the bagdad reflection sequence as tabular mafic intrusions evidence from seismic modeling of mapped exposures two dimensional seismic modeling of exposed rocks implicates subhorizontal tabular mafic intrusions as the likely cause of the unusually prominent reflections that constitute the bagdad reflection sequence brs these basement reflections from depths of 3 15 km are observed on cocorp and other data to extend for some 100 km across the basin and range colorado plateau transition zone in west central arizona the nearby buck mountains fortuitously expose an upended upper crustal block which is used in this study as the basis for a 12 x 6 km seismic model ray tracing through this model produces synthetic reflections from the numerous proterozoic diabase intrusions occurring in the buck mountains these synthetic reflections approximate the scale geometry and character of reflections observed on the cocorp data indicating that the intrusions in the buck mountains furnish a viable geologic analogy for the brs several lines of evidence argue that the major competing hypothesis that tertiary detachment surfaces cause the brs is less satisfactory although we cannot rule out the possibility that the brs is related to hypothetical tertiary intrusions of similar distribution and structure geologic evidence more directly implies that these reflectors are proterozoic in age if so the bagdad reflections serve as strain markers and can be used to constrain subsequent deformation in the arizona transition zone their relatively continuous nature thus argues against widespread disruption of the upper crust in the transition zone during tertiary extension moreover identification of these reflectors as subhorizontal mafic intrusions may have profound consequences for interpretation of similar reflections observed elsewhere by bolstering the contention that mafic intrusions are an important cause of basement reflections in the continental crust

columbia river flood basalts from a centralized crustal magmatic system the columbia river basalt group in the northwestern united states derived from flood basalt eruptions that occurred 16 million years ago exhibits variability in geography and trace element geochemistry that has led to a number of proposed magma origins however the geochemical variability can be explained by a relatively simple model in which magma is derived from a mantle plume that assimilated continental crust in a centralized magma system the columbia river basalt group in the northwestern united states1 comprising about 230 000 cubic kilometres of rock exhibits unusual patterns in lava distribution geochemistry and its apparent relationship to regional tectonics consequently there is little consensus on the origin of its magmas2 3 4 5 6 7 8 9 10 11 12 here we examine the isotopic ratios of sr nd pb and os and trace element abundances in columbia river basalts the results suggest that most of the lava was produced when magma derived from a mantle plume assimilated continental crust in a central magma chamber system located at the boundary between the north american craton and the accreted terranes of idaho and oregon other related basalts are the product of mixing between the mantle plume and different types of regional upper mantle magma was then transported over a wide region by an extensive network of dykes a process that has been identified in other flood basalt provinces as well13 interactions of the plume with surrounding upper mantle and of mantle derived magmas with regional crust provide a relatively simple6 7 9 14 15 model to explain the more unusual features of the main phase columbia river basalts

helium and neon isotopes in the imnaha basalt columbia river basalt group evidence for a yellowstone plume source abstract helium neon and argon isotopic compositions were measured in two flows of the columbia river flood basalt the imnaha basalt has a 3 he 4 he ratio of 11 4 times atmospheric and 20 ne 22 ne and 21 ne 22 ne ratios characteristic of a plume component the measured 3 he 4 he is a lower limit due to possible preferential 3 he loss and or addition of radiogenic 4 he a wanapum basalt flow erupted approximately 2 ma later in the waning stages of volcanism has more morb like noble gases the he nd and sr isotopic compositions of these lavas suggest that the columbia river basalts were derived from the yellowstone plume head which contained both high helium plume material and entrained depleted mantle as the eruptions progressed the plume component in the melting region was gradually diluted or replaced

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

origin of the columbia plateau and snake river plain deflection of the yellowstone plume previous models relating the snake river plain and columbia river basalt northwest united states to the yellowstone plume have not taken into account the effects of the subducted farallon and juan de fuca plates not only did subduction shield the surface from mantle plume activity but it deflected the plume in the direction of plate motion a variety of geophysical and geologic evidence suggests that the plume penetrated the subducted slab at 17 5 ma causing eruption of the columbia river basalt the plate boundary then migrated over the plume and the yellowstone plume has recovered to a more nearly vertical conduit forming the snake river plain and yellowstone plateau

mantle structure beneath the western united states and its implications for convection processes 1 we present tomographic images of the mantle structure beneath the western united states our dynamic north america models of p and s velocity structure dna07 p and dna07 s use teleseismic body waves recorded at 600 seismic stations provided by the earthscope transportable array and regional networks dna07 p and s benefit from the unprecedented aperture of the network while maintaining a dense station distribution providing high resolution body wave imaging of features through the transition zone and into the lower mantle the main features imaged include 1 the juan de fuca subduction system that bottoms at 400 km beneath oregon implying interaction with the yellowstone anomaly 2 a low velocity conduit beneath yellowstone national park that bottoms at 500 km and dips toward the northwest 3 shallow low velocity anomalies upper 200 km beneath the eastern snake river plain esrp and the high lava plains and a deep low velocity anomaly 600 km beneath the esrp but not newberry 4 a low velocity slab gap to 400 km depth immediately south of the mendocino triple junction and south of the gorda slab and 5 high velocity drips beneath the transverse ranges the southern central valley sierra nevada and central nevada these observations reveal extremely heterogeneous mantle structure for the western united states and suggest that we are only just beginning to image the complex interactions between geologic objects the transportable array allows for analysis of the relationships between these anomalies in an internally consistent single tomographic model the dna velocity models are available for download and slicing at http dna berkeley edu

seismically imaged relict slab from the 55 ma siletzia accretion to the northwest united states teleseismic tomography images a large high velocity curtain extending vertically beneath the ca 50 ma challis magmatic trend to maximum depths of 230 600 km we interpret this structure as subducted farallon ocean lithosphere that stalled with the ca 55 ma accretion of the siletzia microplate to north america within the columbia embayment and we consider the regional tectonic implications the abrupt switch ca 53 ma from laramide thrusting and magmatic quiescence to extension and vigorous magmatism in the northwestern united states is evidence for foundering of the fl at subducting farallon slab to account for the imaged curtain foundering apparently occurred by rollback after siletzia accretion terminated subduction within the columbia embayment the magnitude of the high seismic velocity curtain is approximately that expected for 20 50 m y old ocean lithosphere that stalled in the upper mantle ca 50 ma we suggest that the stalled slab became nearly neutrally buoyant as a result of removal of its basaltic crust the melting of which likely contributed to the short lived vigor of challis magmatism after siletzia accretion normal dip cascadia subduction initiated west of siletzia evidenced by arc volcanism in oregon and washington beginning ca 45 40 ma while continued quiescence of the sierra nevada arc suggests persistence of flsubduction to the south kinematically this requires a tear in the subducted farallon slab near the latitude of southern oregon we propose that north to south propagation of this torn slab edge propagated the ignimbrite fl are up across what now is the northern basin and range and ended the laramide orogeny

crustal structure beneath the high lava plains of eastern oregon and surrounding regions from receiver function analysis 1 we analyze teleseismic p to s receiver functions to image crustal structure beneath the high lava plains hlp of eastern oregon and surrounding regions coverage from 206 broadband seismic stations provides the first opportunity to resolve variations in crustal composition thickness and heterogeneity on scales of a few km in depth and tens of km laterally across the hlp region we utilize both h stacking and a new gaussian weighted common conversion point stacking technique we find crust that is 40 km thick beneath the cascades idaho batholith and owyhee plateau and thinner 31 km crust beneath the hlp and northern great basin low poisson s ratios of 0 240 characterize the granitic crust beneath the idaho batholith while the owyhee plateau exhibits values of 0 270 typical of average continental crust the owyhee plateau is a thick simple crustal block with distinct edges at depth the western hlp exhibits high average values of 0 304 typical for regions of widespread basaltic volcanism combined with other geological and geophysical observations the areas of abnormally high poisson s ratios 0 320 and low velocity zones in the crust beneath north central and southern oregon are consistent with the presence of partial melt on either side of the hlp trend suggesting a central zone where crustal melts have drained to the surface perhaps enabled by the brothers fault zone thicker crust and an anomalous n s band of low poisson s ratios 0 252 skirting the steens mountain escarpment is consistent with residuum from a midcrustal magma source of the massive flood basalts supporting the view of extensive mafic underplating and intraplating of the crust from cenozoic volcanism

tertiary calc alkaline magmatism associated with lithospheric extension in the pacific northwest the long standing assumption that calc alkaline magmatism requires contemporaneous subduction is critically reviewed two of many widely spread fields of tertiary calc alkaline magmatism in the pacific northwest one eocene and one miocene provide clear examples of the association between calc alkaline magmatism and lithospheric extension we suggest that the calc alkaline magmatism of these fields is better explained by decompression melting of the subcontinental lithospheric crust and mantle enriched by previous subduction events than by contemporaneous eocene subduction this model proposes that extension will cause calc alkaline dominated magmatism in areas of continental crust with a history of previous subduction as along the western margin of north america currently popular models for the tectonic magmatic evolution of the basin and range province in which subduction driven calc alkaline magmatism is followed by extension related bimodal magmatism may need modification much more of this magmatism may result from extension in the pacific northwest we suggest that subduction beneath the idaho batholith ceased at the end of the cretaceous blocked by the accreted terranes to be reactivated in its present position beneath the cascades during the eocene the various isolated eocene volcanic fields spread across the pacific northwest were due to lithospheric extension not to the presence of a subducting plate immediately below

p wave tomography of the western united states insight into the yellowstone hotspot and the juan de fuca slab abstract we used 190 947 high quality p wave arrival times from 8421 local earthquakes and 1 098 022 precise travel time residuals from 6470 teleseismic events recorded by the earthscope usarray transportable array to determine a detailed three dimensional p wave velocity model of the crust and mantle down to 1000 km depth under the western united states us our tomography revealed strong heterogeneities in the crust and upper mantle under the western us prominent high velocity anomalies are imaged beneath idaho batholith central colorado plateau cascadian subduction zone stable north american craton transverse ranges and southern sierra nevada prominent low velocity anomalies are imaged at depths of 0 200 km beneath snake river plain which may represent a small scale convection beneath the western us the low velocity structure deviates variably from a narrow vertical plume conduit extending down to 1000 km depth suggesting that the yellowstone hotspot may have a lower mantle origin the juan de fuca slab is imaged as a dipping high velocity anomaly under the western us the slab geometry and its subducted depth vary in the north south direction in the southern parts the slab may have subducted down to 600 km depth a slab hole is revealed beneath oregon which shows up as a low velocity anomaly at depths of 100 to 300 km the formation of the slab hole may be related to the newberry magmatism the removal of flat subducted farallon slab may have triggered the vigorous magmatism in the basin and range and southern part of rocky mountains and also resulted in the uplift of the colorado plateau and rocky mountains

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

diurnal cycle of the south pacific convergence zone in 30 years of satellite images 2015 american geophysical union a data set of three hourly south pacific convergence zone spcz location and extent from which ir temperature within the spcz may be obtained for 1980 2012 november april is used to examine the diurnal cycle of the spcz maximum spcz area occurs at 15 00 18 00 local standard time lst two minima in mean ir temperature are evident one at 13 00 16 00 lst nearly coinciding with the maximum in area and the second in the early morning hours at 05 00 07 00 lst when area is at a minimum the morning minimum in mean ir temperature is associated with a peak in deep convection while the afternoon minimum is associated with a peak in midlevel clouds on average the morning minimum in ir temperature dominates in the tropical regions of the spcz while the afternoon ir temperature minimum dominates in the subtropical regions of the spcz the relative strength of the two ir temperature minima is affected by the seasonal cycle intraseasonal variability associated with active madden julian oscillation events and interannual variability due to the el nino southern oscillation the morning ir temperature minimum becomes more dominant when the spcz is more frequently present or when the spcz is shifted toward the equator in terms of cloud height midlevel clouds dominate at all times in all regions of the spcz and peak in abundance between 15 00 18 00 lst low level clouds peak near midnight and then transition to high level clouds which peak between 03 00 06 00 lst just before sunrise

tectonic and magmatic evolution of the northwestern basin and range and its transition to unextended volcanic plateaus black rock range nevada the seismically active eastern and western margins of the northern basin and range have been extensively studied yet the northwestern margin of the province remains incompletely understood the black rock range of northwestern nevada straddles the transition from the basin and range province to the south and east and flat lying volcanic plateaus to the west this poorly understood range preserves a remarkably complete record of cenozoic magmatism and provides an important window into the pre miocene history of the unextended volcanic plateaus of northeastern california and southern oregon geologic mapping and 40ar 39ar geochronology from the northern black rock range document three significant episodes of eocene to middle miocene volcanism eocene 35 ma basalts directly overlie mesozoic granites and arc related volcanic and sedimentary rocks locally erupted oligocene to early miocene 27 21 ma bimodal volcanic rocks comprise the bulk of the cenozoic section and conformably overlie the eocene basalt flows these bimodal units include rhyolitic lavas variably welded rhyolitic ash flows unwelded ash fall deposits and thin basalt flows in the neighboring pine forest range 20 km to the north similar oligocene to early miocene units are overlain by more than 500 m of ca 16 4 ma steens equivalent basalt flows and are capped by ca 16 ma rhyolitic ash flow tuffs in the northern black rock range the ca 16 4 ma middle miocene basalts are absent from the section and a 16 2 ma rhyolitic ash flow tuff directly overlies the early miocene flows basaltic and rhyolitic volcanic products in the northern black rock range span 35 16 ma with many of the oligocene volcanic units derived from local vents and dikes despite the map scale complexities of locally derived lava flows the cenozoic section is broadly conformable and dips gently 5 10 to the northwest the region experi enced no significant tilting between 35 and 16 ma with moderate tilting 5 10 and concomitant uplift occurring after 16 ma this tectonic history is consistent with that of the nearby pine forest and santa rosa ranges where low temperature thermochronology documents footwall exhumation along the range bounding normal faults after 12 ma the velocity structure of the crust beneath the northern black rock range is constrained by a recent geophysical survey seismic reflection refraction and gravity and contains gradients that correspond to basin depths predicted by our geologic mapping together with recently completed geological and geophysical studies from the surrounding region our results suggest that the evolution of the northwestern margin of the basin and range was characterized by long lived and voluminous volcanism without significant tectonism followed by low magnitude 20 extension along high angle normal faults

the fate of the juan de fuca plate implications for a yellowstone plume head beneath the pacific northwest the juan de fuca plate a remnant of the farallon plate continues subducting beneath the north american continent to the east of the cascadia subduction zone lies the yellowstone hotspot track the origins of this track can be traced back to the voluminous basaltic outpourings in the columbia plateau around 17 ma if these basalts are the result of a large melting anomaly rising through the mantle to the base of the north american continent such as a mantle plume head the anomaly would need to punch through the subducting juan de fuca slab here we use teleseismic body wave travel time tomography to investigate the fate of the subducted slab and its possible interaction with a plume head our dataset is derived from the oregon array for teleseismic study oats deployment in oregon and all other available seismic data in this region during the same period in our jdf07 models we image the subducted juan de fuca plate in the mantle east of the cascades beneath oregon where the slab has not been imaged before to a depth of 400 km but no deeper the slab dips similar to 50 degrees e and has a thickness of similar to 75 km immediately beneath the slab we image a low velocity layer with a similar geometry to the slab and extending down to at least similar to 575 km depth in the v model the total length of the high velocity slab is similar to 660 km about 180 km longer than the estimated length of slab subducted since 17 ma assuming similar slab geometry to today this 180 km length of slab would reach similar to 60 km depth comparable to the thickness of continental lithosphere we propose that the absence of the slab below 400 km today is due to the arrival of the yellowstone plume head similar to 17 ma which destroyed the juan de fuca slab at depths greater than the thickness of the continental lithosphere given this scenario the low velocity anomaly beneath the slab is likely the remnant plume head material which has been pulled down by traction with the subducting plate the amplitude of the observed low velocity anomaly is comparable with that expected for plume head material 100 300 degrees c hotter than the surrounding asthenosphere c 2007 elsevier b v all rights reserved

ages of the steens and columbia river flood basalts and their relationship to extension related calc alkalic volcanism in eastern oregon stratigraphic and chemical correlations of tertiary volcanic units in eastern oregon confirm that the steens basalt represents the earliest eruptions of the columbia river flood basalt province field correlations are supported by major and trace element analyses and confirmed by 4 0 ar 3 9 ar dates within the basalt of malheur gorge situated between steens mountain and the southernmost extent of the previously mapped columbia river basalt group the lowest unit correlates with the steens basalt and the conformably overlying middle and upper units correlate with the imnaha and grande ronde basalt formations of the columbia river basalt group new dates indicate that imnaha and grande ronde basalt formations on the columbia plateau 90 of the columbia river basalt group erupted between 16 1 and 15 0 ma these were immediately preceded by the steens basalt a plagioclase phyric tholeiite that erupted above the calculated position of the yellowstone hotspot at 16 6 ma in eastern oregon the flood basalt tholeiites of steens mountain and malheur gorge form a voluminous but brief interlude 16 6 15 3 ma superimposed on the low volume calc alkalic to mildly alkalic volcanism associated with continuing eocene to present east west extension

lithospheric topography tilted plumes and the track of the snake river yellowstone hot spot 1 the trace of the snake river yellowstone hot spot is the world s best example of a mantle plume that has been overridden by continental lithosphere the standard model calls for the plume head to rise under northern nevada and be forced northward to form basalts of the columbia plateau subsequent movement of north america to the southwest over the plume tail created a hot spot trace on the surface we present a new conceptual model for the origin of this feature that resolves inconsistencies in the current standard model and explains the recent documentation of a thermal anomaly in the mantle below yellowstone today that plunges 65 wnw our model implies that the plume tail was forced beneath thinned cratonic lithosphere to the se along with part of the plume head and has remained in this orientation for the last 12 ma we infer that almost all of the volcanism in se oregon and sw idaho prior to 12 ma results from overriding the southern extension of the plume head not the plume tail and that a distinct plume tail hot spot track was not established until formation of the bruneau jarbidge eruptive center around 12 ma the plume tail track may also be controlled by a preexisting structural boundary in lithosphere that is thinner than adjacent lithosphere this model demonstrates the potential importance of lithospheric topography on controlling the surface manifestation of plume volcanism and the complexity that may arise when lithospheric thickness is nonuniform

distribution and geochronology of oregon plateau u s a flood basalt volcanism the steens basalt revisited the timing and petrogenesis of mid miocene flood basalt volcanism in the northwest united states has been extensively addressed yet the chemical characteristics and temporal details of the steens basalt exposed on the oregon plateau are poorly defined steens basalt volcanism has generally been accepted to have occurred at 16 6 ma coeval and or just prior to the onset of columbia river basalt group volcanism to the north new major and trace element analyses and nine 40 ar 39 ar ages ranging from 15 51 0 28 to 16 58 0 18 ma were obtained on oregon plateau flood basalt lava flows from stratigraphic sections in close proximity to steens mountain additionally new 40 ar 39 ar ages were obtained on the uppermost and thirty first lava flow down from the top of the 1 km section of steens basalt exposed at steens mountain and yield eruption ages of 16 59 0 10 and 16 55 0 10 ma respectively field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of steens mountain the chemical and chronologic data presented in this study illustrate that flood basalts with the steens basalt chemical signature erupted across the southern oregon plateau over a much greater timespan than what is typically quoted for the steens mountain type section these data suggest that the main volume of steens basalt volcanism erupted over at least an 1 m y duration from eruptive loci in the vicinity of steens mountain while likely much less voluminous volcanism and lithospheric input of mafic magma appears to have occurred for n2 ma across the oregon plateau these new geochemical and geochronological constraints verify a common temporal link between steens malheur gorge region and columbia river basalt group crbg volcanism this direct temporal link requires that petrogenetic and tectonic models of mid miocene northwestern u s flood basalt volcanism recognize that the northern columbia plateau and southern oregon plateau portions of this mid miocene basalt province were erupting simultaneously during portions of the regional event 2007 elsevier b v all rights reserved

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

the steens mountain oregon geomagnetic polarity transition 3 its regional significance study of the variations of direction and intensity of the geomagnetic field as recorded by the miocene lava flows on steens mountain southeastern oregon has resulted in a detailed description of total field behavior during a reversal in polarity in addition to information about the polarity reversal itself the detailed paleomagnetic record includes several thousand years of geomagnetic history preceding and following the polarity transition at 15 5 ma in order to test the feasibility of using this record as a means of correlation in this part of the western united states comparisons are made of reconnaissance and previously published paleomagnetic records obtained from what has been thought to be the steens basalt or rocks of equivalent age despite the fact that many of these earlier studies were not done in detail and were not intended for correlation purposes convincing similarities among some of the records are evident the steens basalt paleomagnetic record does indeed have potential as a correlation tool during this time of widespread basaltic volcanism additionally paleomagnetic data from flows that were sampled in detail yield a middle miocene paleomagnetic pole at 88 3 n 209 0 95 6 3 for the high lava plains of oregon this pole position is statistically indistinguishable from the earth s rotational axis and implies that no tectonic rotation of this region has occurred since these lava flows were erupted data from selected sites within the coeval part of the columbia river basalt group yield a paleomagnetic pole at 88 7 n 171 6 e 95 4 0 the columbia river basalt group pole is statistically indistinguishable from either the rotational axis or from the high lava plains pole these findings indicate no post 20 ma differential rotation between south eastern washington and south central oregon in contrast to previous interpretations

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

palinspastic restoration of navdat and implications for the origin of magmatism in southwestern north america 1 simultaneous palinspastic restoration of deformation and volcanism illuminates relationships between magmatism and tectonics in western north america using arcgis we retrodeformed the navdat north american volcanic database navdat geongrid org using the western north america reconstruction of mcquarrie and wernicke 2005 from these data sets we quantitatively compare rates of magmatism and deformation and evaluate the age composition and migration of cenozoic volcanism from 36 ma to present these relationships are shown in a series of palinspastic maps as well as animations that highlight migrating extension and volcanism with time western north america is grouped into eight different regions with distinct relationships between strain and volcanism to evaluate competing hypotheses regarding the relationship of extension to continental magmatism a first order observation from this study is that magmatism throughout the basin and range appears to be primarily driven by plate boundary effects notably subducting and foundering slabs as well as slab windows exceptions include the yellowstone hotspot system along the northern border of our study area and late stage 8 ma passive extension related asthenospheric upwelling along the eastern and western margins of the basin and range the palinspastic reconstructions presented here highlight that the classic high angle basin and range faulting that comprises most of the physiographic basin and range province commenced during a magmatic lull more broadly with the exception of the rio grande rift we find that pulses of magmatism lag the onset of extension these observations largely contradict the active rifting model where magmatism triggers basin and range extension

stepwise climate change recorded in eocene oligocene paleosol sequences from central oregon successions of paleosols bounded by erosional surfaces in fluvial sediments of the eocene oligocene strata of central oregon can be interpreted as terrestrial equivalents of the unconformity bound units of sequence stratigraphy in the upper part of the upper eocene clarno formation and in the lower part of the lower eocene lower miocene john day formation truncation surfaces separate otherwise conformable alluvial deposits and allow for stratigraphic subdivision into informal members lower and upper red hill claystones in the clarno formation and lower middle and upper big basin members and lower turtle cove member in the john day formation paleosols in each member show a stepwise change in the degree of weathering of the most strongly developed paleosols kaolinite rich ultisollike paleosols in lower red hill claystones late eocene 42 43 ma smectite rich alfisol like paleosols in the upper red hill claystones late eocene 41 42 ma alfisols and ultisol like paleosols in the lower big

open system magmatic evolution of andesites and basalts from the salmon creek volcanics southwestern idaho u s a abstract the salmon creek volcanics scv comprise 31 26 ma orogenic andesites and mildly alkaline basalts detailed petrologic and geochemical studies of two thick continuous sections of scv lavas reveal cyclic compositional variations apparently produced by a systematic progression of magmatic processes through time the earliest phase of scv activity produced basalts with trace element and isotopic compositions indicating an asthenospheric source enriched in subduction related components following eruption of these basalts an andesitic magma chamber was established within the continental crust near the base of the thickest andesite section compositions of the lavas become more mafic upsection indicating high rates of magma recharge relative to crystallization wall rock assimilation was most significant in the earliest stages of magma chamber initiation and growth as shown by the sharp increase in alkalies and covariant changes in sr pb and nd isotopes in the lowermost andesite lavas further upsection compositional variations in the andesites are more consistent with fractional crystallization as the predominant magmatic process observed phenocryst assemblages in the andesites and mass balance calculations indicate that crystallization was dominated by kaersutitic hornblende with relatively little plagioclase hornblende dominated assemblages suggest that the magmas were hydrous 2 h 2 o in the melt and evolved at moderate pressures in the lower to middle crust 3 15 kbar

the age of the steens reversal and the columbia river basalt group abstract the columbia river basalt group crbg eruptions have a well defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale 40 ar 39 ar ages are presented from lavas erupted in the r 0 through n 1 magnetozones of the crbg and in the transition between r 0 and n 0 four ages from transitionally magnetized lava flows at steens mountain catlow peak and poker jim ridge with a weighted mean age 16 58 0 10 ma 1 and the more precise age 16 654 0 025 ma of the normally magnetized oregon canyon tuff at the top of the catlow peak section show that the oldest crbg magnetozone r 0 correlates with the c5cr chron bayesian statistical analysis applied to data from four flows at catlow peak using the mean age of the steens reversal gives a best and preferred age of the steens reversal of 16 73 0 13 0 08 ma 95 confidence depending on the geomagnetic polarity time scale model the eruption rate from n 0 through r 2 0 34 0 45 ma in the middle and the bulk of the crbg emplacement averaged 0 30 0 41 km 3 a and peaked at a rate 1 1 2 to 4 1 2 times higher during r 2

two stage subduction history under north america inferred from multiple frequency tomography eastward subduction of oceanic tectonic plates has shaped the geologic history of western north america over the past 150 million years the mountain building and volcanism that brought forth the spectacular landscapes of the west are credited to the vast ancient farallon plate which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle here we use finite frequency travel time and amplitude measurements of teleseismic p waves in seven frequency bands to obtain a high resolution tomographic image to approx1 800 km depth we discover several large previously unknown pieces of the plate which show that two distinct stages of whole mantle subduction are present under north america the currently active one descends from the pacific northwest coast to 1 500 km depth beneath the great plains whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent we argue that the separation between them is linked to the laramide era 70 50 myr ago a time of unusual volcanism and mountain building far inland generally explained by an episode of extremely flat subduction

lithosphere asthenosphere interaction beneath the western united states from the joint inversion of body wave traveltimes and surface wave phase velocities summary the relation between the complex geological history of the western margin of the north american plate and the processes in the mantle is still not fully documented and understood several pre usarray local seismic studies showed how the characteristics of key geological features such as the colorado plateau and the yellowstone snake river plains are linked to their deep mantle structure recent body wave models based on the deployment of the high density large aperture usarray have provided far more details on the mantle structure while surface wave tomography ballistic waves and noise correlations informs us on the shallow structure here we combine constraints from these two data sets to image and study the link between the geology of the western united states the shallow structure of the earth and the convective processes in mantle our multiphase dna10 s model provides new constraints on the extent of the archean lithosphere imaged as a large deeply rooted fast body that encompasses the stable great plains and a large portion of the northern and central rocky mountains widespread slow anomalies are found in the lower crust and upper mantle suggesting that low density rocks isostatically sustain part of the high topography of the western united states the yellowstone anomaly is imaged as a large slow body rising from the lower mantle intruding the overlying lithosphere and controlling locally the seismicity and the topography the large e w extent of the usarray used in this study allows imaging the slab graveyard a sequence of farallon fragments aligned with the currently subducting juan de fuca slab north of the mendocino triple junction the lithospheric root of the colorado plateau has apparently been weakened and partly removed through dripping the distribution of the slower regions around the colorado plateau and other rigid blocks follows closely the trend of cenozoic volcanic fields and ancient lithospheric sutures suggesting that the later exert a control on the locus of magmato tectonic activity today the dna velocity models are available for download and slicing at http dna berkeley edu

initial impingement of the yellowstone plume located by widespread silicic volcanism contemporaneous with columbia river flood basalts during the main phase of steens and columbia river united states flood basalt eruptions between 16 7 and 15 0 ma 3900 km 3 of silicic magma erupted from centers dispersed across 25 000 km 2 the largest and oldest silicic centers high rock mcdermitt and lake owyhee caldera complexes lie along a narrow nne trend at the transition between the craton and accreted oceanic crust suggesting that silicic magma generation was greatest where mafic magma intruded more felsic crust less voluminous and younger silicic centers track the spread of mafic dikes to the north the high rock caldera complex forms the western end of the snake river plain yellowstone trend of eastward younging silicic centers we attribute a regional lull in volcanism between ca 23 and 7 ma to lifting of the juan de fuca slab by a mantle plume beginning at ca 25 ma and the distribution of main stage flood basalts and coeval rhyolites to breakthrough of plume material at ca 17 ma to form a smaller plume head centered beneath steens mountain

was the laramide orogeny related to subduction of an oceanic plateau numerous models have been presented to explain the late cretaceous early cenozoic laramide orogeny which affected the foreland region of the western cordillera within the us1 the most attractive models invoke low angle subduction2 4 which can develop for various reasons5 6 evidence from south america indicates that one such reason may be the subduction of buoyant ocean floor we here extend the low angle subduction models by attributing the shallowing of the subduction angle during the laramide orogeny to subduction of a large oceanic plateau of anomalously thick and buoyant oceanic crust pacific plate history indicates that a twin to the hess rise may represent the oceanic plateau that triggered laramide events

segmentation of the farallon slab abstract recent tomography images reveal a complex 3d mantle structure beneath western united states with feature morphology varying rapidly with depth by assimilating plate motion history paleo age of sea floor and paleo geography of plate boundaries in a 3 d numerical model we simulate the farallon juan de fuca subduction during the past 40 ma we find that the highly segmented upper mantle structure of western u s is a direct result of the farallon subduction we show that the tilted horseshoe shaped fast seismic anomaly beneath nevada and utah at 300 600 km depth range is in fact a segment of curled slab subducted since 15 ma and the shallower linear slab beneath the cascades is younger than 5 ma the distinct morphology between these two parts of the subduction system indicates the strong influence of the fast trench rollback since 20 ma the northward migrating jf pa na triple junction and the toroidal flow around slab edges the observed mantle structures are used to constrain the rheology of the upper mantle through matching the shape depth and location of modeled subducted slab segments the inferred viscosity for the asthenosphere is 5 10 19 pa s and those for the transition zone and lower mantle are 1 5 10 21 pa s and 2 10 22 pa s respectively the slab is found to be about 2 orders of magnitude stronger than the ambient mantle above 410 km depth but of similar order of magnitude viscosity in the transition zone

yellowstone hotspot continental lithosphere interaction the snake river plain represents 17 m y of volcanic activity that took place as the north american continent migrated over a relatively fixed magma source or hotspot we present new pb sr and nd data for a suite of 25 basalts collected from western and central snake river plain srp the new isotope data combined with previously published data from the srp provide a traverse of the wyoming craton margin from the 87sr 86sr 0 706 line boundary of western srp with phanerozoic accreted terranes east through the central and eastern srp to the yellowstone plateau low k basalts from the western srp overlain by high k basalts provide a temporal record of regional source variation from 16 8 to 0 2 ma principal component analysis pca of the new and previously published srp basalt pb isotopes reveals that 97 of the total variability is accounted for by mixing between three end members and is consistent with a sublithospheric yellowstone hotspot mantle source with a radiogenic isotope composition similar to the mantle source of the early columbia river basalt group crbg and two continental lithosphere end members heterogeneous in age and composition we use the srp pb sr and nd isotope data to model the yellowstone hotspot continental lithosphere interaction by three component mixing between two continental lithospheric components archean lithosphere cl1 that represents older lithosphere underlying the yellowstone plateau in the east and paleoproterozoic lithosphere cl2 representing the younger lithosphere underlying the srp in the west near the craton margin and a sublithospheric end member representing the yellowstone hotspot pl the results suggest a continuous flow of pl material westward as the na continental lithosphere migrated over the upwelling hotspot along a shoaling gradient in the sub continental mantle lithosphere the model shows a decrease in total lithosphere end members cl1 cl2 and the lithosphere ratio cl1 cl2 from the craton interior at yellowstone toward its western margin consistent with geologic and geophysical evidence that the continental lithosphere beneath the srp decreases in age and thickness from east to west the lithosphere ratio shows step like decreases from yellowstone in the east to the 87sr 86sr 0 706 line in the west indicating that the srp cuts across geochemically distinct parcels of lithospheric mantle consistent with terrane accretion models for the craton margin in the western srp young high k basalts have a lower mass fraction of total lithospheric compared to the underlying low k tholeiites but the same lithosphere ratio consistent with a recent 700 900 ka decrease in lithosphere contribution between eruption of early low and younger high k basalts

tracking changes in crustal thickness during orogenic evolution with sr y an example from the north american cordillera global compilations indicate that the geochemistry of arc magmatism is sensitive to moho depth magmatic products are prevalent throughout the history of cordilleran orogenesis and can be employed to constrain the timing of changes in crustal thickness as well as the magnitude of those changes we investigate temporal variations in crustal thickness in the united states cordillera using sr y from intermediate continental arc magmas our results suggest that crustal thickening began during the late jurassic to early cretaceous and culminated with 55 65 km thick crust at 85 95 ma crustal thicknesses remained elevated until the mid eocene to oligocene after which time crustal thicknesses decreased to 30 40 km in the miocene the results are consistent with independent geologic constraints and suggest that sr y is a viable method for reconstructing crustal thickness through time in convergent orogenic systems

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

geology and 40ar 39ar geochronology of the middle miocene mcdermitt volcanic field oregon and nevada silicic volcanism associated with propagating flood basalt dikes at initiation of the yellowstone hotspot

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

regional study of mineral resources in nevada insights from three dimensional analysis of gravity and magnetic anomalies a three dimensional interpretation of the basins of nevada was developed based on statewide data bases of digital gravity magnetic geologic well and topographic information an iterative technique was applied to isostatic residual gravity anomalies in nevada in order to define the location and shape of pre tertiary basement and to produce a gravity map that reflects variations of density within the pre tertiary basement the dominant feature of the basement gravity of nevada is an enormous area of low gravity that spans the entire state between latitudes 37 n and 40 5 n this regional low strongly correlates with the distribution of middle and late tertiary volcanic rocks and may reflect silicic intrusions within the mid crust and upper crust that are the counterparts of volcanic rocks at the surface although 80 of nevada is covered by cenozoic deposits these deposits are thicker than 1 km over only about 20 of the state the remaining 60 of nevada may have pre tertiary basement rocks within reach of current mineral exploration techniques aeromagnetic profiles from the national uranium resource evaluation nure were analyzed in order to produce a map showing the location of shallow magnetic sources in nevada this analysis shows that 46 of the state has magnetic sources generally mesozoic and cenozoic igneous rocks within 1 km of the surface a linear magnetic anomaly in north central nevada has been interpreted by others as a rift zone active during middle miocene time the rift also is evident in nure magnetic profiles but our interpretation suggests that the magnetic expression of the rift continues south southeast with similar strike to at least 38 n and perhaps to the amagmatic zone lat 37 n the survival since the middle miocene of this narrow crustal feature essentially linear over a distance of 500 km is difficult to interpret in light of later basin and range deformation our analysis of gravity anomalies shows that many deep cenozoic basins are located near the rift yet only two basins cut across it and at least five others change strike near the rift as if to avoid it the rift may have remained linear because it is associated with crustal structures that acted to resist subsequent deformation

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

using 40 ar 39 ar ages of intercalated silicic tuffs to date flood basalts precise ages for steens basalt member of the columbia river basalt group abstract to establish causality between flood basalt eruptions and extinction events and global environmental effects recorded by isotopic excursions in marine sediments highly accurate and precise ages for the flood basalts are required but flood basalts are intrinsically difficult to date we illustrate how 40ar 39ar feldspar ages for silicic tuffs intercalated with and overlying sections of steens basalt the earliest lavas of the middle miocene columbia river basalt group in the northwestern united states provide high precision ages that for the first time make it possible to resolve age differences with stratigraphic position within a section of these flood lavas the stratigraphically lowest rhyolitic tuff a fall deposit yielded an age of 16 592 0 028 ma fcs 28 02 ma and the uppermost the alkali rhyolite ignimbrite tuff of oregon canyon is 16 468 0 014 ma the argon and stratigraphic data indicate that steens basalt eruptions occurred from 16 64 to 16 43 ma in the southern end of its distribution we estimate that the steens mountain geomagnetic reversal occurred at 16 496 0 028 ma 0 18 ma total error our estimates of the timing for initiation of volcanism and volumetric eruptive rates do not seem to support volcanic forcing by the initial stages of columbia river basalt group eruptions as an explanation for the abrupt warming and carbonate dissolution at the beginning of the miocene climatic optimum

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

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

uplift rupture and rollback of the farallon slab reflected in volcanic perturbations along the yellowstone adakite hot spot track field geochemical and geochronological data show that the southern segment of the ancestral cascades arc advanced into the oregon back arc region from 30 to 20 ma we attribute this event to thermal uplift of the farallon slab by the yellowstone mantle plume with heat diffusion decompression and the release of volatiles promoting high k calc alkaline volcanism throughout the back arc region the greatest degree of heating is expressed at the surface by a broad ene trending zone of adakites and related rocks generated by melting of oceanic crust from the farallon slab a hiatus in eruptive activity began at ca 22 20 ma but ended abruptly at 16 7 ma with renewed volcanism from slab rupture occurring in two separate regions the eastern rupture resulted in the extrusion of steens basalt during the ascent and melting of a dry mantle plume source contaminated with depleted mantle the contemporaneous western rupture resulted in renewed subduction melting of a wet mantle source and the rejuvenation of high k calc alkaline volcanism near the nevada california border at 16 7 ma here the initiation of slab rollback is evident in the westward migration of arc volcanism at 7 8 km ma today the uplifted slab is largely missing beneath the oregon back arc region replaced instead by a seismic hole which is bound on the south by the adakite hotspot track we attribute slab destruction to thermal uplift and mechanical dislocation that culminated in rapid tearing of the slab from 17 15 ma and possible foundering and sinking of slab segments from 16 10 ma

quickly erupted volcanic sections of the steens basalt columbia river basalt group secular variation tectonic rotation and the steens mountain reversal 1 the steens basalt now considered part of the columbia river basalt group crbg contains the earliest eruptions of this magmatic episode lava flows of the steens basalt cover about 50 000 km2 of the oregon plateau in sections up to 1000 m thick the large number of continuously exposed quickly erupted lava flows some sections contain over 200 flows allows for small loops in the magnetic field direction paths to be detected for volcanic rocks this detail and fidelity are rarely found outside of the holocene and yield estimates of eruption durations at our four sections of 2 5 ka for 260 m at pueblo mountains 0 5 to 1 5 ka for 190 m at summit springs 1 3 ka for 170 m at north mickey and 3 ka for 160 m at guano rim that only one reversal of the geomagnetic field occurred during the eruption of the steens basalt the steens reversal at approximately 16 6 ma is supported by comparing 40ar 39ar ages and magnetic polarities to the geomagnetic polarity timescale at summit springs two 40ar 39ar ages from normal polarity flows 16 72 0 29 ma 16 61 and 16 92 0 52 ma 16 82 equals 2 error place their eruptions after the steens reversal while at pueblo mountains an 40ar 39ar age of 16 72 0 21 ma 16 61 from a reverse polarity flow places its eruption before the steens reversal paleomagnetic field directions yielded 50 nontransitional directional group poles which combined with 26 from steens mountain provide a paleomagnetic pole for the oregon plateau of 85 7 n 318 4 e k 15 1 a95 4 3 comparison of this new pole with a reference pole derived from crbg flows from eastern washington and a synthetic reference pole for north america derived from global data implies relative clockwise rotation of the oregon plateau of 7 4 5 0 or 14 5 5 4 respectively probably due to northward decreasing extension of the basin and range

mid miocene rhyolite volcanism in northeastern nevada the jarbidge rhyolite and its relationship to the cenozoic evolution of the northern great basin usa we present new physical geochemical geochronologic and oxygen isotope constraints on the mid miocene jarbidge rhyolite in northeastern nevada usa providing new constraints on the tectonomagmatic evolution of the cenozoic northern great basin widespread extension due to rapid collapse of the nevadaplano began at ca 17 16 ma across the northern great basin coeval with this event was compositionally bimodal basalt rhyolite volcanism that is often attributed to the inception of the yellowstone hotspot the most widespread mid miocene volcanic units in northeastern nevada are lavas and domes of the jarbidge rhyolite the thickest and most areally extensive exposures of these lavas include and are found just west of the jarbidge mountains nevada this study focuses on jarbidge rhyolite directly south of the central snake river plain adjacent to the thickest exposures in the vicinity of jarbidge nevada textures on a range of scales indicate that the jarbidge rhyolite consists primarily of phenocryst rich lavas laser 40 ar 39 ar ages for sanidine are consistent with effusive eruption of metaluminous to slightly peraluminous ferroan calc alkalic rhyolite from 16 1 to 15 0 ma prior k ar ages suggest that some activity occurred over a slightly longer duration major and trace element data coupled with new stable and prior radiogenic isotope measurements suggest that jarbidge rhyolite magmas formed primarily via melting of quartzofeldspathic crust the jarbidge rhyolite lavas are geochemically dissimilar from younger snake river plain rhyolites e g lower mgo lower nb higher rb nb and are more similar to coeval rhyolites erupted to the west on or adjacent to the oregon plateau the distribution of the jarbidge rhyolite lavas in northeastern nevada reflects an intimate association with temporally and spatially coincident extension rather than the yellowstone hotspot

chemical discrimination of columbia river basalt flows 1 the homogeneity and distinctive chemical composition of individual columbia river basalt flows are illustrated in a series of chemical plots and the procedure for flow identification is outlined the unique characterization of many individual eruptions permits units to be mapped from feeder dike to flow margins across the columbia plateau for hundreds of kilometers and so provides critical constraints on the physical and chemical evolution of this continental flood basalt province

submarine volcanic morphology of the western galapagos based on em300 bathymetry and mr1 side scan sonar 1 a compilation of high resolution em300 multibeam bathymetric and existing mr1 side scan sonar data was used to investigate the volcanic morphology of the flanks of the western galapagos islands the data portray an assortment of constructional volcanic features on the shallow to deep submarine flanks of fernandina isabela and santiago islands including rift zones and groups of cones that are considered to be the primary elements in constructing the archipelagic apron ten submarine rift zones were mapped ranging in length from 5 to 20 km comparable in length to western canary island rift zones but significantly shorter than hawaiian submarine rift zones a detailed analysis of the northwestern fernandina submarine rift including calculated magnetization from a surface towed magnetic study suggests that the most recent volcanism has focused at the shallow end of the rift small submarine volcanic cones with various morphologies e g pointed cratered and occasionally breached are common in the submarine western galapagos both on rift zones and on the island flanks where no rifts are present at depths greater than 3000 m large lava flow fields in regions of low bathymetric relief have been previously identified as a common seafloor feature in the western galapagos by geist et al 2006 however their source s remained enigmatic the new em300 data show that a number of the deep lava flows originate from small cones along the mid lower portion of the nw submarine rift of fernandina suggesting that the deep flows owe their origin at least in part to submarine rift zone volcanism

nature of the crust beneath northwest basin and range province from teleseismic receiver function data received 30 july 2007 revised 12 april 2008 accepted 2 july 2008 published 14 october 2008 1 we utilized teleseismic receiver function techniques complemented by active source refraction seismology to study the crustal structure and continental rift processes responsible for the development of the northwest corner of the basin and range province in northwest nevada our passive seismic array of 28 short period stations spanning 70 km west to east and 5 broadband usarray transportable array stations that extended our aperture to 230 km provided data on crustal properties independent of the results of our active source refraction experiment combining data from the two experiments provides better constraints on moho depth vp vs ratio and the structure and composition of the crust beneath the northwestern basin and range province than possible from either experiment alone our new data indicate moho depths that vary from 29 5 on the east to 36 5 km in the west along the passive source array and vp vs ratios that systematically span a wide range from 1 68 to 1 83 velocity modeling of data collected from the 300 km wide angle refraction reflection survey provides comparable moho depths of 32 37 km under the same region decreasing vp vs is correlated with decreasing crustal thickness with vp vs ratio 1 80 in northeast california and vp vs 1 74 in the thinner crust of the basin and range province we associate the high vp vs of northeast california with mafic additions to the crust during formation of the modoc plateau in a region of accreted paleozoic island arc terrans similar to those exposed in the eastern klamaths and northwestern sierra nevada unusually low vp vs in conjunction with low absolute vp and vs is observed beneath the central part of our array and may be due to the roots of the sierra nevada batholith now dismembered by basin and range faults back azimuthal stacking of our receiver function data suggests a northwest oriented crustal anisotropy fast direction beneath this region parallel to the northwest to southeast oriented cenozoic extension experienced by the basin and range province and suggestive of a similar flow direction in the middle to lower crust

the influence of a mantle plume head on the dynamics of a retreating subduction zone the geological record and seismic tomography together contain multiple examples of mantle plumes and subducting oceanic slabs interacting however consequences of these interactions and a mechanism for plumes to pass from the subducting to the overriding plate are uncertain we present three dimensional numerical simulations of a retreating subduction zone encountering a mantle plume head we assess the impact a plume head has on the geometry of the trench and whether it can create a window within the subducted slab in our models we consider slab plume head interactions to be a natural consequence of plume head entrainment by plate motion combined with slab retreat strong slabs prevent plume heads from reaching the surface for weak slabs strongly buoyant plume heads cause local trench advance slab windowing and accelerated slab retreat at the margins of the plume head for large plume heads this trench advance may contribute to crustal shortening in the overriding plate opening a slab window creates a potential conduit for transfer of plume material from the subducting plate to the overriding plate without terminating an active magmatic arc away from the point where the plume head tears the slab

genesis of flood basalts and basin and range volcanic rocks from steens mountain to the malheur river gorge oregon the middle and south forks of the malheur river provide a unique mapping corridor connecting two flood basalt successions steens basalt to the south and the basalt of malheur gorge to the north each contains chemically defined subtypes which merge stratigraphically across the north south length of the map area the lowermost flows of steens basalt are stratigraphically equivalent to the lowermost flows of the basalt of malheur gorge the uppermost flows of steens basalt pinch out in the map area but are partly interbedded with the middle and uppermost flows of the basalt of malheur gorge which continue to thicken northward the upper part of the tholeiitic succession is interbedded with a group of previously unrecognized lavas the venator ranch basalt flows tholeiitic volcanism ceased at ca 15 3 ma the last tholeiitic unit erupted is the hunter creek basalt which also thickens northward subsequent younger than 15 3 ma more localized eruptions were dominated by calc alkaline to mildly alkaline lavas associated with basin and range extension local uplift generated deep canyons which were filled by andesitic lavas of the keeney sequence ca 13 10 ma the final eruptive products include the devine canyon tuff ca 9 7 ma the drinkwater basalt ca 6 9 ma and the voltage flow older than 32 000 yr b p major and trace element analyses demonstrate that 1 crystal fractionation was a universal process in the derivation of the

a kinematic model for the formation of the siletz crescent forearc terrane by capture of coherent fragments of the farallon and resurrection plates 1 the volcanic basement of the oregon and washington coast ranges has been proposed to represent a pair of tracks of the yellowstone hotspot formed at a mid ocean ridge during the early cenozoic this interpretation has been questioned on many grounds especially that the range of ages does not match the offshore spreading rates and that the presence of continental coarse clastic sediments is difficult to reconcile with fast convergence rates between the oceanic plates and north america updates to basement geochronology and plate motion history reveal that these objections are much less serious than when they were first raised forward plate kinematic modeling reveals that predicted basement ages can be consistent with the observed range of about 55 49 ma and that the entire basement terrane can form within about 300 km of continental sources for clastic sediments this kinematic model indicates that there is no firm reason to reject the near ridge hotspot hypothesis on the basis of plate motions a novel element of the model is the resurrection plate previously proposed to exist between the farallon and kula plates by including the defunct resurrection plate in our reconstruction we are able to model the farallon hotspot track as docking against the oregon subduction margin starting about 53 ma followed by docking of the resurrection track to the north starting about 48 ma accretion of the farallon plate fragment and partial subduction of the resurrection fragment complicates the three dimensional structure of the modern cascadia forearc we interpret the so called e layer beneath vancouver island to be part of the resurrection fragment our new kinematic model of mobile terranes within the paleogene north american plate boundary allows reinterpretation of the three dimensional structure of the cascadia forearc and its relationship to ongoing seismotectonic processes

diverse mid miocene silicic volcanism associated with the yellowstone newberry thermal anomaly the santa rosa calico volcanic field sc of northern nevada is a complex multi vent mid miocene eruptive complex that formed in response to regional lithospheric extension and flood basalt volcanism santa rosa calico volcanism initiated at 16 7 ma concurrent with regional steens columbia river flood basalt activity and is characterized by a complete compositional spectrum of basalt through high silica rhyolite to better understand the relationships between upwelling mafic magmatism coeval extension and magmatic system development on the oregon plateau we have conducted the first compre hensive study of santa rosa calico silicic volcanism detailed stratigraphic based field sampling and mapping illustrate that silicic activity in this volcanic field was primarily focused along its eastern and western margins at least five texturally distinct silicic units are found in the western santa rosa calico volcanic field including abun dant lava flows near vent deposits and shallow intrusive bodies similar physical features are found in the eastern portion of the volcanic field where four physically distinct units are present the western and eastern santa rosa calico units are characterized by abundant macro and microscopic disequilibrium textures reflecting a complex petrogenetic history additionally unlike other mid mio cene oregon plateau volcanic fields e g mcdermitt the santa rosa calico volcanic field is characterized by a paucity of caldera forming volcanism only the cold springs tuff which crops out across the central portion of the volcanic field was caldera derived major and trace element geochemical variations are present within and between eastern and western santa rosa calico silicic units and these chemical differences coupled with the observed disequilibrium textures illustrate the action of open system petrogenetic processes and melt derivation from heterogeneous source materials the processes and styles of santa rosa calico silicic magmatism are linked to three primary factors local focusing of and thermal and material contributions from the regional flood basalt event lithospheric extension within the northern portion of the northern nevada rift and interaction of mid miocene silicic magmas with pre santa rosa calico lithosphere similar processes and styles of mid miocene silicic volcanism likely occurred across the oregon plateau in regions characterized by both focused lithospheric exten sion and localized mafic magmatism

relationship of epithermal gold deposits to large scale fractures in northern nevada geophysical maps of northern nevada reveal at least three and possibly six large scale arcuate features one of which corresponds to the northern nevada rift that possibly extends more than 1 000 km from the oregonidaho border to southern nevada these features may reflect deep discontinuities within the earth s crust possibly related to the impact of the yellowstone hot spot because mid miocene epithermal gold deposits have been shown to correlate with the northern nevada rift we investigate the association of other epithermal gold deposits to other similar arcuate features in northern nevada mid miocene and younger epithermal goldsilver deposits also occur along two prominent aeromagnetic anomalies west of the northern nevada rift here we speculate that mid miocene deposits formed along deep fractures in association with mid miocene riftrelated magmatism and that younger deposits preferentially followed these preexisting features statistical analysis of the proximity of epithermal gold deposits to these features suggests that epithermal gold deposits in northern nevada are spatially associated with large scale crustal features interpreted from geophysical data

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

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

relation of flat subduction to magmatism and deformation in the western united states flat subduction of the farallon plate beneath the western united states during the laramide orogeny was caused by the combined effects of oceanic plateau subduction and unusually great suction in the mantle wedge the latter of which was caused by the shallowing slab approaching the north american craton root once in contact with basal north america the slab cooled and hydrated the lithosphere upon removal asthenospheric contact with lithosphere resulted in magma production that was especially intense where the basal lithosphere was fertile in what now is the basin and range province and this heating weakened the lithosphere and made it convectively unstable small scale convection has since affected many areas with slab sinking and unloading the western united states elevated into a broad plateau and the weak portion gravitationally collapsed with development of a transform plate boundary the western part of the weak zone became entrained with the pacifi c plate and deformation there is dominated by shear 2 humphreys mwr204 04 1st pages e surface heat flow p wave velocity d seismic velocity at 100 km background s wave velocity 2 100 175 km c s wave velocity

carson pass kirkwood paleocanyon system paleogeography of the ancestral cascades arc and implications for landscape evolution of the sierra nevada california tertiary strata of the central sierra nevada are dominated by widespread voluminous volcanic breccias that are largely undivided and undated the origin of which is poorly understood these dominantly andesitic strata are interpreted to be eruptive products of the ancestral cascades arc deposited and preserved within paleocanyons that crossed the present day sierra nevada before basin and range faulting began there these strata are thus important not only for understanding the paleogeography of the ancestral cascades arc but also for reconstructing the evolution of the sierra nevada landscape our regional scale mapping shows that paleocanyon fills of the central sierra nevada are dominated by intrusions and vent proximal facies along the present day sierran crest with more distal facies extending westward down the paleocanyons vent proximal facies consist of lava domes that collapsed to generate block and ash flow tuffs which in turn were remobilized down canyon to produce coarse grained volcanic mudflow and dilute flow fluvial deposits lava flows are rare instead magmas invaded wet volcaniclastic sediment to form in situ peperite piles that were partly remobilized to form debris flows or block and ash flows with peperite domains detailed mapping and dating of previously undifferentiated tertiary strata in the carson pass kirkwood area of the central sierra nevada has allowed us to identify 6 unconformity bounded sequences preserved within a paleocanyon as deep as 650 m cut into mesozoic granitic basement the carson pass kirkwood paleocanyon trends ne sw with a paleo transport direction roughly parallel to the modern mokelumne river drainage toward the sw sequence 1 consists of oligocene silicic ignimbrites sourced from nevada sequences 2 through 6 consist of dominantly andesitic rocks of the miocene ancestral cascades arc with 40 ar 39 ar ages ranging from 14 69 0 06 ma to 6 05 0 12 ma these new dates provide constraints on the ages of unconformities vertical relief on the unconformities within the paleocanyon ranges from 12 to 303 meters with paleoslope gradients range from 3 to 48 no evidence exists for widening or deepening of the paleocanyon into mesozoic basement during tertiary time so we infer that unconformity 1 the paleocanyon floor and walls was inherited from cretaceous time the deepest unconformities within the paleocanyon fill reincised into granitic basement in the early miocene unconformity 2 between 14 and 10 ma unconformity 5 and between 10 and 6 ma unconformity 6 the paleocanyon was also beheaded cut off from sources to the east by ca 10 ma we suggest that the early miocene reincision records tectonism related to the onset of arc magmatism in the sierra nevada the middle miocene reincision may record the onset of basin and range faulting in the central sierra the late miocene reincision may correspond to uplift attendant with the northward sweep of the triple junction through the latitude of the central sierra paleocanyons of the central sierra differ from those of the northern and southern sierra by showing steeper local paleorelief and the bouldery stream deposits attest to higher axial gradients than envisioned for other parts of the range this may indicate that the uplift history of the range is not uniform from segment to segment

upper mantle seismic structure beneath the pacific northwest a plume triggered delamination origin for the columbia river flood basalt eruptions abstract we invert teleseismic p and s body waves constrained by an ambient noise surface wave model and moho depth inferred from receiver function analysis gao et al 2011 to image mantle structures continuously from the surface to the base of the upper mantle the major structures coincide with prominent geological features we focus on a ne oregon structure termed here the wallowa anomaly which coincides with the source area for the 16 ma columbia river flood basalt eruptions and a circular area of topographic relief created during and after these eruptions resolution tests indicate that the curtain like structure previously interpreted as farallon lithosphere connects with the wallowa anomaly above 150 km along the northeast margin of the wallowa anomaly this connection along with the pre flood basalt magmatic and tectonic history of the pacific northwest lead us to conclude that arrival of the yellowstone plume to south central oregon initiated delamination of remnant farallon lithosphere from the base of ne oregon exposing ocean crust to yellowstone asthenosphere this hypothesis accounts for the propagation of flood basalt volcanism far north of the yellowstone hotspot track and for the high silica composition of most of the flood basalt magmas

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

the role of oceanic plateau subduction in the laramide orogeny the cause of the laramide phase of mountain building remains uncertain conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant hess or shatsky oceanic plateaux independent verification of these models has remained elusive because the putative ridges or plateaux are no longer at the earth s surface inverse convection models have identified two prominent seismic anomalies on the recovered farallon plate here we combine inverse convection models with reconstructions of plate motions to show that these seismic anomalies coincide palaeogeographically with the restored positions of the shatsky and hess conjugate plateaux as they subducted beneath north america specifically the distribution of laramide crustal shortening events tracked the passage of the shatsky conjugate beneath north america whereas the effects of the hess conjugate subduction were restricted to the northern mexico foreland belt we propose that continued subduction caused the oceanic crust to undergo the basalt eclogite phase transformation during which the shatsky conjugate lost its extra buoyancy and was effectively removed increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward followed by regional scale surface rebound we conclude that laramide uplift resulted from the removal rather than emplacement of the shatsky conjugate

thermal structure beneath the snake river plain implications for the yellowstone hotspot basaltic magmatism associated with the yellowstone hotspot has been widely attributed to upwelling of a mantle plume yet the temporal and spatial distribution of these magmas and their compositional characteristics are distinctive from oceanic hotspot magmatism fundamental questions concern the influence of continental cratonic lithosphere in producing the differences and the extent to which upper plate processes contribute to magma production to better understand scenarios of melt generation p t conditions are estimated for segregation of primitive snake river plain srp basalts from the mantle combined with analysis of trace element and seismic constraints we conclude from this that 1 melt production was concentrated at depths between roughly 70 100 km 2 mantle temperature was only slightly higher than ambient conditions with a maximum potential temperature of 1450 c and 3 the mantle source was relatively fertile mg 90 these results suggest that the seismically imaged plume below yellowstone is significantly cooler than upwellings beneath hawaii iceland and many other oceanic hotspots our findings in combination with other geochemical and geodynamic considerations are permissive of magma generation within the ancient lithospheric mantle keel associated with the wyoming craton plume contributions while not excluded involve physical and geochemical implications that suggest they are subordinate

intermediate composition magma production in an intracontinental setting unusual andesites and dacites of the mid miocene santa rosa calico volcanic field northern nevada abstract the mid miocene santa rosa calico volcanic field sc of northern nevada provides an outstanding example of the role open system magmatic processes play in producing calc alkaline and tholeiitic andesite dacite magmas in an intracontinental setting sc volcanism commenced at 16 7 ma and is associated with the initial manifestations of the yellowstone hotspot the columbia river steens flood basalt event s and the formation of the northern nevada rift locally a diverse package of magmatic products ranging from tholeiitic basalt to high si rhyolite was produced during an 2 myr duration within this package are the products of at least four distinct intermediate composition magmatic systems that may represent as much as 40 of the sc volcanic pile these help differentiate the sc from contemporaneous oregon plateau volcanic fields e g mcdermitt lake owyhee northwest nevada that are dominated by bimodal basalt rhyolite assemblages all sc intermediate units are characterized by textural and mineralogic complexities including xenoliths and xenocrysts of local crust and crystal clots of plagioclase clinopyroxene orthopyroxene oxide sc intermediate units are dominantly tholeiitic but include lava flows with transitional to calc alkaline affinities relative to locally erupted steens basalt sc intermediate lava flows have similar elemental enrichments and depletions but dissimilar sr and nd isotopic compositions these isotopic differences coupled with the abundant disequilibrium features and variable incompatible element ratios indicate that open system magmatic processes played a major role in the genesis of the intermediate units sc silicic magmas were produced primarily via upper crustal melting of chemically and isotopically heterogeneous cretaceous granitoid interaction between fractionating mafic steens flood basalt magmas and the more evolved crustal melts assimilation of local upper crust provides a general template for the generation of sc tholeiitic and calc alkaline intermediate magmas between system heterogeneities primarily reflect magma versus assimilation dominated mixing these complex processes and the resulting production and eruption of sc intermediate composition melts were stimulated by incipient focused lithospheric extension and driven by the virtually continuous local availability of upwelling steens basalt over at least a 2 myr duration these results demonstrate that at least some of the mid miocene and younger calc alkaline magmas that are found across the pacific northwest and are unrelated to subduction processes were formed as a direct result of open system processes affecting regional flood basalt magmas rather than just melting of previously enriched via subduction lithospheric mantle as suggested by previous workers

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