geology of panamint valley saline valley pull apart system california palinspastic evidence for low angle geometry of a neogene range bounding fault geological studies support the interpretation that northern panamint valley and saline valley southeast california form paired pull apart basins on opposite sides of the right slip hunter mountain fault zone eight to ten km of late cenozoic net slip can be established on the hunter mountain fault zone palinspastic reconstruction of northern panamint valley indicates that the valley was formed by movement on a shallow crustal low angle normal fault of 0 15 degree west dip during the last 3 0 ma this interpretation appears to contradict the notions that little extension is accomodated in the uppermost crust by low angle faulting and that the most recent extension in the basin and range province is accomodated exclusively by high angle faulting saline valley however is interpreted to have formed by extension on closely spaced rotated planar normal faults thus within one geometric system of paired pull apart basins extension appears to have been accommodated in the shallow crust in two different ways

late paleoproterozoic extension and a paleostress field reconstruction of the north china craton abstract mafic dyke swarms and aulacogens are major anorogenic extensional events in the late paleoproterozoic north china craton ncc the n nnw mafic dyke swarms are widespread in the ncc whose ages span between 1 83 and 1 77 ga the similar ages and orientations of 1 8 ga dyke swarms in the ncc demonstrate that the amalgamated ncc experienced widespread extension at this time based on the width statistics of dyke swarms on ten survey lines an average crustal extension ratio of 0 35 was found for the ncc the small magnitude of overall extension suggests that the mafic dyke swarms were emplaced into the elastic fractures and indicates that the ncc had become a brittle plate prior to the emplacement of the mafic dyke swarms precisely dated mafic dyke swarms when used as paleostress indicators can be employed in the paleostress field reconstruction of precambrian cratons two dimensional finite element modeling 2 d fem of the ncc in which the various blocks were assigned densities and elastic constants shows that north south compression favors dyke intrusion along generally n nw lines and that deviations in dyke trends can be explained by the effects of boundary constraints and the physical properties of the crust the best fitting model can be considered a plausible representation of the tectonic force acting on the ncc that produces the intraplate stress field that is most consistent with the observed orientation of dyke swarms the results of modeling of the late paleoproterozoic stress field suggest a common tectonic setting for the emplacement of mafic dyke swarms in the central orogenic zone western and east blocks of the ncc the results also show that the north south tectonic forces play an important role in determining the paleostress field in the ncc the widespread extension of the ncc resulted from the north south tectonic forces which may be related to the break up of the late paleoproterozoic supercontinent the paleostress field modeling provides a possible approach to consider the supercontinent paleostress reconstruction and to reveal the mechanisms of the supercontinent break up

early miocene change in direction of least principal stress southwestern united states conflicting inferences from dikes and metamorphic core detachment fault terranes previous investigations have suggested that the direction of least principal stress in the western united states has shifted from northerly to easterly between 50 to 30 ma an effort is made to more tightly constrain the time of this shift using igneous dike host rock systems in which the isotopic age of both host and dike is known and differs by less than 10 ma in such systems the possibility that magma intruded along old fractures unrelated to the state of stress at the time of intrusion is minimized and the axis of least principal stress is considered to be normal to the strike of the subvertical dike during the time interval between the ages of the dike and host rock these systems indicate that a northerly least principal stress orientation persisted more recently than previously believed until probably after 30 ma possibly about 26 ma and conceivably as recently as 18 ma in some areas the data are insufficient and too loosely bracketed in time to decide how fast the change in state of stress occurred in any one place and if by systematic rotation or abrupt shift however the change could be diachronous earlier in the southeast and later in the northwest major easterly trending magmatic and tectonic elements of tertiary age in the great basin of utah and nevada are compatible with the dike data however numerous metamorphic core complexes and detachment fault terranes seem to indicate more or less easterly extension throughout the time period represented by the dikes this inconsistency is not clearly resolvable but may reflect differences between the regional plate stress field and gravitational stresses in a crust overthickened by mesozoic compression

structural control of young basaltic fissure eruptions in the plateau basalt area of the arabian plate northeastern jordan abstract in the el hamadieh region part of the large intracontinental basalt plateau extending nw se from syria to jordan and saudi arabia in the northern part of the arabian plate young volcanic edifices formed by fissure eruptions have been investigated all geometrical dimensions and parameters of the ridges have been measured the field relationships documented the data evaluated and the structural patterns analysed using the attitude of the fissures as palaeostress indicators a stress strain model is introduced showing the superimposition of stress fields magma was extruded at the intersection of two regional structures the ne sw striking amman hallabat fold belt and the nw se trending azraq sirhan graben a first effusion with a relatively fluid magma took place in reactivated fold related conjugated hk 0 shear fractures with a compressional 1 stress trending nw se a second phase of fissure effusions produced a more viscous lava these fissures represent a hk 0 planes with an acute angle about the fold axis and b planes of normal faults along the southwestern shoulder of the azraq sirhan graben in the position of fold related reactivated conjugated 0 hl shear planes the geometrical relationships between the amman hallabat folds the azraq sirham faults and the volcanic fissures indicate a rotation of the local stress field with the originally nw se trending compressional 1 and the ne sw trending 3 stress changing into a ne sw acting tensional 1 stress

currents in the mantle and the geology of continents abstract this paper is the first of a series which have considered the possible effects of currents in the mantle upon continents it mentions effects of aging in oceans upon the direction of slope of coastal peneplains when oceans are young the coasts are cliffs from which peneplains dip downward on the inland side as the margins separate from the ridge they cool and the slopes reverse it suggests how jeffreys arguments against convection currents breaking the lithosphere can be avoided and discusses the possibility that upwelling has penetrated beneath the southwestern united states

rbsr and oxygen isotopic study of alkalic rocks from the trans pecos magmatic province texas implications for the petrogenesis and hydrothermal alteration of continental alkalic rocks rb sr and o isotopic data for mid tertiary alkalic rocks from the trans pecos magmatic province of west texas demonstrate that hydrothermal alteration and fluid rock cation exchange interactions have affected the isotope geochemistry of these rocks strontium and o isotopic data for late stage minerals in an alkali basalt hawaiite sill record two episodes of fluid rock interactions firstly sr isotopic data for calcite precipitated in open spaces suggest that most of the sr incorporated in this mineral was derived from the surrounding silicate minerals however o was derived from heated formation fluids 18o 13 t 100 c in isotopic equilibrium with surrounding cretaceous marine limestones secondly sr isotopic data for analcime and natrolite lie along mixing lines between a magmatic 87sr 86sr and a cretaceous marine 87sr 86sr oxygen isotopic data for natrolite suggest fluids in equilibrium with this mineral were isotopically light 18o 8 very similar to present day geothermal waters of the trans pecos region 18o 6 to 9 these data suggest that later meteoric fluids introduced sr with a cretaceous marine 87sr 86sr ratio into minerals with significant cation exchange capacity dilute hcl leaching experiments demonstrate the removal of this labile or exchangeable sr from the alkali basalt rb sr isotopic data for the leached alkali basalt and handpicked calcite record a crystallization age of 42 ma consistent with k ar data for an unaltered alkali basalt hawaiite dike from the same area 42 6 1 3 ma leaching experiments on one phonolite suggest the sr isotopic variability in unleached phonolite and nepheline trachyte samples may be attributed to sr in secondary calcite and zeolites which have an upper cretaceous marine 87sr 86sr ratio rb sr isotopic data for leached phonolite and sanidine separate yield an age of 36 5 0 8 ma within analytical uncertainty of a k ar biotite age 36 0 1 1 ma of another phonolite these leaching experiments demonstrate that the rb sr isotopic systematics of hydrothermally altered continental alkalic rocks may be significantly improved providing more reliable geochronologic and isotopic tracer information necessary in constructing precise models of mantle sources

synchronous opening of the rio grande rift along its entire length at 25 10 ma supported by apatite u th he and fission track thermochronology and evaluation of possible driving mechanisms one hundred and forty seven new apatite u th he ahe ages are presented from 32 sample locations along the flanks of the rio grande rift in new mexico and colorado these data are combined with apatite fission track aft analyses of the same rocks and modeled together to create well constrained cooling histories for rio grande rift flank shoulders the data indicate rapid cooling due to extension from ca 28 to 5 ma in the sawatch range ca 28 ma to quaternary in the sangre de cristo mountains ca 25 to 5 ma in the albuquerque basin and ca 25 to 10 ma in the southern rio grande rift in southern new mexico rapid cooling of rift flanks followed the oligocene ignimbrite flare up and the northern section of the rio grande rift in colorado exhibits semicontinuous cooling since the oligocene overall however rift flank cooling along the length of the rift was out of phase with high volume magmatism and hence is inferred to have been driven mainly by exhumation due to faulting although each location preserves a unique cooling history when combined with existing ahe data from the gore range in northern colorado and the sandia mountains in new mexico together these data indicate that extension and exhumation of rift shoulders were synchronous along 850 km of the length of the rio grande rift from 25 to 10 ma these time space constraints provide an important new data set with which to develop geodynamic models for initiation and evolution of continental rifting models involving northward propagation of rifting and colorado plateau rotation are not favored as primary mechanisms driving extension instead a geodynamic model is proposed that involves upper mantle dynamics during multistage foundering and rollback of a segment of the farallon plate near the laramide hinge region that extended between the wyoming and se new mexico high velocity mantle domains the first stage of flat slab removal accompanied ca 40 20 ma volcanism in the san juan and mogollon datil ignimbrite centers which initiated asthenospheric upwelling and circulation a second stage involved a ca 30 25 ma detachment of remaining fragments of the farallon slab intensifying asthenospheric upwelling and focusing it along a n s trend beneath colorado and new mexico by 25 ma the north american lithosphere had become weakened critically along this narrow zone so that extension was accelerated resulting in the observed 25 10 ma cooling indicated by the thermochronologic data this developed a central graben with increased fault related high strain rates and resulted in maximum sediment accumulation in the rio grande rift our geodynamic model thus involves oligocene removal of parts of the farallon slab beneath the ignimbrite centers followed by a major oligocene miocene slab break that instigated the discrete n s rio grande rift through focused upper mantle convection beneath the southern rocky mountain rio grande rift region

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

mechanics of the giant radiating mackenzie dyke swarm a paleostress field modeling 1 the 1 27 ga mackenzie dyke swarm of the canadian shield is a giant radiating dyke swarm that gradually swings in orientation from n s in the focal area to nw se trends in peripheral areas in this paper we propose a new model the plug model that accounts for the paleostress contribution to the mechanism of emplacement for the mackenzie dyke swarm in the canadian shield the 1 27 ga stress field on the canadian shield calculated by the plug model explains the radiating nature of the mackenzie dyke swarm around the coppermine river lava field by local stress concentrations the parallel nature of the dyke swarm at distance more than 1000 km from the focal source can be explained by the existence of a regional tectonic stress field created by ridge push acting on the southeast margin of the canadian shield from the grenville ocean the thin elastic plate and two dimensional cross section modeling suggest that the interaction between stresses from a mantle upwelling and the grenville ocean spreading play an important role in the intrusion mechanism of the mackenzie dyke swarm the change in dyke orientation from n s trending to nw se trending is caused by coupling between resistance from the focal area plug area and a grenville ocean ridge push

topography the geoid and compensation mechanisms for the southern rocky mountains the southern rockies of colorado are anomalously high elevations greater than 2800 m topographically rough implying active uplift and underlain by significant low velocity anomalies in the upper mantle that suggest an intimate relationship between mantle geodynamic processes and the surface topography the region is in isostatic equilibrium i e near zero free air gravity anomaly however the poor correlation between the high topography and crustal thickness makes the application of simple compensation models e g pure heiskanen or pratt hayford problematic knowledge of how the current topography of the rockies is isostatically compensated could provide constraints on the relative role of sublithospheric buoyancy versus lithospheric support here we evaluate the geoid and its relationship to the topography using the geoid to elevation ratio gtr in the spatial domain and the admittance in the frequency domain to constrain the mechanism of compensation we separate the upper mantle geoid anomalies from those with deeper sources through the use of spherical harmonic filtering of the egm2008 geoid we exploit the fact that at wavelengths greater than the flexural wavelength where features are isostatically compensated the geoid topography ratio can be used to estimate the depth of compensation and the elastic thickness of the lithosphere the results presented below indicate that the main tectonic provinces of the western united states have moderate geoid topography ratios between 3 5 and 5 5 m km 3 9 for the southern rockies 4 25 for the colorado plateau and 5 2 for the northern basin and range suggesting shallow levels of isostatic compensation in terms of the elastic thickness of the lithosphere our results indicate an elastic thickness of less than 20 km these value support the notion that a major portion of the buoyancy that has driven uplift resides at depths less than 100 km and that upper mantle processes such as small scale convection may play a significant role in the buoyant uplift of the southern rockies as well as other actively uplifting areas of the western united states further support for this hypothesis is provided by high coherence for the geoid topography relationship for nearly all wavelengths between 50 and 1000 km

the unusual 3d interplay of basement fault reactivation and fault propagation fold development a case study of the laramide age stillwell anticline west texas usa abstract subsurface fault geometries have a systematic influence on folds formed above those faults we use the extraordinarily well exposed fold geometries of the laramide age stillwell anticline in west texas usa to develop a strain predictive model of fault propagation fold formation the anticline is a 10 km long nw trending ne vergent asymmetric fold system with an axis that displays a map view left stepping en echelon pattern we integrated field observations geologic and structural data cross sections and 2d kinematic modeling to establish an unusual 3d two stage model of contractional fold formation including 1 reverse reactivation of a pre existing nw striking sw dipping left stepping en echelon normal fault system in paleozoic basement rocks to generate monoclinal flexures in overlying layered cretaceous carbonate rocks and 2 the formation of a subsequent flat ramp fault system that propagated horizontally along a mechanically weak clay rich cretaceous unit before ramping up at the hinge of the pre existing monocline system strain is focused within the forelimb of the system in front of the propagating fault tip and is accommodated by a combination of interlayer slip flat ramp faulting and fracturing proximal to planes of slip this strain predictive model can be applied to similar less well exposed contractional systems worldwide and provides a new unusual example of laramide age contractional deformation

stress history and tectonic development of the rio grande rift new mexico paleostress determinations from radiometrically dated dikes indicate ene wsw regional extension began by about 32 ma in new mexico and perhaps earlier in the southern part of the state the extension direction maintained this orientation until sometime after 23 ma when it rotated clockwise to an e w or wnw ese direction which is the present orientation the spreading direction in the rio grande rift however has been e w since the beginning of rifting about 31 ma and reflects control of spreading by preexisting structural grain ne sw extension occurred in the grants pie town area of the colorado plateau and in the roofs of batholiths in the socorro and questa areas concurrently with e w spreading in the rio grande rift nearly orthogonal orientations of late eocene oligocene 38 32 ma paleostress indicators in southern new mexico indicate a nearly isotropic middle tertiary stress field both the western and eastern boundaries of the zone of e w spreading along the rio grande rift in northern new mexico shifted 60 200 km eastward sometime between 23 and 16 ma perhaps coeval with rotation of the direction of regional extension this eastward shift also coincided approximately with the renewal of volcanism following an early miocene 22 17 ma lull during the late miocene 9 5 ma the eastern boundary of e w spreading shifted back to the present eastern margin of the rio grande rift the n s to nne ssw orientation of the least principal horizontal stress in the southern great plains province has persisted since at least 28 ma stress province boundaries can be remarkably sharp with stress orientations changing by 90 within 30 km or less the ne trending jemez lineament transects the colorado plateau rio grande rift basin and range and southern great plains stress provinces but except for a transition zone along the present southeastern boundary of the colorado plateau both paleostress and contemporary stress reflect the respective stress provinces not the lineament chemical analyses of dated dikes reveal highly variable compositions but indicate that true basalts are more common among early rift 20 ma volcanic rocks than is generally recognized a histogram constructed from nearly 200 radiometric ages of igneous rocks in new mexico between 104 w and 108 w longitude indicates that volcanism along the rio grande rift has been episodic with peaks at 37 34 31 27 26 22 17 13 12 6 and 5 0 ma

late cenozoic uplift in southern asia and the american west rationale for general circulation modeling experiments geologic evidence suggests significant increases in uplift rates and absolute elevation in several areas during the late cenozoic especially in the last 10 m y regions of major uplift include the tibetan plateau and himalayan mountains of southern asia and the sierra nevada colorado plateau basin and range rocky mountains and high plains of the american west uplift occurred at a scale large enough to affect regional and global climate in a number of ways several of which we evaluate by general circulation model experiments in accompanying papers

postdepositional tilt of the miocene pliocene ogallala group on the western great plains evidence of late cenozoic uplift of the rocky mountains the rocky mountains and adjacent western great plains share a common history of late cenozoic stream incision both epeirogenic uplift and climate change with no associated uplift have been proposed as the cause of thissubcontinental scale erosional episode however the lack of a well defined cenozoic paleoelevation history for the region has hampered our ability to distinguish between the two a tilt analysis of the cheyenne tablelands in the western great plains of wyoming and nebraska provides us with a datum from which postdepositional changes in slope can be determined miocene to earliest pliocene gravels of the ogallala group 17 5 5 ma cap the tablelands which currently tilt down to the east at slopes as great as 10 2 however paleohydraulic analysis of the ogallala gravels indicates depositional slopes of 10 3 to 10 4 implying a postdepositional increase in tilt if a hinge point at the eastern edge of the cheyenne tablelands is assumed this tilt translates into differential uplift of 680 m 815 410 m at the western edge of the great plains flexural isostatic rebound of the tablelands due to known young erosion in surrounding basins only produces a few hundred meters of uplift therefore even if all of the recent erosion in the region can be attributed to climate change the resulting rebound is insufficient to account for the observed uplift of the tablelands thus the tilting of the cheyenne tablelands is most consistent with broad wavelength tectonic uplift centered under the rocky mountains initiated during ogallala deposition and continuing since deposition ceased

chemical transport across the itcz in the central pacific during an el nino southern oscillation cold phase event in march april 1999 we examine interhemispheric transport processes that occurred over the central pacific during the pem tropics b mission ptb in march april 1999 by correlating the observed distribution of chemical tracers with the prevailing and anomalous windfields the intertropical convergence zone itcz had a double structure during ptb and interhemispheric mixing occurred in the equatorial region between itcz branches the anomalously strong tropical easterly surface wind had a large northerly component across the equator in the central pacific causing transport of aged polluted air into the southern hemisphere sh at altitudes below 4 km elevated concentrations of chemical tracers from the northern hemisphere nh measured south of the equator in the central pacific during ptb may represent an upper limit because the coincidence of seasonal and cold phase enso conditions are optimum for this transport stronger and more consistent surface convergence between the northeasterly and southeasterly trade winds in the southern hemisphere sh resulted in more total convective activity in the sh branch of the itcz at about 6os the middle troposphere between 4 7 km was a complex shear zone between prevailing northeasterly winds at low altitudes and southwesterly winds at higher altitudes persistent anomalous streamline patterns and the chemical tracer distribution show that during ptb most transport in the central pacific was from sh to nh across the equator in the upper troposphere seasonal differences in source strength caused larger interhemispheric gradients of chemical tracers during ptb than during the complementary pem tropics a mission in september october 1996

cenozoic thermal mechanical and tectonic evolution of the rio grande rift careful documentation of the cenozoic geologic history of the rio grande rift in new mexico reveals a complex sequence of events at least two phases of extension have been identified an early phase of extension began in the mid oligocene about 30 ma and may have continued to the early miocene about 18 ma this phase of extension was characterized by local high strain extension events locally 50 100 regionally 30 50 low angle faulting and the development of broad relatively shallow basins all indicating an approximately ne sw 25 extension direction consistent with the regional stress field at that time extension events were not synchronous during early phase extension and were often temporally and spatially associated with major magmatism a late phase of extension occurred primarily in the late miocene 10 5 ma with minor extension continuing to the present it was characterized by apparently synchronous high angle faulting giving large vertical strains with relatively minor lateral strain 5 20 which produced the modern rio grande rift morphology extension direction was approximately e w consistent with the contemporary regional stress field late phase graben or half graben basins cut and often obscure early phase broad basins early phase extensional style and basin formation indicate a ductile lithosphere and this extension occurred during the climax of paleogene magmatic activity in this zone late phase extensional style indicates a more brittle lithosphere and this extension followed a middle miocene lull in volcanism regional uplift of about 1 km appears to have accompanied late phase extension and relatively minor volcanism has continued to the present we have estimated geotherms and calculated lithospheric strength curves for the two phases of extension using geologic data to constrain earlier events and geophysical data to constrain the modern geotherm and crustal structure a high geotherm was deduced for early phase extension resulting in a shallow crustal brittle ductile transition and negligible mantle strength the lithosphere cooled after early phase extension resulting in a deeper crustal brittle ductile transition and perhaps more significantly a considerable zone of mantle strength immediately beneath the moho these results indicate that early phase extensional style was controlled by a crustal decollement near the brittle ductile transition which was prevented during late phase extension by significant strength in the uppermost mantle late cenozoic uplift of the rift zone cannot be explained by crustal thinning during extension and geotherm evolution predicted from simple cooling however this uplift does not appear to be restricted to the rift zone and pliocene to recent volcanism and heat flow data suggest that uplift may be caused by magmatic thickening of the crust perhaps unrelated to rifting the complex interrelationship among regional and local prerifting synrifting and postrifting events in the rio grande rift suggests that rifting at least in this region should not be considered in isolation of other geologic events

new insights into the fluvial geomorphology and its effects on paleofloods in the black hills south dakota abstract identification and characterization of a relict landscape in the black hills of south dakota has provided insight into the postuplift history 30 ma to present a new watershed decomposition technique has been developed that utilizes high resolution digital elevation models and traditional long valley profiles to identify the location and extent of the relict landscape as well as its geomorphic characteristics results identified an expansive relict landscape at elevation in the center of the black hills that contains fluvially mature features such as low gradient channels and broad flood valleys moreover the relict landscape and associated watersheds that drain it are undergoing active fluvial incision via knickzone migration this observation suggested that the black hills were once much more subdued than today at present channel confinement lack of potential floodwater storage and steepened stream gradients exists in reaches of active incision downstream of the relict landscape large paleofloods influenced by these processes have been well documented and have been restricted to the narrow and steep canyons that drain the relict landscape therefore active fluvial incision into the black hills influences the ability of more recent holocene large floods to leave substantial evidence of their occurrence inside incised canyon walls and should also be considered when studying watershed factors influencing large flood events on the flanks of the black hills

tectonic geomorphometrics of the western united states speculations on the surface expression of upper mantle processes the topography of the western united states provides a classic field laboratory for investigations of the relationship between surface features and sub crustal dynamic processes the interpretation of recently collected high resolution seismic images of the upper mantle beneath the central colorado rocky mountains substantiates the notion that much of the high elevation coincides with thin or attenuated continental crust with respect to predicted airy crustal thicknesses necessitating topographic support by anomalously buoyant mantle this is highly suggestive that broad scale topographic features may be correlated with buoyancy variations in the upper mantle in an attempt to sharpen our understanding of the underlying geodynamics we evaluate the correlation between the surface topographic character and data sets that provide information about density variations indicative of buoyancy in the upper mantle including the lithospheric geoid upper mantle seismic velocity anomalies and crustal lg q our general conclusion is that mantle buoyancy is driving differential surface uplift throughout the western united states and this driver of topography is manifested by measureable anomalies in the topographic roughness at short wavelengths tens of kilometer and elevated spectral power in the topography at longer several hundred kilometers wavelengths a provocative conclusion is that the long recognized physiographic provinces of the colorado plateau rocky mountains and rio grande rift are also neotectonic provinces that are related to convective processes and related buoyancy in the upper mantle

delineation of groundwater provenance in a coastal aquifer using statistical and isotopic methods southeast tanzania rapid population growth and urbanization has placed a high demand on freshwater resources in southeast costal tanzania in this paper we identify the various sources of groundwater and the major factors affecting the groundwater quality by means of multivariate statistical analyses using chemical tracers and stable isotope signatures the results from hierarchical cluster analyses show that the groundwater in the study area may be classified into four groups a factor analysis indicates that groundwater composition is mainly affected by three processes accounting for 80 6 of the total data variance seawater intrusion dilution of groundwater by recharge and sewage infiltration the hydrochemical facies of shallow groundwater was mostly of the na ca cl type although other water types were also observed the deep groundwater samples were slightly to moderately mineralized and they were of the nahco3 type this water type is induced mainly by dissolution of carbonate minerals and modified by ion exchange reactions the signal from the stable isotope composition of the groundwater samples corresponded well with the major chemical characteristics in the shallow groundwater both high nitrate and high chloride concentrations were associated with localized stable isotope enrichments which offset the meteoric isotopic signature this is inferred to be due to the contamination by influx of sewage as well as intrusion by seawater the depleted stable isotope values which coincides with a chemical signature for the deep aquifer indicates that this deep groundwater is derived from infiltration in the recharge zone followed by slow lateral percolation this study shows that a conceptual hydrogeochemical interpretation of the results from multivariate statistical analysis using hca and fa on water chemistry including isotopic data provides a powerful tool for classifying the sources of groundwater and identifying the significant factors governing the groundwater quality the derived knowledge generated by this study constitutes a conceptual framework for investigating groundwater characteristics this is a prerequisite for developing a sound management plan which is a requirement for ensuring a sustainable exploitation of the deep aquifer water resource in the coastal area of tanzania

genetic stratigraphic sequences in basin analysis ii application to northwest gulf of mexico cenozoic basin the northwest gulf of mexico cenozoic sedimentary wedge illustrates the application of genetic stratigraphic sequence analysis and documents several general conclusions 1 sequences defined by regional marine flooding are the principal genetic stratigraphic units of the basin fill continental margins are characterized by repetitive episodes of basin margin offlap punctuated by periods of transgression and marine flooding of the depositional platform 2 continental margin outbuilding is concentrated at one or more shelf edge deltaic depocenters separated by interdeltaic bights depocenters remain fixed during a depositional episode but commonly relocate during transgression and flooding 3 a distinct syndepositional structural style in prograding continental margins results in sporadic uplift of a basin fringing peripheral bulge and accentuates preservation of shelf margin facies along zones of extensional normal faulting and enhanced subsidence 4 genetic stratigraphic sequences commonly reflect an evolving interplay among two or even three variables for example early cenozoic gulf sequences are most closely related to tectonic events of the intraplate source terrane which in turn affect rate and location of sediment supply and basin margin response to loading late cenozoic sequences more closely reflect proposed eustatic cycles

chemical and thermal zonation in a mildly alkaline magma system infiernito caldera trans pecos texas postcollapse lavas of the infiernito caldera grade stratigraphically upward from nearly aphyric high silica rhyolite 76 sio2 to highly prophyritic trachyte 62 sio2 plagioclase clinopyroxene orthopyroxene magnetite ilmenite and apatite occur as phenocrysts throughout the sequence sanidine biotite and zircon are present in rocks with more than about 67 sio2 major and trace elements show continuous variations from 62 to 76 sio2 modeling supports fractional crystallization of the observed phenocrysts as the dominant process in generating the chemical variation temperatures calculated from coexisting feldspars pyroxenes and fe ti oxides agree and indicate crystallization from slightly more than 1100 c in the most mafic trachyte to 800 c in high silica rhyolite the compositional zonation probably arose through crystallization against the chilled margin of the magma chamber and consequent rise of more evolved and therefore less dense liquid mineral compositions vary regularly with rock composition but also suggest minor mixing and assimilation of wall rock or fluids derived from wall rock mixing between liquids of slightly different compositions is indicated by different compositions of individual pyroxene phenocrysts in single samples liquid solid mixing is indicated by mineral compositions of glomerocrysts and some phenocrysts that apparently crystallized in generally more evolved liquids at lower temperature and higher oxygen fugacity than represented by the rocks in which they now reside glomerocrysts probably crystallized against the chilled margin of the magma chamber and were torn from the wall as the liquid rose during progressive stages of eruption assimilation is indicated by rise of oxygen fugacity relative to a buffer from more mafic to more silicic rocks calculation of density and viscosity from the compositional and mineralogical data indicates that the magma chamber was stably stratified lower temperature but more evolved thus less dense rhyolite overlay higher temperature less evolved and therefore more dense progressively more mafic liquids the continuity in rock and mineral compositions and calculated temperature viscosity and density indicate that compositional gradation in the magma chamber was smoothly continuous any compositional gaps must have been no greater than about 2 sio2

compositional changes in trans pecos texas magmatism coincident with cenozoic stress realignment the composition and inferred sources of cenozoic magmas in trans pecos texas changed abruptly between 32 and 28 ma coincident with a fundamental change in stress regime magmas emplaced between 48 and 32 ma comprise extended differentiation suites beginning with relatively evolved basaltic magmas these rocks have low nb and ta compared to zr hf la ba and k characteristics typical of continental volcanic arcs however the texas rocks are more alkalic have higher concentrations of incompatible elements and have higher ratios of nb and ta to zr hf la ba and k than is typical of rocks from arcs near trenches these differences reflect relative position within the cordilleran arc trace element models involving partial melting in the mantle wedge combined with fractional crystallization and assimilation of upper and lower crust can account for most of the observed trace element concentrations and ratios trans pecos magmatism may have involved smaller degrees of partial melting than is typical of arcs located closer to the subduction zone magmas emplaced after 31 ma were either bimodal alkali basalt rhyolite or exclusively basaltic they have high absolute abundances of incompatible trace elements and high ratios of nb and ta to zr hf la ba and k the mafic rocks are typical of continental rift or ocean island basalts trace element models using nb ta y yb hf and zr reproduce the range of concentrations and ratios observed in these rocks and require very small degrees of partial melting variable amounts of fractional crystallization but little or no assimilation the change in magma compositions and sources is best illustrated by a drop in zr nb from between 18 and 6 25 in the earlier arc rocks to between 6 25 and 2 in the later rift related rocks stress regime appears to play an important role in the process of magma generation and evolution the change in trace element compositions indicative of tectonic setting supports interpretations of paleostress data that activity up to 31 ma occurred in a continental volcanic arc whereas later activity occurred in a setting of intraplate extension the magmatic change may have been simultaneous with the change in stress regime or may have lagged by as much as 3 4 m y

the basin and range province origin and tectonic significance the basin and range province is a vast arid tract of regionally corrugated angular topography of high relief in the western cordillera it is characterized by evenly spaced parallel mountain ranges and intervening desert basins fig ure la the range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward interrupted here and there by low fault scarps that paral lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges thermal springs located at or near range bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting in the southern part of the province especially in southeastern california and southwestern arizona range fronts have been worn back by erosion leaving a thin veneer of gravel on an erosion cut bedrock surface that slopes gently outward the range bounding faults of these mountain blocks are buried at the outer edge of such pediments often at considerble distances from the erosional remnants of the ranges themselves the american physiographer n m fenneman 1928 1931 named the basin and range province and defined its general boundaries as thus circum scribed the province includes some 800 000 km2 of area in eight western states later students pardee 1950 lawrence 1976 reynolds 1979 eaton 1979b have observed that many of the fundamental geological and geo physical characteristics of the province are found well beyond the boundaries drawn by fenneman which were based on physiography alone as a tec tonophysical entity its areal extent is greater than 1 million km2 more than 10 of the area of the united states figure 1 fenneman 1931 subdivided the province into five physiographic sections the largest of which is the great basin see figure 1b it is not as its name implies a single regional depression with a common topographic center but is characterized instead by isolated networks of interior drainage divisible into

geomorphic and tectonic forcing of late cenozoic warping of the colorado piedmont late cenozoic warping of the colorado piedmont involved interplay of tectonic forcing river erosion and isostatic response to erosion modeled erosional isostasy closely replicates the observed pattern of deformation but accounts for only about half its magnitude the remainder reflects tectonic rock uplift that increases southward across the piedmont likely reflecting proximity to the northward propagating rio grande rift this differential uplift triggered differential erosion concentrated on southern piedmont river systems particularly the arkansas river which led in turn to differential isostatic rock uplift focused on the arkansas drainage covariation of tectonic uplift erosion and isostatic compensation across the piedmont reflects a positive feedback between uplift induced erosion and erosion induced isostasy which has progressed to the point that isostatic uplift is approximately equal to the initial tectonic forcing

origins of topography in the western u s mapping crustal and upper mantle density variations using a uniform seismic velocity model to investigate the physical basis for support of topography in the western u s we construct a subcontinent scale 3 d density model using 1000 estimated crustal thicknesses and s velocity profiles to 150 km depth at each of 947 seismic stations crustal temperature and composition are considered but we assume that mantle velocity variations are thermal in origin from these densities we calculate crustal and mantle topographic contributions typical 2 uncertainty of topography is 500 m and elevations in 84 of the region are reproduced within error remaining deviations from observed elevations are attributed to melt variations in crustal quartz content and dynamic topography compositional variations in the mantle while plausible are not necessary to reproduce topography support for western u s topography is heterogeneous with each province having a unique combination of mechanisms topography due to mantle buoyancy is nearly constant within 250 m across the cordillera relief there 2 km results from variations in crustal chemistry and thickness cold mantle provides 1 5 km of ballast to the thick crust of the great plains and wyoming craton crustal temperature variations and dynamic pressures have smaller magnitude and or more localized impacts positive gravitational potential energy gpe anomalies 2 1012n m calculated from our model promote extension in the northern basin and range and near the sierra nevada negative gpe anomalies 3 1012n m along the western north american margin and yakima fold and thrust belt add compressive stresses stresses derived from lithospheric density variations may strongly modulate tectonic stresses in the western u s continental interior

a tectonic redefinition of the southern rocky mountains abstract the southern rocky mountains of the western united states physiographically defined and described by n m fenneman nearly 60 yrs ago are tectonically redefined and extended they are shown to constitute the crestal range of a mammoth continental arch like feature here named the alvarado ridge its axis trends south from casper wyoming at least as far south as el paso texas beyond which the ridge begins to lose morphological identity maximum elevations along the crest of the ridge exceed 4 2 km the summit ranges are bordered on either side by gently sloping rises that extend outward for at least 1300 km falling to elevations of less than 400 m modest rift structures along the ridge axis continue beyond the southern terminus of the mountains before playing out a major sediment filled axial graben exists over the southern two thirds of the ridge but equivalent parts of it farther north were stripped of their neogene fill by erosion in the headwaters areas of the colorado and north platte rivers the maximum elevation of earlier laramide mountains in this area has been estimated to have been no more than 2 km in colorado half that of the present range related topography was nearly obliterated by erosion prior to late eocene time lateral stream planation produced a southeast sloping major late eocene erosion surface across the region that had what was probably an isostatically adjusted average maximum elevation of less than 900 m present day elevations and relief on the ridge crest are the result of steep crestal normal faulting pronounced block uplift and regional arching with extensional strain limited to an axial corridor less than 200 km wide this episode of mountain building began in middle miocene time 17 to 12 ma culminating in latest miocene and early pliocene time between 7 and 4 ma ago debris from the newly elevated range the southern rocky mountains sensu lato was shed along the full length of the neogene alvarado ridge down parts of both rises as well as into its medial graben the uplifted and tilted east rise the great plains ramp has an extensive aggradational cover that spread eastward to distances of 700 km and more in middle miocene to earliest pliocene time its fluviatile lacustrine and eolian sediments are undergoing dissection today but a sufficient portion remains to define the unmodified profile of the east rise as well as the original length of the ridge it is coincidentally similar in configuration and scale to the west rise of the mid atlantic ridge the topography of the crestal mountain ranges on both structures is also remarkably similar the southern rocky mountains are as high as the alps but are not the product of lithospheric compression and crustal thickening rather they reflect profound epeirogenic uplift driven by lithospheric thinning and accompanied by extensional strain and differential vertical jostling along the crest of the ridge

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

upper mantle shear structure beneath the colorado rocky mountains a tomographic inversion of teleseismic shear wave delays has been performed using data collected in the 1992 program for array seismic studies of the continental lithosphere rocky mountain front experiment the shear wave residuals used as data were corrected for known crustal sediment and topographic differences and thus were sensitive to mantle shear velocity variations beneath the colorado array large differences in shear wave travel time are observed across colorado and into western kansas with a maximum difference of near 5 sec between the central rockies in colorado and a station in western kansas the shear wave residuals were inverted for shear wave velocity variations in the upper 350 km of the mantle beneath colorado and kansas large differences in mantle shear velocity are found between the rockies and the kansas great plains the average shear velocity in the upper 200 km is about 9 greater beneath kansas than beneath the rockies at greater depths lateral variations are far smaller and not as well correlated with surface tectonics in a qualitative sense there appear to be three mantle structures present beneath the study area beneath the rockies the upper mantle is extremely slow both to the west and east of the rockies we find intermediate mantle shear velocity there is then an abrupt increase in mantle shear velocity near the colorado kansas border we interpret the abrupt increase in mantle shear velocity near the colorado kansas border to mark the boundary of cratonic lithosphere the slow velocity zone beneath the rockies appears to be upwelled asthenosphere as is seen beneath the rio grande rift to the south to match the seismic contrast between the mantle beneath kansas and the colorado rockies requires a temperature contrast of at least 350 c as well as partial melt beneath the rockies the hot mantle beneath the rockies implies that the mantle density is low and the high topography of the rockies is at least partially due to buoyant mantle

an automated method for producing synoptic regional maps of river gradient variation procedure accuracy tests and comparison with other knickpoint mapping methods abstract the study of abrupt changes in longitudinal river profiles or knickpoints is currently approached through an empirical power law the slope area relationship results based on digital elevation model dem analyses and stream extractions are generally intended to determine crustal uplift rates and identify transient landscape conditions in this article we present an alternative geomorphometric method for locating knickpoints and knickzones based on local slope gradient and curvature attributes intended as a rapid regional scale automated knickpoint detection technique the accuracy of this slope curvature method is tested on two digital elevation grids nasa s srtm ground resolution of 90 m resampled here to 75 m and the aster dem 15 m in the sierra nacimiento new mexico usa a basement cored mountain range recently exhumed by waves of headward drainage integration in response to remote tectonic deformation in the adjacent rio grande rift out of every 10 gradient anomalies detected by the srtm derived numeric routine up to 8 are certifiable knickpoints recognized among a population of georeferenced occurrences surveyed in the field an independent comparison with the slope area method provided a further accuracy test which was particularly useful at sites that could not be validated in the field for practical reasons given the low tectonic activity of the study area the majority of knickpoints was also found to coincide with lithologic boundaries making it difficult without further geomorphological data to single out dynamic knickpoints directly caused by the upstream propagation of channel instabilities relating to base level change

saltwater intrusion and nitrate pollution in the coastal aquifer of dar es salaam tanzania dar es salaam quaternary coastal aquifer is a major source of water supply in dar es salaam city used for domestic agricultural and industrial uses however groundwater overdraft and contamination are the major problems affecting the aquifer system this study aims to define the principal hydrogeochemical processes controlling groundwater quality in the coastal strip of dar es salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater quality major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors dissolution of calcite and dolomite weathering of silicate minerals seawater intrusion due to aquifer overexploitation and nitrate pollution mainly caused by the use of pit latrines and septic tanks high enrichment of na and cl near the coast gives an indication of seawater intrusion into the aquifer as also supported from the na cl signature on the piper diagram the boreholes close to the coast have much higher na cl molar ratios than the boreholes located further inland the dissolution of calcite and dolomite in recharge areas results in ca hco3 and ca mg hco3 groundwater types further along flow paths ca2 and na ion exchange causes groundwater evolution to na hco3 type from the phreeqc simulation model it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals the results of this study provide important information required for the protection of the aquifer system

mid cenozoic stress evolution and magmatism in the southern cordillera texas and mexico transition from continental arc to intraplate extension orientations of dikes veins faults and slickenlines reveal the evolution of stress during eocene to miocene magmatism in the southern cordillera where most thoroughly studied by us in trans pecos texas magmatism began at about 48 ma shortly after the cessation of laramide folding dikes and veins that formed from then until about 32 ma strike dominantly east northeast this indicates that the least principal stress 3 was north northwest additional data suggest that the maximum principal stress 1 was east northeast the stress field changed to 1 vertical and 3 east northeast i e east northeast extension at least by 28 ma and probably by 31 ma dikes and veins that formed between 31 and 17 ma when all magmatism ceased in texas strike north northwest this change marks the beginning of regional basin and range extension however major normal faulting exclusively of high angle type did not begin until about 24 ma a similar stress change marked by a similar change in dike and vein orientations occurred throughout the southwestern united states and northern mexico the time of change is not well constrained in texas but available information allows it to have occurred at the same time thoughout the southern cordillera we suggest the earlier stress field is related to east northeast convergence between the farallon and north american plates the change in stress is approximately coincident with collision of the east pacific rise and paleotrench extension may be related to the change from a convergent to a transform margin along the western edge of north america the changes in the stress field are accompanied by changes in the sources and compositions of magmas erupted in texas contemporaneity of the changes in stress and magmatism indicates that they are related combined with regional age patterns paleostress and geochemical data indicate that pre 31 ma magmatism in the southern cordillera occurred in a subduction related continental volcanic arc subsequent magmatism occurred in an environment of intraplate extension of the basin and range province

mantle dynamics isostasy and the support of high terrain dynamic topography is commonly understood to be deflection of the earth s surface that results from convection of the mantle because different authors use the words dynamic topography differently topography designated as dynamic may amount to only a few hundred meters or may exceed 2000 m for most regions however surface heights computed on the assumption that the lithospheric column is in isostatic equilibrium provide good approximations to observed topography the small free air gravity anomalies and still smaller isostatic anomalies 2000 m of dynamic topography moreover simple examples show that if flow induced stresses maintain a given density distribution then the resultant surface deflections can be smaller than those that would exist in isostatic equilibrium in light of confusion over the term dynamic topography we offer readers simple tools that we hope will enable them to diagnose the component of topography that results from flow induced stresses and to distinguish it from topography that is compensated isostatically

the impact of inter bed cohesion on fold related fracture development stillwell anticline west texas usa abstract the interpretation of fracture networks in contractional folds is challenging due to the range of factors that control fracture formation we use outcrop based analysis of fractures in plan view pavements and in a 9 bed cross sectional exposure to evaluate the fracture system within cretaceous limestone layers of a laramide fold in west texas the stillwell anticline opening mode extension fractures veins at high angles to bedding dominate the fracture population although shear fractures and faults cut bedding at low angles within the forelimb analysis of extension fractures reveals nw striking axial parallel and ne striking axial perpendicular fracture sets interpreted to have formed during contractional folding a third n striking fracture set formed during subsequent basin and range extension and a fourth ese striking fracture formed due to unloading during exhumation fracture fill textures suggest that many fracture apertures increased during exhumation the relative abundances of the four fracture sets and the intensity of each set vary from bed to bed in cross section because beds display no significant differences in mechanical strength and there is no correlation between bed thickness and fracture intensity we attribute this bed to bed variability to differences in cohesion between beds bed decoupling when combined with low extensional

upper mantle structure beneath the eastern colorado plateau and rio grande rift revealed by bouguer gravity seismic velocities and xenolith data 1 the lithospheric architecture of the colorado plateau rio grande rift transition zone reflects a narrow corridor of active tectonism and magmatism between the tectonically stable colorado plateau and the stable interior of north america in this paper we analyze bouguer gravity data crustal thickness variations xenolith data and seismic wave speed structure to argue that the rio grande rift and southeastern colorado plateau is underlain by a low density upper mantle province that does not trend along upper crustal tectonic boundaries but rather is correlated with regions of middle to late tertiary magmatism we model uppermost mantle density contrasts using bouguer gravity along five parallel profiles across the colorado plateau and validate our models by converting modeled density contrasts to seismic velocity variations for comparison with regional seismic data as well as comparing modeled densities to calculated densities from xenoliths we find that upper crustal features and crustal thickness variations do not explain long wavelength 200 km variations in bouguer gravity across the colorado plateau rio grande rift a large 200 440 km wide region of low uppermost mantle density is modeled beneath the eastern colorado plateau margin jemez lineament rio grande rift and parts of the westernmost great plains this region has a density contrast of 90 to 30 kg m3 relative to the adjacent colorado plateau and great plains provinces and coincides with previously imaged regions of low upper mantle seismic velocities in situ densities inferred from xenolith data in the colorado plateau support the modeled upper mantle density variations and support the idea that the eastern and western margins of the plateau have been significantly modified by metasomatism we suggest that the region of low upper mantle density likely reflects some combination of temperature variations partial melt contents and compositional variations related to tertiary magmatic activity and or thinned lithosphere the relationship of this zone of modified upper mantle to extension in the rio grande rift remains enigmatic however particularly given the lack of spatial correlation between the trends of upper mantle structures and surficial extensional structures

hydrogeochemical assessment of a coastal aquifer using statistical and geospatial techniques case study of mombasa north coast kenya the geochemistry of a coastal aquifer was assessed using statistical and geospatial analysis tools for the pre monsoon rainy and post monsoon seasons data were obtained from both the field and laboratory analysis of water samples statistical methods such as correlation coefficients piper plots factor analysis and mixing index were used to gain insights into the geochemistry while geospatial tools were used to create contours to understand the spatial distribution of the measured groundwater parameters of the coastal aquifer the measured groundwater levels ranged from 0 84 to 30 08 m above mean sea level the electrical conductivities and total dissolved solids values were observed to have perfectly correlated with each other groundwater salinities were generally high as over 94 of the water samples tested exceeded the who drinking water limit of 750 s cm and 500 mg l respectively the groundwater ph was generally slightly alkaline but could be slightly acidic in the rainy season the na k mg2 cl and so4 2 were observed to have high impacts on the geochemistry and also had tendencies to form similar trends ec tds and nacl values above 1000 mg l in the groundwater were observed to generally skew towards the ocean during the rainy season the principal process influencing the geochemistry was found to be seawater intrusion while mineral dissolutions and rainwater percolation play lesser roles the aquifer predominantly comprises na cl waters of marine origin the study shows the growing importance and applicability of integrated statistical and geospatial approaches for better understanding of groundwater and geochemistry of aquifers

geochronology of some canarian dike swarms contribution to the volcano tectonic evolution of the archipelago we present new conventional k ar and 39ar 40ar age data and geometric observations on dike swarms which intrude the islands of la gomera tenerife and fuerteventura canary archipelago the younger dated dikes of la gomera 5 4 5 2 ma were emplaced by a radial fissural structure independently superimposed on an older one dated between 10 5 and 7 0 ma the dated dike swarms of both the anaga and teno peninsula of tenerife are contemporaneous 5 8 3 6 ma while ages of 0 43 ma were measured on two dikes intruding the pedro gil ridge the dikes of the basal complex of fuerteventura give well defined 39ar 40ar plateaus when they are situated far away from basic and ultrabasic plutons and disturbed 39ar 40ar age spectra when they are situated in the vicinity of the plutons these disturbances are probably due to a thermal metamorphism the results of both the conventional k ar and 39ar 40ar stepwise heating methods show the existence of an important magmatic phase between 24 and 17 ma the dike swarms of the basal complex are partly contemporaneous with other dike systems intruding the basaltic series i of fuster et al 1968a this phase seems to correspond to the final stages of activity on the main nne ssw volcano tectonic axis following this a volcanic edifice was built on the nw part of jandia peninsula mainly between 15 4 and 14 2 ma the similarity of the ages obtained on dikes and lava flows shows the importance of fissural volcanism in these islands this study shows that dating of dike swarms reveals the existence of major eruptive phases an important part of the products of which have been eroded brief episodes of fissural volcanic activity sometimes probably as short as 200 000 years may be correlated with the migration of the lithosphere over a fixed hot spot the north south 3 5 ma old present and the nne ssw 24 17 ma old oriented dike swarms of la palma staudigel et al in prep and fuerteventura may represent the regional paleostress orientation plane 1 2 which affected the lithosphere during their intrusion although other phenomena may control the orientation of dikes these directions are concordant with the known contemporaneous nw african tectonics

early basin and range development in trans pecos texas and adjacent chihuahua magmatism and orientation timing and style of extension magmatism during early basin and range extension in trans pecos texas consisted of widespread but volumetrically minor alkali basalts 24 17 ma an alkalic mafic to intermediate composition stratovolcano 28 27 ma and a dominantly rhyolitic possibly bimodal caldera complex in adjacent chihuahua 31 28 ma this magmatism contrasted sharply with the much more voluminous and compositionally more diverse eocene oligocene magmatism the age and orientation of dikes emplaced during this magmatic episode have been used to determine the orientation of least principal stress 3 an ene orientation of 3 similar to early extension throughout the basin and range province can be well documented by 28 ma and possibly as early as 31 ma faulting consisting almost exclusively of movement along high angle normal faults was underway by 23 ma some initial extension was taken up along preexisting wnw trending structures oblique to ene oriented 3 the least principal stress probably changed to wnw as it did elsewhere in the basin and range province but the timing of this change is not well constrained in texas ene and ne striking silver copper lead veins in precambrian and cretaceous red beds may have formed during the later basin and range extension the present basin and range topography of this region is the result of movement along high angle and dominantly nnw trending faults established early under 3 oriented ene and reactivated after 3 changed to wnw

receiver functions in the western united states with implications for upper mantle structure and dynamics 1 investigations into mechanisms driving surface tectonics commonly search for mantle sources but few observations constrain flow in the upper mantle and transition zone here variations in the upper mantle discontinuities at 410 km and 660 km below the western united states are revealed through mapping depths of compressional to shear wave conversions recorded by broadband seismometers the resulting image exhibits 20 and 30 km of topography on the 410 and 660 km discontinuities similar depth variations to those seen where subducting slabs of lithosphere reach the transition zone the pattern of discontinuity topography imaged here does not correlate with the surface tectonics of the rocky mountains colorado plateau or basin and range province providing no support for upward and downward flow at transition zone depths controlling surface topography and deformation in this region at least at scale lengths of a few hundred kilometers furthermore we find no clear correlation between the depths of the 410 and 660 km discontinuities undulations on the surfaces of both discontinuities appear to be spaced at distances of 800 km if the topography were due only to lateral temperature differences such differences would be comparable to those where slabs sink and might suggest separate convective flow above and below the transition zone alternatively the topography may reflect lateral variations in composition variations in the sharpness of converted phases across the region offer some support for compositional heterogeneity but the lack of a correlation between sharpness and depth casts doubt on this explanation for the variations in depth

stress orientations during oligocene volcanism in trans pecos texas timing the transition from laramide compression to basin and range tension orientations of dikes and hydrothermal veins indicate that east northeast compression occurred during the main pulse of volcanism in west texas 39 to 32 m y ago direction of compression was essentially the same as that during eocene laramide deformation the observations support a continental arc tectonic setting the beginning of extension between 32 and 30 m y ago coincides with a regional change in style of magmatism from intrusions and eruptions spanning a wide range in chemical composition to basaltic volcanism in texas and to bimodal but dominantly rhyolitic volcanism in northern mexico

the mean meridional transport circulation of the troposphere in an idealized gcm abstract large numbers of particle trajectories are used to characterize the mean meridional transport circulation of an idealized general circulation model gcm the gcm has a uniform land surface no topography or land sea contrasts and no seasonal cycle the trajectories are analyzed using both lagrangian mean statistics and an approach based on the green s function of the tracer transport equation it is shown that the distribution of particle trajectories provides an estimate of the ensemble mean green s function for the inviscid transport equation lagrangian means have a number of problems that render their interpretation difficult including boundary effects and selection of the appropriate averaging period the green s function on the other hand provides a quantitative description of the transport circulation that is easy to visualize and interpret the results demonstrate that within the idealized model the atmosphere can be divided into three parts the southern hemisphere extratropics the

the volcanic section at nazas durango mexico and the possibility of widespread eocene volcanism within the sierra madre occidental descriptions of volcanic rocks in the sierra madre occidental of western mexico have thus far emphasized the widespread and nearly continuous cover of ash flow tuffs and other units attributed to the oligocene ignimbrite flare up however much less attention has been given to the pre oligocene volcanic sequence beneath this ash flow blanket at nazas durango on the eastern flank of the sierra madre occidental a well exposed section includes voluminous felsic volcanic rocks of both eocene and oligocene age the oldest igneous rocks at nazas include a small exposure of intermediate volcanic breccias and lava flows that overlie cretaceous limestones and apparently were deformed with them during laramide tectonism more commonly the limestones are overlain by tertiary volcanic rocks and continental clastic deposits the tertiary volcanic section is about 800 m thick not including intercalated fanglomerates the section records three distinct magmatic episodes from 51 to 40 ma comprising felsic ash flow tuffs and intermediate lava flows and domes a pulse at about 30 ma comprising voluminous felsic ash flow tuffs with an aggregate thickness of about 500 m and from 24 to 20 ma comprising alkalic basalts the ahuichila formation a molasse type conglomerate up to 200 m thick that underlies the tertiary volcanic section was deposited during or just after the laramide deformation the santa ines formation is a widespread fanglomerate up to 110 m thick that underlies the basalts and apparently accumulated during normal faulting eocene volcanism in the nazas area is characterized by interfingering felsic ash flow tuffs and intermediate lava flows and domes a similar sequence of eocene volcanic rocks has been mapped in central chihuahua state about 350 km to the northwest of nazas felsic tuffs interbedded with intermediate volcanic rocks older than 40 ma are also exposed at tayoltita durango about 200 km southwest of nazas these occurrences suggest that a wider spectrum of compositions and volcanic styles existed during the eocene than during the oligocene in the sierra madre occidental eocene volcanism was more typical of orogenic magmatic belts developed at continental margins whereas the oligocene activity was dominated by voluminous felsic ash flow tuffs erupted during a transition of tectonic setting from subduction along a continental margin to intraplate extension in addition to these three areas there are several other localities in western mexico having volcanic rocks with reported ages between 40 and 53 ma it is possible that the volcanic field during the eocene in mexico was comparable in extent to that of the oligocene the eocene volcanism in western mexico was in part contemporaneous with the challis volcanic episode of northwestern united states and its extension into western canada however eocene magmatism in the pacific northwest apparently developed in an extensional tectonic setting whereas in mexico no evidence for eocene extensional deformation is known

impact of rock uplift on rates of late cenozoic rocky mountain river incision 1 the high relief of the modern rocky mountain landscape formed in the late cenozoic by downcutting of a fluvial network that links a series of easily eroded sedimentary basins across relatively resistant crystalline cores of adjacent ranges using a numerical model of fluvial erosion and the flexural isostatic response to the associated unloading we first calculate the expected pattern and pace of incision caused by rock uplift related to migration of the yellowstone hot spot and to growth of the northern portion of the rio grande rift calculated incision rates are 60 m myr and total depth of erosion of sedimentary basins is 300 m well below the long term incision rates and amounts of erosion interpreted from the geologic record broad scale tilting of the region toward the east accomplished by a gradient in rock uplift of 1 km along the north south axis of the central rockies declining to zero 1000 km to the east can account for the additional erosion needed to match observations in each modeling scenario stream incision is nonsteady with rock uplift outpacing erosion for 1 myr in perimeter basins and 1 5 myr in interior basins three factors dominate the spatial and temporal pattern of regional landscape evolution 1 the time since uplift began 2 the uplift pattern and 3 the distribution of relatively resistant bedrock within the region our results suggest that the spatial variability in late cenozoic exhumation can be explained by a long lived transience in the stream network response to these various late cenozoic geophysical events

crustal structure gravity anomalies and heat flow in the southern rio grande rift and their relationship to extensional tectonics abstract the rio grande rift is a neogene structure which has undergone a two phase history of extension and volcanism significant amounts of new data are available in the southern rio grande rift region which is the area of this study the first phase of extension 15 30 ma was of the order of 30 50 and was directed ne sw the second phase 3 10 ma involved extension of about 10 which was directed e w crustal structure in the region of the rift can be mapped using recent seismic and gravity data and the observed thinning associated with the rift is consistent with extension of about 10 however heat flow data in the southern rift cannot be explained simply in terms of extension both the heat flow and uplift history of the area require crustal thickening possibly by intrusion in the lower crust a process not directly related to extension

sedimentation and tectonism in the laramide tornillo basin of west texas abstract the tornillo basin of west texas and adjacent mexico is the southernmost laramide age intermontane basin in the north american cordillera the basin is bounded on the west by the chihuahua tectonic belt a southern extension of the cordilleran fold and thrust belt and on the east by the marathon uplift a laramide style basement cored uplift sedimentological evidence indicates that tectonism began in this region during late cretaceous time ca 70 ma and ended during middle eocene time ca 50 ma the basin experienced two major pulses of fluvial sedimentation the first during middle to late maastrichtian time and the second during late paleocene to early eocene time high sedimentation rates during these periods correspond to the influx of coarse extra basinal detritus high mudstone content in the basin alluvium and thick tabular fluvial sand bodies these episodes probably reflect pulses of tectonism a lull in sedimentation during early paleocene time resulted in a thin sequence of alluvium with very high mudstone content thin lenticular fluvial sand bodies and well developed paleosols the basin experienced left lateral transpressive deformation during the second phase of tectonism the sedimentary sequence in the tornillo basin is compatible with the two phase model of laramide tectonism developed for the central and southern rocky mountains

evidence for shallow isostatic compensation of the southern rocky mountains from rayleigh wave tomography in the past decade seismologists have found that the topography in colorado correlates with seismic velocities in the upper mantle not with crustal thickness and postulated that isostatic compensation of the southern rocky mountains takes place largely in the mantle to test the validity of this hypothesis we use rayleigh wave phase and amplitude data recorded in the rocky mountain front passcal experiment to obtain crustal thickness and shear wave structure in the crust and upper mantle the thickest crust in the model is beneath one of the two most elevated regions the san juan volcanic field the other most elevated region in central colorado the sawatch range is underlain by crust of anomalously low velocity the seismically defined mantle lithosphere is thicker beneath the great plains than beneath the colorado plateau and is largely absent beneath the southern rocky mountains but the correlation of topography with mantle velocities is weaker than the correlation with crustal anomalies we construct a model that matches the observed bouguer gravity anomaly primarily with variations in crustal thickness and density by assuming that density anomalies are proportional to the observed velocity anomalies the anomalous mantle may also contribute to maintaining regional isostasy in colorado but its contribution may be much less important than the crust

hydrochemical characteristics of aquifers near sutherland in the western karoo south africa the western karoo depends entirely on groundwater for domestic and agricultural use an understanding of the chemical evolution of the groundwater provides insight into the interaction of water with the environment and contributes to better resource management descriptive statistics correlation matrices and factor analysis together with stable isotope data were used to gain an understanding of the hydrochemical processes of the groundwaters in the fractured rocks around sutherland in the western karoo the main processes influencing the groundwater chemistry are salinisation mineral precipitation and dissolution cation exchange and human activity the stable isotope data indicate that the infiltration of evaporated water is partly responsible for the occurrence of saline subsurface waters the ionic oxygen and hydrogen stable isotope composition of the groundwaters suggests that the effects of localised topography are one of the overriding factors controlling the groundwater chemistry in higher lying areas ca hco3 2 type waters are prevalent while in topographical flat areas nacl type waters dominate in areas where water collects close to or at the surface saline soils are formed from these areas salts are leached to the subsurface during significant recharge periods these findings are important in that they contribute to the identification of suitable locations for developing groundwater resources that could yield a relatively better quality water q 2001 elsevier science b v all rights reserved

post paleozoic alluvial gravel transport as evidence of continental tilting in the u s cordillera the western united states contains three thin but remarkably widespread alluvial conglomeratic units that record episodes of large scale tilting across the u s cordilleran orogen in post paleozoic time these units are 1 the shinarump conglomerate of late triassic age exposed in northern arizona and adjacent parts of utah nevada and new mexico 2 lower cretaceous gravel deposits that overlie the morrison formation throughout the rocky mountain region and 3 the gravel rich parts of the miocene pliocene age ogallala group in western nebraska and adjacent southeastern wyoming paleoslopes of the rivers depositing these units were in the range of 10 24 to 10 23 based on paleohydraulic calculations however depositional thickness trends of these units are not sufficient to have generated such steep paleoslopes thus long wavelength tilting of the earth s surface must have occurred for these gravels to be transported although these units were deposited adjacent to large tectonic features including an evolving and migrating continental arc and for the ogallala group the northward propagating rio grande rift the tilting occurred over wavelengths too broad to be directly generated by these features these widespread gravel units attest to the interplay between the creation of subduction related isostatic and dynamic topography and continental sedimentation hence paleotopography as determined from calculated transport gradients of sedimentary deposits provides a

kinematic evolution of the eastern chalk draw fault during basin and range extension evaluating the role played by preexisting structural fabrics in along strike displacement patterns abstractbasin and range extensional structures in trans pecos texas have been interpreted to be strongly influenced by preexisting structures including igneous centers in the big bend region these igneous centers are interpreted to have deflected propagating faults as they are rheologically strong relative to the surrounding rocks we conducted detailed field mapping and analysis of fracture and fault data along the eastern segment of the chalk draw fault in big bend national park to evaluate its kinematic evolution and interaction with paleogene plutons our primary results document the following key relationships 1 the rosillos laccolith and other paleogene plutons in the study area are pervasively fractured dominantly by northwest striking mode i fractures which generally do not extend into adjacent sedimentary rock sequences 2 throw on the eastern chalk draw fault is strongly asymmetric decreasing rapidly as it enters the rosillos laccolith and 3 analysis of fault slip data in the regio

wintertime transport of reactive trace gases from east asia into the deep tropics author s donets valeria atlas el pan ll schauffler sm honomichl s hornbrook rs apel ec campos t hall sr ullmann k bresch jf navarro m blake dr