakes m 6 seated on the active detachment beneath the outer and lesser himalaya segmentation also prevails for moderate and small magnitude earthquakes concentrated in a narrow himalaya seismic belt hsb straddling northern lesser himalaya and southern higher himalaya numerical calculations of stress distribution favour that the degree of seismicity in the hsb is a good proxy to the presence of mid crustal ramp connecting the locked section of active detachment to aseismically slipping detachment beneath higher himalaya further gap or diffused pattern in the concentrated seismicity in the hsb in agreement with mapped high electrical conductive structure suggests ramp structure may be absent where underthrusting delhi hardwar ridge interacts with the himalaya arc in the nappe dominated tectonics the accommodation of the accumulating strains on the listric thrust faults produces increased frequency of moderate magnitude earthquakes and thereby possibly influences stress level on the detachment this may explain relatively less frequent occurrences of large earthquake in the kangra chamba region compared to that in the garhwal himalaya the low level seismicity in the simla region may also be influenced by the active kaurik chango rift in the higher himalaya as the former casts a stress shadow on the latter

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

long distance longitudinal transport of gravel across the cordilleran thrust belt of montana and idaho two newly identified middle eocene paleovalleys 100 km long preserved on top of the southwest montana reentrant of the cordilleran fold and thrust belt indicate long lived longitudinal flow across the thrust belt and resolve a long standing debate about the source of the voluminous quartzite debris in the upper cretaceous to lower tertiary divide harebell and pinyon conglomerates of montana idaho and wyoming geologic mapping stratigraphic provenance and geochronologic studies revealed that eocene volcanic and sedimentary rocks in the paleovalleys are as thick as 2 km onlap preexisting bedrock and interfinger with well rounded conglomerate derived from formations exposed only to the west the middle eocene paleovalleys are the youngest expression of a major paleoriver system that transported sediment toward the foreland during the sevier orogeny an eocene subcrop map shows that the headwaters of the eocene paleovalleys coincided with structural culminations in the thrust belt that supplied sediment to the divide conglomerate of the upper cretaceous to lower tertiary beaverhead group ultimately the lemhi pass and hawley creek paleovalleys provided several thousand cubic kilometers of quartzite debris to the pinyon and harebell conglomerates of northwest wyoming 200 350 km away and formed the northwest half of a giant longitudinal drainage system sevier contraction not the rising idaho batholith first uplifted vast culminations beneath the headwaters of this river system

upper crustal compressional wave velocity in the garhwal himalaya a value of 5 2 km s is obtained for compressional wave velocity in the upper crust in the vicinity of the main central thrust where it crosses the yamuna and bhagirathi valleys in the garhwal himalaya the data used consisted of the arrival times of compressional waves recorded at five portable stations from nine earthquakes occurring in the region

electrical resistivity cross section across the garhwal himalaya proxy to fluid seismicity linkage abstract magnetotelluric mt measurements along a profile cutting across the garhwal himalaya of india are inverted to obtain 2 d electrical resistivity structures of the himalayan wedge and of the underthrusting indian plate the imaged resistivity cross section is dominated by a low angle north east dipping intra crustal high conducting layer ic hcl with an average thickness of 5 km at transition from the lesser himalaya to the higher himalaya the ic hcl is marked by a ramp structure across which its top jumps from a depth of 8 km to 13 km high conductivity of the layer is caused by pounding of upward propagating metamorphic fluids trapped by tectonically induced neutral buoyancy in compression regime of the himalaya the mechanical weakening effects of the fluids counteract the fault normal stresses thereby facilitating thrust type earthquakes on a plane imaged as the top of the ic hcl it is suggested that in the himalaya collision belt like the active subduction zone the active seismic plane forming seat of large and great earthquakes is located a few kilometers above the top of the down going plate in this tectonic setting the high conductance ramp symbolizes a block of low shear strength and high strain which under the deviatoric stresses release accentuated stresses into the brittle crust thereby generating small but more frequent earthquakes in the narrow himalayan seismic belt in response to either the co seismic pumping or the stress transfer during inter seismic period the upward infiltration of fluid fluxes into the over pressurized zones sufficiently reduces the shear strength of local thrusts and shear zones turning these into locales of concentrated seismicity

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

geochemistry of precambrian carbonates vii belt supergroup montana and idaho usa carbonates from the 1100 1450 ma old proterozoic belt supergroup were collected from stratigraphic sections throughout montana and idaho usa the sampled sequences in ascending stratigraphic order include the newland altyn spokane greyson transition empire wallace helena siyeh snowslip shepard and libby formations an increase in the degree of postdepositional alteration of belt limestones is reflected in a diminution of sr and mg contents an increase in mn and depletion in 13c and 18o two diagenetic trends can be resolved for the limestones one affecting the presumed originally aragonite rich sediments includes carbonates from the lower belt newland formation in contrast the middle belt carbonate wallace helena siyeh formations may have been originally of high mg calcitic mineralogy projection of the alteration trends for the lower and middle belt limestones suggest 21 smow and 2 5 to 1 0 pdb as the best preserved values for the 18o and 13c of seawater respectively both comparable to results from other mesoproterozoic carbonate sequences the oxygen isotope data for limestones show a regional westward depletion of 8 in 18o possibly reflecting a higher temperature of postdepositional alteration in the western belt basin this depletion in 18o is accompanied by a comparable decrease in 13c values most likely because a higher proportion of carbon was incorporated from co2 generated by thermal cracking of hydrocarbons at depth dolostones in the belt basin are dominantly micritic with good preservation of depositional textures chemically and isotopically their alteration trends mimic those of limestones leading to comparable projected best values for 13c 87sr 86sr values of belt carbonates range between 0 70484 and 0 74991 progressive diagenesis as indexed by decreasing concentrations of sr and depletions in 18o and 13c results in an increase in 87sr 86sr values the least radiogenic measurement from a lower belt newland limestone sample appears to fit reasonably well into the general trend for proterozoic seawater as discussed in mirota and veizer 1994 this and the consistency of 18o and 13c in limestones with other coeval sequences suggests that the lower and middle belt carbonate sections are marine deposited in an environment that was not continuously separated from the open ocean

structural controls and evolution of gold silver and ree bearing copper cobalt ore deposits blackbird district east central idaho epigenetic origins the cu co au ag ni ree ore deposits of the blackbird district east central idaho have previously been classified as besshi type vms sedex and iocg deposits within an intact stratigraphic section new studies indicate that across the district mineralization was introduced into the country rocks as a series of structurally controlled vein and alteration systems quartz rich and biotite rich veins and alteration zones and minor albite and siderite veinlets maintain consistent order and sulfide mineral associations across the district both early and late quartz veins contain chalcopyrite and pyrite whereas intermediate stage tourmaline biotite veins host the cobaltite barren early and late albite and late carbonate generally siderite form veins or are included in the quartz veins ree minerals principally monazite allanite and xenotime are associated with both tourmaline biotite and late quartz veins the veins are in mineralized intervals along axial planar cleavage intrafolial foliation and shears mineralized intervals are hosted by a variety of metasedimentary rocks including three phyllitic units of mesoproterozoic age and two schistose units all of these units are s tectonites in the footwall of a regional thrust fault specifically the district lies within an oblique thrust ramp containing a series of structural horses three domains in a duplex system the deposits span the three domains and are hosted by metamorphic rocks that range from lower amphibolite facies in the structurally upper domain to lower middle greenschist facies in the lower domain an inverted metamorphic sequence early quartz and biotite veins were introduced during progressive folding and prolonged peak metamorphic conditions and they underwent late tectonic retrograde recrystallization and metamorphic mineral growth to the same extent as the country rocks in each domain where little subsequent deformation occurred early veins are discordant to bedding but where folding was polyphase and fabrics are penetrative mineralized zones are concordant with metamorphic compositional layering late quartz veins in the zones are associated with retrograde minerals and textures and are only locally deformed 40 ar 39 ar dating of unoriented muscovite from the selvage of a late quartz vein yields a late cretaceous age of about 83 ma the time of retrograde metamorphism associated with introduction of late quartz veins textural data at all scales indicate that the host sites for veins and the tectonic evolution of both host rocks and mineral deposits were kinematically linked to late cretaceous regional thrust faulting heat fluids and conduits for generation and circulation of fluids were part of the regional crustal thickening the faulting also juxtaposed metaevaporite layers in the mesoproterozoic yellowjacket formation over blackbird district host rocks we conclude that this facilitated chemical exchange between juxtaposed units resulting in leaching of critical elements cl k b na from metaevaporites to produce brines scavenging of metals co cu etc from rocks in the region and finally concentrating metals in the lower plate ramp structures although the ultimate source of the metals remains undetermined the present cu co au ag ni ree blackbird ore deposits formed during late cretaceous compressional deformation

compositional variation of glauconites in upper cretaceous paleogene sedimentary iron ore deposits in south eastern western siberia abstract glauconite occurs either as unaltered greenish or as altered brownish variety in upper cretaceous palaeocene sediments in the southeastern corner of western siberia studied section within the bakchar iron ore deposit includes ipatovo slavgorod gan kino and lyulinvor formations which are represented by sandstones siltstones claystones and oolitic ironstones of coastal marine facies the origin of unaltered glauconite is explained by the verdissement theory transgressions during lower coniacian santonian and campanian favored the formation of unaltered glauconites in dysoxic to anoxic conditions subaerial exposure of glauconite resulted in leaching of potassium oxidation of iron and formation of iron hydroxides in upper coniacian maastrichtian and palaeocene glauconite ultimately converts to leptochlorite and hydrogoethite by this alteration abundant microscopic gold inclusions besides sulphides sulphates oxides and silicates characterize this glauconite mineral inclusions include precious rare metals and non ferrous metals the concentration of gold in glauconite may be as high as 42 9 ppb abundant inclusions of various compositions in glauconites indicate enrichment of marine sediments in precious and non precious metals while major element composition of glauconites is affected by subaerial exposure the broadly similar micro inclusions in both altered and unaltered varieties are possibly related to the comparatively immobile nature of ree and trace elements

tracking deep crust by zircon xenocrysts within igneous rocks from the northern alxa china constraints on the southern boundary of the central asian orogenic belt abstract the southern boundary of the central segment of the central asian orogenic belt caob with the precambrian alxa block is not well constrained due to poor recognition of deep crust statistical analysis of zircon xenocrysts within igneous rocks from the northern alxa and its adjacent regions was applied to resolve this problem we compiled new and previously published geochronological zircon age data obtained by shrimp sims tims and la icp ms for 316 igneous rock samples of which 61 samples contain zircon xenocrysts new and previously published hf isotopic compositions of these zircon xenocrysts are combined with zircon ages in this study the zircon xenocrysts are mainly contained within permian rocks from the yabulai nurgong honggueryulin ynh zone and igneous rocks from the northwestern margin of the north china craton wncc a few xenocrysts were also found in permian igneous rocks from the zongnaishan shalazhashan zs zone neo proterozoic and paleo proterozoic intrusive rocks in the ynh zone as well as devonian to carboniferous granitoids and volcanic rocks from southern mongolia altogether we analyzed more than 270 zircon xenocrysts and considered only ages that are less than 10 discordant xenocryst ages within the permian igneous rocks from the zs zone are mainly around ca 350 ma ca 600 ma and ca 1400 ma the oldest age of zircon xenocrysts in this zone is similar to those of zircon xenocrysts from the caob 1 1 ga by contrast abundant zircon xenocrysts within permian igneous rocks from the ynh zone show highly variable age populations at ca 2 6 2 1 ga 1 8 1 6 ga 930 750 ma and 460 300 ma zircon xenocrysts from the zs zone have positive e hf t values of 6 3 to 13 9 whereas those from the ynh zone display highly variable e hf t values from 16 1 to 11 6 from the southern caob and the zs zone to the ynh zone the zircon xenocrysts show a significant shift from juvenile to crustal hf isotopic compositions suggesting that the area between the zs and the ynh zones constitutes the southern boundary of the caob in the deep crust within this region our study indicates that statistical analysis or isotopic mapping of zircon xenocrysts is an effective method to trace the nature of the deep continental crust and to separate between newly formed orogenic domain and ancient cratons

substrate control on formation and maturation of glauconites in the middle eocene harudi formation western kutch india formation and maturation of glauconites within the early middle eocene harudi formation built up in a lagoon to shelf transition in western kutch india is addressed taking recourse to a combined sedimentological micropalaeontological and mineralogical investigation the glauconite is authigenic and its occurrence coincides with maximum flooding the glauconite is more matured within the fecal pellets and is less matured within the intra particle pores of bioclasts namely foraminifera ostracoda gastropoda and bryozoa sem eds xrd and geochemical studies clearly document that the infilling within the intra particle pores of bioclasts belong to nascent to slightly evolved glauconite 5 wt k2o while the pellet belongs to slightly evolved to evolved glauconite 5 7 wt k2o based on chemical characteristics and sem investigations the glauconitization process can be best explained by the verdissement theory the glauconite possibly formed as initial authigenic glauconitic smectite precipitates while the pellet matured subsequently by addition of k the process was hindered in case of infillings the maturation process also involved addition of si and release of al with or without release of mg the glauconite maturation was facilitated in case of fecal pellets because of higher porosity inter granular nature of pores and availability of necessary elements in semi confined micro environments in contrast the maturation was aborted at an early stage in case of infillings because of the minute closed nature of the pores and prevalent alkaline conditions not allowing dissolution of calcitic tests a low negative cerium anomaly suggests glauconite formation in sub oxic micro environments created by decay of organic matters within the fecal pellets and bioclasts a clear case of substratum control on glauconitization becomes apparent

pre rodinian mesoproterozoic supercontinental rifting along the western margin of laurentia geochemical evidence from the belt purcell supergroup abstract recent efforts at supercontinental reconstruction for the late precambrian e g the sweat hypothesis moores 1991 have commonly invoked an uncertain lacustrine interpretation for deposition of the mesoproterozoic belt purcell supergroup with the belt basin placed centrally within the assembled landmass the utility of the belt purcell system in addressing proterozoic continental configurations has improved with refined constraints on the timing and setting of deposition contrary to a lacustrine interpretation elemental carbon sulfur c s relationships and stratigraphic s isotope trends for black shales of the helena embayment of western montana the easternmost extension of belt deposition reveal that the early belt basin received at least episodic fluxes of open marine water into a variably restricted setting in the present study this model of restricted marine deposition was tested and corroborated by generation of an analogous s isotope stratigraphy for the time equivalent prichard formation located to the west of the embayment within the main belt basin data for both pyrite and coexisting pyrrhotite show comparatively high 34 s values and systematic s isotope shifts in excess of 20 over stratigraphic intervals spanning several hundred meters suggesting limited and temporally varying sulfate availability in a bacterially driven marine system additional geochemical and sedimentological proxies e g 13 c of carbonate lithologies diagnostic abiotic carbonate precipitates and the presence of glauconite further support a repeated or continued marine linkage throughout belt deposition a model for a restricted marine setting when viewed in light of independent tectonic evidence i e syndepositional mafic magmatism with peaks in activity at approximately 1470 and 1370 ma hydrothermal activity mineralization along basin bounding high angle normal faults and high rates of sedimentation that resulted in sediment thicknesses of up to 16 km defines a history of active and likely episodic rifting for the belt basin collectively these observations are temporally consistent with new models for precambrian supercontinent cyclicity and suggest a complex history for western laurentia that includes both episodes of convergence and at least incipient rifting prior to the final assembly of rodinia this relationship reflects a stepwise continental assembly perhaps analogous to the formation of gondwana and the subsequent assembly of pangea

lower edge of locked main himalayan thrust unzipped by the 2015 gorkha earthquake faults are unlocked by earthquakes analysis of seismic data from the 2015 nepal earthquake shows that only part of the main himalayan thrust fault was unzipped by the quake leaving much of the fault locked and ready to slip in a future event

evolution patterns of glaucony maturity a mineralogical and geochemical approach abstract integrated mineralogical and geochemical analyses were carried out on a wide variety of glauconitic grains from 25 western european localities covering the whole range of glaucony maturity and ranging in age between the cretaceous and the pliocene a comprehensive data set documents the major evolution patterns of glaucony and the relationships between glauconitic minerals and their substrate components increasing glaucony maturity involves potassium and rubidium enrichment the progressive evolution from a k poor glauconitic smectite to a k rich glauconitic mica is accompanied by the overall decrease of distance between peaks 001 and 020 the width of basal reflection 001 however is proposed here as a more powerful tool for the discrimination of glauconitic minerals at different stages of maturity this parameter is especially efficient for the differentiation of evolved and highly evolved glaucony the range of initial substrate composition appears to control significantly major and trace element variations during glaucony maturation contrasting patterns are observed in glauconitic grains as a function of the proportion of carbonate versus siliciclastic substrate components particularly the distribution patterns of mgo and sio2 in glaucony associated with carbonate host rocks suggest progressive uptake of these elements in the structure of glauconitic minerals with increasing degree of maturation by contrast a decrease in sio2 and ai2o3 with increasing maturity may be observed when the substrate for glauconitization is represented by siliciclastic material fe shows a positive correlation with k2o in glaucony with low maturity but an opposite trend in evolved glaucony therefore fe cannot be regarded as a reliable indicator of glaucony maturity maturation of glauconitic minerals results invariably in ca and sr depletion irrespective of the original composition of the precursor material as a consequence the proportion of ca and sr may be considered as an inverse relative criterion of glauconitic mineral evolution for a given source area

four cordilleran paleorivers that connected sevier thrust zones in idaho to depocenters in california washington wyoming and indirectly alaska upper cretaceous sandstones from 17 localities from california to southeastern alaska united states contain unexpectedly large populations of detrital zircons with proterozoic u pb ages with age peaks at 1800 1650 and 1380 ma these peaks are indicative of a sediment source region in the southern part of the proterozoic belt supergoup basin in central idaho which hosts 1800 1650 ma detrital zircons and which was intruded by rift related 1380 ma bimodal plutons and sills belt rocks were strongly uplifted and eroded during late cretaceous sevier shortening and fed four paleoriver systems the lemhi pass hawley creek river system flowed east and sourced the beaverhead harebell pinyon nonmarine megafan in the cordilleran foreland basin the kione river flowed southwest to northern california where it sourced a very large ca 82 80 ma 600 m thick delta and submarine fan complex within the northern great valley forearc basin considerable kione detritus also transited the forearc basin to reach the franciscan trench sourcing a pulse of deposition and subduction accretion in central california and even part of southern california the swakane river flowed northwest out of idaho into washington sourcing the protolith for the high grade swakane gneiss more speculatively a yakutat river may have flowed northwest and deposited yakutat strata in a trench off washington or british columbia before those rocks were translated north to southeastern alaska recognition of a major source area in central idaho for zircons with an uncommon age of 1380 ma helps constrain the ca 85 65 ma paleogeography and paleotectonics of major sectors of the north american convergent margin orogen

compositional variability of glauconites within the upper cretaceous karai shale formation cauvery basin india implications for evaluation of stratigraphic condensation abstract a detailed mineral chemical investigation of glauconite within the condensed section deposits of the cretaceous karai shale formation cauvery basin india reflects a wide spectrum in chemical composition related to origin and evolution in different substrates stratigraphic condensation and post depositional alteration fe and mg rich glauconite comprising up to 60 of the sedimentary rocks occurs as replaced forms of fecal pellets as infillings within pores and chambers of bioclasts including those of foraminifera ostracoda bryozoa and algae and as altered forms of mica exhibiting vermiforms authigenic precipitation of k and fe poor glauconite followed by addition of fe and k into the lattice and concomitant release of al and si explains the origin of glauconite pellets and infillings the origin of glauconite vermiforms in partly degraded mica involves only the second stage of fe and k addition glauconite pellets and vermiforms exhibit sharply defined alteration zones along peripheries to form rims and in proximity to cracks or cleavages with reduced k2o and fe2o3 total and enhanced al2o3 and sio2 related to late stage meteoric water actions cores of glauconite pellets and unaltered zones of vermiforms reflect evolved characteristics with 6 k2o typical of a condensed section while other glauconite varieties occurring at the same stratigraphic level exhibit slightly evolved nature not consonant with stratigraphic condensation increasing abundance of glauconite pellets from the bottom to the top of the transgressive systems tract accompanied by slight increase in k2o within their cores reflects the effect of stratigraphic condensation on the evolution of glauconite high fe2o3 total content of glauconite in the karai shale formation may be related to upwelling although the fe may be contributed partly by the biotite substrate mossbauer spectroscopy of glauconites reveals significant total fe substitution in both tetrahedral and octahedral sites detailed mineral chemical analysis enables us to distinguish stratigraphically significant glauconite within the karai shale formation from the rest of the glauconite notwithstanding its wide compositional range

neoarchean and paleoproterozoic crystalline basement rocks of north central idaho constraints on the formation of western laurentia zircon u pb analyses of 18 orthogneisses from the clearwater and priest river complexes of northern idaho have identified important exposures of neoarchean and paleoproterozoic basement rocks in northwest laurentia all samples have ages that fall into two tightly defined age ranges in the neoarchean from 2 67 to 2 65 ga and in the paleoproterozoic from 1 88 to 1 84 ga the neoarchean orthogneisses show no evidence of older components whereas some of the paleoproterozoic orthogneisses have xenocrystic zircon cores with ages overlapping with those of the neoarchean gneisses collectively these two packages of rocks represent the two main periods of crust formation in northwest laurentia one wholly juvenile in the neoarchean and the other a mixture of juvenile paleoproterozoic and inherited neoarchean components based on existing data the ages of orthogneisses in the clearwater and priest river complexes are identical and probably represent one continuous crustal block we refer to this basement domain as the clearwater block the paleoproterozoic 1 86 ga magmatism described here is distributed throughout the northwest laurentian margin and does not coincide with the proposed trend of the great falls tectonic zone therefore a model of a single linear arc preserved within the great falls tectonic zone is inadequate in describing the majority of the known 1 86 ga crust in the region

origin of the inverted metamorphism of the lower himalayas central nepal abstract the southern slope of the central nepal himalayas is divided geologically by the main central thrust zone into the underlying midland metasediment zone lower himalayas in the south and the overlying himalayan gneiss zone higher himalayas in the north the former zone consists of a thick pile of various kinds of metasediments folded into an anticlinorium and the latter zone consists of high grade crystalline rocks with a north dipping homoclinal structure and is further overlain by the tethys himalayas composed of non or slightly metamorphosed tethyan sediments the main central thrust zone shows a characteristic lithology such as calcareous schist quartzite phyllitic schist and augen gneiss and is intensely mylonitized the mineral composition and paragenesis of the rocks in the present area indicate an inverted metamorphism from the midland metasediment to the main central thrust zones that is the metamorphic grade of the midland metasediment zone increases upward toward the thrust zone from the chlorite to the biotite zones of the greenschist facies and the greenschist amphibolite transitional zone is attained in the thrust zone the himalayan gneiss zone is apparently much higher in metamorphic grade than the thrust zone on the basis of the compositional zoning of garnets and muscovite compositions however it is concluded that the himalayan gneisses suffered a polymetamorphism the younger metamorphism resulted in a retrograde metamorphism of the himalayan gneisses together with the above mentioned inverted metamorphism and an anatectic melting in depth this was caused by shear heating due to the thrust movement along the main central thrust zone during the alpine tectonism the older metamorphism which seems to be precambrian in age was much higher in metamorphic grade than the younger one and formed the pyrope rich cores of garnets in the himalayan gneisses the maximum metamorphic temperature of 550 c estimated from the mineral paragenesis in the thrust zone is compatible with that caused by shear heating due to the thrusting with thrusting velocity of 5 10 cm yr maximum stress of 300 bars and thrusting distance of 250 km bird 1978

high iron glaucony from the continental shelf of the yellow sea off the southwestern korean peninsula abstract massive fractured and porous types of glauconies are found in subtidal sand and semi consolidated sand mud deposits from the continental shelf of the southeastern yellow sea thermal treatments on powdered glaucony increase the x ray intensity near 10 a suggesting the presence of some expandable layers ferric iron and mg 0 35 0 54 atoms progressively substitute for al from porous to massive glauconies a higher proportion of expandable layers is also observed with apparent increases from porous to massive type the clay preserved is ordered disordered massive to fractured type and interstratified illite smectite porous type containing 7 27 expandable layers glauconies from the yellow sea apparently form after deposition in reducing environments at the interface between dissolving clay minerals and degrading biogenic debris