a new map of the geographic extent of the socorro mid crustal magma body for 35 years strong reflected phases from the sill like mid crustal socorro magma body smb have been observed on microearthquake seismograms recorded in the central rio grande rift new mexico in 1979 the lateral extent of the smb was estimated at 1700 km2 by mapping reflection points for 220 observed szs arrivals we have remapped the magma body using 697 pzp 2169 szp and 2589 szs reflections observed on seismograms recorded between 1975 and 1995 by new mexico tech seismic networks these data indicate that the smb covers an area greater than 3400 km2 by comparing the distribution of observed reflection points with the distribution of all possible reflection points limits can be set for the northern and southern boundaries of the smb the 80 km north south extent of the smb falls just inside a 100 km north south range of enhanced seismicity near socorro this area the socorro seismic anomaly ssa covers 5000 km2 and has the same elliptical shape as the outline of the smb based on observed reflection points the close spatial relation between the ssa and the mid crustal magma body along their northern and southern boundaries suggests that the observed seismicity can be used to place limits on the eastern and western boundaries of the magma body using the distribution of earthquakes within the ssa over the last 34 years the maximum east west extent of the smb is 60 km our reflection based value is 50 km our data indicates that the upper surface of the magma body displays no regional dip considering timing errors maximum possible relief on the magma body surface is 0 50 km

incorporating socorro magma body reflections into the earthquake location process when network station spacing is large relative to event depths constraining focal depths using direct arrivals is difficult to help overcome this problem we incorporated the szs szp and pzp socorro magma body reflections into the earthquake location process to find a velocity model appropriate for combining direct and reflected arrivals we jointly inverted for 75 hypocenters and a one dimensional velocity model we used over 1000 direct arrivals and over 400 reflected arrivals solving for p velocity vp and poisson s ratio v above the approximate base of the seismogenic zone vp and v below the base of the seismogenic zone depth to the upper surface of the magma body and station corrections high velocity and low velocity initial models converged to the same solutions without damping we found that vp and v may both decrease slightly below about 10 km depth magma body depth is near 18 75 km a close fit between observed and modeled reflected phase arrival times and an even lateral distribution of positive and negative reflected phase residuals indicate that the magma body upper surface is nearly flat in the socorro area combining direct and reflected phases reduces average focal depth error by more than a factor of 3 and average origin time error by a factor of 2 compared with using only direct phases this is because reflected arrivals are more sensitive to changes in focal depth than direct arrivals and combining upgoing direct and downgoing reflected times reduces the trade off between focal depth and origin time for this method to be successful phases must be carefully identified to help identify secondary phases we used relative arrival time curves plotting phase minus p times versus s minus p times picking reflection pairs szp and szs szp and pzp or szs and pzp identified on a single seismogram significantly reduces the chances of phase misidentification the szp szs and szp pzp pairs also help prevent trade offs between reflector depth and focal depths

comparison of vibroseis and explosive source methods for deep crustal seismic reflection profiling in the basin and range province direct comparison of low fold high energy explosive and high fold lower energy vibroseis methods for acquiring deep crustal seismic reflection data in the basin and range province suggests that the high fold common midpoint cmp method there does not provide the best possible image of lower crustal structure during the recent acquisition of a vibroseis profile in the basin and range province we fired single deep shot holes to obtain a coincident single fold explosive section within the upper crust upper 3 s the explosive source and vibroseis records are nearly equivalent for record times below 3 s however comparison of the explosive source gathers and the coincident final 60 fold vibroseis section demonstrates that low fold explosive profiling provides a higher quality image of the midcrust to lower crust 3 10 s the higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives making them less sensitive to common noise sources whereas deeper than 4 5 s the vibroseis energy levels on individual source efforts fall to that of ambient noise levels the explosions provide signal generated energy exceeding ambient noise levels down to 18 19 s although individual reflections can be correlated on explosion and vibroseis shot gathers reflection events on the 60 fold vibroseis stack do not correlate to those on the single fold explosion profile suggesting that the high fold cmp method in our study did not maintain the integrity of the weak lower crustal reflected arrivals reasons why the high fold cmp method apparently failed include complex even time varying statics nonhyperbolic moveout at long offsets and the difficulty in resolving stacking velocities with data having low signal to noise ratios reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower energy vibroseis section

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

crustal extension and magmatic processes cocorp profiles from death valley and the rio grande rift new crustal scale information on the interaction between normal faulting and magmatic activity is provided by recent cocorp deep seismic reflection profiles in death valley california and by reprocessing of the cocorp data from the socorro area of the rio grande rift new mexico the most striking feature on the seismic sections from these areas is a prominent subhorizontal series of reflectors at mid crustal depth previous studies have suggested that thin tabular magma bodies lie within the mid crustal reflective zones in addition because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the cocorp data these mid crustal horizons are here inferred to be detachments or zones of tectonic decoupling upper crustal cenozoic faults do not appear to penetrate deeper than 15 km in death valley and 13 km in the rio grande rift in death valley these faults are relatively planar with moderate dips 20 to 35 and appear to bound large basement rocks one such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central death valley to the surface location of a 690 000 yr old basaltic cinder cone listric and low to moderate angle normal faults are evident on the reprocessed new mexico data and constitute the structural component of upper crustal extension in particular a listric master fault traceable to depths as great as 13 km is inferred to underlie the albuquerque basin unlike the death valley data no faults are observed to merge with the socorro magma body per se rather subhorizontal to moderately west dipping packages of reflections are imaged between the base of the faulted upper crust 13 km depth and the mid crustal magma body about 20 km depth the middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion events seen in the middle and lower crust are generally subhorizontal and prominent layering is observed a band of reflections attributed to the crust mantle boundary is evident on most seismic sections the upper mantle appears seismically transparent on some of the profiles the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections at least one of which is a layer of magma this association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes

a lower crustal extension to a midcrustal magma body in the rio grande rift new mexico a prominent and spatially localized phase arriving approximately 4 s after the initial compressional arrival repeatedly observed on seismograms recorded in the central rio grande rift from intermediate and deep focus earthquakes at distances near 90 is identified as a probable compressional to shear wave conversion from a lower crustal root of the socorro midcrustal magma body smb this phase is inconsistent with previously determined crustal models containing a simple sill like midcrustal magma body with a negligible conduit system finite element synthetic seismogram modeling suggests that a partially molten root with a western boundary dipping steeply to the east extends several kilometers from the 19 km deep upper surface of the magma body down into the lower crust of the southwestern portion of the albuquerque belen basin the location and geometry of this feature suggest that the intracrustal intrusion of mantle magmas responsible for the ongoing inflation of the smb and associated seismicity is occurring via an off axis conduit system located beneath the active margin of the albuquerque belen basin

seismic depth imaging of normal faulting in the southern death valley basin motivated by the need to image faults to test cenozoic extension models for the death valley region of the western basin and range province an area of strong lateral velocity variations we examine the geometry of normal faulting in southern death valley by seismic depth imaging we analyze cocorp death valley line 9 to attain an enhanced image of shallow fault structure to 2 5 km depth previous work used standard seismic processing to infer normal faults from bed truncations displacement of horizontal reflectors and diffractions we obtain a detailed velocity model by nonlinear optimization of first arrival times picked from shot gathers examine the unprocessed data for fault reflections and use a kirchhoff prestack depth imaging procedure to handle lateral velocity variations and arbitrary dips properly fault plane reflections reveal the listric true depth geometry of the normal fault at the black mountains range front in southern death valley this is consistent with the concept of low angle ex

strain offsets with monotonous damped oscillations during the 1986 izu oshima volcano eruption the sequential process of the 1986 izu oshima volcano eruption japan was recorded by a volumetric strain meter near the erupted summit the records show occasional strain offsets accompanying long period damped oscillations which we interpret as responses of the volcanic system to perturbations in magmatic pressure the polarities of strain offsets were contractive for all events before the enormous ground deformation of november 21 due presumably to the intensive dike intrusion on the contrary all events after that had expansive polarities the characteristic frequencies of oscillations were 42 6 mhz for the events before changing abruptly to half that after the ground deformation in our model which is a one dimensional magma filled conduit the boundary condition at the top of the conduit changed from a closed to an open end upon a void formation at the top of the magma head due to its descent immediately after the dike intrusion before the magma escape from the reservoir as a dike magma was episodically supplied from the reservoir to the conduit and thus contraction events were generated after the magma escape magma left in the conduit was episodically drained back to the reservoir to generate expansion events this episodic magma supply or drainage was modeled by a prescribed pressure change at the bottom of the conduit we solved the free oscillation problem of the conduit pipe with an elastic sidewall to calculate the resultant volumetric strain outside the pipe physical properties of the magma and the excitation mechanism are inferred from the application of our formulation to the observation the conduit length and fluid acoustic velocity are estimated to be 10 km and 430 m s respectively suggesting that the fluid is composed of a gas liquid mixture if the conduit radius is taken to be 40 m and if the damping of oscillation occurred by viscous dissipation the fluid viscosity is 108 109 pa s a reasonable estimate for the basaltic magma the applied pressure changes at the bottom of the conduit are in a range from 105 to 106 pa with risetimes of the order of 4 to 100 s

neogene basins of the northern rio grande rift partitioning and asymmetry inherited from laramide and older uplifts abstract three asymmetric neogene basins in the northern rio grande rift of new mexico and colorado the san luis basin the upper arkansas river graben and the blue river graben are tilted toward large flanking normal faults and lie astride the similarly asymmetric late cretaceous early tertiary laramide san juan san luis sawatch and front range gore range uplifts respectively the steep thrust faulted side of each uplift is on the same side as the down rotated side of each of the neogene basins in addition the direction of stratal tilt changes northward across the villa grove accommodation zone from east in the san luis basin to west in the upper arkansas river graben this accommodation zone coincides approximately with the northward change from the east directed san juan san luis uplift to the west directed sawatch uplift these observations supported by seismic reflection studies across the san luis basin and studies of several other superimposed pairs of rift basins and laramide uplifts suggest that the basin bounding normal faults are listric and merge at depth with the older thrusts which are also listric and root into the crust at about 15 16 km the blue river graben is complicated by lack of basin fill and a thrust history along the west side of the gore range that is at least as old as late paleozoic nonetheless the neogene valley is demonstrably tilted west and lies astride an overall west directed thrust system similar to other thrust and basin relationships in the northern rio grande rift

deep seismic reflection evidence for the role of extension in the evolution of continental crust summary deep seismic reflection profiling as well as geologic studies indicate that extensional basins are a common feature of continental crust the wide spread occurrence of extensional basins combined with published models on the effects of extension on lower crustal rocks suggests that extensional processes play an important role in the evolution of continental cmst extension is now recognized to have been the last major tectonic event to affect approximately 50 of the united states this observation and the preservation of extensional features in areas which have experienced subsequent episodes of compression suggests that extensional processes may lead to a strengthening of continental crust in areas of active extension such as the western united states seismic and petrologic data as well as theoretical modeling of heat flow data suggests that the lower crust may be predominantly intrusive igneous material emplaced during extension and that the present mohorovicic discontinuity formed during extension although the interpretation of the various data is somewhat speculative we suggest that the volume of continental material in some areas has doubled as a result of extension thus extension may result in the addition of a significant amount of new material to the continents

tectonic cycles in southern africa abstract the recurrence of major geological events within the 3600 ma stratigraphic record of southern africa suggests a periodicity of320 25ma for at least the last 3200 ma this periodicity is reflected in major episodes of continental volcanism granitoid plutonism orogeny reciprocity of terrestrial and marine dominated sequences platform carbonate sedimentation glaciation and first order changes in sea level these serve to define 11 completed megacycles each introduced by a major rifting event accompanied by listric faulting alluvial sedimentation in restricted fault bound troughs volcanism and a type granitoid plutonism prior to 3200 ma continental rift magmatism occurred subaqueously due to very high early archean sea levels resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics the long term reciprocity of terrestrial and marine dominated sequences records eustatic first order changes in sea level with highstands characterized by generally humid warm continental climates low relief and tropical afforestation on the other hand lowstand climates were hot and arid with high relief and widespread desertification thick marine sequences accumulate in areally extensive epeirogenic basins without significant volcanism or faulting and commonly are divided into an early phase of open shelf sedimentation followed by deposition in a brackish marine foreland basin within openshelf environments an early phase of biochemical precipitation and or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin marine dominated sequences display a deterministic stacking of lithofacies important glacial epochs commonly occur midway through first order regression sometimes with lesser events closely following peak transgression potentially they display 320 ma periodicity but with an unexplained mid proterozoic gap glaciation is related to the combined effect of climatic cooling associated with first order regression and depletion of atmospheric co2 by biological uptake associated with platform carbonate development terrestrial volcanism is believed to be the main contributor of atmospheric co2 climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis

curie point depth estimates from aeromagnetic data from death valley and surrounding regions california aeromagnetic data were analyzed to determine the curie point depth cpd by power density spectral and three dimensional inversion methods within and surrounding death valley in southern california we calculated the cpd for 0 5 regions using 2d power density spectral methods and found that the cpds varied between 8 and 17 km however the 0 5 region may average areas that include shallow and deep cpds and because of this limitation we used the 3d inversion method to determine if this method may provide better resolution of the cpds the final 3d model indicates that the depth to the bottom of the magnetic susceptible bodies varies between 5 and 23 km even though both methods produced roughly similar results the 3d inversion method produced a higher lateral resolution of the cpds the shallowest cpds occur within the central and southern death valley panamint valley coso geothermal field and the tecopa hot springs region deeper 15 km cpds occur over outcropping granitic and precambrian lithologies in the panamint range grapevine mountains black mountains and the argus range the shallowest cpd occurs within the central death valley over a possible seismically imaged magma body and slightly deeper values occur within the panamint valley southern death valley and tecopa hot springs the shallow cpd values suggest that partially molten material may also be found in these latter regions the cpd computed heat flow values for the region suggest that the entire area has high heat flow values 100 mw m 2 on the other hand locally extremely high values 200 mw m 2 occur within the panamint valley the southern and central death valley and tecopa hot springs region these locally high heat flow values may be related to midcrustal magma bodies but additional geophysical experiments are needed to determine if the magma bodies exist

limitations of quantitative analysis of deep crustal seismic reflection data examples from glimpce amplitude preservation in seismic reflection data can be obtained by a relative true amplitude rta processing technique in which the relative strength of reflection amplitudes is preserved vertically as well as horizontally after compensating for amplitude distortion by near surface effects and propagation effects quantitative analysis of relative true amplitudes of the great lakes international multidisciplinary program on crustal evolution seismic data is hampered by large uncertainties in estimates of the water bottom reflection coefficient and the vertical amplitude correction and by inadequate noise suppression processing techniques such as deconvolution f k filtering and migration significantly change the overall shape of amplitude curves and hence calculation of reflection coefficients and average reflectance thus lithological interpretation of deep crustal seismic data based on the absolute value of estimated reflection strength alone is meaningless the relative strength of individual events however is preserved on curves generated at different stages in the processing we suggest that qualitative comparisons of relative strength if used carefully provide a meaningful measure of variations in reflectivity simple theoretical models indicate that peg leg multiples rather than water bottom multiples are the most severe source of noise contamination these multiples are extremely difficult to remove when the water bottom reflection coefficient is large 0 6 a condition that exists beneath parts of lake superior and most of lake huron

moho reflectivity and seismic signal penetration abstract analysis of amplitude decay and frequency change is applied to determine limits of seismic signal penetration on deep seismic reflection data collected by lithoprobe and the consortium for continental reflection profiling cocorp changes in spectral frequency content are efficiently monitored with averaged instantaneous frequency and bandwidth results from montana show pronounced increases in average frequency and slight drops in amplitude associated with a die out of reflections at the moho results from quebec show amplitude and frequency change occurring through the moho on lines with and without moho reflections these contrasting observations confirm both adequate signal penetration and that the differences in moho reflectivity are representative of geologic differences

application of revised ray tracing migration to imagine lateral variations of seismic fabric corresponding to different tectonic styles in the northern apennines abstract migration of zero offset seismic sections of deep crust can be done with methods based on ray tracing we modify the classical ray tracing migration method rtm introducing a consistency check to control whether back propagated rays satisfy the condition of strict normal incidence at the migrated reflector a synthetic test shows the effectiveness of the method in particular the control of normal incidence allows elimination of physically inconsistent reflectors from the migrated section then rtm is applied to a crustal seismic profile acquired in central italy using a velocity model obtained from wide angle data that reproduces the gross structures of the apenninic crust the lateral variation of the seismic fabric shown from the migrated section reveals the presence of coexisting extensional and compressional tectonic regimes kinematic diffraction modelling gives additional information about both the distribution of seismic velocities and major active geodynamic processes in the upper lithosphere the migrated section supports the subdivision of northern apennines in two tectonic regions a stretched upper plate tuscany and northern thyrrenian supported by a rise of the asthenosphere and a downwarped lower plate adria subducted below the mountain belt

modeling of highly anisotropic crust and application to the altiplano puna volcanic complex of the central andes 1 the altiplano puna volcanic complex apvc is located in the central andes and covers an area over 50 000 km2 the style of volcanism is predominately caldera forming ignimbrite eruptions active between 10 and 1 ma chmielowski et al 1999 used teleseismic events recorded from seven portable broadband seismic stations deployed in the apvc to identify a 1 km thick very low velocity zone at a depth of 19 km based on the correlation with surface volcanics the extremely low shear velocities of 1 km s or less and the depth of the layer the low velocity zone was interpreted to be a magma body using local events recorded during the same seismic deployment we have improved the crustal model for the apvc and detected seismic anisotropy in the crust above the low velocity zone using a hexagonal symmetry anisotropy code we computed synthetic receiver functions for many models of layered anisotropic media and found two different models consistent with the data in both models approximately 20 30 anisotropy is present in a 3 km thick surface low velocity layer and 15 20 anisotropy is present in the remaining crust above the buried low velocity zone the first model has the 1 km thick low velocity zone at 17 km while the alternate model has the low velocity zone at 14 km depth overlain by a 4 km thick transition zone of intermediate velocities this anisotropy may be due to the fracture system by which magma migrates to the surface from the midcrustal sill like magma body

osmium isotope evidence for volcanism across the wuchiapingian changhsingian boundary interval abstract two negative carbon isotope excursions 3 5 6 5 across the wuchiapingian changhsingian boundary wcb are observed globally sections in china canada and iran however the causative mechanism of these excursions is debated here high resolution osmium isotope 187os 188os or osi chemostratigraphy of four globally correlated wcb sections 3 in china meishan shangsi and lianyuan and 1 in canada buchanan lake show two separate osi excursions to less radiogenic compositions that are coincident with the carbon isotope shifts for two of the south china sections lianyuan meishan in contrast only a single osi excursion to less radiogenic compositions coinciding with the earliest changhsingian carbon isotope shift is observed for the shangsi and buchanan lake sections the osi shift is interpreted to reflect increased unradiogenic os input from basaltic magmatism in south china possibly related to the emeishan large igneous province lip however 187os 188os data suggest that only the earliest changhsingian volcanism had global impact on both the ocean and atmosphere the lack of any evidence for a biotic event associated with the wcb therefore may have been due to the more regional rather than global impact of volcanism during the latest wuchiapingian in contrast during the earliest changhsingian volcanism was sufficient to cause a more global signal in the ocean osmium record but was inadequate or too prolonged to drive any significant environmental change volcanism however may have provided the isotopically light carbon that drove the negative carbon isotope excursions across the wcb

kinematic thermal and petrological model of the central alps lepontine metamorphism in the upper crust and eclogitisation of the lower crust abstract seismic and seismological studies as well as gravimetric models indicate that a slab of european lithospheric mantle and lower crust is currently underthrust below the apulian crust we assume a simple kinematic model in which the lower and upper subducted european crusts are decoupled along a decollement the lower crust goes into subduction without deformation the upper crust deforms by pure shear with a horizontal compressional axis the total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry different distributions of erosion being used the computed temperature field is steady state if the kinematic model applies during a minimum time of 40 myr for a convergence rate of 8 mm yr equilibrium mineral assemblages and densities are determined from the computed p t conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust assuming local isostasy the density model fits the average topographic profile across the central alps the p t t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure oligocene metamorphism in the lepontine dome the peak calculated temperature for the deepest non subducted crustal rocks is 600 c for a pressure of 0 8 gpa near the lower limit of high pressure amphibolites we thus propose that the lepontine metamorphism corresponds to the steady state thermal regime however either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt the computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km we conclude that the moho limiting the deepest part of the root may correspond to the eclogitisation phase change lower crust eclogites have a density comparable to or higher than that of the mantle depending on their chemical composition 3 37 for andesitic eclogites 3 56 for gabbroic eclogites thus andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink

stress magnitude strain rate and rheology of extended middle continental crust inferred from quartz grain sizes in the whipple mountains california knowledge of the magnitude of differential stress and strain rate during the formation of mylonitic shear zones in metamorphic core complexes provides constraints on the mechanical behavior of the middle continental crust during extension we analyzed the differential flow stress during the mylonitization of quartzofeldspathic rocks in the whipple mountains california using grain size piezometers and kinetic laws for grain growth mylonitic gneisses collected from two widely separated transects have grain sizes that cluster in a range from 32 to 61 m analysis of grain growth kinetics indicates that mylonitization of the gneisses continued during cooling to temperatures 500 c compatible with estimates from two feldspar thermometry quartz grain size piezometers suggest that the mylonitization occurred under differential stresses 1 3 of 40 150 mpa or maximum shear stresses of 20 75 mpa extrapolation of quartzite flow laws to 500 c indicates that the mylonitization occurred at strain rates faster than 10 14 s 1 these estimates suggest that the mylonitic zone within the whipple mountains had an effective viscosity of the order of 1018 4 1020 4 pa s these low viscosities and rapid strain rates combined with seismic reflection data showing that continental crust is layered suggest that more realistic physical models of extension of the continental lithosphere should treat the lithosphere as a heterogeneous distribution of high viscosity regions separated by low viscosity zones

relation of deformation and multiple intrusion in the death valley extended region california with implications for magma entrapment mechanism the crystalline core of the black mountains crustal section death valley california exposes a tremendous volume of miocene plutonic rock intruded at a depth of 10 13 km few plutons were intruded above 10 km prior to unroofing by tectonic denudation in the eastern part of the core a brittle detachment fault separates predominantly miocene hanging wall strata from the midcrustal miocene 11 6 and 8 7 ma plutonic complex and early proterozoic basement in the west mylonitic lineations and foliations locally well developed within the 11 6 ma intermediate mafic willow spring pluton are cut by dikes which emanate from an 8 7 ma silicic plutonic complex the younger silicic complex throughout the crystalline core exhibits few ductile deformational structures published thermal and barometric studies from both plutonic bodies indicate similar midcrustal 10 13 km emplacement depths at ambient temperatures just above 300 c the significant difference in densities of these magmas argues against a density control for magma entrapment also the country rock above and below the plutonic complex contains no apparent differences from field observations that would suggest a change in density the depth of entrapment corresponds well with the expected depth for the crustal strength maximum determined from laboratory experiments the similar emplacement depths but contrasting styles of deformation of the two plutonic bodies further suggests that entrapment may have been controlled by a high strength barrier represented by the brittle ductile transition late hypabyssal intrusions and associated volcanism are linked to diachronous rapid unroofing of the range block all show a northwest progression paralleling the regional extension direction thus when migration of magma through the high strength barrier did occur it was apparently related to increased strain rates which allowed magma ascent by fracture exploitation rheological stratification of the crust may have played an important if not major role in trapping magmas in the middle crust in this area

geometry of the garlock fault zone based on seismic relection data detailed examination of shot gathers from line 3 of the consortium for continental reflection profiling mojave experiment indicates a low velocity high amplitude seismic event that may originate from a south dipping portion of the garlock fault zone part of this event was previously interpreted as a subhorizontal surface located in the midcrust our analysis indicates however that the distance to this event varies consistently with the distance of the shot and receivers from the fault zone and that the velocities are too low for seismic waves traveling through predominantly crystalline rocks these characteristics are more consistent with the presence of a subvertical reflector located near the garlock fault zone than with the previously inferred subhorizontal reflector beneath the seismic line

application of avhrr imagery and grace measurements for calculation of actual evapotranspiration over the quaternary aquifer lake chad basin and validation of groundwater models the paper presents an application of noaa avhrr and grace data for studying the regional hydrogeology of the quaternary aquifer in the lake chad basin the actual evapotranspiration et over the phreatic aquifer is computed using the s sebi algorithm and then compared to the et estimates derived from the grace land water solutions both the grace solutions and the s sebi results are similar in description of the actual et over the lake chad basin avhrr data helped to overcome non uniqueness in the modelling of the piezometry anomalies particularly in regional groundwater models the harr piezometric dome appeared to be an area of relatively high effective recharge while the chari baguirmi and the kadzell piezometric depressions and bahr el gahzal were probably areas of excess et over recharge