a different perspective on glauconite as a chronometer for geologic time scale studies there are advocates of the use of glauconite as a chronometer for geologic time scale studies who insist that if the geohistory of the sample in question is known in detail one can ascertain before hand that the sample will yield a radiometric result that is accurate in the geological sense examples are presented indicating that while some glauconite samples may yield the correct age within their assigned analytical uncertainties when compared to high temperature minerals others clearly do not and are too young by varying degrees the outright acceptance of published results on glauconite can result in erroneous age estimates of various biostratigraphic levels and stage or epoch boundaries

stratigraphy composition and form of the deccan basalts western ghats india in the western ghats between latitudes 18 20 n and 19 15 n 7000 km2 of deccan basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts using over 70 measured vertical sections major and trace element analyses of nearly 1200 samples and rare earth and87sr 86sr determinations for over 60 samples we divide the basalt into three subgroups and ten formations in this paper we describe the seven principal formations in the area and the most prominent individual flows

evolution of the lithosphere in the area of the rhine rift system the rhine rift system rrs forms part of the european cenozoic rift system ecris and transects the variscan orogen permo carboniferous troughs and late permian to mesozoic thermal sag basins crustal and lithospheric thicknesses range in the rrs area between 24 36 km and 50 120 km respectively we discuss processes controlling the transformation of the orogenically destabilised variscan lithosphere into an end mesozoic stabilised cratonic lithosphere as well as its renewed destabilisation during the cenozoic development of ecris by end westphalian times the major sutures of the variscan orogen were associated with 45 60 km deep crustal roots during the stephanian early permian regional exhumation of the variscides was controlled by their wrench deformation detachment of subducted lithospheric slabs asthenospheric upwelling and thermal thinning of the mantle lithosphere by late early permian times when asthenospheric temperatures returned to ambient levels lithospheric thicknesses ranged between 40 km and 80 km whilst the thickness of the crust was reduced to 28 35 km in response to its regional erosional and local tectonic unroofing and the interaction of mantle derived melts with its basal parts re equilibration of the lithosphere asthenosphere system governed the subsidence of late permian mesozoic thermal sag basins that covered much of the rrs area by end cretaceous times lithospheric thicknesses had increased to 100 120 km paleocene mantle plumes caused renewed thermal weakening of the lithosphere starting in the late eocene ecris evolved in the pyrenean and alpine foreland by passive rifting under a collision related north directed compressional stress field following end oligocene consolidation of the pyrenees west and northwest directed stresses originating in the alps controlled further development of ecris the rrs remained active until the present whilst the southern branch of ecris aborted in the early miocene extensional strain across ecris amounts to some 7 km plume related thermal thinning of the lithosphere underlies uplift of the rhenish massif and massif central lithospheric folding controlled uplift of the vosges black forest arch

landsliding and the evolution of normal fault bounded mountains much of the tectonic and climatic history in high relief regions such as the mountains of the western u s basin and range province is contained in the morphology of hillslopes drainage networks and other landforms that range in scale from 10 1 to 101km to understand how these landforms evolve we have developed a numerical landscape evolution model that combines a detailed tectonic displacement field with a set of physically based geomorphic rules bedrock landsliding long recognized as a significant geomorphic process in mountainous topography is for the first time explicitly included in the rule set in a series of numerical experiments we generate synthetic landscapes that closely resemble mountainous topography observed in the basin and range the production of realistic landscapes depends critically on the presence of bedrock landslides and landsliding yields rates of long term erosion that are comparable in magnitude to those of fluvial erosion the erosive efficiency of bedrock landsliding implies that hillslopes may respond very quickly to changes in local base level and that fluvial erosion is the rate limiting process in steady state experimental landscapes our experiments generate power law distributions of landslide sizes somewhat similar to both field and laboratory observations thus even a simple model of bedrock landsliding is capable of quantitatively reproducing mountainous topography and landslide distributions and represents a significant step forward in our understanding of the evolution of normal fault bounded ranges

high resolution fault displacement mapping from three dimensional seismic data evidence for dip linkage during fault growth abstract detailed mapping using high resolution three dimensional seismic data has revealed a number of sub horizontal anomalies in the distribution of vertical displacement throw on the planes of growth faults in the gulf of mexico recognition of these anomalies is highly sensitive to the interval at which the fault displacements are sampled because they represent local decreases in throw which are confined to small discrete parts of the fault planes the distribution of the anomalies is inconsistent with model displacement fields of quasi elliptical concentric contours and is therefore incompatible with models of fault growth by uniform slip distribution and radial tip line propagation an alternative model is proposed whereby the evolution of a fault plane is established by the propagation and linkage of precursor fault segments in the dip direction dip linkage overlap and linkage of fault tips in the dip direction results in relay structures that are sub parallel to fault strike and therefore displacement minima that are sub horizontal on normal and thrust faults however since they are orthogonal to the main slip direction on the fault these structures have a low preservation potential and are therefore unlikely to be well resolved on cross sectional seismic profiles

the relationship between the hawaiites and basalts of the itcha volcanic complex central british columbia volumetrically subordinate alkaline mafic lava flows form a late capping stage over the earlier felsic lavas that form the shield of the itcha volcanic complex ivc of the anahim volcanic belt avb in central british columbia b c the mafic capping stage of the ivc is dominated by hawaiites which are the earliest of the mafic lavas and are succeeded by alkali olivine basalts aob and then by basanites the alkali olivine basalts can be subdivided into high intermediate and low mgo aob groups all of which share similar hfse ratios e g nb zr with the hawaiites high al contents and sr zr ratios indicate that hawaiites and fe rich evolved aob were derived from primitive aob parental magmas by crystal fractionation of a wehrlitic assemblage at pressures on the order of 8 to 10 kbar high si and low fe contents indicate that the majority of the evolved aob lavas however do not represent an intermediate stage in the liquid line of descent to hawaiites but were most likely produced by gabbroic fractionation from primitive aob magmas at relatively low pressures the parental magmas of the majority of these lavas were distinct from those of the observed high mgo basalts having higher hfse contents and being more si under saturated the high al high sr zr signature of high pressure fractionation of a clinopyroxene dominated assemblage in the ivc is shared by hawaiites of other alkaline volcanic suites of the canadian cordillera such as the edziza volcanic complex in northern b c and appears to be a feature of hawaiites in many localities including hawaii and iceland viscosities calculated for both high and low pressure crystal fractionation models suggest that aphyric hawaiites are residual liquids escaped from a wehrlitic crystalline network at elevated pressures possibly at the base of the crust

global volcanism biological mass extinctions and the galactic vertical motion of the solar system summary the recent findings of nearly matching long term cyclicity in biological mass extinctions geomagnetic reversals impact cratering and other terrestrial processes have evoked a major controversy we report here a compilation and analysis of major global magmatic episodes showing a significant enhancement of volcanic activity with a periodicity of 33 million years for the last 250 million years these magmatic episodes match extremely well the best available astronomical estimates of the periodic 31 5 myr galactic disc crossing events during vertical motions of the solar system these events also have a close linkage with the marine biological mass extinctions and other geological rhythms it is argued that the prolonged volcanic activity instead of impact cratering may have been the more immediate primary cause for profound climatic and other environmental deterioration sufficient to create biological crises on a global scale the volcanic periodicity scheme is well supported by the evidence of recent increases in volcanic activity however the alternative hypothesis of the death star nemesis can be rejected since it does not explain the recent increase in volcanism and impact cratering during the last 4 3 myr in view of its quieter apehelion position lying beyond the oort cloud of comets

rift basins in western margin of india and their hydrocarbon prospects with special reference to kutch basin the western continental margin of india can be classed as a divergent or passive margin the western continental shelf is an extensive carbonate bank bombay offshore basin passing into clastic sediments on the north and south three craton margin embayed basins kutch cambay and narmada in the northern part of the shelf are filled predominantly with clastic sediments these basins occupy grabens bounded by faults diverging seaward the grabens were formed by three rift systems along major precambrian tectonic trends the rifting developed sequentially from north to south around the saurashtra horst kutch basin was formed in the early jurassic followed by cambay basin in early cretaceous time and the narmada in the late cretaceous it appears that these rifting ev nts occurred at successive stages during the northward migration of the indian plate after its break from gondwanaland in late triassic or early jurassic it is inferred that these rift basins opened up successively as a result of the counterclockwise drift of the indian craton bombay offshore and cambay are two major oil producing basins in the western margin these basins are characterized by high geothermal gradients attributed to the shallowness of the mantle in this region oil has not been found in kutch basin which is mainly an onshore mesozoic basin the basin depocenter shifted offshore at the northwestern part of the continental shelf where the shelf is wide

signals of degeneration of the sub crustal part of the indian lithosphere since the break up of gondwanaland abstract currently the continental fragments of the erstwhile gondwanaland supercontinent are widely separated a relative comparison of magsat anomalies of these fragments reveals a striking similarity across all the rifted continental margins except at the western and southern margins of india it reflects that magnetized crust has largely remained unchanged elsewhere since the break up of gondwanaland a similar analysis of satellite derived free air gravity anomalies also indicate a similar continuity across other rifted margins except those of india these anomalies characterise the existing differences in the crustal and upper mantle structures and causal dependence on the relative mobilities of the gondwanic continents it is evident that after the break up only the indian continental structure has suffered a dramatic change possibly due to its super northward drift from antarctica during late cretaceous early tertiary

cenozoic postrift domal uplift of north atlantic margins an asthenospheric diapirism model postrift domal uplifts are a distinct feature of the northern north atlantic rifted margins uplifted regions southern norway finnmark svalbard northern united kingdom occur with a typical spacing of 900 km the amount of domal uplift is substantial 1 2 km thermochronological geomorphological and seismic stratigraphic data suggest a coincident neogene onset of uplift postdating north atlantic breakup and predating onset of glaciation the norwegian domes are characterized by strongly negative gravity anomalies and reduced mantle p wave and s wave velocities previous models for uplift around the north atlantic e g glacial erosion intraplate stresses secondary convection fail to explain the above characteristics we present an asthenospheric diapir model based on a rayleigh taylor instability wherein a hot icelandic asthenosphere layer meets cold cratonic lithosphere the generated asthenospheric diapirs and associated partial melting offer a viable explanation for updoming application of this model to the south norwegian dome succesfully explains most of the currently available data

the origin of large local uplift in extensional regions large localized uplift is commonly observed in continental regions undergoing extension these observations can be modelled by planar high angle normal faulting of an elastic upper crust overlying an inviscid lower crust isostasy provides the necessary driving force the model quantifies the role of flexural rigidity density variations in the crust and erosion and deposition of sediment

magnetovariational study over a seismically active area in the deccan trap province of western india this paper presents the results of a magnetovariational study undertaken over a basalt covered region of western india which has been experiencing a swarm of seismic activity since early 1986 the most striking feature of the transient variations is the anomalous enhancement of the east west y component at stations bounding the seismically active belt from the nature of the horizontal transfer functions and the anomalous induction arrows it is inferred that this behaviour is related to the presence of a resistive body embedded in a north south oriented conductive belt consistent with an evolutionary model of deccan volcanism and other geophysical data it is suggested that the central resistive block coincident with the concentrated belt of earthquake epicentres represents either a dormant volcanic plug or a plutonic body rooted in upwarped conducting mantle to account for the phase relationship between the anomalous y variations and the normal north south x variations it is proposed that the upwarped asthenosphere with its associated high geothermal anomaly acts as a conducting path to channel induced current from the region of the cambay triple junction located towards the northwest of the array the existence of the strong induced currents in the region of the triple junction conforms with the induction arrow pattern observed at the array stations

late pleistocene holocene slip rate of the zhangye thrust qilian shan china and implications for the active growth of the northeastern tibetan plateau 1 we derive a slip rate for a thrust at the central qilian shan mountain front by combining structural investigations satellite imagery topographic profiling luminescence dating and 10be exposure dating the seismically active zhangye thrust transects late pleistocene alluvial fan deposits and forms a prominent north facing scarp the fault consists of two segments that differ in orientation scarp height and age a series of loess covered terraces records the uplift history of the western thrust segment loess accumulation on all terraces started at 8 5 1 5 kyr and postdates terrace formation gravels from the highest terrace yielded a 10be exposure age of 90 11 kyr which dates the onset of faulting with a displacement of 55 60 m derived from fault scarp profiles this yields a vertical slip rate of 0 64 0 08 mm yr 1 along the eastern thrust segment three 10be ages from the uplifted alluvial fan constrain that faulting started at 31 5 kyr together with a displacement of 25 30 m this leads to a vertical faulting rate of 0 88 0 16 mm yr 1 a dip estimate of 40 to 60 for the fault plane combined with lower and upper limits of 0 6 and 0 9 mm yr 1 for the vertical slip rate gives minimum and maximum horizontal shortening rates of 0 4 and 1 1 mm yr 1 across the zhangye thrust our results are consistent with geologic and gps constraints which suggest that nne directed shortening across the northeastern tibetan plateau is distributed on several active faults with a total shortening rate of 4 to 10 mm yr 1

role of climate changes for wind gap formation in a young actively growing mountain range wind gaps in actively growing mountain ranges are unique geomorphological features testifying to the competition between tectonics and fluvial incision although it is clear that these landforms reflect the defeat of rivers during sustained rock uplift the role of climate changes in their formation has never been explored here we use a coupled tectonics landscape evolution model to show that temporal changes in precipitation rate exert an important control on wind gap formation in models with a constant precipitation rate rivers flowing across a growing range are either defeated at an early stage or they abandon their valleys very late if at all if precipitation varies wind gaps form mostly 100 200 ka after a transition to drier conditions because of sediment aggradation upstream of the range our results suggest that the pliocene quaternary aridification of central asia contributed to wind gap formation in active mountain ranges in the foreland of northeastern tibet this article is protected by copyright all rights reserved

how sediment promotes narrow rifting application to the gulf of california 1 if the gravitational stress changes related to crustal thinning are large enough to affect the mode of extension then sedimentation should also affect that mode we propose that the weight of sediments reduces the difference in crustal buoyancy forces caused by local crustal thinning allowing a rift to extend more easily in a narrow rift mode we examine the effect of deposition of large amounts of nonlocally derived sediment on extensional style using two dimensional regional scale numerical experiments of extending thick continental crust with varied initial thermal conditions geometries rheological parameters and sedimentation properties depending on initial model conditions the onset of rifting in thick continental crust occurs in the narrow rifting wide rifting or core complex mode with continued extension all cases eventually transition to a narrow rifting mode for a system in wide rift mode moderate to fast sedimentation shortens the time needed to transition to a narrow rift in the gulf of california changes in extensional style correlate with sediment thickness with an earlier transition to narrow rifting in the north versus the south we compare our model results to the extensional history of the gulf of california where sediments may have caused the northern and north central domain of the gulf delfin tiburon and guaymas segments to transition to narrow rifting before the south central domains alarcon segment

a search for iridium in the deccan traps and inter traps it has been suggested that flood basalts in the deccan india might be associated with events at the cretaceous tertiary boundary ktb a search for iridium in 47 samples from lava flows and inter trap sediments in the deccan yields negative results concentrations are not statistically different from zero with a minimum detection level on the order of 0 1 ng g 1 ppb this does not help to constrain the debate on the internal vs external origin of ktb boundary events since for instance ir could have been outgassed from the magma and original concentrations of only 0 05 ng g 1 prior to outgassing are sufficient to account for the worldwide ir excess mass at the ktb

crustal structure along mehmadabad billimora profile in the cambay basin india from deep seismic soundings abstract deep seismic sounding dss investigations were carried out along a 220 km long n s profile from mehmadabad 22 50 n 72 46 e to billimora 20 46 n 72 58 e in the cambay basin india for studying the detailed crustal structure in the region seismic refraction as well as narrow and wide angle reflection travel time data were obtained for various crustal boundaries starting from the sedimentary section down to the moho by continuous profiling analysis of reversed reflection and refraction travel time data has yielded crustal velocity depth functions for the mehmadabad broach and broach navsari sections along the profile which are significantly different from each other the average velocities at depths in the magmadabad broach section have been found to be considerably lower than those in the broach navsari section by about 25 at 5 km 17 at 10 km 7 at 20 km and 2 at the moho depth and even deeper down in the upper mantle the presence of a relatively thicker sedimentary section especially in the jambusar broach area as revealed by the reversed refraction data analysis is responsible for this gross difference in the average velocity functions for the two parts of the profile and the same have therefore been used for obtaining the detailed crustal depth along the profile the cambay basin which is well known to be bounded by step faults on the eastern and western margins is also dissected into seven major crustal blocks in the n s direction from mehmadabad to billimora the various crustal blocks identified are from the north 1 i mehmadabad block 2 ii tarapur block 3 iii petlad katana block 4 iv jambusar broach block 5 v kosamba block 6 vi surat block and 7 vii navsari billimora block these blocks are displaced up or down relative to the adjacent blocks due to movements along the faults bounding them most of the faults have been found to extend quite deep and to affect the moho boundary considerably as is quite evident from the relief of the moho found along the profile further the seismic activity observed in the region during historical as well as modern times suggests that these deep faults may even be currently active the jambusar broach block which is well known as a broad regional syncline with the maximum thickness of cenozoic sediments has been found to be a major graben revealing a maximum crustal thickness of 38 40 km the granitic basement in this graben is also the deepest at 6 0 6 5 km depth towards the north and south of this major graben the moho as well as the granitic basement are found to be at relatively shallower depths in various crustal blocks the boundary velocity at the moho has been found to be 8 1 8 2 km sec whereas at the granitic basement level there seems to be a lateral velocity variation from 5 7 to 6 3 km sec the crustal depth section obtained in the present study reveals a close relationship between the deep and shallow structural features in this region the deccan traps which form the basement of the overlying tertiary sediments over most part of the profile vary in thickness from 1 0 to 1 8 km however the trap surface rises to very shallow depths on the southern part of the profile the boundary velocity in the deccan traps has been found to vary between 4 8 and 5 2 km sec in the anklesvar kosamba area there is also evidence for the presence of a 1 2 km thick layer of mesozoic sediments with a velocity of about 4 0 km sec underlying the 1 1 km thick deccan traps layer

a new theory of the origin and evolution of the deccan traps india abstract the deccan traps india represent one of the largest continental flood basalts and its origin and phases of evolution have remained puzzling for the earth science community an integrated analysis of available geochronological geochemical and geophysical data of post mesozoic volcanism clearly rejects the dominating theory of origin from a hot spot e g la reunion at about 23 s 55 e in the indian ocean the deccan traps show imprints of three events belonging to a worldwide peak magmatism series matching with a galactic catastrophism cyclicity of 33 m y study further reveals that the deccan magmas were not directly derived by the partial melting of the deep mantle but appear to have their origin at a depth of less than 40 km as a consequence of crustal rifting and secondary melting caused by the rise of geoisotherms due to frictional heating generated at the lithospheric asthenospheric boundary during india s fast south north journey from antarctica

the growth of fault bounded tilt blocks a series of uniformly tilted fault bounded blocks is a common feature in actively extending regions such as the basin and range province if the tilted blocks were produced by rigid domino style rotation one would predict large voids at either end of a series of these blocks using tilt data and a simple flexural calculation we suggest that much of the apparent rigid behavior could also be produced by internal block deformation in our model of normal fault growth isostatic elastic uplift of the footwall is coupled with hanging wall downdrop within the region between faults resulting in the appearance of a tilted rigid block we present tilt data sampled at varying distances from several block defining faults within the northeast basin and range province tilt measurements between a series of 30 km spaced block defining faults are found to be uniform while tilts between more widely spaced faults exhibit a pattern of tilt that diminishes to zero in less than 30 km using a simple flexural calculation for internal block deformation we show that for this region the patterns of tilt are consistent with a flexural length scale of 8 12 km and deflections of 2 4 km these estimates are compatible with both the lower limit to seismicity and basin depth determined from earthquake and seismic reflection studies

bargen continuous gps data across the eastern basin and range province and implications for fault system dynamics we collected data from a transect of continuous global positioning system gps sites across the eastern basin and range province at latitude 39 n from 1997 2000 intersite velocities define a region 350 km wide of broadly distributed strain accumulation at 10 nstr yr 1 on the western margin of the region site egan 10 km north of ely nevada moved at a rate of 3 9 0 2 mm yr 1 to the west relative to site cast which is on the colorado plateau velocities of most sites to the west of ely moved at an average rate of 3 mm yr 1 relative to cast defining an area across central nevada that does not appear to be extending significantly the late quaternary geological velocity field derived using seismic reflection and neotectonic data indicates a maximum velocity of egan with respect to the colorado plateau of 4 mm yr 1 also distributed relatively evenly across the region the geodetic and late quaternary geological velocity fields therefore are consistent but strain release on the sevier desert detachment and the wasatch fault appears to have been anomalously high in the holocene previous models suggesting horizontal displacement rates in the eastern basin and range near 3 mm yr 1 which focused mainly along the wasatch zone and intermountain seismic belt may overestimate the holocene wasatch rate by at least 50 per cent and the quaternary rate by nearly an order of magnitude while ignoring potentially major seismogenic faults further to the west

migration of activity within normal fault systems examples from the quaternary of mainland greece abstract we examine five areas of mainland greece where active extension occurs on sub parallel systems of normal faults and where geomorphological and stratigraphic evidence indicates that the faulting has migrated basinwards into the original hanging walls in several cases within the late quaternary by comparing fault slip rates estimated from geomorphological data with current extension rates known from geodetic measurements it appears that the newest faults can account for effectively all the present day motions fault migration of this sort is easy to recognize in young systems close to sea level because vertical movements of footwalls and hanging walls are obvious and reveal which faults are currently most active but is less easy to confirm away from reliable reference levels or in older terrains with poorer time resolution it is probably more common than is appreciated and has a profound effect on syn rift sedimentation and erosion patterns fault migration is probably an inevitable consequence of the interplay between stresses generated by the fault related topography and the ultimate strength of major faults it is likely to be further encouraged in places where lower crustal flow or rotations about a vertical axis are important however it is not clear why migration should preferentially occur into the hanging walls as observed in central greece

footwall topographic development during continental extension 1 we examine the progressive development of footwall topography associated with a set of active normal faults in the northeastern basin and range province of the western united states fault length and displacement increase monotonically from northeast to southwest in the study area allowing us to track both variations in footwall morphology with increasing displacement and along strike changes in morphology on a single fault we show that patterns of catchment area footwall relief and catchment outlet spacing vary predictably and are related to the growth of the range bounding normal fault array in this semiarid region full parsing of footwall drainage area and removal of antecedent topography do not occur until fault arrays grow beyond two crustal scale segments multiple segment faults with lengths of up to 150 km have footwall relief that is limited to 1000 m in the center of the footwall and that decays to zero at the fault tips over a length scale of 15 km we hypothesize that this relatively uniform footwall relief is erosionally limited and reflects the efficacy of surface processes in removing footwall material in the center of the footwall if the fault array grows by relatively steady propagation of the tips we suggest that the 15 km length scale required to reach uniform relief is related to a timescale of relief generation by the fault tip propagation rate while such propagation rates are poorly known an average rate of 10 mm yr 1 would imply footwall relief generation over a timescale of 1 myr

melting of heterogeneous mantle in a slab window environment examples from the middle tertiary masset basalts queen charlotte islands british columbia voluminous basalts dominate a middle tertiary bimodal volcanic assemblage in the queen charlotte islands british columbia canada these moderately enriched mid ocean ridge basalt morb like magmas were associated with significant crustal extension related to the subduction of the pacific farallon ridge and the initiation of a slab window environment which persisted for 35 ma these asthenospheric melts were derived from a low degree of melting of a heterogeneous peridotite source mainly in the spinel stability field as inferred from fractional melting inversion calculations the basaltic magmas in turn gave rise to both tholeiitic and calc alkaline volcanic strata up to 3 5 km thick in local grabens the tholeiitic basalts resemble morb with rare earth element ree patterns ranging from slightly depleted to enriched in light ree lree the calc alkaline basaltic andesites are enriched in lree but are not strongly depleted in nb the rocks have overlapping nd sr and pb isotopic compositions sim

spatial variations in catchment averaged denudation rates from normal fault footwalls we test the spatial correspondence between rock uplift in active normal fault footwalls in the western usa and catchment averaged denudation rates from cosmogenic radionuclide crn analysis we find that denudation rates vary along strike depending on the fault length and displacement for the 18 km long sweetwater fault foot wall relief is largely inherited from the prefaulting topography and denudation rates are only partly reflective of fault displacement for the 130 km long wassuk fault all inherited topography has been removed by erosion in response to fault displacement denudation rates however show little along strike consistency and no correlation with catchment size or morphology we argue that in the early stages of fault growth crn derived denudation rates reflect inherited prefaulting relief with an overprint of fault controlled erosion in later stages of fault growth rates are decoupled from fault displacement through the impact of stochastic landsliding events and longer term measures of denudation such as low temperature thermochronometry that integrate over multiple earthquakes may provide a better record of spatial variations in rock uplift

the basin and range province origin and tectonic significance the basin and range province is a vast arid tract of regionally corrugated angular topography of high relief in the western cordillera it is characterized by evenly spaced parallel mountain ranges and intervening desert basins fig ure la the range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward interrupted here and there by low fault scarps that paral lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges thermal springs located at or near range bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting in the southern part of the province especially in southeastern california and southwestern arizona range fronts have been worn back by erosion leaving a thin veneer of gravel on an erosion cut bedrock surface that slopes gently outward the range bounding faults of these mountain blocks are buried at the outer edge of such pediments often at considerble distances from the erosional remnants of the ranges themselves the american physiographer n m fenneman 1928 1931 named the basin and range province and defined its general boundaries as thus circum scribed the province includes some 800 000 km2 of area in eight western states later students pardee 1950 lawrence 1976 reynolds 1979 eaton 1979b have observed that many of the fundamental geological and geo physical characteristics of the province are found well beyond the boundaries drawn by fenneman which were based on physiography alone as a tec tonophysical entity its areal extent is greater than 1 million km2 more than 10 of the area of the united states figure 1 fenneman 1931 subdivided the province into five physiographic sections the largest of which is the great basin see figure 1b it is not as its name implies a single regional depression with a common topographic center but is characterized instead by isolated networks of interior drainage divisible into

super mobility of hot indian lithosphere negi j g pandey o p and agrawal p k 1986 super mobility of hot indian lithosphere tectonoph vsics 131 147 156 the great mobility of the indian subcontinent during the last 180 million years 9000 km of south north motion with an anticlockwise rotation of about 60 o has long been a major puzzle for theorists of plate tectonics a relative analysis of the available heat flow data the estimated temperature depth regime of india and the gravity anomalies reveals that the indian lithosphere does not have the characteristics of a typical shield zone and that it has higher radioactivity smaller viscous drag decidedly low viscosity of 10 p lower density and a well defined low velocity zone as compared to other shield areas of the world these factors appear to contribute significantly to the faster northward movement of the thin indian lithosphere as compared to the almost immobile african plate

geomagnetic reversals from impacts on the earth the impact of a large extraterrestrial object on the earth can produce a geomagnetic reversal through the following mechanism dust from the impact crater and soot from fires trigger a climate change and the beginning of a little ice age the redistribution of water near the equator to ice at high latitudes alters the rotation rate of the crust and mantle of the earth if the sea level change is sufficiently large 10 meters and rapid in a few hundred years then the velocity shear in the liquid core disrupts the convective cells that drive the dynamo the new convective cells that subsequently form distort and tangle the previous field reducing the dipole component near to zero while increasing the energy in multipole components eventually a dipole is rebuilt by dynamo action and the event is seen either as a geomagnetic reversal or as an excursion sudden climate changes from other causes such as volcanic eruptions could also trigger reversals this mechanism may not be the sole cause of geomagnetic reversals but it can account for the rapid drop of the dipole component preceding a reversal the predominance of multipole components during a transition the associations of microtektites temperature drops and extinctions with reversals and the possible correlation between peaks in the geomagnetic reversal rate and the times of mass extinctions the model may also account for the long term changes in the average rate of reversals we make several testable predictions

impact induced melting and the development of large igneous provinces abstract we use hydrodynamic modelling combined with known data on mantle melting behaviour to examine the potential for decompression melting of lithosphere beneath a large terrestrial impact crater this mechanism may generate sufficient quantity of melt to auto obliterate the crater melting would initiate almost instantaneously but the effects of such massive mantle melting may trigger long lived mantle up welling that could potentially resemble a mantle hotspot decompression melting is well understood it is the main method advocated by geophysicists for melting on earth whether caused by thinned lithosphere or hot rising mantle plumes the energy released is largely derived from gravitational energy and is outside but additive to the conventional calculations of impact modelling where energy is derived solely from the kinetic energy of the impacting projectile be it comet or asteroid the empirical correlation between total melt volume and crater size will no longer apply but instead there will be a discontinuity above some threshold size depending primarily on the thermal structure of the lithosphere we estimate that the volume of melt produced by a 20 km diameter iron impactor travelling at 10 km s may be comparable to the volume of melt characteristic of terrestrial large igneous provinces 106 km3 similar melting of the mantle beneath an oceanic impact was also modelled by roddy et al int j impact eng 5 1987 525 the mantle melts will have plume like geochemical signatures and rapid mixing of melts from sub horizontal sub crater reservoirs is likely direct coupling between impacts and volcanism is therefore a real possibility that should be considered with respect to global stratigraphic events in the geological record we suggest that the end permian siberian traps should be reconsidered as the result of a major impact at 250 ma auto obliteration by volcanism of all craters larger than 200 km would explain their anomalous absence on earth compared with other terrestrial planets in the solar system

major circular structure beneath southern lake huron defined from potential field data newly compiled and merged canadian and united states potential field data for the area of southern lake huron reveal a large circular structure that has a diameter that may extend to 100 km the aeromagnetic image shows a circular central high with a diameter of 6 km located at fat 43 14 21 n long 82 19 88 w surrounded by a principal ring with a diameter of 50 km and indications of conformable arcuate structure extending to a diameter of about 100 km the prevailing northeast striking aeromagnetic signature of the grenville front and adjacent grenville front tectonic zone is interrupted and may be overprinted by the circular feature the bouguer gravity field shows a strong positive anomaly that extends southeast from the grenville front and terminates in an arcuate complex of positive anomalies that correlate with the aeromagnetic field the gravity field illustrates the circular structure with an annular low and central high as well as positive extensions that follow the foliate pattern of magnetic anomalies marking the outer area of the feature there is no strong evidence of the feature in the bathymetry data of southern lake huron although the general coastline of the southern end of lake huron closely follows the outline of the main circular feature the interruption of the grenville trends the clear regular circular signature of the feature and the interpreted magnetic source depths at the precambrian surface suggest an event possibly as young as early paleozoic in the absence of basement samples the origin of the feature is unconfirmed however the scale and potential field anomaly pattern of the feature are comparable to several of the larger impact structures exposed on the canadian shield

bombay gravity high and eruption of deccan flood basalts india from a shallow secondary plume abstract the western margin of india especially the area surrounding the gravity high of bombay is covered by a thick pile of deccan flood basalts the massive eruption is one of the largest continental volcanic events at the cretaceous tertiary biological mass extinction boundary 65 ma ago the area is characterized by several hidden tectonic features and their origin and evolution have remained controversial in the search for magma source structure attention was focused on an unusual oval shaped positive gravity anomaly over the bombay coastal zone the gravity analysis indicates the presence of two subsurface structures at depths of 4 5 0 5 km and 18 2 km these depths correspond to the top of the crustal basaltic layer and an upwarped moho discontinuity respectively the positive gravity anomaly in three dimensional analysis reveals a possible magma conduit structure and the near absence of the granitic layer the unusually shallow moho layer coincides with the region of a broad high heat flow anomaly average heat flow more than 80 mw m2 the geophysical signatures from the neighborhood show that the asthenosphere has welled up considerably and is located at a depth of about 40 km high heat flow and asthenospheric depth estimates suggest melting conditions at very shallow depths it is suggested that the highly differentiated magma associated with the major eruptive phase of deccan traps did not originate at the d layer it is hypothesized that magma was brought to the surface by a major secondary plume at a much shallower depth within the lithosphere and possibly at the crust mantle boundary as a consequence of subcrustal erosion and secondary melting caused by the extreme velocity of india during the late cretaceous and early tertiary substantial melting of the base of the lithosphere due to frictional heating might have resulted in the accumulation of a large quantity of magma below the thinned lithosphere at the pre existing fracture zone near the continental margin the presence of a magmatic conduit structure above the moho derived from the inversion of gravity data and other tectonic conditions suggests a possible triggering of the magma reservoir by a large bolide impact 10 km diameter at the cretaceous tertiary boundary near bombay

evaluation of reservoir temperatures and local utilization of geothermal waters of the konkan coast india abstract geochemical studies on fifteen geothermal manifestations 38 70 c from the konkan coast geothermal province of india have been used to evaluate the reservoir temperatures activity studies of the minerals and the waters present in the reservoirs suggest that the thermal waters are in equilibrium with montmorillonite kaolinite and quartz at about 100 c reservoir temperatures of these geothermal systems as estimated by geochemical thermometers are 88 to 128 c and thus too low for economic electricity production

slip on normal faults induced by surface processes after the cessation of regional extension insights from three dimensional numerical modelling abstract in regions of active extension normal faulting generates topography that is continuously modified by erosion sediment transport and deposition as shown by previous numerical models with full coupling between tectonics and surface processes the redistribution of mass at the earth s surface accelerates the rate of faulting by affecting the stress state of the crust it remains unknown however how fault slip evolves as a result of ongoing surface processes if regional extension as the main driver of faulting ceases here we use three dimensional finite element modelling to show that surface processes acting on normal fault bounded mountain ranges may sustain fault slip for millions of years even after regional extension has stopped the models consist of two successive phases during the first phase the normal fault accumulates displacement owing to an extensional boundary condition while erosion and sediment deposition are active on the model surface at the beginning of the second phase extension of the model is stopped while the surface processes remain active the results show that in most models normal faulting continues during the second phase at rates of 20 to 70 m ma for more than 1 ma in some experiments normal slip is maintained for 3 ma whereas in other models a short phase of normal faulting is followed by slow reverse slip the maximum amount of normal slip in different experiments reaches up to 90 m during the second model phase if erosion is intensified by increasing the diffusion constant by a factor of 5 the fault accumulates the additional normal slip at a faster rate i e during a shorter time period in contrast a five fold variation of the fluvial erosion constant does not significantly affect the fault slip evolution variations of the fault dip and length have a similar effect on the duration of the phase with additional normal slip as variations of the diffusion constant the fault slip evolution is correlated with the temporal evolution of the erosion and sedimentation rates which decrease more or less gradually after the end of extension ultimately the fault slip behaviour is controlled by the evolution of the differential stress which varies through time due to the redistribution of mass on the model surface that is induced by erosion sediment transport and deposition our results imply that individual normal faults may remain active even if regional extension ceases because surface processes continue to modify the fault generated topography

geochemistry of the miocene mount noel volcanic complex british columbia and comparison with the columbia river basalt the 19 6 ma mount noel volcanic complex mnvc is located about 150 km east of the garibaldi volcanic belt and adjacent to the chilcotin plateau basalts in southwestern british columbia the mnvc unconformably overlies jurassic to cretaceous metasedimentary rocks and lower cretaceous coast plutonic rocks the complex consists of flat lying columnar jointed basalt flows along with volcaniclastic debris flows and minor pyroclastic rocks basaltic flows have olivine fo 60 to fo 85 and plagioclase an 60 to an 80 phenocrysts with clinopyroxene microphenocrysts major trace and rare earth element analyses of the mnvc indicate the flows can be divided into two groups the largest group comprises tholeiitic to transitional basalt whereas a smaller subset is alkaline basalt rare earth element patterns are slightly enriched in the light rare elements in the transitional group and more strongly enriched in the alkaline basalts in tectonic discriminant diagrams all basalts plot in the within plate fields or in overlap areas typical of continental flood basalts alkaline basalts found only at the base of the thick sequence of tholeiitic to transitional basalts are characterized by high p 2 o 5 sr and ba contents the mnvc tholeiitic to transitional basalts are comparable to the chilcotin group basalts and to the lowermost basalts of the columbia river group the origin of mnvc basalts is best interpreted as volcanism behind the pemberton volcanic arc

mantle plumes and the periodicity of magnetic field reversals many geological phenomena show long term cyclical behavior of particular interest are mass extinctions and geomagnetic field reversals which may have correlated periodicities of approximately 30 myr we propose that these cycles are governed by periodic instability of the thermal boundary layer d at the base of the mantle as this mobile layer thickens by thermal diffusion it becomes dynamically unstable and hot material erupts from it the layer now thinner extracts heat from the core at a greater rate increasing the energy supply and hence the reversal frequency of the dynamo the hot material from the d layer rises via mantle plumes to the surface where it causes increased volcanic activity which affects the climate and biota

geological and biological consequences of giant impacts it seems inescapable that large impacts have been an ongoing phenomenon throughout geological time in the phanerozoic the estimated frequency of impact by bodies of 5 km diameter or more is on the order of 1 0 every 1 00 myr the 5 km lower limit is chosen arbitrarily but it is shown in this review that such a body is certainly large enough to cause very large terrestrial effects a small decrease in this size limit would lead to a large increase in the frequency of occurrence it may be claimed that bodies of a size likely to have drastic and sudden effects on niany physical and biological features of the earth have arrived with a frequency comparable to the average duration of a biostratigraphic stage nothing is said of the interval between arrivals which may be random i e stochastic currently there is not enough geological evidence to allow recognition of other than random spacing between a few positively identified events during the phanerozoic since impacts by extraterrestrial bodies have been clearly established it is reasonable to postulate that they must leave a record in the stratigraphic column why then do we currently have so few horizons that have been claimed to contain evidence of a global impact event since the effects we are considering are unquestionably global in this paper we bring together evidence from several disciplines that

body wave tomography of western canada abstract in this study we have produced p and s wave velocity models for western canada using 23 420 delay times measured on vertical component seismograms and 15 805 delay times measured on transverse component seismograms respectively from a range of permanent and temporary networks resolution is best in southwestern british columbia and along the canoe northwestern alberta southern yukon and northwest territories and batholiths northwestern bc arrays where the station density is the highest and fair elsewhere we focus our attention on two distinct features 1 the transition from phanerozoic to cratonic mantle in northwestern canada and 2 the complex tectonic environment at the northern terminus of the cascadia subduction zone where the plate boundary changes from convergent to transform we find that the main transition from phanerozoic to cratonic mantle in northwestern canada occurs at the cordilleran deformation front and represents a sharp jump in seismic velocity from 2 to 2 over a distance of 50 km in northern cascadia we have imaged and characterized the signature of the subducting juan de fuca plate and observed evidence of subduction beyond the northern edge of the slab we also demonstrate that the anahim hotspot track is underlain by a 2 low velocity zone possibly extending to 400 km beneath nazko cone that appears to be the source of volcanism in this area consequently we associate the source of magmatism in this area to a mantle scale rather than lithospheric scale process

slip acceleration on normal faults due to erosion and sedimentation results from a new three dimensional numerical model coupling tectonics and landscape evolution abstract although it is well accepted that mass redistribution on earth s surface due to erosion and sedimentation affects the rate of crustal deformation the influence of surface processes on the slip behaviour of individual faults remains poorly understood here we use the new tool casqus which combines landscape evolution modelling with three dimensional tectonic models to investigate how erosion and sediment deposition affect fault behaviour in extensional tectonic regimes a systematic set of experiments with models consisting of an isostatically responding elastic upper crust containing a single normal fault shows that the slip accumulation on the fault is accelerated by erosion and sedimentation depending on fault geometry extension rate and parameters controlling erosion and sediment deposition the slip rate of the normal fault is up to 15 higher when surface processes are taken into account the slip acceleration can be explained by an increase in the differential stress caused by erosion of the fault footwall and sedimentation in the hanging wall which alter the faulting induced flexure of the crust we observe a similar slip rate increase in a more complex model of a growing horst bounded by three faults of different length our results show that a positive feedback exists between surface processes and tectonics and imply that geologically derived fault slip rates that integrate over several earthquake cycles may contain a contribution from surface processes

chronostratigraphy of eocene volcanism central british columbia onset and termination of eocene felsic volcanism in the chilcotin plateau of central british columbia is constrained between 54 6 and 46 6 ma by 33 new u pb and 40ar 39ar isotopic age determinations dates were obtained from representative felsic coherent and fragmental volcanic rocks that comprise the ootsa lake group the resulting chronostratigraphy shows that magma compositions evolved from felsic to intermediate with no spatial migration of the volcanic activity rhyolitic compositions are oldest and are overlain by dacitic rocks with varied phenocrysts assemblages in many parts of the chilcotin plateau the eocene stratigraphy is capped by distinctive vitreous black dacite lavas which are contemporaneous with andesitic lavas of the endako group in the nechako plateau to the north crystallization ages from ootsa lake group rocks of the chilcotin plateau overlap age determinations from correlative rocks of the nechako plateau and southern bc collectively this geochronological dataset supports p

three dimensional numerical modeling of slip rate variations on normal and thrust fault arrays during ice cap growth and melting 1 changes in the volumes of ice caps considerably alter the stress state of the lithosphere by generating a transient signal that is added to the tectonic background stress field these stress field changes in turn affect crustal deformation and in particular the slip behavior of existing faults here we use three dimensional finite element models to investigate how arrays of normal and thrust faults near a growing and subsequently melting ice cap are influenced in their slip evolution the results show that regardless of fault dip both types of faults experience a decrease in their slip rate during ice cap advance and an increase in their slip rate during ice cap retreat if they are located beneath the ice cap in contrast faults outside the ice cap that are loaded on their footwall or hanging wall only show the opposite pattern their slip rate increases during glacial loading and decreases during subsequent unloading if the load is located along strike of the fault that is at one of its tips the slip behavior of normal and thrust faults is different the normal fault shows a slip rate increase during unloading the thrust fault during loading our results explain the location and timing of deglaciation induced paleoearthquakes in scandinavia and the contrasting slip histories reported from normal faults in the basin and range province which are located at different positions relative to the former yellowstone ice cap more generally our findings imply that a uniform slip behavior of faults in formerly glaciated regions should not be expected

on a tectonic mechanism for regional sealevel variations abstract no satisfactory tectonic explanations have yet been offered for the apparent sealevel fluctuations of about 1 cm 1000 years and a magnitude of up to a few hundred meters that have been proposed by vail et al 1 we propose a mechanism that does appear to be able to explain these changes if horizontal stresses of the order of a few kilobars occur in the lithosphere and if changes in these stress fields occur on geological time scales the proposed model is one of an interaction between these stresses and the deflections of the lithosphere caused by sedimentary loading apparent sealevel changes of up to 100 m can be produced at the flanks of the sedimentary basins by this interaction the mechanism is most effective for young margins that are subject to rapid sediment loading by its nature the tectonic model can explain contemporaneous fluctuations in apparent sealevel in neighbouring depositional environments in principle it implies the possibility of regional correlations in different basinal settings

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

terrestrial maria the origins of large basalt plateaus hotspot tracks and spreading ridges large lava plateaus which form abruptly within plates without apparent tectonic cause appear to be the terrestrial equivalents of lunar maria deep erosion of lava plateaus reveals that their magma chambers are gabbro and granophyre complexes the classic lopoliths the close association of the deccan plateau with the timing of the terminal cretaceous boundary clay suggests that it is an impact crater large enough to cause pressure relief melting in the asthenosphere basalt then flooded the crater to form a lava lake the terrestrial equivalent of a lunar mare part of the crater rim survives in the amirante ridge the carlsberg oceanic ridge and chagos laccadive hotspot track formed simultaneously with the deccan plateau several other mesozoic and cenozoic lava plateaus are parts of similar arrays of simultaneous features one of those is the southern oregon part of the columbia plateau with its associated continental rifting in the northern basin and range and the yellowstone hotspot track along t

coastal ecosystem responses to late stage deccan trap volcanism the post k t boundary danian palynofacies of mumbai bombay west india abstract the deccan trap continental flood basalt eruptions of india occurred c 67 63 ma thus spanning the cretaceous tertiary boundary 65 ma deccan eruptions were coeval with an interval of profound global environmental and climatic changes and widespread extinctions and this timing has sparked controversy regarding the relative influence of deccan volcanism upon end cretaceous catastrophic events if deccan trap activity was capable of affecting global ecosystems evidence should be present in proximal indian sedimentary facies and their palaeontological contents the impact of late stage deccan volcanism upon biota inhabiting mumbai bombay island s post k t boundary lagoonal systems is documented here sediments or intertrappeans which accumulated within these lagoons are preserved between trap lavas that characterise the closing stages of this flood basalt episode mumbai island formation intertrappean faunal and floral communities are conspicuously distinct from those common to many pre k t boundary late maastrichtian intertrappeans across the deccan province the latter sedimentary intercalations mostly developed in cognate semiarid palustrine ecosystems by contrast those around mumbai evolved in sheltered peripheral marine settings within subsiding continental margin basins unique to this late deccan stage and under an increasingly humid danian climate geochemical analyses reveal that mumbai sedimentation and diagenesis were intimately related to local explosive volcanic and regional intrusive activity at c 65 63 ma although tectonic and igneous events imprinted their signatures throughout these sedimentary formations organisms usually sensitive to environmental perturbations including frogs and turtles thrived critically palynofacies data demonstrate that whilst plant material deposition was responsive to environmental shifts there were no palpable declines in floral productivity following mumbai pyroclastic discharges therefore it is implausible that this late stage explosive volcanism influenced major ecosystem collapses globally

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

growth of faults by accumulation of seismic slip the accommodation of large strains in the upper crust is largely achieved by the accumulation of displacement on faults observation shows that as a fault accumulates displacement it grows in size i e its surface area and its length increase here we address the question for an increase in the amount of displacement on a fault by how much would the length of the fault change it is argued by cowie and scholz 1992a that the displacement on a fault is linearly related to the length of the fault this simple result is expanded upon in this paper by assuming that a fault accumulates displacement by repeated earthquakes two different approaches are presented the first approach considers the balance that must be achieved between the energy available for deformation and the work involved in creating new fault surface area as the fault grows the available energy is provided by changes in strain energy when the fault slips the second approach is to construct a geometrical model for fault growth using the scaling relationship between the slip during a single earthquake and the length of the rupture the total displacement on a fault is the sum of the slips contributed by many earthquakes the usefulness of these two approaches is that the growth of a fault over geologic time can be described by parameters that can be obtained from earthquake and fault data the models presented here predict that 1 the maximum amount a fault can grow in a single earthquake that ruptures the entire fault is of the order of 1 of its previous length and 2 the work of faulting may account for 10 of the total energy available during an earthquake the energy lost from the system is accounted for by work done against friction and seismic radiation consequences of fault growth for the segmentation and thus seismogenic potential of a fault over geologic time are discussed using predictions of the theory

back arc convection model for columbia river basalt genesis abstract genesis of the columbia river basalt group crbg may be accounted for by the progressive thermal erosion of an attenuated heterogenous subcontinental lithosphere scl by back arc convection behind the cascade arc assembly of the source components of the crbg began during the early mesozoic when slices of former oceanic lithosphere were isolated below terranes accreted to the continental margin subsequent lithospheric shearing accompanying northward translation of the terranes allowed the injection of small degree partial melts from the asthenosphere which solidified in the scl as pyroxenite veins the scl source of the crbg underwent a further episode of shearing accompanied by attenuation during the eocene and oligocene when it was caught between contrasting right lateral terrane translation and e w extension regimes in the northern cordillera and basin and range province respectively the attenuation resulted in the raising of the geotherm close to that of the pyroxenite and iherzolite solidi melting was triggered by back arc convection behind the cascade arc the pyroxenite was the first component mobilised constituting the principal source of the imnaha formation continued back arc convection resulted in the failure of the scl and the generation of the voluminous grande ronde formation from pyroxenite and the surrounding scl the change to wanapum volcanism marks the stage of exhaustion of in situ sources and their replacement with a combination of proterozoic scl from the east and asthenospheric material by back arc convection the saddle mountains formation was derived entirely from proterozoic scl small magma volumes resulted in a susceptibility to local contamination such that the distinct isotopic arrays of the saddle mountains formation comprise three mixing lines and do not have any age significance

normal faulting driven by denudational isostatic rebound fluvial piracy in the guadix baza basin southeast spain promoted erosion of a high volume of sediments during the late quaternary after the former internal drainage of this basin changed to external due to headward erosion by the guadalquivir river as a response to load release this basin underwent uplift which in turn enhanced fluvial incision and erosional processes differential erosion within the guadix baza basin resulted in dissimilar isostatic rebound we numerically model the lithospheric response to sediment unloading and show a total rising of 15 m in the guadix subbasin compared to 2 m in the baza subbasin this differential uplift is likely to have been accommodated along the baza normal fault which has been active throughout the quaternary modeling results suggest that 13 of the total quaternary fault throw directly arises from the differential isostatic readjustment between subbasins this example is one of the first estimates of fault slip partitioning between tectonic and isostatic effects due to unloading driven by river incision

late cretaceous and paroxysmal cretaceous tertiary extinctions the various geological signatures at cretaceous tertiary time including iridium and other associated elements microspherules and shock deformation features are compatible with the suggestion that the transition is marked by a period of intense volcanism the volatile emissions from this volcanism would lead to acid rain reduction in the alkalinity and ph of the surface ocean global atmospheric temperature changes and ozone layer depletion these environmental effects coupled with those related to the major sea level regression of the late cretaceous provide the framework for an explanation of the selective nature of the observed extinction record

the satah mountain and baldface mountain volcanic fields pleistocene hot spot volcanism in the anahim volcanic belt west central british columbia canada the satah mountain and baldface mountain volcanic fields smvf bmvf comprise more than three dozen small volcanic centers and erosional remnants thereof these fields are located in the chilcotin highland of west central british columbia canada and are spatially associated with the anahim volcanic belt avb a linear feature of alkaline to peralkaline plutonic and volcanic centers of miocene to holocene ages the avb has been postulated to be the track of a hot spot passing beneath the westward moving cordilleran lithosphere we test the avb hot spot model by applying whole rock 40ar 39ar geochronology n 24 and geochemistry whole rock chemical compositions of volcanic rock samples n 59 from these two fields suggest a strong geochemical affinity with the nearby itcha range shield volcano however smvf and bmvf centers are mostly small in volume 1 km3 and differ in composition from one another even where they are in close spatial proximity trace element and ree patterns of mafic avb lavas are similar to ocean island basalts oib suggesting a mantle source for these lavas the age ranges for the smvf n 11 2 21 to 1 43 ma and bmvf n 7 3 91 to 0 91 ma are largely coeval with the itcha range the distribution of volcanoes in these two volcanic fields is potentially consistent with the postulated avb hot spot track eruption rates in the smvf were high enough to build an elongated ridge that deviates from the e w trend of the avb by almost 90 this deviation might reflect the mechanisms and processes facilitating magma generation and ascent through the lithosphere in this tectonically complex region and may also indicate interaction of the potential hot spot with pre existing fracture systems in vicinity of the itcha range

common parent magma for miocene to holocene mafic volcanism in the northwestern united states primitive high alumina olivine tholeiites have been documented from a wide area of cenozoic volcanism in the northwestern united states several new localities extend the geographic range of these rocks to the cascade range of northern oregon the picture gorge region of north central oregon and the powder river volcanic field of northeastern oregon these new sites also extend the age range over which these primitive magmas were generated in the northwestern united states to between 16 ma and the present all of the high alumina olivine tholeiites are of remarkably uniform chemical and mineralogical composition and are similar to primitive ocean ridge and island arc tholeiites the wide distribution in time and space of this magma type and uniform primitive composition suggest that a relatively homogeneous oceanic mantle source underlies much of the northwestern united states basalts from the different volcanic provinces exhibit varied evolutionary trends away from this parental composition reflecting the distinct magmatic processes involved in each province trace element and rare earth element abundances in the primitive high alumina olivine tholeiite lavas record the involvement of crustal material in their genesis it is argued that this chemical signature was acquired by assimilation of crustal material by a primitive magma such as mid ocean ridge basalt morb en route to the surface

along strike variations of the slip direction on normal faults insights from three dimensional finite element models normal faults in nature exhibit a systematic variation of the slip direction along strike with pure dip slip at their centres that changes gradually to oblique slip near their tips here we evaluate the variation of the slip direction along normal faults by three dimensional finite element modelling the model results reveal a nearly linear increase of the strike slip component over two thirds of the distance from the fault centre to its tips which results in an increasing obliqueness of the slip vector along strike excluding the fault tips the relationship between lateral slip and fault length can be approximated by a power law function which shows that the strike slip component is proportional to the dip slip and to the fault dip the development of the sinistral and dextral slip components is caused by the orientation of the minimum principal stress which is deflected toward the fault centre in both footwall and hanging wall a comparison with two faults in the central apennines italy shows good agreement between modelled and measured slip vectors the occurrence of converging slip patterns implies that caution is advised when inferring oblique extension regional stress trajectories and palaeostress fields from slip vectors measured away from the fault centre

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin

landscape evolution at the margin of the basin and range streams erode new basins in extensional mountain ranges where tectonic asymmetry facilitates divide migration ranges in the northern basin and range of the united states form a space for time series that illustrates stream incision and divide migration following footwall uplift accelerating basin growth and changes in the amount and direction of volume loss during basin expansion stream erosion exceeds hillslope retreat which elevates summit plateaus

a possible k t boundary bolide impact site offshore near bombay and triggering of rapid deccan volcanism abstract the temporal coincidence of a major biological mass extinction including dinosaurs the well known iridium excess anomaly at the cretaceous tertiary k t boundary and the eruption of deccan flood basalts at about 65 ma has aroused global interest among geologists and biologists it is widely debated whether the mass extinction and iridium anomaly are due to an asteroid impact or the massive outpouring of extensive deccan volcanism an oval shaped unusual positive gravity anomaly 10 000 km 2 in area near bombay has attracted our attention during a search for an impact site near deccan basalts a detailed gravity interpretation indicates the presence of a fossil conduit structure of 12 km height extending from a shallow crust mantle boundary at 18 km to an approximate depth of 6 km from the surface the conduit structure with a maximum diameter of about 35 km at its base may originate from cracking of a weak pre deccan trap shallow upwarped mantle the structure may have been caused by a bolide impact which triggered the eruption of massive flood basalts deccan traps on the western margin of the fast moving indian plate an impact in this locality can explain the sudden detachment of the arcuate seychelles block from india as well as the large scale reorganisation of plate boundaries in the indian ocean our hypothesis of impact triggered volcanism at 65 ma advocates a bimodal cause for the mass extinction at the k t boundary extraordinary geothermal and structural conditions of the nearby region are also discussed as circumstantial evidence to support the twin cause mechanism by weakened features and the presence of partial melt at subcrustal depth

escarpment evolution on high elevation rifted margins insights derived from a surface processes model that combines diffusion advection and reaction experiments with a surface processes model of large scale 1 1000 km long term 1 100 m y erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent the model describes changes in topographic form as a result of simultaneous short and long range mass transport representing hillslope diffusive processes and fluvial transport advection respectively fluvial entrainment is modeled as a first order kinetic reaction which reflects the credibility of the substrate and therefore the fluvial system is not necessarily carrying at capacity one dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are 1 antecedent topography drainage 2 the timescale or equivalently a length scale in the fluvial entrainment reaction 3 the flexural response of the lithosphere to denudation and 4 the relative efficiencies of the short and long range transport processes when rainfall and substrate lithology are uniform a significant amount of discharge draining over the escarpment top causes it to degrade only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions an additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment this corresponds to a substrate that is hard to detach by fluvial erosion and therefore to fluvial erosion that is not transport limited continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide it is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology low flexural rigidities promote steep and slowly retreating escarpments for given rainfall and substrate conditions the morphology of a retreating model escarpment is determined by the ratio of the short range diffusive and long range advective transport efficiencies a low ratio which is interpreted to correspond to a relatively arid climate and weathering limited conditions promotes steep sharp topped escarpments with straight main slopes and escarpment retreat occurs over a wide range of height scales a high ratio interpreted to correspond to a more humid temperate climate produces a convex upper slope and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock however resistant caprocks are not an essential requirement for model scarps to retreat we conclude that the inferred controls and model behavior are both consistent with the present day morphology of rifted continental margins and with modern conceptual models of landscape evolution

an alternative model for the growth of faults conventional growth models suggest that faults become larger due to systematic increases in both maximum displacement and length we propose an alternative growth model where fault lengths are near constant from an early stage and growth is achieved mainly by increase in cumulative displacement the model reconciles the scaling properties of faults and earthquakes and predicts a progressive increase in fault displacement to length ratios as a fault system matures this growth scheme is directly applicable to reactivated fault systems in which fault lengths were inherited from underlying structure and established rapidly the model may also apply to some non reactivated fault systems near constant fault lengths during subsequent growth are attributed to retardation of lateral propagation by interaction between fault tips the model is validated using kinematic constraints from growth strata which are displaced by a system of reactivated normal faults in the timor sea nw australia

geomorphology of active faulting and seismic hazard assessment new tools and future challenges by witold zuchiewicz and jerzy zasadni 2012 intended for this volume young tectonics of the outer western carpathians in the light of some morphometric indices in this paper we test the applicability of some morphometric indices commonly used in tectonic geomorphological studies to the area of the outer western carpathians representing an alpine fold and thrust belt this belt was shaped largely in miocene times and shows traces of pliocene quaternary tectonic activity consisting mainly in some remnant folding and faulting this is indicated by scarce examples of faulting within quaternary strata numerous cases of clast fracturing owing to palaeoearthquakes of relatively large magnitudes as well as warping and tilting of pleistocene and holocene strath terraces analysis of selected morphometric indices valley floor width valley height ratio riverbed gradients stream length gradient index basin elongation ratios and others conducted for the entire region and some chosen test areas in the westernmost part of the outer western ac c ep te d m an u sc r ip t accepted manuscript 24 carpathians proved their usefulness in detecting zones showing tendencies to young surface uplift however in the case of the stream length gradient index sl hack s index a word of caution should be added since the index is extremely sensitive to lithologic contrasts in the bedrock and its applicability to young tectonic analysis of fold and thrust belts appears to be strongly limited dr andrzej piotrowski apiot pgi gov pl szczecinie panstwowy instytut geologiczny szczecin poland and dr klaus reicherter rwth aachen university germany

understanding conspicuous gravity low over the koyna warna seismogenic region maharashtra india and earthquake nucleation a paradigm shift the continued seismicity in koyna warna region of the western part of maharastra india and its relationship with subsurface structures concealed below thick volcanic sequences are studied in detail using gravity field along with newly available deep scientific drilling results this seismically active zone is marked by a large conspicuous negative gravity anomaly the causes of which are yet to be fully understood recent findings from the boreholes drilled in the koyna gupta et al in int j earth sci 104 1511 1522 2015 and killari seismic zones both of which penetrated the thick deccan volcanic cover and the underlying archean crystalline basement have motivated us to revisit the bouguer gravity field over this region using a newly developed finite element method of regional residual separation our study reveals the presence of two thick low density low velocity crustal zones below the koyna warna region the shallower one between 5 and 13 km depth and the deeper one between 35 and 43 km depth just above the moho both of these zones appear to contain mantle metasomatised and fractionated magmatic material respectively interestingly the hypocenters of all m 5 koyna earthquakes occur within the upper low velocity low density zone we also suggest high order crustal exhumation below this region which led to the removal of the entire sedimentary and granitic upper crustal column this process has brought denser mid crustal lithological facies close to the surface quaternary uplifting and movement of fault blocks along the old as well as newly created fault planes seem to be still continuing a paleo rift may have existed beneath this region below which moho temperatures 600 c and mantle heat flow 31 mw m2 are still high

late miocene ductile extension and detachment faulting mykonos greece the geometry and style of structural relations at map to microscopic scale on mykonos cyclades aegean sea greece suggests ductile extension and detachment faulting during the late miocene this island is dominated by granitic rocks that have a well developed shallowly dipping mylonitic foliation and a northeast southwest trending mineral elongation lineation kinematic indicators in these rocks exhibit a top to the northeast down dip sense of shear textural relations suggest that conditions during mylonitic deformation were greenschist to amphibolite facies and published k ar geochronology suggests a late miocene age for this deformation event mylonitic rocks lie in the footwall of a low angle fault and fault striae parallel to the elongation lineation suggest kinematic coordination between mylonitic deformation and movement along the fault translation along the fault juxtaposes younger tilted sedimentary rocks of low metamorphic grade upon the once deeper mylonitic rocks the style and geometry of deformational fabrics and map relations on this island are strikingly similar to those observed in metamorphic core complexes of the western united states

displacement rates of normal faults previous estimates of displacement rates on individual faults have been limited to neotectonic faults and averaged over time intervals of about 200 kyr or less1 2 3 4 5 these estimates have been highly variable which has led to a belief that longer term displacement rates on individual faults are likely to be variable as well here we report estimates of long term normal fault displacement rates averaged over time intervals ranging from 1 to 40 myr and based on observed decreases in displacement of progressively younger horizons intersected by syn sedimentary faults we find that displacement rates are remarkably stable over these longer time periods and within a given fault system the rates are strongly dependent on the relative size of the fault as measured by cumulative vertical displacement taken together these results indicate that faults become large relative to nearby faults by having higher displacement rates even when small rather than as a consequence of having been active for longer our analyses also show that high regional strain rates tend to be accommodated by high fault displacement rates rather than high fault densities

structure and evolution of the western continental margin of india deduced from gravity seismic geomagnetic and geochronological studies abstract available geochronological gravity seismic and geomagnetic data on the western continental margin of india between 8 20 n are presented based on the data three models describing the evolution of the crustal structure are developed model 1 describes the evolution of the crustal structure along 19 n the main basic igneous activity that accompanied the tectonic events since the late cretaceous resulted in the formation of rifts along the coast thus giving rise to the present day structure the acid volcanic event that preceded the above activity at about 93 ma ago is responsible for the observed crustal structure along 13 30 n shown in model 2 this event resulted in the formation of the st mary group of islands along this latitude counterclockwise rotation of the indian plate during the tertiary period caused rifting of the continental blocks giving rise to the laccadive kerala graben bounded by the chagos laccadive ridge on the west and the west coast fault on the east vertical movements of the rifted blocks promoted the deposition of both continental and marine sediments on the coast west of the west coast fault this tectonic event was accompanied by upwarping of the mantle crust thus giving rise to the present day structure shown in model 3

amirante basin western indian ocean possible impact site of the cretaceous tertiary extinction bolide if an impact event caused the mass extinctions and geochemical anomalies at the cretaceous tertiary boundary it probably occurred in an oceanic area however no convincing impact site has yet been discovered whereas late cretaceous magnetic lineations in other oceans show no obvious signs of disturbance at the tertiary boundary the end cretaceous african plate boundary in the indian ocean provides evidence of major tectonic reorganization at or shortly after magnetostratigraphic chron c29r immediately south of the microcontinental seychelles bank the amirante basin has a roughly circular shape of about 300 km diameter is partially ringed by enigmatic arc and trench structures and is located within oceanic crust of late cretaceous age it is therefore a possible impact site extensive chaotic slump structures apparently exist at the appropriate level on the east african continental margin and they may indicate its proximity to the mega earthquake focus and or giant tsunamis in the somali basin by triggering readjustments along the indian african and antarctic african plate boundaries and thus altering the regional balance of driving forces the impact may have affected plate motions

a lead and strontium isotopic study of the anahim volcanic belt british columbia additional evidence for widespread suboceanic mantle beneath western north america mantle derived alkali basalts and their peralkaline differentiates of the late cenozoic anahim volcanic belt occur in a 500 km long east west trending belt that cuts across the predominantly north northwest structural grain of the accreted terranes in southern british columbia pb and sr isotopic ratios of these rocks indicate that heterogeneous suboceanic depleted mantle is present in their source region beneath alexander stikinia and kootenay terranes this widespread suboceanic mantle source is characterized 206pb 204pb 18 62 19 18 207pb 204pb 15 52 15 63 208pb 204pb 38 15 38 68 and 87sr 86sr 7026 7033 these values are similar to those of the source for northeast pacific ocean seamounts that is the anahim source is not as depleted as the morb source in the northeast pacific no evidence exists for the presence of radiogenic subcontinental mantle in the zone of melting beneath the anahim belt locally very radiogenic initial isotopic ratios from bella bella and king island centers on the coast and the wells gray clearwater centers on the far eastern end of the anahim belt reflect contamination from a radiogenic precambrian crustal component derived from alexander and kootenay terranes respectively the mantle source for anahim belt volcanic rocks appears isotopically continuous with the source for alkali basalts in the basin and range of the western united states and distinct from the subcontinental mantle source proposed for some snake river plain and columbia river basalt group lavas

a radiometric age for the cretaceous tertiary boundary based upon k ar rb sr and u pb ages of bentonites from alberta saskatchewan and montana bentonites associated with coal seams at the north american cretaceous tertiary k t boundary in the red deer valley of alberta in the frenchman valley of saskatchewan and near hell creek montana were dated by the k ar rb sr and u pb methods based upon palynological stratigraphic correlation and the presence of a sharp iridium anomaly the three sampled bentonite horizons all occur less than 1 m above the established k t boundary there is evidence of possible systematic errors giving dating results outside nominal statistical error values despite possible constant errors the weighted mean of nine average dates is calculated as 64 3 1 2 ma 2 standard deviation an unweighted averaging produces a grand mean age of 64 4 1 2 ma

peritidal sedimentary depositional facies and carbon isotope variation across k t boundary carbonates from nw adriatic platform the k t boundary in the karst region italy and slovenia is well documented in correspondence to shallow water limestones its evidence is usually related to palaeontological data disappearance of cretaceous taxa and subsequent appearance of tertiary ones palaeomagnetism ch 29r iridium anomaly and negative shift of 13 c in italy the padriciano section highlights the k t boundary included in the basal breccia of a peritidal cycle recent investigations brought to light a short well exposed new stratigraphic sequence containing the k t boundary 100 m east of padriciano as demonstrated by lithological palaeontological and new isotopic data the recording of a new peritidal setting denotes stressed environmental conditions the findings of the biota mostly correspond to the lagoonal limestones of the peritidal cycle it consists of small opportunistic thin shelled foraminifers ostracods and gastropods at padriciano the environmental stability occurred after episodesofstrongfreshwaterinfluence theintraclasts oncolitesandstromatoliteshavebeenobservedinthepadricianosection the broken shell fragments algae and foraminifera have been found associated with fenestral fabric these features indicate deposition in a shallow marine intertidal to supratidal environment algal boundstones and microbial stromatolitic laminae crenulated domal laminae and crenulatedconical laminae havebeen studied in petrographicsections for microstructures the k t boundary section has been analysed for the carbon and oxygen isotopic ratios the 13 c values range from 3 62 to 10 01 v pdb and the 18 o values range from 3 85 to 5 47 v pdb this extreme depletion in the 13 c record has already been reported from other k t boundary sections both in the padriciano area italy and in slovenia dolenja vas sections and may be related to global climate change but also to local environmental changes occurring in the nwadriatic platform 2007 elsevier b v all rights reserved

active tectonics of the aegean region the aegean sea and its surrounding regions comprise one of the most rapidly deforming parts of the alpine himalayan mountain belt though the deformation of the belt as a whole is related to thc northward move ment of africa arabia and india relative to eurasia the tectonics of the aegean region itself is dominated by strike slip and extensional motions the actively deforming part of the aegean region is not large about 700 x 700 km2 compared with that in asia yet it has had an influence on our views of continental tectonics disproportionate to its size and comparable perhaps only to tibet fundamental contributions from the aegean have been made to many topics including