an extensive crustally derived 3325 to 3310 ma silicic volcanoplutonic suite in the eastern pilbara craton evidence from the kelly belt mcphee dome and corunna downs batholith abstract new sensitive high resolution ion microprobe zircon u pb dating indicates that the eastern pilbara craton contains an extensive suite of variably deformed 3325 to 3310 ma rhyolites high level quartz feldspar porphyries and granitoids these range in composition from na rich trondhjemite low k rhyolite to k rich monzogranite with silica rich 68 biotite hornblende bearing porphyritic granodiorites and monzogranites most abundant initial volcanism accompanied regional extension at 3325 ma and was followed by intrusion of high level porphyritic stocks and emplacement of massive to foliated batholithic granitoids at 3315 ma the compositions of this volcano plutonic suite suggest that the most likely source for parent magmas is older continental crust with tonalite trondhjemite granodiorite suite compositions crustally derived silicic volcanic rocks and granitoids have traditionally been considered rare in pre 3 0 ga granitoid greenstone terranes evidence from the eastern pilbara indicates that magmatic recycling of continental crust has occurred at least as early as 3 3 ga

a garnet two mica granite coyote mountains southern arizona geologic setting uranium lead isotopic systematics of zircon and nature of the granite source region the 58 m y old pan tak granite considered representative of the widespread early tertiary garnet two mica granites of south central arizona consists of biotite and muscovite biotite granite intruded by garnet muscovite and garnet biotite granite and associated pegmatite textural evidence indicates that muscovite is a primary magmatic mineral the granite was intruded into jurassic dioritic to granitic rocks and paleozoic metasedimentary rocks during a latest cretaceous and early tertiary regional metamorphic episode part of the pluton was subsequently mylonitized during an early or middle tertiary deformational episode of metamorphic core complex aspect the fabrics produced by these two metamorphic episodes differ in distribution style orientation and probably age the zircon population of the pan tak granite consists of a mixture of small euhedral crystals and larger rounded crystals with or without overgrowths uranium lead isotopic ratios of five size fractions of discordant zircon define a chord with a lower concordia intercept age of 58 2 m y and a late precambrian upper intercept 207 pb 206 pb ages range from 214 m y for the finest fraction to 434 m y for the coarsest fraction the pan tak discordia is interpreted as a mixing line between 58 m y old magmatic zircon and an older zircon component largely or entirely of late precambrian age geologic and isotopic evidence indicates that this older zircon component was not assimilated from the country rocks but rather was inherited from the source material of the granite the presence of this inherited zircon indicates that the pan tak magma was produced by melting of crustal rocks containing late precambrian zircon this source material was probably the 1 7 to 1 4 b y old crystalline basement of south central arizona generation of the pan tak and related granites by crustal anatexis was one aspect of a latest cretaceous to early tertiary orogenic episode in south central arizona

origin of mesozoic and tertiary granite in the western united states and implications for pre mesozoic crustal structure 2 nd and sr isotopic studies of unmineralized and cu and mo mineralized granite in the precambrian craton in the cordilleran region of the western united states mesozoic and tertiary peraluminous granitic rocks display regional variations in initial 143nd 144nd end end 10 to 12 in southern arizona 17 to 19 in the northern great basin ngb and 30 in the northern rocky mountains initial 87sr 86sr values are between 0 710 and 0 721 and show no regional pattern metaluminous granitic rocks have a wider range of end values extending from values similar to those of the peraluminous granites to much higher values the 87sr 86sr values are mostly fairly low between 0 705 and 0 710 except in the ngb where values as high as 0 7157 are observed no systematic differences between the end or 87sr 86sr values of cu or mo mineralized and unmineralized granite were discerned except for cu mineralized granite in eastern nevada and mo mineralized granite in colorado which have end values higher 0 and lower 10 0 respectively than unmineralized granite in the same region comparison to end values of exposed precambrian rock suggests that the peraluminous granite and the mo granite in colorado were derived exclusively from felsic precambrian basement rocks and that the regional variations in the end values reflect the regional variation in the average crustal age the nd data confirm that the precambrian basement underlying the ngb and eastern california is isotopically distinct from precambrian crust in the remainder of the western united states the similarity between the end values of peraluminous granite and precambrian crust also suggests that the high 147sm 144nd 0 13 and the low total light rare earth element lree abundances characteristic of peraluminous granite in southern arizona were imposed during the chemical evolution of the magmas metaluminous granite are interpreted to have formed via mixing of mantle derived magma and large proportions of low 87sr 86sr granulite facies lower crust except in the eastern ngb where the mantle magmas mixed with a lower crustal source with a significantly higher 87sr 86sr ratio ree abundance patterns for metaluminous granite in the ngb are characterized by extreme lree enrichment which supports the proposed origin for these rocks by mantle crust mixing or by remelting of the lower crust alone no systematic difference exists between the sources of cu or mo mineralized and unmineralized metaluminous granite but the data suggest that the cu sources are in the mantle and the mo sources are in preexisting crust overall the nd data indicate that continental interior granite in the western united states was primarily derived from preexisting crust and although changes in the thermal structure of the continental mantle may have triggered magma formation the resulting granites do not represent significant juvenile additions to the continental crust

spatial and temporal relationships between mid tertiary magmatism and extension in southwestern arizona cenozoic magmatism in southwestern arizona which is within the basin and range tectonic province occurred almost entirely between 15 and 25 ma volcanic rocks typically consist in ascending order of 1 a thin sequence of mafic to intermediate lava flows 2 voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows and 3 basalt and andesite volcanic rock sequences rest disconformably on pre tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism prevolcanic clastic debris is interpreted as a consequence of local early normal faulting in most regions tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism extension generally ended before about 17 ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated transition zone tectonic province which is adjacent to the colorado plateau rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 ma extension was extreme in this belt whereas it was generally moderate to slight in other parts of southwestern arizona large magnitude extension was not associated with areas of greatest igneous activity and rapid cooling and exhumation of core complexes postdated local magmatism these relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust except for young extension in this northwest trending belt there are no apparent regional migration trends for either magmatism or extension within southwestern arizona lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension the long duration and large magnitude of extension adjacent to the transition zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally

tropopause inversion layer seasonal and latitudinal variations and representation in standard radiosonde data and global models 1 previous publications have given information on the seasonal and latitudinal variations of the tropopause inversion layer til as seen in high resolution radiosonde data sets when soundings are averaged using the tropopause as a reference level this paper presents a more quantitative analysis of the latitudinal and seasonal structure of the til than has been given previously to do this we define the region over which the static stability relaxes from its overshoot value at the tropopause to its local minimum in the stratosphere this region is seen to increase monotonically in thickness from about 1 km at low latitudes to about 4 to 5 km at high latitudes when the seasons are defined as winter djf spring mam summer jja and fall son the transition from tropical behavior occurs a little poleward of 20 n in both djf and mam and moves to a little poleward of 30 n in jja and son somewhat surprisingly it is also shown that almost identical information about the til can be derived from standard radiosonde data for our period of analysis because of their reporting of the tropopause and other significant levels but caution needs to be used in doing this since the number of reported significant levels has varied significantly over the long term and with some distinct discontinuities finally we discuss what sort of information on the til can be obtained from global models given their relatively coarse vertical resolution

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

double tropopause formation in idealized baroclinic life cycles the key role of an initial tropopause inversion layer 1 recent studies have shown that double tropopauses exist in all seasons and at all longitudes in the midlatitudes as of yet the key mechanism responsible for their formation is not known in this study we explore the connection between double tropopauses and midlatitude baroclinic eddies this is investigated in the context of idealized life cycle experiments the key finding of this study is that large areas of double tropopauses form spontaneously at the nonlinear stage of the life cycle evolution provided an extratropical tropopause inversion layer is present in the balanced initial temperature profile we also show that the areas covered with double tropopauses grow as the strength of the initial tropopause inversion layer is increased without such a layer as in canonical examples of baroclinic life cycles much studied in the literature no double tropopause formation occurs in agreement with observations double tropopauses in our life cycle experiments form predominantly in areas of cyclonic flow at upper levels however the air masses that end up between the two tropopauses are found to originate from high latitudes this appears to differ from a recently published case study where the air between double tropopauses was shown to originate partly from low latitudes such a discrepancy suggests that more than one pathway may exist to advect air masses between the two tropopauses

a quasi geostrophic diagnosis of the zonal flow associated with cut off lows over south africa and surrounding oceans the zonal flow associated with cut off lows cols comprises two jet streaks of different spatial extents the smaller scale jet streak located north of the cols forms as a result of meridional divergence of vorticity advection and it is quasi stationary relative to the cols it dissipates as the cols do the same the larger scale jet streak gives rise to anticyclonic and equatorward rossby wave breaking rwb as it propagates southeasterly to the base of the ridge south of the col and then northeasterly beyond that point as the jet streak propagates it brings with it the anticyclonic barotropic shear that causes the rossby waves to break its propagation is caused by zonal momentum advection by the zonal flow from jet streak entrance to its exit as it propagates its northwesterly southeasterly orientation changes to one that is more zonal to become south westerly northeasterly at the end of the col life cycle this change in orientation is due to meridional advection of zonal momentum where the meridional flow advects momentum southward northward at the jet streak entrance exit the jet streaks form a split jet structure and the winds between the streaks is decelerated by vorticity advection convergence because the flow and col and rwb life cycle are coupled understanding the dynamics that underlie the changes in the col ambient flow contributes to resolving the outstanding rwb col causality problem

miocene calc alkaline magmatism calderas and crustal extension in the kofa and castle dome mountains southwestern arizona two widespread lower miocene rhyolite ash flow tuffs in the kofa and castle dome mountains of southwestern arizona are products of caldera forming eruptions these closely erupted tuffs the tuff of yaqui tanks and the tuff of ten ewe mountain are approximately 22 ma in age and their eruptions culminate a 1 to 2 m y long burst of calc alkaline volcanic activity centered on the northern castle dome mountains exotic blocks of proterozoic and mesozoic crystalline rocks up to 20 m across are present in exposures of the tuff of yaqui tanks exposed in the central castle dome mountains and the southern kofa mountains a single thick cooling unit of the tuff of ten ewe mountain that includes thick lenses of mesobreccia marks the location of the younger caldera that extends from palm canyon in the western kofa mountains eastward more than 7 km along strike to the central part of the range the tuffs show rapid thinning away from their inferred sources they were probably associated with high volume 100 kms eruptions large residual bouguer gravity anomalies one beneath each inferred caldera are interpreted as batholithic rocks or low density caldera fill caldera related volcanism in the kofa region occurred during a transition in extensional tectonic regimes from a regime of east west trending uplifts and basins to a regime manifest primarily by northwest striking normal faults a narrow corridor of folding and strike slip faulting formed during volcanism in the southern kofa mountains upper oligocene or lower miocene coarse sedimentary rocks along the southern flank of the chocolate mountains anticlinorium in the southern castie dome mountains mark the periphery of a basin similar to other early and middle tertiary basins exposed in southern california the volcanic section of the kofa region was dissected by high angle normal faults related to northeast southwest oriented crustal extension typical of the southern basin and range province

the key role of lower level meridional shear in baroclinic wave life cycles a series of idealized nonlinear life cycle experiments is performed to compare changes in life cycle behavior caused by upper level and near surface meridional shear of the initial zonal wind it is shown that both the eddy kinetic energy and the zonal flow accelerations produced during a cycle of growth and equilibration respond primarily to the meridional shear of the zonal wind near the surface and only weakly to shear near the tropopause near the critical shear for transition from anticyclonic to cyclonic life cycle behavior the zonal flow accelerations are minimized and the eddy persistence is maximized above this critical shear eddy breaking on the poleward side of the jet increases and strong cyclonic zonal wind shears are generated the influence of baroclinic shears is minimized by using dipolar wind anomalies that are zero near the center of a basic baroclinic jet and by taking advantage of the fact that the life cycle response is very sensitive to small changes in the magnitude of initial meridional shear the small baroclinic shears contribute to the differences in the sense that a cyclonic shear decreasing with height is slightly more effective in inducing cyclonic behavior than is a barotropic cyclonic shear with the same surface value upper tropospheric eddy momentum fluxes by linear normal modes are also much more sensitive to lower tropospheric meridional shear than to upper tropospheric meridional shear the primary response of normal modes to lower tropospheric meridional shear is to change the momentum flux at all levels

tropopause folds and synoptic scale baroclinic wave life cycles abstract the linear stability of three midlatitude zonal mean states of varying baroclinicity and barotropy is examined using a primitive equation stability analysis under the anelastic approximation each of these basic states includes both tropospheric and stratospheric regions with realistic properties compared with those of the earth s atmosphere in midlatitudes in all three cases the fastest growing synoptic scale modes delivered by the linear stability analysis have wavelengths of approximately 4000 km the nonlinear evolution of these synoptic scale modes is explored using a three dimensional anelastic finite difference model whose zonal scale is chosen to be equal to the wavelength of the fastest growing mode of linear theory during the nonlinear evolution of the wave deep tropopause folds are shown to form generically and the depth to which the fold penetrates the troposphere is seen to increase with the baroclinicity of the mean state these folds descending along the sloping frontal zone

climatological features of global multiple tropopause events 1 this study examines various climatological features related to multiple tropopause events mt events the analysis is based on the lapse rate definition of the tropopause and is performed on a radiosonde data subset taken from the integrated global radiosonde archive database the global statistics of mt events are analyzed taking into consideration both their seasonal and geographical variations our results are in moderate qualitative agreement with those of earlier studies they reinforce the analytical findings of other researchers but at the same time highlight important differences in both the number and position of the maximum occurrence of mt events we found a latitudinal band of multiple tropopause occurrence in the northern hemisphere and three centers in the southern hemisphere which coincided with identified zones of maximum cyclogenesis the climatological features of pressure temperature and vertical separation of mt events revealed the complexity of these phenomena which behave very differently according to latitude and season

vs and density structure beneath the colorado plateau constrained by gravity anomalies and joint inversions of receiver function and phase velocity data 1 the colorado plateau is a physiographic province in the western us with an average elevation of 1 9 km where in contrast to neighboring provinces there is little evidence of large scale tectonic deformation or magmatism recent availability of earthscope usarray seismic data allow us to better examine the crust and upper mantle structure beneath the region and test proposed explanations for the plateau s uplift and relative stability using phase velocities for fundamental mode rayleigh waves and p receiver functions we perform over 800 joint inversions for 1 d shear wave velocity vs profiles sampling the plateau and surrounding regions down to 150 km depth we image a sharp change in crustal thickness at the western edge of the colorado plateau with a more gradual increase eastward moving into the rocky mountains a relatively thick 100 km lithosphere beneath the plateau extends into the rocky mountains to the north we use empirical scaling relations to estimate densities from our vs results and predict the associated gravity anomalies which are inconsistent with the observed distribution of the bouguer gravity anomalies we somewhat reconcile the prediction and observations by assuming that lateral density variations below 50 km can be ignored and the lithospheric root is therefore neutrally buoyant while there is some evidence for small scale convection and lithospheric removal at its edges the shape of the lithospheric mantle anomaly is consistent with a large scale uplift of the plateau by heating since removal of the farallon slab we conclude that the lithospheric root is key to the long term stability of the colorado plateau leading to a colder stronger crust

transport and mixing in the extratropical tropopause region in a high vertical resolution gcm part i potential vorticity and heat budget analysis abstract a high vertical resolution general circulation model gcm output has been analyzed to clarify transport and mixing processes in the extratropical tropopause region the high resolution gcm with a vertical resolution of about 300 m above the extratropical upper troposphere allows simulation of fine atmospheric structures near the tropopause such as the extratropical tropopause transition layer extl and the tropopause inversion layer til the high resolution gcm realistically simulates fine thermal and dynamic structures in the extratropical tropopause region the thickness and maximum stability of the simulated til are consistent with observations the high resolution output was analyzed using a zonal mean potential vorticity pv equation to identify dominant transport processes in the extratropical tropopause region in the northern hemisphere extratropics during winter mean downward advection sharpens the pv gradient between the tropopause and 20 k below it whereas latitudinal variati

shortening in the upper plate of the buckskin rawhide extensional detachment fault southwestern u s and implications for stress conditions during extension commonwealth post graduate research award u s geological survey national cooperative geologic mapping program under statemap g11ac20455

the double tropopause and its dynamical relationship to the tropopause inversion layer in storm track regions using high resolution dynamic limb sounder observations and era interim reanalysis this study demonstrates that the warm conveyor belt wcb is a mechanism responsible for the relationship between the double tropopause dt and the tropopause inversion layer til a relationship recently suggested in the literature based on idealized model simulations of baroclinic disturbances using these data sets spatial and temporal characteristics of the dt til relationship are examined over a 3 year period 2005 2008 in the extratropics results from satellite data show that as the til increases in strength so does the frequency of the dt regardless of season or hemisphere the inverse relationship is found in the tropics using only dt profiles zonal composites of wind relative vorticity and temperature from reanalysis data show that as the til increases in strength the upper tropospheric circulation switches from cyclonic to anticyclonic and the upward vertical motion increases this result suggests the wcb as a mechanism since it is on the anticyclonic side of the jet and is characterized by the movement of tropical air poleward and upward from the surface to verify this relationship the vertical and horizontal development of a synoptic scale baroclinic system is analyzed over a 4 day period results show the equatorward extension of the polar tropopause and thus the formation of the dt due to the strengthening of the til in the region of vertical motion associated with the wcb moreover this result suggests that air movement within the dt could originate from high latitudes when associated with a baroclinic disturbance

tertiary structure and thermal history of the harquahala and buckskin mountains west central arizona implications for denudation by a major detachment fault system the harquahala and buckskin mountains lie in the footwall of the whipple buckskin bullard detachment system in the harquahala mountains mesozoic fabric and structure are progressively more intensely overprinted by penetrative tertiary deformation toward the northeastern pan of the range tertiary mylonitic deformation is recognized by the presence of deformed miocene mafic dikes and characteristic textural features lineations in the mylonite trend 040 060 and megascopic kinematic indicators mostly indicate top to the northeast sense of shear the crest of an antiform defined by tertiary mylonitic fabric does not coincide with the topographic crest of the range reorientation of mesozoic foliation in the vicinity of this antiform suggests that it originated as two shear zones as opposed to a single zone that was then bent the northeastern part of the brown s canyon granite acts as a large low strain lozenge in the southeast limb of the foliation arch mylonitic foliation along the southeast side of the range above this lozenge is strongly oblique to the trend of the detachment fault but the relative orientation is consistent with left oblique normal shear evidence of tertiary plastic deformation is absent southwest of sunset pass minor low angle normal faults in the southwestern harquahala mountains and little harquahala mountains dip northeast the low angle normal faults are cut by nw trending high angle right oblique faults in the central and southwestern harquahala mountains 40ar 39ar age spectra from k feldspar muscovite and hornblende and total gas ages from biotite indicate late cretaceous to early tertiary cooling to argon closure temperatures biotite and k feldspar from the area northeast of sunset pass record rapid early to middle miocene cooling reconstruction of the original geometry of the detachment system based on thermal differences indicated by contrasting cooling histories and the orientation of an early miocene dike swarm indicate that the initial dip of the detachment fault was most probably between 30 and 40 thus the harquahala mountains are a lilted block exposing of the order of a 10 km section through the pre tertiary crust heterogeneous proterozoic gneiss sparse paleozoic and mesozoic metasedimentary rocks and an oligocene plutonic complex are extensively overprinted by tertiary mylonitic fabrics in the buckskin mountains hornblende 40ar 39ar cooling ages suggest that most of these rocks were below hornblende closure temperature by early tertiary time except in the vicinity of the oligocene plutonic complex feldspar and biotite 40ar 39ar cooling ages suggest that the footwall of the whipple detachment system experienced a more uniform cooling history in the buckskin mountains than in the harquahala mountains cooling ages between about 13 and 20 ma are recorded throughout the range with no consistent spatial pattern of ages the eagle eye detachment fault southeasternmost extension of the whipple buckskin bullard fault system becomes a transfer fault along the southeastern side of the harquahala mountains this fault separates two distinct extensional terranes on the northwest approximately 50 km of crustal extension has been accommodated on the whipple fault system while on the southeast a similar total magnitude of crustal extension is accommodated by a series of smaller normal faults in the adjacent big horn lake pleasant extended terrane as the deformation becomes more distributed maximum slip on any one normal fault decreases to the point where no single fault has enough slip to denude crustal levels at which tertiary plastic deformation has occurred

evolution of lithospheric strength after thrusting modeling of an eroding thrust belt indicates that immediately after thrusting the lithosphere is about twice as strong as unthickened cratonic lithosphere but after 10 to 20 m y it becomes weaker than cratonic lithosphere reaching a minimum of 40 to 70 of cratonic strength 30 to 100 m y after thrusting this may explain why rifting of thrust belts commonly occurs several tens of million years after thrusting ceases relative weakening results from 1 elevation of the geotherm caused by burial of radioisotope enriched surface rocks and 2 replacement of strong mantle rocks by weak crustal rocks in the lithospheric column low denudation rates enhance weakening by allowing radioisotope enriched rocks to remain buried longer our modeling indicates that 1 magmatism after thrusting is favored by high concentrations of heat producing elements in the lower plate and by slow denudation rates and 2 factors that affect denudation rate such as climate and the development of external drainage can significantly influence evolution of a thrust belt

early miocene mylonitization and detachment faulting south mountains central arizona the south mountains of central arizona are one of the geologically simplest metamorphic core complexes of the north american cordillera an early miocene age of mylonitization is indicated by crosscutting relationships between mylonitic fabric and a composite pluton dated at 22 25 ma by rb sr u th pb and k ar techniques the kinematic agreement and close temporal association of mylonitization and detachment faulting support models in which the two processes are related to an evolving crustal shear zone that accommodated mid tertiary continental extension 19 references 3 figures 2 tables

a climatology of multiple tropopauses derived from gps radio occultations with champ and sac c 1 a global climatology of multiple tropopauses mt is discussed based on global positioning system gps radio occultation ro data from the german champ challenging minisatellite payload and the us argentinian sac c satelite de aplicaciones cientificas c satellite mission for the period may 2001 april 2005 in this study we present first detailed investigations about the geographical and temporal distribution of mt during different seasons the thickness of the layer between the lowest first and highest last tropopause has a strong annual cycle in the vicinity of the subtropical jet stj stream region values vary between 4 5 km during winter and 2 3 km during summer respectively whereas higher differences were found on the northern hemisphere it is shown that the occurrence distribution of mt is in good agreement with the mean climatological location of the stj 30 40 on both hemispheres in particular during winter time

static stability in the extratropical tropopause region abstract idealized axisymmetric anomalies of potential vorticity pv on a midlatitude f plane and their related response in terms of balanced wind and temperature are investigated with special focus on the static stability in the tropopause region the pv anomalies are specified such that they can be interpreted as the result of conservative advection in the tropopause region across the gradients of a prescribed background atmosphere with piecewise constant buoyancy frequency squared n2 related cyclones and anticyclones are treated identically except for the sign of the tropopause potential temperature anomaly composite profiles of n2 are computed for which the thermal tropopause is used as a common reference level and where the number of cyclones in the composite equals the number of anticyclones one obtains a pronounced peak of n2 just above the tropopause and slightly enhanced values below the tropopause in comparison with the background profile within the framework of pv inversion various mechan

origin of bimodal volcanism southern basin and range province west central arizona miocene volcanic rocks in the castaneda hills area west central arizona are interbedded with continental clastic sedimentary rocks minor limestone gravity glide blocks of precambrian and paleozoic rocks and monolithologic megabreccia the sedimentary and volcanic units dip to the southwest and are offset by northwest trending listric and high angle normal faults the listric faults coalesce at the rawhide detachment fault which overlies mylonitic gneiss the volcanic suite is strongly bimodal rocks with 55 to 71 wt sio 2 are rare on the basis of age geomorphic position and petrography five volcanic units can be distinguished older basalts 18 7 and 16 5 m y old quartz bearing basalts 13 7 and 12 4 m y old rhyolite lavas and tuffs 15 1 to 10 3 m y old mesa forming basalts 13 1 to 9 2 m y old and megacryst bearing basalts 8 6 to 6 8 m y old most of the basalts contain groundmass olivine and titanaugite phenocrysts and are alkali olivine basalts many rhyolites contain more than 75 wt sio 2 the initial whole rock sr isotopic composition of the basalts indicates that they are partial melts of an isotopically vertically heterogenous mantle the chemical composition of some of the megacrysts in megacryst bearing basalts with 87 sr 86 sr i equal to 7035 and 7038 supports a high pressure mantle origin the low 7034 sr ratio and lack of evidence for mixing with young rocks indicate that the quartz bearing basalts were also derived from the mantle other basalts with 87 sr 86 sr i 0 705 probably were derived from old lithospheric mantle with a high rb sr ratio and do not appear to be contaminated with old upper crustal material the rhyolites have initial sr isotopic ratios of 0 7093 and 0 7141 these ratios indicate that the rhyolites were not differentiated from the basalts partial melting of 1 3 b y old lower crustal material with rb sr 0 10 to 0 19 satisfactorily explains the isotopic ratios of the rhyolites granulite which may constitute the lower crust in this part of arizona has rb sr ratios similar to those required to produce the rhyolites k substituted for na during cooling and devitrification in some of the rhyolites partial melting of upper mantle peridotite and old lower crustal granulite from 19 to 7 m y ago in the castaneda hills area produced the bimodal volcanic suite the nearly contemporaneous production of basaltic and rhyolitic magma from the earth9s crust and mantle requires extremely heterogenous source regions asthenospheric upwelling associated with basin range extensional tectonism probably produced the heating event that caused partial melting and basaltic magma generation at different levels in the mantle partial melting in the lower crust to produce rhyolitic magmas probably was caused by the intrusion of basalt magma the basaltic and rhyolitic magmas formed in separate source regions rose independently and erupted at the same time and place

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

hematite and mn oxide u th he dates from the buckskin rawhide detachment system western arizona gaining insights into hematite u th he systematics

spatiotemporal development of irreversible mixing in midlatitude baroclinic wave life cycles morphology energetics and nonisentropic mixing activity the significance of nonisentropic irreversible mixing processes is diagnosed for idealized simulations of synoptic scale baroclinic wave life cycles along the subtropical jet stream using a nonhydrostatic anelastic mesoscale model subject to a free slip surface boundary condition a variety of morphological features of mixing is identified such as a mesoscale columnar vortex associated with the onset of frontal fracture episodic overturnings along the surface fronts wrinkling of the tropopause and injection of tropospheric air into the stratosphere the evolution of the degree of nonisentropic irreversible mixing is first analyzed by computing the change of the base component of potential energy that cannot be converted into kinetic energy the structure of the mixing activity is also diagnosed through inspection of spatiotemporal changes of entropy to demonstrate that the surface fronts are by far the most active regions of such activity this activity is found to be primarily longitudinal extending from the surface to the lower stratosphere to form a three dimensional spiral in the synoptic scale cyclone and along the fronts however an exceptional region also exists along the warm front where the structure becomes primarily transverse in the mature phase of frontal development in a model including an explicit representation of small scale turbulence in all simulations the net transfer of mass and heat across the tropopause is from troposphere to stratosphere the maximum transfer occurs when the observed climatological level of stratification contrast is assumed between stratosphere and troposphere the same climatological choice leads to a minimum net irreversible mixing which occurs primarily at earth s surface

inertia gravity waves spontaneously generated by jets and fronts part i different baroclinic life cycles abstract the spontaneous generation of inertia gravity waves in idealized life cycles of baroclinic instability is investigated using the weather research and forecasting model two substantially different life cycles of baroclinic instability are obtained by varying the initial zonal jet the wave generation depends strongly on the details of the baroclinic wave s development in the life cycle dominated by cyclonic behavior the most conspicuous gravity waves are excited by the upper level jet and are broadly consistent with previous simulations of o sullivan and dunkerton in the life cycle that is dominated by anticyclonic behavior the most conspicuous gravity waves even in the stratosphere are excited by the surface fronts although the fronts are no stronger than in the cyclonic life cycle the anticyclonic life cycle also reveals waves in the lower stratosphere above the upper level trough of the baroclinic wave these waves have not been previously identified in idealized simulations the sensiti

transport and mixing of chemical air masses in idealized baroclinic life cycles 1 the transport mixing and three dimensional evolution of chemically distinct air masses within growing baroclinic waves are studied in idealized high resolution life cycle experiments using suitably initialized passive tracers contrasting the two well known life cycle paradigms distinguished by predominantly anticyclonic lc1 or cyclonic lc2 flow at upper levels stratosphere troposphere exchange differs significantly between the two life cycles specifically transport from the stratosphere into the troposphere is significantly larger for lc2 typically by 50 due to the presence of large and deep cyclonic vortices that create a wider surf zone than for lc1 in contrast the transport of tropospheric air into the stratosphere is nearly identical between the two life cycles the mass of boundary layer air uplifted into the free troposphere is similar for both life cycles but much more is directly injected into the stratosphere in the case of lc1 fourfold approximately however the total mixing of boundary layer with stratospheric air is larger for lc2 owing to the presence of the deep cyclonic vortices that entrain and mix both boundary layer air from the surface and stratospheric air from the upper levels for lc1 boundary layer and stratospheric air are brought together by smaller cyclonic structures that develop on the poleward side of the jet in the lower part of the middleworld resulting in correspondingly weaker

conditions for flow in the continental crust 1 earthquake focal depth distributions and effective elastic thickness variations on the continents suggest that in old cold regions such as shields the lower continental crust can be stronger than the upper mantle and imply that the upper mantle can flow even when the lower crust does not however in other places substantial flow in the lower crust does occur particularly near extensional metamorphic core complexes we suggest that the lower crust only flows when its viscosity is significantly reduced either by heating from igneous intrusions or by the addition of water in both cases the important characteristics of the flow are a consequence of the limited time over which the viscosity reduction occurs the timescale is controlled either by conductive cooling of intrusions or by the separation of melt from its matrix which removes the water

subsurface evidence for the sudden integration of the salt river across the internally drained basin and range province arizona usa abstract the dynamics of river systems are most easily studied by observing their present geomorphology exposed at earth s surface however the bulk of these often long lived transportation and depositional systems is mostly buried at depth and hidden from view accessing these subsurface archives is often not possible the phoenix basin of arizona usa with its rich collection of water well drilling information offers a unique opportunity clast assemblages of sand and gravel deposits contained in subsurface drill cuttings enable estimates of sediment provenance and importantly reveal changes of provenance over time this study represents more than a decade of drill cuttings analysis and presents a method for evaluating the depositional history of how the phoenix basin developed a large external river system that crossed a long lived internally drained basin this configuration developed after basin and range faulting and extension had ceased allowing the basin to fill to capacity with sediment when placed in the context of a series of new isopach maps of the subsurface ancestral salt river deposits our clast assemblage analysis supports the conclusion that the salt and verde rivers appeared suddenly in the geologic record the ancestral salt river deposits and other nearby streams crossing the basin each possess a unique clast assemblage that can be used to fingerprint sediments with similar sources we use these observations to draw conclusions about the age and origin of these streams used in conjunction with other types of analyses such as cosmogenic burial dating and detrital zircon analyses determining clast assemblages of drill cuttings in the context of mapping these assemblages is a powerful new method for estimating the source of sediment and for understanding how the phoenix basin and basins in general fill with sediment this information is of value in modeling both basin development and groundwater resources in similar contexts in arizona and elsewhere

denudation of metamorphic core complexes and the reconstruction of the transition zone west central arizona constraints from apatite fission track thermochronology apatite fission track thermochronologic results from transects across the basin and range and transition zone provinces in west central arizona provide constraints on the denudational history and structural framework of the region apatite fission track ages decrease from 21 to 14 ma in the harcuvar mountains and from 16 to 13 ma in the buckskin rawhide mountains in the slip direction sw ne of detachment faults in the lower plates of the metamorphic core complexes mean lengths of confined fission tracks from the core complexes are all 14 m indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone 40 c m y for lower plate rocks apparent apatite ages in the transition zone province generally increase from 25 ma to 100 ma away from the basin and range province this trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the weaver mountains and the colorado plateau gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the phillips kirkland drill hole reveal parts of denuded mesozoic and cenozoic apatite partial annealing zones these paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks the reconstructions indicate that the major faults in the transition zone province have relative displacements of 1 km and that offset on them occurred mostly after 25 ma these data also indicate that most of the rocks now exposed in the transition zone of west central arizona were not exposed until miocene time

a numerical assessment of slab strength during high and low angle subduction and implications for laramide orogenesis a finite element computer program for one dimensional heat flow was modified to apply to two dimensional subduction where conductive heat transport parallel to the upper and lower plates was neglected one dimensional finite element columns were cut displaced and reassembled to model heat advection calculated temperature depth dependent pressure stress levels associated with dislocation creep in olivine at constant strain rate and a 1 gpa deviatoric stress maximum were used to calculate the form of the brittle plastic failure envelope for profiles across subducted slabs the failure envelopes were then used to calculate maximum sustainable compression parallel to the slab model results suggest the following 1 in high angle subduction without frictional heating and horizontal heat flow temperatures along the interplate contact zone remain below about 120 c which is far below temperatures necessary for olivine plasticity without substantial frictional heating or lateral heat flow from the adjacent magmatic arc the subducted slab will gain rather than lose strength before its upper surface contacts asthenosphere in addition viscous coupling due to olivine plasticity and associated ablation of the upper plate will be unimportant slab strength reduction due to heating occurs rapidly after the top of the slab comes into contact with asthenosphere 2 in low angle subduction the top of the slab remains in contact with overlying lithosphere and does not contact asthenosphere slab strength reduction due to conductive heating from above occurs slowly because the upper plate is cooled by the lower plate and is the source of progressively less heat temperatures along the interplate contact zone may be sufficient for olivine plasticity at large distances from the trench and viscous coupling between plates and associated ablation of the upper plate may be important in this tectonic setting 3 low angle subduction for distances of over 1500 km during the laramide orogeny in southwestern north america appears viable at 50 ma slab strength beneath the black hills of south dakota is predicted to have been comparable to or greater than strength at the trench

episodic caldera volcanism in the miocene southwestern nevada volcanic field revised stratigraphic framework 40ar 39ar geochronology and implications for magmatism and extension the middle miocene southwestern nevada volcanic field swnvf is a classic example of a silicic multicaldera volcanic field in the great basin more than six major calderas formed between 15 and 7 5 ma the central swnvf caldera cluster consists of the overlapping silent canyon caldera complex the claim canyon caldera and the timber mountain caldera complex active from 14 to 11 5 ma and centered on topographic timber mountain locations of calderas older than the claim canyon caldera source of the tiva canyon tuff are uncertain except where verified by drilling younger peralkaline calderas black mountain and stonewall mountain formed northwest of the central swnvf caldera cluster we summarize major revisions of the swnvf stratigraphy that provide for correlation of lava flows and small volume tuffs with the widespread outflow sheets of the swnvf new laser fusion 40 ar 39 ar isotopic ages are used to refine and revise the timing of eruptive activity in the swnvf the use of high sensitivity mass spectrometry allowed analysis of submilligram sized samples with analytical uncertainties of 0 3 1 permitting resolution of age differences as small as 0 07 ma these results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the swnvf major caldera episodes belted range crater flat paintbrush timber mountain and thirsty canyon groups erupted widespread ash flow sheets within 100 300 k y time spans and pre and post caldera lavas erupted within 100 300 k y of the associated ash flows peak volcanism in the swnvf occurred during eruption of the paintbrush and timber mountain groups when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1 35 m y separated by a 750 k y magmatic gap peralkaline and metaluminous magmatism in the swnvf overlapped in time and space the peralkaline tub spring and grouse canyon tuffs erupted early and the peralkaline thirsty canyon group tuffs and stonewall flat tuff erupted late in the history of the swnvf flanking the central volumetrically dominant peak of metaluminous volcanism magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data particularly in the overlapping grouse canyon and area 20 calderas of the silent canyon caldera complex volcanism in the swnvf coincided with the miocene peak of extensional deformation in adjoining parts of the great basin although regional extension was concurrent with volcanism it was at a minimum in the central area of the swnvf where synvolcanic faulting was dominated by intra caldera deformation significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the swnvf between the paint brush and timber mountain caldera episodes larger magnitude detachment faulting in the bullfrog hills southwest of the central swnvf caldera cluster followed the climactic timber mountain caldera episode postvolcanic normal faulting was substantial to the north east and south of the central swnvf caldera cluster but the central area of peak volcanic activity remained relatively unextended in postvolcanic time volcanism and extension in the swnvf area were broadly concurrent but swnvf area were broadly concurrent but in detail they were episodic in time and not coincident in space

k metasomatism and detachment related mineralization harcuvar mountains arizona the bullard detachment fault a regional low angle normal fault exposed in the harcuvar mountains of west central arizona separates lower plate mylonitic rocks and chloritic breccia from upper plate volcanic and sedimentary rocks areally extensive k metasomatism has converted upper plate mafic flows and felsic ash flow tuffs into rocks with 8 to 12 wt k 2 o 2 o and a simple k feldspar hematite quartz mineralogy the secondary k feldspar is very pure or 95 to 99 5 monoclinic and structurally similar to orthoclase differences in 18 o values between secondary k feldspar replacing sanidine phenocrysts 9 to 11 and k feldspar replacing groundmass 11 to 14 in the tuffs imply differential o exchange with migrating fluids whole rock 18 o values for tuffs 10 to 14 and mafic flows 6 to 9 do not therefore represent primary igneous values the rocks apparently became k metasomatized and 18 o enriched while interacting with low to moderate temperature neutral to alkaline oxidizing brines that accumulated in an extensional basin above the detachment fault cu au ag mineralization is concentrated in faults and fissures in metasomatized mafic flows above the detachment fault fluid inclusion studies show that mineralizing fluids had minimum temperatures of 290 to 330 c along the detachment fault 240 to 290 c in mafic flows above the detachment fault and 100 to 130 c in barite calcite mn oxide veins farther from the detachment fault the predominant mineralizing fluids near the detachment fault and in the mafic flows were saline brines with 13 to 17 equivalent wt naci more dilute brines with 6 to 12 equivalent wt naci formed the barite calcite mn oxide veins inferred 18 o values of the mineralizing fluids range from 3 for high temperature quartz sulfide veins to 5 for lower temperature barite calcite mn oxide veins the high salinities oxygen isotope compositions and geologic setting indicate that the mineralizing fluids were basinal brines the mineralizing fluids apparently evolved from early deep level reducing basin brines to a later stage marked by the influx of higher level oxidizing basin brines relatively minor amounts of less evolved lower 18 o meteoric water entered the system during the very late stages of mineralization paragenetic relations and geochemical and isotopic data indicate that mineralization was superimposed on previously k metasomatized rocks mineralization and k metasomatism may be indirectly linked however because both occurred during detachment faulting and both involved basinal brines specifically k metasomatism of upper plate units liberated elements such as cu pb zn mn sr and na that were incorporated into the mineralizing basin brines multiple fluid regimes during detachment faulting are indicated because basin brine dominated mineralization overprinted lower plate mylonitic rocks and breccia that had probably previously equilibrated with igneous or metamorphic fluids at deeper levels of the detachment system

relationship between mesozoic and cenozoic tectonic features in west central arizona and adjacent southeastern california the maria fold and thrust belt mftb is a narrow belt of mesozoic crustal shortening that trends east west across west central arizona and adjacent southeastern california it is characterized by generally south vergent folds and thrusts that commonly displace proterozoic crystalline rocks over deformed and metamorphosed paleozoic and mesozoic strata the mftb is cut by a south to southeast trending belt of mid tertiary extensional deformation extension was characterized by large displacements on low angle normal faults known as detachment faults and by isostatic uplift of mylonitic midcrustal rocks now exposed in metamorphic core complexes the geometry and style of extensional deformation change along the extensional belt and reveal the influence of the mftb several extensional features are spatially coincident with the root zone of mftb thrusts 1 an areally extensive west northwest trending belt of denuded tertiary mylonitic fabrics in the whipple and harcuvar metamorphic core complexes 2 a style of extension characterized by minimum extensional dismemberment of the upper plate and maximum denudation and uplift of deep seated lower plate rocks 3 an abrupt bend in the belt of arched uplifted rocks below detachment faults and 4 an abrupt bend in the trend of the breakaway zone of the detachment faults and an associated lateral ramp in the detachment fault system the first two features may be the result of isostatic uplift of a crustal root downward protruding moho bulge inherited from crustal thickening within the mftb and the other two features may reflect at the time of detachment fault initiation the influence of stresses in the crust associated with flexural support of the buoyant mesozoic crustal root

thermal evolution of monte blanco dome low angle normal faulting during gulf of california rifting and late eocene denudation of the eastern peninsular ranges footwall rocks of the canada david detachment fault northeastern baja california record late eocene earliest oligocene and late neogene cooling events previously unrecognized in the region biotite 40 ar 39 ar ages of 65 ma reflect slow cooling through 350 c 5 to 10 m y later than is typical in the region multiple diffusion domain modeling of k feldspar 40 ar 39 ar release spectra shows very slow cooling 1 c m y from 65 to 45 ma accelerated cooling from 315 c to 215 c between 45 and 33 ma records at least 3 4 km of denudation that we relate to east side up reactivation of late cretaceous structures that generally follow the oceanic continental suture previously established rivers flowing west from mainland mexico apparently became further entrenched during this uplift and continued to supply distinctive rhyolitic detritus to the coast ultimately surface uplift disrupted and rerouted the extraregional rivers some 2 to 6 m y before the cooling event ended footwall rocks remained nearly isothermal from 30 to 15 10 ma when renewed rapid cooling 33 17 c m y began in response to footwall exhumation by top to the west low angle normal faulting that accommodated rift related extension in what finally became the gulf of california apatite fission track and u th he ages of 5 ma and 4 ma respectively record final detachment related cooling through 110 c to 70 c thermal kinematic modeling suggests that 5 7 km of late neogene tectonic denudation and 10 12 km of horizontal extension were necessary in order to unroof the samples by 2 ma additional extension of unknown magnitude has probably occurred subsequently geodetically measured horizontal extension rates are considerably higher than the long term extension rate that can be attributed to detachment faulting

palinspastic restoration of navdat and implications for the origin of magmatism in southwestern north america 1 simultaneous palinspastic restoration of deformation and volcanism illuminates relationships between magmatism and tectonics in western north america using arcgis we retrodeformed the navdat north american volcanic database navdat geongrid org using the western north america reconstruction of mcquarrie and wernicke 2005 from these data sets we quantitatively compare rates of magmatism and deformation and evaluate the age composition and migration of cenozoic volcanism from 36 ma to present these relationships are shown in a series of palinspastic maps as well as animations that highlight migrating extension and volcanism with time western north america is grouped into eight different regions with distinct relationships between strain and volcanism to evaluate competing hypotheses regarding the relationship of extension to continental magmatism a first order observation from this study is that magmatism throughout the basin and range appears to be primarily driven by plate boundary effects notably subducting and foundering slabs as well as slab windows exceptions include the yellowstone hotspot system along the northern border of our study area and late stage 8 ma passive extension related asthenospheric upwelling along the eastern and western margins of the basin and range the palinspastic reconstructions presented here highlight that the classic high angle basin and range faulting that comprises most of the physiographic basin and range province commenced during a magmatic lull more broadly with the exception of the rio grande rift we find that pulses of magmatism lag the onset of extension these observations largely contradict the active rifting model where magmatism triggers basin and range extension

triple junction instability as cause for late cenozoic extension and fragmentation of the western united states regional extension in the western united states began soon after 30 m y b p the time of first interaction between the pacific and north american plates previous explanations for extension include lithospheric active processes and asthenospheric active processes the former are favored because of coincidence of timing with lithospheric interactions relative timing of volcanism and deformation and coincidence with pre existing structures although extension has occurred in part in a back arc setting it cannot be attributed to age of subducted crust or azimuth of subduction because both attributes favor back arc contraction characteristic behavior before 40 m y b p the mendocino triple junction has been unstable since its inception the continental margin was relatively straight before 30 m y b p and it has become more convex westward as the triple junction has migrated northward the continental margin and arc have been anchored to the subducting slab farallon juan de fuca whereas the triple junction must move parallel to the san andreas transform the combined result has been the northwestward and clockwise movement of coastal blocks relative to the continental interior and the eastward stepping of the san andreas transform relative to the coast these effects result from the unstable geometry of the mendocino triple junction lack of a subducted slab slab window beneath the extended lithosphere enables asthenospheric rise but does not cause extension

introduction to special section magmatism and extension the relationship between magmatism and the formation of continental rift zones is the subject of much controversy in particular the cause and effect relationships between magmatism and extension and the mode of generation of magma during the process of extension are still hotly debated this controversy served as the theme of a symposium on cenozoic magmatism in the colorado river extensional corridor and adjacent areas and a field trip held as part of the geological society of america cordilleran rocky mountain section meeting in reno nevada metcalf et al 1993 it was clear from data presented at the symposium that a considerable amount of new information has become available regarding magmatism and extension since the last special section on this topic published by the journal of geophysical research june 1989

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

latest cretaceous and early tertiary orogenesis in south central arizona thrust faulting regional metamorphism and granitic plutonism rocks in a number of mountain ranges in southernmost central arizona are juxtaposed by thrust faults regionally metamorphosed and intruded by garnet two mica granites field relations and k ar and u pb isotopic geochronology indicate that thrust faulting metamorphism and granitic plutonism were closely related aspects of a latest cretaceous and early tertiary orogenic episode the regionally metamorphosed rocks chiefly greenschist facies quartzofeldspathic schists were derived from jurassic and cretaceous sedimentary volcanic and plutonic rocks in five ranges these metamorphic rocks are overlain along synmetamorphic thrust faults by precambrian gneiss or late jurassic or late cretaceous plutonic rocks in three additional ranges upward increase of textural grade within the metamorphic rocks and analogy with the areas of exposed thrust faults indicate the likelihood of a concealed thrust fault flanking the range seven of these eight thrust faults have single lower plates but one thrust is underlain by a duplex consisting of several imbricate structural sheets separated by mylonitic tectonic slides areal structural relations and the regional arrangement of distinctive jurassic igneous rock units strongly suggest that one large range the baboquivari mountains and several smaller ranges are fensters in a thrust fault system of regional extent and that several adjacent ranges are allochthonous the regionally metamorphosed rocks are intruded by synmetamorphic to postmetamorphic early tertiary leucocratic granites characterized by various combinations of accessory biotite muscovite and garnet at least two of these granites contain inherited precambrian zircon indicative of generation by crustal anatexis k ar and or u pb isotopic ages of premetamorphic granites metamorphic rocks thrust zone mylonites synmetamorphic granites and metamorphogenic fissure veins indicate that the orogenic episode commenced in late cretaceous time and culminated in early tertiary time 58 to 60 m y ago the observed and inferred close spatial and temporal relations between thrust faulting regional metamorphism and granitic plutonism lead to the following hypothesis for orogenesis in south central arizona crustal compression caused overthrusting of crystalline rocks resulting in crustal thickening and crustal thickening and conductive and magmatic heat flux from the mantle together set up a thermal regime within which regional metamorphism and the generation and emplacement of the anatectic granites took place

geologic setting and tertiary structural evolution of southwestern arizona and southeastern california volcanic and sedimentary rocks and structures record the tertiary structural evolution of the lower colorado river region in southwestern arizona and southeastern california a late eocene or early oligocene prior to 33 ma episode of faulting is indicated by medium to coarse grained arkosic rocks in the chocolate and southern trigo mountains much of the area was relatively quiescent tectonically during extrusion of volcanic rocks from 33 to 22 ma but the southernmost part was periodically uplifted and eroded into its underlying crystalline rocks a major episode of extensional deformation and tilting occurred after deposition of welded tuff about 22 ma and affected the entire area from the palo verde mountains on the west to the kofa mountains on the east extension related faulting quickly waned and had largely ceased by about 20 ma in the kofa mountains elsewhere the timing is poorly constrained by 13 ma thick alluvial fans filled many grabens and half grabens among tilted fault blocks throughout the area volcanism in the lower colorado river region may have been coincident with a broad structural depression now oriented east west the northern limit of the volcanic terrane defines a tilt domain boundary the northern boundary reaching from the new water mountains in arizona to the little chuckwalla mountains in california ultimately evolved to separate a terrane of relatively untilted crystalline horsts on the north from a series of east or northeast dipping fault blocks on the south the southern margin is less well defined but is subparallel to the northern boundary and to the chocolate mountains anticlinorium

stratigraphic relations and u pb geochronology of the upper cretaceous upper mccoy mountains formation southwestern arizona a previously unrecognized angular unconformity divides the jurassic and cretaceous mccoy mountains formation into a lower and an upper unit in the dome rock mountains and livingston hills of western arizona the lower unit of the mccoy mountains formation consists of generally fine grained quartzose and volcaniclastic strata that were deposited after cessation of middle jurassic explosive volcanism the basal contact of the lower unit is disconformable in most places but locally it has been interpreted to be gradational with the underlying silicic volcanic rocks the upper unit is a fining upward sequence of quartzo feldspathic and arkosic conglomerate and sandstone that records uplift of a northern source terrane a tuff in the lower part of the upper unit has a u pb crystallization age of 79 2 ma late cretaceous rocks of the lower unit are deformed by pre 80 ma thrust faults of the generally southward vergent maria fold and thrust belt which bounds the outcrop belt of the mccoy mountains formation on the north the upper unit is exposed only south of the fold and thrust belt we interpret the intraformation unconformity in the mccoy mountains formation to have developed where rocks of the lower unit were deformed adjacent to the southern margin of the maria fold and thrust belt the upper unit of the formation is interpreted as a foreland basin deposit that was shed southward from the actively rising and deforming fold and thrust belt the apparent absence of an equivalent unconformity in the mccoy mountains formation in adjacent california is presumably a consequence of the observed westward divergence of the outcrop belt from the fold and thrust belt continued southward shortening deformed the entire formation under greenschist and locally amphibolite facies conditions soon after the upper unit was deposited tectonic burial beneath the north vergent mule mountains thrust system in the latest late cretaceous 70 ma marked the end of mesozoic contractile deformation in the area

stratigraphic effects of change from internal to external drainage in an extending basin southeastern nevada observations from southern white river valley southeastern nevada shed light on sediment dispersal and geomorphic development during and after extensional faulting that formed the valley fine grained lacustrine and alluvial sediments of the late neogene white river narrows formation were deposited in a subsiding half graben while coeval coarse grained deposits were confined to within 1 km of the active fault the syntectonic deposits are overlain along an erosional unconformity by post tectonic uniformly coarse grained alluvial plain and fluvial terrace deposits this sequence conforms to a recent proposal that fine grained intervals in extensional basins are deposited syntectonically whereas coarse grained intervals are deposited during post tectonic progradation of marginal clastic wedges the stratigraphy and geomorphology of the study area suggest that post tectonic progradation of coarse clastic sediment resulted from incorporation of the white river valley into the colorado river drainage system meadow valley wash also links previously closed young but presently quiescent extensional basins to the colorado river system by contrast most nearby basins that contain quaternary scarps remain internally drained maintenance of internal drainage by active subsidence therefore may have been important in ponding of coarse grained sediments next to the fault after tectonic subsidence ceased the basin drainage became southward this action resulted in post tectonic progradation of coarse grained clastic deposits as sediment dispersal systems responded to the new external base level regional comparisons however indicate that depending on local rates of tectonic subsidence and erosion a basin may become externally drained syntectonically which can cause syntectonic deposits to be coarse grained in addition a basin may remain closed long after tectonic activity ceases causing fine grained deposits to continue to accumulate in a quiescent basin we conclude that the interplay between internal and external drainage which depends not only on the rate of local fault activity but also regional tectonics climate and other factors is a significant factor in the stratigraphic evolution of continental extensional basins

investigation of double tropopause spatial and temporal global variability utilizing high resolution dynamics limb sounder temperature observations 1 this study examines the seasonal variation of the double tropopause dt using data from the high resolution dynamics limb sounder the combination of high data density and high vertical resolution available with this satellite instrument allows for the detailed analysis of fine scale structures such as the dt the spatial distribution of dt frequency is examined for all seasons from 2005 to 2007 global analysis of this distribution reveals that dts have a strong tendency to follow the zonal wind pattern and are present all year over the andes moreover during northern hemisphere winter spring there is a 15 decrease in the dt frequency over the eastern pacific and western atlantic two regions of wave breaking produced from a weakening of the zonal flow significant dt thickness values are also present over these regions but are found to extend further poleward than the corresponding frequency pattern a time series of dt frequencies is analyzed and shows an increase in daily frequencies during 2006 that highlights the interannual variability of this thermal structure for the first time dt duration is investigated in the extratropics utilizing a hovmoller diagram of dt frequency this representation highlights two preferred regions of formation the pacific and atlantic the slope of the feature gives it a speed of 18 m s in the northern hemisphere and a slightly faster speed for the southern hemisphere these speeds and the corresponding structures highlight a potential connection between upper tropospheric waves and dts

an analysis of frontogenesis in numerical simulations of baroclinic waves abstract using a primitive equation pe model we revisit two canonical flows that were previously studied using a semigeostrophic equation sg model in a previous paper the authors showed that the pe and the sg models can have significantly different versions of the large scale dynamics here they report on the implications of this difference for frontogenesis the program for the study of frontogenesis developed by b j hoskins and collaborators is followed to show how in the pe version of the canonical cases the surface warm front develops before the cold front and why the upper level front is a long nearly continuous feature going from ridge to trough the frontogenesis experienced by an air parcel is computed following the parcel to illustrate better the mechanisms involved as the present calculations are carried out longer than most previous ones the relation of the upper frontogenesis to the formation of the upper level cutoff cyclone is also examined trajectory and three dimensional g

implications of low temperature cooling history on a transect across the colorado plateau basin and range boundary west central arizona fission track ages of apatite and zircon from metamorphic plutonic and sedimentary rocks along a 80 km transect across the colorado plateau basin and range boundary in west central arizona show differences in the low temperature cooling histories between the provinces the transect extends from cypress mountain in the colorado plateau transition zone to the eastern buckskin mountains in the basin and range along the northeast margin of the basin and range province metamorphic rocks exposed in the footwall of a major detachment fault system yield zircon and apatite fission track ages of 16 10 ma these ages are similar to k ar fusion ages of biotite and age minima of k feldspar on 40ar 39ar age spectra and collectively indicate rapid cooling one k feldspar age spectrum has an age maximum of about 22 8 ma an age minimum of 11 8 ma and a spectrum whose shape is suggestive of reheating possibly in middle miocene time the heating event was probably related to hydrothermal activity during emplacement of cu and mn deposits in and above the detachment fault zone effects of this heating are only locally detected in rocks above the detachment fault in the poachie range fission track ages of apatite and zircon are 60 50 and 80 70 ma respectively the disparity between the apatite and zircon ages indicates that the rocks cooled slowly in late cretaceous and early tertiary time probably due to gradual uplift and erosion total uplift and denudation in the area of the poachie range since cretaceous time is 6 km or more to the northeast of the range fission track ages of apatite and zircon increase and diverge indicating that apparent uplift decreased in that direction the apatite ages from the poachie range are concordant with early tertiary hornblende ages determined in other studies of lower plate rocks near the southwest end of the transect the ages represent cooling of crystalline rocks after cretaceous regional metamorphism and magmatism near bagdad 20 km northeast of the poachie range 2 km or less of erosion has occurred since intrusion of high level plutons and dikes and caldera formation in late cretaceous time remnants of an erosion surface that developed in middle tertiary time are preserved in the transition zone volcanic and sedimentary rocks at least as old as early miocene were deposited on the erosion surface and filled valleys cut into it dissection of these deposits began about 8 ma we interpret these data combined with those from other studies to indicate that in cretaceous time southward thrusting and later extensive magmatism in the middle crust led to thickening and heating of the crust the cretaceous igneous rocks at bagdad are high level manifestations of this magmatism uplift and slow cooling occurred in late cretaceous and early tertiary time in late oligocene and early miocene time during northeast southwest extension middle crustal rocks moved southwest put from beneath the southwest margin of the transition zone tectonic denudation rapidly exposed the crust that had been brought up from a depth of 10 km or more and rapidly cooled in the eastern buckskin and harcuvar mountains middle miocene reheating occurred locally in the lower plate along the detachment and in nearby parts of the upper plate

fine scale structure of the extratropical tropopause region 1 a vertically high resolved climatology of the thermal and wind structure of the extratropical tropopause region is presented the climatology is based on data from 80 u s radiosonde stations covering the period 1998 2002 time averages for each radiosonde station are computed using the tropopause as a common reference level for all vertical profiles within the mean a strong inversion at the tropopause in the mean vertical temperature gradient is uncovered that is temperature strongly increases with altitude within the lowermost stratosphere this tropopause inversion layer exists on average throughout the investigated extratropics about 30 n to 70 n accordingly the static stability parameter shows considerably enhanced values within the lowermost extratropical stratosphere compared to typical extratropical stratospheric values further aloft conventional averages are not able to capture the tropopause inversion layer mean profiles of the horizontal wind show behavior qualitatively corresponding to thermal wind balance winter and summer exhibit distinctly different climate states in the extratropical tropopause region an approximated potential vorticity is considered and found to be close to well mixed within the troposphere as well as within the tropopause inversion layer this suggests the view of the tropopause inversion layer as representing a dynamically active atmospheric layer some potential implications are discussed

migration of tertiary volcanism in the southwestern united states and subduction of the mendocino fracture zone abstract a compilation of 417 isotopic dates on mid and late cenozoic igneous rocks from the southwestern united states shows that volcanism migrated northward with time the principal locus of volcanism at any given time was an east west band that corresponded closely with the calculated position of the subducted mendocino fracture zone mfz under the north american plate this correspondence supports the theory that volcanism was triggered by subduction of the mfz which was a major 1 km north facing topographic step in the farallon plate both volcanism and the mfz moved northward at about 3 1 cm year andesites and rhyolites show close correspondence to mfz passage but many basalts were erupted significantly later cooling dates on basement rocks in southern arizona cluster at the time the mfz passed under that area earlier models proposing rapid late tertiary steepening of the subducted farallon plate were based on a westward sweep of volcanism our compilation shows no evidence for such a westward sweep

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

tertiary sedimentary rocks associated with the harcuvar core complex in arizona u s a insights into paleogeographic evolution during displacement along a major detachment fault system abstract geologic mapping and analysis of oligocene miocene sedimentary rocks in the upper plate of the buckskin rawhide detachment fault system west central arizona reveals a complex paleogeographic history during fault displacement involving shifting sediment source areas and multiple drainage reversals within the study area four upper plate fault blocks are capped by homoclinal sedimentary sections that display fanning dip relationships indicating concurrent tilting and sedimentation during deposition of these strata upper plate sedimentation was intermittently confined to separate half graben while at other times rapid rates of aggradation relative to fault displacement resulted in burial of ridges separating sub basins four sedimentary assemblages recognized within the study area can be correlated between fault blocks on the basis of lithologic similarity stratal position provenance paleocurrent data and sparse geochronologic constraints the basal assemblage consists of lacustrine rocks and interfingering fluvial strata composed of detritus derived from the granitic terrane surrounding the northern part of the study area these sediments were deposited during the earliest stages of tilting of upper plate fault blocks and inferred displacement along the buckskin rawhide detachment fault system the lower assemblage which directly overlies the basal assemblage consists of fine grained lacustrine deposits sandy conglomerate and breccia during lower assemblage deposition mass flow dominated alluvial fans were derived from source areas consisting mainly of mesozoic and paleozoic rocks exposed to the south of the study area these source areas may have been associated with the breakaway fault of the detachment system or with fault blocks stranded above structurally elevated parts of the footwall tilting of fault blocks continued during deposition of the lower assemblage and strongly affected dispersal patterns and lithofacies distributions lacustrine sediments were deposited mainly within large deep sub basins and with few exceptions lower assemblage streams followed half graben axes the middle assemblage consists of conglomerate and sandstone deposited by an extensive south directed stream system that probably flowed off undistended parts of the hanging wall across extended parts and locally onto the footwall tilting of upper plate fault blocks continued during middle assemblage deposition but rapid burial of escarpments by sediment permitted relatively unobstructed southward drainage the upper assemblage consists of sandy conglomerate deposited by a northeast directed system of broad shallow streams these deposits display a variety of clast types including tertiary mylonitic sedimentary and volcanic rocks that were eroded from the upwarped footwall of the core complex and overlying klippen upper assemblage streams maintained relatively consistent northeasterly flow even during the early stages of deposition when some fault blocks were still rotating

high precision 40ar 39ar sanidine geochronology of ignimbrites in the mogollon datil volcanic field southwestern new mexico 40ar 39ar age spectra have been obtained from 85 sanidine separates from 36 ignimbrites and one rhyolitic lava in the latest eocene oligocene mogollon datil volcanic field of southwestern new mexico of the 97 measured age spectra 94 yield weighted mean plateau ages each giving single spectrum 1 precision of 0 25 0 4 0 07 0 14 ma replicate plateau age determinations for eight different samples show within sample 1 precisions averaging 0 25 plateau ages from multiple n 3 8 samples of individual ignimbrites show 1 within unit precision of 0 1 0 4 0 04 0 13 ma this within unit precision represents a several fold improvement over published k ar data for the same ignimbrites and is similar to the range of precisions reported from single crystal laser fusion studies a further indication of the high precision of unit mean 40ar 30ar ages is their close agreement with independently established stratigraphic order two samples failed to meet plateau criteria apparently due to geologic contamination by older feldspars effects of minor contamination are shown by six other samples which yielded slightly anomalous plateau ages 40ar 39ar plateau ages permit resolution of units differing in age by 0 5 0 15 ma or less this high resolution combined with paleomagnetic studies has helped to correlate ignimbrites among isolated ranges and has allowed development of an integrated timestratigraphic framework for the volcanic field mogollon datil ignimbrites range in age from 36 2 to 24 3 ma ignimbrite activity was strongly episodic being confined to four brief 2 6 m y eruptive episodes separated by 1 3 m y gaps ignimbrite activity generally tended to migrate from the southeast toward the north and west

tropospheric intrusions associated with the secondary tropopause 1 deep intrusions of tropospheric air into the lower stratosphere above the subtropical jet are investigated using new observations and meteorological analyses these intrusions are characterized by low ozone concentration and low static stability the low ozone layer is consistently observed from ozonesonde profiles and satellite remote sensing data from aura hirdls the intruding layer occurs along and under the poleward extending tropical tropopause which becomes the secondary tropopause in middle to high latitudes the association of the ozone and the thermal structure provides evidence for the physical significance of the subtropical tropopause break and the secondary tropopause the core of the intruding layer is typically between 370 and 400 k potential temperature 15 km but the vertical extent of the intrusion can impact ozone above 400 k the lower boundary of the overworld two intrusion events over the continental united states in the spring of 2007 are analyzed to show the spatial extent and the temporal evolution of the intruding air mass these examples demonstrate the effectiveness of potential temperature lapse rate i e static stability as a diagnostic for the intrusion event comparison with the potential vorticity field is made to show the complementarity of the two dynamical fields the static stability diagnostic provides a tool to map out the horizontal extent of the intruding layer and to investigate its evolution furthermore the diagnostic makes it possible to forecast the intrusion event for field studies

space time patterns and tectonic controls of tertiary extension and magmatism in the great basin of the western united states structural and stratigraphic relations in the great basin indicate widespread pre middle miocene crustal extension that appears to define two north trending belts most extension in these belts was oligocene age but locally it began earlier or lasted into early miocene time the eastern belt straddles the nevada utah border and includes the snake range nevada area with its southern end near 37 5 n and its western edge at the seaman butte mountains breakaway the southern boundary of the eastern belt is occupied by the 26 15 ma caliente caldera complex and approximately coincides with the present east west trending margin of the great basin north of saint george utah crust north of this boundary extended approximately east west before volcanism began at 30 32 ma but to the south extension began after about 15 ma this boundary may have been a rooted zone of left slip faults that allowed the footwall of the stampede detachment to move west relative to unextended terrain to the south the eastern margin of the eastern belt is probably located near the present eastern edge of the great basin but its northern end is poorly defined the western belt runs from the funeral and grapevine mountains california to the ruby mountains nevada and north northeast to the albion range idaho tens of kilometers of crustal extension occurred at least locally in both belts but magnitude of extension is poorly known for large areas of each tertiary volcanism in the great basin began in the north in eocene time with predominantly effusive volcanism and swept southward ending with voluminous oligocene miocene ignimbrite eruptions from calderas in an irregular discontinuous belt between marysvale utah and reno nevada a result of the south ward migration of volcanism is that the onset of extension in both belts was syn or post volcanic in the north but was pre volcanic in the south late paleogene extension and crustal magmatism coincided in both time and space only locally where south migrating magmatism overlapped active north south trending extensional belts most calderas in the southern great basin formed in previously extended belts or on their margins southward migration of ignimbrite sources was apparently blocked by unextended crust to the south in contrast volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to or during crustal extension this is consistent with observations in the southern basin and range where volcanism and crustal extension were generally synchronous and volcanism was dominantly effusive thus caldera formation may be controlled by the distribution of upper crustal extension although the physical mechanism for this control remains speculative the space time patterns of late paleogene extension in the great basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin but being driven by gravitational collapse of thick crust space time patterns of tertiary volcanism in the basin and range also appear to conform to patterns of thermal weakening or destruction of the subducted slab both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts because both predict close spatial and temporal association of extension and magmatism which is not generally observed

dynamical formation of an extra tropical tropopause inversion layer in a relatively simple general circulation model 1 the key factors contributing to the formation and maintenance of the recently discovered extra tropical tropopause inversion layer are presently unclear in this study it is shown that such a layer can form as a consequence of the turbulent dynamics of synoptic scale baroclinic eddies alone in the absence of explicitly parameterized small scale radiative convective processes a simple general circulation model initialized from a state of rest and driven with idealized forcings is found to spontaneously develop an inversion layer above the tropopause under a wide variety of parameter choices and model resolutions furthermore such a model is able to capture qualitatively both the latitudinal and in part the seasonal dependence of the observed tropopause inversion layer however the inability of our simple model to capture some detailed quantitative features strongly suggests that other physical processes beyond balanced synopticscale dynamics are likely to play an important role citation son s w and l m polvani 2007 dynamical formation of an extra tropical tropopause inversion layer in a relatively simple general circulation model geophys res lett 34 l17806 doi 10 1029 2007gl030564

generation of mesoscale gravity waves in upper tropospheric jet front systems abstract multiply nested mesoscale numerical simulations with horizontal resolution up to 3 3 km are performed to study the generation of mesoscale gravity waves during the life cycle of idealized baroclinic jet front systems long lived vertically propagating mesoscale gravity waves with horizontal wavelengths 100 200 km are simulated originating from the exit region of the upper tropospheric jet streak in a manner consistent with past observational studies the residual of the nonlinear balance equation is found to be a useful index in diagnosing flow imbalance and predicting wave generation the imbalance diagnosis and model simulations suggest that balance adjustment as a generalization of geostrophic adjustment is likely responsible for generating these mesoscale gravity waves it is hypothesized that through balance adjustment the continuous generation of flow imbalance from the developing baroclinic wave will lead to the continuous radiation of gravity waves

overview tectonic implications of cenozoic volcanism in coastal california cenozoic volcanic rocks of coastal california were erupted west of the magmatic arc trend related to subduction along the continental margin two assemblages representing discrete pulses of mid tertiary oligocene miocene and mid miocene volcanism occupied relatively compact tracts as restored palinspastically prior to disruption and dispersal by tectonic displacements within the neogene san andreas transform system a third assemblage records migratory post mid miocene volcanism at centers located progressively farther north the distribution and petrologic character of all three assemblages reflect slab window magmatism triggered by decompression melting of upwelling mantle paleogeographic reconstructions based on analysis of magnetic anomaly patterns offshore and restoration of on land features prior to san andreas transform slip and associated transrotational deformation indicate the paleotectonic positions of the cenozoic volcanic fields before structural disruption the pulses of mid tertiary and mid miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise trench encounters associated with subduction of the vancouver farallon and monterey arguello plates respectively along the continental margin off southern california migratory post mid miocene volcanism in central california accompanied the incremental expansion of a growing slab window as the mendocino triple junction migrated northward along the continental margin mid miocene columbia river basalt volcanism in the pacific northwest was coeval with the mid miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the san andreas transform as a coherent plate boundary columbia river volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the pacific plate

structural reinterpretation of the ajo mining district pima county arizona based on paleomagnetic and geochronologic studies the ajo mining district of southern arizona is divided into two main structural blocks by the gibson arroyo fault the eastern camelback mountain block contains the late cretaceous early tertiary porphyry copper deposit which has been previously thought to be associated with the displaced apex of a large intrusion exposed by deeper erosion in the western cardigan peak block however unpublished u pb data support a mid tertiary age for the western intrusion paleomagnetic and geologic evidence indicates that the ore deposit has been tilted to the south a total of approximately 120 degrees 68 degrees before and another 55 degrees after emplacement of the overlying locomotive fanglomerate and ajo volcanics the ajo volcanics have k ar ages 24 25 m y slightly older than those from the large intrusion of the cardigan peak block 20 23 m y paleomagnetic directions are consistent with the southward tilting of the volcanic rocks but they suggest that the remanent magnetization and perhaps the k ar dates of the western intrusion were reset by the emplacement of dikes younger than the locomotive fanglomerate and ajo volcanics these and other geologic relations indicate the following sequence of mid tertiary events in the district 1 emplacement of the western intrusion 2 movement along the gibson arroyo fault 3 unroofing and perhaps tilting of the pluton asymp 70 degrees to the south along with the camelback mountain block 4 syntectonic depositions of the locomotive fanglomerate and the ajo volcanics 5 continued uplift and tilting to the south totaling 40 degrees to 60 degrees 6 intrusion of the youngest dikes with attendant alteration and remagnetization of the host rocks and 7 minor oblique movement along the gibson arroyo fault

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

the mechanical feasibility of low angle normal faulting abstract low angle normal faulting is precluded by elementary rock mechanics arguments however the existence of many low angle normal faults in regions such as western north america has long suggested that this familiar theory is incomplete initiation of a low angle normal fault requires the stress tensor within the brittle upper crust to have inclined principal axes which necessitates a substantial shear stress within the vertical plane nonetheless no one has previously identified any specific extensional stress field which is suitably oriented to allow shear failure of previously unfractured rock at a low angle dip of 30 in preference to the steeper dip of 50 60 predicted by standard theory despite previous claims to the contrary this study shows for the first time that appropriately oriented stress fields can exist however they require the shear stress to locally reach 100 mpa near the base of a 10 km thick brittle layer this is only possible under extreme conditions as it requires dramatic lateral variations in the state of stress across the extending region it is suggested that a shear stress of this order can develop due to the combined effects of lower crustal flow which imparts a horizontal shear traction at the base of the brittle layer and loading which imparts shear traction in the vertical plane associated with the isostatic response to changes in heat flow caused by changes to the geometry of subducting slabs beneath an extending region regional patterns of low angle normal faulting in western north america are thus interpreted in terms of changes to the geometry of subduction of the farallon plate

late stage slip history of the buckskin rawhide detachment fault and temporal evolution of the lincoln ranch supradetachment basin new constraints from the middle miocene sandtrap conglomerate

association of double tropopause events with baroclinic waves 1 we herein propose a method for identifying breaks in the subtropical tropopause as found in the era interim reanalysis data the method uses the identification of double tropopauses and allows the quantification of the extension of the overlap between the tropical and extratropical tropopauses the correlations between the meridional extension of the superposition of tropopauses and the fields of geopotential potential vorticity or potential temperature reveal baroclinic wave patterns similar wave patterns were also identified in the potential temperature fields derived from gps radio occultation cosmic data the zonal propagation velocity of the anomalies in the meridional extension of the overlap was estimated using hovmoller diagrams the estimated zonal velocities suggest that the variability in the superposition of the tropopauses is associated with baroclinic rossby wave patterns in the subtropical upper troposphere and lower stratosphere

the impact of coriolis approximations on the environmental sensitivity of idealized extratropical cyclones the precise influence of climate change on extratropical cyclone genesis and evolution is an important but as yet unsolved problem given their physical and economic impact on a large portion of the planet s population however extratropical cyclones are also affected by the competing influences of forcing mechanisms at a wide range of spatial scales complicating the problem while the advent of idealized numerical modeling has allowed great strides in addressing these complications and achieving some qualitative consensus in the literature there is still some quantitative disagreement about response magnitude and where local maxima and minima in the response may be located thus the advantages inherent in the variety of idealized numerical modeling methods used to address this problem are also a drawback as it can be difficult to draw one to one comparisons across experiments although the effects of particular model architecture choices such as microphysical and cumulus schemes are well documented others are less understood in this study we examine the role of coriolis approximations by comparing a new set of etc sensitivity experiments using a linear plane approximation to an existing set of extratropical sensitivity experiments using a constant f plane approximation etcs within the new plane experiment are found to generally decrease in strength with temperature as measured by both minimum sea level pressure and maximum eddy kinetic energy eke a small increase in eke is observed at the warmest temperatures likely due to diabatic influences disrupting flow within the warm conveyor belt while seemingly contradictory to the previous f plane results the two experiments are instead found to be qualitatively similar upon further inspection with an offset of approximately 8 k this offset is primarily due to the coriolis approximations although the initial stability profile affected by the coriolis approximation has a marginal influence

double tropopauses and the tropical belt connected to enso a detailed analysis of double tropopause dt occurrences requires vertically well resolved accurate and globally distributed information on the troposphere stratosphere transition zone here we use radio occultation observations from 2001 to 2018 with such properties we establish a connection between el nino southern oscillation enso phases and the distribution of dts by analyzing the global and seasonal dt characteristics the seasonal distribution of dts reveals several hotspot locations such as near the subtropical jet stream and over high mountain ranges where dts occur particularly often in this study we detect a higher number of dts during the cold la nina state while warmer el nino events result in lower dt rates affecting the structure of the tropopause region close to the nino 3 region this relates to a much lower first lapse rate tropopause altitude during la nina and corresponds to an apparent narrowing of the tropical belt there

new age determinations of central colorado plateau laccoliths utah recognizing disturbed k ar systematics and re evaluating tectonomagmatic relationships published k ar ages of laccoliths of the henry and la sal mountains range from 23 to 56 ma with a bimodal distribution in the eocene and late oligocene epochs eocene ages require a period of magmatism during the laramide orogeny a time of little or no igneous activity in the western united states at these latitudes on the basis of 40 ar 39 ar and fission track ages we conclude that intrusive activity occurred during middle to late oligocene and early miocene time 31 20 ma in the henry la sal and abajo mountains these results imply a genetic link to magmatism in the reno marysvale and san juan volcanic fields which lie to the west and east of the colorado plateau respectively we interpret the small volume of the laccoliths relative to the surrounding volcanic fields to reflect differences in crustal composition and tectonic history of the plateau and adjacent areas eocene k ar ages result from excess 40 ar or contamination by xenocrysts excess argon is manifested by trapped argon with 40 ar 36 ar ratios greater than atmosphere 295 5 resulting in total gas or apparent k ar ages that are too old laser fusion experiments and microprobe data show that some samples with eocene or older total gas ages and uncorrelated inverse correlation plots are affected by radiogenic argon residing in incompletely outgassed hornblende xenocrysts all such samples contain middle to late oligocene magmatic hornblende crystals as revealed by laser fusion experiments xenocrysts may be derived from amphibolite xenoliths common in these rocks mixing calculations indicate that only 1 4 contamination may be required to increase k ar ages by 10 15 ma for xenocrysts outgassed as much as 75 the 40 ar 39 ar method is capable of identifying small degrees of magmatic contamination by older k bearing phases

jurassic to holocene tectonics magmatism and metallogeny of northwestern mexico the present metallic distribution in northwestern mexico is the culmination of superposed magmatism tectonism erosion and burial over more than 150 m y detailed palinspastic reconstructions of preextensional configurations the first study of its kind for this region clarify the interplay among these features on the present distribution and character of mineralized geologic systems this new synthesis goes beyond previous metallogenic investigations of northwestern mexico by separating events into specific timing and structural relationships and by restoring the geology to its preextensional configuration metallogenic factors such as enrichment preservation and erosion play major roles in the present distributions and for the first time are related to the overall metallogenic framework of northwestern mexico the analysis concludes that modern metallogenic patterns are the result of the complex superposition and subsequent redistribution of geologic systems in a way that is related directly to the regional history rather than simply metallic belts or interpreted angle of a subducting slab three main extensional events in the oligocene holocene have been restored and the palinspastic distributions have been analyzed reconstructions reveal the following 1 mineralization events igneous centers and sedimentary sequences are continuous across the gulf of california and other areas with large amounts of extension 2 middle tertiary gold silver mineralization in baja california may be the western part of the sierra madre oc present address bhp billiton avenida america vespucio sur 100 8th floor las condes santiago chile e mail john mark g staude bhpbilliton com cidental metallogenic province thus expanding the previously recognized extent of this province s mineralization 3 late cretaceous early tertiary porphyry copper deposits and intrusive centers form a narrower belt than previously noted and are traceable for over 400 km with parts of the belt buried beneath the younger sierra madre occidental volcanic fields 4 interpreted alignments of older geologic features including lineaments of ore deposits are displaced in the reconstructions 5 sedimentary rock hosted gold deposits and low angle detachment gold systems are closely related and occur around core complexes by using structurally restored time slices it becomes clear that older deposit types tend to be those formed at greater depths and more proximal to intrusions whereas younger deposits formed at shallower depths are less eroded and are more commonly volcanic rock hosted these characteristics express themselves in the regional distribution of deposit types second mineralization of widely differing ages is spatially superposed commonly associated with coeval magmatic and tectonic events the structural and magmatic events together with paleodistribution of ore deposits define a new framework to interpret the metallogenic history of northwestern mexico

cenozoic mineral deposits and subduction related magmatic arcs in mexico spatial distribution of isotopic ages of igneous rocks in northern mexico shows that a magmatic arc commenced at 140 m y b p near a paleotrench progressed 1 000 km eastward by 40 m y b p and then regressed to within 200 km of the continental margin by 18 m y b p during progression a calcic and calc alkaline continental margin volcano plutonic regime developed but farther east high k calc alkaline and alkaline facies formed in northeastern mexico and west texas following regression calc alkaline volcanism was renewed on the pacific margin at 18 m y b p and continued until cessation of subduction by initiation of transform movements between pacific and american plates benioff zone depth estimates from k 57 5 relationships suggest decreasing dip during progression and a variable dip variable depth regime from 140 to 16 m y b p both the dip angle convergence rate relationship and independent convergence rates require increasing convergence but decreasing sinking rates of subducted oceanic slab from 80 to 42 m y b p followed by decreasing convergence and sinking rates from 42 to 16 m y b p changes in subduction dynamics are attributed to plate reorganization at 44 m y b p and the approach of the east pacific rise differing hydrothermal ore assemblages in zones subparallel to the paleotrench are constrained by magma distribution in space and time cu mo w ores 106 to 40 m y old in calc alkaline magmas formed during progression whereas mo and caf 2 deposits coincide with the alkaline belt during arc stillstand above the deepest subduction depths and pb zn ag deposits 49 26 m y b p were formed during late arc progression and early regression these geodynamic variations plus sr rb and s isotopic characteristics of the arc terrane suggest that processes associated with subduction and some crustal contamination rather than local compositional anomalies in crust or mantle account for different magma and mineral deposit suites southern mexico exhibits fe and cu zones whereas the overlapping calc alkaline trans mexican chiapanecan arc of late miocene to holocene age contains several base and precious metal deposits

the extratropical tropopause inversion layer global observations with gps data and a radiative forcing mechanism global characteristics of the extratropical tropopause inversion layer identified in radiosonde observations by birner are studied using high vertical resolution temperature profiles from gps radio occultation measurements the gps data are organized according to the height of the thermal tropopause in each profile and a temperature inversion layer above the tropopause with an average magnitude of 3 5 k is found to be a ubiquitous climatological feature the gps data show that the inversion layer is present for all seasons in both hemispheres spanning the subtropics to the pole and there is not strong longitudinal structure dependence of the inversion layer on upper troposphere vorticity is studied while anticyclones exhibit a substantially stronger inversion than cyclones as expected from balanced dynamics the inversion is evident for all circulation types radiative transfer calculations indicate that strong gradients in both ozone and water vapor near the tropopause contribute to the inversion significant absorption of both longwave and shortwave radiation by ozone occurs warming the region above the tropopause water vapor near and immediately above the tropopause contributes to cooling effectively enhancing the inversion

timing and tectonic setting of tertiary low angle normal faulting and associated magmatism in the southwestern united states major episodes of tertiary low angle normal faulting in the southwestern united states south of latitude 37 n moved northward with time occurring about 25 million years ago in southern arizona 20 million years ago in the mojave desert region and 10 to 15 million years ago in the las vegas area in most areas faulting and associated volcanism occurred when the mendocino triple junction lay to the west this correspondence probably resulted from the unstable configuration of the triple junction east west to northeast southwest horizontal stretching as plates pulled apart at the triple junction prevented development of a hole in the lithosphere at the triple junction the regional strain history predicted by this model is consistent with observed tertiary low angle faults strike slip faults and folds

tectonics of mid tertiary extension along a transect through west central arizona large magnitude miocene extension in west central arizona occurred primarily along three imbricate northeast dipping normal faults the structurally highest of these faults the gently dipping buckskin rawhide detachment fault accommodated approximately 66 km of crustal extension whereas the two structurally lower faults accommodated a total of about 20 km extension due to this large magnitude extension an area at the earth s surface that was 10 to 20 km wide is now over a 100 km wide and crystalline rocks with mid tertiary mylonitic fabrics uncovered by detachment faulting are exposed over roughly 2000 km in the harcuvar metamorphic core complex most of the upper plate of the buckskin rawhide detachment fault was largely undeformed by internal extension only the thin tapered end of the upper plate was highly extended during extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded forming extension parallel folds in the lower plate low flexural strength characterized the lower plate during denudation and a highly mobile low viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere

timing rate and magnitude of slip on the buckskin rawhide detachment fault west central arizona we present thermochronologic and geochronologic data that constrain the slip history of the buckskin rawhide detachment fault in west central arizona one of the largest extensional fault systems in the north american cordillera u th he zircon and apatite thermochronology integrated with 40ar 39ar geochronology of postdetachment volcanic rocks indicate that large magnitude extension associated with the detachment fault initiated at 21 20 ma and continued until 12 11 ma in the southwestern portion of the buckskin rawhide metamorphic core complex u th he footwall cooling ages from the breakaway zone in the western bouse hills to upper greenschist facies mylonites in the southern buckskin mountains indicate that the slip rate on the detachment fault was 3 1 5 1 km myr during the early miocene space time patterns of hanging wall tilting suggest that at 17 16 ma a secondary detachment fault breakaway developed 12 km northeast of the primary detachment fault breakaway proximal conglomerates deposited in a supradetachment basin adjacent to the secondary breakaway scarp were displaced 6 11 km northeast in the middle miocene by the buckskin rawhide detachment fault at a slip rate of 1 2 2 7 km myr the total displacement across the detachment fault in the southwestern portion of the core complex is 24 10 km well short of the previous estimate of 66 8 km across the entire core complex based on these data and new observations we propose that total displacement on the buckskin rawhide detachment fault system increases in the slip direction to 40 50 km at the northeastern end of the exposed footwall corresponding to time averaged slip rates that ranged from 2 km myr to 6 km myr across the entire core complex

illumination of rheological mantle heterogeneity by the m7 2 2010 el mayor cucapah earthquake 1 major intracontinental strike slip faults tend to mark boundaries between lithospheric blocks of contrasting mechanical properties along much of their length both crustal and mantle heterogeneities can form such boundaries but the role of crustal versus mantle strength contrasts for localizing strain sufficiently to generate major faults remains unclear using the crustal velocity field observed through the global positioning system gps in the epicentral area of the m7 2 2010 el mayor cucapah earthquake baja california we find that transient deformation observed after the event is anomalously small in areas of relatively high seismic velocity in the shallow upper mantle 50 km depth this pattern is best explained with a laterally heterogeneous viscoelastic structure that mimics the seismic structure the mantle of the southern colorado river desert scrd and peninsular ranges pr which bound the fault system to its east and west respectively have anomalously high viscosity and seismic velocity we hypothesize that compared with the rest of the san andreas fault saf system to its north the strike slip fault system in northern baja california is narrow because of the presence of the pr and scrd high viscosity regions which bound it

insights into the geotectonic evolution of the southern rhodope as inferred from a combined ams microtextural and paleomagnetic study of the tertiary symvolon and vrondou plutons abstract the present study focuses on the tertiary granitoids of the rhodope massif northern greece namely the early miocene symvolon pluton and the composite miocene oligocene vrondou pluton in this paper we provide new anisotropy of magnetic susceptibility ams data from symvolon pluton discussed along with previously published ones from vrondou as well as new paleomagnetic data from samples widely distributed over both studied regions combined with macroscopic observations and microtextural examinations the magnetic fabric study pointed out anisotropic bodies with strong preferred orientations of the magnetic grains however the paleomagnetic vectors corrected for anisotropy effect remain unchanged two groups of magnetic lineations were identified a prevailing ne sw trending linear fabric visible mainly in the early miocene plutons and a second nw se trending fabric clearly imprinted in the mid oligocene eastern part of the vrondou pluton macro and microtextural study revealed the presence of magmatic textures mainly in eastern vrondou and sub solidus plastic deformation textures in symvolon and western vrondou paleomagnetic results displayed a complex pattern with prevailing clockwise rotations reinforcing the existing pattern for n greece and supporting the capacity of granitic bodies to reliably record the earth s magnetic field independently of their anisotropy