strain softening along the mct zone from the sikkim himalaya relative roles of quartz and micas abstract in the darjeeling sikkim himalaya two distinct faults form the main central thrust mct the structurally higher mct1 and the lower mct2 each has accommodated translation greater than 100 km the lower mct2 places greater himalayan amphibolite grade paro lingtse gneiss over lesser himalayan greenschist grade daling metapelites the mct2 is folded by the underlying lesser himalayan duplex and is exposed at different structural positions of the fold at pelling the mct2 zone is exposed as a 373 m thick nw dipping fault zone that exposes 19 m of hanging wall mylonitized lingtse gneiss the lingtse protolith shows evidence of amphibolite grade plastic deformation features in quartz and feldspar within the hanging wall mylonite zone hwmz quartz and feldspar have undergone grain size reduction by different deformation mechanisms and feldspars are sericitized suggesting the presence of fluids during deformation we estimate a temperature of 300 c within the fault zone during fluid assisted retrogression and deformation reaction softening of feldspars produced a large proportion of intrinsically weak matrix this in combination with development of a strong foliation defined by parallel mica grains resulted in strain softening along the mct2 zone and concentrated the deformation along a thin zone or zones

cenozoic tectonic evolution in the central andes in northern chile and west central bolivia implications for paleogeographic magmatic and mountain building evolution a review of available stratigraphic structural and magmatic evolution in northernmost chile and adjacent peru and bolivia shows that in this region 1 compression on the paleogene intra arc during the middle eocene incaic phase formed the nne ssw oriented incaic range along the present day precordillera and western cordillera and 2 post incaic tectonic conditions remained compressive until present contrasting with other regions of the andes where extensional episodes occurred during part of this time lapse a late oligocene early miocene peak of deformation caused further uplift the incaic range formed a pop up structure bounded by two thrusts systems of diverging vergencies it represented a major paleogeographic feature that separated two domains with different tectonic and paleogeographic evolutions and probably formed the andean water divide this range has been affected by intense erosion and was symmetrically flanked by two major basins the pampa del tamarugal and the altiplano magmatic activity remained located along the previous late cretaceous early eocene arc with slight eastward shift further compression caused westvergent thrusting and uplift along the western eastern cordillera bounding the altiplano basin to the east by another pop up shaped ridge eastward progression of deformation caused eastvergent thrusting of the eastern cordillera and subandean zone

rheomorphic ignimbrites of the rogerson formation central snake river plain usa record of mid miocene rhyolitic explosive eruptions and associated crustal subsidence along the yellowstone hotspot track rogerson graben usa is critically placed at the intersection between the yellowstone hotspot track and the southern projection of the west snake river rift eleven rhyolitic members of the re defined 420 m thick rogerson formation record voluminous high temperature explosive eruptions emplacing extensive ashfall and rheomorphic ignimbrite sheets yet each member has subtly distinct field chemical and palaeomagnetic characteristics new regional correlations reveal that the brown s view ignimbrite covers 3300 km2 and the wooden shoe ignimbrite covers 4400 km2 and extends into nevada between 11 9 and 8 ma the average frequency of large explosive eruptions in this region was 1 per 354 ky about twice that at yellowstone the chemistry and mineralogy of the early rhyolites show increasing maturity with time possibly by progressive fractional crystallisation this was followed by a trend towards less evolved rhyolites that may record melting and hybridisation of a mid crustal source region contemporaneous magmatism induced crustal subsidence of the central snake river basin is recorded by successive ignimbrites offlapping and thinning up the n facing limb of a regional basin margin monocline which developed between 10 59 and 8 ma the syn volcanic basin topography contrasted significantly with the present day elevated yellowstone hotspot plateau concurrent basin and range extension produced the n trending rogerson graben early uplift of the shoshone hills 10 34 ma was followed by initiation of the shoshone fault and an e sloping half graben 10 3 10 1 ma the graben asymmetry then reversed with initiation of the brown s bench fault 8 ma which remained intermittently active until the pliocene

lithospheric extension near lake mead nevada a model for ductile flow in the lower crust small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions in this study we examine the boundary between the unextended colorado plateau and a strongly extended domain in the basin and range province in the lake mead nevada region bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required finite element modeling of newtonian flow and power law creep shows that flow over the length scale of the eastern basin and range 500 km or more corresponding to upper crustal extension by a factor of 1 4 3 over 10 m y requires effective viscosities less than 10 18 10 20 pa s for ductile channels 10 25 km thick flow over shorter length scales 150 km may be accommodated with effective viscosities as high as 10 21 pa s modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory derived power law creep rates the longer scale flow may require elevated crustal temperatures more than 700 c depending on the composition and material properties assumed under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep this work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions and thereby explaining the relative uniformity in gravity and topography between such regions

crust and upper mantle structure beneath the pacific northwest from joint inversions of ambient noise and earthquake data 1 we perform a joint inversion of phase velocities from both earthquake and ambient noise induced rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the pacific northwest we focus particularly on the areas affected by mid miocene to present volcanic activity the joint inversion combined with the high density seismic network of the high lava plains seismic experiment and data from the earthscope transportable array provides outstanding resolution for this area in oregon we find that the pattern of low velocities in the crust and uppermost mantle varies between the high lava plains physiographic province and the adjacent northwestern basin and range these patterns may be due to the presence of the brothers fault zone which separates the clockwise rotating northwest basin and range from the relatively undeformed areas further north further to the east the owyhee plateau snake river plain srp and northeastern basin and range are characterized by high crustal velocities though the depth extent of these fast wave speeds varies by province of particular interest is the mid crustal high velocity sill previously only identified within the srp we show this anomaly extends significantly further south into utah and nevada we suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid crustal sill structures within the srp

evolution of microstructures in precambrian shear zones an example from eastern india abstract shear zones are areas of intense deformation in localized zones which can be used as natural laboratories for studying deformation characteristics metre to micro scale structures that develop in response to a progressive simple shear in a shear zone are characterized by a protracted history of deformation and are immensely useful in delineating the history of progressive deformation to decipher these localized zones of deformation and to establish the continuous non coaxial character of deformation detail microstructural studies are very useful singhbhum shear zone ssz a regional precambrian tectonic dislocation zone in eastern india depicting a top to south thrust movement of the hanging wall provides a scope for studying microstructural characteristics developed in response to a progressive shear at mid crustal level ssz is characterized by intense stretching lineation isoclinal folds shear planes superposed schistosity and deformed quartz veins quasi plastic qp deformation mechanisms were predominantly active in the ssz the overprinting relationship between the earlier and later schistosity with a consistent sense of shear indicates that earlier schistosity is transposed to later schistosity through the intermediate stages of crenulation cleavage during a progressive non coaxial deformation the recrystallization of quartz in mylonitic quartzite suggests protracted history of deformation the analysis of the character of quartz grains of both the porphyroclasts and recrystallized grains suggests that strain was partitioned between the most intensely deformed central part of the shear zone and the shear related deformation zone outside the central part of the shear zone

teleseismic time functions for large shallow subduction zone earthquakes broadband vertical p wave records are analyzed from 63 of the largest shallow subduction zone earthquakes which have occurred in the circum pacific in the last 45 yr most of the records studied come from a common instrument the pasadena california benioff 1 90 seismometer propagation and instrument effects are deconvolved from the p wave records using a damped least squares inversion to obtain the teleseismic source time function the inversion has the additional constraint that the time function be positive everywhere the period band over which the time functions are considered reliable is from 2 5 to 50 sec fourier displacement amplitude spectra computed for each of the 1 90 p wave trains indicate spectral slopes measured between 2 and 50 sec of 1 0 to 2 25 with an average value of 1 5 these values assume an average attenuation of t 1 0 the seismic moments derived from the p wave time functions compare well with other published values for earthquakes having moments smaller than 2 5 10 28 dyne cm m w 8 2 because the 1 90 seismometer has little response at very long periods this technique underestimates the moments of the very largest events the time functions are characterized using five parameters 1 spectral slope between 2 and 50 sec 2 roughness of the time function 3 multiplicity of sources 4 pulse widths of individual sources and 5 overall signal duration the 63 earthquakes studied come from 15 subduction zones with a wide range in the ages of subducted lithosphere convergence rates and maximum size of earthquakes comparing the time function parameters with age rate and m w of the subduction zone does not yield obvious global trends however most of the subduction zones do behave characteristically and can be grouped accordingly

kinematic analysis of fractures in the great rift idaho implications for subsurface dike geometry crustal extension and magma dynamics 1 extension across the southern great rift of the eastern snake river plain esrp idaho was measured to calculate the dimensions of underlying dikes and interpret magmatic and extensional processes cumulative rift perpendicular extension ranges from 0 64 to 4 50 m along the 14 km long kings bowl segment from 1 33 to 4 41 m along the 14 km long new butte segment and from 0 74 to 1 57 m along the 4 km long minidoka segment along strike of each segment extension increases toward coeval vents each rift segment is interpreted to be underlain by a subsurface dike whose dimensions are calculated using buoyancy equilibrium and boundary element models dikes are calculated to have tops that are 950 530 m deep bottoms that are 23 31 km deep and widths that taper to zero from a maximum of 2 21 m modeling suggests that the kings bowl dike has a maximum probable width of 8 m and a volume of 2 km3 about 400 times the volume of its coeval lava flow dike widths and ages at the southern great rift provide evidence for a holocene esrp strain rate of about 1 to 3 10 16 s 1 which is as much as an order of magnitude slower than strain rates in the adjacent seismically active basin and range province eruptive fissures are present where rift width is 1650 m this corresponds to a depth to dike top of 700 m which we propose was the depth where vesiculation initiated thus increasing magma pressure and inducing eruption

kinematics and timing of three superposed extensional systems east central idaho evidence for an eocene tectonic transition cenozoic crustal extension in east central idaho began about 50 ma and continues at present three distinct episodes characterize one of the longest intervals of cenozoic extension yet documented in the north america cordillera crosscutting relationships between ne striking normal faults and volcanic rocks regional dike trends and slickenline data indicate nw se extension during peak eocene volcanism about 49 48 ma episode 1 ne striking normal faults with at most a few kilometers of offset formed in an intraarc setting during rapid ne subduction of oceanic plates under the pacific northwest north to nnw striking and west dipping normal faults with offsets up to 10 15 km formed during a younger middle eocene to oligocene basin forming event episode 2 this newly documented episode was the most important extensional event in east central idaho and began during the waning phases of challis volcanism wsw ene to sw ne extension during episode 2 was nearly perpendicular to the extension direction during episode 1 and perpendicular to the grain of the idaho montana fold and thrust belt the flip in extension direction between episode 1 and episode 2 is tightly constrained by 40ar 39ar age determinations to have taken place at the end of eocene challis magmatism about 46 48 ma i infer that plate boundary forces controlled the geometry of normal faults and dikes during episode 1 whereas internal stresses within previously thickened crust drove major sw to wsw directed extension during episode 2 a drop in convergence rates between the north american and farallon plates between 59 ma and 42 ma stock and molnar 1988 may coincide with the onset of gravitational spreading during episode 2 and may also explain the abrupt end of eocene magmatism in the pacific northwest miocene and younger sw dipping basin and range faults episode 3 extended the region in a ne sw direction although faults formed during episode 2 and episode 3 are not parallel slickenlines indicate only small changes in slip vector trends suggesting little rotation of the extension direction in east central idaho since 46 ma

what sets topographic relief in extensional footwalls we use three large normal fault arrays in the northeastern basin and range province western united states to document catchment development and relief production during fault growth fault slip and slip rates increase systematically along strike from zero at the fault tips catchment relief and across strike range width both increase as slip accumulates but reach maximum values at a distance of 15 km from the fault tips and remain uniform along strike over much of the footwalls catchment outlet spacing also increases away from the fault tips but does not reach a uniform value and may vary by a factor of 5 6 along strike we infer that catchments first elongate in the across strike direction as slip accumulates and the range half width increases once the half width reaches its maximum value continued catchment growth is possible only by along strike capture which increases outlet spacing but not relief the close correspondence between catchment relief and range half width suggests that geomorphically limited hillslope and channel gradients are achieved within the 15 km tip zone thus the limiting factor in footwall development is the width of the range which is controlled by two external agents the geometry and spacing of the major faults and the elevations of base level on both flanks

temperature of the plume layer beneath the yellowstone hotspot recent studies show that the yellowstone hotspot is associated with a plume like lowvelocity pipe that ascends from the transition zone to the lithosphere asthenosphere boundary where the plume is sheared to the southwest by north american plate motion rayleigh wave tomography shows this plate sheared plume layer has an extremely low s wave velocity of 3 8 0 1 km s at 80 km depth 0 15 0 3 km s lower than the velocity observed beneath normal mid ocean ridges to constrain the temperature of the plume layer a grid search with respect to grain size and temperature is performed to fi observed rayleigh wave phase velocities this search fi nds that the excess temperature of the plume layer is 55 80 c at 95 confi dence for two different temperature velocity and two different melt velocity models confi rming that a thermal mantle plume exists

crustal structure and thickness along the yellowstone hot spot track evidence for lower crustal outflow from beneath the eastern snake river plain 1 receiver functions from seismic stations about the yellowstone hot spot track are migrated to depth using a vp vs map constructed from stacking of the direct and free surface moho reverberations i e h k analysis and a shear velocity tomogram constructed from surface wave measurements the thickest crust 48 54 km resides in the wyoming province beneath the sampled laramide age blocks and the thinnest crust 32 37 km resides in the montana basin and range province the eastern snake river plain esrp crust is thickest 47 km at its ne end beneath the young calderas and thinnest 40 km at its sw end beneath the older twin falls caldera two esrp crustal thickness domains are found 1 at the older twin falls and picabo calderas the mean esrp crust is 4 km thicker with respect to its margins and 2 adjacent to the heise caldera field the mean esrp crust is 4 km thicker with respect to its se margin crust but no thicker with respect to its nw margin crust this lobe of anomalously thick crust is explained as resulting from lower crustal outflow from beneath the heise caldera field confirmation of these crustal thickness variations is provided by inspection of common conversion point ccp stacks that delineate several secondary features the top of a thick high velocity 3 9 km s lower crust layer within the wyoming province up to 17 km thick and a paired negative and positive amplitude arrival at 12 km depth and 18 km depth beneath the yellowstone caldera this paired arrival would be consistent with a low velocity zone perhaps associated with magma staging beneath the caldera our most important finding is that the magmatic loads injected into the esrp crust over the last 4 12 myr in tandem with the esrp crustal viscosity structure have been sufficient to drive significant outflow of the esrp lower crust

soil engineering properties and failure mechanisms of shallow landslides in soft rock materials abstract tertiary soft rock strata exposed on the eastern side of changbai mountain are landslide prone strata in recent years shallow landslides have frequently occurred along the highway in this region leading to great challenges in highway construction and safe operations national highway 302 which is under construction crosses an old landslide group close to the village of xinyan a shallow landslide that recently occurred in this section the xinyan landslide which occurred in a road embankment composed of soft rock materials is studied herein as a case study the field survey identifies the geological characteristics and current conditions of the landslide area and confirms an inheritance relationship between the xinyan landslide and previous landslides through a laboratory geotechnical test mineral composition analysis and microstructure analysis of landslide soils it is found that the examined soft rock materials exhibit both dispersion and expansion the coupling effect of soil expansion and dispersion contribute to the formation of seepage channels and to the degradation of soil properties under the combined effects of these factors local shear failure first occurs at the weakest toe of the embankment then unloading effects and strain softening lead to the progressive propagation and expansion of the sliding surface finally the failure of the xinyan landslide enters a progressive failure mode from the slope toe to the interior area therefore this study reveals that expansibility dispersivity extremely low shear strength levels softening behavior and preferential flows are the main causes of the repeated failure of the gentle soft rock slopes in this region these results may serve as a good reference for the prevention and treatment of similar soft rock landslides occurring in the yanbian region or worldwide

density and lithospheric strength models of the yellowstone snake river plain volcanic system from gravity and heat flow data abstract the structure and composition of the yellowstone snake river plain ysrp volcanic system were analyzed using gravity data taken at over 30 000 stations in the ysrp and surrounding region additional constraints were provided by tomographic seismic velocity models new heat flow and temperature information gps derived strain rates earthquake locations and chemical analyses of volcanic rocks p wave velocity models and velocity density data based on petrologic information were used to constrain three dimensional density models rheology and strength properties were calculated at selected locations and compared to earthquake focal depths results of this study suggest that the lower crust of the snake river plain srp has been thickened by the addition of an underplated layer composed primarily of clinopyroxene having a density of 3200 kg m3 a mid crustal high density sill is composed of a series of gabbroic lenses inter fingering with the granitic upper crust this geometry yields a bulk composition comparable to diorite and a density of 2900 kg m3 the mid crustal sill varies from 4 to 11 km in thickness resulting in a series of se nw trending gravity highs observed along the axis of the srp the mid crustal sill extends up to 20 km southeast of the srp volcanic field and causes asymmetry of the gravity field the yellowstone plateau volcanic field density model reveals low density partial melt 10 km beneath the caldera that shallows under the northeastern caldera and continues laterally 20 km north of the caldera boundary and notably increases the previously estimated size of the magma reservoir by 20 the caldera melt body has a density of 2520 kg m3 and a significantly lower value of 2470 kg m3 for the northeastern caldera melt body southwest of yellowstone the crustal section occupied by the mid crustal sill in the srp and the partial melt in yellowstone constitute a transition area between the active yellowstone magma system and the now volcanically quiescent srp with a density of 2820 kg m3 strength models show that the crust of the ysrp becomes progressively stronger and cooler with increasing distance from yellowstone and tectonic earthquakes within the yellowstone caldera are unlikely to nucleate below 4 6 km depth thus limiting the maximum magnitude of earthquakes to m 6 5

shear wave velocities and crustal structure of the eastern snake river plain idaho the lithospheric velocity structure beneath the eastern snake river plain esrp is inferred from data on rayleigh wave dispersion periods 6 s to 40 s and teleseismic shear wave delays as well as other data from refraction profiling heat flow volcanic history and geology the unusual velocity structure indicates a 12 km thick high velocity layer depth 8 to 20 km with p and s velocities of 6 54 and 3 87 km s respectively this overlies a 21 km thick lower crust with p and s velocities of 6 82 and 3 61 km s respectively p velocities and layer thicknesses are based on our interpretation of refraction data braile et al 1979 and are similar to the results of sparlin et al 1979 upper mantle s velocity is 4 1 4 2 km s and a lid appears to be absent or weakly developed although the mild shear velocity inversion in the lower crust might result from viscous effects possibly due to partial melt an anomaly in mineral composition is the preferred explanation this may be a major depletion of quartz as a result of its fusion into the voluminous rhyolitic magmas which erupted onto the esrp between 10 and 6 m y ago the overlying high velocity layer is thought to comprise acidic and basic intrusions of late cenozoic age and archean granitoid rocks whose velocity and density may have been increased by thermal metamorphism related to plutonism

mass transfer along the yellowstone hotspot track ii kinematic constraints on the volume of mantle derived magma abstract this paper evaluates the neogene kinematic evolution of the eastern snake river plain esrp to estimate the volume of mafic magma transferred from mantle to crust during passage over the yellowstone hotspot four major kinematic processes affected the esrp including densification due to emplacement of gabbro into the middle crust subsidence and consequent filling of a 6 km deep basin with volcanic and plutonic rocks 20 extension via dike injection or faulting ductile attenuation and lower crustal flow from esrp to the adjacent basin and range two kinematic models involving these processes are proposed overall the two models are viable only if the initial crustal thickness were 43 47 km thick otherwise the lost mass of the esrp is not balanced by the excess mass in the basin and range in the first model densification eruption and extension by dike injection the esrp crust receives mantle derived gabbro equivalent to a 16 km thick layer but would need to lose 10 km of crust via lower crustal flow in the second model densification eruption and extension by faulting and ductile attenuation the esrp crust receives a 9 4 km equivalent thickness of mantle derived gabbro and would need to lose 6 km of crust via lower crustal flow the cumulative amount of mantle derived gabbro in the esrp crust derived by this kinematic analysis equivalent to a 9 4 16 km thick layer is nearly identical to the amount estimated independently using a mass balance petrologic model suggesting it is a robust result

recent magmatotectonic activity in the eastern snake river plain island park region revealed by sar interferometry abstract synthetic aperture radar interferometry insar has been applied in this study to address crustal deformation in a 10 000 km 2 region located immediately west of the yellowstone hotspot insar results show that surface movements in the study area were non linear and episodic during the period of observation 1993 2006 the island park region and its adjacent eastern snake river plain esrp were characterized by northeast trending zones of uplift 1 cm yr 1 and subsidence 1 cm yr 1 with various extents through time the western edge of yellowstone caldera experienced episodes of subsidence 1 cm yr 1 during 1997 2000 and uplift 3 cm yr 1 during 2004 2006 differential surface movements of varying rates were also detected between 1993 and 2006 in the vicinity of basin and range normal faults to the north of henrys fork caldera throughout the study area surface displacements across the island park region the esrp and the adjacent basin and range province generally reversed the movement direction in 2004 in concert with displacement reversal to uplift in the yellowstone caldera crustal deformation in the general vicinity of major basin and range faults is interpreted to reflect diffuse extensional strain adjacent to the deeper segments of faults rather than near surface slip the northeast trending displacement zones in the esrp and the island park region may indicate folding in response to converging zones of extension in the surrounding basin and range province surface displacements of the yellowstone caldera are interpreted to reflect migration of magma or hydrothermal fluids the inverse relation between vertical displacements in yellowstone and its surrounding regions may reflect an upper crustal flexural response or a large scale movement of hydrothermal fluids though neither hypothesis is completely supported by the processed data

planar high angle faulting in the basin and range geodetic analysis of the 1983 borah peak idaho earthquake geodetic elevation changes record the broad scale deformation associated with the m 7 0 october 28 1983 borah peak idaho earthquake on the lost river fault the crest of the lost river range rose 0 2 m and adjacent thousand springs valley subsided 1 0 m in relation to reference points 45 km from the main shock epicenter the deformation was modeled by dislocations in an elastic half space a planar fault with uniform dip slip of 2 05 0 10 m dipping 47 o 2osw and extending to a vertical depth of 13 3 1 2 km fits the geodetic data best and is also consistent with the main shock hypocenter and fault plane solution the geodetic moment is 2 6 0 5 x 1019 n m 2 6 0 5 x 1026 dyn cm and the estimated static stress drop is 2 9 0 4 mpa 29 4 bars tests for coseismic slip on listric faults which flatten with depth and on detachments horizontal faults or shear zones showed fits to the geodetic data that are inferior to those for planar high angle faults no detectable coseismic slip occurred on the mesozoic white knob thrust fault although the low angle thrust sheet intersects the south end of lost river fault near the 1983 mainshock epicenter if the high angle lost river fault abuts a flat lying detachment fault or shear zone such a structure must lie at depths of 12 km near the brittle ductile transition where stick slip behavior gives way to creep the depth and geometry of faulting at borah peak is similar to that inferred from seismic and geodetic evidence for the 1954 m 7 2 fairview peak nevada and the 1959 m 7 3 hebgen lake montana events suggesting that if detachments are active at these localities they are deep and most likely slip by creep

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

lithospheric heating by mantle plumes summary a broad topographic swell is associated with the hawaiian hotspot a kinematic model is developed assuming that downstream of the hotspot this swell is produced by lithospheric heating by a mantle plume material from the model plume flows radially in the asthenosphere and eventually heats the lower lithosphere upstream from the hotspot the model swell owes its existence mainly to hot material at asthenospheric depths the model predicts extensive lithospheric heating downstream from the hotspot interpreted data to tell if the lower lithosphere behaves in this way are lacking

late quaternary fault segmentation from analysis of scarp morphology a thorough knowledge of the lateral extent of individual prehistoric ruptures along a fault is necessary to assess the locations and geologic characteristics of boundaries between independent faultrupture segments paleoseismic studies employing only trenching may yield insufficient data to evaluate the lateral extent of individual rupture events systematic mapping of fault scarps and degradation equation modeling of relative scarp ages on the lemhi fault a range bounding normal fault in east central idaho provide a method to assess rupture extent and boundaries between rupture segments the youngest latest quaternary surface ruptures occurred at distinctly different times on adjacent segments of the fault the boundaries between rupture segments differ in characteristics although most are at intersections of the currently active range bounding fault and mid tertiary normal faults several major structural and tectonic discontinuities did not form segment boundaries in the most recent rupture cycle or probably in the previous cycle the lemhi fault illustrates both the importance of preexisting structures in controlling extent of ruptures and the need for detailed analysis along an entire fault zone

lower crustal flow in an extensional setting constraints from the halloran hills region eastern mojave desert california postextensional uplift and tilting of quaternary lake sediments in the halloran hills eastern mojave desert california suggest that regional scale flow within the lower crust is occurring in response to tectonic loads created during upper crustal extension in late miocene time the concept that significant flow within the lower crust may occur for millions of years after the cessation of extension indicates that topographic gradients can be modified by regional scale flow within the lower crust and that direct constraints on lower crustal flow can be obtained by quantitative analysis of rates of postextensional surface tilting an analytic solution developed in this paper combines viscous flow in the lower crust flexural bending within the midcrust and isostasy into an invertible linear differential equation that describes the topographic response to upper crustal extension assuming that there was no topographic relief across the halloran hills region prior to extension inversion of modern topographic data indicates that crustal thinning above the halloran hills detachment increases westward with a listric geometry and that little crustal thinning has occurred east of the western flank of clark mountain in good agreement with the known geometry of the fault and the location of its breakaway zone incorporation of quaternary tilting data indicates that the modern viscosity beneath the halloran hills assuming a 10 km lower crustal channel is a maximum of 1019 pa s and that the viscosity of the lower crust has decreased by at least 1 to 2 orders of magnitude since 8 ma this corresponds to a temperature increase of at least 75 to 100 c at the moho in our opinion the most likely source of this temperature increase is a regional scale thermal event within the underlying mantle and diffusion of heat upward into the lower crust if correct these results have important implications for the way in which crustal extension is linked to mantle heating within the basin and range province

ignimbrites of the eastern snake river plain evidence for major caldera forming eruptions the eastern snake river plain is a predominantly rhyolitic province analogous to the present day yellowstone plateau volcanic field but older and further evolved the widespread quaternary basaltic lavas that now blanket the eastern snake river plain appear to have erupted after major rhyolitic activity ceased and represent less than 20 of the total volume of volcanic rock near the north central margin of the plain the rhyolites show a general age progression that becomes younger northeastward toward yellowstone three major ignimbrite sheets comparable in magnitude to those of the yellowstone plateau field have been correlated on both sides of the eastern snake river plain from the arco and pocatello areas to margins of the yellowstone plateau these 4 3 to 6 5 ma old ignimbrites are referred to herein as the heise volcanic field studies of facies changes and lateral variations in the three major ignimbrites of the heise field permit identification of related calderas that are now buried beneath the plain petrologic studies suggest that although the ignimbrites overlap compositionaliy each was derived from an isotopicaliy distinct magma that contained a significant component of crustal material

the yellowstone snake river plain seismic profiling experiment crustal structure of the eastern snake river plain seismic refraction profiles recorded along the eastern snake river plain esrp in southeastern idaho during the 1978 yellowstone snake river plain cooperative seismic profiling experiment are interpreted to infer the crustal velocity and attenuation q 1 structure of the esrp travel time and synthetic seismogram modeling of a 250 km reversed refraction profile as well as a 100 km detailed profile indicate that the crust of the esrp is highly anomalous approximately 3 to 6 km of volcanic rocks with some interbedded sediments overlie an upper crustal layer compressional velocity approx 6 1 km s which thins southwestward along the esrp from a thickness of 10 km near island park caldera to 2 to 3 km beneath the central and southwestern portions of the esrp an intermediate velocity approx 6 5 km s layer extends from approx 10 to approx 20 km depth a thick approx 22 km lower crust of compressional velocity 6 8 km s a total crustall thickness of approx 42 km and a p sub n velocity of approx 7 9 km s is observed in the esrp similar to the western snake river plain and the rocky mountains provinces high attenuation is evident on the amplitude corrected seismic data due to low q values in the volcanic rocks q sub p more 20 to 200 and throughout the crust q sub p 160 to 300 based on these characteristics of the crustal structure and volcanic age progression data it is suggested that the esrp has resulted from an intensitive period of intrusion of mantle derived basaltic magma into the upper crust generating explosive silicic volcanism and associated regional uplift and caldera collapse this activity began about 15 m y ago in southwestern idaho and has migrated northeast to its present position at yellowstone subsequent cooling of the intruded upper crust results in the 6 5 km s velocity intermediate layer crustal subsidence and periodic basaltic volcanism as represented by the esrp complete the sequence of crustal evolution less

a new kinematic evolutionary model for the growth of a duplex an example from the rangit duplex sikkim himalaya india abstract the lesser himalayan duplex lhd is a prominent structure through much of the lesser himalayan fold thrust belt in the darjeeling sikkim himalaya a component of the lhd is exposed in the rangit window as the rangit duplex rd the rd consists of ten horses of the upper lesser himalayan sequence gondwana buxa upper daling the duplex varies from hinterland dipping in the north through an antiformal stack in the middle to foreland dipping in the south the ramgarh thrust rt is the roof thrust and based on a balanced cross section the main himalayan sole thrust is the floor thrust at a depth of 10 km and with a dip of 3 5 n retrodeformation suggests that the rd initiated as a foreland dipping duplex with the early ramgarh thrust as the roof thrust and the rt as the floor thrust the rt became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent rt reactivation this kinematic history best explains the large translation of the overlying mct sheets the restoration suggests that rd shortening is 125 km and the original gondwana basin extended 142 km northward of its present northernmost exposures within the window

lithosphere scale thrusting in the western u s cordillera as constrained by sr and nd isotopic transitions in neogene volcanic rocks the western idaho suture zone juxtaposes continental and oceanic terranes with distinctive sr and nd isotopic signatures across a sharp isotopic transition coincident with the mesozoic sup 87 sr sup 86 sr 0 706 line msl late cretaceous to eocene granitic plutons that define the msl are allochthonous and were transported eastward by approximately 150 km during late mesozoic to tertiary ca 100 55 ma thrusting sr and nd isotopic compositions of neogene rhyolites and basalts from southwestern idaho southeastern oregon and northern nevada define isotopic transitions coincident with the msl there are continental affinities east of long 116 degree 30minutesw and oceanic affinities west of 119w between these longitudes intermediate isotopic compositions imply derivation of magmas from both continental and oceanic mantle basalts and crustal rhyolites sources it is inferred that the basal decollement of the mesozoic and tertiary contractional orogen dips westward below depths of segregation of basaltic magmas and that oceanic lithospheric mantle was thrust eastward over a shelf of old isotopically evolved continental lithospheric mantle that now protrudes approximately 150 km west of the msl

isostatic rebound active faulting and potential geomorphic effects in the lake lahontan basin nevada and california the high shoreline of the late pleistocene sehoo lake in the lahontan basin is used as a passive strain marker to delineate the magnitude and character of regional deformation since 13 ka the elevations of 170 high shoreline sites document that the once horizontal equipotential shoreline which traverses almost 4 of latitude and 3 of longitude is now deflected vertically about 22 m most of the deformation is attributed to isostatic rebound but a small down to the north regional tilting also appears to contribute to the overall deformation pattern active faults locally offset the high shoreline but cannot explain the regional upwarping attributed to isostatic rebound since 13 ka preliminary models of the rebound yield an upper mantle viscosity of 10 18 pa s that implies a maxwell relaxation time of about 300 yr the rapid earth response coupled with the rapid fall in lake level at the end of pleistocene time may have acted to divert some of the major rivers flowing into the basin from one terminal subbasin to another the regional deformation caused by the rebound may also have acted to control the present location of honey lake these shoreline data therefore support the potential for a link be

extension of the yellowstone plateau eastern snake river plain and owyhee plateau formation of the late cenozoic volcanic province comprising the owyhee plateau eastern snake river plain and yellowstone plateau has been accompanied by east northeast directed crustal extension a new vector of 45 mm yr n56 degree e for the migration of silicic volcanism across the volcanic province is calculated if migration of volcanism reflects west southwest continental drift over a mantle plume a zone of crustal extension must separate the volcanic province from the more slowly moving north american craton space time relations of basin fill in the adjacent basin and range province provide evidence for a zone of extension about 125 km wide coincident with and east of coeval silicic volcanism since 16 ma the zone of extension has migrated along with silicic volcanism maintaining its position between the province and the unextended craton

evaluation of models for the origin of miocene low δ18o rhyolites of the yellowstone columbia river large igneous province two models have been proposed for the origin of the low 18o rhyolites of the snake river plain yellowstone hotspot trend the first involves a single multicyclic caldera system or multiple nested or overlapping systems in which the early erupted products become hydrothermally altered and are subsequently melted to form low 18o rhyolites bindeman and valley 2001 watts et al 2011 the second model appeals to a crustal magma source which was hydrothermally altered in a thermal event prior to the onset of miocene magmatism boroughs et al 2005 we critically evaluate the plausibility of each and provide thermal temporal and volume constraints which must be included in any model for the petrogenesis of low 18o rhyolites of the central snake river plain although the models are not mutually exclusive we conclude that a component of pre existing low 18o crust is required in the central snake river plain where the greatest volumes of strongly 18o depleted rhyolite occur this component could be provided by meteoric hydrothermally altered granites of the idaho batholith which are cut by the central snake river plain

strain rates and contemporary deformation in the snake river plain and surrounding basin and range from gps and seismicity we used new horizontal global positioning system gps velocities along with earthquakes faults and volcanic features to assess how strain is accommodated in the northern basin and range province we estimated horizontal velocities for 132 stations within the snake river plain and the surrounding basin and range from gps phase data collected from 1994 to 2007 these velocities show regional scale clockwise rotation suggestive of driving forces beyond those associated with the yellowstone hotspot within the western centennial tectonic belt the gps measurements indicate that the basin and range is extending at a rate an order of magnitude greater than the snake river plain which explains its low seismicity between these two regions we discern the centennial shear zone a ne trending zone of right lateral shear with estimated slip rates that increase northeastward from 0 9 0 3 mm yr in the sw to 1 7 0 2 mm yr in ne we interpret the new gps velocities to indicate 1 right lateral shear may be accommodated by strike slip earthquakes on ne trending faults in the centennial shear zone 2 three basin and range faults lost river lemhi and beaverhead terminate at the snake river plain margin and 3 extension in the snake river plain occurs at a much lower rate than the rate of normal faulting in the western centennial tectonic belt

crustal structure of the eastern snake river plain determined from ray trace modeling of seismic refraction data ray trace travel time modeling of the seismic refraction record sections for a profile from near soda springs idaho to near mckay idaho was used to derive a crustal model across the eastern snake river plain esrp the derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the esrp the interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the esrp when compared with the upper crust beneath the adjoining northern rocky mountain and basin and range provinces the most prominent features of the crustal structure inferred by the ray trace modeling are as follows 1 the northwest margin of the esrp was modeled as a fault structure downthrown on the srp side se with an offset of greater than 4 km the southeast margin conversely appears to be downwarped with possible minor faulting paleozoic sedimentary rocks are present beneath the cenozoic volcanics of the esrp possibly to and beyond 40 km from the southeastern margin of the esrp near blackfoot idaho 2 the modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the esrp 3 an intermediate 6 5 km s layer occurring beneath the esrp interpreted from a refraction profile located along the axis of the esrp was found to be localized within the esrp margins this layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region 4 a density model of the crust across the esrp was prepared with the densities selected using the interpreted seismic velocities as a constraint the gravity field calculated from this model resulted in a good match to observed gravity data over the eastern snake river plain

magmatism and extension the thermal and mechanical effects of the yellowstone hotspot currently there is a parabolic shaped pattern of seismicity and latest quaternary faulting which centers on the axis of the eastern snake river plain srp and has its apex at the yellowstone plateau the pattern of activity surrounds a region of aseismicity that includes the eastern srp geologic evidence suggests that this pattern has migrated in tandem with rhyolitic volcanism induced by the yellowstone hotspot during its 16 m y trek across western north america similarly there is a migration in the onset of basaltic lava flows and the emplacement of up to 12 km of midcrustal mafic intrusions that follows the track of rhyolitic volcanism these observations suggest that the intrusion of magmas has a significant influence on the pattern of seismicity and faulting we present a one dimensional finite difference thermomechanical model that accounts for the observed pattern of increased faulting followed by fault quiescence within the circum eastern srp in this model mafic magmas are intruded into a lithosphere that is already extending the intrusions heat the surrounding rocks resulting in locally increased strain rates as the intruded magmas solidify the length of time required to return strain rates to their pre intrusion level is then determined the model assumes constant horizontal tectonic forces and maps strain rate as a function of yield strength and time since intrusion we vary model parameters such as crustal thickness initial geothermal gradient and amount of magma intruded in order to assess how they affect turnaround time for strain rates the amount and timing of intruded material allowed by the model are constrained by seismic refraction data as well as by regional geochronologic and petrologic studies we assume intruded rock rheology to be a maryland diabase mantle rheology we base on dry olivine we represent the crust by both dry granite and dry anorthosite rheologies our results suggest that 2 to 3 m y appears to be a reasonable length of time for strain rates to return to levels present before a midcrustal mafic intrusion equivalent in magnitude to the yellowstone intrusion this corresponds closely to the length of time between the onset of along axis accelerated faulting and subsequent fault quiescence assuming a hotspot velocity of 3 5 to 4 0 cm yr relative to the north american plate

subsidence of a volcanic basin by flexure and lower crustal flow the eastern snake river plain idaho the eastern snake river plain esrp is a linear volcanic basin interpreted by many workers to reflect late cenozoic migration of north america over the yellowstone hotspot thermal subsidence of this volcanic province with respect to yellowstone has been documented by several workers but no one has characterized subsidence with respect to the adjacent basin and range province this paper documents crustal flexure along the northwest edge of the esrp uses flexure to model the dimensions of a dense load beneath the basin and presents evidence in support of density driven subsidence and lower crustal flow away from the basin crustal flexure adjacent to the esrp is reflected by the attitudes of mesozoic fold hinges and neogene volcanic rocks fold hinges formed with a subhorizontal plunge and a trend perpendicular to the esrp but now show a southward plunge near the esrp of as much as 20 25 we present a contour map of equal fold plunges proximal to the esrp that shows flexure is roughly parallel to and extends 10 20 km north of the average edge of the esrp flexural profiles indicate the minimum amount of esrp subsidence with respect to the basin and range subsidence ranges from 4 5 to 8 5 km the structural contour map and published seismic and gravity data were used to develop and constrain flexural subsidence models these models indicate the flexed crust is very weak flexural parameter of 4 10 km interpreted to be a result of the high heat flow of the esrp assuming subsidence was induced by emplacement of a dense crustal layer beneath the esrp a midcrustal sill identified in previous seismic surveys is too wide and probably too thin to produce the measured flexure new dimensions include a thickness of 17 25 km and a half width of 40 50 km which place the edge of the sill beneath the edge of the esrp the dimensions of the esrp sill are based on isostatic compensation in the lower crust because compensation in the asthenosphere requires an unreasonable sill thickness of 30 km and because esrp seismic gravity and heat flow data support lower crustal compensation density driven lower crustal flow away from the esrp is proposed to accommodate subsidence and maintain isostatic equilibrium timing of subsidence is constrained by esrp exploratory wells where 6 6 ma rhyolites at a depth of 1 5 km indicate most subsidence occurred prior to their emplacement and by strong spatial correlations between plunge contours and quaternary volcanic rift zones two processes interpreted to contribute to the load include an extensive midcrustal mafic load emplaced at 10 ma which provided the heat source for the initial rhyolitic volcanism on the esrp and continuing localized loads from dikes and sills associated with quaternary basalts widespread 10 ma magmatism and subsidence conflicts with simple time transgressive migration of the yellowstone hotspot indicating a need for revision of the hotspot paradigm

spatio temporal moment tensor inversion for multiple shocks an application to the deep 1972 banda sea earthquake abstract a waveform inversion scheme is presented for multiple shocks an earthquake source is modeled as a sequence of point sources with different focal mechanisms each point source is defined by an onset time relative spatial coordinates and a moment tensor all the point sources are allowed to occur at only grid points with the same interval on a single flat plane and to have the same trapezoidal time history unknown source parameters are obtained by an iterative procedure each iteration step consists of two substeps first onset times and relative spatial coordinates of subevents with respect to a reference point are determined second seismic moment tensors of the subevents are obtained by a conventional linear moment tensor inversion a statistical test of a change in the focal mechanism during the source process is made using confidence ellipses of principal stress axes for each subevent we apply the inversion to wwssn teleseismic long period p waveforms from a deep focus earthquake of the 1972 banda sea mb 6 6 the result suggests that the earthquake consisted of two major subevents with a time interval of 7 5 s and that the main rupture connecting the two subevents propagated westward by approximately 20 km the best double couple seismic moment is 5 7 1018 nm and 2 0 1018 nm for the first and the second subevents respectively differences between directions of the p and t axes of the two major subevents may suggest that the main rupture of the earthquake propagaged along a northward convex fault surface

strength and rheology of the western u s cordillera effective elastic thickness te depends primarily on temperature composition and state of stress of the lithosphere in this paper we examine high resolution spectral estimates of te and their relationships to regional heat flow age of the lithosphere seismic properties stress orientations and earthquake focal depths of the western u s cordillera the relationship of te to heat flow indicates that ductile flow accommodates long term 106 to 108 years isostatic response at different levels of the crust and upper mantle depending principally on age and by implication bulk composition of the lithosphere isostatic response is primarily controlled by the upper mantle in archean lithosphere of the middle rocky mountains whereas te depends on lower crustal flow in early proterozoic lithosphere of the colorado plateau the yellowstone snake river plain volcanic field and significantly extended regions in the basin range and northern rocky mountains are associated with latest proterozoic aged lithosphere and indicate middle to upper crustal control of long term te we also show that azimuthal variations of te reflect deviatoric stress in the lithosphere te is found empirically to approximate the 95th percentile focal depth of background seismicity the latter relationship is inconsistent with brittle ductile control of focal depth indicating that another rheological transition e g from stick slip to stable sliding frictional behavior is responsible tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere moreover we find a spatial correlation of the intermountain seismic belt to a marked transition in te implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current day plate boundary interactions

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

mixing primitive and evolved olivine tholeiite magmas in the eastern snake river plain idaho abstract butte 5206 a small monogenetic olivine tholeiite basalt otb shield in the eastern snake river plain esrp erupted during late pleistocene time along the arco big southern butte volcanic rift zone geochemical data from butte 5206 and other basalts that erupted in the vicinity including correlated samples from coreholes are evaluated for possible mechanisms of magma evolution in the esrp thermodynamic and mass balance models demonstrate that the evolution of butte 5206 magmas reflect a two stage process 1 fractional crystallization of relatively primitive magma possibly at multiple crustal levels and 2 mixing of primitive and evolved liquids in a shallow reservoir prior to eruption application of this model to other basalts and local eruptive centers in the region suggest that similar processes are active during the petrogenesis of many esrp basalts and that the process of intrusion and mixing is cyclic throughout pleistocene to holocene time

iterative deconvolution of complex body waves from great earthquakes the tokachi oki earthquake of 1968 abstract a method of body wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone a sequence of point sources each being characterized with the seismic moment the onset time and the location are iteratively derived from observed records at multi stations where the two dimensional extent of the source location is taken into account a modification is made of the iterative method of kikuchi and kanamori on the formulation of inversion procedure to facilitate the computation using this method we analyse long period p waves of the tokachi oki earthquake of 1968 mw 8 2 and obtain several significant subevents with time durations of 10 s their spatio temporal distribution shows that the rupture process consists of three characteristic stages a a stage of introductory rupture b a stage of main rupture and c a stage of aftershocks the main rupture takes place in the form of clustering around a few sites of the fault plane the largest subevent occurs in the northwestern corner the stress drop associated with this event is estimated to be 200 bars one order of magnitude higher than the stress drop averaged over the entire fault plane the sum of the seismic moments of the individual subevents amounts to 2 3 1028 dyn cm which approximately coincides with the one estimated from the analysis of long period surface waves this implies that the source of the tokachi oki earthquake consists of several major subevents with time durations of 10 s in addition to other minor subevents

contemporary seismicity in the eastern snake river plain idaho based on microearthquake monitoring microearthquake monitoring by a permanent network operated since 1971 and two recent temporary surveys performed by the idaho national engineering laboratory suggest that infrequent small magnitude earthquakes are characteristic of eastern snake river plain esrp seismicity although a total of only 19 earthquakes have been observed to date and not all have well constrained focal depths their relatively shallow occurrence at depths of 8 km is consistent with the hypothesis that elevated crustal temperatures in the esrp confine the brittle portion of the crust and hence seismogenesis to the upper 6 to 10 km a composite focal mechanism of two microearthquakes located near the axis of the esrp indicates normal faulting with a minor component of strike slip motion the northeast southwest trending t axis indicated by this composite mechanism is consistent with the direction of extension indicated by holocene volcanic rift zones within the esrp and normal faults in the adjacent basin and range province to the northwest based on this low level of small magnitude seismicity deviatoric stresses appear to be small in the esrp

fallout tuffs of trapper creek idaho a record of miocene explosive volcanism in the snake river plain volcanic province a 900 m thick section of tuffaceous sedimentary rock vitric fallout tuff and ash flow tuff is well exposed along trapper creek in south central idaho this section provides nearly continuous exposure through the fill of the goose creek basin a major north trending miocene extensional basin located along the southern margin of the snake river plain volcanic province srpvp some 51 separate units of vitric fallout tuff are recognized in the trapper creek section petrographic and chemical characteristics of these vitric tuffs indicate that most are from srpvp sources new 40ar 39ar laser fusion dating along with prior isotopic age determinations show that the trapper creek tuffs span the period ca 13 9 8 6 ma chemical correlation indicates that fallout tuffs in the central part of the trapper creek section 12 5 10 0 ma are from sources in the bruneau jarbidge volcanic field of the srpvp centered 100 km west of trapper creek underlying fallout tuffs may have had sources in the owyhee humboldt field of the srpvp centered 200 km west of trapper creek while overlying fallout tuffs interlayered with several ash flow tuffs had a relatively proximal source possibly in the proposed twin falls volcanic field centered 60 km north of trapper creek the trapper creek tuffs provide insight into the characteristics of explosive silicic volcanism within the srpvp during middle late miocene time from ca 13 9 to ca 9 5 ma major eruptions those depositing 1 5 m of fallout tuff were frequent about one event per 200 k y their products display a trend toward the eruption of progressively less evolved higher temperature silicic magma after 12 5 ma this trend to higher temperature eruptions termed the cougar point flare up culminated in the eruption of high temperature 1000 c plagioclase rich magma during the period 10 5 9 5 ma in contrast to these eruptions later 7 0 ma major silicic eruptions within the srpvp were characterized by the lower temperature 850 c of the erupted magma and by the longer intervals about one event per 500 600 k y between eruptions variations in the character of srpvp explosive silicic eruptions may reflect changes in the structure composition or state of stress in the crust beneath the eastward propagating srpvp or perhaps changes in the yellowstone hot spot plume that may drive the srpvp volcanism

layered mafic sill complex beneath the eastern snake river plain evidence from cyclic geochemical variations in basalt the eastern snake river plain in southern idaho western united states is characterized by 1 2 km of pleistocene to late pliocene basalt overlying rhyolite caldera complexes cyclic variations in the chemical composition of basalts from 1136 m of scientific drill core show that the parent magmas of these lavas evolved by crystal fractionation at shallow to intermediate crustal depths punctuated by episodic recharge with more primitive compositions and assimilation of adjacent wall rock we have identified 10 upward fractionation cycles and four reversed cycles assimilation of sialic crust was limited and mainly affects the oldest basalts which directly overlie rhyolites we infer that the crystal fractionation and or recharge cycles took place in a series of sill like intrusions at intermediate crustal depths that now form a layered mafic intrusion that underlies the eastern snake river plain at depth this layered sill complex is represented by the 10 km thick basaltic sill that has been imaged seismically at 12 22 km depth the association of this mid crustal sill complex with geochemical fractionation cycles in basalt supports the concept that exposed layered mafic intrusions may be linked to overlying basalt provinces that have since been removed by erosion

the zagros hinterland fold and thrust belt in sequence thrusting iran abstract the collision of the iranian microcontinent with the afro arabian continent resulted in the deformation of the zagros orogenic belt the foreland of this belt in the persian gulf and arabian platform has been investigated for its petroleum and gas resource potentials but the zagros hinterland is poorly investigated and our knowledge about its deformation is much less than other parts of this orogen therefore this work presents a new geological map stratigraphic column and two detailed geological cross sections this study indicates the presence of a hinterland fold and thrust belt on northeastern side of the zagros orogenic core that consists of in sequence thrusting and basement involvement in this important part of the zagros hinterland the in sequence thrusting resulted in first and second order duplex systems mode i fault bend folding fault propagation folding and asymmetric detachment folding which indicate close relationships between folding and thrusting study of fault bend folds shows that layer parallel simple shear has the same role in the southeastern and northwestern parts of the study area e 23 4 9 1 a major lateral ramp in the basement beneath the talaee plain with about one kilometer of vertical offset formed parallel to the sw movement direction and perpendicular to the major folding and thrusting

source retrieval for mantle earthquakes by iterative deconvolution of long period p waves abstract long period p waves of eighteen deep and intermediate depth earthquakes in a moment range 1 10 26 3 10 28 dyn cm are analysed by the method of kikuchi and fukao this issue to iteratively derive point dislocations on a plane the fault plane is defined as the plane of point dislocations that gives the best fit between the synthetic and observed waveforms for dip slip type earthquakes the fault plane is found to dip more steeply than the auxiliary plane in general point dislocations tend to cluster into one or a few subevents we characterize the largest subevent by its maximum moment rate m 0 m and its half duration from which the associated stress drop can be roughly estimated for earthquakes shallower than 400 km half duration is approximately constant 5 s so that stress drop increases from 1000 bar as m 0 m increases the value of for the great banda sea earthquake of 1963 h 100 km is exceptionally large 10 s for which 700 bar for earthquakes deeper than 400 km increases from 3 s to 9 s as m 0 m increases so that is approximately constant 700 bar stress drop so obtained approximates to an overall stress drop with a possible exception of the several largest earthquakes the distribution of point dislocations and the moment rate function indicate that rupture most often expands in a fan shaped fault area and that a large part of the rupture front ceases propagation in a coherent manner a sharp contrast in stress drop behavior across a depth of 400 km may be related to the olivine spinel transition within the downgoing slab of lithosphere

gravity anomalies and flexure of the lithosphere at the middle amazon basin brazil the middle amazon basin is a large paleozoic sedimentary basin on the amazonian craton in south america it contains up to 7 km of mainly shallow water sediments a chain of bouguer gravity highs of approximately 40 to 90 mgals transects the basin roughly coincident with the axis of maximum thickness of sediment the gravity highs are flanked on either side by gravity lows of approximately 40 mgals the observed gravity anomalies can be explained by a steeply sided zone of high density in the lower crust varying in width from 100 to 200 km wide within this region the continental crust has been intruded replaced by more dense material to more than half its original thickness of 45 50 km the much wider sedimentary basin results from regional compensation of the subsurface load and the subsequent load of accumulated sediments by flexure of the lithosphere the observed geometry of the basin is consistent with an elastic lithosphere model with a mechanical thickness of 15 20 kin although this value is lower than expected for a stable cratonic region of early proterozoic age it is within the accepted range of effective elastic thicknesses for the earth rapid subsidence during the late paleozoic may be evidence of a second tectonic event or lithospheric relaxation which could lower the effective mechanical thickness of the lithosphere the high density zone in the lower crust as delineated by gravity and flexural modeling has a complex sinuous geometry which is narrow and south of the axis of maximum sediment thickness on the east and west margins and wide and offset to the north in the center of the basin the linear trough geometry of the basin itself is a result of smoothing by regional compensation of the load in the lower crust

intraplate deformation and microplate tectonics of the yellowstone hot spot and surrounding western u s interior 1 contemporary deformation of the yellowstone hot spot and surrounding western united states is analyzed using tectonic microplate modeling employing constraints from gps observations corrected for postseismic deformation of m7 earthquakes fault slip rates and earthquake focal mechanisms we focus primarily on the kinematics of the yellowstone hot spot and the eastern snake river plain volcanic field esrp and secondarily on basin range and columbia plateau provinces our results reveal southwest motion of the yellowstone plateau excluding localized volcanic deformation at 0 9 0 1 mm a that decreases to 0 8 0 1 mm a in the esrp block the southwest to west motion of the yellowstone esrp introduces shear in the northern rocky mountain block which is translating east at 0 78 0 08 mm a there is 0 5 mm a differential motion between the esrp and the block at its northern boundary and none at the southern boundary the eastern basin range block moves west at 3 0 0 1 to 4 6 0 1 mm a while velocities of the western basin range microplates rotate to a northwest direction accompanied by a transition from normal to oblique shear deformation columbia plateau block velocities are notably eastward an effect enhanced by postseismic relaxation following the m9 1700 cascadia subduction zone paleoearthquake this study reveals that the overall motion of the western united states is characterized by clockwise rotation with westward extension and northwest shear in the basin range province northeast to east contraction in the columbia plateau and southwest extension of the yellowstone snake river plain block that is driven by the high gravitational potential of the yellowstone swell

tectonics of the yellowstone hotspot wake in southwestern montana deposits off a neogene paleovalley provide a geologic datum in southwestern montana that brackets the age of faulting associated with the yellowstone hotspot ages of paleovalley deposits on top of the modern blacktail ruby tendoy and centennial ranges and in adjacent valleys range from 16 to 2 ma at least 2 km of offset has occurred since emplacement of the timber hill basalt 6 0 0 1 ma much may have occurred after deposition of the 2 0 ma huckleberry ridge tuff which is now found in isolated outcrops as far north as the ruby range gravel clasts within the paleovalley have a northern provenance whereas volcanic and volcaniclastic rocks were derived from the snake river plain to the south we propose that uplift associated with passage of the hotspot caused the drainage reversal

lithospheric evolution of the andean fold thrust belt bolivia and the origin of the central andean plateau we combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central andean plateau between 188 and 208 s from late cretaceous to present by integrating geophysical results of upper mantle structure crustal thickness and composition with recently published structural stratigraphic and thermochronologic data we emphasize the importance of both the crust and upper mantle in the evolution of the central andean plateau four key steps in the evolution of the andean plateau are as follows 1 initiation of mountain building by 70 ma suggested by the associated foreland basin depositional history 2 eastward jump of a narrow early fold thrust belt at 40 ma through the eastward propagation of a 200 400 km long basement thrust sheet 3 continued shortening within the eastern cordillera from 40 to 15 ma which thickened the crust and mantle and established the eastern boundary of the modern central andean plateau removal of excess mantle through lithospheric delamination at the eastern cordillera altiplano boundary during the early miocene appears necessary to accommodate underthrusting of the brazilian shield replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the eastern cordillera and altiplano at 24 23 ma and further volcanism recorded by 12 7 ma crustal ignimbrites 4 after 20 ma deformation waned in the eastern cordillera and interandean zone and began to be transferred into the subandean zone long term rates of shortening in the fold thrust belt indicate that the average shortening rate has remained fairly constant 8 10 mm year through time with possible slowing 5 7 mm year in the last 15 20 myr we suggest that cenozoic deformation within the mantle lithosphere has been focused at the eastern cordillera altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination d 2005 elsevier b v all rights reserved

quaternary tectonic setting of the 1983 borah peak earthquake central idaho the 1983 borah peak earthquake was accompanied by extensive surface faulting along a part of the lost river fault that has abundant evidence of latest quaternary last 15 000 yr offset this fault and two similar range front normal faults along the lemhi range and beaverhead mountains lie in an area of basinand range structure in central idaho that is part of a roughly v shaped belt of latest quaternary surface faulting that extends from the wasatch fault through the yellowstone area to the lost river fault the position of this belt may be related to the outward migration of a thermal front associated with the northeastward progression of late cenozoic silicic volcanism along the yellowstonesnake river plain axis the central segments of the lost river lemhi and beaverhead faults have been active more recently and probably more active throughout quaternary time than the southern and northern segments the main 1983 surface faulting occurred in an area of high structural relief along a central segment of the lost river fault that has ruptured in latest quaternary time which suggests that comparable areas along other range fronts in the area should be regarded as likely sites of future surface faulting other perspective of fault behavior suggest additional possible sites and all segments of the range front faults are regarded as capable of surface faulting

distribution of active faulting along orogenic wedges minimum work models and natural analogue abstract numerical 2 d models based on the principle of minimum work were used to examine the space time distribution of active faulting during the evolution of orogenic wedges a series of models focused on thin skinned thrusting illustrates the effects of arid conditions no erosion unsteady state conditions accretionary influx greater than erosional efflux and steady state conditions accretionary influx balances erosional efflux on the distribution of fault activity for arid settings a general forward accretion sequence prevails although a significant amount of internal deformation is registered the resulting fault pattern is a rather uniform spread along the profile under fixed erosional efficiency settings the frontal advance of the wedge front is inhibited reaching a steady state after a given forward propagation then the applied shortening is consumed by surface ruptures over a narrow frontal zone under a temporal increase in erosional efficiency i e transient non steady state mass balance conditions a narrowing of the synthetic wedge results a rather diffuse fault activity distribution is observed during the deformation front retreat once steady balanced conditions are reached a single long lived deformation front prevails fault activity distribution produced during the deformation front retreat of the latter scenario compares well with the structural evolution and hinterlandward deformation migration identified in southern bolivian subandes ssa from late miocene to present this analogy supports the notion that the ssa is not in steady state but is rather responding to an erosional efficiency increase since late miocene the results shed light on the impact of different mass balance conditions on the vastly different kinematics found in mountain ranges suggesting that those affected by growing erosion under a transient unbalanced mass flux condition tend to distribute deformation along both frontal and internal faults while others under balanced conditions would display focused deformation on a limited number of steady structures

a new interpretation of deformation rates in the snake river plain and adjacent basin and range regions based on gps measurements we evaluate horizontal global positioning system gps velocities together with geologic volcanic and seismic data to interpret extension shear and contraction within the snake river plain and the northern basin and range province u s a we estimate horizontal surface velocities using gps data collected at 385 sites from 1994 to 2009 and present an updated velocity field within the stable north american reference frame snarf our results show an ene oriented extensional strain rate of 5 9 0 7 x 10 sup 9 yr sup 1 in the centennial tectonic belt and an e oriented extensional strain rate of 6 2 0 3 x 10 sup 9 yr sup 1 in the intermountain seismic belt combined with the northern great basin these extensional strain rates contrast with the regional north south contraction of 2 6 1 1 x 10 sup 9 yr sup 1 calculated in the snake river plain and owyhee oregon plateau over a 125 x 650 km region tests that include dike opening reveal that rapid extension by dike intrusion in volcanic rift zones does not occur in the snake river plain at present this slow internal deformation in the snake river plain is in contrast to the rapidly extending adjacent basin and range provinces and implies shear along boundaries of the snakemore river plain we estimate right lateral shear with slip rates of 0 5 1 5 mm yr along the northwestern boundary adjacent to the centennial tectonic belt and left lateral oblique extension with slip rates of 10 sup 6 yrs the low strain field in the snake river plain and owyhee oregon plateau would extend through the quaternary less

the role of magma overpressure in suppressing earthquakes and topography worldwide examples in an extending terrane basaltic magma supplied at a pressure greater than the least principal stress overpressure may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting as vertical dikes intrude they press against their walls in the direction opposite the least principal stress and increase its magnitude the emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses this process tends to equalize the stresses and thus diminishes the deviatoric stress difference between maximum and minimum stresses that creates faults and causes earthquakes observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks

static and dynamic support of western united states topography article history accepted 7 october 2013 available online 31 october 2013 editor p shearer isostatic and dynamic models of earth s surface topography can provide important insights into the driving processes of tectonic deformation we analyze these two estimates for the tectonically active western united states using refined structural models derived from earthscope usarray for the crust use of recent moho depth measurements and crustal density anomalies inferred from passive source seismology improve isostatic models however seismically determined lithospheric thickness variations from lithosphere asthenosphere boundary lab maps and lithospheric and mantle density anomalies derived from heat flow or uppermost mantle tomography do not improve isostatic models substantially perhaps this is a consequence of compositional heterogeneity a mismatch between thermal and seismological lab and structural complexity caused by smaller scale dynamics the remaining non isostatic dynamic component of topography is large topography anomalies include negative residuals likely due to active subduction of the juan de fuca plate and perhaps remnants of formerly active convergence further south along the margin our finding of broad scale positive residual topography in the basin and range substantiates previous results implying the presence of anomalous buoyancy there which we cannot fully explain the colorado plateau does not appear dynamically anomalous at present except at its edges many of the residual topography features are consistent with predictions from mantle flow computations this suggests a convective origin and important interactions between vigorous upper mantle convection and intraplate deformation

a forward approach to the problem of nonlinear viscoelasticity and the thickness of the mechanical lithosphere the physical significance of the mechanical lithosphere is clarified using a nonlinear maxwell flexure model this model reconciles high temperature creep data for rock with the observed long term strength of the mechanical lithosphere an approximate correspondence is established between the nonisothermal maxwell model and an elastic flexure model to define an effective lithosphere thickness he the model calculations are essentially forward since he does not depend in a significant way upon the nominal or prescribed value of beam thickness depending rather upon such variables as temperature and creep activation energy the effective thickness of the maxwell model though a function of time is stable enough over appropriate time scales to explain the long term integrated strength of the lithosphere the strength index is defined as the ratio of axial stress to the axial stress that would be present in a purely elastic fiber suffering the same strain the base of the effective lithosphere is marked by a critical isopleth of strength c 0 3 this critical level though not its position is independent of time since loading the strength index also maps out zones of predominant rheology the narrowness of the transition zone between the elastic lithosphere and the viscous asthenosphere explains the success of the elastic plate in modeling many examples of lithosphere flexure the elastic plate approximation may apply best where the lithosphere is relatively young and hot isolated weak zones in the continental crust may or may not weaken the lithosphere as a whole depending upon the distribution of these zones and the manner in which they deform in general such weakening is likely to be most effective when the base of the crust occurs at midlithospheric depths