response of daily and annual shell growth patterns of the intertidal bivalve phacosoma japonicum to holocene coastal climate change in japan abstract it is well known that the latitudinal distribution of shallow marine mollusks around the japanese islands has changed episodically during the holocene in response to climate change however few studies have investigated the biotic response of individual species to holocene climate change at daily to annual scales of temporal resolution holocene shells of the venerid phacosoma japonicum recovered from various sites in japan preserve lunar daily and annual growth increments in the outer shell layer and hinge plate as also observed in modern shells sclerochronological analysis of 29 shells with known 14 c ages revealed that the life history traits e g length of the growth period and the mean lunar daily growth rate at a given age have changed markedly over the past 8000 years the intra annual patterns of lunar daily increments and shell oxygen isotope records of the fossil shells can be compared with those of modern specimens from various locations which are constrained by annual patterns of seawater temperature and summer autumn precipitation rates furthermore the temporal trends recorded in the fossil shells show a strong correlation with climate change in mid latitude parts of east asia over the past 8000 years indicating that p japonicum has changed its life history traits in response to holocene terrestrial and coastal climate change

cerro toledo rhyolite jemez volcanic field new mexico 40ar 39ar geochronology of eruptions between two caldera forming events the cerro toledo rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera forming ignimbrites the tshirege member and otowi member of the bandelier tuff in the jemez volcanic field new mexico to provide a chronologic framework for geochemical and isotopic studies on these rhyolites which record the evolution of the bandelier magma system during this interval a 40 ar 39 ar geochronology study was undertaken pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing bandelier tuff analyzed crystal populations range from being fairly homogeneous juvenile material to very heterogeneous mixed juvenile and xenocrystic assemblages in most cases dominant groups of juvenile sanidine crystals define 40 ar 39 ar ages which agree with stratigraphic constraints plagioclase analyses are distinctly more scattered and do not typically define reasonable ages the 40 ar 39 ar ages for the two members of the bandelier tuff yield an interval of 380 20 k y between these caldera forming eruptions during this interval nine major pyroclastic pumice units were deposited in the sections studied for which six yield isochron ages one a weighted mean age one a maximum age and one no reliable age due to lack of sanidine 40 ar 39 ar dates on pumice fall units within the cerro toledo rhyolite tephra indicate that eruptive activity occurred at 1 59 1 54 1 48 1 37 and 1 22 ma 40 ar 39 ar dating of cerro toledo rhyolite domes indicates these were erupted within the caldera at 1 54 1 45 1 38 1 34 and 1 27 ma the dates obtained indicate that eruptive activity occurred throughout the 380 k y interval between the two members of the bandelier tuff but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca 1 54 1 48 and 1 38 1 34 ma the interval from 1 34 to 1 38 ma was particularly active 7 of 18 units dated are these ages

volcanism and sedimentation along the western margin of the rio grande rift between caldera forming eruptions of the jemez mountains volcanic field north central new mexico usa abstract the cerro toledo formation ctf a series of intracaldera rhyolitic dome complexes and their associated extracaldera tephras and epiclastic sedimentary deposits records the dynamic interplay between volcanic tectonic and geomorphic processes that were occurring along the western margin of the rio grande rift between major caldera forming eruptions of the bandelier tuff 1 65 1 26 ma the alamo canyon and pueblo canyon members differ significantly despite deposition within a few kilometers of each other on the pajarito plateau these differences highlight spatial distinctions in vent sources eruptive styles and depositional environments along the eastern side of the jemez mountains volcanic field during this ca 400 000 year interval intercalated pyroclastic fall deposits and sandstones of the pueblo canyon member reflect deposition with a basin thick alamo canyon member deposits of block and ash flow tuff and pyroclastic fall deposits fill a paleovalley carved into coarse grained sedimentary units reflecting deposition along the mountain front chemistry and ages of glass from fall deposits together with clast lithologies of sedimentary units allow correlation of outcrops subsurface units and sources dates on pyroclastic fall deposits from alamo canyon record deep incision into the underlying otowi member in the southern part of the pajarito plateau within 100 k y of the toledo caldera forming eruption reconstruction of the ctf surface shows that this period of rapid incision was followed by aggradation where sediments largely filled pre existing paleocanyons complex sequences within the upper portion of the otowi member in outcrop and in the subsurface record changes in the style of eruptive activity during the waning stages of the toledo caldera forming eruption

a gis based volcanic hazard and risk assessment of eruptions sourced within valles caldera new mexico abstract the objective of this study is to evaluate the spatial extent of a possible future eruption using a gis based volcanic hazard tool designed to simulate pyroclastic fallout and density currents pdcs as well as lava flows and to assess the social and economic vulnerabilities of the area at risk simulated pyroclastic fallout deposits originating from the el cajete crater within the valles caldera jemez mountains volcanic field new mexico are calibrated to isopach and lithic isopleth maps of the lower and upper el cajete as constructed by wolff et al 2011 the change in the axial orientation of fallout deposits between the lower and upper el cajete is best matched using seasonal variations in wind speed and direction based on modern atmospheric records the calibration of pdcs is based on the distribution and run out of the battleship rock ignimbrite once calibrated hazards are simulated at a second vent location determined from probability distributions of structural features the resulting hazard simulation maps show the potential distribution of pyroclastic fallout pdcs and lava flows indicating areas to the s se of valles caldera to be at greatest risk to assess hazard preparedness social vulnerability is evaluated for all census designated places cdps within the study site based on methods by the hazards and vulnerability research institute twenty four variables were selected as proxies of social vulnerability and a principal component analysis was used to generate eight components which account for 64 of the total variance the eight component scores are summed into a final score for each cdp and the standard deviations from the mean of the scores are mapped with the cdps allowing for an easy visualization of areas that are considered more socially vulnerable economic vulnerability is evaluated through a multi criteria evaluation of population infrastructure road types and land use each variable is categorized and assigned a value representing relative vulnerability based on cost and importance the variables are assigned weights relative to one another through a pairwise comparison and summed together into a final map showing the distribution of economic vulnerability which is useful when used with the hazard simulation maps for targeting areas for mitigation to reduce economic loss in order to evaluate the overall risk the hazard simulation maps and vulnerability assessments are aggregated through weighted linear combination and pairwise comparison matrices creating a total of three risk maps although the actual maps provide greater detail overall based on the criteria chosen the risk maps show that pyroclastic fallout has the greatest impact effecting areas up to 50 km s se of the valles caldera including highly vulnerable cities such as los alamos and white rock the pdc and lava flow hazards however impact significantly smaller areas primarily disturbing forested land the methodology presented in this paper allows for a robust analysis of the risks posed by eruptions sourced from the valles caldera and is especially useful in focusing mitigation strategies to reduce the loss from such hazardous events

geochronology of the midas low sulfidation epithermal gold silver deposit elko county nevada the midas deposit is the largest known high grade au ag vein deposit located along the northern nevada rift it belongs to a suite of middle miocene low sulfidation epithermal systems associated with magmatism and faulting along the rift interpretation of new 40ar 39ar dates for volcanic rocks and hydrothermal minerals related to gold mineralization and additional isotopic dates throughout the midas region constrain the timing of volcanic tectonic and hydrothermal activity the midas hydrothermal system developed following a change from mafic dominated bimodal volcanism and basin formation to felsic volcanism folding and faulting at about 15 6 ma from 15 6 to 15 2 ma sediments and tuffs were deposited on a relatively impermeable rhyolite flow during this period faulting and tilting of the volcanic edifice created pathways for hydrothermal fluids that flowed to the surface forming sinters and hydrothermal breccias approximately 200 kyr after the change in volcano tectonic regime oblique slip faulting took place along zones of preexisting weakness creating dilational zones and additional channelways for mineralizing fluids at 15 4 ma high grade veins formed in fault zones throughout the region depositing at least 4 million ounces moz of gold and 40 moz of silver the timing of high grade au ag mineralization is identical to the age of rhyolite intrusions whose source magma chamber likely provided the heat necessary to drive the hydrothermal system the age of an unaltered tuff that unconformably overlies opalized sediments establishes that tilting of the units and the hydrothermal system had ceased by 15 2 ma the temporal and spatial coincidence of rhyolite volcanism faulting and high grade mineralization may reflect the importance of contributions from deeper fluid reservoirs containing magmatic components or highly exchanged meteoric waters

reconstruction of the most recent volcanic eruptions from the valles caldera new mexico abstract products of the latest eruptions from the valles caldera new mexico consist of the el cajete pyroclastic beds and battleship rock ignimbrite a sequence of pyroclastic fall and density current deposits erupted at 55 ka capped by the later banco bonito flow erupted at 40 ka and collectively named the east fork member of the valles rhyolite the stratigraphy of the east fork member has been the subject of conflicting interpretations in the past a long running investigation of short lived exposures over a period of many years enables us to present a more complete event stratigraphy for these eruptions than has hitherto been possible the volume of rhyolitic magma erupted during the 55 ka event may have been more than 10 km3 and for the 40 ka event can be estimated with rather more confidence at 4 km3 during the earlier event plinian eruptions dispersed fallout pumice over much of the valles caldera the southern jemez mountains and the rio grande rift we infer a fallout thickness of several decimeters at the site of the city of santa fe and significant ash fall in eastern new mexico in contrast pyroclastic density currents were channeled within the caldera moat and southwestward into the head of canon de san diego the principal drainage from the caldera simultaneous or rapidly alternating pyroclastic fallout and density current activity characterized the 55 ka event with density currents becoming more frequent as the eruption progressed through two distinct stages separated by a brief hiatus one early pyroclastic surge razed a forest in the southern caldera moat in a similar manner to the initial blast of the may 18 1980 eruption of mt st helens ignimbrite outflow from the caldera through the drainage notch may have been restricted in runout distance due to steep rugged topography in this vicinity promoting mixing between flows and air and the formation of phoenix clouds lavas erupted during both the 55 and 40 ka events were largely confined to the caldera moat any future rhyolitic eruptions of similar magnitude in the southern or western parts of the valles caldera will likely affect similar areas

stratigraphic placement and age of modern humans from kibish ethiopia in 1967 the kibish formation in southern ethiopia yielded hominid cranial remains identified as early anatomically modern humans assigned to homo sapiens however the provenance and age of the fossils have been much debated here we confirm that the omo i and omo ii hominid fossils are from similar stratigraphic levels in member i of the kibish formation despite the view that omo i is more modern in appearance than omo ii 40ar 39ar ages on feldspar crystals from pumice clasts within a tuff in member i below the hominid levels place an older limit of 198 14 kyr weighted mean age 196 2 kyr on the hominids a younger age limit of 104 7 kyr is provided by feldspars from pumice clasts in a member iii tuff geological evidence indicates rapid deposition of each member of the kibish formation isotopic ages on the kibish formation correspond to ages of mediterranean sapropels which reflect increased flow of the nile river and necessarily increased flow of the omo river thus the 40ar 39ar age measurements together with the sapropel correlations indicate that the hominid fossils have an age close to the older limit our preferred estimate of the age of the kibish hominids is 195 5 kyr making them the earliest well dated anatomically modern humans yet described

stream incision and terrace development in frijoles canyon bandelier national monument new mexico and the influence of lithology and climate abstract the record of stream incision and terrace development in frijoles canyon nm was investigated to examine the controls on channel evolution in this region frijoles canyon developed after deposition of the voluminous ignimbrites of the tshirege member of the bandelier tuff at ca 1 22 ma which provides excellent temporal constraints on canyon evolution base level fall along the rio grande the master drainage in the region contributed to initial incision of the stream development of a major knickpoint in basalt near the rio grande however has effectively isolated most of the stream channel from direct effects of this base level change since the early or middle pleistocene late quaternary evolution of the channel has included development of strath and fill terraces that record the effects of climatic variations and local lithologic variations within the tuff on channel processes a fill terrace underlain by up to 13 m of coarse gravelly alluvium records significant aggradation in the early to middle holocene following a major regional climate change aggradation probably resulted from large increases in sediment supply in the holocene relative to the late pleistocene which was perhaps related to variations in intense summer rains rapid incision in the middle to late holocene has stranded the underlying strath up to 21 m above the modern channel and local rates of incision of up to 4 m ka are more than 10 times the average rates since 1 22 ma holocene incision displays great spatial variability associated with local lithologic variations and results in a stream profile that has evolved rapidly in the last 10 000 years the high rates of incision occur where easily eroded nonwelded tuff is exposed in the canyon bottom whereas a more resistant vapor phase altered tuff unit upstream has impeded incision incision may also have been affected by migration of knickpoints within the tuff paired strath terraces record periods of local base level stability and lateral erosion that were terminated by episodes of downcutting the downcutting was probably controlled by rare high magnitude floods capable of stripping the pervasive gravel armor and exposing nonwelded tuff beneath the stream bed no tectonic influence on late quaternary terrace development or stream incision has been recognized despite frijoles canyon being crossed by the pajarito fault zone a major bounding structure of the rio grande rift instead climatic influences on sediment supply and flood characteristics mediated by lithologic variations in the canyon bottom have controlled late quaternary stream profile evolution

late thermal evolution of proterozoic rocks in the northeastern llano uplift central texas abstract the llano uplift of central texas usa contains large exposures of polydeformed grenvillian metaigneous and metasedimentary rocks that were intruded by late to post orogenic granites new u pb and 40 ar 39 ar single mineral ages and rb sr mineral isochrons document the retrograde cooling history that followed grenville age metamorphism in the north central and northeastern llano uplift and explain discrepancies between previous u pb crystallization ages and younger determinations based on rb sr and k ar isotopic systems dating has been carried out in the valley spring gneiss vsg and the post tectonic lone grove pluton lgp in three areas of the llano uplift from west to east vsg rocks in the babyhead area north of llano texas yielding hornblende muscovite and biotite 40 ar 39 ar ages and rb sr mineral isochron ages from 1098 to 1014 ma a sample from the lgp at petrick quarry yielding u pb titanite and hornblende and biotite 40 ar 39 ar ages from 1096 to 1076 ma and vsg samples from the inks lake area in the eastern llano uplift yielding u pb titanite hornblende and biotite 40 ar 39 ar and rb sr mineral isochron ages from 1087 to 1029 ma titanite from the oatman granite to the southwest of these areas yields a u pb age of 1119 6 ma these ages combined with closure temperature estimates document cooling in the llano uplift from 1100 1000 ma at rates between 5 and 14 c ma 1 although rapid uplift following early high pressure metamorphism has important regional thermal effects the cooling rates documented in this study are interpreted to represent the relaxation of local geotherms following local magmatic activity the low temperature history documented in this study is probably unrelated to uplift rates this supports the suggestion that magmatic heating was an important factor in the late high temperature low pressure metamorphism of the llano region it further suggests that the late pressure and temperature conditions recorded in the rocks were likely not synchronous across the region

gas saturation and evolution of volatile and light lithophile elements in the bandelier magma chamber between two caldera forming eruptions we have studied the distribution of light and volatile elements li be b h2o f cl in matrix glasses and melt inclusions in quartz and sanidine phenocrysts from tephras of the cerro toledo rhyolite which were erupted from the bandelier magma chamber between two caldera forming eruptions at 1 608 ma and 1 225 ma since the small volume tephras of the cerro toledo rhyolite record chemical evolution of the upper parts of the magma chamber during this 383 kyr period we can track volatile evolution in the magma during its differentiation specifically the development of a separate fluid phase from the magma the partitioning of volatile and light elements between this fluid and the magma and the timing of the second caldera eruption at 1 225 ma the melt inclusion data reveal that li be and b were progressively enriched with time at the top of the magma chamber showing a similar three fold enrichment by contrast f cl and f2o show more variable behavior f was enriched 3 6 times in the magma and cl increased 2 5 times while water concentrations remained constant evidence for a separate fluid phase includes 1 comparatively small magmatic enrichment factors for cl and h2o elements which are known to partition into fluid 2 decline of the cl be and cl b ratios in melt inclusions with time indicating preferential loss of cl and 3 maximum cl contents of 2900 3000 ppm in melt inclusions which are concentrations at or close to the saturation limit for cl in a two phase gas saturated magma the lack of temporal h2o enrichment in the magma suggests that it became gas saturated at an early stage the cl fluid magma partition coefficient was initially low due to the presence of a low salinity single phase vapor the fluid is calculated to have increased its salinity from 0 76 wt equivalent nacl to 3 1 wt nacl by crystallization and cl enrichment in the magma consequently the fluid was transformed to two immiscible phases as a result the cl partition coefficient increased and the cl content in the magma was buffered at 2900 3000 ppm overpressures in the magma chamber have been modeled as a function of crystallization total pressure and depth h2o solubility in rhyolite resurgence crystallization contraction and initial fluid mass for the large amounts of crystallization 40 70 required by the geochemical relations unrealistically large overpressures 75 mpa are indicated unless 1 the magma existed as a foam or 2 the chamber was able to degas passively and thus buffer the buildup of pressure to maintain realistic overpressures in the magma chamber 50 mpa an equilibrium was required between gas buildup by crystallization and volatile loss by passive degassing the change from a one phase vapor to a two phase immiscible fluid before the upper bandelier tuff eruption may have promoted partial or complete sealing of the magmatic hydrothermal system permitting significant overpressures to develop the result could have been a cataclysmic caldera forming eruption

the 40ar 39ar age constraints on the duration of resurgence at the valles caldera new mexico 1 the 40ar 39ar ages for rocks from the valles caldera new mexico imply that resurgent uplift of the caldera floor occurred within 27 27 ka of caldera collapse the structural resurgent dome of the valles caldera was uplifted approximately 1000 m above the surrounding caldera floor during resurgence the upper bandelier tuff which yields an age of 1 256 0 010 ma 2 ages relative to fish canyon tuff sanidine at 28 02 ma is the lower constraint on the timing of resurgence the oldest postcollapse ring fracture dome cerro del medio is the upper time constraint and yields an age of 1 229 0 017 ma for one of its oldest flow lobes therefore resurgent uplift probably occurred within 54 ka at a minimum rate of 1 9 cm yr sanidine phenocrysts from rhyolites of the postcollapse intracaldera deer canyon and redondo creek members yield 40ar 39ar single crystal laser fusion ages ranging from 1 229 0 013 ma to 1 283 0 017 ma deer canyon member n 8 and 1 208 0 017 ma to 1 239 0 017 ma redondo creek member n 4 with one exception from each unit these ages are statistically indistinguishable from the upper bandelier tuff indicating that eruption of these postcollapse rhyolites probably commenced shortly after caldera formation melt inclusion hosted excess argon was not found to have a measurable effect on the age of sanidine from the upper bandelier tuff or the deer canyon member

sediment accumulation on top of the andean orogenic wedge oligocene to late miocene basins of the eastern cordillera southern bolivia a large volume of tertiary synorogenic sediment accumulated on top of the eastern cordillera of southern bolivia as the andean orogenic wedge was shortened thickened and uplifted oligocene to upper miocene strata were deposited in five basins that were separated by active north trending fold thrust structures of the then frontal part of the orogenic wedge these coarse grained deposits recorded accumulation in the most proximal sector of the andean foreland basin system the wedge top depozone analyses of depositional systems sediment dispersal patterns and clast provenance of 0 6 2 3 km thick oligocene to upper miocene wedge top successions demonstrate that faultpropagation and fault bend folds commonly isolated individual basins while serving as primary sediment sources growth strata which formed by progressive fold limb rotation indicate thrust fault displacement and related folding concurrent with deposition alluvial fans defined most basin margins whereas braided streams or small lakes occupied basin axes diagnostic stratigraphic units confined to individual basins suggest that streams were rarely able to cut across growing folds to connect adjacent basins growth strata and crosscutting and onlapping relationships between contractional structures and wedge top strata delineate the chronology of fold thrust deformation in the eastern cordillera five new 40 ar 39 ar dates and previously published k ar dates ranging from 30 to 8 ma define an oligocene phase of west vergent backthrusting followed by primarily east vergent thrusting during miocene time timing of displacement on two east vergent thrusts is determined by 40 ar 39 ar ages of tuffs within adjacent growth strata sequences of the tupiza formation conglomerate 16 14 0 06 ma and oploca formation 13 33 0 15 ma 15 7 2 4 ma these ages combined with basin depositional histories demonstrate synchronous and out of sequence thrust displacement during miocene shortening upper crustal contractional deformation and wedge top deposition terminated in the eastern cordillera during late miocene time as the thrust front propagated eastward into the subandean zone continued thrust front migration produced the present day configuration in which eastern cordillera wedge top basins originally developed above the toe of the orogenic wedge at relatively low elevations are now 250 km west of the active thrust front and at 3 km elevation the modern wedge top depozone overlies the active frontal part of the orogenic wedge and consists of strata in thrustbounded subandean zone basins and sediment overlying blind structures beneath the westernmost chaco plain in general wedge top deposits become highly susceptible to erosional recycling as the orogenic wedge propagates forward and the wedge surface is uplifted nevertheless eastern cordillera wedge top deposits have been preserved for 10 30 m y in southern bolivia such long term preservation may reflect the inability of drainage systems to remove sediment mass from this low precipitation region of the central andes retention of sediment mass within the orogenic belt may promote critical thrust wedge conditions in which propagation of the thrust front is favored

paleomagnetic study of late miocene through pleistocene igneous rocks from the southwestern usa results from the historic collections of the u s geological survey menlo park laboratory 1 seventy sites from the southwestern united states provide paleomagnetic results that meet certain minimum criteria and can be considered for the time averaged field initiative tafi the virtual geomagnetic poles for these 70 units are circularly distributed and their mean is nearly coincident with the rotational axis when other published data for the southwestern united states are included n 146 the virtual geomagnetic poles are again circularly distributed but their mean is significantly displaced from the rotational axis whichever of these data sets is used the mean poles for normal and reversed polarity data differ by 170 and are not antipodal at greater than 95 confidence when the data are separated into specific age groups the 95 confidence limits about the mean poles for the brunhes matuyama combined gauss gilbert and late miocene intervals all include the rotational axis angular dispersion about these four mean poles increases systematically with increasing age and is consistent with paleosecular variation model g

dating soils and alluvium with cosmogenic 21ne depth profiles case studies from the pajarito plateau new mexico usa abstract 21 ne depth profiles measured from amalgamated samples of sand sized quartz from the quaternary bandelier tuff were used to estimate inheritance corrected exposure ages in stream terraces and an alluvial fan on the pajarito plateau stable alluvium and associated soils display a downward decreasing type i nuclide profile pattern bioturbated colluvium with uniform type iii nuclide concentrations has buried these alluvial surfaces another possible profile type with downward increasing nuclide concentrations type ii is described but was not found on the pajarito plateau we present mathematical models for each of the profile patterns relating nuclide concentration to inheritance time averaged bulk density and either exposure time or burial rate curve fits of type i profiles are used to estimate the age of alluvial landforms a burial correction technique for deposits buried by younger colluvium is also presented inheritance and burial corrected 21 ne surface exposure ages of 144 17 ka and 63 8 ka for two pajarito plateau stream terraces agree within error with independent soil profile development index dates of 160 ka and 68 78 ka a soil overlying the 50 60 ka el cajete pumice yields an inheritance corrected 21 ne exposure age of 44 11 ka that also agrees closely with a 14 c age on soil charcoal of 41 ka these results demonstrate that reliable cosmogenic 21 ne exposure ages can be obtained for fine grained alluvial surfaces with the depth profile technique

effusive eruptions from a large silicic magma chamber the bearhead rhyolite jemez volcanic field nm abstract large continental silicic magma systems commonly produce voluminous ignimbrites and associated caldera collapse events less conspicuous and relatively poorly documented are cases in which silicic magma chambers of similar size to those associated with caldera forming events produce dominantly effusive eruptions of small volume rhyolite domes and flows the bearhead rhyolite and associated peralta tuff member in the jemez volcanic field new mexico represent small volume eruptions from a large silicic magma system in which no caldera forming event occurred and thus may have implications for the genesis and eruption of large volumes of silicic magma and the long term evolution of continental silicic magma systems 40ar 39ar dating reveals that most units mapped as bearhead rhyolite and peralta tuff the main group were erupted during an 540 ka interval between 7 06 and 6 52 ma these rocks define a chemically coherent group of high silica rhyolites that can be related by simple fractional crystallization models preceding the main group minor amounts of unrelated trachydacite and low silica rhyolite were erupted at 11 9 and 8 ma respectively whereas subsequent to the main group minor amounts of unrelated rhyolites were erupted at 6 1 and 1 5 ma the chemical coherency apparent fractional crystallization derived geochemical trends large areal distribution of rhyolite domes 200 km2 and presence of a major hydrothermal system support the hypothesis that main group magmas were derived from a single large shallow magma chamber the 540 ka eruptive interval demands input of heat into the system by replenishment with silicic melts or basaltic underplating to maintain the bearhead rhyolite magma chamber although the volatile content of main group magmas was within the range of rhyolites from major caldera forming eruptions such as the bandelier and bishop tuffs eruptions were smaller volume and dominantly effusive bearhead rhyolite domes occur at the intersection of faults and are cut by faults suggesting that the magma chamber was structurally vented preventing volatiles from accumulating to levels high enough to trigger a caldera forming eruption

thermochronology of the grenville orogeny in west texas abstract the carrizo mountain group cmg comprises the westernmost exposures of the metamorphic core of the grenville orogen in texas unlike the correlative llano uplift 500 km to the east the cmg underwent one phase of medium p t metamorphism peak t was attained during d2 in all cmg exposures the peak metamorphic paleotemperatures decrease to the northwest in the overall direction of tectonic transport from staurolite to biotite grade garnet in the southeast carrizo mountains grew isothermally and probably during decompression i e part of a clockwise p t loop at ca 520 c in between biotite grade d1 and d3 events hornblende in this region cooled below closure t during d2 at 1057 6 ma within error of muscovite closure in the northwest carrizo mountains at 1059 2 ma hornblende in higher grade exposures to the southeast cooled through closure t as late as 1002 7 ma muscovite in the garnet grade and higher exposures closed to ar diffusion at 992 4 ma to 981 3 ma biotite and partially recrystallized hornblende from the northwest carrizo mountains cooled through closure in this same time period we interpret these data as signifying 1 ar loss due to peak metamorphism in hornblende southeast carrizo mountains and muscovite northwest carrizo mountains at ca 1059 ma 2 post tmax cooling through 500 c in the higher grade outcrops until ca 1000 ma and 3 cooling of all cmg exposures through 300 c due to uplift along the streeruwitz thrust and erosion from 1000 to 980 ma these ages significantly post date known tectonism in the llano uplift and extend the duration of grenville orogenesis in texas by ca 115 million years the 1000 980 ma time period is a time of thrusting in frontal regions of the grenville province of canada and the northeastern us and shearing in the natal province south africa suggesting ultimate closure of the grenville orogen and supporting recent rodinia reconstructions

sedimentologic and geomorphic evidence for seesaw subsidence of the santo domingo accommodation zone basin rio grande rift new mexico basin analysis of facies patterns and basin geomorphology reveals a complex subsidence pattern for part of the rio grande rift in north central new mexico united states the santo domingo basin study site is located in the accommodation zone between the east tilted northern albuquerque basin and the west tilted espanola basin half graben upper miocene through lower pleistocene basin fill sediment records a longitudinal south flowing ancestral rio grande and flanking piedmont stream systems chronology of deposition is provided by 40ar 39ar geochronological study of lava flows and pyroclastic deposits interbedded with the sediment down to the east faults concentrated in the western basin illustrate increasing displacement toward the north and merge with structures forming the western margin of the espanola basin down to the west faults in the central and eastern basin exhibit southward increasing displacement and become the principal structures along the eastern margin of the northern albuquerque basin the latitudinal overlap of these fault systems in the santo domingo basin gives the appearance of a symmetrical graben with anticlinal bedding attitudes analysis of the distribution of sedimentary facies high level geomorphic surfaces and pleistocene terraces indicates however that the basin subsided in a seesaw fashion rather than as a symmetrical graben during the late miocene and early pliocene the course of the ancestral rio grande was deflected to the west suggesting strong westward tilting of the basin in late pliocene and pleistocene time however eastward tilting brought the river into the eastern basin at that time an east sloping pediment was eroded across west dipping strata in the western basin pediment gravel and tuff present as unconformity bounded strata in the western basin correlate with continuously aggraded conformable basin fill strata to the east a return to westward tilting since middle pleistocene time is suggested by unpaired terraces consistent with net westward migration of the rio grande lack of post middle pleistocene motion on faults in the eastern basin and numerous fault displacements of the pediment gravel in the western basin

argon isotopic analysis of breccia veins from the roter kamm crater namibia and implications for their thermal history the rocks exposed in the rim of the 2 5 km wide and 3 7 ma old roter kamm crater in southwest namibia are cut by breccia veins that macroscopically resemble and were originally described as pseudotachylytes the veins were later shown to be cataclasites with no evidence for melting 40ar 39ar data for vein and host rock samples indicate a low grade metamorphic event at around 300 ma but provide no evidence for an impact age the samples have suffered 5 7 ar loss which we associate with the impact event all the samples record similar ranges of possible timetemperature conditions and there are no resolvable differences between the results for the vein and the host rock samples as would be expected if frictional heating played an important role in breccia formation modeling the 40ar 39ar data assuming instantaneous impact heating followed by extended cooling and coupling these results to published data on fluid inclusions quartz precipitation shock effects and crater degradation suggest that the veins reached maximum temperatures of 230 290 c during impact and never approached melting temperatures of the precursor rocks

age and stratigraphic relationships of pre and syn rift volcanic deposits in the northern puertecitos volcanic province baja california mexico geologic mapping of volcanic strata of the northern puertecitos volcanic province pvp in northeastern baja california mexico performed in conjunction with 40 ar 39 ar analysis and petrochemical study documents the miocene geologic history of a well preserved volcanic succession within the northern sierra santa isabel and its relationship to the evolving pacific north america plate boundary subduction related volcanic deposits well exposed in profile along the northern margin of the pvp in the informally named santa isabel wash region span pre 17 to 15 ma minor rift related volcanism occurred at 12 5 and 9 ma prior to voluminous pvp forming volcanism at 6 6 5 ma isochron ages typically exhibit precision 1 for plagioclase of 2 7 and for anorthoclase and sanidine of 2 5 and replicate analysis of an internal anorthoclase standard indicate 1 2 reproducibility within a given irradiation and 2 5 for samples irradiated separately improved local correlations made possible by the rich stratigraphic section preserved in santa isabel wash help constrain the relationships of several widespread pyroclastic flow deposits in northeastern baja california these correlations are important for both paleomagnetic studies within the region and for establishing geologic ties across the gulf of california the combined mapping and age results imply that most extensional deformation in the study area is post 6 ma although some earlier faulting and the development of the pre 6 ma matomi accommodation zone are also documented results support a transitional plate boundary model which implies that much of the pacific north america relative plate motion north of delfin basin i e the northernmost gulf of california is accommodated on n to nnw striking faults developed during late miocene ene directed extension the model predicts a zone of divergence east of the pvp which provides a structural mechanism for the positions and jumps of nearby gulf of california spreading centers upper and lower tiburon and delfin basins since 6 ma and relates major pulses of pvp volcanism at 6 and 3 ma to these offshore spreading center adjustments results also imply that most extensional deformation in santa isabel wash is the result of incorporation of the pvp into the gulf extensional province 2 3 ma due to northwestward propagation of the guaymas fracture zone rotational deformation north of the pvp may have begun contemporaneously with this adjustment along the gulf extensional province rift margin

interpretation of u th he single grain ages from slowly cooled crustal terranes a case study from the transantarctic mountains of southern victoria land low temperature thermochronologic techniques e g apatite fission track aft thermochronology and u th he dating constrain near surface t t paths and are often applied to uplift denudation and landscape evolution studies samples collected in vertical profiles from granitic walls on either side of the ferrar glacier southern victoria land antarctica were analyzed using aft thermochronology and apatite u th he dating to further constrain the lowest temperature thermal history of this portion of the transantarctic mountains aft central ages vary systematically with elevation and together with track length information define a multi stage cooling denudation history in the cretaceous and early tertiary apatite u th he single grain age variation with elevation is not as systematic with considerable intra sample age variation although many complicating factors e g u and th rich micro inclusions fluid inclusions variation in crystal size particle ejection correction zonation and particle ejection correction implantation of he into a crystal or impediment of he diffusion out of a crystal and 147sm derived particles may contribute to age dispersion we found that variation in single grain ages correlated with cooling rate samples that cooled relatively quickly have less variation in single grain ages whereas samples that cooled relatively slowly 3 c m y or resided within an u th he partial retention zone heprz prior to more rapid cooling have a comparatively greater variation in ages decay of u and th via particle emission creates a 4he concentration profile dependent upon the initial parent u th within a crystal variation of single grain ages for samples with non homogeneous u th distributions will be enhanced with long residence time in the partial retention zone i e slow cooling because of the relative importance of loss via volume diffusion and loss via particle ejection with respect to the u th zonation and the grain boundary correction of ages for particle ejection ft correction factor typically assumes uniform u and th distribution within the crystal and when applied to a population of crystals with different u and th distributions will enhance the variation in ages most complicating factors listed above for apatite u th he ages result in ages that are too old we propose that if considerable variation in u th he single grain ages exists that a weighted mean age is determined once outlier single crystal ages are excluded using the criterion of chauvenet or a similar approach we suggest that the true age or most representative age for that age population lies between the minimum u th he age and the weighted mean age we apply this approach coupled with composite age profiles to better constrain the t t history of the profiles along the ferrar glacier significant intra sample variation in single crystal apatite u th he ages and other minerals dated by the u th he method should be expected especially when the cooling rate is slow the variation of u th he single crystal ages is therefore another parameter that can be used to constrain low temperature thermal histories