ca sr and 87sr 86sr geochemistry of disseminated calcite in himalayan silicate rocks from nanga parbat influence on river water chemistry trace amounts of disseminated calcite were identified in gneiss schist and granite bedrock sampled from the raikhot watershed and other locations within the nanga parbat massif of northern pakistan the calcite grains occur interstitially within individual silicate minerals at grain boundaries and as fracture fillings that transect the mineralogic fabric of the rock disseminated calcite composes is 0 29 wt of silicate rocks sampled in the raikhot watershed and has ca sr mol nmol and 87sr 86sr ratios that range from 0 878 to 5 33 and from 0 794 039 to 0 930 619 respectively elsewhere in the nanga parbat region disseminated calcite composes 3 6 wt of the silicate rock samples and has ca sr mol nmol and 87sr 86sr ratios that range from 0 535 to 3 33 and from 0 715 757 to 0 771 244 respectively for all samples the 87sr 86sr ratios of disseminated calcite are similar to the 87sr 86sr ratios measured in the silicate host rock within the partially glaciated raikhot watershed the rapid weathering of disseminated calcite with high ca sr and 87sr 86sr ratios has a strong influence on the chemical composition of stream water and exceeds contributions from silicate mineral dissolution comparisons of disseminated calcite compositions with source water chemistry throughout the himalaya suggest that disseminated calcite may be a more important component of himalayan silicate rocks than previously recognized therefore calculations relating the sr isotope geochemistry of himalayan rivers to atmospheric co2 consumption should consider this widespread and compositionally variable carbonate end member

silicate weathering rates decoupled from the 87sr 86sr ratio of the dissolved load during himalayan erosion this study establishes that carbonate weathering dominates the supply of sr with elevated 87sr 86sr ratios in the bhote kosi sun kosi river system central nepal a major himalayan tributary to the ganges the dissolved load of the bhote kosi sun kosi displays a rapid increase in the 87sr 86sr ratio in its lower section downstream of the main central thrust a feature common to other himalayan tributaries of the ganges mass balance analysis of the contributing sources to the dissolved load identifies the weathering of palaeoproterozoic impure carbonates comprised largely of dolomite and mica from the lesser himalaya as the most significant lithological control upon elevated 87sr 86sr ratios in situ laser ablation analysis of the dolomite component yields 87sr 86sr ratios of 0 93 to 1 11 indistinguishable from bulk rock this confirms the presence of a highly soluble source of radiogenic strontium and suggests that diffusive exchange during a protracted metamorphic history has homogenised sr isotopes between silicate and carbonate phases within the assemblage weathering of the carbonate component of such rocks has yielded unusually high 87sr 86sr at significant sr fluxes in the dissolved load approximately 60 of the impact of the bhote kosi sun kosi on the marine 87sr 86sr ratio can be traced to weathering of this carbonate since similar lithologies outcrop within the catchments of most himalayan tributaries that feed the ganges and brahmaputra rivers the presence of such radiogenic carbonates undermines the inference that the impact of these rivers on sr isotopes in seawater is indicative of high silicate weathering rates that result from the uplift of tibet and the himalaya

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

topography relief and trmm derived rainfall variations along the himalaya 1 along the southern himalayan topographic front the indian summer monsoon modulates erosive processes and rates to investigate the influence of topography and relief on rainfall generation and resultant erosion we processed satellite rainfall amounts for the last 8 years 1998 2005 from the tropical rainfall measurement mission trmm based upon a spatial resolution of 5 5 km for the himalaya we identify 1 the spatial distribution of rainfall and 2 the large scale relationships between topography relief and rainfall locations our results show two distinct rainfall maxima along strike in the himalaya the first outer rainfall peak occurs along the southern margin of the lesser himalaya within a narrow band of mean elevation 0 9 0 4 km and mean relief 1 2 0 2 km the second discontinuous inner band typically occurs along the southern flank of the greater himalaya elevation and relief both 2 1 0 3 km

complex anisotropy in d beneath the eastern pacific from sks skks splitting discrepancies article i nfo although observations of seismic anisotropy in the lowermost mantle are abundant the mechanism which generates anisotropy in the d layer is not well understood most observational constraints on d anisotropy come from the splitting of shear waves that propagate nearly horizontally through the lowermost mantle however anisotropy in the d layer can also give rise to discrepancies in shear wave splitting for sks and skks phases for the same event station pair as these phases sample different regions of the lowermost mantle here i report observations of strongly discrepant sks skks splitting observed at broadband stations in western mexico and california in particular strong skks splitting with fast polarization directions near 60 and delay times up to 3 s is observed for a group of raypaths that sample a region of the d layer beneath the eastern pacific ocean a comparison of skks splitting with sks splitting observed from the same events as well as with sks splitting observed at a variety of backazimuths at each station suggests that the anomalous anisotropic structure is in the lowermost mantle my preferred model for the unusual anisotropic geometry in this region is large shear deformation in d at the edge of a region where slab material impinges upon the core mantle boundary resulting in lattice preferred orientation of lower mantle minerals

abnormal monsoon years and their control on erosion and sediment flux in the high arid northwest himalaya abstract the interplay between topography and indian summer monsoon circulation profoundly controls precipitation distribution sediment transport and river discharge along the southern himalayan mountain front shf the higher himalayas form a major orographic barrier that separates humid sectors to the south and arid regions to the north during the indian summer monsoon vortices transport moisture from the bay of bengal swirl along the shf to the northwest and cause heavy rainfall when colliding with the mountain front in the eastern and central parts of the himalaya precipitation measurements derived from passive microwave analysis ssm i show a strong gradient with high values at medium elevations and extensive penetration of moisture along major river valleys into the orogen the end of the monsoonal conveyer belt is near the sutlej valley in the nw himalaya where precipitation is lower and rainfall maxima move to lower elevations this region thus comprises a climatic transition zone that is very sensitive to changes in indian summer monsoon strength to constrain magnitude temporal and spatial distribution of precipitation we analyzed high resolution passive microwave data from the last decade and identified an abnormal monsoon year amy in 2002 during the 2002 amy violent rainstorms conquered orographic barriers and penetrated far into otherwise arid regions in the northwest himalaya at elevations in excess of 3 km asl while precipitation in these regions was significantly increased and triggered extensive erosional processes i e debris flows on sparsely vegetated steep hillslopes mean rainfall along the low to medium elevations was not significantly greater in magnitude this shift may thus play an important role in the overall sediment flux toward the himalayan foreland using extended precipitation and sediment flux records for the last century we show that these events have a decadal recurrence interval during the present day monsoon circulation hence episodically occurring amys control geomorphic processes primarily in the high elevation arid sectors of the orogen while annual recurring monsoonal rainfall distribution dominates erosion in the low to medium elevation parts along the shf

silicate versus carbonate weathering in the himalaya a comparison of the arun and seti river watersheds abstract we studied the water chemistry of two large and geologically differing himalayan watersheds in order to maximize the contrast between silicate versus carbonate weathering effects on river chemistry our previous research involved the seti river of westernmost nepal geologically typical of many rivers in nepal in draining mixed carbonate silicate lithologies including abundant carbonate rocks of the lesser himalayan sequence for comparison the arun river was chosen for study because south of the himalayan front it drains almost exclusively greater and lesser himalayan silicate rocks despite this dominance of silicate rocks carbonate weathering probably of metamorphic calcite in great himalayan paragneisses is clearly in evidence in many arun watersheds weathering of silicate rocks exposed all along the arun south of the range front has a small impact on mainstem river chemistry the mainstems of both the seti and arun systems are dominated by weathering of carbonate rocks although the contribution of silicate weathering is more visible in arun mainstem chemistry the carbonate weathering source to the arun mainstem is probably both limestone in the tethyan sequence widely exposed in northern headwaters of the system and metamorphic calcite within the greater himalayan sequence a number of small watersheds along the arun and seti appear to be carbonate free they probably provide the best constraints to date on the ca na and mg na ratios of waters draining himalayan silicate rocks two critical parameters for calculation of co 2 consumption by silicate weathering in the himalaya the observed ca na and mg na ratios would produce slightly higher estimates of silicate weathering fluxes than previous studies the geologic contrasts between the seti and the arun produce large differences in the 87 sr 86 sr ratio of each mainstem unlike the major element chemistry the seti mainstem displays much higher 87 sr 86 sr ratios than the arun mainstem the opposite of the expected relationship since radiogenic silicate rocks of the greater and lesser himalaya are so widely exposed along the arun 87 sr 86 sr ratios of the arun mainstem never exceed 0 734 and show little downstream change as the mainstem passes through silicate rocks of the greater and lesser himalaya 87 sr 86 sr ratios of the seti mainstem increase sharply from 0 725 to 0 785 when the river enters the belt of metacarbonate rocks of the northern lesser himalayan sequence a pattern also displayed by other himalayan rivers such as the kali gandaki and bhotse khola metacarbonate rocks including those of the lesser himalaya are a major source of radiogenic sr in modern himalayan rivers and probably have been key players in elevating marine 87 sr 86 sr ratios since the early miocene

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

frequency dependent shear wave splitting and heterogeneous anisotropic structure beneath the gulf of california region abstract the gulf of california region has undergone a major evolution over the past 15 ma since subduction of the now extinct farallon plate ceased and transform motion began leading to the initiation of rifting and the opening of the gulf of california 5 ma ago the character of dynamic processes in the upper mantle beneath the region remains poorly understood but constraints on seismic anisotropy in the upper mantle can shed light on contemporary mantle flow processes in order to characterize more fully the anisotropic structure of the upper mantle beneath the region i present measurements of sks splitting at 14 broadband stations of the nars baja array at periods between 8 and 50 s the measured splitting parameters t at nars baja stations exhibit a high degree of complexity with dramatic lateral variations significant backazimuthal variations at individual stations and a large number of well constrained null measurements over a range of backazimuths the dependence of splitting parameters on frequency is evaluated by applying a series of bandpass filters to high signal to noise sks arrivals i find that while fast directions are generally insensitive to the sks frequency content delay times are frequency dependent at several stations the measurements presented in this study indicate that the anisotropic structure of the upper mantle beneath the gulf of california region is complex and exhibits a high degree of both lateral and vertical heterogeneity the observed splitting is consistent with a scenario in which a large scale reorganization of the mantle flow field has been inhibited by the presence of a stalled fragment of oceanic lithosphere associated with farallon subduction the pattern of mantle flow appears to be controlled by more localized processes likely including small scale buoyant upwelling associated with partial melting beneath the central part of the gulf of california rift

tectonic implications of post 30 ma pacific and north american relative plate motions the pacific plate moved northwest relative to north america since 42 ma the rapid half rate of pacific farallon spreading allowed the ridge to approach the continent at about 29 ma extinct spreading ridges that occur offshore along 65 of the margin lonsdale 1991 document that fragments of the subducted farallon slab became captured by the pacific plate and assumed its motion prior to the actual subduction of the spreading ridge this plate capture process can be used to explain much of the post 29 ma cordilleran north america extension strike slip and the inland jump of oceanic spreading in the gulf of california the pacific and north american contact zone lengthened with each successive plate capture event underpinning the parts of western north america directly inland with a strong plate undergoing pacific relative motion we suggest that much of the post 29 ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent the plate capture hypothesis is distinctly different from theories involving shallow slab gaps kinematic problems associated with shallow slab gap models cause us to question them this conclusion is consistent with seismic refraction interpretations that suggest there is an inclined layer with high velocities like that of basalt or gabbro at the base of the continental crust beneath much of the californian margin and the documented reduction of slab pull forces and density associated with young subducting slabs thermal and rheologic modeling suggests that coastal california was a strong zone at all depths allowing it to be firmly linked to pacific motion our model shows that deformed regions such as the basin and range and borderland provinces developed in predicted weak parts of the crustal section but they have been incompletely linked to the deep plate across the ductile middle and lower crustal layer

tertiary metamorphic core complexes in sonora northwestern mexico several ranges encompassing more than 35 000 km 2 of sonora mexico contain distinctly lineated and foliated granitic and metamorphic rocks that constitute the lower plates of metamorphic core complexes penetrative deformation is characterized by gently dipping mylonitic foliation across which northeast trending stretching lineation is everywhere developed prominent northwest trending fractures dikes and normal faults are orthogonal to the lineation most kinematic indicators in lower plate mylonitic rocks record top to the southwest sense of shear upper plate stratigraphic sequences include mesozoic supracrustal rocks tertiary volcanic and sedimentary rocks and allochthonous precambrian basement tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults previously published u pb and k at ages from lower plate granitic orthogneisses upper plate volcanic sequences and crosscutting dikes constrain the time of mylonitic deformation and detachment faulting in several of these areas to late oligocene early miocene partitioning of extensional strain in sonora was influenced by pre tertiary crustal structure the belt of core complexes developed across two contrasting blocks of continental crust separated by the n60ow striking mojave sonora megashear portions of the southern papago block northeast of the megashear consisting of jurassic magmatic arc rocks and upper jurassic cretaceous siliciclastic and carbonate strata resting upon a concealed tectonically fragmented precambrian basement were especially susceptible to crustal attenuation some core complexes of the southern papago block occur within zones trending northwest that may coincide with late jurassic lineaments in the caborca block southwest of the megashear core complex related rocks and structures have not been identified where surface exposures of middle proterozoic basement and overlying upper proterozoic paleozoic platform strata are common however extensional mylonitic fabrics are locally developed along the margins of a tertiary two mica granite batholith core complexes on both sides of the megashear appear to be preferentially developed where tertiary granites have intruded regions of crust with basement disrupted by pre tertiary structures sonoran core complexes preserve an extensional tectonic history comparable with that described from core complexes farther north in the united states and canadian cordillera the timing of mid crustal extension in sonora 25 18 ma is contemporaneous with the timing of core complex development in arizona nevada and utah extension occurred later in these areas than in the pacific northwest british columbia region but earlier than in the

major ion chemistry of the ganga brahmaputra river system weathering processes and fluxes to the bay of bengal the ganga brahmaputra one of the world s largest river systems is first in terms of sediment transport and fourth in terms of water discharge a detailed and systematic study of the major ion chemistry of these rivers and their tributaries as well as the clay mineral composition of the bed sediments has been conducted the chemistry of the highland rivers upper reaches of the ganga the yamuna the brahmaputra the gandak and the ghaghra are all dominated by carbonate weathering ca mg and hco3 account for about 80 of the cations and anions in the lowland rivers the chambal the betwa and the ken hco3 excess over ca mg and a relatively high contribution of na k to the total cations indicate that silicate weathering and or contributions from alkaline saline soils and groundwaters could be important sources of major ions to these waters the chemistry of the ganga and the yamuna in the lower reaches is by and large dictated by the chemistry of their tributaries and their mixing proportions illite is the dominant clay mineral about 80 in the bedload sediments of the highland rivers kaolinite and chlorite together constitute the remaining 20 of the clays in the chambal betwa and ken smectite accounts for about 80 of the clays this difference in the clay mineral composition of the bed sediments is a reflection of the differences in the geology of their drainage basins the highland rivers weather acidic rocks whereas the others flow initially through basic effusives the ganga brahmaputra river system transports about 130 million tons of dissolved salts to the bay of bengal which is nearly 3 of the global river flux to the oceans the chemical denudation rates for the ganga and the brahmaputra basins are about 72 and 105 tons km yr 1 respectively which are factors of 2 to 3 higher than the global average the high denudation rate particularly in the brahmaputra is attributable to high relief and heavy rainfall

shear wave splitting observations at the regions of northern baja california and southern basin and range in mexico 1 recent installation of 3 component broadband digital stations around the gulf of california allowed us to make shear wave splitting observation from records of 73 sks and skks phases from 30 events recorded at 12 stations since january of 2001 stations in the southern basin and range province yield upper mantle fast shear wave direction from ne sw to e w consistent with both local direction of miocene extension and north american absolute plate motion the shear at the pacific north american plate limit seems to control tightly the anisotropy pattern of the closest station ne70 the anisotropic pattern along the peninsular range east of the former trench between the north american plate and the now subducted farallon plate is almost uniform and is interpreted as asthenospheric flow induced by the sinking fragment of the farallon plate although frozen anisotropy associated with in situ captured fragment of this plate cannot be ruled out

a theoretical model coupling chemical weathering rates with denudation rates uplift of the himalayas has been proposed to have locally accelerated chemical weathering thus leading to enhanced co2 sequestration and global cooling this hypothesis assumes that rapid erosion exposes fresh highly reactive minerals at earth s surface empirical studies quantifying the relationship between erosion and weathering have produced apparently conflicting results where the nature of the relationship is dependent on the weathering regime of the sampled landscapes we derive a quantitative model that defines this relationship across the range of weathering regimes from supply limited to kinetically limited conditions the model matches trends in field data collected by others and reconciles apparently conflicting results the model also demonstrates that as erosion rates increase potential increases in weathering rate from the exposure of fresher materials are offset by the decrease in the total volume of minerals exposed due to thinner regolith we conclude that the relationship between weathering and erosion is one of diminishing returns in which increases in erosion rate lead to progressively smaller increases in weathering rate indeed at the highest erosion rates weathering rates may decline the ability therefore of accelerated uplift and erosion to stimulate greater co2 sequestration may be significant in landscapes eroding at rates of 10 102 t km 2 yr 1 however where erosion rates are greater than 102 t km 2 yr 1 increases in denudation may not be matched by increases in chemical weathering finally our results suggest that watersheds with regolith thicknesses of 0 5 m will yield the greatest solute fluxes

seismic anisotropy observations in the mexicali valley baja california mexico a study of seismic anisotropy was performed using data from earthquakes of the mexicali valley the investigated region encompasses the cerro prieto geothermal field cpgf one of the most important fields in the world the results showed that at most of our stations the average polarization directions of the fast s waves range from n14 w to n17 e a n s polarization direction was obtained for the whole area by averaging the polarization directions from all stations used in terms of the eda hypothesis this average trend agrees with the postulated state of stress for southern california and with fault plane solutions for events of the mexicali valley notorious deviations from the n s global trend were found southeast of the cpgf with polarization trends between n25 e and n67 e and in the geothermal field with polarization directions between n7 w and n14 w the polarization results for these zones indicated stress conditions that are different from the more regional stress pattern the delay times that were measured between the fast and slow shear waves reached values of up to 0 6 sec with a mean value of 0 35 sec consistent with our polarization results the larger delay times 0 2 0 6 sec were found in the cpgf smaller or null values were observed at the periphery of the study area no temporal trends in the delay times were apparent as shown by data from the two stations that recorded the larger number of events overall we conclude that the splitting effects of this study result from a shallow anisotropy volume the splitting results are thus interpreted as caused by the preferred orientation of vertical fluid filled microcracks aligned in a direction that is parallel to the regional stress field the stronger splitting effects that were observed in the area of the cpgf were found consistent with the geothermal reservoir that is embedded in the sedimentary cover of the zone at depths of 1 to 4 5 km from the surface we thus believe that such marked splitting effects have a direct relation with the reservoir of the cpgf

surface wave tomography of the gulf of california 1 we measured interstation fundamental mode rayleigh wave phase velocities using data from the nars baja seismic network located around the gulf of california a region average shear velocity model and a set of azimuthally anisotropic phase velocity maps are obtained from these data the average shear velocity structure shows a strong low velocity zone underlying a thin lid and the data are suggestive of low velocities down into the transition zone the phase velocity maps display signatures of sedimentary layers crustal thickness variations upwelling under the plate boundary and the presence of the subducted farallon microplate remnants beneath the gulf the upper mantle features inferred from this study provide new seismic evidence on the tectonic evolution of the region

seismic anisotropy in northern and central gulf of california region mexico from teleseismic receiver functions and new evidence of possible plate capture 1 we present anisotropic models for the shallow lithosphere in the northern and central gulf of california obtained from receiver function rf analysis this region has undergone intense plate reorganization over the past 25 ma that led to the transfer of the plate boundary east of the modern baja california peninsula posterior to the cease of the subduction of the farallon plate beneath the north american plate our main interest in modeling the seismic anisotropy is to characterize rock fabric inherited from past and recent tectonism and to provide new information for understanding the tectonic processes involved in the evolution of the local plate boundary we calculate rfs from teleseismic p waves recorded by the network of autonomously recording seismographs nars baja array and we stack them in 20 wide back azimuthal bins to enhance coherent information these rfs yield significant transverse energy whose azimuthal variations are not consistent with that expected for a crust with dipping moho therefore we interpret the conspicuous rfs energy of the transverse component as convincing evidence of anisotropy we present forward models obtained by fitting the radial and transverse rfs with synthetic seismograms calculated with a reflectivity code our analysis suggests substantial variations in the structure of the shallow lithosphere along the baja california peninsula and the presence of stalled fragments of the extinct farallon plate beneath its central part the results at station ne81 in sonora are interpreted as frozen crustal anisotropy inherited from miocene basin and range extension consistent with previous observations of the upper mantle anisotropy beneath this station

chemical weathering in granitic environments abstract factors controlling chemical weathering in granitic environments are deduced using an extensive database from the literature based on a transition state theory model we evaluated the effects on chemical weathering of runoff temperature and ph the dependence between temperature and runoff and chemical weathering is complicated by other factors such as the presence depth of soil cover soil cover favors chemical weathering when it is thin and rich in weatherable minerals in contrast it reduces chemical weathering when is thick and poor in primary minerals processes that increase the contact time and the surface of the water rock interactions such as physical denudation increase chemical weathering rates in granitic environments the effect of temperature can be quantified assuming an apparent activation energy value of 48 7 kj mol to describe chemical weathering of silica in granitic crystalline environment under high runoff conditions

small scale sublithospheric continental mantle deformation constraints from sks splitting observations summary it is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle and that this is a relatively small scale phenomenon compared with anisotropy of the oceanic mantle during recent years shear wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy the magnitude of the splitting delay times can often rule out a crustal source of anisotropy however it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere it is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon here we describe a way to constrain the depth of the anisotropy by studying the lateral variation of the splitting parameters at neighbouring seismic stations we use sks and skks phases recorded at the nars nl array a relatively dense network of broad band stations separated by about 50 km fresnel zones at different depths are calculated for these phases the depth of the anisotropy is constrained by the criterion that fresnel zones corresponding to different splitting observations should not overlap variability in the splitting measurements for events with different directions of approach recorded at one station and for single events recorded at various stations provides evidence for a non homogeneously distributed source of anisotropy located below the array at different depths we show that this variability in the measurements cannot be due to effects of crustal scattering our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth this suggests that the deformation in this region is subject to significant lateral variations

crustal thickness of the peninsular ranges and gulf extensional province in the californias we estimate crustal thickness along an east west transect of the baja california peninsula and gulf of california mexico and investigate its relationship to surface elevation and crustal extension we derive moho depth estimates from p to s converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at 318n latitude depth to the moho is 33 63 km near the pacific coast of baja california and increases gradually toward the east reaching a maximum depth of 40 64 km beneath the western part of the peninsular ranges batholith the crust then thins rapidly under the topographically high eastern peninsular ranges and across the main gulf escarpment crustal thickness is 15 18 62 km within and on the margins of the gulf of california the moho shallowing beneath the eastern peninsular ranges represents an average apparent westward dip of 258 this range of moho depths within the peninsula ranges as well as the sharp east west gradient in depth in the eastern part of the range is in agreement with earlier observations from north of the international border the moho depth variations do not correlate with topography of the eastern batholith these findings suggest that a steeply dipping moho is a regional feature beneath the eastern peninsular ranges and that a local airy crustal root does not support the highest elevations we suggest that moho shallowing under the eastern peninsular ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the gulf extensional province that commenced in the late cenozoic support of the eastern peninsular ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and or upper mantle

chemical and strontium oxygen and carbon isotopic compositions of carbonates from the lesser himalaya implications to the strontium isotope composition of the source waters of the ganga ghaghara and the indus rivers samples of precambrian carbonate mostly dolomite outcrops collected across the lesser himalaya have been analysed for their mineralogy chemical composition and isotope ratios of sr o and c to assess the extent of their preservation and their role in contributing to the high radiogenic strontium isotope composition of the source waters of the ganga ghaghara and the indus their sr concentrations range from 20 to 363 ppm 18opdb 1 4 to 12 8 and mn 11 2036 ppm the petrography of the samples their low sr concentrations and wide range of 18o values are suggestive of their postdepositional alteration the 87sr 86sr of the bulk samples and their carbonate fractions are similar to one another with values ranging from 0 7064 to 0 8935 and are generally more radiogenic than that of contemporaneous seawater comparison of the 87sr 86sr and sr ca ratios among the carbonates and silicates from the lesser himalaya and the source waters of the ganga ghaghara and the indus shows that the values for the source waters overlap with those of the silicates but are much higher than those in carbonates an upper limit of carbonate sr in the various source waters is calculated to be between 6 and 43 assuming that all the ca in the rivers is of carbonate origin the results show that on the average weathering of the precambrian carbonates is unlikely to be a major contributor to the highly radiogenic strontium isotope composition of these source waters however they can be a dominant supplier of radiogenic sr to some rivers on a regional scale the silicate sr component in some of the source waters of the ganga bhagirathi bhilangna alaknanda and ganga ghaghara kali and sarju and the indus sutlej was calculated from the ca na sr na ratios and strontium isotope compositions of these rivers and the silicate endmember these calculations suggest that 33 89 of sr in the bhagirathi bhilangna alaknanda ganga and sarju rivers is of silicate origin whereas in the kali and the sutlej it is much lower only 8 the remaining sr to all these waters has to be supplied from other sources such as weathering of carbonates and evaporites this study underscores the importance of weathering of silicates carbonates and evaporites in contributing to the sr mass balance and 87sr 86sr of the source waters of the ganga ghaghara and the indus the present day silicate and carbonate sr contributions to the sr budget of the rivers vary considerably but among the major source waters of the ganga silicate sr exerts a more dominant control on their sr abundance and 87sr 86sr

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

strontium isotopes and rubidium in the ganga brahmaputra river system weathering in the himalaya fluxes to the bay of bengal and contributions to the evolution of oceanic87sr 86sr the concentrations of rb and sr and87sr 86sr isotopic ratios have been measured in the dissolved load of the ganga brahmaputra g b river system the ganga was sampled extensively from its source at gangotri in the higher himalaya to patna on the alluvial plains the brahmaputra was sampled in its stretch in assam in india the average sr concentration in the ganga at patna is 1 2 mol l and that in the brahmaputra at goalpara is 0 73 mol l the mean87sr 86sr ratios are 0 7239 and 0 7192 respectively the87sr 86sr in the ganga source waters the alaknanda the bhagirathi and their tributaries range between 0 7300 and 0 7986 considerably higher than the global average runoff value of 0 7119 the high87sr 86sr in the ganga source waters result from the intense weathering of precambrian granites and gneisses enriched in radiogenic sr the sr isotope systematics of the ganga waters is dominated by silicate weathering whereas carbonate weathering plays a significant role in their major ion chemistry the g b system transports about 910 million moles of dissolved sr annually to the bay of bengal with an average87sr 86sr of 0 7213 the sr transported by the g b system is about 2 7 of the global dissolved sr flux to the oceans via rivers model calculations reveal that the g b system has contributed significantly to the sr isotope evolution of seawater during the past 20 ma the flux of rb through the g b system is 24 million moles per year this is about 5 of the global river supply of rb to the oceans nearly twice the contribution of water via the g b system to the oceans our study suggests that the marine geochemistries of sr and rb and possibly u may have been influenced considerably by the himalayan orogeny

lithologic and climatologic controls of river chemistry abstract the chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff in all 101 different rivers each draining a primary lithology were examined across the united states puerto rico and iceland to minimize seasonal effects only rivers with at least two years of data were used basaltic catchments were examined in the greatest detail in a survey of hawaiian island watersheds the average river chemistries could be related to the distribution of soil associations within each catchment an analysis of cation activity relationships among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium ion exchange control on cation ratios among different lithologies unique weathering rate relationships were developed with yields at typical present day runoff rates 1 100 cm y increasing in the order sandstones granites basalts shales and carbonates the bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering thus a warm wet climate or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal

rayleigh wave dispersion measurements reveal low velocity zones beneath the new crust in the gulf of california rayleigh wave tomography provides images of the shallow mantle shear wave velocity structure beneath the gulf of california low velocity zones lvzs are found on axis between 26 and 50 km depth beneath the guaymas basin but mostly off axis under the other rift basins with the largest feature underlying the ballenas transform fault we interpret the broadly distributed lvzs as regions of partial melting in a solid mantle matrix the pathway for melt migration and focusing is more complex than an axis centered source aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments we also consider the existence of solid lower continental crust in the gulf north of the guaymas basin where the association of the lvzs with asthenospheric upwelling suggests lateral flow assisted by a heat source these results provide key constraints for numerical models of mantle upwelling and melt focusing in this young oblique rift

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

the effect of organic matter on chemical weathering study of a small tropical watershed nsimi zoetele site cameroon abstract the effect of organic matter during soil water interaction is still a debated issue on the controls of chemical weathering in a tropical environment in order to study this effect in detail we focused on the weathering processes occurring in a small tropical watershed nsimi zoetele south cameroon this site offers an unique opportunity to study weathering mechanisms in a lateritic system within a small basin by coupling soil and water chemistry the lateritic cover in this site can reach up to 40 m in depth and show two pedological distinct zones unsaturated slope soils on the hills and or elevated areas and water saturated soils in the swamp zone which represent 20 of the basin surface the study present chemical analysis performed on water samples collected monthly from different localities between 1994 1997 and on soil samples taken during a well drilling in december 1997 the results suggest the existence of chemical and spatial heterogeneities of waters in the basin colored waters flooding the swamp zone have much higher concentrations of both organic matter i e doc and inorganic ions e g ca mg al fe th zr than those from springs and groundwater from the hills nevertheless these organic rich waters present cation concentrations na ca mg k which are among the lowest compared to that of most world rivers the main minerals in the soils are secondary kaolinite iron oxi hydroxides quartz and accessory minerals e g zircon rutile we mainly focused on the mineralogical and geochemical study of the swamp zone soils and showed through sem observations the textural characterization of weathered minerals such as kaolinite zircon rutile and the secondary recrystallization of kaolinite microcrystals within the soil profile water chemistry and mineralogical observations suggest that hydromorphic soils of the swamp zone are responsible for almost all chemical weathering in the basin thus in order to explain the increase of element concentration in the organic rich waters we suggest that organic acids enhance dissolution of minerals such as kaolinite goethite and zircon and also favors the transport of insoluble elements such as al fe ti zr and ree by chemical complexation sio2 aq concentrations in these waters are above saturation with respect to quartz dissolution of phytholithes amorphous silica may be responsible for this relatively high sio2 aq concentration al mg ratios obtained for the soil and the mengong river waters show that a significant amount of al does not leave the system due to kaolinite recrystallisation in the swamp zone soils geochemical data obtained for this watershed show the important contribution of vegetation and organic matter on chemical weathering in the swamp zone quantitatively we propose that the increasing amount in total dissolved solid tds due to organic matter and vegetation effect is about 35 in summary this interaction between soils and waters occurs mostly in soils that are very depleted in soluble elements thus the low concentration of major elements in these water is a direct consequence of the depleted nature of the soils

anisotropy beneath california shear wave splitting measurements using a dense broadband array we have determined the shear wave splitting parameters for a dense network of broad band stations in the western united states using high quality sks and skks waveforms with particularly high spatial resolution in the southern california region covered by the trinet seismic network the alignment of most fast polarization directions can be explained by plate tectonic extensional and compressional events we find that the overall pattern of fast directions agrees well with the pn anisotropy model by hearn that images the uppermost mantle furthermore the measured fast directions are generally orthogonal to the maximum horizontal compressive stress directions as determined from shallow crustal stress indicators world stress map this suggests that the pattern of anisotropy in the western us is predominantly uniform throughout the crust and upper mantle and that a 100 150 km thick layer as estimated from the sks delay time assuming 4 per cent anisotropy of anisotropic material has experienced coherent strain conditions and has undergone a similar deformation history a more detailed investigation reveals small scale lateral variations in anisotropy that are manifested by minor differences in splitting parameters between closely located stations as well as between sks and skks for the same station event pairs we also identify a contrast in splitting parameters between central the greater bay area and southern california in central california our measurements show evidence for variation of splitting parameters with backazimuth while in southern california the pattern of measurements can be fit adequately with a single layer anisotropy model this contrast dominates any consistent effect of the san andreas fault saf we can fit the variation of splitting parameters as a function of polarization azimuth for some stations in the vicinity of the saf better with a two layer anisotropy model than a single layer model with one thin layer having a fast direction parallel to the saf however many alternative models which could incorporate dipping axes of anisotropy lateral variation of anisotropy or a more continuous variation of fast direction with depth would be able to produce a similar fit

chemical denudation and silicate weathering in himalayan glacier basins batura glacier pakistan original article can be found at http www sciencedirect com science journal 00221694 copyright elsevier b v doi 10 1016 s0022 1694 02 00036 7 full text of this article is not available in the uhra

glaciers show direct linkage between erosion rate and chemical weathering fluxes field studies suggest a linkage between high physical erosion rates and rates of chemical denudation mechanical erosion by temperate glaciers is commonly an order of magnitude higher than in mountainous fluvial catchments leading to an expectation that chemical weathering fluxes should also be high from glacierized basins yet solute fluxes from glaciers are not found to be higher than non glacierized catchments application of a model of silicate weathering from glaciers based on mineral surface area production and mineralogy shows that solute fluxes are consistent with the low temperatures dilute water chemistry and high mineral surface area production in these environments low temperatures reduce silicate weathering rates this effect explains the difference between silica fluxes from glaciers and from non glacierized basins as in laboratory flow through reactors glacial solute flux should depend on surface area production and mineral weathering rate constants the surface area production is significant a typical glacial erosion rate and grain size distribution produces on the order of 104 km2 of mineral surface area per square kilometer per year this new surface area is highly reactive because mineral weathering rates decline with surface age application of the reactor model yields results largely consistent with measured solute fluxes for the example of bench glacier alaska the model underpredicts potassium fluxes probably due to accelerated initial dissolution of biotite strained by abrasion the success of the model in predicting other silicate weathering fluxes reflects the far from equilibrium conditions in glacial runoff such that mineral weathering rate constants are not limited by saturation state in a small data set from glacial catchments both annual silica fluxes and mean concentrations increase with water discharge this suggests that mineral surface area increases with water discharge from glaciers an effect plausibly linked to erosion rates those glaciers for which both erosion rate and silica flux data are available support the idea that production of new reactive mineral surface area by glacial erosion drives silicate weathering fluxes

the himalayas organic carbon burial and climate in the miocene cooling ages of rock in the himalayas imply that rapid exhumation between the main central thrust system and the south tibetan detachment system occurred between 21 and 17 ma the generation of relief and enhanced weathering which followed this event may have resulted in a pronounced increase in the delivery of dissolved strontium carbon phosphorus and other chemical weathering products to the ocean richter et al 1992 the increased supply of nutrients stimulated productivity in oceanic upwelling zones and expansion of the oxygen minimum zone leading to enhanced burial and preservation of organic matter in the monterey formation and other deposits from this interval a downdraw of atmospheric co2 associated with enhanced chemical weathering rates and organic matter burial may have led to global cooling and the expansion of the antarctic ice sheet by 15 ma the above scenario differs from the monterey hypothesis of vincent and berger in that co2 downdraw is primarily via silicate weathering rather than organic carbon burial and that organic carbon burial is driven by increased delivery of nutrients to the ocean rather than by stronger upwelling a carbon mass balance calculation which assumes that river fluxes have been increasing over the last 40 ma predicts that absolute organic carbon burial increased over this interval while at the same time the fraction of carbon buried as organic matter versus carbonate decreased this implies that the organic carbon cycle has acted as a net source of co2 to the atmosphere over the late cenozoic

shear wave splitting along a nascent plate boundary the north anatolian fault zone summary the north anatolian fault zone nafz is a transform structure that constitutes the boundary between the anatolian plate to the south and the eurasia plate to the north we analysed the properties of the upper mantle strain field and mantle anisotropy in the vicinity of nafz via splitting of sks and skks phases we used data from the north anatolian fault naf passive seismic experiment this is the first study that analyses the upper mantle anisotropy in this region and our results indicate that the observed upper mantle strain field is uniform underneath the array with consistent ne sw polarization directions for fast split waves the measured lag times between the arrivals of the fast and slow split waves varies from 0 5 to 1 6 s for the study area smaller lag times are obtained consistently in the eastern part of the array however we do not observe any significant variation in either the polarization directions or the delay times across the plate boundary nafz the uniformity of the fast polarization directions throughout the study area and the strength of anisotropy favour an asthenospheric source for the anisotropy the regional tectonic framework favours a sw direction of asthenospheric flow due to the forces acting on the upper mantle exerted by the slab roll back taking place along the aegean and the cyprean subduction zones

relative contributions of silicate and carbonate rocks to riverine sr fluxes in the headwaters of the ganges abstract exhumation of the himalayan tibetan orogen is implicated in the marked rise in seawater 87sr 86sr ratios since 40 ma however both silicate and carbonate rocks in the himalaya have elevated 87sr 86sr ratios and there is disagreement as to how much of the 87sr flux is derived from silicate weathering most previous studies have used element ratios from bedrock to constrain the proportions of silicate and carbonate derived sr in river waters here we use arrays of water compositions sampled from the head waters of the ganges in the indian and nepalese himalaya to constrain the end member element ratios the compositions of tributaries draining catchments restricted to a limited range of geological units can be described by two component mixing of silicate and carbonate derived components and lie on a plane in multicomponent composition space key elemental ratios of the carbonate and silicate components are determined by the intersection of the tributary mixing plane with the planes na 0 for carbonate and constant ca na for silicate the fractions of sr derived from silicate and carbonate sources are then calculated by mass balance in sr ca mg na composition space comparison of end member compositions with bedrock implies that secondary calcite deposition may be important in some catchments and that dissolution of low mg trace calcite in silicate rocks may explain discrepancies in sr ca na mg covariation alternatively composition dependent precipitation or incongruent dissolution reactions may rotate mixing trends on cation ratio diagrams however the calculations are not sensitive to transformations of the compositions by incongruent dissolution or precipitation processes provided that the transformed silicate and carbonate component vectors are constrained silicates are calculated to provide 50 of the dissolved sr flux from the head waters of the ganges assuming that discrepancies between ca mg na covariation and the silicate rock compositions arise from addition of trace calcite if the ca mg na mixing plane is rotated by composition dependent secondary calcite deposition this estimate would be increased moreover when 87sr 86sr ratios of the sr inputs are considered silicate sr is responsible for 70 16 1 of the 87sr flux forcing changes in seawater sr isotopic composition since earlier studies predict that silicate weathering generates as little as 20 of the total sr flux in himalayan river systems this study demonstrates that the significance of silicate weathering can be greatly underestimated if the processes that decouple the water cation ratios from those of the source rocks are not properly evaluated

weathering processes in the ganges brahmaputra basin and the riverine alkalinity budget abstract we present river chemistry data for a network of rivers draining the western and central nepal himalaya our sampling locations cover the system from the sources of rivers in tibet to the gangetic plain water samples were collected throughout the year including the monsoon season for rivers in nepal and for the ganges and brahmaputra in bangladesh rivers draining the north himalaya are characterized by low discharge under a cold and arid climate main stream waters have 13 c dic near 0 and high so42 so42 hco3 ratios xso4 with values around 40 eq and high cl ca is the dominant cation ca2 cations 55 to 75 eq after correction of sodium by chloride dissolved sulfate is produced in waters from the tethian sedimentary series whereas chlorine is related to thermal waters dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution 13 c dic values of up to 3 9 show that metamorphic co2 is an important weathering agent rivers of the north himalayan basins have about 50 of their alkalinity derived from carbonate dissolution 20 from biogenic activity and 30 from metamorphic co2 on the south flank of the himalaya rivers are more depleted in 13 c and have on average lower xso4 most rivers have 13 c dic and xso4 values compatible with a simple mixing between soil co2 and sulfate derived from sulfide oxidation during the monsoon discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced xso4 and 13 c decrease while ca cations increases implying enhanced dissolution of pedogenic calcite as a whole the g b riverine flux of alkalinity derived from silicate weathering is around 2 7 1011 mol year modest at the global scale sulphuric acid controls 20 30 of the weathering reaction in the brahmaputra and 6 to 9 in the ganges na and k balance 60 to 65 of the silicate derived alkalinity flux and the long term co2 consumption by ganges and brahmaputra is near 6 4 1010 mol year the flux of metamorphic co2 converted to alkalinity via weathering reactions is ca 1 1010 mol year

geodynamics of the gulf of california from surface wave tomography the gulf of california which forms part of the pacific north american plate boundary is an ideal place to investigate upper mantle dynamics in a continental rifting area with 19 seismic stations located around the gulf the nars baja experiment 2002 2008 was designed to image its crustal and mantle structure here we present results of a tomographic inversion of love and rayleigh interstation phase velocity measurements for a radially anisotropic shear velocity model of the gulf of california this study confirms the overall low shear wave velocities in the upper 200km of the mantle found in other rayleigh wave studies and the presence of a positive shear wave velocity anomaly at depths of roughly 80 160km beneath the central gulf zhang et al 2009 in addition we find that the horizontal shear velocity vsh is generally higher than the vertical shear velocity vsv for the northern gulf however there is strong indication for vsv vsh in the 40 60km depth range this region also has anomalously low shear wave velocities down to 100km depth combining these observations we suggest that the low velocity anomaly and the negative radial anisotropy vsh vsv beneath the northern gulf are related to vertical flow of low velocity material in an area of a slab window either by the vertical alignment of olivine crystals or by a fabric of vertically oriented sheets of melt the high velocity anomaly beneath the central gulf is interpreted as a remnant slab fragment which inhibits vertical flow from the deeper mantle our tomographic results indicate that the formation of the gulf of california cannot be explained by simple models of crustal extension or dynamic upwelling instead its structure and geodynamics are caused by the cessation of subduction by stalled microplates in the central and southern gulf and the presence of a slab window in the north 2012 elsevier b v

subduction control on the curie isotherm around the pacific north america plate boundary in northwestern mexico gulf of california preliminary results abstract spectral analysis of namag magnetic data enabled us to estimate depths to the curie isotherm along the pacific north america plate boundary the estimated depths for the gulf of california and southern baja california peninsula correlate quite well with the depths to the moho available from previous studies based on receiver function and seismic refraction in northernmost baja california and southwestern usa no correlation is observed between depths to the curie isotherm and depths to the mantle obtained from receiver function analysis depths to the curie isotherm are shallower along the coastal plains of sonora and sinaloa differences between receiver function based mantle depths and curie depths are equally large however there is more consistency with depths established by seismic refraction studies four zones with shallow curie isotherm are identified in southern gulf of california gs1 gs2 gs3 and gs4 gs1 is located at the gulf of california mouth the alarcon rise is located between gs1 and gs2 pescaderos basin is located northwest of gs2 gs3 stretches from the carmen basin to the farallon basin gs4 extends from tiburon island up to northern guaymas basin northernmost shallow curie depth zone gn1 comprises upper and lower delfin basins as well as the consag and wagner basins a northern prolongation also covers pinacate volcanic center the southern shallow curie isotherm zones are associated with lithospheric asthenospheric material upwelling zones gn1 also includes a convective component related to the younger magmatism of the pinacate volcanic field deeper curie depths are found at southern baja california peninsula interpreted as associated with thicker continental or stretched crust it is possible that the crust is underlain or underplated by magnetic oceanic crust as suggested by satellite magnetic anomalies centered at the magdalena shelf a contrasting pattern features northern baja california deep curie zones to the west contrasting with relative shallower curie isotherm areas to the east the prolongation towards the peninsula of the extinct shirley transform fracture zone roughly separates the two domains observed along the peninsula and suggests that the curie isotherm along the peninsula is controlled by differences subduction of the guadalupe and magdalena microplates major differences pointed out between northern and southern gulf of california might indicate that subduction also played a key role at the beginning of the opening of the gulf of california however currently the curie pattern is controlled by the rifting processes shallow curie zones correlating quite well with the rift basins and the lithosphere asthenosphere upwelling material

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

role of river suspended material in the global carbon cycle the reaction of ca derived from silicate weathering with co2 in the world s oceans to form carbonate minerals is a critical step in long term climate moderation ca is delivered to the oceans primarily via rivers where it is transported either as dissolved species or within suspended material the relative importance for climate moderation of riverine dissolved ca vs suspended ca transport stems from the total ca flux and its climate dependence data in the literature suggest that within uncertainty global riverine dissolved ca flux is equal to suspended material ca flux to determine how these fluxes depend on temperature and rainfall a 40 yr field study was performed on 4 catchments

geologic control of sr and major element chemistry in himalayan rivers nepal our study of the seti river in far western nepal shows that the solute chemistry of the river and its tributaries is strongly controlled by geology the seti flows through four distinct terranes starting with the tethyan sedimentary series tss and greater himalayan series ghs tss ghs waters display 87 sr 86 sr ratios of 0 73 and high sr and ca consistent with the composition of limestone and marble common in these terranes the 87 sr 86 sr ratio and mg increase markedly as the river passes into the lesser himalayan series lhs where tributaries have 87 sr 86 sr ratios from 0 75 to 1 02 and high sr ca and mg the high mg in lhs waters correlate with high 87 sr 86 sr ratios which we attribute to weathering of highly radiogenic 0 71 0 82 dolostones tributaries to the seti river draining the largely carbonate free dadeldhura thrust sheet dts have ratios near 0 74 but low sr ca and mg and therefore have little impact on seti mainstem chemistry mass balance calculations and camg weathering indices show that carbonate weathering accounts for 70 of total dissolved solids to the seti river sr ca ratios of river waters provide a minimum estimate of the carbonate weathering contribution to sr due to partitioning of sr and ca during incongruent dissolution and reprecipitation of calcite overall we attribute high 87 sr 86 sr ratios in the seti river and its tributaries to the weathering of metacarbonates especially dolostones in the upper nawakhot group which have exchanged sr with silicates during metamorphism our modeling of sr fluxes in the seti river indicates that the tss ghs accounts for 36 39 of the sr the lhs for 40 53 and 8 23 for the dts prior to exposure of lhs rocks at 12 ma tss and ghs carbonates with low 87 sr 86 sr ratios dominated himalayan rivers we attribute the elevated 87 sr 86 sr ratios of himalayan paleorivers during the late miocene and pliocene to exposure and weathering of lhs metacarbonates copyright 2000 elsevier science ltd

chemical characteristics and suspended sediment load of meltwaters from a himalayan glacier in india abstract meltwaters draining from the snout of the chhota shigri glacier eastern lahul valley himachal pradesh india were analysed for major cations and anions the glacier lies on the southern slope of the himalayas at an elevation ranging between 4000 and 5000 m the drainage area of the basin is 40 km 2 while the glacier area is 10 km 2 water quality was determined during the summer ablation period from 17 july to 17 august 1987 meltwaters from the glacial surface site 1 and 2 have low solute content electrical conductivity 5 0 5 92 s cm 1 ca 2 0 73 mgl 1 mg 2 0 08mgl 1 na 0 36mgl 1 k 0 34mgl 1 whereas after passing through the glacier waters are chemically enriched electrical conductivity 23 2 29 0 s cm 1 ca 2 0 9 9 9mgl 1 mg 2 0 5 0 35mgl 1 na 0 7 0 8mgl 1 k 0 4 1 4 mgl 1 the variations of solute concentration in meltwaters at the snout is related to its discharge the water movement within the chhota shiggi glacier appears to be through the subglacial and englacial channels

models of tracer breakthrough and permeability in simple fractured porous media tailing and bimodal behaviour of tracer breakthrough curves from tracer tests conducted in fractured porous media are commonly presented as deviations from the fickian model for homogeneous porous media tailing is mainly attributed to 1 tracer storage brought about by diffusion between mobile and static regions of fluid 2 a concentration of flow towards the wider aperture and thus more permeable fracture zones and 3 the high variance in fracture conductivity and consequent mixing of tracer bi or multi modality has been attributed to solute following a few highly permeable flow paths systematic numerical simulations of flow and transport in geometrically simple fractured porous media were conducted using a 2d finite difference flow code and a particle tracking transport model as a simplification only advective dispersion was considered the modelling study produced a large variety of tracer breakthrough curves including two patterns commonly seen in field data the backward tailed uni modal type and the gaussian type the study demonstrates that different types of breakthrough might be characteristic of particular sets of conceptual models for heterogeneities and as such may provide a useful pointer in the application and interpretation of tracer tests

the role of disseminated calcite in the chemical weathering of granitoid rocks accessory calcite present at concentrations between 300 and 3000 mg kg 1 occurs in fresh granitoid rocks sampled from the merced watershed in yosemite national park ca usa loch vale in rocky mountain national park co usa the panola watershed ga usa and the rio icacos puerto rico calcite occurs as fillings in microfractures as disseminated grains within the silicate matrix and as replacement of calcic cores in plagioclase flow through column experiments using de ionized water saturated with 0 05 atm co2 produced effluents from the fresh granitoid rocks that were dominated by ca and bicarbonate and thermodynamically saturated with calcite during reactions up to 1 7 yr calcite dissolution progressively decreased and was superceded by steady state dissolution of silicates principally biotite mass balance calculations indicate that most calcite had been removed during this time and accounted for 57 98 of the total ca released from these rocks experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution the lack of excess ca and alkalinity indicated that calcite had been previously removed by natural weathering the extent of ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposed in granitoid rocks high ca na ratios relative to plagioclase stoichiometries indicate excess ca in the yosemite loch vale and other alpine watersheds in the sierra nevada and rocky mountains of the western united states this ca enrichment correlates with strong preferential weathering of calcite relative to plagioclase in exfoliated granitoids in glaciated terrains in contrast ca na flux ratios are comparable to or less than the ca na ratios for plagioclase in the subtropical panola and tropical rio icacos watersheds in which deeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase during transport limited weathering these results indicate that the weathering of accessory calcite may strongly influence ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces

the indus river system india pakistan major ion chemistry uranium and strontium isotopes the indus river is one of the large river systems draining the himalaya we report in this paper the major ion chemistry sr and u isotope systematics of the indus system particularly its headwaters the results show that 1 on an average about a third of the cations in the waters can be from silicate weathering however most of the ca mg is likely to be from the weathering of carbonates and evaporites 2 the 87sr86sr of the waters ranges between 0 7085 and 0 7595 the higher values 0 72 are typical of the tributaries draining the precambrian granite gneissic terrains the 87sr86sr of the indus main channel throughout its entire stretch shows only minor variations 0 7104 0 7116 the indus transports 8 8 106 mol sr to the arabian sea annually with a 87sr86sr of 0 7111 if our results of sr concentration and 87sr86sr measured at thatta can be considered typical of the indus throughout the year and 3 the u concentration in the indus and its tributaries is generally high 0 37 10 3 g 1 1 the source for the high uranium can be the weathering of granites zones of uranium mineralisation and black shales the indus results when compared with our earlier data on the ganga brahmaputra show that in all these three river systems carbonate weathering is the dominant source of ca mg and hco3 and that u concentrations are high in the case of 87sr86sr the indus waters are less radiogenic however its tributaries draining the precambrian granites gneisses have 87sr86sr in excess of 0 72 similar to that in the rivers of the ganga system the low 87sr86sr of the indus 0 7111 suggests that its contribution to the sr isotope evolution of oceans since the cenozoic is less significant than that of the ganga brahmaputra

the relationship between upper mantle anisotropic structures beneath california transpression and absolute plate motions we calculated sks splitting parameters for the california integrated seismic network in southern california we also estimated splitting in the upper 100 km using azimuthal anisotropy determined from surface waves the inferred splitting from surface waves in the mantle lithosphere is small on average 0 2 s compared with sks splitting 1 5 s and obtains a maximum value 0 5 s in the transpressive region of the big bend south of and aligned with the san andreas fault saf in contrast the sks splitting is approximately e w and is relatively uniform spatially either side of the big bend of the saf these differences suggest that most of the sks splitting is generated much deeper down to 300 400 km than previously thought probably in the asthenosphere fast directions align with absolute plate motions apm in northern and southeastern california but not in southwestern california we interpret the parallelism with apm as indicating the sks anisotropy is caused by cumulative drag of the asthenosphere by the overlying plates the discrepancy in southwestern california arises from the diffuse boundary there compared to the north where relative plate motion has concentrated near the saf system in southern california the relative motion originated offshore in the borderlands and gradually transitioned onshore to the saf system this has given rise to smaller displacement across the saf 160 180 km compared with central and northern california 400 500 km thus in southwestern california the inherited anisotropy from prior north american apm has not yet been overprinted by pacific apm

s wave polarization inversion of the 15 october 1979 23 19 imperial valley aftershock evidence for anisotropy and a simple source mechanism s polarizations from the 16 near source strong motion stations of the 15 october 1979 23 19 imperial valley aftershock have been analyzed for constraining the source mechanism they show a significant delay of the east component relative to the north component at most stations assuming that this is an effect of anisotropy with a horizontal symmetry axis in the upper 4 km of sediments where the rays are dominantly vertical the measured fast s polarizations are mainly oriented between n0 e and n25 e and show a mean delay of 0 25 s between slow and fast s this is consistent with an hexagonal anisotropy due to vertical cracks induced by the regional compressional stress field and parallel to it with a mean fluid filled crack density of 0 1 to 0 2 the records once corrected for the splitting effect show s polarizations stable in time which have been used to estimate the fault mechanism parameters this is done by applying a new inversion method which is based on a model probability estimate the error function is related to the polarization misfits and the associated joint probability function of the data and parameter is computed in the whole model space the inversion of the corrected s polarizations provides a unique and well constrained solution for the imperial valley aftershock mechanism strike 145 5 dip 85 10 slip 155 15 which is compatible with the available p polarities and is quite similar to the main shock mechanism

crustal thickness estimates for baja california sonora and sinaloa mexico using disperse surface waves 1 dispersed surface waves of regional events recorded at network of autonomously recording seismographs nars baja and red sismica de banda ancha resban networks located over the baja california peninsula sonora and sinaloa mexico were used to estimate shear wave elastic models and crustal thickness we analyzed fundamental modes of surface waves with period between 10 and 40 s multiple filter analysis and the inversion method described by herrmann and ammon 2003 was used crustal thickness estimates for the peninsular ranges of northern baja california agree with those obtained by previous studies in the peninsular ranges of northern baja california we analyzed dispersion of surface waves with northwest southeast travel paths along the east and west sides of the baja california peninsula as well as a northwest southeast travel path along the western sides of the mexican states of sonora and sinaloa it was found that the crustal structure east of the baja california peninsula is similar to the structure of sonora and sinaloa the correlation between those two structures suggests dextral offset of the order of 275 25 km if we consider baja california peninsula as a rigid body moving toward the northwest relative to the north america plate this displacement between the structures is in agreement with the displacement determined by the dating of miocene deposits located in san felipe on the baja california peninsula pacific plate and isle tiburon located west of sonora north america plate

spatial variations in chemical weathering and co2 consumption in nepalese high himalayan catchments during the monsoon season the major ion chemistry of the marsyandi basin and six of its tributaries in the nepalese himalaya have been investigated during the monsoon months of 2002 weekly water samples taken at 10 river monitoring stations in the annapurna watershed over the course of 4 months provide chemical weathering data for the region at an unprecedented temporal and spatial resolution the river chemistry of all but one basin is heavily dominated by carbonate weathering which compared to silicate weathering contributes 80 to 97 of the total solute load this prevalence is due to a combination of a intrinsically faster dissolution kinetics of carbonates b relatively high runoff and c glacial meltwater and low temperatures at high altitudes resulting in enhanced carbonate solubilities monitoring stations with headwaters in the tethyan sedimentary series tss are particularly carbonate rich and slightly supersaturated with respect to calcite through half of the monsoon season silicate weathering in the tss is driven largely by sulfuric acid and therefore does not contribute significantly to the drawdown of atmospheric co2 with respect to the tributaries in the greater himalayan sequence ghs carbonate weathering is practically as predominant as for the tss in spite of the largely felsic lithology of the ghs relative to the tss the primary proton source in the ghs has shifted with at least 80 of the protons derived from carbonic acid averaged over the whole field area the co2 fluxes based on silicate derived ca and mg are considerably lower than the global average assuming that this study area is representative of the entire range we conclude that in situ weathering of the high himalayas does not represent a significant sink of atmospheric carbon dioxide despite the presence of a watershed south of the ghs that is characterized by a four times higher co2 consumption rate than the global average silicate weathering rates of all basins appear to be climate controlled displaying a tight correlation with runoff and temperature given the extremely low chemical weathering under transport limited conditions in high altitude crystalline terrains outside of the monsoon season this would result in virtually no chemical exhumation for 2 3 of the year in such a cold and arid climate north of the rain shadow cast by the high himalayas 2009 elsevier ltd all rights reserved

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

a review of the geodynamic evolution of flat slab subduction in mexico peru and chile abstract subducting plates around the globe display a large variability in terms of slab geometry including regions where smooth and little variation in subduction parameters is observed while the vast majority of subduction slabs plunge into the mantle at different but positive dip angles the end member case of flat slab subduction seems to strongly defy this rule and move horizontally several hundreds of kilometers before diving into the surrounding hotter mantle by employing a comparative assessment for the mexican peruvian and chilean flat slab subduction zones we find a series of parameters that apparently facilitate slab flattening among them trench roll back as well as strong variations and discontinuities in the structure of oceanic and overriding plates seem to be the most important however we were not able to find the necessary and sufficient conditions that provide an explanation for the formation of flat slabs in all three subduction zones in order to unravel the origin of flat slab subduction it is probably necessary a numerical approach that considers also the influence of surrounding plates and their corresponding geometries on 3d subduction dynamics

reconciling the elemental and sr isotope composition of himalayan weathering fluxes insights from the carbonate geochemistry of stream waters determining the relative proportions of silicate vs carbonate weathering in the himalaya is important for understanding atmospheric co2 consumption rates and the temporal evolution of seawater sr however recent studies have shown that major element mass balance equations attribute less co2 consumption to silicate weathering than methods utilizing ca sr and 87sr 86sr mixing equations to investigate this problem we compiled literature data providing elemental and 87sr 86sr analyses for stream waters and bedrock from tributary watersheds throughout the himalaya mountains in addition carbonate system parameters pco2 mineral saturation states were evaluated for a selected suite of stream waters the apparent discrepancy between the dominant weathering source of dissolved major elements vs sr can be reconciled in terms of carbonate mineral equilibria himalayan streams are predominantly ca2 mg2 hco3 waters derived from calcite and dolomite dissolution and mass balance calculations demonstrate that carbonate weathering contributes 87 and 76 of the dissolved ca2 and sr2 respectively however calculated ca sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and pco2 gradients in the himalaya the state of calcite and dolomite saturation was evaluated across these gradients and the data show that upon descending through the himalaya 50 of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas co2 stream water ca mg and ca sr ratios decrease as the degree of supersaturation with respect to calcite increases and mg2 ca2 and hco3 mass balances support interpretations of preferential ca2 removal by calcite precipitation on the basis of patterns of saturation state and pco2 changes calcite precipitation was estimated to remove up to 70 of the ca2 originally derived from carbonate weathering accounting for the nonconservative behavior of ca2 during riverine transport brings the ca sr and 87sr 86sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock thereby permitting consistency between elemental and sr isotope approaches to partitioning stream water solute sources these results resolve the dissolved sr2 budget and suggest that the conventional application of two component ca sr and 87sr 86sr mixing equations has overestimated silicate derived sr2 and hco3 fluxes from the himalaya in addition these findings demonstrate that integrating stream water carbonate mineral equilibria divalent cation compositional trends and sr isotope inventories provides a powerful approach for examining weathering fluxes

active tectonics of northeastern sonora mexico southern basin and range province and the 3 may 1887 mw 7 4 earthquake north south striking and west dipping basin and range province normal faults form the western edge of the sierra madre occidental plateau in northeastern sonora these faults and associated half grabens extend over a distance of more than 300 km between the san bernardino basin in the north and the sahuaripa basin in the south an earthquake in 1887 ruptured three neighboring segments of this major fault zone our field mapping in this region indicates that the surface rupture of the 1887 earthquake extends farther to the south and is considerably longer 101 4 km end to end length than previously reported a compilation of the seismicity in the epicentral region of the 1887 earthquake shows the epicenters to be distributed in well defined clusters at the northern end of the 1887 surface rupture in the step overs between the three rupture segments on a neighboring fault in the west fronteras fault and on fault segments farther south along the same fault zone granados region the distribution of seismicity correlates well with calculated changes in coulomb failure stress resulting from the 1887 earthquake

strong tectonic and weak climatic control of long term chemical weathering rates the relationships among climate physical erosion and chemical weathering have remained uncertain because long term chemical weathering rates have been difficult to measure here we show that long term chemical weathering rates can be measured by combining physical erosion rates inferred from cosmogenic nuclides with dissolution losses inferred from the rock to soil enrichment of insoluble elements we used this method to measure chemical weathering rates across 22 mountainous granitic catchments that span a wide range of erosion rates and climates chemical weathering rates correlate strongly with physical erosion rates but only weakly with climate implying that by regulating erosion rates tectonic uplift may significantly accelerate chemical weathering rates in granitic landscapes

shear wave splitting beneath western united states in relation to plate tectonics we have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western united states fast polarization directions and delay times t show spatial variations that are coherent within geologic provinces stations located near the san andreas fault show clear evidence for fault parallel anisotropy in the crust and upper mantle 115 125 km thickness this can be explained by the finite strain associated with the relative plate motion between the north american and pacific plates the lateral extent of this strain field is probably narrow to the west because stations 55 km west of the san andreas fault do not show fault parallel anisotropy in southern california station lac located 80 km east of the san andreas fault shows large fault parallel anisotropy this suggests that the pacific north american plate boundary in the mantle might be displaced to the east in southern california a deeper e w oriented fast direction of anisotropy underlies the fault parallel anisotropic layer in the vicinity of the san andreas fault an e w fast feature is also present beneath the western basin and range and the foothills of the sierra nevada although local variations are present the magnitude of delay times suggests that this feature resides in the asthenosphere we interpret this feature as the asthenospheric flow in the slabless window left behind the farallon plate the flow induced anisotropy may partially be frozen in at shallow depths station orv is located near the southern edge of the gorda slab where no anisotropy is detected the absence of anisotropy at this location could therefore mark a boundary between farallon associated flow and regions where e w oriented asthenospheric flow did not occur the lack of evidence for ne sw fast orientation within the walker lane shear belt of western nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the san andreas fault stations located over the young subducting gorda plate mark a change in the fast direction to nearly ne sw this direction aligns well with the maximum compressive stress direction in the overlying north american plate and the ne sw directed internal shearing of the gorda plate the anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions this implies that the anisotropic thickness may include some of the asthenosphere alternatively using a higher anisotropy of 8 can bring thicknesses in line with other measures of lithospheric thicknesses the correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method and that asthenospheric flow may be a significant contributor to seismic anisotropy

chemical weathering in glacial environments do glaciers enhance or inhibit chemical weathering rates relative to other environments the importance of glaciers in the global carbon cycle and climate change hinges on the answer we show that catchments occupied by active alpine glaciers yield cation denudation rates greater than the global mean rate but do not exceed rates in nonglacial catchments with similar water discharge silica denudation rates are distinctly lower in glacier covered catchments than in their nonglacial counterparts because sediment yields are high from glaciers this suggests that water flux rather than physical erosion exerts the primary control on chemical erosion by glaciers potassium and calcium concentrations are high relative to other cations in glacial water probably due to dissolution of soluble trace phases such as carbonates exposed by comminution and cation leaching from biotite preferential weathering of biotite may result in higher 87 sr 86 sr in glacial runoff than expected from whole rock compositions thus although glaciers do not influence total chemical denudation rates at a given runoff they may yield compositionally distinctive chemical fluxes to the oceans disruption of mineral lattices by grinding increases dissolution rates this and high surface area should make glacial sediments exceptionally weatherable weathering of glacial erosion products in environments beyond the glacier margin deserves attention because it may figure prominently in global chemical cycles

stress orientation and anisotropy based on shear wave splitting observations in the cerro prieto fault area baja california mexico digital seismograms continuously recorded from 1988 to 1992 by two stations of the resnom seismic network in northern baja california mexico were used to search for probable shear wave anisotropic characteristics in the region of the cerro prieto fault shear wave splitting was identified in many of the three component records analyzed we measured the polarization direction of the leadings wave inside thes wave window as well as the delay times between fast and slow phases on those records displaying shear wave splitting for station cpx which is nearest the imperial valley region to the north the preferred polarization direction found in this study azimuth 180 10 coincides with the direction of the regional maximum compressive stress determined for the region this polarization direction can be interpreted in terms of the extensive dilatancy anisotropy model as the effect of vertical parallel aligned cracks the preferred polarization direction measured at lmx however gives an azimuth of 45 5 thus it appears that faults and fractures aligned oblique to the main tectonic trend have a greater influence on the anisotropic characteristics of the crust south of cerro prieto volcano than that of the regional stress field time delays between slow and fasts waves observed at cpx appear constant from 1988 to 1992 while delays measured at lmx for the same interval indicate a small increase with time which cannot be attributed to azimuthal variations of paths

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

worldwide distribution of continental rock lithology implications for the atmospheric soil co2 uptake by continental weathering and alkalinity river transport to the oceans 1 the silicate rock weathering followed by the formation of carbonate rocks in the ocean transfers co2 from the atmosphere to the lithosphere this co2 uptake plays a major role in the regulation of atmospheric co2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks this leads us to develop the first world lithological map with a grid resolution of 1 1 this paper analyzes the spatial distribution of the six main rock types by latitude continents and ocean drainage basins and for 49 large river basins coupling our digital map with the gem co2 model we have also calculated the amount of atmospheric soil co2 consumed by rock weathering and alkalinity river transport to the ocean among all silicate rocks shales and basalts appear to have a significant influence on the amount of co2 uptake by chemical weathering

temperature dependence of silicate weathering in nature how strong a negative feedback on long term accumulation of atmospheric co2 and global greenhouse warming estimation of the temperature dependence of natural feldspar weathering in two catchments at different elevations yields an apparent arrhenius activation energy of 18 4 kcal mol 77 0 kj mol much higher than most laboratory values this finding supports recent suggestions that hydrolytic weathering of silicate minerals may consume carbonic acid and thereby remove atmospheric carbon dioxide more rapidly with increasing temperature than previously thought this result provides a stronger negative feedback on long term greenhouse warming than has been assumed in most models of global carbon cycling the present estimate was determined from the ratio of feldspar weathering rates determined by geochemical mass balance in the southern blue ridge mountains of north carolina united states temperature a function of elevation is the only factor that differs between the two catchments parent rock type aspect hillslope hydrology and vegetation type and successional stage are the same in both

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

microstructural and seismic properties of the upper mantle underneath a rifted continental terrane baja california an example of sub crustal mechanical asthenosphere the gulf of california rift is a young and active plate boundary that links the san andreas strike slip fault system in california to the oceanic spreading system of the east pacific rise the xenolith bearing lavas of the san quintin volcanic area provide lower crust and upper mantle samples from beneath baja california peninsula the microstructures crystallographic preferred orientations cpo and petrology of the san quintin xenoliths suggest that the continental lithosphere in this region has undergone several stages of deformation recrystallisation and melt rock interaction melt rock interactions have led to enrichment in olivine while fine grained microstructures suggest intense deformation in an active shear zone in the shallow upper mantle in this study we highlight the effect of the fine grained mylonitic shear zone development in the upper mantle as an important process of weakening of continental lithosphere the results of the microstructural study show a reduction in cpo strength with increasing grain size reduction most cpos are consistent with dominant slip on the 0kl 100 system as a consequence corresponding seismic anisotropies decrease for both p and s waves with increasing grain size reduction the shallow crystallographic fabric can be related to active shear zones which accommodate the relative motion between the northern baja terrane and the pacific plate estimates of the strain rate stress and viscosity indicate that the shallow mantle beneath northern baja is thermally and chemically lithospheric but mechanically has similar viscosity as the asthenosphere the northern baja terrane is an interesting case of continental crust lying directly on low viscosity upper mantle 2012 elsevier b v all rights reserved

climatic and tectonic controls on chemical weathering in the new zealand southern alps abstract climatic and tectonic controls on the relative abundance of solutes in streams draining the new zealand southern alps were investigated by analyzing the elemental and sr isotope geochemistry of stream waters bedload sediment and hydrothermal calcite veins the average relative molar abundance of major cations and si in all stream waters follows the order ca2 50 si 22 na 17 mg2 6 k 5 for major anions the relative molar abundance is hco3 89 so42 7 cl 4 weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters but nearly all streams have a carbonate dominated ca2 and hco3 mass balance stream water ca sr and 87sr 86sr ratios vary from 0 173 to 0 439 mol nmol and from 0 7078 to 0 7114 respectively consistent with the ionic budget these ratios lie solely within the range of values measured for bedload carbonate ca sr 0 178 to 0 886 mol nmol 87sr 86sr 0 7081 to 0 7118 and hydrothermal calcite veins ca sr 0 491 to 3 33 mol nmol 87sr 86sr 0 7076 to 0 7097 streams draining regions in the southern alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain 10 more ca2 and 30 more sr2 from carbonate weathering compared to streams draining regions in drier more stable landscapes similarly streams draining glaciated watersheds contain 25 more sr2 from carbonate weathering compared to streams draining non glaciated watersheds the highest abundance of carbonate derived solutes in the most physically active regions of the southern alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock which contains trace amounts of calcite estimated to weather 350 times faster than plagioclase in this environment in contrast regions in the southern alps experiencing lower rates of uplift and erosion have a greater abundance of silicate versus carbonate derived cations these findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters using the southern alps as a model for assessing the role of active tectonics in geochemical cycles this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater