miocene extension and extensional folding in an anticlinal segment of the black mountains accommodation zone colorado river extensional corridor southwestern united states 1 recent studies demonstrate that rifts are characterized by linked tilt domains each containing a consistent polarity of normal faults and stratal tilt directions and that the transition between domains is typically through formation of accommodation zones and generally not through production of throughgoing transfer faults the mid miocene black mountains accommodation zone of southern nevada and western arizona is a well exposed example of an accommodation zone linking two regionally extensive and opposing tilt domains in the southeastern part of this zone near kingman arizona east dipping normal faults of the whipple tilt domain and west dipping normal faults of the lake mead domain coalesce across a relatively narrow region characterized by a series of linked extensional folds the geometry of these folds in this strike parallel portion of the accommodation zone is dictated by the geometry of the interdigitating normal faults of opposed polarity synclines formed where normal faults of opposite polarity face away from each other whereas anticlines formed where the opposed normal faults face each other opposed normal faults with small overlaps produced short folds with axial trends at significant angles to regional strike directions whereas large fault overlaps produce elongate folds parallel to faults analysis of faults shows that the folds are purely extensional and result from east northeast stretching and fault related tilting the structural geometry of this portion of the accommodation zone mirrors that of the black mountains accommodation zone more regionally with both transverse and strike parallel antithetic segments normal faults of both tilt domains lose displacement and terminate within the accommodation zone northwest of kingman arizona however isotopic dating of growth sequences and crosscutting relationships show that the initiation of the two fault systems in this area was not entirely synchronous and that west dipping faults of the lake mead domain began to form between 1 m y to 0 2 m y prior to east dipping faults of the whipple domain the accommodation zone formed above an active and evolving magmatic center that prior to rifting produced intermediate composition volcanic rocks and that during rifting produced voluminous rhyolite and basalt magmas

chemical weathering in high sediment yielding watersheds new zealand 1 we have determined the chemical erosion yields for fifteen watersheds in new zealand ranging in size from 12 2 to 2928 km2 these rates coupled with previously measured physical erosion yields allow us to compare these two modes of landscape denudation the physical erosion yields are some of the highest measured in the world although in most instances the chemical erosion yields are only a small fraction of the total erosion yields the absolute values are very high our data strongly support the notion that chemical erosion rates are greatly influenced by the yield of physical erosion and that the rapid production of fresh surfaces as a result of high physical erosion rates and subsequent denudation is critical to the high chemical erosion yields observed

eocene onset and late miocene acceleration of cenozoic intracontinental extension in the north qinling range weihe graben insights from apatite fission track thermochronology abstract intracontinental extension in central china is accommodated by cenozoic grabens flanking the ordos block such as the weihe graben which accommodate 5 6 mm yr of extension to define the onset and evolution of major cenozoic extension we examine the cooling history of exhumed footwall rocks in the north qinling range adjacent to the weihe graben we present results from 47 new apatite fission track aft samples primarily from two vertical transects of 1 5 2 5 km relief located on the northern flank of the north qinling footwall aft ages that get progressively older with increasing distance from the active range bounding normal fault suggest that cenozoic uplift and southward tilting was a response to range parallel extension correlations between aft ages and both elevation and track lengths combined with thermal modeling of representative samples reveal that the north qinling experienced two major stages of cenozoic exhumation 1 relatively slow exhumation in response to a small magnitude of extension that initiated at 50 ma 2 relatively rapid exhumation in response to a large magnitude of accelerated extension that initiated at 10 ma in addition small changes in cooling rate at 35 ma and 25 ma may reflect minor changes in faulting rate we interpret initial extensional at 50 ma to be a far field effect of initial india asia continental collision in contrast accelerated extension after 10 ma along the qinling is likely linked to the upward and outward growth of the tibetan plateau alternatively lower crustal flow beneath the qinling may have progressively accelerated exhumation in late miocene time

cenozoic tectonism in the central basin and range magnitude rate and distribution of upper crustal strain a wealth of pre cenozoic geologic markers across the central basin and range province defined mainly by thrust faults developed within a miogeoclinal wedge makes it one of the best areas on earth to reconstruct large scale continental extension we use this 100 000 km 2 region bounded by the southern sierra nevada on the west and the colorado plateau on the east to illustrate the concepts of correlation and misalignment for regional strain markers and of strain compatibility and kinematic viability in palinspastic reconstructions the results indicate 250 to 300 km of west northwest motion of the sierra away from the plateau accommodated by both crustal thinning and north south shortening tertiary intermontane basin deposits and mineral cooling ages of deeply exhumed rocks constrain the overall kinematics of motion most of the westward motion occurred between 16 and 5 ma at rates near 2 cm yr slowing to 1 to 1 5 cm yr in the last 5 ma we have quantified the partitioning of strain between vertical crustal thinning via normal faults and map view plane strain via conjugate strike slip faults by placing a grid of 10 x 10 km elements on a retrodeformed map of the region and measuring the increase in area of grid elements between the undeformed and present day grids this analysis yields a maximum finite elongation of the basin and range at 36 n to 37 n of 3 4 oriented n73 w map view area balance shows that 20 percent of this elongation is compensated by north south shortening and 80 percent by crustal thinning this yields an average thinning factor for the upper crust of 2 7 between the sierra and plateau inconsistent with the hypothesis that neogene deformation in the central basin and range is predominantly dextral shear plane strain these results in concert with the observation that the southern sierra nevada has similar crustal thickness to the central basin and range support the hypothesis of large scale eastward flow of sierran deep crust during extension

thermal effects of normal faulting during rifted basin formation 2 the lugano val grande normal fault and the role of pre existing thermal anomalies abstract we investigate the thermal consequences of rift related normal faulting and compare the results with a well studied natural example the lugano val grande normal fault southern alps only limited heating of the crust is caused by lithospheric thinning in the simple but realistic situation where heat conduction is substantially faster than heat advection no major thermal disturbance is associated with the downward movement of the hanging wall radiometric ages and fault rocks associated with the lugano val grande normal fault demonstrate that cooling rather than heating affected the crust during normal faulting this pattern is not compatible with such a simple numerical model and is explained by a waning thermal anomaly induced by a magmatic intrusion immediately preceding or overlapping with the first stages of normal faulting the magmatic body must have been emplaced at depths greater than 15 18 km and probably started to cool in the carnian i e few million years before the onset of normal faulting along the lugano val grande fault

chemical weathering river geochemistry and atmospheric carbon fluxes from volcanic and ultramafic regions on luzon island the philippines abstract we investigated rates of chemical weathering of volcanic and ophiolitic rocks on luzon island the philippines luzon has a tropical climate and is volcanically and tectonically very active all factors that should enhance chemical weathering seventy five rivers and streams 10 draining ophiolites 65 draining volcanic bedrock and two volcanic hot springs were sampled and analyzed for major elements alkalinity and 87sr 86sr cationic fluxes from the volcanic basins are dominated by ca2 and mg2 and dissolved silica concentrations are high 500 1900 m silica concentrations in streams draining ophiolites are lower 400 900 m and the cationic charge is mostly mg2 the areally weighted average co2 export flux from our study area is 3 89 0 21 106 mol km2 yr or 5 99 0 64 106 mol km2 yr from ophiolites and 3 58 0 23 106 mol km2 yr from volcanic areas uncertainty given as 1 standard error s e this is 6 10 times higher than the current best estimate of areally averaged global co2 export by basalt chemical weathering and 2 3 times higher than the current best estimate of co2 export by basalt chemical weathering in the tropics extrapolating our findings to all tropical arcs we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments which amount to 1 of the global exorheic drainage area chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long term carbon cycle

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

the influence of continental ice atmospheric co2 and land albedo on the climate of the last glacial maximum the contributions of expanded continental ice reduced atm9spheric co2 and changes in land albedo to the maintenance of the climate of the last glacial maximum lgm are examined a series of experiments is perform ed using an atmosphere mixed layer ocean model in which these changes in boundary conditions are incorporated either singly or in combination the model used has been shown to produce a reasonably reajistic simulation of the reduced temperature of the lgm manabe and broccoli 1985 by comparing the results from pairs of ex periments the effects of each of these envirornnental changes can be determined expanded continental ice and reduced atmospheric co2 are found to have a substantial impact on global mean temperature the ice sheet effect is confined almost ex clusively to the northern hemisphere while lowered co2 cools both hemispheres changes in land albedo over ice free areas have only a minor thermal effect on a global basis the reduction of co2 content in the atmosphere is the primary contributor to the cooling of the southern hemisphere the model sensitivity to both the ice sheet and co2 effects is characterized by a high latitude amplification and a late autumn and early winter maximum

sierra nevada basin and range transition near reno nevada two stage development at 12 and 3 ma relative and absolute elevations of the sierra nevada and adjacent basin and range province timing of their differentiation and location amount and timing of strike slip movement between them are controversial the provincial boundary near reno developed in two stages 1 at ca 12 ma the 700 km 2 verdi boca sedimentary basin formed across what was to become the boundary probably as a result of a small magnitude but regional extensional episode that affected much of the western basin and range 2 at 3 ma the basin was complexly faulted and folded during a larger magnitude extensional episode that established the modern sierran structural and topographic boundary in this area the boundary is really a transition zone with a western edge along the donner pass california fault zone which is farther west than previously placed both episodes appear to have resulted from east west extension only which suggests that northwest motion of the sierra nevada relative to the basin and range shown by geodetic data began after 3 ma or was taken up farther east

structural geologic evolution of the butte district montana the butte district contains two porphyry cu mo deposits and the younger zoned main stage base metal lode deposits that formed in the period from 66 to 62 ma synchronous with laramide compressional thrust faulting and folding in southwest montana new geologic mapping of the butte district reveals three distinct episodes of deformation 1 gentle 20 10 ne tilting produced during laramide compressional deformation uplift and exhumation during and after ore formation in the late cretaceous to paleocene 66 53 ma 2 moderate 10 50 nw tilting of eocene age produced by ne striking normal faults that are limited to the northwest part of the district and synchronous with the eruption of the lowland creek volcanics and 3 miocene to present gentle 15 ese tilting in the hanging walls of the continental east ridge and rocker normal faults stratigraphic relationships isotopic ages hornblende barometry and fluid inclusion data indicate that exposures of the butte granite which hosts mineralization crystallized at 6 to 9 km depth and that early porphyry cu mo deposits formed at similar depths of 5 to 9 km a period of rapid uplift and erosion removed about 7 km of rock from above the ore deposit between 66 to 64 ma and 53 ma but was mostly completed by 59 ma estimated exhumation rates range from 0 5 to 2 mm yr and uplift was likely most rapid between 63 5 and 62 ma during main stage mineralization which occurred at depths of 2 to 6 km the butte granite and boulder batholith were emplaced at 79 to 76 ma into thrust fault ramps which later became inactive near butte as laramide crustal shortening moved eastward in the helena salient the accelerated period of uplift and unroofing is attributed to renewed deformation along the montana disturbed belt south of butte that uplifted the south end of the boulder batholith and produced 20 of gentle ne tilting episode i in the district the exhumation history of the deposit is therefore broadly contemporaneous with ore formation and is consistent with the general transition from lithostatic to hydrostatic conditions that is evident from the telescoping of early porphyry cu mo ores cut by the main stage lode veins within a 5 m y period the laramide compressional regime near butte evolved into an extensional environment characterized by normal faulting the normal faults were synchronous with eruption of the 53 to 48 ma lowland creek volcanics which directly overlie the main stage veins in the northwest part of the district structural reconstruction of the district at the time of mineralization produces orientations of quartz porphyry dikes porphyry cu mo veinlets main stage veins and alteration zonal patterns that are steeply dipping and symmetrical similar to observations from other upright or restored porphyry copper deposits early porphyry cu mo veins and dikes are oriented e w and likely originated as tensile hydrofractures above a ductile magma carapace during laramide shortening additionally during the laramide compressional phase two conjugate fault systems occupied by main stage veins are normal oblique slip faults that record minor e w and vertical shortening and n s extension these normal faults with minor offset likely resulted from brittle failure caused by doming above a volatile rich and buoyant granitic magma chamber and cupola at depth the structural evolution of the butte district therefore reflects the interaction of a far field compressive stress regime with local buoyant magmatic forces

crustal structure of the northwestern basin and range province and its transition to unextended volcanic plateaus 1 the northwestern margin of the basin and range province is characterized by a transition from low magnitude 20 extension in northwestern nevada to relatively unextended volcanic plateaus in northeastern california seismic velocity and potential field modeling provides new control on the mesozoic to present tectonic evolution of this poorly understood portion of the u s cordillera we document 20 crustal thinning associated with basin and range extension from a crustal thickness of 37 km under northeastern california to 31 km under northwestern nevada that is consistent with the amount of extension recorded in the upper crust in northwestern nevada suggesting the crustal response to extension was relatively homogeneous over the entire crustal column our modeling also shows a well defined 80 km wide zone of unusually low upper crustal velocities 5 9 6 1 km s that coincide with the surface location of sparse cretaceous granites locating the elusive northern extension of the sierra nevada batholith through northwestern nevada for the first time in the subsurface combining geological and geophysical data we reconstruct the late cretaceous to present crustal evolution of this region documenting an interplay between magmatic addition to the crust erosional exhumation sedimentation and extension that has reversed the direction of crustal thinning from a west facing continental margin to an east facing interior basin margin over this time interval finally we find no evidence in northwestern nevada for unusually thick crust 40 km prior to basin and range extension

miocene crustal extension and uplift in southeastern nevada constraints from fission track analysis the gold butte block of southeastern nevada appears to be a cross section through the upper approximately 17 km of colorado plateau crust tilted eastward and tectonically denuded via top to the west normal faulting in tertiary time the east side of the block exposes a steeply dipping cratonic paleozoic section 2 to 3 km thick proterozoic basement to the west is continuously exposed for an apparent thickness of approximately 15 km structurally below the basal cambrian unconformity apatite fission track dating and confirmed track length distributions have been used to determine the time of unroofing of the gold butte block and hence the timing of one episode of extension in this part of the basin and range province samples collected in as east west transect along the block indicate an abrupt transition from a period of relative thermal and tectonic stability to the onset of rapid cooling the fission track data indicate that the unroofing of the gold butte block was rapid and that the crustal section was brought to within a few kilometers of the surface at ca 15 ma

basin and range crustal and upper mantle structure northwest to central nevada we present an interpretation of the crustal and uppermost mantle structure of the basin and range of northwestern nevada based on seismic refraction wide angle reflection near vertical reflection and gravity data in comparison to most previous estimates we find that the crust is somewhat thicker 32 36 km versus 22 30 km and the uppermost mantle velocity is somewhat higher 8 0 km s versus 7 3 7 9 km s along our transects the crust is thinnest 32 km in the carson sink buena vista valley region and increases by 2 4 km to the west and east respectively there is considerable complexity throughout the crust where velocities range from of 2 5 km s at the surface to 7 4 km s in the lowermost crust variations in velocity and structure of the upper crustal layers reveal apparent basement velocity depressions areas of lower velocities extending up to 10 km in depth that underlie some surface ranges as well as the basins the middle crust rises from about 20 km beneath central nevada to within 12 km of the surface beneath the area of thinnest crust and is characterized by a modest 0 1 km s change in velocity and low velocity gradients these midcrustal layers mark the onset of high crustal reflectivity and the apparent limiting depth to which basin and range faults can be traced in near vertical reflection profiles suggesting that these midcrustal layers represent the transition between the brittle and ductile zones of the crust the lower crust is more structurally complex with layers thickening and thinning in a systematic manner with the upper crustal layers generally where there are velocity depressions in the upper crust the lower crust is thickest and shallowest the geometry of these lower crustal layers derived from refraction modeling coincides with changes in the crustal reflectivity determined from the consortium of continental reflection profiling reflection data the lower crustal layer is unusually high in velocity 7 4 km s and is likely the layer identified as mantle in some previous studies we do not identify the 7 4 km s layer as mantle because 1 there is an underlying layer with a velocity 8 0 km s that is more consistent with the worldwide average velocity for the upper mantle and 2 the 7 4 km s layer does not correspond to the reflection moho gravity modeling and comparison to existing seismic models show a general consensus in many aspects with respect to crustal structure this new model forms the basis for speculation on some of the processes associated with rifting of the basin and range province one such process lithospheric magmatism is inferred from the strong attenuation of transmitted seismic waves which occurs at the same interface at which high amplitude bright spot reflections originate unlike previous models the overall structure and velocity of the crust and uppermost mantle of our new model are similar to other regions worldwide which have undergone high degrees of extension

effect of vegetation on an ice age climate model simulation a growing number of studies suggest that vegetation changes can significantly influence regional climate variations herein we utilize a climate model genesis with a land surface vegetation package to evaluate the potential role of the very large vegetation changes that occurred during the last glacial maximum lgm in particular we focus on the potential response to a significant reduction in the area of tropical rainforest simulations employed a global vegetation reconstruction for the lgm and climate long range investigation mapping and prediction climap sea surface temperature sst estimates results indicate that expansion of dryland vegetation causes a 15 30 additional lgm cooling for australia 0 4 c and africa 0 9 c respectively turnover from conifer to tundra also causes cooling of 2 4 c or more in western europe and siberia however for the largest rainforest area amazon basin inclusion of realistic vegetation increased modeled temperatures 2 4 c and decreased precipitation by 10 35 these latter results are similar to those obtained with sensitivity experiments of the effects of future amazon deforestation initial assessment of the potential effect of decreased stomatal resistance due to lower ice age co2 levels indicates little significant response to this effect comparison of model predicted low elevation lgm temperature changes with estimates from proxy data indicate that inclusion of realistic vegetation estimates for the lgm results in slightly more than 50 agreement between models and data for low elevation sites in low mid latitudes data at variance with model predictions would appear to be explainable by considering additional changes in vegetation ice age dust or a 1 2 c cooling below climap values this conclusion is at variance with a 3 4 c tropical cooling suggested by some studies for explaining estimated land temperature changes during the lgm in some western european sites model temperatures are colder than proxy data by 2 8 c this model data discrepancy may be explained by less sea ice in the subpolar north atlantic than stipulated by climap a conclusion consistent with new marine data from that region

the sou hills a barrier to faulting in the central nevada seismic belt bedrock zones transverse to regional structures may indicate barriers at depth that inhibit the propagation of fault ruptures in the basin and range province the tertiary bedrock of the sou hills separates dixie and pleasant valleys and is transverse to the trends of physiography and historic surface faulting in central nevada four lines of evidence indicate that the sou hills are a barrier to faulting in the seismic belt first total late cenozoic vertical displacement on range bounding faults decreases toward the sou hills second analyses of landforms that reflect rates of relative uplift show that quaternary uplift decreases where range bounding faults meet the sou hills third the most recent prehistoric faulting south of the transverse zone is several thousand years younger than faulting to the north fourth patterns of late quaternary fault scarps in the sou hills are similar to rupture patterns observed at the termination of faults elsewhere

patterns and timing of late quaternary faulting in the great basin province and relation to some regional tectonic features large scale surface faulting events each tens of kilometers long and involving more than a meter of displacement of late quaternary age have not been uniformly distributed but have been concentrated in subprovincial and smaller belts and areas within the great basin province furthermore faulting apparently has not been uniformly distributed over time the average recurrence interval of large scale faulting events on individual faults in the province is measured in thousands or tens of thousands of years in contrast to such long recurrence intervals in the central nevada and eastern california seismic belts large scale faulting events have occurred during the last 111 years in an apparently coherent sequential belt filling pulse of activity in which events were separated by only a few years or a few decades the questions are where are other pulses of large scale faulting likely to occur and what controls the localization of such belts and areas characterized by higher rates of faulting two examples of tectonic subprovinces that may play a role in delimiting smaller zones of faulting are the black rock carson sink zone of extension and the central nevada downwarp the central nevada seismic belt lies along the dividing line between these two subprovinces and the northwest margin of the black rock carson sink zone of extension coincides with the margin of a fingerlike belt of dense faulting in northwestern nevada

geometry of cenozoic extensional faulting dixie valley nevada precise definition of geometric relationships between individual basins and ranges may help to reveal the mechanical processes of basin and range cenozoic extensional faulting at depth previous studies have attempted to identify simple horsts and grabens tilted crustal blocks with planar faulting or tilted crustal blocks with listric faulting in the shallow crust normal faults defining these crustal blocks may root 1 individually in the ductile lower crust 2 in regional or local low angle detachment faults or 3 in igneous intrusions or decoupling surfaces produced by the intrusions the present study in dixie valley west central nevada makes use of a seismic reflection survey gravity models seismograms from earthquakes occurring on december 16 1954 and geometrical block models these data show a structurally asymmetric basin bounded by a single zone of faulting on the northwest and by a downbowed and step faulted floor to the southeast the northwest bounding fault is moderately dipping 50 and planar to a depth of 3 km the southeast boundary is step faulted and altogether the faults indicate an extension of 20 across the valley at the rate of 0 38 mm y for the last 8 my synthetic earthquake seismograms confirm a focal depth of 15 km and fault dip of 62 for the fairview peak earthquake and suggest that the focal depth of the dixie valley earthquake was also 15 km instead of the previously reported 40 km local microearthquakes cluster around 10 15 km the geometrical block models indicate that crustal horst graben faulting and planar high angle normal faults rooted in a low angle detachment surface do not readily account for development of the subsidiary step faults found in dixie valley extension of the crust by intrusion may develop high angle faults and with further intrusion may develop the subsidiary faults and produce a complex sagged asymmetric graben like dixie valley

role of inherent geological and climatic characteristics on landscape variability in the tectonically passive western ghat india abstract how does lithological variation affect the landscape and its evolution is a fundamental issue in modern and post orogenic terrains in the absence of active tectonic uplift spatial variation of rock resistance and rainfall distribution are thought to primarily control the fluvial and hillslope processes however these lithologic and climatic controls are difficult to decouple owing to the fact that these post orogenic terrains endure long and complex geomorphic history to examine the relative importance of lithology and rainfall in a post orogenic setting we analyzed west flowing drainage basins in the western ghat wg with a multi proxy analysis we relied on digital topography sediment yield and hydrological data we evaluated topographic relief hypsometry river long profiles and sediment yield to elaborate the importance of spatial variability of rock resistance in the long term evolution of these drainage basins the dominant increase of relief 200 1500 m and decrease of sediment yield 3 32 1 91 ton month km2 from north to south of the escarpment is directly attributed to the rock erodibility and structural characteristics field verification suggests that despite high rainfall condition the channels are mostly detachment limited and therefore lack the abrasive tools we further showed that the harder lithologies sustain higher steepness on the contrary the spatial distribution of satellite derived rainfall showed a negligible effect on the long profile evolution our analysis is also supported by the existing knowledge of chemical denudation rate and implicates that rock hardness and weatheribility defines the topographic characteristics with little or no effect of rainfall in this passive margin setting

u th he dating of apatite a potential thermochronometer abstract we found a gem quality crystal of durango fluorapatite to have a 4he content consistent with complete retention of radiogenic helium since its formation at about 31 ma isothermal heating and step heating analysis reveal 4he loss to occur systematically by volume diffusion at low temperatures the linear low temperature portion of the diffusion data yields an activation energy of 38 5 8 1 kcal mol and a frequency factor of in d 0 a 2 16 4 2 8 sec 1 corresponding to a closure temperature of 105 c 30 c cooling rate 10 c m y it appears that u th he dating of apatite might represent a useful new thermochronometer with a range similar to that of fission track dating of apatite from these results we infer that a number of the too young u th he dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low temperature thermal history of their host rocks

geophysical confirmation of low angle normal slip on the historically active dixie valley fault nevada the december 16 1954 dixie valley earthquake ms 6 8 followed the nearby fairview peak earthquake ms 7 2 by 4 min 20 s waveforms from the fairview peak event contaminate those from the dixie valley event making accurate fault plane solutions impossible a recent geologic study of surface rupture characteristics in southern dixie valley suggests that the dixie valley fault is low angle 30 along a significant portion of the 1954 rupture to extend these observations into the subsurface we conducted a seismic reflection and gravity experiment our results show that a portion of the dixie valley ruptures occurred along a fault dipping 25 to 30 as such the dixie valley event may represent the first large low angle normal earthquake on land recorded historically our high resolution seismic reflection profile images the rupture plane from 5 to 50 m depth medium resolution reflections as well as refraction velocities show a smoothly dipping fault plane from 50 to 500 m depth stratigraphic truncations and rollovers in the hanging wall show a slightly listric fault to 2 km depth gravity profiles conservatively constrain maximum basin depth and define overall geometry extension along the low angle section may have occurred in two phases during the cenozoic current fault motion postdates a 13 to 15 ma basalt imaged in the hanging wall and inherits from a fault formed during an earlier extensional pulse concentrated at 24 2 to 24 4 ma the earlier extension suggests extraordinary slip rates as high as 18 mm yr resulting in the formation of the low angle fault break sections of the dixie valley fault where there is no evidence for current low angle slip correlate well with areas where no pre 15 ma slip has been documented

early jurassic deformation in the pine forest range northwest nevada and implications for cordilleran tectonics triassic and jurassic rocks in the black rock desert of northwest nevada comprise part of the mesozoic volcanic arc successions found throughout the westernmost u s cordillera detailed structural studies in the pine forest range located in the western black rock desert provide new insight into the jurassic structural history of this part of the mesozoic arc system regional jurassic deformation and metamorphism in the pine forest range affected a thick 10 5 km section of lower paleozoic through triassic strata that were tilted after deformation and are now exposed in approximately cross sectional view at high stratigraphic levels deformation produced a slaty cleavage and relatively open folds both of which are concentrated in rheologically incompetent rock types and was accompanied by metamorphism at low grade at deeper stratigraphic levels the regional foliation intensifies and becomes more pervasive folds tighten and metamorphic grade increases through greenschist into amphibolite grade at the deepest stratigraphic levels the rocks are amphibolite grade l s tectonites with a pronounced mylonitic fabric the 2 km thick zone of mylonites is interpreted as a ductile shear zone that formed when the stratigraphic section was still upright whereas the less deformed younger rocks are interpreted to occupy the upper plate of the shear zone and to record a strain gradient away from the shear zone a variety of structural and regional relations indicate that the shear zone is a ductile thrust fault along which younger rocks moved to the northwest in present day coordinates with respect to older rocks the timing of regional jurassic deformation and metamorphism is constrained by relations in and around two plutons dated by the u pb zircon method the 201 1 ma big creek pluton was intruded as a sill like body along the upper levels of the shear zone and is associated with a variety of features indicating syntectonic intrusion including internal textural features in plutonic rocks and metamorphic changes in wall rocks with proximity to the pluton the 185 1 ma theodore pluton which intrudes across part of the shear zone has a thermal aureole which statically overprints the shear zone fabric deformation was thus ongoing at 201 ma and over by 185 ma the early jurassic age of regional shortening deformation in the pine forest range contrasts with the middle to late jurassic 170 150 ma age of regional shortening documented in many other parts of the mesozoic arc system including the klamath mountains sierra nevada and mojave desert regions of california it also contrasts with the record of early jurassic extensional tectonism documented in the sierra nevada mojave desert and western arizona segments of the mesozoic arc system these conflicting relations indicate a complex tectonic evolution for the jurassic u s cordillera plate margin and lend support to the idea that disparate parts of this plate margin were juxtaposed along younger strike slip faults

reappraisal of sea ice distribution in atlantic and pacific sectors of the southern ocean at 18 000 yr bp the question of the distribution of winter and summer sea ice cover during the last glacial maximum 18 000 yr bp is important in climatic modelling of the glacial world we have now compared satellite derived 5 yr monthly averages of per cent summer sea ice cover in the atlantic and pacific sectors of the southern ocean with the distribution of reported sedimentological indicators of sea ice cover at the sediment water interface these data strongly suggest that sediment type cannot be used to identify summer sea ice limits neither in recent sediments nor in sediments of the last glacial maximum rather the data seem to indicate that sediment type can be used to identify spring sea ice limits for these two time intervals

evidence for large magnitude post eocene extension in the northern shoshone range nevada and its implications for the structural setting of carlin type gold deposits in the lower plate of the roberts mountains allochthon the northern shoshone and toiyabe ranges in north central nevada expose numerous areas of mineralized paleozoic rock including major carlin type gold deposits at pipeline and cortez paleozoic rocks in these areas were previously interpreted to have undergone negligible postmineralization extension and tilting but here we present new data that suggest major post eocene extension along west dipping normal faults tertiary rocks in the northern shoshone range crop out in two w nw trending belts that locally overlie and intrude highly deformed lower paleozoic rocks of the roberts mountains allochthon tertiary exposures in the more extensive northern belt were interpreted as subvertical breccia pipes intrusions but new field data indicate that these pipes consist of a 35 8 ma densely welded dacitic ash flow tuff informally named the tuff of mount lewis interbedded with sandstones and coarse volcaniclastic deposits both tuff and sedimentary rocks strike n s and dip 30 to 70 e the steeply dipping compaction foliation in the tuffs was interpreted as subvertical flow foliation in breccia pipes the southern belt along mill creek previously mapped as undivided welded tuff includes the tuff of cove mine 34 4 ma and unit b of the bates mountain tuff 30 6 ma these tuffs dip 30 to 50 east suggesting that their west dipping contacts with underlying paleozoic rocks previously mapped as depositional are normal faults tertiary rocks in both belts were deposited on paleozoic basement and none appear to be breccia pipes we infer that their present east tilt is due to extension on west dipping normal faults some of these faults may be the northern strands of middle miocene ca 16 ma faults that cut and tilted the 34 0 ma caetano caldera 40 east in the central shoshone range 5 km south of mill creek but further mapping is necessary to trace the faults through the highly deformed paleozoic rocks that surround the isolated tertiary outcrops significant post eocene extensional faulting in the northern shoshone range may have important implications for both the structure of the roberts mountains allochthon and the exposure of potentially mineralized rocks in its lower plate both of which were likely east tilted and repeated by west dipping faults together with overlying tertiary rocks

the global middle and late miocene and the deep earth model for earlier orogenies abstract the 11 ma middle and late miocene interval forms only 2 of phanerozoic time but includes two broadly contemporaneous global orogenies one reaching across southern eurasia and the other spanning the circumpacific these two global orogenies caused many global events and because they are close to us in time causality links between these orogenies and these far scattered events are much more apparent than in older orogenies these teleconnections include establishment of the present world oceanic current system the beginnings of oceanic cooling at 14 ma an increase in hemipelagic mud bordering the continents changes in continental tilts and river systems great biological changes increased desertification replacement of c 3 by c 4 grasses a major shift in siliceous microfossils from the atlantic to the pacific at 15 ma and first development of the earliest hominids in east africa starting at 10 ma in addition spreading rates of oceanic ridges increased volcanism increased there was movement along many transform faults and continental elevations increased short bulleted statements set forth these near and far field events immediate causes were increased rates of plate motions and or changes of directions only a strong short 11 ma pulse of heat from the earth s core provides the logical ultimate source of this energy only such a pulse of heat from the core has sufficient magnitude and acts globally thus the deep earth cycle is a plausible easy to understand overarching explanation that should be increasingly considered by geologists and geophysicists studying the outer crust a flow diagram links major earth surface features on the continents to the deep earth alternative explanations for these events are considered and seem totally inadequate

the chronology of cenozoic volcanism and deformation in the yerington area western basin and range and walker lane high precision 40 ar 39 ar isotopic ages obtained from cenozoic volcanic rocks and subvolcanic intrusions document the age of initiation and the temporal evolution of extensional and strike slip faulting in the western basin and range province in the northern wassuk range faulting began between ca 26 and 24 7 ma both normal and strike slip faults are bracketed between 23 1 and 22 2 ma and between 15 and 14 ma these ages document inception of the ancestral walker lane a northwest trending zone of right transtensional faulting in western nevada that separated extending crust on the east from the unextended sierra nevada block on the west at lat 39 n we speculate that the southwesterly migrating episodic oligocene early miocene east west extensional faulting in the basin and range thinned and weakened the crust allowing right slip faults to develop in the walker lane in response to san andreas right shear in california southwest of the walker lane there was no faulting prior to 15 ma here in the yerington district andesitic magmatism began at ca 15 ma and was followed by 150 east west extension along closely spaced 1 2 km normal faults with up to 4 km of offset each proffett 1977 these faults tilted older cenozoic rocks 35 40 w our new 40 ar 39 ar ages substantially revise earlier k ar ages of the timing of extension and establish that andesite lava flows cut by normal faults are 13 8 15 ma and that these faults are intruded by 12 6 13 0 ma dacites rapid extension is thus bracketed to a 0 7 1 7 m y interval at 95 confidence indicating local east west strain rates of 2 4 10 14 s 5 10 mm yr following this period lower rates of extension prevailed near yerington along more widely spaced normal oblique slip faults that localized clastic sedimentation of the wassuk group between 11 and 8 ma these faults and sedimentary rocks are more abundant southwest of yerington in a belt parallel to the walker lane in previously little extended crust from 7 ma to present normal right oblique slip faults with a lower rate of extension than the previous two periods produced the modern ranges near yerington and extend 100 km southwest of the walker lane which continues to be the locus of strike slip faulting thus since 15 ma the margin of the basin and range has moved progressively 100 km west creating the broad walker lane belt and lower strain rates near yerington

controls on intense silicate weathering in a tropical river southwestern india the silicate weathering rate swr and associated carbon dioxide consumption rate ccr in tropical silicate terrain is assessed through a study of the major ion chemistry in a small west flowing river of peninsular india the nethravati river the specific features of the river basin are high mean annual rainfall and temperature high runoff and a precambrian basement composed of granitic gneiss charnockite and minor metasediments the water samples n 56 were collected from three locations along the nethravati river and from two of its tributaries over a period of twelve months chemical weathering rate cwr for the entire watershed is calculated by applying rainwater correction using river chloride as a tracer chemical weathering rate in the nethravati watershed is estimated to 44 t km 2 y 1 encompassing a swr of 42 t km 2 y 1 and a maximum carbonate contribution of 2 t km 2 y 1 this swr is among the highest reported for granito gneissic terrains the assessed ccr is 2 9 10 5 mol km 2 y 1 the weathering index re calculated from molecular ratios of dissolved cations and silica in the river suggests an intense silicate weathering leading to kaolinite gibbsite precipitation in the weathering covers the intense swr and ccr could be due to the combination of high runoff and temperature along with the thickness and nature of the weathering cover the comparison of silicate weathering fluxes with other watersheds reveals that under similar morpho climatic settings basalt weathering would be 2 5 times higher than the granite gneissic rocks c 2012 elsevier b v all rights reserved

tertiary extension in the old woman mountains area california evidence from apatite fission track analysis apatite fission track ages and confined track length distributions from the old woman mountains area southeastern california record evidence for periods of rapid cooling during the late cretaceous following mesozoic orogenesis and during miocene crustal extension apatites from the clipper marble and turtle mountains give fission track ages between 62 and 69 ma as a result of cooling of these upper crustal rocks after regional late cretaceous plutonism and metamorphism in the piute mountains apatite fission track ages decrease from 70 ma with mean track lengths of 13 5 m to 14 m from west to east intermediate aged samples from the piute mountains have mean track lengths between 11 and 13 m due to their extended residence in the apatite annealing zone during the early tertiary this pattern is the result of 20 30 of westward tilting of the piute mountains block as indicated by the dip of tertiary strata the apatite data indicate that tilting occurred after 18 ma and caused rapid cooling of rocks exposed on the eastern side of the mountain range knowing the amount of tilting and the apparent paleotemperatures across the piute mountains allows us to estimate that the synextensional geothermal gradient was 50 20 c km apatites from the northeastern old woman mountains yield fission track ages of 68 ma with mean track lengths of 13 8 m these samples record cooling after batholith emplacement at 73 ma in the northwestern old woman mountains apatite fission track ages decrease to 20 ma and have mean track lengths 15 m indicating very rapid cooling we interpret this age pattern to be the result of minor eastward tilting of the old woman mountains block after 18 19 ma or flexural unroofing of the northwestern old woman mountains due to movement on the ship mountains fault extension in the old woman mountains area occurred between 19 and 16 ma which is coincident with extension in the colorado river extensional corridor and later than major extension in the mojave extensional belt

erosion of deccan traps determined by river geochemistry impact on the global climate and the 87sr 86sr ratio of seawater abstract the impact of the deccan traps on chemical weathering and atmospheric co2 consumption on earth is evaluated based on the study of major elements strontium and 87sr 86sr isotopic ratios of the main rivers flowing through the traps using a numerical model which describes the coupled evolution of the chemical cycles of carbon alkalinity and strontium and allows one to compute the variations in atmospheric pco2 mean global temperature and the 87sr 86sr isotopic ratio of seawater in response to deccan trap emplacement the results suggest that the rate of chemical weathering of deccan traps 21 63 t km2 yr and associated atmospheric co2 consumption 0 58 2 54 106 mol c km2 yr are relatively high compared to those linked to other basaltic regions our results on the deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of co2 consumption during weathering of basalts according to the relationship f r f c 0 exp ea r 1 t 1 298 where f is the specific co2 consumption rate mol km2 yr rf is runoff mm yr c0 is a constant 1764 mol l ea represents an apparent activation energy for basalt weathering with a value of 42 3 kj mol determined in the present study r is the gas constant and t is the absolute temperature k modelling results show that emplacement and weathering of deccan traps basalts played an important role in the geochemical cycles of carbon and strontium in particular the traps led to a change in weathering rate of both carbonates and silicates in carbonate deposition on seafloor in sr isotopic composition of the riverine flux and hence a change in marine sr isotopic composition as a result deccan traps emplacement was responsible for a strong increase of atmospheric pco2 by 1050 ppmv followed by a new steady state pco2 lower than that in pre deccan times by 57 ppmv implying that pre industrial atmospheric pco2 would have been 20 higher in the absence of deccan basalts pco2 evolution was accompanied by a rapid warming of 4 c followed after 1 myr by a global cooling of 0 55 c during the warming phase continental silicate weathering is increased globally since weathering of continental silicate rocks provides radiogenic sr to the ocean the model predicts a peak in the 87sr 86sr ratio of seawater following the deccan traps emplacement the amplitude and duration of this spike in the sr isotopic signal are comparable to those observed at the cretaceous tertiary boundary the results of this study demonstrate the important control exerted by the emplacement and weathering of large basaltic provinces on the geochemical and climatic changes on earth

high resolution climate simulation of the last glacial maximum the climate of the last glacial maximum lgm is simulated with a high resolution atmospheric general circulation model the ncar ccm3 at spectral truncation of t170 corresponding to a grid cell size of roughly 75 km the purpose of the study is to assess whether there are significant benefits from the higher resolution simulation compared to the lower resolution simulation associated with the role of topography the lgm simulations were forced with modified climap sea ice distribution and sea surface temperatures sst reduced by 1 c ice sheet topography reduced co2 and 21 000 bp orbital parameters the high resolution model captures modern climate reasonably well in particular the distribution of heavy precipitation in the tropical pacific for the ice age case surface temperature simulated by the high resolution model agrees better with those of proxy estimates than does the low resolution model despite the fact that tropical ssts were only 2 1 c less than the control run there are many lowland tropical land areas 4 6 c colder than present comparison of t170 model results with the best constrained proxy temperature estimates noble gas concentrations in groundwater now yield no significant differences between model and observations there are also significant upland temperature changes in the best resolved tropical mountain belt the andes we provisionally attribute this result in part as resulting from decreased lateral mixing between ocean and land in a model with more model grid cells a longstanding model data discrepancy therefore appears to be resolved without invoking any unusual model physics the response of the asian summer monsoon can also be more clearly linked to local geography in the high resolution model than in the low resolution model this distinction should enable more confident validation of climate proxy data with the high resolution model elsewhere an inferred salinity increase in the subtropical north atlantic may have significant implications for ocean circulation changes during the lgm a large part of the amazon and congo basins are simulated to be substantially drier in the ice age consistent with many but not all paleo data these results suggest that there are considerable benefits derived from high resolution model regarding regional climate responses and that observationalists can now compare their results with models that resolve geography at a resolution comparable to that which the proxy data represent

long term erosion rates of panamanian drainage basins determined using in situ 10be abstract erosion rates of tropical landscapes are poorly known using measurements of in situ produced 10 be in quartz extracted from river and landslide sediment samples we calculate long term erosion rates for many physiographic regions of panama we collected river sediment samples from a wide variety of watersheds n 35 and then quantified 24 landscape scale variables physiographic climatic seismic geologic and land use proxies for each watershed before determining the relationship between these variables and long term erosion rates using linear regression multiple regression and analysis of variance anova we also used grain size specific 10 be analysis to infer the effect of landslides on the concentration of 10 be in fluvial sediment and thus on erosion rates cosmogenic 10 be inferred background erosion rates in panama range from 26 to 595 m my 1 with an arithmetic average of 201 m my 1 and an area weighted average of 144 m my 1 the strongest and most significant relationship in the dataset was between erosion rate and silicate weathering rate the mass of material leaving the basin in solution none of the topographic variables showed a significant relationship with erosion rate at the 95 significance level we observed weak but significant correlation between erosion rates and several climatic variables related to precipitation and temperature on average erosion rates in panama are higher than other cosmogenically derived erosion rates in tropical climates including those from puerto rico madagascar australia and sri lanka likely the result of panama s active tectonic setting and thus high rates of seismicity and uplift contemporary sediment yield and cosmogenically derived erosion rates for three of the rivers we studied are similar suggesting that human activities are not increasing sediment yield above long term erosion rate averages in panama 10 be concentration is inversely proportional to grain size in landslide and fluvial samples from panama finer grain sizes from landslide material have lower 10 be concentration than fine grained fluvial sediment large grains from both landslide and stream sediments have similarly low 10 be concentrations these data suggest that fluvial gravel is delivered to the channel by landslides whereas sand is preferentially delivered by soil creep and bank collapse furthermore the difference in 10 be concentration in sand sized material delivered by soil creep and that delivered by landsliding suggests that the frequency and intensity of landslides influence basin scale erosion rates

melting history of antarctica during the past 60 000 years marked changes in the surface water hydrology of the southern ocean during the past 60 kyr are revealed by a detailed comparison of the oxygen isotopic composition of planktonic and benthic foraminifera from sediment cores and the surface water temperature estimated by a transfer function derived from the distribution of diatoms in the same sediments from 35 to 17 kyr bp the southern ocean polar front was covered by a melt water lid containing a significant contribution from melting icebergs calved from antarctic ice shelves these icebergs may have originated from a succession of surges of the ice shelves

exhumation of the central wasatch mountains utah 2 thermokinematic model of exhumation erosion and thermochronometer interpretation 1 the wasatch fault is a 370 km long normal fault in utah that marks the boundary between the stable colorado plateau to the east and the extending basin and range to the west understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones e g east africa rift transantarctic mountains we explore the thermokinematics of footwall exhumation and erosion in the cottonwood intrusive belt of the central wasatch mountains emphasis is placed on using low temperature thermochronometers to quantify 1 the spatial and temporal variability of exhumation and erosion rates 2 the geometry of footwall tilt 3 the fault dip angle and 4 the magnitude and duration of exhumation these processes are investigated using two dimensional 2 d thermal kinematic models coupled with cooling rate dependent kinetic models which predict exhumed apatite fission track aft and u th he ages the range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0 2 and 2 0 mm yr 1 footwall tilt hinge positions between 15 and 40 km distance from the fault a single planar normal fault with dip angles of 45 and 60 and exhumation magnitudes of up to 15 km at the fault simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat producing layers erosion maintains a constant topographic profile the kinematic and exhumation history of the wasatch mountains is investigated by comparing model predicted thermochronometer ages to observed aft zft and u th he ages predicted and observed ages are compared using a reduced chi square analysis to determine a best fit kinematic model for the wasatch mountains the preferred model includes exhumation occurring on either a 45 or 60 dipping fault a footwall hinge located a minimum of 20 25 km from the fault and a step decrease deceleration in the footwall exhumation rate at the fault from 1 2 to 0 8 mm yr 1 at around 5 ma the model also suggests an exhumation duration of 12 myr 2 myr index terms 1035 geochemistry geochronology 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8109 tectonophysics continental tectonics extensional 0905 8130 tectonophysics heat generation and transport keywords exhumation erosion normal faults numerical modeling heat flow thermochronometers

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

tectonic versus chemical weathering controls on the composition of fluvial sands in tropical environments the compositions of fluvial sands collected in mountainous regions along the volcanic front in central america and along the andean front in venezuela and colombia correlate with the tectonic setting of their source terrains the sands derived from the central american volcanic arc are compositionally distinct from those derived from the andean terrains and minor variations in tectonic style can also be discriminated subtle differences in tectonic setting and source rock composition are recognizable only when modification by chemical weathering is slight they are strongest in sands from low order streams draining mountainous areas with modest alluvial plain development sands that have been exposed to weathering for extended periods during alluvial storage are progressively enriched in chemically stable phases with increased weathering sands from continental terrains converge toward a supermature quartz arenite end member and the imprint left by source rock composition and tectonic setting is obscured in intraoceanic terrains a similar but much less pronounced enrichment in stable phases relative to less stable phases is observed similarly overprinting the tectonic signal although detailed tectonic information is easily lost sands derived from intraoceanic terrains are distinguishable from those derived from continental terrains even after long exposure to intense chemical weathering conditions

geochemical fluxes and weathering of volcanic terrains on high standing islands taranaki and manawatu wanganui regions of new zealand abstract sediment fluxes from high standing oceanic islands hsis such as new zealand are some of the highest known milliman j d and syvitski j p m 1992 geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers j geol 100 525 544 recent geochemical work has suggested that along with their extremely high physical weathering yields many new zealand watersheds also have very high chemical weathering yields in new zealand the magnitude of both the physical and chemical weathering yields is related to the lithology of the watershed most of the previous work on this topic has been undertaken in southern alps watersheds of schist and greywacke and in east cape watersheds of semi consolidated marine sediments and greywacke we recently sampled north island watersheds in the taranaki and manawatu wanganui regions which have been subjected to volcanism since the miocene we sampled watersheds that contain both volcanic and sedimentary rocks a series of water and sediment samples was collected and analyzed for major minor and trace elements this was done to quantify the weathering intensities in the watersheds and to establish the relationship between physical and chemical weathering yields in volcanic lithologies our results reveal distinct chemical signatures for the different regions waters draining the taranaki region volcanics are significantly enriched in k and depleted in ca2 and sr2 compared to waters draining the manawatu wanganui region volcanics which also traverse expanses of sedimentary siltstones and mudstones the ca2 and sr2 depletions may reflect the relative absence of caco3 in the taranaki region watersheds in addition sediment samples from the taranaki region show significant enrichment in ti al ca fe mn mg ca and p and depletion in si and rb compared to those of the manawatu wanganui region from total dissolved solids concentrations and mean annual water discharge we calculate chemical weathering yields of 60 240 tons km 2 a 1 these weathering yields fall within the middle to upper range of those previously documented for the southern alps 93 480 tons km 2 a 1 and east cape 62 400 tons km 2 a 1 calculated silicate weathering yields of 12 33 6 tons km 2 a 1 and co2 consumption of 852 2390 103 mol km 2 a 1 for the rivers draining the taranaki volcanic region are higher than those previously reported for watersheds hosted in sedimentary and metamorphosed rock terrains on hsis co2 consumption is found to be within the range previously measured for the basaltic terrains of the deccan traps 580 2450 103 mol km 2 a 1 and reunion island 1300 4400 103 mol km 2 a 1 our calculated chemical weathering yields demonstrate the importance of hsis particularly those with volcanic terrains when considering global geochemical fluxes

large scale fractures related to inception of the yellowstone hotspot during middle miocene time western north america was subject to flood basalt volcanism dike swarm injection and broad scale fracturing and folding of the crust we propose a simple model to account for these events and for a regional pattern of geologic and geophysical features aeromagnetic maps reveal some of the most important elements of this pattern which are several narrow arcuate anomalies here referred to as the northern nevada rifts these rifts extend hundreds of kilometers across nevada and are likely caused by highly magnetic middle miocene mafic dikes with the aid of filtering techniques the anomalies can be traced into oregon together with other geologic features such as fold axes dike swarms and faults they produce a spoke like pattern fanning over 2208 of arc that converges toward a point near the oregon idaho border lat 448n a possible cause for this pattern is a point source of stress at the base of the crust related to the formation of the yellowstone hotspot the spoke like pattern however does not persist at large distances from the emerging hotspot several hundred kilometers to the south the northern nevada rifts deviate significantly 308 from a radial trend we show that a simple model imposing a point source of stress at the base of the crust and a regional stress field aligned with the presumed middle miocene stress direction fits the observed fracture pattern it thus accounts for both the radial pattern present near the nascent hotspot and the far field pattern due to regional stresses

evaluation of controls on silicate weathering in tropical mountainous rivers insights from the isthmus of panama the isthmus of panama comprises a lithologically diverse andesitic oceanic arc of late cretaceous to holocene age it has large spatial variation in rainfall displays a large range of physical erosion rates and therefore is an ideal location to examine silicate weathering in the tropics we use a multiyear data set of river chemistry for a 450 km transect across the cordillera central of west central panama to investigate controls on chemical weathering in tropical small mountainous rivers sea salt corrected cation weathering yields casil mgsil na k range over more than an order in magnitude from 3 1 to 31 7 t km2 yr while silicate weathering yields casil mgsil na k si range from 6 9 to 69 5 t km2 yr watershed lithology is the primary control on riverine chemistry but landscape topographic character and land cover and or land use also influence solute delivery potential strong statistical links of small mountainous river chemical weathering fluxes with rainfall and physical weathering rates attest to the importance of runoff and erosion in maintaining elevated bedrock weathering rates co2 consumption ranges from 155 103 mol km2 yr to 1566 103 mol km2 yr in the upper range of global rates leading us to suggest that andesite terrains should be considered separately when calculating removal of co2 from the atmosphere via silicate weathering

synchronous climate changes in antarctica and the north atlantic central greenland ice cores provide evidence of abrupt changes in climate over the past 100 000 years many of these changes have also been identified in sedimentary and geochemical signatures in deep sea sediment cores from the north atlantic confirming the link between millennial scale climate variability and ocean thermohaline circulation it is shown here that two of the most prominent north atlantic events the rapid warming that marks the end of the last glacial period and the bolling allerod younger dryas oscillation are also recorded in an ice core from taylor dome in the western ross sea sector of antarctica this result contrasts with evidence from ice cores in other regions of antarctica which show an asynchronous response between the northern and southern hemispheres

mantle plume influence on the neogene uplift and extension of the u s western cordillera espite its highly extended and thinned crust much of the western cordillera in the united states is elevated more than 1 km above sea level therefore this region cannot be thought of as thick crust floating isostatically in a uniform mantle rather the lithospheric mantle and or the upper asthenosphere must vary in thickness or density across the region utilizing crustal thickness and density constraints we modeled the residual mass deficit that must occur in the mantle lithosphere and asthenosphere beneath the western cordillera a major hot spot broke out during a complex series of cenozoic tectonic events that included lithospheric thickening backarc extension and transition from a subduction to a transform plate boundary we suggest that many of the characteristics that make the western cordillera unique among extensional provinces can be ttributed to the mantle plume that created the yellowstone hot spot

simple energy balance model resolving the seasons and the continents application to the astronomical theory of the ice ages a one level seasonal energy balance climate model which has explicit two dimensional land sea geography is introduced and its properties analyzed the model distinguishes land from sea surface strictly by the local thermal inertia employed the transport is governed by a smooth latitude dependent diffusion mechanism the model s seasonal cycle is solved for with and without ice feedback in each case good agreement with real data is accomplished with minimal tuning of the adjustable parameters the model sensitivity for small solar constant changes is in agreement with that of other accepted models however when the solar constant is lower a few percent the ice cap grows discontinuously from covering greenland and vicinity to a size three times larger and mostly into northern canada a similar discontinuous growth occurs when the orbital elements are changed to favor cool summers in the northern hemisphere this discontinuous sensitivity is discussed in the context of the milankovitch theory of the ice ages and the associated branch structure is shown to be analogous to the so called small ice cap instability known from simpler models although the discontinuity in sensitivity is somewhat conjectural both linear and nonlinear models have their largest sensitivity to orbital parameter changes in the northern portions of the large continents in the western hemisphere this also corresponds to the ice sheet boundary suggesting that the land sea configuration is crucial to large glacier growth irrespective of moisture budgets other experiments and implications are also discussed the remarkable paper by hays et al 1976 hereafter his presented compelling empirical evidence that continental ice sheet volume changes and sea surface temperature variations in the last 700 000 years are correlated with the changes in the earth s orbital elements theoretical arguments for such a connection date back to the last century but the most detailed and convincing arguments are due to milankovitch 1941 and the theory usually is associated with his name if the hypothesis is correct the theory and observations provide the cleanest experiment to determine the sensitivity of the earth s climate to truly external influences such a measure of climate sensitivity is of obvious importance considering the recent interest in estimating the change in climate due to other forcings such as the enhanced greenhouse effect due to increased levels of co2 in the atmosphere e g ramanathan 1981 hansen et al 1981 manabe and stouffer 1980 in effect climate models might be tested or calibrated by this natural experiment climate modeling has progressed dramatically in the last decade with the construction and testing of several general circulation models gcm s although some interesting ice age experiments have been done with these models gates 1976 manabe and hahn 1977 they still include only crude parameterizations of the slower components such as the oceans these numerical models are the ultimate tool that can be brought to bear upon the climate problem however they are almost as complicated as the climate itself hence experiments must be carefully designed and the interpretation is often difficult due to the expense of obtaining definitively long runs in parallel with gcm construction has been the development of x permanently at applied research corporation landover maryland 20785

late stage evolution of the chemehuevi and sacramento detachment faults from apatite u th he thermochronometry evidence for mid miocene accelerated slip the unique low temperature sensitivity of the apatite u th he ahe technique provides valuable new information relating to the late stage thermal evolution of metamorphic core complexes in the southern basin and range province in all the ranges studied the data show greater complexity than previously seen from other thermochronometers in the chemehuevi and northern sacramento mountains a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall this is followed by essentially age invariant segments of 15 1 ma reflecting an increase in the rate of detachment fault movement the timing of the change suggested by the data agrees with recently published ahe results from the nearby harcuvar mountains located 125 km to the southeast suggesting that an underlying regional mechanism triggered this change plate reconstructions indicate that the pioneer mendocino fracture zone migrated northward through the area at this time leaving a slab window in its wake we suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this combined with the extensional strain regime present resulted in an increased rate of slip within the actively extending metamorphic core complexes this agrees well with timing constraints for extension onset in the central basin and range province of southern nevada and suggests that weakening of the southern basin and range province at 15 1 ma was partially responsible for the onset of extension in the central basin and range these findings demonstrate that improved constraints on changes in extension rate within the basin and range province should be possible through the application of the ahe technique to other metamorphic core complex footwalls

crustal scale recycling in caldera complexes and rift zones along the yellowstone hotspot track o and hf isotopic evidence in diverse zircons from voluminous rhyolites of the picabo volcanic field idaho abstract rhyolites of the picabo volcanic field 10 4 6 6 ma in eastern idaho are preserved as thick ignimbrites and lavas along the margins of the snake river plain srp and within a deep 3 km borehole near the central axis of the yellowstone hotspot track in this study we present new o and hf isotope data and u pb geochronology for individual zircons o isotope data for major phenocrysts quartz plagioclase and pyroxene whole rock sr and nd isotope ratios and whole rock geochemistry for a suite of picabo rhyolites we synthesize our new datasets with published ar ar geochronology to establish the eruptive framework of the picabo volcanic field and interpret its petrogenetic history in the context of other well studied caldera complexes in the srp caldera complex evolution at picabo began with eruption of the 10 44 0 27 ma u pb tuff of arbon valley tav a chemically zoned and normal 18o 18o magma 7 9 unit with high zoned 87sr 86sri 0 71488 0 72520 and low e nd 0 18 and e hf 0 28 the tav and an associated post caldera lava flow possess the lowest end 0 23 indicating 40 60 derivation from the archean upper crust normal 18o rhyolites were followed by a series of lower 18o eruptions with more typical lower crustal sr nd hf isotope ratios and whole rock chemistry the voluminous 8 25 0 26 ma west pocatello rhyolite has the lowest 18o value 18omelt 3 3 and we correlate it to a 1 000 m thick intracaldera tuff present in the inel 1 borehole with published zircon ages 8 04 8 35 ma and similarly low 18o zircon values the significant 4 5 decrease in magmatic 18o values in picabo rhyolites is accompanied by an increase in zircon 18o heterogeneity from 1 variation in the tav to 5 variation in the late stage low 18o rhyolites a trend similar to what is characteristic of heise and yellowstone and which indicates remelting of variably hydrothermally altered tuffs followed by rapid batch assembly prior to eruption however due to the greater abundance of low 18o rhyolites at picabo the eruptive framework may reflect an intertwined history of caldera collapse and coeval basin and range rifting and hydrothermal alteration we speculate that the source rocks with pre existing low 18o alteration may be related to 1 deeply buried and unexposed older deposits of picabo age or twin falls age low 18o volcanics and or 2 regionally abundant late eocene challis volcanics which were hydrothermally altered near the surface prior to or during peak picabo magmatism basin and range extension specifically the formation of metamorphic core complexes exposed in the region could have facilitated the generation of low 18o magmas by exhuming heated rocks and creating the large water rock ratios necessary for shallow hydrothermal alteration of tectonically rift zones and volcanically calderas buried volcanic rocks these interpretations highlight the major processes by which supereruptive volumes of magma are generated in the srp mechanisms applicable to producing rhyolites worldwide that are facilitated by plume driven volcanism and extensional tectonics

reappraisal of the relationship between the northern nevada rift and miocene extension in the northern basin and range province the northern nevada rift is a prominent mafic dike swarm and magnetic anomaly in north central nevada inferred to record the middle miocene 16 5 15 0 ma extension direction in the northern basin and range province in the western united states from the 245 250 rift direction basin and range extension is inferred to have shifted 45 clockwise to a modern direction of 290 300 during the late miocene the region surrounding the northern nevada rift was actively extending while the rift formed and these domains are all characterized by extension oriented 280 300 this direction is distinctly different from the rift direction and nearly identical to the modern basin and range direction although the rate structural style and distribution of basin and range extension appear to have undergone a significant change in the late miocene ca 10 ma the overall spreading direction does not middle miocene extension was directed perpendicular to the axis of the thickest crust formed during mesozoic shortening and this orientation may reflect gravitational collapse of this thick crust orientation of northern nevada rift dikes may reflect a short lived regional stress field related to the onset of yellowstone hotspot volcanism

a coupled climate model simulation of the last glacial maximum part 2 approach to equilibrium the climate of the last glacial maximum lgm is simulated with a coupled climate model the simulated climate undergoes a rapid adjustment during the first several decades after imposition of lgm boundary conditions as described in part 1 and then evolves toward equilibrium over 900 model years the climate simulated by the coupled model at this period is compared with observationally based lgm reconstructions and with lgm results obtained with an atmosphere mixed layer slab ocean version of the model in order to investigate the role of ocean dynamics in the lgm climate global mean surface air temperature and sea surface temperature sst decrease by about 10 c and 5 6 c in the coupled model which includes ocean dynamics compared to decreases of 6 3 and 3 8 c in slab ocean case the coupled model simulates a cooling of about 6 5 c over the tropics which is larger than that of the climap reconstruction 1 7 c and larger than that of the slab ocean simulation 3 3 c but which is in reasonable agreement with some recent proxy estimates the ocean dynamics of the coupled model captures features found in the climap reconstructions such as a relative maximum of ocean cooling over the tropical pacific associated with a mean la nina like response and lead to a more realistic sst pattern than in the slab model case the reduction in global mean precipitation simulated in the coupled model is larger 15 than that simulated with the slab ocean model 10 in conjunction with the enhanced cooling some regions such as the usa and the mediterranean region experience increased precipitation in accord with proxy paleoclimate evidence the overall much drier climate over the ocean leads to higher sea surface salinity sss in most ocean basins except for the north atlantic where sss is considerably lower due to an increase in the supply of fresh water from the mississippi and amazon rivers and presumably a decrease in salt transport by the weakened north atlantic overturning circulation the north atlantic overturning stream function weakens to less than half of the control run value the overturning is limited to a shallower depth less than 1000 m and its outflow is confined to the northern hemisphere in the southern ocean convection is much stronger than in the control run leading to a stronger overturning stream function associated with enhanced antarctic bottom water formation as a result southern ocean water masses fill the entire deep ocean the antarctic circumpolar current acc transport through the drake passage increases by about 25 the acc transport despite weaker zonal winds is enhanced due to changes in bottom pressure torque the weakening of the overturning circulation in the north atlantic and the accompanying 30 decrease in the poleward ocean heat transport contrasts with the strengthening of the overturning circulation in the southern ocean and a 40 increase in heat transport as a result sea ice coverage and thickness are affected in opposite senses in the two hemispheres the lgm climate simulated by the coupled model is in reasonable agreement with paleoclimate proxy evidence the dynamical response of the ocean in the coupled model plays an important role in determining the simulated and undoubtedly the actual lgm climate

late pleistocene variations in antarctic sea ice i effect of orbital insolation changes the effect of orbitally induced insolation changes on antarctic sea ice cover are examined by means of a dynamic thermodynamic seaice model results are compared with modified climap 18 000 b p sea ice reconstructions calculations suggest that changes in insolation receipt had only a minor influence on pleistocene sea ice distributions the small response can be explained by a number of factors albedo effects reduce the insolation perturbation at the surface some of the shortwave radiation entering the ocean contributes to bottom ablation rather than lateral melting the radiation perturbation at the upper surface of the ice must go to warming the surface to the melting point before melting ensues and finally the relatively high heat capacity of open water dampens the surface temperature response to altered seasonal insolation perturbations

geology geography and humans battle for dominance over the delivery of fluvial sediment to the coastal ocean abstract sediment flux to the coastal zone is conditioned by geomorphic and tectonic influences basin area and relief geography temperature runoff geology lithology ice cover and human activities reservoir trapping soil erosion a new model termed bqart in recognition of those factors accounts for these varied influences when applied to a database of 488 rivers the bqart model showed no ensemble over or underprediction had a bias of just 3 across six orders of magnitude in observational values and accounted for 96 of the between river variation in the long term 30 years sediment load or yield of these rivers the geographical range of the 488 rivers covers 63 of the global land surface and is highly representative of global geology climate and socioeconomic conditions based strictly on geological parameters basin area relief lithology ice erosion 65 of the between river sediment load is explained climatic factors precipitation and temperature account for an additi

diachroneity of basin and range extension and yellowstone hotspot volcanism in northwestern nevada some of the earliest volcanic rocks attributed to the yellowstone hotspot erupted from the mcdermitt caldera and related volcanic centers in northwestern nevada at 17 15 ma at that time extensional faulting was ongoing to the south in central nevada leading some to suggest that the nascent hotspot caused or facilitated middle miocene basin and range extension regional geologic relationships indicate that the total magnitude of extension in northwestern nevada is low compared to the amount documented in central nevada and that extension was accommodated almost entirely by the widely spaced high angle normal fault systems that formed the modern ranges the santa rosa range is the closest major range to the mcdermitt caldera it was tilted 15 east and uplifted as an intact crustal block by 8 km of slip along a west dipping normal fault on the west side of the range apatite fission track data from the santa rosa footwall block indicate that faulting and extension was ongoing ca 7 5 ma and began as early as 10 ma data from the pine forest range to the west indicate a similar 8 10 ma age for extensional faulting basin and range extension in northwestern nevada is therefore significantly younger than 17 15 ma hotspot volcanism this timing argues against a direct link between the yellowstone hotspot and the initiation of extension casting additional doubt on the role of the hotspot in extension across the broader basin and range province

thermochronological constraints on the timing and magnitude of miocene and pliocene extension in the central wassuk range western nevada apatite fission track and u th he thermochronological data provide new constraints on the timing of faulting and exhumation of the wassuk range western nevada where east dipping normal faults have accommodated large magnitude ene wsw oriented extension extensional deformation has resulted in the exhumation of structurally coherent fault blocks that expose sections of preextensional mostly granitic upper crust in the grey hills and central wassuk range these fault blocks display westward tilts of 60 and expose preextensional paleodepths of up to 8 5 km based on the structural reconstruction of tilted preextensional tertiary andesite flows that unconformably overlie mesozoic basement rocks apatite fission track and u th he thermochronological data from the fault blocks constrain the onset of rapid footwall exhumation at 15 ma fission track modeling results indicate rapid fault block exhumation occurred between 15 and 12 ma which is in agreement with miocene volcanic rocks that bracket the tilting history in addition fission track and u th he data suggest reduced rates of cooling following major extension as well as renewed cooling related to active high angle faulting along the present day range front starting at 4 ma thermochronological data from structurally restored fault blocks indicate a preextensional miocene geothermal gradient of 27 5 c km the thermochronological constraints on the timing of extensional faulting and the eruptive history in the wassuk range imply a model for extension where crustal heating and volcanism precede the onset of rapid large magnitude extension and where synextensional magmatism is suppressed during the highest rates of extension

miocene extension in the east range nevada a two stage history of normal faulting in the northern basin and range the east range in northwestern nevada is a large east tilted crustal block bounded by west dipping normal faults detailed mapping of tertiary stratigraphic units demonstrates a two phase history of faulting and extension the oldest sedimentary and volcanic rocks in the area record cumulative tilting of 30 45 e whereas younger olivine basalt fl ows indicate only a 15 20 e tilt since ca 17 13 ma cumulative fault slip during these two episodes caused a minimum of 40 extensional strain across the east range and quaternary fault scarps and seismic activity indicate that fault motion has continued to the present day apatite fi ssion track and u th he data presented here show that faulting began in the east range ca 17 15 ma coeval with middle miocene extension that occurred across much of the basin and range this phase of extension occurred contemporaneously with middle miocene volcanism related to the nearby northern nevada rifts suggesting a link between magmatism and extensional stresses in the crust that facilitated normal faulting in the east range younger fault slip although less well constrained began after 10 ma and is synchronous with the onset of low magnitude extension in many parts of northwestern nevada and eastern california these fi ndings imply that rather than migrating west across a discrete boundary late miocene extension in western nevada is a distinct younger period of faulting that is superimposed on the older middle miocene distribution of extended and unextended domains the partitioning of such middle miocene deformation may refl ect the infl uence of localized heterogeneities in crustal structure whereas the more broadly distributed late miocene extension may refl ect a stronger infl uence from regional plate boundary processes that began in the late miocene

thickness of the chemical weathering zone and implications for erosional and climatic drivers of weathering and for carbon cycle feedbacks quantitative understanding of variability in weathering fluxes on the modern earth is limited because little is known about where the most important weathering reactions take place this is partly because the locus of weathering is difficult to measure empirically inverse analysis of a parametric model presented here provides first order constraints on variability in the thickness of the zone of active weathering results suggest that the effective thickness of the weathering zone varies relatively little across several orders of magnitude of denudation rate at low to moderate denudation rates reactions in soils may dominate weathering fluxes at the catchment scale but the contribution from soil weathering decreases at higher denudation rates consequently increased erosion leads to higher weathering fluxes sustained by progressively greater contributions from weathering in bedrock the effect of climate temperature and runoff on weathering fluxes is apparently weaker at low denudation rates than at high denudation rates such that erosion and potentially associated bedrock weathering may be important for maintaining climate stabilizing feedbacks in earth s carbon cycle

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

relationship between mechanical erosion and atmospheric co2 consumption in the new zealand southern alps to examine the influence of mountain uplift on the long term carbon cycle we used geochemical hydrologic and suspended load data for 12 streams draining the new zealand southern alps to quantify rates of erosion weathering and atmospheric co 2 consumption rapid uplift in the western southern alps elevates mechanical erosion rates by a factor of 13 relative to those on the tectonically stable eastern side 125 10 8 vs 9 4 10 8 g km 2 yr respectively similarly the average chemical weathering rate is 5 times higher on the western compared to eastern side of the mountain range 9 8 10 7 vs 2 0 10 7 g km 2 yr respectively however because the proportion of stream water ca 2 and mg 2 from carbonate weathering increases as the rate of mechanical erosion increases the long term atmospheric co 2 consumption rate on the western side is 2 times higher than that on the eastern side 14 10 4 vs 6 9 10 4 mol km 2 yr respectively and only 1 5 times higher than the global mean value 9 10 4 mol km 2 yr data for major world rivers including himalayan rivers provide a consistent interpretation regarding the relationship between mechanical erosion intensity and the ratio of silicate to carbonate weathering thus we conclude that mountain building increases atmospheric co 2 consumption rates by only a factor of 2 which is much lower than previous estimates

molybdenum vanadium and uranium weathering in small mountainous rivers and rivers draining high standing islands abstract rivers draining high standing islands hsis and small mountainous rivers smrs are known to have extremely high sediment fluxes and can also have high chemical weathering yields which makes them potentially important contributors to the global riverine elemental flux to the ocean this work reports on the riverine concentrations ocean flux and weathering yields of molybdenum mo vanadium v and uranium u in a large number of small but geochemically important rivers using 338 river samples from ten lithologically diverse regions these redox sensitive elements are used extensively to infer paleo redox conditions in the ocean and mo and v are also important rock derived micronutrients used by microorganisms in nitrogen fixation unlike in large river systems in which dissolved mo has been attributed predominately to pyrite dissolution mo concentrations in these rivers did not correlate with sulfate concentrations v was found to correlate strongly with si in terrains dominated by silicate rocks but this trend was not observed in primarily sedimentary regions many rivers exhibited much higher v si ratios than larger rivers and rivers draining young quaternary volcanic rocks in nicaragua had much higher dissolved v concentrations mean 1306 nm than previously studied rivers u concentrations were generally well below the global average with the exception of rivers draining primarily sedimentary lithologies containing carbonates and shales fluxes of u and mo from igneous terrains of intermediate composition are lower than the global average while fluxes of v from these regions are higher and up to two orders of magnitude higher in the nicaragua rivers weathering yields of mo and v in most regions are above the global mean despite lower than average concentrations measured in some of those systems indicating that the chemical weathering of these elements are higher in these smr watersheds than larger drainages in regions of active boundaries with andesite dacite lithologies rivers draining young pleistocene rocks had higher concentrations than did older miocene pliocene rocks of a similar composition this work shows that weathering yields of mo v and u from smrs are slightly higher than from large rivers and the age of igneous lithologies in these regions exhibits a measurable control on riverine concentrations of these elements

cenozoic tectonic evolution of the white mountains california and nevada the white mountains represent the westernmost range of the central northern basin and range province they are situated to the east of the unextended sierra nevada and represent a crustal block that is bounded along its western flank by the high angle white mountains fault zone the fault zone accommodates up to 8 km of total dip slip displacement investigation of the structural and thermal history of the white mountains indicates a two stage cenozoic tectonic evolution preextensional miocene volcanic rocks preserved along the eastern side of the range unconformably overlie mesozoic granitic basement and currently dip up to 25 to the east recording the total cenozoic tilt of the crustal block apatite fission track and u th he thermochronological data indicate that the white mountains underwent rapid exhumation and eastward tilting in the middle miocene starting at ca 12 ma geologic mapping 1 10 000 fault kinematic analysis and dating of younger volcanic sequences show that following middle miocene east west extension the white mountains have been dominated by right lateral transtensional deformation related to the walker lane belt the eruption of late miocene and pliocene volcanic sequences in the eastern white mountains postdates the majority of the uplift of the range as evidenced by infilling of paleodrainages and the presence of east directed flow fabrics fault kinematic indicators from the white mountains fault zone are characterized by apparent overprinting of dip slip fault motion indicators by right lateral slickenfibers and fault striations demonstrating that the range bounding fault system along the western side of the white mountains was reactivated as a dextral strike slip fault system at the northern and southern ends of the range pliocene right lateral transtension along this northwest southeast trending fault systems resulted in the formation of northeast trending pull apart basins that truncate the mountain range and transfer strike slip displacement eastward from the owens valley fault zone to the fish lake valley fault zone the inception of strike slip faulting in fish lake valley occurred at ca 6 ma as constrained by late miocene volcanic units right lateral faulting on the western side of the white mountains occurred at ca 3 ma and is distinctly younger than the faulting in the fish lake valley area indicating a westward migration of transcurrent deformation through time

style and age of late oligocene early miocene deformation in the southern stillwater range west central nevada paleomagnetism geochronology and field relations paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late oligocene to early miocene episode of large magnitude extension in the southern stillwater range and adjacent regions of west central nevada the southern stillwater range was the site of an oligocene to early miocene volcanic center comprising 1 28 3 to 24 3 ma intracaldera ash flow luffs lava flows and subjacent plutons associated with three calderas 2 24 8 to 20 7 ma postcaldera silicic dikes and domes and 3 unconformably overlying 15 3 to 13 ma dacite to basalt lava flows plugs and dikes the caldera related luffs lava flows and plutons were tilted 60 70 either west or east during the initial period of cenozoic deformation that accommodated over 100 extension directions of remanent magnetization obtained from these extrusive and intrusive caldera related rocks are strongly deflected from an expected miocene direction in senses appropriate for their tilt a mean direction for these rocks after tilt correction however suggests that they were also affected by a moderate 33 4 11 8 component of counterclockwise vertical axis rotation paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24 8 to 20 7 ma postcaldera dikes and domes in detail an apparent decrease in rotation with decreasing age of individual isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24 4 and 24 2 ma the onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked deformation was not driven by magma buoyancy however because tilting equally affected the caldera systems of different ages including their plutonic roots it is more likely that regional extension was focused in the southern stillwater range due to magmatic warming and reduction of tensile strength of the brittle crust faults that accommodated deformation in the southern stillwater range initially dipped steeply and cut deeply to expose more than 9 km of crustal section the exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early miocene brittle ductile transition

two phase westward encroachment of basin and range extension into the northern sierra nevada 1 structural geophysical and thermochronological data from the transition zone between the sierra nevada and the basin and range province at latitude 39 n suggest 100 km westward encroachment of basin and range extensional deformation since the middle miocene extension accommodated primarily by east dipping normal faults that bound west tilted range forming fault blocks varies in magnitude from 150 in the wassuk and singatse ranges to the east geological and apatite fission track data from exhumed upper crustal sections in the wassuk and singatse ranges point to rapid footwall cooling related to large magnitude extension starting at 14 15 ma farther to the west geological and thermochronological data indicate a younger period of extension in the previously unextended pine nut mountains the carson range and the tahoe truckee depression initiated between 10 ma and 3 ma and incipient post 0 5 ma faulting to the west of the tahoe truckee area these data imply the presence of an extensional breakaway zone between the singatse range and the pine nut mountains at 14 15 ma forming the boundary between the sierra nevada and basin and range at that time in addition fission track data imply a miocene preextensional geothermal gradient of 27 5 c km 1 in the central wassuk range and 20 5 c km 1 in the singatse range much higher than the estimated early tertiary gradient of 10 5 c km 1 for the sierra nevada batholith this might point to a significant increase in geothermal gradients coupled with a likely decrease in crustal strength enabling the initiation of extensional faulting apatite fission track geophysical and geological constraints across the sierra nevada basin and range transition zone indicate a two stage coupled structural and thermal westward encroachment of the basin and range province into the sierra nevada since the middle miocene

transition from contraction to extension in the northeastern basin and range new evidence from the copper mountains nevada abstract new mapping structural analysis and 40ar 39ar dating reveal an unusually well constrained history of late eocene extension in the copper mountains of the northern basin and range province in this area the northeast trending copper creek normal fault juxtaposes a distinctive sequence of metacarbonate and granitoid rocks against a footwall of upper precambrian to lower cambrian quartzite and phyllite correlation of the hanging wall with footwall rocks to the northwest provides an approximate piercing point that requires 8 12 km displacement in an ese direction this displaced fault slice is itself bounded above by another normal fault the meadow fork fault which brings down a hanging wall of dacitic to rhyolitic tuff that grades conformably upward into conglomerate these relationships record the formation of a fault bounded basin between 41 3 and 37 4 ma the results are consistent with a regional pattern in which volcanism and extension swept southward from british columbia to southern ne

the role of shortwave radiation in the summer decay of a sea ice cover heat balance models of ice leads and the underlying water column were used to investigate the role of shortwave radiation in the summer decay of a sea ice cover because of the potential importance of positive feedback between decreasing ice concentration and increasing solar input to the ocean particular emphasis was given to the treatment of heat transfer and lateral melting in leads the rate of heat loss to floe edges was calculated from lead temperatures using an empirical boundary layer parameterization coefficients for this parameterization were obtained from field measurements in the dynamically active ice cover of the greenland sea during the 1984 marginal ice zone experiment and in the static ice of mould bay northwest territories field coefficients were found to be considerably larger than those previously obtained in the laboratory suggesting that heat transfer rates near floe edges were controlled more by winds and currents than by local density differences model studies of individual leads indicate that heat losses to the atmosphere cause lateral melt rates to asymptotically approach a constant value as lead size increases thus although very wide leads transmit considerable energy to the underlying ocean they have relatively little effect on lateral melting other results provide information on the relative importance of horizontal heat transfer rates ice thickness floe perimeter atmospheric forcing and oceanic heat flux in general the calculations indicate that much of the shortwave radiation entering the ocean is absorbed below the ice where it contributes to bottom ablation rather than lateral melting this suggests that previous work has seriously overestimated the magnitude of the feedback between ice concentration and incident shortwave radiation

kinematics of the northern walker lane an incipient transform fault along the pacific north american plate boundary in the western great basin of north america a system of dextral faults accommodates 15 25 of the pacific north american plate motion the northern walker lane in northwest nevada and northeast california occupies the northern terminus of this system this young evolving part of the plate boundary offers insight into how strike slip fault systems develop and may reflect the birth of a transform fault a belt of overlapping left stepping dextral faults dominates the northern walker lane offset segments of a w trending oligocene paleovalley suggest 20 30 km of cumulative dextral slip beginning ca 9 3 ma the inferred long term slip rate of 2 10 mm yr is compatible with global positioning system observations of the current strain field we interpret the left stepping faults as macroscopic riedel shears developing above a nascent lithospheric scale transform fault the strike slip faults end in arrays of n striking normal faults suggesting that dextral shear diffuses into extension in the great basin coeval extension and dextral shear have induced slight counterclockwise fault block rotations which may ultimately rotate riedel shears toward the main shear zone at depth thus facilitating development of a throughgoing strike slip fault

post laramide removal of the farallon slab western united states i propose that post laramide removal of the subhorizontally subducting farallon slab occurred by buckling downward along an approximately east northeast trending axis this process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the united states and propagation of the separated edges toward the central axis of downwelling accompanied by aesthenosphic upwelling behind the trailing edges initial buckling probably began near 50 ma and slab removal was complete by 20 ma this model is based primarily on the space time evolution of the ignimbrite flare up a major mid tertiary igneous event of mantle origin which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central nevada from the north and southeast post laramide uplift extension establishment of the cascadia subduction zone and active magmatism may be consequences of lithosphere scale modifications caused by the laramide removal of the slab and the resulting asthenospheric upwelling

the influence of continental ice sheets on the climate of an ice age the climate influence of the land ice that existed 18 000 years before present 18k b p is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean simulated climates are obtained from two versions of the model one with the land ice distribution of the present and the other with that of 18k b p in the northern hemisphere the tropospheric flow field is strongly influenced by the laurentide ice sheet and features a split flow straddling the ice sheet with a strong jet stream forming the southern branch the northern branch of the flow brings very cold air over the north atlantic ocean where thick sea ice is maintained the distribution of sea surface temperature sst difference between the two experiments in the northern hemisphere resembles the difference between the sst at 18k b p and at present as estimated by the climap project 1981 the 18k b p ice sheets have very little influence upon atmospheric temperature and sst in the southern hemisphere this is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation hydrologic changes in the model climate are also found with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in north america and eurasia these findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum

faulting in a propagating continental rift insight from the late miocene structural development of the abert rim fault southern oregon usa abstract new geological mapping and 40ar 39ar ages reveal a temporal progression of faulting along a major extensional fault the abert rim fault at the active margin of the basin and range province in the western u s the onset of extensional deformation near lake abert coincided with widespread basaltic volcanism fault cross cutting relationships and tilt of volcanic layers demonstrate that nw striking faults formed between 8 9 and 7 5 ma and were subsequently cut by the nne striking abert rim fault sequential restoration of cross sections indicate that the abert rim fault south of 42 40 n latitude had 250 m of stratigraphic separation prior to deposition of the rattlesnake tuff at 7 ma low values of net extension 4 and long term rates of geologic deformation 1 mm yr suggest that regional extension since 10 ma has occurred primarily by diking we model province wide along and across strike expansion within the margin of the basin and range province as regional dilation accompanying northward propagation of the walker lane transform the growth of this extensional province is superimposed on a previously unrecognized early miocene volcanic landscape marked by volcanism between 21 4 and 22 3 ma dike orientations within one of these early miocene volcanoes and evidence for the existence of as much as 500 m of paleo topography prior to widespread deposition of the steens basalts at 16 ma suggests that late miocene faults near lake abert may reactivate an older structural fabric

thermochronometrically constrained anatomy and evolution of a miocene extensional accommodation zone and tilt domain boundary the southern wassuk range nevada 1 apatite ahe and zircon zhe u th he thermochronometric data from the southern wassuk range wr coupled with 40ar 39ar age data from the overlying tilted tertiary section are used to constrain the thermal evolution of an extensional accommodation zone and tilt domain boundary ahe and zhe data record two episodes of rapid cooling related to the tectonic exhumation of the wr fault block beginning at 15 and 4 ma extension was accommodated through fault block rotation and variably tilted the southern wr to the west from 60 70 in the central wr to 15 35 in the southernmost wr and pine grove hills and minimal tilting in the anchorite hills and along the mina deflection to the south middle miocene geothermal gradient estimates record heating immediately prior to large magnitude extension that was likely coeval with the extrusion of the lincoln flat andesite at 14 8 ma geothermal gradients increase from 19 4 c km to 65 20 c km toward the mina deflection suggesting that it was the focus of middle miocene arc magmatism in the upper crust the decreasing thickness of tilt blocks toward the south resulted from a shallowing brittle ductile transition zone postmagmatic middle miocene extension and fault block advection were focused in the northern and central wr and coincidentally moderated the large lateral thermal gradient within the uppermost crust

chronology of polyphase extension in the windermere hills northeast nevada fission track and 40 ar 39 ar dating and chemical correlation of volcanic strata exposed in the windermere hills and northern pequop mountains northeast nevada indicate a protracted polyphase history of tertiary late eocene late miocene extension along the northern margin of a major cordilleran metamorphic core complex early extension is recorded by a west tilted half graben filled with early oligocene 34 79 0 18 39 18 0 12 ma sedimentary rocks in the eastern windermere hills above the lowangle black mountain detachment fault the early oligocene half graben conformably overlies a widespread suite of late eocene 39 18 0 12 40 38 0 06 ma calc alkaline volcanic rocks reflecting a temporal link between early extension at a high structural level and the end of the ignimbrite flare up these strata are cut by east west striking normal faults which are exposed along and parallel to the northern margin of the metamorphic complex available age data e g between 14 93 0 08 and 34 79 0 18 ma permit the interpretation that the east west striking faults formed at the same time as or after large magnitude unroofing of highgrade rocks we interpret the east west striking faults to accommodate differential uplift of greenschist grade metamorphic rocks in the upper crust above a lateral ramp in a west northwest directed mylonitic shear zone subsequent extension in the windermere hills is defined by deep rapidly filled half grabens of middle miocene 7 42 2 0 to 14 93 0 08 ma age that unconformably overlie older faults and synextensional deposits these are the youngest half grabens in the region and are inferred to be initiated by extensional stresses imparted to the base of the lithosphere by a laterally spreading mantle plume e g the yellowstone hotspot located in southeastern oregon at this time

the northern walker lane refraction experiment pn arrivals and the northern sierra nevada root abstract in may 2002 we collected a new crustal refraction profile from battle mountain nevada across western nevada the reno area lake tahoe and the northern sierra nevada mountains to auburn ca mine blasts and earthquakes were recorded by 199 texan instruments extending across this more than 450 km long transect the use of large mine blasts and the ultra portable texan recorders kept the field costs of this profile to less than us 10 000 the seismic sources at the eastern end were mining blasts at barrick s goldstrike mine the goldstrike mine produced several ripple fired blasts using 8000 44 000 kg of anfo each a daily occurrence first arrivals from the larger goldstrike blasts are obvious to distances of 300 km in the raw records first arrivals from a quarry blast west of the survey near watsonville ca located by the northern california seismic network with a magnitude of 2 2 can be picked across the recording array to distances of 600 km the watsonville blast provides a western source nearly reversing the goldstrike blasts a small earthquake near bridgeport ca also produced pickable p wave arrivals across the transect providing fan shot data arrivals from m5 events in the mariana and kuril islands also appear in the records this refraction survey observes an unexpectedly deep crustal root under the northern sierra nevada range over 50 km in thickness and possibly centered west of the topographic crest pn delays of 4 6 s support this interpretation at battle mountain nevada we observe anomalously thin crust over a limited region perhaps only 150 km wide with a moho depth of 19 23 km pn crossover distances of less than 80 km support this anomaly which is surrounded by observations of more normal 30 km thick crust a 10 km thick and high velocity lower crustal pillow is an alternative hypothesis but unlikely due to the lack of volcanics west of battle mountain large mine and quarry blasts prove very effective crustal refraction sources when recorded with a dense receiver array even over distances exceeding 600 km new elastic synthetic seismogram modeling suggests that pn can be strong as a first arrival easing the modeling and interpretation of crustal refraction data fast eikonal computations of first arrival time can match pickable pn arrival times

the reconstruction of late quaternary antarctic sea ice distribution the use of diatoms as a proxy for sea ice the study of annual diatom transfer from the southern ocean surface to the sea floor using time series sediment traps combined with the mapping of diatom assemblages from surface sediments results in the definition of a proxy for past variations of sea ice extent the application and robustness of the proposed proxy are demonstrated by the study of late pleistocene sediment cores recovered from different environmental regimes in the southern ocean relative amounts of the sea ice indicator diatoms fragilariopsis cylindrus and fragilariopsis curta of 3 in diatom assemblages preserved in sediments are representative of the presence of seasonal winter sea ice summer sea ice occurrence is signalled by a distinct drop in biogenic sedimentation rate as a result of strongly reduced biogenic export in the presence 3 of diatom assemblage of the deep temperature diatom fragilariopsis obliquecostata the spatial and temporal distribution pattern of the sea ice proxy indicates significant sea ice variations during marine isotope stages 1 and 5 in the present antarctic zone of the southern ocean during glacial maxima the antarctic winter sea ice was extended into the present polar front zone while the antarctic summer sea ice was most probably expanded into the area of the present winter sea ice edge during the short climatic optima at the onset of interglacial periods the winter sea ice extent was distinctly reduced

the mount perkins block northwestern arizona an exposed cross section of an evolving preextensional to synextensional magmatic system the steeply tilted mount perkins block northwestern arizona exposes a cross section of a magmatic system that evolved through the onset of regional extension new 4oar 39ar ages of variably tilted 0 90 o volcanic strata bracket extension between 15 7 and 11 3 ma preextensional intrusive activity included emplacement of a composite miocene laccolith and stock trachydacite dome complex and east striking rhyolite dikes related volcanic activity produced an 18 16 ma stratovolcano cored by trachydacite domes and flanked by trachydacite trachyandesite flows and 16 ma rhyolite flows similar compositions indicate a genetic link between the stratovolcano and granodioritic phase of the laccolith magmatic activity synchronous with early regional extension 15 7 14 5 ma generated a thick felsic volcanic sequence a swarm of northerly striking subvertical rhyolite dikes and rhyolite domes field relations and compositions indicate that the dike swarm and felsic volcanic sequence are cogenetic modes of magma emplacement changed during the onset of extension from subhorizontal sheets east striking dikes and stocks to northerly striking subvertical dike swarms as the regional stress field shifted from nearly isotropic to decidedly anisotropic with an east west trending horizontal least principal stress preextensional trachydacitic and preextensional to synextensional rhyolitic magmas were part of an evolving system which involved the ponding of mantle derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower levels major extension halted this system by generating abundant pathways to the surface fractures which flushed out preexisting crustal melts and hybrid magmas remaining silicic melts were quenched by rapid upper crustal cooling induced by tectonic denudation these processes facilitated eruption of mafic magmas accordingly silicic magmatism at mount perkins ended abruptly during peak extension 14 5 ma and gave way to mafic magmatism which continued until extension ceased

timing of cenozoic volcanism and basin and range extension in northwestern nevada new constraints from the northern pine forest range eocene middle miocene volcanic rocks in the northern pine forest range nevada are ideally situated for reconstructing the timing and style of volcanism and extensional faulting in the northwesternmost part of the basin and range province a conformable sequence of cenozoic volcanic and sedimentary strata in the northern pine forest range dips 30 w and 11 new 40 ar 39 ar ages from this sequence defi ne 3 major episodes of volcanic activity pre tertiary basement and older ca 38 ma tertiary intrusive rocks are overlain unconformably by oligocene ca 30 23 ma basalt fl ows and dacitic to rhyolitic ash fl ow tuffs interbedded with fi ne grained tuffaceous sedimentary rocks oligocene rocks are overlain by 550 m of ca 17 16 ma basalt fl ows equivalent to the steens basalt in southern oregon and basalt fl ows are capped by a thin 16 3 ma ignimbrite that likely is correlative with either the idaho canyon tuff or the tuff of oregon canyon the northern pine forest range is bounded to the east by a major down to the east normal fault that dips 40 e with well developed fault striations indicative of dip slip motion this fault initiated at an angle of 70 and was rotated 30 during uplift of the range a suite of 17 apatite fi ssion track ages from the pine forest footwall block demonstrates that exhumation uplift and slip on the range bounding fault began ca 12 11 ma and continued until at least 7 ma with moderate slip since then the pine forest range did not undergo signifi cant extension before or during peak oligocene and miocene volcanism and similar geologic relationships in nearby ranges suggest that a larger region of northwestern nevada was also little extended during this interval basin and range faulting in northwestern nevada appears to have begun no earlier than 12 ma making it distinctly younger than deformation in much of central and southern nevada where peak extension occurred in the middle miocene or earlier

the northern nevada rift regional tectono magmatic relations and middle miocene stress direction as defined by the most recent aeromagnetic surveys the north northwest trending northern nevada rift zone extends for at least 500 km from southern nevada to the oregon nevada border at several places along the rift the magnetic anomaly is clearly related to north northwest trending dikes and flows that based on new radiometric dating erupted between 17 and 14 ma and probably during an even shorter time interval the tectonic significance of the rift is dramatized by its length its coincidence in time and space at its northern terminus with the oldest silicic caldera complex along the yellowstone hot spot trend and its parallelism with the subduction zone along the north american coast prior to the establishment of the san andreas fault the northern nevada rift is also equivalent in age trend and composition to feeder dikes that fed the main eruptive pulse 95 volumetrically off the columbia river flood basalts in northern oregon 15 5 16 5 ma because of these similarities both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south southeast and north northwest respectively from a central mantle plume the site of the initial breaching of the north america plate by this plume is probably the mcdermitt volcanic center at the north end off the rift near the oregon nevada border the present north northwest trend of the rift and its internal elements such as dikes and lava filled grabens record the orientation of the arc normal extensional stress in this back arc region at the time of emplacement paleomagnetic evidence presented by others and interpreted to indicate block rotations at three sample localities is not consistent with either a rotation of dikes within the rift or with a regional rotation of the entire rift the present north northwest trend of the rift reflects the state of stress in the basin and range during middle miocene time and is consistent with stress indicators of similar age throughout the basin and range and rio grande rift provinces