annually resolved late holocene paleohydrology of the southern sierra nevada and tulare lake california here we present 2000 year long annually resolved records of streamflow for the kings kaweah tule and kern rivers in the southwestern sierra nevada of california and consequent lake level fluctuations at tulare lake in the southern san joaquin valley the integrated approach of using moisture sensitive tree ring records from the living blended drought atlas to reconstruct annual discharge and then routing this discharge to an annual tulare lake water balance model highlights the differences between these two types of paleoclimate records even when subject to the same forcing factors the reconstructed streamflow in the southern sierra responded to yearly changes in precipitation and expressed a strong periodicity in the 2 8 year range over most of the reconstruction the storage capacity of tulare lake caused it to fluctuate more slowly masking the 2 8 year streamflow periodicity and instead expressing a strong periodicity in the 32 64 year range over much of the record although there have been longer droughts the 2015 water year represents the driest in the last 2015 years and the 2012 2015 drought represents the driest 4 year period in the record under natural conditions simulated tulare lake levels would now be at about 60 m which is not as low as what occurred multiple times over the last 2000 years this long term perspective of fluctuations in climate and water supply suggests that different drought scenarios that vary in terms of severity and duration can produce similar lake level responses in closed lake basins

thin crust and active upper mantle beneath the southern sierra nevada in the western united states abstract analysis of seismic refraction wide angle reflection travel time and seismic amplitude data collected during the 1993 southern sierra nevada continental dynamics sscd project resolves a small crustal root 40 42 km thick centered 80 km west of the sierran topographic crest the sscd experiment consisted of a west east profile across the sierra nevada at approximately 36 5 n and a north south profile extending the length of owens valley located eastward of the sierran topographic crest two dimensional finite difference travel time inversion of p g and p n arrivals resolves upper crustal velocities of 6 0 to 6 4 km s 1 within the sierran batholith and basin and range an upper crustal west dipping wedge of higher velocities 6 8 7 2 km s 1 consistent with ophiolitic material underlying the great valley sedimentary sequence and higher velocities in the lower crust beneath the basin and range 6 8 7 0 km s 1 than those beneath the sierran batholith 6 6 km s 1 low average p n velocities 7 6 7 9 km s 1 and a laminated transitional moho imaged beneath the sierran batholith also differ from the higher p n velocities 7 9 8 0 km s 1 and sharp first order moho observed beneath the basin and range the crust decreases in thickness both westward of the root to 28 34 km beneath the great valley and eastward to 35 km beneath the highest sierran topography and decreases further to 27 30 km beneath the basin and range crustal thickness also appears to increase southeast to northwest from 29 30 km beneath the garlock fault in the south to 38 40 km beneath the north end of chalfant valley juxtaposition of the crustal model with previous p wave tomography models of the south ernmost sierra nevada upper mantle reveal that the thickest sierran crust on the west east profile overlies a pronounced upper mantle high velocity anomaly 5 whereas the region of laminated moho overlies a flanking upper mantle low velocity region 3 the upper mantle velocity anomalies relatively low p n and relatively flat moho observed beneath the sierran crest suggest that the recent uplift of the sierra nevada is due to asthenospheric flow and or lithospheric thinning beneath the southeastern sierra nevada and basin and range

seismic structure of the lithosphere from teleseismic converted arrivals observed at small arrays in the southern sierra nevada and vicinity california 21 well distributed teleseisms 30 to 100 distance were recorded by mixed broad band bb and short period sp seismic arrays at mineral king mk and horseshoe meadow hm in the southern sierra nevada and at darwin plateau dp between the inyo and argus ranges these arrays permit identification and separation of direct p and s arrivals reflections from topography scattered energy and arrivals from different back azimuths multipath arrivals p to s conversions can be identified from beamed bb or sp seismograms least squares time domain processing recovered single event receiver functions from these beamed seismograms converted phases attributable to the moho are clear and uncomplicated at dp ps p times of 3 9 4 2 s correspond to crustal thicknesses of 31 33 km assuming a mean p crustal velocity of 6 2 km s and a poisson s ratio of 0 255 ps p times at hm eastern sierra are 3 9 4 6 s 32 37 km excepting some unusual seismograms from events at back azimuths of 225 239 mk western sierra times for ps p show a strong e w asymmetry 3 9 4 1 s 31 33 km from the east and 4 8 5 3 s 39 42 km from the west the mk arrivals from the east are multiple and substantially weaker by comparison with hm and dp these results confirm the absence of a thick crust under the southern high sierra inferred from both a refraction experiment coincident with this experiment and some earlier studies at dp the moho event follows an intracrustal negative polarity event defining the top of an s wave low velocity zone this feature dips west under the sierran crest at hm but is absent farther west at mk this feature appears to be a manifestation of extensional strain and thus indicates that the surficially undeformed sierra overlies a tectonized lower crust sub moho energy is absent under the basin and range dp but is conspicuous under the high sierra at positive arrivals 7 3 s mk and 9 s hm after the p these arrivals might be from the base of a low velocity low density upper mantle body supporting the sierra

paleotopography in the western u s cordillera issue 2 of volume 312 of the american journal of science brings together several new papers bearing on the history of orogenesis paleotopography and paleoclimate in a region whose geologic record has been more important to the modern development of ideas about these processes than any other in

history and controls of subsidence in the late cretaceous tertiary great valley forearc basin california abstract analysis of the upper cretaceous tertiary fill of the great valley forearc basin of california reveals a complicated history of subsidence partitioned in time and space western oceanward parts of the basin record subsidence then uplift in apparent response to the angle and rate of descent of the underlying subducting plate specifically uplift in the forearc basin corresponds to the onset of the laramide orogeny and inferred low angles of subduction in contrast eastern arcward parts of the basin display subsidence histories suggestive of thermal contraction and compatible with continentward migration of sierran arc magmatism in the late cretaceous these results imply that subsidence histories of forearc basins are likely to be much more complicated and less predictable than those of rifted continental margins and foreland basins where single subsidence mechanisms predominate and time varying subduction is not an underlying controlling factor

the uplift of the sierra nevada and implications for late cenozoic epeirogeny in the western cordillera tilted cenozoic strata in the eastern great valley california record progressive late cenozoic uplift of the sierra nevada the magnitude of tilting is combined with other geologic data using a technique pioneered by grant and others to infer the rate and timing of post late miocene uplift between the kings river and the feather river angular unconformities of late neogene age have been reported throughout the east central great valley typically stratigraphic units of late oligocene through miocene pliocene age beneath the unconformities dip approximately 1 2 1 6 southwest middle to late pliocene units above the unconformities dip approximately 0 6 0 9 southwest and progressively overlap younger units from east to west these angular unconformities are interpreted to represent the onset of late cenozoic uplift of the sierra nevada due to westward tilting dated volcaniclastic units in the east central sacramento valley bracket the onset of tilting there between 3 4 and 8 4 ma stratigraphic and geologic relations from the stanislaus river area eastern san joa quin valley suggest that the late cenozoic tilting probably began approximately 5 ma previously published tilt data from the san joaquin valley combined with new work in the eastern sacramento valley indicate that the rate of post late miocene tilting has been approximately uniform in the east central great valley early to middle pleistocene units typically dip 0 5 0 7 southwest middle pleistocene units dip 0 2 0 4 middle to late pleistocene units dip 0 1 0 15 late pleistocene units dip 0 05 0 1 if it is assumed that major westward tilting began 5 ma these data indicate an approximately uniform tilting rate of 0 28 per million years these data have important implications for models of the uplift simultaneous inception of tilting in the east central great valley at approximately 5 ma does not support models that link uplift of the sierra nevada to the migration of the mendocino triple junction late cenozoic tilting of the sierra nevada block does not appear to coincide temporally with the onset of major basin and range extension the uplift may be due to thinning of the mantle lithosphere beneath the sierra nevada but the tilt data do not afford a direct test of this hypothesis post late miocene tilting of the sierra nevada is temporally associated with late cenozoic uplift of the northern basin and range uplift of the cascades uplift of the colorado plateau and uplift of the southern rocky mountains supporting the contention that tilting of the sierra nevada may have occurred as part of a cordillera wide uplift event

coupled crust mantle dynamics and intraplate tectonics two dimensional numerical and three dimensional analogue modeling 1 tectonic deformation of some intraplate regions may be caused by the rayleigh taylor rt instability of dense subcrustal lithosphere mantle lithosphere as it descends into the mantle we report on a series of scaled three dimensional 3 d analogue and 2 d numerical experiments of coupled crust mantle dynamics that study such a process the models investigate the effects of two geometries of rt instability linear versus axisymmetric and use different rheological stratifications of the crust the growth of the rt instability is strongly influenced by crustal rheology while rt growth rates are not greatly influenced by the geometry of the mantle lithosphere instability itself the surface is experiments having a viscous only crust are characterized by significant crustal contraction and thickening above the mantle downwelling and extension and thinning in adjacent regions in experiments with a brittle upper crust the surface deformation is much more subdued however strong mantle flow induced deformation still occurs in the ductile lower crust beneath the quiescent brittle upper crust when the lower crust is relatively strong localized styles of extensional and contractional structure develop in the brittle upper crust owing to the greater degree of coupling between upper crust and the mantle instability in all the experiments a portion of the ductile lower crustal material is entrained within the downgoing rt instability deep into the mantle the interplay between dynamic topography and crustal thickening thinning induced by the underlying mantle flow governs various time dependent phases of subsidence and uplift of the model surface the experiments help to account for a number of first order tectonic behaviors of the lithospheres of earth and other terrestrial planets for example intraplate orogens and basins complex localized tectonic structure in intraplate settings and deep crustal seismic fabrics may arise as the variable crustal response to underlying mantle lithosphere rt instabilities

mantle flow tectonics the influence of a ductile lower crust and implications for the formation of topographic uplands on venus convective flow within the interiors of terrestrial planets is a primary source of large scale topographic and tectonic features we treat the crust and mantle as a layered viscous half space and find solutions for flow driven by a buoyancy force distribution within the mantle and by relief at the surface and crust mantle boundary the crust initially responds to mantle upwelling with uplift but continued flow causes significant crustal thinning and subsidence downwelling leads to initial subsidence and subsequent crustal thickening and surface uplift changes in crustal thickness are driven by 1 vertical normal stresses due to mantle flow and 2 shear coupling of horizontal mantle flow into the crust the degree and time scale of crustal deformation are sensitive to crustal thickness and viscosity and the presence of elastic layers in the upper crust and mantle deformation can occur on time scales less than several hundred million years for reasonable values of these parameters strong or elastic upper crustal layers enhance crustal thinning or thickening while large viscosity contrasts between the lower crust and upper mantle or elastic mantle layers tend to diminish deformation for time dependent mantle flow due to a rising or sinking diapiric body crustal deformation depends upon the ratio of diapir radius to crustal thickness and the ratio of crustal to mantle viscosity a sinking diapir can lead to crustal thickening elevated topography and a specific sequence of time varying deformation while regions such as beta regio have long been suggested to result from convective upwelling in the venus mantle we suggest that a mantle downwelling origin is consistent with many of the characteristics of the compressional mountain ranges of western ishtar terra

topographic response to mantle lithosphere removal in the southern sierra nevada region california geological studies of mantle xenoliths entrained in late neogene quaternary lavas from the southern sierra nevada region and regional geophysical studies suggest that the highdensity mantle lithosphere that formed beneath the sierra nevada batholith in conjunction with arc magmatism is being convectively removed as a drip structure this structure as imaged seismically is roughly cylindrical in shape with a diameter of 100 km and extends to 225 km depth centered above this structure is a region 120 km in diameter that is undergoing active subsidence relative to adjacent regions such subsidence is seen in the active fluvial alluvial sediment flooding of mountainous topography of the southwestern sierra and in the development of the adjacent tulare lake basin of the san joaquin valley dynamic modeling of such upper mantle drip structures predicts a phase of overlying surface subsidence during the most vigorous phase of drip formation the southern sierra upper mantle drip and the overlying crust appear to be in this phase of their dynamically coupled evolution

deep drop down basin in the southern dead sea abstract the large dead sea graben is located within the dead sea fault a plate boundary of the transform type the graben is composed of two basins the northern one is occupied by a lake about 300 m deep and has a sedimentary fill of about 6 km the southern dead sea basin however is unusually deep with about 14 km of sedimentary fill the geometry of the southern dead sea basin is anomalous along the entire dead sea fault we suggest that the southern dead sea basin was formed during the first stage of the formation of the dead sea fault when the tips of propagating faults one from the collision front in the north and one from the red sea in the south met in this area the fault tips overlapped and curved towards each other isolating a block of crust and lithosphere that dropped into the mantle geophysical data indicate that the basin is probably bordered on all sides by vertical faults that cut deep into basement the deep part of the southern dead sea basin is not a pull apart but instead a drop down basin the salton trough in california is probably another example of such a basin

nonuniform surface uplift of the andean plateau revealed by deuterium isotopes in miocene volcanic glass from southern peru abstract proposals for rapid late miocene surface uplift driven by large scale lithospheric removal beneath the central andean plateau have been based largely on temperature sensitive paleoaltimeters both the magnitude and mechanism of this proposed pulse of uplift have been challenged first climatic general circulation models support protracted uplift with predicted temperature and isotopic shifts enhanced by attainment of threshold elevations second tectonic models in which surface elevations are compensated by regional contraction and crustal thickening question the need for lithospheric removal and predict broadly coeval uplift of the entire plateau we present hydrogen isotope data using a novel temperature insensitive volcanic glass proxy from continuous well dated lower to middle miocene basin fill in the western cordillera of the northern plateau that show a rapid decrease in d values 62 8 at 19 16 ma with extremely negative values continuing into the pliocene we propose that the basin reached its current elevation by 16 ma 6 myr earlier than proposed for the central plateau the rapid decrease in d values is consistent with punctuated surface uplift of 2 2 3 7 km between 19 and 16 ma whereas the 3 7 km upper estimate assumes a static climate similar to modern the 2 2 km lower estimate conservatively incorporates modeled changes in the isotopic composition of precipitation associated with elevation change comparison of these results to existing paleoelevation estimates from the andean plateau facilitates a tentative reconstruction of earliest middle miocene paleotopography showing a central depression with flanking hinterland and thrust belt highlands this apparent pattern of nonuniform plateau uplift contradicts shortening proportional topographic growth over tens of myr we propose that temporally and spatially irregular surface uplift may be linked to pronounced local variability in crustal shortening and or piecemeal removal of dense mantle lithosphere insofar as these irregularities reflect the scale of heterogeneity for surface uplift processes available paleoelevation estimates point to a principal uplift mechanism that varies over relatively short

anisotropy and vp vs in the uppermost mantle beneath the western united states from joint analysis of pn and sn phases pn and sn phases are valuable for resolving velocity structure in the mantle lid as they propagate horizontally right below the moho relatively few sn tomography attempts have been made compared to pn because sn is often highly attenuated or buried in p wave coda usarray has greatly increased data coverage for regional phases and both pn and sn are routinely picked by network analysts here we jointly invert pn and sn arrival time residuals with a modified time term analysis and a regularized tomography method and present new maps of crustal thickness uppermost mantle p velocity perturbations vp vs ratios and azimuthal anisotropy strength and orientation beneath the western united states the results indicate partially molten mantle below the snake river plain and the colorado plateau the seismic structure of the top 40 km of the mantle below the colorado plateau differs from that seen at greater depths in other studies such as surface wave or teleseismic body wave tomography whereas the snake river plain anomaly just below the moho is comparable to structures seen at about 200 km depth pn fast axes provide complementary information to sks shear wave splitting observations and our analysis indicates that in several regions in the western united states the orientation of azimuthal anisotropy changes with depth in the upper mantle however we have so far been unable to resolve shear wave splitting directly in sn waveforms which seem to be dominated by sn sv energy

lithospheric instability beneath the transverse ranges of california recent high resolution seismic experiments reveal that the crust beneath the san gabriel mountains portion of the transverse ranges thickens by 10 15 km contrary to earlier studies associated with the transverse ranges there is an anomalous ridge of seismically fast upper mantle material extending at least 200 km into the mantle this high velocity anomaly has previously been interpreted as a lithospheric downwelling both lithospheric downwelling and crustal thickening are associated with the oblique convergence of pacific and north america plates across the san andreas fault though it seems likely that the lithospheric downwelling is driven at least partly by gravitational instability of the cold lithospheric mantle we show by means of numerical experiment that the balance between buoyancy forces that drive deformation and viscous stresses that resist deformation determines the geometry of crustal thickening and mantle downwelling we use a simple two layered lithospheric model in which dense lithospheric mantle overlies relatively inviscid and less dense asthenosphere and is overlain by buoyant crust external plate motion drives convergence which is constrained by boundary conditions to occur within a central convergent zone of specified width a fundamental transition in the geometry of downwelling is revealed by our experiments for slow convergence or low crustal viscosity downwelling occurs as multiple sheets on the margins of the convergent zone for fast convergence or crust that is stronger than mantle lithosphere a single downwelling occurs beneath the center of the convergent zone this complexity in the evolution of the system is attributed to the interaction of crustal buoyancy with the evolving gravitational instability in order for a narrow downwelling slab to have formed beneath the transverse ranges within the last 5 myr the effective lithospheric viscosity of the convergent region is at most about 10 20 pa s

active megadetachment beneath the western united states geodetic data interpreted in light of seismic imaging seismicity xenolith studies and the late quaternary geologic history of the northern great basin suggest that a subcontinental scale extensional detachment is localized near the moho to first order seismic yielding in the upper crust at any given latitude in this region occurs via an m7 earthquake every 100 years here we develop the hypothesis that since 1996 the region has undergone a cycle of strain accumulation and release similar to slow slip events observed on subduction megathrusts but yielding occurred on a subhorizontal surface 5 10 times larger in the slip direction and at temperatures 800 c net slip was variable ranging from 5 to 10 mm over most of the region strain energy with moment magnitude equivalent to an m7 earthquake was released along this megadetachment primarily between 2000 0 and 2005 5 slip initiated in late 1998 to mid 1999 in northeastern nevada and is best expressed in late 2003 during a magma injection event at moho depth beneath the sierra nevada accompanied by more rapid eastward relative displacement across the entire region the event ended in the east at 2004 0 and in the remainder of the network at about 2005 5 strain energy thus appears to have been transmitted from the cordilleran interior toward the plate boundary from high gravitational potential to low via yielding on the megadetachment the size and kinematic function of the proposed structure in light of various proxies for lithospheric thickness imply that the subcrustal lithosphere beneath nevada is a strong thin plate even though it resides in a high heat flow tectonic regime a strong lowermost crust and upper mantle is consistent with patterns of postseismic relaxation in the southern great basin deformation microstructures and low water content in dunite xenoliths in young lavas in central nevada and high temperature microstructures in analog surface exposures of deformed lower crust large scale decoupling between crust and upper mantle is consistent with the broad distribution of strain in the upper crust versus the more localized distribution in the subcrustal lithosphere as inferred by such proxies as low p wave velocity and mafic magmatism

subduction evolution and mantle dynamics at a continental margin central north island new zealand 1 central north island new zealand provides an unusually complete geological and geophysical record of the onset and evolution of subduction at a continental margin whereas most subduction zones are innately two dimensional north island of new zealand displays a distinct three dimensional character in the back arc regions specifically we observe mariana type subduction in the back arc areas of central north island in the sense of back arc extension high heat flow prolific volcanism geothermal activity and active doming and exhumation of the solid surface evidence for emplacement of a significant percent of new lithosphere beneath the central north island comes from heat flux of 25 mw km of strike of volcanic zone and thinned crust underlain by rocks with a seismic wave speed consistent with underplated new crust seismic attenuation qp 1 is high 240 and rhyolitic and andesitic volcanism are widespread almost complete removal of mantle lithosphere is inferred here in pliocene times on the basis of the rock uplift history and upper mantle seismic velocities as low as 7 4 0 1 km s in contrast southwestern north island exhibits chilean type back arc activity in the sense of compressive tectonics reverse faulting low heat flow thickened lithosphere and strong coupling between the subducted and overriding plates this rapid switch from mariana type to chilean type subduction occurs despite the age of the subducted plate being constant under north island moreover stratigraphic evidence shows that processes that define the extensional back arc area the central volcanic region are advancing southward into the compressional system wanganui basin at about 10 mm yr we link the progression from one system to another to a gradual and viscous removal of thickened mantle lithosphere in the back arc regions thickening occurred during the miocene within the taranaki fault zone the process of thickening and convective removal is time and temperature dependent and has left an imprint in both the geological record and geophysical properties of central north island which we document and describe

delamination in collisional orogens thermomechanical modeling 1 modes of mantle delamination in collision zones are discussed in the light of 2d numerical modeling freely evolving thermomechanical models take into account phase changes in the mantle and melting in the crust distinct modes are identified in which orogens undergo mantle delamination concurrently or after collision delamination propagates along the moho of the subducted plate together with the retreating trench provided that slab pull is sufficient and that the meta stability of the crust lithospheric mantle interface is overcome topography is an instantaneous response to delamination and migrates with the focused and localized separation between crust and lithospheric mantle delamination front early exhumation of high pressure rocks is followed by exhumation of high temperature partially molten rocks convective stabilization of delamination outlasts slab break offs and impedes renewed build up of mechanically strong lithospheric mantle by static cooling parameter studies investigate the sensitivity of delamination to the age of the subducted oceanic plate the collision rate the mantle rheology and explore the effects of melting and sedimentation mantle flow patterns that form around delamination and subduction are sensitive to and often limited by the semi permeable 670 km spinel perovskite transition demonstrating that phase changes are essential factors delamination may account for the formation of complex boundary zones between continents such as between africa and europe in the mediterranean where transient compression and extension the lack of mantle lithosphere and the exhumation of high grade metamorphic core complexes are features also obtained in our modeling results

preservation exhumation of ultrahigh pressure subduction complexes abstract ultrahigh pressure uhp metamorphic terranes reflect subduction of continental crust to depths of 90 140 km in phanerozoic contractional orogens rocks are intensely overprinted by lower pressure mineral assemblages traces of relict uhp phases are preserved only under kinetically inhibiting circumstances most uhp complexes present in the upper crust are thin imbricate sheets consisting chiefly of felsic units serpentinites dense mafic and peridotitic rocks make up less than 10 of each exhumed subduction complex roundtrip prograde retrograde p t paths are completed in 10 20 myr and rates of ascent to mid crustal levels approximate descent velocities late stage domical uplifts typify many uhp complexes sialic crust may be deeply subducted reflecting profound underflow of an oceanic plate prior to collisional suturing exhumation involves decompression through the p t stability fields of lower pressure metamorphic facies scattered uhp relics are retained in strong refractory watertight host minerals e g zircon pyroxene garnet typified by low rates of intracrystalline diffusion isolation of such inclusions from the recrystallizing rock matrix impedes back reaction thin aspect ratio ductile deformed nappes are formed in the subduction zone heat is conducted away from uhp complexes as they rise along the subduction channel the low aggregate density of continental crust is much less than that of the mantle it displaces during underflow its rapid ascent to mid crustal levels is driven by buoyancy return to shallow levels does not require removal of the overlying mantle wedge late stage underplating structural contraction tectonic aneurysms and or plate shallowing convey mid crustal uhp decollements surfaceward in domical uplifts where they are exposed by erosion unless these situations are mutually satisfied uhp complexes are completely transformed to low pressure assemblages obliterating all evidence of profound subduction

210 po and 210 pb distribution dissolved particulate exchange rates and particulate export along the north atlantic us geotraces ga03 section vertical profiles of 210po and 210pb in the water column were measured in the dissolved phase 51 m particles at seven stations along the us geotraces north atlantic zonal transect ga03 mass balance calculations were employed to assess nuclide exchange rates at the dissolved small particle interface and between small and large particles and to quantify export with settling large particles in the surface ocean 210po scavenging is linearly correlated with the concentration of particulate organic carbon poc in large particles supporting the role of biogenic particle in 210po bioaccumulation and export in stations near the coast this link is more complex due to the variable source of biogenic material and temporal changes in the surface biogeochemical and physical conditions at depth 210po exhibits significant widespread deficit with respect to 210pb that could in part be attributed to in situ 210po scavenging and may be related to surface biological productivity as previously reported the 210pb scavenging rates in the surface ocean were higher at ocean margins at depth 210pb scavenging increases with depth and eastward due to the increase of adsorption sites available in the benthic layers and to a regional contribution of benthic 210pb scavenging and or particle flux respectively the benthic nepheloid layer bnl and the hydrothermal tag plume distinctly enhance 210pb scavenging due to increased surface adsorption in association with resuspended or freshly formed particles in contrast 210po is not seen to be significantly scavenged in these environments due to its relatively short half life and the long residence time of particles

tectonic control on southern sierra nevada topography california in this study we integrate the apatite u th he thermochronometric technique with geomorphic structural and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late cenozoic faulting in the southern sierra nevada the geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low relief landscape little affected by internal deformation to a more complex landscape affected by numerous faults regionally the relict landscape surface is readily resolved by age elevation relationships of apatite he ages coupled to geomorphology these relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting on the basis of 63 new apatite he ages and stratigraphic data from proximal parts of the san joaquin basin we resolve two sets of normal faults oriented approximately n s and approximately nw quaternary west side up normal faulting along the n s breckenridge kern canyon zone has resulted in a southwest step over from the owens valley system in the controlling structure on the regional west tilt of sierran basement this zone has also served as a transfer structure partitioning neogene quaternary extension resulting from normal displacements on the nw fault set this fault system for the most part nucleated along late cretaceous structures with late cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region

current sierra nevada north america motion from very long baseline interferometry implications for the kinematics of the western united states we use geodetic measurements from very long baseline interferometry to estimate the motion of the sierra nevadan microplate which is composed of the sierra nevada and the great valley the motion of the sierra nevadan microplate relative to the north american plate is described by a right handed rotation of 0 61 m y about lat 32 n long 128 w this euler pole lies only 10 southwest of the sierra nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis it further predicts a velocity of the eastern edge of the sierra nevada at 38 0 n 119 3 w relative to stable north america of 11 1 mm yr toward n36 3 w quoted uncertainties are plus or minus one standard error which accounts for about one fourth of the velocity between the pacific and north american plates and is 25 clockwise of many prior estimates the velocity nearly parallels the boundary between the sierra nevada and the great basin which implies that current motion within the great basin results in a rotational noncoaxial deformation we use this velocity to estimate how motion is distributed across the broad deforming zone taking up pacific north america plate motion we find that the vector sum of strike slip along the san andreas fault and motion of the sierra nevada relative to stable north america taken up by deformation within the great basin differs little from the pacific north america plate velocity the difference can be described at 36 n along the san andreas fault by a vector of 6 mm yr directed toward n20 w this vector resolves into components of 5 mm yr parallel to the fault and 2 mm yr perpendicular to the fault with 95 confidence intervals of 0 to 10 mm yr and 1 to 5 mm yr respectively the component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault perpendicular shortening we conclude that motion previously inferred to be taken up by deformation other than strike slip along the san andreas fault or deformation within the great basin is much smaller than previously thought

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

magnetotelluric evidence of lithospheric mantle thinning beneath the southern sierra nevada a wideband 0 01 1000 s magnetotelluric survey across the southern sierra nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10 20 km the eastern zone underlies the highest topography of the range and extends eastward this eastern conductive zone extends to depths in excess of 100 km based on model sensitivities and lies well below the moho depth of 32 38 km from seismological studies therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt such an interpretation is consistent with gravitational seismological and geologic evidence estimates of the partial melt fraction range from 2 to 5 at depths of 40 70 km which are consistent with fractions of melt inclusions observed in xenoliths from the eastern sierra nevada this partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations the other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes the western conductive zone also straddles the moho and extends into the mantle beneath western foothills of the sierra nevada and the great valley sediments to the west however xenoliths from this region indicate high velocity crustal rocks to depths in excess of 60 km and we therefore attribute the enhanced conductivity to graphitic metasediments and or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the sierran plutons

using tracer thermochronology to measure modern relief change in the sierra nevada california abstract we examine relief change in two major sierra nevada drainages the kings and the san joaquin the modern distribution of bedrock cooling ages is derived by fitting isochrones to a dense distribution of existing cooling ages this analysis indicates southwest tilting of the range by 3 4 0 8 which is consistent with other geologic evidence for tilting after 5 ma the detrital cooling ages from the kings show an excess of young ages relative to what would be expected from uniform erosion whereas the san joaquin drainage shows excess old ages these results indicate increasing relief in the kings the results in the san joaquin are more difficult to interpret due to the dense network of dams which trap sediment however we analyze the effect of storage behind the dams and suggest steady or declining relief in the san joaquin the residence time of sediment in these drainages indicates that these relief estimates are averaged over the last 100 to 10 ka we suggest that relief change is related to localized surface uplift in the southern sierra nevada including the kings drainage caused by ongoing lower lithosphere removal the san joaquin drainage may be north of the lithospheric anomaly which is consistent with stagnating or decaying relief there

three dimensional seismic model of the sierra nevada arc california and its implications for crustal and upper mantle composition a three dimensional p wave velocity model of south central california from the coast ranges to the sierra nevada shows that the crust under most of the southern sierra nevada batholith has seismic velocities 5 9 6 3 km s below the continental average the crust is not much thicker on average about 35 km than in the adjacent great valley and basin and range province apart from a small northward thickening crustal root under the western sierra nevada that reaches a depth of 42 km crustal velocities above the continental average are observed beneath much of the great valley due to a high velocity body underlying the sedimentary basin and the foothills metamorphic belt 6 4 7 0 km s upper mantle velocities are generally low 7 8 km s but span a wide range 7 4 8 2 km s we display the velocity model in several cross sections and maps of moho depth and average crustal velocity the measured velocities in the upper and mid crust of the sierra nevada batholith are in good agreement with laboratory measurements on sierra nevada tonalites after corrections for density and temperature peridotite xenoliths from the eastern sierra nevada suggest strong upper mantle anisotropy which could explain some of the velocity heterogeneity in the sierra nevada mantle by the time cretaceous subduction related magmatism ceased the sierra nevada arc must have had a thick mafic lower crust yet a principal result of our work is that today the batholith has a crust of mainly felsic composition throughout a subcrustal layer with velocities below normal pn velocities 7 6 km s may indicate the presence of lower crustal material in eclogite facies

rayleigh and s wave tomography constraints on subduction termination and lithospheric foundering in central california the crust and upper mantle structure of central california have been modified by subduction termination growth of the san andreas plate boundary fault system and small scale upper mantle convection since the early miocene here we investigate the contributions of these processes to the creation of the isabella anomaly which is a high seismic velocity volume in the upper mantle there are two types of hypotheses for its origin one is that it is the foundered mafic lower crust and mantle lithosphere of the southern sierra nevada batholith the alternative suggests that it is a fossil slab connected to the monterey microplate a dense broadband seismic transect was deployed from the coast to the western sierra nevada to fill in the least sampled areas above the isabella anomaly and regional scale rayleigh and s wave tomography are used to evaluate the two hypotheses new shear velocity vs tomography images a high velocity anomaly beneath coastal california that is sub horizontal at depths of 40 80 km east of the san andreas fault a continuous extension of the high velocity anomaly dips east and is located beneath the sierra nevada at 150 200 km depth the western position of the isabella anomaly in the uppermost mantle is inconsistent with earlier interpretations that the isabella anomaly is connected to actively foundering foothills lower crust based on the new vs images we interpret that the isabella anomaly is not the dense destabilized root of the sierra nevada but rather a remnant of miocene subduction termination that is translating north beneath the central san andreas fault our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern sierra nevada where we find a lower crustal low vs layer consistent with a small amount of partial melt the high elevations relative to crust thickness and lower crustal low vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a 3 4 ma pulse of basaltic magmatism

influence of lithology on hillslope morphology and response to tectonic forcing in the northern sierra nevada of california 1 many geomorphic studies assume that bedrock geology is not a first order control on landscape form in order to isolate drivers of geomorphic change e g climate or tectonics yet underlying geology may influence the efficacy of soil production and sediment transport on hillslopes we performed quantitative analysis of lidar digital terrain models to examine the topographic form of hillslopes in two distinct lithologies in the feather river catchment in northern california a granodiorite pluton and metamorphosed volcanics the two sites separated by 2 km and spanning similar elevations were assumed to have similar climatic histories and are experiencing a transience in landscape evolution characterized by a propagating incision wave in response to accelerated surface uplift c 5 ma responding to increased incision rates hillslopes in granodiorite tend to have morphology similar to model predictions for steady state hillslopes suggesting that they adjust rapidly to keep pace with the incision wave by contrast hillslopes in metavolcanics exhibit high gradients but lower hilltop curvature indicative of ongoing transient adjustment to incision we used existing erosion rate data and the curvature of hilltops proximal to the main channels where hillslopes have most likely adjusted to accelerated erosion rates to demonstrate that the sediment transport coefficient is higher in granodiorite 8 8 m2 ka 1 than in metavolcanics 4 8 m2 ka 1 hillslopes in both lithologies get shorter i e drainage density increases with increasing erosion rates

small scale convective instability and upper mantle viscosity under california thermal calculations and convection analysis constrained by seismic tomography results suggest that a small scale convective instability developed in the upper 200 kilometers of the mantle under california after the upwelling and cooling of asthenosphere into the slab window associated with the formation of the san andreas transform boundary the upper bound for the upper mantle viscosity in the slab window 5 x 10 19 pascal seconds is similar to independent estimates for the asthenosphere beneath young oceanic and tectonically active continental regions these model calculations suggest that many tectonically active continental regions characterized by low upper mantle seismic velocities may be affected by time dependent small scale convection that can generate localized areas of uplift and subsidence

plate mantle coupling and continental flooding the flooding records of continents have been computed in dynamically self consistent models of plates and convection platforms flood following rapid translation of continental plates as may occur following supercontinent breakup in these models continental flooding is primarily controlled by deep mantle sources and continental hypsometry is strongly influenced by dynamic topography some models especially bottom heated ones predict more extensive flooding than has been observed during the phanerozoic however the computed flooding can be made consistent with the observations if the viscosity of the upper regions of the convecting system are significantly reduced compared to the deep regions the fluid is driven predominantly by internal heating or the convection is confined to a depth appreciably less than the width of the non subducting plate

rayleigh taylor instability under a shear stress free top boundary condition and its relevance to removal of mantle lithosphere from beneath the sierra nevada 1 the separation of zones of apparent downwelling flow at the ends of the sierra nevada suggests a relatively large wavelength 500 km of unstable growth but rayleigh taylor instabilities for plausible rheological structures with a fixed top boundary condition require much shorter wavelengths 100 km for maximum growth rates to understand this difference we analyze analytical solutions and perform numerical 2 d plane strain experiments for rayleigh taylor instability of a dense layer overlying a less dense substratum representing the instability between the mantle lithosphere and the underlying asthenosphere focusing on the effects of a shear stress free boundary condition at the top the overall effect of this condition is an enhancement of growth rate factors at long wavelengths which depends greatly on the exponential viscosity variation with depth of the layer with large or little variation across the unstable layer the solutions approximate those with a fixed top boundary condition or for constant viscosity respectively an intermediate zone showing the enhanced growth rates includes ratios of layer thickness to viscosity e folding length h l of 1 8 for newtonian viscosity and 1 4 for nonlinear viscosity the free top condition is likely applicable to geologic situations where the lower crust is weak olivine flow laws and low temperature estimates at 35 km depth 255 355 c place the sierra nevada viscosity scaling ratio h l between 5 and 9 thus longer wavelengths than commonly assumed for rayleigh taylor instabilities seem permissible when viscosity decreases with depth and the top surface of the layer is only weakly constrained

late paleozoic to early mesozoic mafic ultramafic complexes from the northern north china block constraints on the composition and evolution of the lithospheric mantle abstract the late paleozoic to early mesozoic mafic ultramafic complexes from the northern north china block ncb are used to constrain the isotopic composition and evolution of mantle reservoirs during different tectonic stages prior to the initiation of lithosphere destruction in the late mesozoic zircon u pb ages show that emplacement of the mafic ultramafic complexes occurred in at least three stages middle devonian ca 395 ma late carboniferous early permian 308 276 ma and triassic 250 220 ma most of the middle devonian complexes display low initial 87sr 86sr ratios of 0 70467 0 70492 weak to moderate negative end t values from 6 3 to 1 5 and weakly negative to weakly positive zircon ehf t values from 11 5 to 5 5 indicating that the parent magma was derived from slightly enriched lithospheric mantle the late carboniferous early permian complexes are characterized by low initial 87sr 86sr ratios of 0 70521 0 70604 significant negative end t values from 14 1 to 9 3 and zircon ehf t values from 17 0 to 10 5 and were probably derived from metasomatized lithospheric mantle the early triassic complexes exhibit initial 87sr 86sr ratios of 0 70656 0 70825 and significant negative end t values from 17 1 to 8 2 indicating their derivation from enriched lithospheric mantle however the late triassic mafic ultramafic complexes display weakly negative to weakly positive end t values of 4 4 to 0 5 and zircon ehf t values from 2 9 to 1 7 indicating that magma sources likely resulted from mixing of depleted asthenospheric mantle and enriched lithospheric mantle zircon ages geochemistry and sr nd hf isotopic studies suggest that the lithospheric mantle beneath the northern ncb had a slightly enriched signature during the middle devonian but it became significantly enriched during the late carboniferous to early permian due to metasomatism of lithospheric mantle during the southward subduction of the paleo asian oceanic plate after final closure of the paleo asian ocean and amalgamation of the mongolian arc terranes with the northern ncb in the latest permian to earliest triassic enriched lithospheric mantle still existed beneath the northern ncb during the early triassic however in the late triassic involvement of asthenospheric mantle became significant owing to strong upwelling of asthenospheric mantle and post collisional lithospheric delamination in the early late triassic these results suggest that the cratonic lithospheric mantle beneath the northern ncb underwent multiple stages of modification prior to its destruction and thinning initiation of lithospheric destruction and thinning in the northern ncb likely occurred during the early late triassic which is much earlier than in other parts of the ncb where it occurred in the late mesozoic it appears likely that lithospheric destruction and thinning of the ncb initially started at the northern and eastern margins of the craton as a result of post collisional lithospheric delamination and then spread to the interior of the craton

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

topography of the overriding plate during progressive subduction a dynamic model to explain forearc subsidence overriding plate topography provides constraints on subduction zone geodynamics we investigate its evolution using fully dynamic laboratory models of subduction with techniques of stereoscopic photogrammetry and particle image velocimetry model results show that the topography is characterized by an area of forearc dynamic subsidence with a magnitude scaling to 1 44 3 97 km in nature and a local topographic high between the forearc subsided region and the trench these topographic features rapidly develop during the slab free sinking phase and gradually decrease during the steady state slab rollback phase we propose that they result from the variation of the vertical component of the trench suction force along the subduction zone interface which gradually increases with depth and results from the gradual slab steepening during the initial transient slab sinking phase the downward mantle flow in the nose of the mantle wedge plays a minor role in driving forearc subsidence

contemporary vertical velocity of the central basin and range and uplift of the southern sierra nevada 1 we estimate the present day vertical velocity field in the central basin and range and sierra nevada mountains and consider causative processes we analyzed data from 16 continuously operating global positioning system stations finding that relative vertical velocity precision wrms scatter is 0 25 mm yr these data demonstrate that the southern sierra nevada is experiencing uplift of 0 5 mm yr relative to a local reference frame in southern nevada and 1 mm yr relative to stations in the northern mojave desert as a possible source of these motions we explored post seismic viscoelastic relaxation following the 1872 owens valley and 1952 kern county earthquakes with models of laterally homogeneous lower crust and upper mantle viscosity post seismic deformation from these two earthquakes cannot entirely explain the data set lateral variations in viscosity or contributions from other regional earthquakes may also be important

rayleigh taylor instabilities with anisotropic lithospheric viscosity summary rocks often develop fabric when subject to deformation and this fabric causes anisotropy of physical properties such as viscosity and seismic velocities we employ 2 d analytical solutions and numerical flow models to investigate the effect of anisotropic viscosity av on the development of rayleigh taylor instabilities a process strongly connected to lithospheric instabilities our results demonstrate a dramatic effect of av on the development of instabilities their timing location and most notably their wavelength are strongly affected by the initial fabric specifically we find a significant increase in the wavelength of instability in the presence of av which favours horizontal shear we also find that an interplay between regions with different initial fabric gives rise to striking irregularities in the downwellings our study shows that for investigations of lithospheric instabilities and likely of other mantle processes the approximation of isotropic viscosity may not be adequate and that av should be included

using hilltop curvature to derive the spatial distribution of erosion rates 1 erosion rates dictate the morphology of landscapes and therefore quantifying them is a critical part of many geomorphic studies methods to directly measure erosion rates are expensive and time consuming whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents if hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates in this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high resolution 1 m digital elevation data published and new cosmogenic radionuclide analyses in the feather river basin california suggest that erosion rates vary by over an order of magnitude 10 to 250 mm kyr 1 hilltop curvature increases with erosion rates allowing calibration of the hillslope sediment transport coefficient which controls the relationship between gradient and sediment flux having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature additionally we show that hilltop curvature continues to increase with rising erosion rates after gradient limited hillslopes have emerged hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient providing soil production on hilltops can keep pace with erosion finally hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels