magnetotelluric observations in the western ouachita mountains southeastern oklahoma the first magnetotelluric mt analysis of the ouachita mountains region is presented magnetotelluric data acquired at 19 sites along a 60 km profile in southeastern oklahoma were used to image the western extension of the ouachita mountains and to determine the poorly known subsurface interaction between the pennsylvanian tishomingo belton uplift and the subsurface extension of the exposed western ouachita mountains drill hole data geologic mapping seismic reflection profiles and 1 d and 2 d mt derived models indicate that lying beneath the low resistivity gulf coastal plain sediments are 2 3 km of deep water lower pennsylvanian jackfork group sediments and 6 8 km of ouachita facies lithologies mainly consisting of the stanley group beneath the profile s northern section are 2 4 km of atoka formation sediments probably deposited within the arkoma basin that underlie thrusted zones of the stanley group the most unique feature is a high resistivity zone beneath stations 7 9 interpreted to be p

enhanced mantle conductivity from sulfide minerals southern sierra nevada california petrographic studies of peridotitic xenoliths entrained in late quaternary basalts from beneath the southern sierra nevada have revealed the presence of accessory sulfide minerals along grain boundaries and fractures equilibration temperatures from the xenoliths are sufficiently high that the molten sulfides coexist with the basaltic melt sulfides are extremely conductive relative to the solid matrix or the basaltic melt so a small fraction can increase the bulk conductivity of the mantle appreciably previous estimates of 2 5 partial melt from magnetotelluric measurements can be plausibly reduced to less than 1 such low melt percentages have longer residence times in the mantle and are more consistent with the volumetrically minor late quaternary basalt flows and the primitive basalt compositions

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

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

lithospheric heterogeneity in the kyrgyz tien shan imaged by magnetotelluric studies 1 a 450 km long north south magnetotelluric profile spanning the tien shan from kazakhstan to western china reveals lateral variations in the resistivity of the mantle lithosphere to depths of 140 km minimum changes of one order of magnitude in this depth range result from variations in temperature or composition or both higher resistivities beneath a central portion of the range where the moho is half as deep as elsewhere in the tien shan indicate a strong lithospheric block we propose that this block protects this region of thin crust from appreciable deformation and instead transmits stresses due to india asia plate convergence from the southern to the northern parts of the range

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

fluid generation and pathways beneath an active compressional orogen the new zealand southern alps inferred from magnetotelluric data 1 forty one wideband magnetotelluric mt soundings were collected in a 150 km long transect across the southern alps of the central south island of new zealand an active compressional orogen decomposed mt impedance tensors vertical magnetic field relations and reconnaissance soundings at two locations off line imply an approximately two dimensional geometry here with average regional geoelectric strike of n40 e similar to surface geologic trends two independent two dimensional inversion algorithms were applied to the mt data and both imply a concave upward u shaped middle to lower crustal conductive zone beneath the west central portion of the island the average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along strike conductive strands narrower than the transverse magnetic cross strike mode mt data can resolve the deep crustal conductor under the southern alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust fluid interconnection and electrical conduction are promoted by shear deformation the conductor rises northwestward toward the trace of the alpine fault but attains a near vertical configuration at a depth of 10 km and reaches close to the surface 5 10 km inland of the fault trace itself the transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle ductile transition in uplifting schist and approach the surface through induced hydrofractures the high grade schist becomes resistive after depletion of fluids and continues to extrude toward the alpine fault shallow extensions of the deep high conductivity are coincident with modern hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance to the southeast high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent southern alps orogen which also exhibits evidence for expulsion of high temperature fluids the higher conductivity inferred along strike possible anisotropy could reflect more efficient fluid interconnection in this higher strain direction as well as possible contributions by sheared fluid deposited graphite conductivity of the uppermost mantle of the south island is low consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by p wave delay studies

internal structure of the sierra nevada batholith based on specific gravity and gravity measurements about 6 000 specific gravity sg measurements of samples collected from nearly 200 granitic plutons comprising the central sierra nevada batholith yield a sg contour map across the batholith from 36 25 to 38 north latitude with notable exceptions sg decreases from values generally greater than 2 7 in the west to less than 2 6 over a few small areas of high silica high potassium granites near the east edge a good correlation between measured sg and analyzed weight percent sio2 enables estimation of average silica variations across the batholith the average sg is 2 69 corresponding to an average of 68 wt sio2 for the 18 000 km central part of the batholith a 1 km gridded version of the sg measurements has been used to generate a series of synthetic gravity maps assuming that the sg of rocks at the surface extends unchanged to various depths the synthetic map computed for a depth of 10 km shows the best correspondence with the isostatic residual gravity map indicating that variations in the observed gravity residuals are largely caused by sg variations of the plutonic rocks exposed at the surface that apparently extend downward to an average depth of about 10 km although the 10 km synthetic gravity map gives the best overall fit to the observed gravity data comparison of individual anomalies indicates that the bottoms of the plutons as defined by sg variations at the surface are generally shallower along the west edge of the sierra nevada 7 2 km and deeper in the younger and more felsic eastern part 12 3 km these depths do not necessarily represent a distinct base of the sierra nevada batholith they may indicate the depth below which density homogenization occurs either by igneous or possibly structural processes

trapping of aqueous fluids in the deep crust free aqueous fluids in the deep ductile crust have residence times there which are very short in geological terms if not consumed by retrograde metamorphic reactions they empty very rapidly upwards this can be shown by applying the theory of magma percolation in a ductile host to the problem of aqueous fluid percolation below the brittle ductile transition thermal activation of ductility is strong enough to facilitate the rapid evolution of porosity and consequently allow rapid fluid transport in the cooler crust above the brittle ductile transition however this mechanism vanishes as a consequence fluid overpressures resulting from the accumulation of fluid in the transition zone must induce hydraulic fracturing rather than pore inflation in regions of horizontally compressive regional stress the commonest situation in stable continental crust these fractures will be horizontal aqueous fluids rising through or liberated from the lower crust will therefore probably accumulate at least initially at the brittle ductile transition in horizontally extensive reservoirs with high horizontal permeability but low vertical permeability

flexural isostasy and uplift of the sierra nevada of california the sierra nevada of california has experienced several kilometers of uplift in the last 10 m y but its present elevation is isostatically compensated by a crustal root that has probably existed since late cretaceous flexural isostasy can reconcile these observations basin and range faulting and extension starting 10 m y ago allowed rapid uplift of an eroded and regionally compensated magmatic arc underlain by an excess crustal root a quantitative model of an unbroken elastic plate buoyed up by the excess root provides an explanation for the estimated 10 m y paleotopography of the range the same model fits the present topography when broken shear and fiber stresses not transmitted at the location of the owens valley fault zone the broken plate model predicts the appropriate amount of westward tilt of the sierra nevada and can explain the pronounced eastward tilt of the white and inyo mountains the position where paleo and present drainages in the sierra cross over in elevation is the most sensitive indicator of the elastic thickness required of the lithosphere an effective elastic thickness of 50 km best fits the observed crossover distances and is consistent with the amount of post 10 m y uplift calculated fiber and shear stresses suggest that actual extension of the crust is more important in permitting the uplift than the mere presence of normal faults part of the residual isostatic gravity anomaly pattern high gravity over the eastern great valley and western sierra nevada low residual gravity over the high sierra may be explained by the continuing flexural support of departure from local isostatic equilibrium the model implies that before 10 m y ago the sierra should have been marked by a negative isostatic gravity anomaly of more than 100 m gal flanked by isostatic gravity highs such large isostatic anomalies have not been observed to date over eroded arcs but the sierra has uniquely low heat flow and presumably therefore high lithospheric strength and the sierran root is at the upper limit of size that could be isostatically suppressed

resistive dry lower crust in an active orogen nanga parbat northern pakistan abstract conductivity models beneath the nanga parbat haramosh massif nphm derived from magnetotelluric soundings reveal that there is no widespread interconnected conductive aqueous fluid to minimum depths of 40 km below sea level given the continuing tectonic denudation young granitic and migmatitic bodies indicating partial melt at shallow crustal depths and active seismicity this result is surprising in light of similar studies in active tectonic regimes elsewhere away from the nphm models reveal the usual conductive lower crust we propose that deep magmatic and metamorphic fluids are produced beneath nphm in isolated zones but that the active deformation permits their escape through the brittle ductile transition a magnetotelluric survey in an area as complicated as nanga parbat required the development of methodologies for utilizing standard two dimensional inversions in a three dimensional environment we show here how to identify which parts of the magnetotelluric responses are adequately represented with two dimensional approximations unlike previous efforts we do not attempt to create a set of generic rules that may be applicable to all geologic environments instead a procedure is outlined that can be tailored to each interpretation one important result of this work is that magnetotelluric data along a profile can be used to constrain structure off the ends of the profile

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

india s hidden inputs to tibetan orogeny revealed by hf isotopes of transhimalayan zircons and host rocks abstract to better understand tibetan orogenesis which involves complex tectonic processes here we report the first integrated analysis of hf isotopes of zircon separates and their host rocks from the transhimalayan batholiths the rocks studied include 19 granitoids from different parts of the gangdese batholith the largest transhimalayan intrusive complex and five postcollisional adakites that occurred as plugs or dikes cutting the gangdese batholith in the southern lhasa terrane southern tibet published zircon hf isotope data from other transhimalayan granitoids and the linzizong volcanic rocks are also synthesized the magmatic zircons crystallizing between ca 200 and 15 ma show depleted mantle type hf isotopic characteristics throughout the mesozoic but shift markedly in the paleogene the hf isotopic shift is interpreted as tracking the evolving progress of himalayan sediment subduction driven by the approaching indian continent and thus signals the initiation of the india asia collision that we infer to have occurred by 55 ma our data furthermore indicate that southern tibet underwent significant crustal thickening during ca 45 and 30 ma before emplacement of the postcollisional adakites that exhibit hf nd isotopic systematics suggesting binary mixing of melts from the juvenile gangdese mafic crust and incorporated himalayan sediments in the petrogenesis

magnetotelluric evidence for crustal suture zones bounding the southern great valley california a geoelectric section inferred from a regional magnetotelluric study across the coast ranges the great valley and the sierra nevada reveals significant variations in electrical resistivity zones of lower resistivity interpreted at depths from 10 km to at least 30 km lie near mapped geologic boundaries between the coast ranges and the great valley and beneath the eastern side of the great valley the former boundary is inferred by others to separate the subduction complex of the coast ranges from the mafic basement of the great valley the lower resistivities are most likely associated with metasediments trapped between the coast ranges ophiolite and the former oceanic crust beneath the great valley the latter boundary is problematic but may be evidence for a deep metasedimentary section trapped between the ophiolites beneath the great valley and the granitic rocks of the sierra nevada the lack of change in the magnetotelluric phase across the great valley indicates that a suture zone marked by lower resistivities is unlikely to be present beneath the valley however this does not preclude the existence of a resistive suture zone

reinterpretation by ray tracing of a transverse refraction seismic profile through the california sierra nevada part i abstract this study reanalyzes a refraction seismic profile from nts nevada to san francisco transverse to the high sierra nevada three models of the earth9s crust with an asymmetrical mountain root constructed by different authors are tested by the use of ray tracing model cqm carer et al 1970 with a maximum crustal thickness of 35 km centered about 50 km west of the crest of the sierra models a and b pakiser and brune 1980 with a mountain root extending to a depth of 55 km below the sierra in the latter guided and diffracted waves are used to explain seismic p n energy in the shadow zone of the root the preferred model a has a high velocity layer dipping from west to east in the upper crust while model b does not the present study demonstrates how fallacies in interpretation of complicated structures can occur if all ray types are not quantitatively traced the influence of the p wave gradient in the upper mantle on the length of the shadow zone is shown to be important and it is demonstrated that a modified cqm model explains the observed travel times when diffracted waves are included cqm does not however explain why the p n travel times on a separate refraction profile along the sierra nevada from a truckee earthquake are so great model b interprets the p n signals at 200 km as diffracted waves but the computed travel times do not fit the observations the dipping wave guide in model a does not work as expected and perhaps should be ruled out

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

can in situ measurements of mantle electrical conductivity be used to infer properties of partial melts 1 constraints on bulk conductivity from magnetotelluric measurements and petrological analyses of late quaternary peridotite xenoliths from the southern sierra nevada allow evaluation of models commonly used to relate electrical conductivity to the physical and chemical state of the upper mantle in these models two conductive melts basalt and sulfide are embedded in a resistive matrix bounds on the amount of sulfide 0 06 0 4 and the bulk conductivity 0 03 0 1 s m place constraints on the degree of interconnection between the melts because the sulfide melt is very conductive even a small fraction of well connected melt results in a bulk conductivity larger than 0 1 s m similarly completely disconnected melts result in bulk conductivities much less than 0 03 s m the only models which matched both the bulk conductivity and sulfide bounds consisted of a small fraction 1 interconnected basalt melt with a discontinuous sulfide phase such a texture is observed in laboratory experiments with much larger sulfide melt fractions but has not been reported for small melt fractions a variant of the hashin shtrikman model and a hybrid model consisting of cascaded hashin shtrikman calculations were successful in matching the magnetotelluric and petrologic constraints with a model that appropriately simulates the melt interconnectivity we suggest that electrical conductivity may be used to infer in situ melt properties in the mantle

effects of three dimensional structure on magnetotelluric sounding curves significant errors may result when applying one dimensional 1 d or two dimensional 2 d interpretation methods to magnetotelluric mt data collected in three dimensional 3 d environments both depths and resistivities can be grossly incorrect if 1 d or 2 d methods are applied in a 3 d setting we present examples of mt sounding curves generated using a 3 d modeling program madden and park 1982 and illustrate some interpretation pitfalls if 3 d effects are not considered the 3 d effects discussed herein are attributed to a surface layer heterogeneity and can be readily identified in mt data from a well designed mt survey the mt survey must include stations chosen to yield information about regional structure alternatively carefull examination of a geologic map will help the intepreter estimate the regional effects present in the data

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

thermal evolution of the sierra nevada tectonic implications of new heat flow data eight new heat flow measurements in the southern sierra nevada constrain models of thermal evolution and lithospheric structure low reduced heat flows 18 to 21 mw m in the southwest sierra nevada are consistent with previous results from the northwestern and central sierra nevada and extend the known region of linear heat flow heat production correlation an additional 150 km to the south isostatic residual gravity and measured rock densities are also linearly correlated in the central sierra nevada suggesting a general association between the upper crustal distribution of density and heat production the linear residual gravity density relation implies that isostatic residual gravity anomalies have upper crustal sources and therefore is evidence against a flexural model of cenozoic sierra nevada uplift new reduced heat flows measured in the southeast sierra nevada are relatively high 32 to 57 mw m and correlate spatially with a region of high seismicity that includes extensional earthquake swarms this correlation supports the view that basin and range extensional tectonics and associated magmatic processes are encroaching on the eastern sierra nevada in the stable southwest sierra nevada as in the central and northwestern sierra nevada the persistence of low reduced heat flow at the surface is consistent with a thermal origin for the cenozoic uplift of the sierra nevada provided the conductive lithosphere is at least 60 to 90 km thick simple order of magnitude calculations show that thermal uplift models combining mechanisms such as advective warming and thinning of the mantle lithosphere advection of basaltic magma and heat into the crust simple thermal expansion and eclogite to basalt phase conversion can account for the timing and amount of cenozoic sierra nevada uplift thermal models do not need the assumptions of unusual lithospheric strength and crustal buoyancy required by mechanical models for cenozoic uplift driven by a mesozoic crustal root

lithospheric dismemberment and magmatic processes of the great basin colorado plateau transition utah implied from magnetotellurics 1 to illuminate rifting processes across the transition zone between the extensional great basin and stable colorado plateau interior we collected an east west profile of 117 wideband and 30 long period magnetotelluric mt soundings along latitude 38 5 n from southeastern nevada across utah to the colorado border regularized two dimensional inversion shows a strong lower crustal conductor below the great basin and its transition zone in the 15 35 km depth range interpreted as reflecting modern basaltic underplating hybridization and hydrothermal fluid release this structure explains most of the geomagnetic variation anomaly in the region first measured in the late 1960s hence the transition zone while historically included with the colorado plateau physiographically possesses a deep thermal regime and tectonic activity like that of the great basin the deep crustal conductor is consistent with a rheological profile of a brittle upper crust over a weak lower crust in turn on a stronger upper mantle jelly sandwich model under the incipiently faulted transition zone the conductor implies a vertically nonuniform mode of extension resembling early stages of continental margin formation colorado plateau lithosphere begins sharply below the western boundary of capitol reef national park as a resistive keel in the deep crust and upper mantle with only a thin and weak moho level crustal conductor near 45 km depth several narrow steep conductors connect conductive lower crust with major surface faulting some including modern geothermal systems and in the context of other great basin mt surveying suggest connections between deep magma sourced fluids and the upper crustal meteoric regime the mt data also suggest anisotropically interconnected melt over a broad zone in the upper mantle of the eastern great basin which has supplied magma to the lower crust consistent with extensional mantle melting models and local shear wave splitting observations we support a hypothesis that the transition zone location and geometry ultimately reflect the middle proterozoic suturing between the stronger yavapai lithosphere to the east and the somewhat weaker mojave terrane to the west we conclude that strength heterogeneity is the primary control on locus of deformation across the transition zone with modulating force components

thermobarometric constraints on the depth of exposure and conditions of plutonism and metamorphism at deep levels of the sierra nevada batholith tehachapi mountains california we present thermodynamic estimates of pressures temperatures and volatile activities in variably deformed gabbroic to granitic cretaceous 115 100 ma batholithic and framework rocks of the tehachapi mountains southernmost sierra nevada california al contents of hornblende in granitoids imply igneous emplacement at 8 kbar in the southernmost tehachapi mountains with lower pressures 3 7 kbar to the north metamorphic pressures and temperatures for garnet bearing paragneisses and metaigneous rocks were estimated on the basis of garnet hornblende plagioclase quartz and garnet biotite plagioclase quartz thermobarometers disparate results for the metaigneous rocks from the latter system point to the difficulty of applying pelite based thermobarometers to rocks of contrasting composition and mineralogy preferred pressures cluster at 7 1 9 4 and 3 6 4 3 kbar incomplete knowledge of reaction histories however limits our interpretation of the lower pressures because they are minimum estimates the 4 kbar samples are all from a small area and if our interpretation is correct they imply a local more shallow event superimposed on crust once residing at deeper structural levels garnet hornblende and garnet biotite temperatures are less coherent likely owing to retrograde fe mg exchange and range from 570 to 790 c the majority of the rocks are igneous and affected by recrystallization and metamorphism during subsolidus cooling they are not granulites country rock paragneisses are typically migmatized at peak metamorphic conditions near that of the wet granite solidus 690 c veinlike paragenesis of garnet in the metaigneous rocks suggests formation related to the presence of a fluid phase thermodynamic estimates of volatile activities in these garnet bearing assemblages suggest variable mostly co 2 rich fluid compositions in the absence of any pervasive fluid flux the igneous rocks of the tehachapi mountains were thus intruded at depths of 30 km making them the deepest known exposed components of the cretaceous sierra nevada batholith metamorphism occurred at these great depths and perhaps locally after 15 km of uplift before 87 ma implying an uplift rate of 1 2 mm yr a minimum uplift rate is 0 6 mm yr this original uplift and possible subsequent uplift events may have been related to underthrusting of a block of rand schist from what is now the southeast with concomitant widespread ductile deformation the deduced pressure temperature and uplift history is similar to those of high pressure high temperature cretaceous batholithic rocks in salinia and the san gabriel mountains but direct correlation is not wan anted when compared with higher level intrusive rocks from analogous portions of the sierra nevada batholith to the north the tehachapi rocks reveal a deep batholith that is more heterogeneous and somewhat more mafic on average but displaying a similar level of isotopic hybridization involving mantle and crustal sources the batholith is quartz rich at these levels suggestive of a weak ductile middle crust susceptible to prolonged deformation and possible delamination

mantle heterogeneity beneath eastern california from magnetotelluric measurements 1 a 300 km long magnetotelluric profile across the great valley sierra nevada and california basin and range reveals minimum resistivity differences of 1 order of magnitude at depths from moho at 30 35 km to depths greater than 200 km horizontal wavelengths of 25 km are seen for these changes at depths shallower than 100 km and the isabella high seismic wave speed anomaly is present in the section between depths of 100 and 200 km as a minimum tenfold resistivity increase a transformation from electrical resistivity to temperature and melt fraction using a petrological model and laboratory measurements reveals that partial melt is relatively uncommon and confined primarily to depths between 30 and 100 km the remainder of the observed resistivity variation is attributed to lateral variations in temperature predicted melt fractions match those inferred from miocene quaternary xenoliths but modeled temperatures are 200 c higher than inferred from thermobarometric analyses of the xenoliths possible causes for this discrepancy include an incorrect starting material for the petrological model exotic conductive phases higher conductivity due to oxidation state and or iron content and diffusion of ions through the solid matrix i show that hydrogen diffusion or higher oxygen concentrations are plausible explanation for the higher model temperatures less than 200 ppm h 106 si in olivine would increase the conductivity of the matrix sufficiently to allow temperatures inferred from the resistivity section to match those from the xenoliths a possible source of the hydrogen would be fragments of sinking metasomatized mantle lithosphere that melted during cenozoic delamination

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

evidence for hydraulic heterogeneity and anisotropy in the mostly carbonate prairie du chien group southeastern minnesota usa abstract in southeastern minnesota paleozoic bedrock aquifers have typically been represented in groundwater flow simulations as isotropic porous media to obtain a more accurate hydrogeologic characterization of the ordovician prairie du chien group a new approach was tested combining detailed geologic observations particularly of secondary porosity with hydraulic data lithologic observations of the depositional and erosional history of the carbonate dominated bedrock unit constrained characterization of both primary matrix and secondary porosity from outcrops and core hydrostratigraphic data include outcrop and core observations along with core plug permeability tests hydrogeologic data include discrete interval aquifer tests borehole geophysics water chemistry and isotope data and dye trace studies results indicate that the prairie du chien group can be subdivided into the shakopee aquifer at the top consisting of interbedded dolostone sandstone and shale and the underlying oneota confining unit consisting of thickly bedded dolostone the boundary between these two hydrogeologic units does not correspond to lithostratigraphic boundaries as commonly presumed groundwater flow in the shakopee aquifer is primarily through secondary porosity features most commonly solution enlarged bedding planes and sub horizontal and vertical fractures regional scale preferential development of cavernous porosity and permeability along specific stratigraphic intervals that correspond to paleokarst were also identified along with a general depiction of the distribution of vertical and horizontal fractures the combination of outcrop and core investigations along with borehole geophysics discrete interval aquifer tests water chemistry and isotope data and dye trace studies show that the prairie du chien group is best represented hydrogeologically as heterogeneous and anisotropic furthermore heterogeneity and anisotropy within the prairie du chien group is mappable at a regional scale 15 000 km2

electrical resistivity structures and tectonic implications of main karakorum thrust mkt in the western himalayas nne pakistan abstract we discovered a conductive zone along main karakoram thrust which could be an indication of flat subduction of kohistan island arc beneath the eurasian plate kohistan island arc collided with the karakoram block of the eurasian plate in the early cretaceous however according to findings of many researchers the subduction ceased about 75 ma ago the presence of the conductive zone is an indication of current magmatism or hydrothermal fluids maximum low frequency band data from fourteen sites with recording periods of 10 2 103 s was acquired along a profile crossing mkt our results reveal the existence of multiple low resistivity zones beneath the region extending from shallow to the depths of more than 100 km these low resistivity zones might be a signature of the ongoing magmatic activities or hydrothermal fluids along the shyok suture zone in addition we discovered another large conductive body towards the south of the study area which could be a result of uprising magmatic plumes generated by the subducting indian plate along the indian suture zone and their entrapment in the overlying kohistan block

is extension in death valley accommodated by thinning of the mantle lithosphere beneath the sierra nevada california while relatively rapid and large scale extension occurred in the death valley region of california during neogene time little or no shallow extension occurred in the adjacent sierra nevada this contrast in tectonic history has often been extrapolated to include the entire lithosphere but geophysical and geologic observations indicate that more extension of the mantle lithosphere has occurred under the sierra than under the death valley region upper mantle seismic velocities observed beneath the high sierra are lower than those observed in other regions with comparable surface heat flow this discrepancy could be resolved if the mantle lithosphere beneath the high sierra had become warmer presumably by tectonic thinning in the last 10 m y upper mantle seismic velocities averaged topography and bouguer gravity anomalies all are consistent with the presence of thinner mantle lithosphere beneath the high sierra than beneath the california portion of the basin and range province to the east this suggests that extension of the crust near death valley might be accommodated at a deeper level by thinning of the mantle lithosphere beneath the sierra nevada the extension in the crust of the california basin and range province and the thinning of the mantle lithosphere under the high sierra appear to share the same bounds in time and space the uplift of the high sierra occurred over the past 9 m y which coincides with most of the extension that occurred in the california basin and range province because the orientation of extension in the california basin and range province is inferred to be approximately n60 w from geologic geodetic and in situ stress measurements the northern and southern edges of the death valley extensional subprovince may extend n60 w from the inferred northern and southern limits of west dipping low angle normal faults of the death valley region pronounced changes in the averaged topography and bouguer gravity anomaly across these two bounds both in the basin and range province and in the sierra nevada support a connection between the tectonics of both regions the geomorphic history of the southern sierra suggests an up to the north warp of the sierra across this southern bound during latest cenozoic time hence extension near death valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the sierra nevada this geometry requires

geophysical constraints on partial melt in the upper mantle this paper addresses the conditions under which partial melt can exist in the mantle in order to be observed as a geophysical anomaly typical observed anomalies are high electrical conductivity of the order of 0 1 s m or greater velocity decreases of 7 10 seismic q values less than 100 and a frequency band for seismic effects in the region near 1 hz existing theories of electrical conduction in partial melts and of frequency dependent seismic properties together with recent measurements of melt electrical conductivity viscosity and partial melt texture can be used to establish requirements for melt to be observed by geophysical methods from electrical anomalies mainly sensitive to melt volume and its interconnection one can require a minimum melt fraction of several percent at temperatures close to the solidus 1150 1300 c however seismic models demand only a small volume in very flattened shapes aspect ratio 0 001 melt fraction 0 1 further if melt configuration permits seismic dissipation in bulk that is there exist flattened voids intersecting more or less equant voids then it is possible to infer melt fractions for elastic anomalies that are consistent with the several percent required for electrical anomalies observed equilibrium textures of partly melted peridotite together with inferred melt solid surface energies suggest that melt on a grain size scale in a gravitational field segregates into a strongly anisotropic pattern thus if partial melt causes mantle geophysical anomalies it should exist in a variety of void shapes and probably of sizes while the association of electrical and elastic anomalies with indications of reduced density volcanism and high heat flow makes the hypothesis of partial melting an attractive explanation the minimum physical requirement is for existence of relatively high temperature