tertiary extensional faulting and evolving ductile brittle transition zones in the northern snake range and vicinity new insights from seismic data new geologic mapping and high resolution seismic data from east central nevada shed light on the evolution of the northern snake range decollement nsrd and its relation to the present basins and ranges we suggest that the nsrd originated in oligocene time as a flat 20 km wide 7 km deep zone of decoupling between localized ductile stretching below and more widely distributed high angle normal faulting above younger east dipping normal faults cut the western part of the nsrd forming the tilted fault blocks and highly asymmetric grabens that constitute the duck creek range duck creek valley and schell creek range spring valley and folded the nsrd to its present antiformal geometry these youngest faults flatten into pervasive subhorizontal reflectors at a depth of 10 to 15 km that we interpret as the youngest ductile brittle transition

mesozoic tectonics and metamorphism in the pequop mountains and wood hills region northeast nevada implications for the architecture and evolution of the sevier orogen discussion and reply the pequop mountains wood hills east humboldt range region northeast nevada exposes a nearly continuous cross section of precambrian to mesozoic strata representing middle to upper crustal levels of the mesozoic hinterland of the sevier orogen these rocks preserve the transition from unmetamorphosed mesozoic upper crust to partially melted middle crust integration of new structural metamorphic and u pb thermochronologic data from the wood hills and pequop mountains coupled with a regional tectonic reconstruction reveals substantial cretaceous metamorphism contraction and extension in the sevier hinterland in northeast nevada we report two phases of contraction not previously recognized that are accommodated by top to the southeast thrust faults the windermere and independence thrusts contraction was succeeded by two phases of extension along west rooted normal faults the late cretaceous pequop fault and tertiary mary9s river fault system the earliest phase of thrust faulting resulted in as much as 30 km of crustal thickening and an estimated minimum of 69 km of shortening along an inferred fault called the windermere thrust the timing of this thrusting event is bracketed between late jurassic ca 153 ma and late cretaceous 84 ma relaxation of crustal isotherms following and perhaps during thrusting resulted in barrovian style metamorphism of footwall rocks and partial melting of metapelite at deep levels peak metamorphism was attained ca 84 ma and by this time hinterland crustal thickening had reached a maximum during 84 75 ma another minor pulse of shortening and thickening along the independence thrust was followed by partial exhumation of the metamorphic rocks and as much as 10 km of crustal thinning along the pequop fault thus the interval from 84 to 75 ma in northeast nevada marks a fundamental and apparently permanent change from horizontal contraction to extension in the upper to middle crust in the hinterland final exhumation of the metamorphic rocks was accomplished by the tertiary mary9s river fault system our data indicate that much of the metamorphism and some of the contraction in the sevier hinterland in northeast nevada which was previously thought to be largely late jurassic is actually cretaceous in age furthermore the data indicate that widespread metamorphism of the middle crust is a byproduct of tectonic burial and that hinterland and foreland thrust faulting were coeval suggesting that thrust faults in the sevier orogen do not form a simple foreland younging sequence

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

incremental 40ar 39ar thermochronology of mylonitic rocks from the northern snake range nevada a 40ar 39ar thermochronologic study of muscovite biotite and potassium feldspar from footwall rocks of the northern snake range decollement nevada was undertaken in order to constrain the timing and temperatures of mylonitic deformation during extension and the subsequent cooling uplift history of these mylonitic rocks mylonitic deformation has been bracketed between 37 ma and about 24 ma and minimum temperatures during ductile thinning were about 300 c mineral cooling ages along the east flank of the range suggest a minimum tmax gradient of about 55 c ma this steep thermal gradient is the result of a localized heat source and or shearing of colder rocks with older cooling histories over hotter rocks with younger cooling histories although mylonitic deformation may have spanned a 13 m y period 37 ma to 24 ma deformation was not synchronous throughout the range on the west flank of the range age spectra from muscovite and microcline samples indicate that rocks were at temperatures well below 325 c and deformation had ceased by 30 ma while on the east flank of the range lower plate rocks were at temperatures above 325 c and ductile deformation was still ongoing this is also reflected in chrontours of published k ar muscovite ages along with 40ar 39ar ages which monotonically decrease in age from west to east these data provide evidence for a temperature difference across the range within the same structural horizon from about 30 to 24 ma these relations suggest a lateral thermal gradient the east flank of the range cooling more slowly than the west flank alternatively the data suggest either an eastward dip of lower plate units prior to the onset of tertiary mylonitic deformation or that layering dipped eastward due to deformation in an east dipping top to the east shear zone both cases result in differential cooling of lower plate units as they pass through a series of subhorizontal isotherms the 40ar 39ar data along with published finite strain structural and quartz petrofabric data provide constraints on the formation and geometric evolution of metamorphic core complex detachment faults

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

rapid eocene extension in the robinson district white pine county nevada constraints from 40ar 39ar dating precise 40ar 39ar ages on pretectonic syntectonic and posttectonic rhyolites from the robinson mining district which is within a highly extended domain in east central nevada indicate that extreme extension 400 occurred in fewer than 900 k y during the eocene the duration of extension is tightly delimited by an eocene lacustrine unit that was horizontal prior to extension and by younger rhyolitic units the rhyolites represent at least five eruptive episodes including the preextensional and synextensional rhyolite of white hill emplaced between 37 6 and 37 4 ma and the postextensional rhyolite of garnet hill emplaced at 36 7 ma although some of it is perhaps as old as 37 1 ma thus the duration of extreme extension at robinson was between 0 9 and 0 3 m y comparable to or shorter than other highly extended areas in western north america the extensional domain in east central nevada is a composite feature produced by localized episodic rapid extension that began in early eocene time and continued to at least middle miocene time

geology and k ar geochronology of the paradise peak mine and the relationship of pre basin and range extension to early miocene precious metal mineralization in west central nevada the paradise peak mine is a major gold silver mercury deposit located in the southwestern part of the paradise range near the eastern edge of the walker lane in the western great basin nevada the tertiary volcanic rocks of the area can be divided into a sequence of 26 to 24 ma intermediate lavas a sequence of 24 to 22 ma silicic ash flow tuffs and a sequence of 20 to 15 ma intermediate lavas we classify these rocks as the older middle and younger sequences sedimentary rocks and latites and basalts 12 ma old or younger locally overlie the younger intermediate lavas south of the mine silicified rhyolite tuff in the lower part of the middle tuff sequence is the principal host of the precious metal ore most ore occurs under an acid leached zone of alunitic alteration and is contained in hydrothermal breccias that crosscut early quartz pyrite and alunite alteration field relations and k ar ages of hypogene alunite indicate that precious metal mineralization and alunitic and silicic alteration formed at about 19 to 18 ma several million years after eruption of the main host rock mineralization formed during high angle faulting related to crustal extension low angle normal faults are present 5 km northeast of the paradise peak mine but they formed more than 2 ma after formation of the paradise peak deposit silicic and alunitic alteration and precious metal mineralization are generally absent in areas of low angle faulting regional stratigraphic relations and k ar ages indicate that volcanism changed from silicic ash flow tuffs to intermediate lavas at about 20 to 19 ma regionally extensive angular unconformities indicate that a period of pre basin and range crustal extension occurred between about 22 to 19 ma this extension was penecontemporaneous with the shift in the style of volcanism and with gold silver mineralization in the paradise peak mine and in the goldfield and tonopah districts of western nevada the close temporal and spatial relationships of precious metal mineralization with pre basin and range extension suggest that extension was a major factor in the genesis of early miocene precious metal deposits in the western great basin

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

geometry and kinematics of the grant range brittle detachment system eastern nevada u s a an end member style of upper crustal extension documenting the range of styles of normal faulting is fundamental to understanding crustal extension here geologic mapping field relationships and deformed and restored cross sections illustrate the geometry and kinematic development of a system of west vergent detachment faults in the grant range in eastern nevada faults exhibit brecciation and stratigraphic cutoff angles of 5 15 at all structural levels and deform a 10 km thick section of paleozoic and paleogene rocks the fault system is folded across an anticlinal culmination which grew during extension as indicated by progressively increasing interlimb angles and incision in the axial zone the eastern limb consists of an imbricate stack of faults that were emplaced from bottom to top in the western limb several faults exhibit apparent thrust relationships the oldest faults are cut by a 29 ma dike and the highest preserved fault cuts 32 ma volcanic rocks that restore to paleodepths of 1 km retrodeformation of folding and minimal structural relief and angularity across a paleogene unconformity indicate the faults were active at 5 15 angles retrodeformation of offset indicates 49 km 98 extension we propose a model of stationary sustained isostatic uplift and incision at the culmination axis a fixed hinge with updip excision producing bottom to top growth of the imbricate stack and downdip excision producing apparent thrust relationships the fault system exhibits similarities to core complex detachment systems though it is confined to upper crustal levels and there are no preserved high angle or listric normal faults indicating a unique extension style dominated by low angle excision

dating mylonitic deformation by the 40ar 39ar method an example from the norumbega fault zone maine strongly mylonitic rocks associated with the regionally extensive norumbega fault zone in south central maine provide an excellent opportunity for testing the effects of mylonitization on argon isotopic systems in muscovite 40ar 39ar muscovite age spectra from samples outside the zone of mylonitization are relatively undisturbed and have well defined early carboniferous plateau ages in contrast to these nonmylonitized samples all age spectra for muscovite from the mylonites are highly discordant they are characterized by young ages at low extraction temperatures which systematically increase to ages that equal the plateau ages for muscovite collected outside the mylonite zone detailed petrographic observations suggest that these systematic discordances reflect a mixing of argon components from older relict muscovite porphyroclasts and fine grained white mica aggregates that recrystallized during mylonitic deformation total gas ages of five different grain size fractions separated from the same mylonite sample become progressively younger with decreasing grain size indicating a larger component of the recrystallized grains in the finer grain size fractions although the three finest grain size fractions give different total gas ages and do not overlap in age for most of their release spectra their initial increments do coincide at approximately 290 ma this indicates a minimal older age contribution from the relict porphyroclasts in the initial increments and suggests the 290 ma age provides a good estimate for the time of mineral growth associated with mylonitic deformation these data combined with kinematic analysis reveal that the segment of the norumbega fault zone studied the sandhill corner fault is a late carboniferous early permian dextral strike slip fault a lack of significant offset in regional early carboniferous mineral age patterns across the fault suggests that displacement was probably less than 30 km this study demonstrates that 40ar 39ar dating methods can be used to date deformational events effectively as long as several important criteria are met first and foremost samples must be well characterized prior to analysis dynamic recrystallization must have occurred at or below the closure temperature of the mineral to be analyzed regional cooling patterns must also be established through detailed thermochronology so that mineral ages and age spectra from the deformed rocks can be compared to regional cooling ages of the same mineral finally the effects of excess argon must be negligible

history and kinematics of cenozoic extension in the northern toiyabe range lander county nevada since earliest oligocene time 15 km of extension has occurred along a major gently dipping normal fault system and a younger more steeply dipping system of normal faults in the northern toiyabe range in central nevada the bernd canyon fault is the structurally lowest of the gently dipping faults in this area it dips 12 28 west and is similar to well known detachment faults paleozoic rocks in the hanging wall are cut by subsidary gently dipping normal faults and are tilted 40 to more than 90 to the east the early oligocene intracaldera tuff of hall creek is the oldest cenozoic unit present in the hanging wall and is tilted 28 58 eastward successively younger units have systematically decreasing dips and record a history of progressive possibly episodic extension bedding in greenschist grade lower paleozoic rocks in the footwall of the bernd canyon fault intersects the fault at angles of 5 to 25 and is arched across the range unlike many other detachment faults the bernd canyon fault contains early oligocene and younger volcanic and sedimentary units in the footwall that may be correlated to units in the hanging wall and they record a similar tilting history sequential untilting of successively older tertiary units suggests that the bernd canyon fault was initiated at a dip of 50 60 and was rotated to a dip of 25 30 during progressive extension before being cut and further rotated by younger faults restoration of tertiary units also suggests that the doming of paleozoic units across the range is a tertiary feature and that associated low bedding to fault angles in the footwall of the bernd canyon fault are the result of penetrative inhomogeneous simple shear deformation of the footwall including faults outside the area of detailed study 20 km of east west extension occurred across a domain that is now 45 km long north south by 35 km wide east west extension apparently began in earliest oligocene time before or perhaps synchronously with the onset of volcanism and it continued into the pliocene epoch tilt directions of oligocene volcanic rocks calcite veining near the detachment and elongate fossils in footwall units indicate that the extension direction on the detachment and related faults may have been nearly east west to possibly east northeast west southwest changing tilt directions in younger rocks suggest that the extension direction switched to west northwest east southeast at some time palinspastic restoration of normal faults also suggests that oligocene basins were initially bounded by high angle faults and had topographically elevated footwalls the restoration implies that the styles of miocene to recent basin and range faulting and pre miocene pre basin and range faulting are closely similar

a vegetation and fire history in a subalpine woodland and rain forest region solomons jewel lake tasmania solomons jewel lake occurs in an area of subalpine woodland with patches of cool temperate rain forest and the conifer athrotaxis cupressoides which is found mainly along stream channels a pollen record shows that the main vegetation types have not varied throughout the last 4000 years but subtle changes are evident in some elements apart from a reduction in eucalyptus for about 300 years beginning around 3500 bp the sclerophyll vegetation has remained relatively unchanged a small decline in rainforest taxa began around 1700 bp followed by an expansion of sphagnum from 1200 bp then athrotaxis cupressoides and cyperaceae from 800 900 bp these trends probably represent a chain of events associated with a cooling which reduced rain forest and allowed wetland areas to expand these then provided additional habitat for athrotaxis cupressoides which expanded because the wetlands provided additional protection from fire

timing of tertiary extension in the railroad valley pioche transect nevada constraints from 40ar 39ar ages of volcanic rocks time space relations of extension and volcanism place critical constraints on models of basin and range extensional processes this paper addresses such relations in a 130 km wide transect in the eastern great basin bounded on the east by the ely springs range and on the west by the grant and quinn canyon ranges stratigraphic and structural data combined with 40ar 39ar isotopic ages of volcanic rocks document a protracted but distinctly episodic extensional history field relations indicate four periods of faulting only one of these periods was synchronous with nearby volcanic activity which implies that volcanism and faulting need not be associated closely in space and time based on published dates and the analyses reported here the periods of extension were 1 prevolcanic pre 32 ma 2 early synvolcanic 30 to 27 ma 3 immediately postvolcanic about 16 to 14 ma and 4 pliocene to quaternary the break between the second and third periods is distinct the minimum gap between the first two periods is 2 ma but the separation may be much larger temporal separation of the last two periods is only suggested by the stratigraphic record and cannot be rigorously demonstrated with present data the three younger periods of faulting apparently occurred across the entire transect the oldest period is recognized only at the eastern end of the transect but appears to correlate about 150 km northward along strike with extension in the northern snake range kern mountains area therefore the oldest period also is regional in extent but affected a different area than that affected by younger periods this relation suggests that distinct extensional structures and master detachment faults were active at different times the correlation of deformation periods of a few million years duration across the railroad valley pioche transect suggests that the scale of active extensional domains in the great basin may be greater than 100 km across strike

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

tertiary structural development of selected valleys based on seismic data basin and range province northeastern nevada reflexion seismic data in railroad diamond mary s river and goshute valleys provide information on their structural development that cannot be deduced solely from outcrop and well data these valleys contain tertiary sediments that in dip section define an asymmetrical basin bounded along the eastern flank by a major listric normal fault with about 3 0 4 5 km of displacement the west flank is defined by a gentle east dipping ramp seismically the trace of the listric fault is interpreted to dip westward and sole into the palaeozoic section exploiting regionally recognized mesozoic decollement surfaces the tertiary depocentre adjacent to this fault shifted from west to east with continued slippage through time the greatest movement occurring in the miocene and post miocene in the strike direction the valleys are separated into at least two subbasins by an east west structurally high axis the axis is postulated to be the result of a tear fault associated with movement along the listric normal fault tertiary stratigraphy varies between valleys and between sub basins in a given valley all the valleys contain miocene and younger rocks however not all sub basins contain the pre miocene section suggesting a complex scheme of structural development

statistical modelling of annealing kinetics of fission tracks in zircon reassessment of laboratory experiments abstract we remodel the annealing kinetics of fission tracks fts in zircon to fit data from 43 laboratory experiments in three earlier publications using spontaneous ft lengths in zircon from the nisatai dacite heating temperature and time are in the ranges of 350 912 c and 10 3 104 h respectively the data sets illustrate a transition in annealing process at a certain temperature zone in order to express the annealing behavior involving the possible transitional process with improved goodness of fit two types of models are constructed the hybrid linear model that consists of the fanning linear model at high temperature and the parallel linear model at low temperature connected with a transitional temperature zone and the parallel curvilinear model that approximates smoothly the transition in the annealing process the validity of the extrapolation of the model to geological time scale is confirmed by comparison with some deep borehole core data the length estimates by the hybrid linear model agree well with the borehole data whilst the estimates by the parallel curvilinear model show a significant disagreement with highly annealed borehole samples although this model provides good agreement with the experimental data for the hybrid linear model estimates of the partial annealing zone corresponding to isothermal heating for 106 and 107 years are 281 352 c and 262 330 c where the top and bottom of the partial annealing zone are defined by the mean length reduction ratio of 0 4 and 0 8 respectively

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 zero damage model for fission track annealing in zircon a zircon fission track annealing model is calculated on the basis of annealing experiments from the literature with induced tracks in decay event damage free zircon samples empirically derived parallel and fanning equations for this zero damage model yield an excellent fit to the data with the fanning model providing slightly better statistical parameters a comparison between annealing models with fanning iso annealing lines but different decay event damage densities reveals that annealing temperatures and closure temperatures for the estimated partial annealing zone are highest for the zero damage model compilations of existing geologic constraints on the zircon partial annealing zone on one hand and the zircon closure temperature on the other show that these constraints do not or only partly overlap with curves of proposed models for the zircon partial annealing zone and closure temperature this finding is consistent with the fact that the annealing behavior of zircon from long duration temperature evolutions is increasingly influenced by the accumulated decay event damage zircon samples of young age or low u content show a behavior closest to the predictions of the zero damage model and are in the predicted range of published models with low decay event damage density for thermal events of more than 10 myr duration however constraints from field studies show marked differences from proposed partial annealing zone boundaries of the zero or low damage models the applicability of the zero damage model is threefold 1 it predicts correct closure temperatures in the case of very rapid cooling across the partial annealing zone where basically no decay event damage is accumulated 2 it predicts an uppermost boundary for complete annealing of a mixture of zircon components of different age as found in sedimentary samples and in this case may be used as a thermometer 3 it represents an important reference for the establishment of a more comprehensive model of zircon fission track annealing that also includes the influence of decay event damage for such a model two different equations are discussed however additional detailed experimental and field data are needed for a more robust annealing model that includes the influence of decay event damage annealing

relations between hinterland and foreland shortening sevier orogeny central north american cordillera the tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens the classic central cordilleran foreland fold and thrust belt in the united states late jurassic to early tertiary sevier belt and the more internal zones to the west central nevada thrust belt provide data critical to understanding the developmere of internal and external parts of orogens the garden valley thrust system part of the central nevada thnkst belt crops out in south central nevada within a region generally considered to be the hinterland of the jurassic to eocene sevier thrust belt the thrust system consists of at least four principal thrust plates composed of strata as young as pennsylvanian in age that are unconformably overlain by rocks as old as oligocene suggesting that contraction occurred between those times new u pb dates on intrusions that postdate contraction combined with new paleomagnetic data showing significant tilting of one area prior to intrusion suggest that regionally these thrusts were active before 85 100 ma the thrust faults are characterized by long relatively steeply dipping ramps and associated folds that are broad and open to close upright and overturned although now fragmented by cenozoic crustal extension individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike we correlate the structurally lowest thrust of the garden valley thrust system the golden gate mount irish thrust southward with the gass peak thrust of southern nevada this correlation carries the following regional implications at least some of the slip across jurassic to mid cretaceous foreland thrusts in southern nevada continues northward along the central nevada thrust belt rather than noaheastward into utah this continuation is consistent with age relations which indicate that thrusts in the type sevier belt in central utah are synchronous with or younger than the youngest thrusts in southern nevada this in turn implies that geometrically similar sevier belt thrusts in utah must die out southward before they reach nevada that slip along the southem nevada thrusts is partitioned

geochronology of blueschists from west central baja california and the timing of uplift in subduction complexes a geochronologic study of blueschists from west central baja california has provided constraints on the timing of high pressure low temperature metamorphism and the subsequent uplift of this subduction complex mineral separates from coherent blueschists and blueschist blocks enclosed in serpentinite melange were analyzed by 40 ar 39 ar fission track and rb sr methods 40 ar 39 ar age spectra for white mica separated from four blueschist blocks and an overprinted eclogite block in serpentinite matrix melange show slow cooling gradients between 115 95 ma rb sr apparent ages for one block are concordant with these results 40 ar 39 ar ages of blue amphibole are primarily the result of the degassing of white mica intergrowths as confirmed by microprobe analyses apatite fission track ages indicate a blueschist block and an eclogite block cooled to a temperature below 100 c at 22 ma and 32 ma respectively subduction related metamorphism of the coherent blueschists occurred in la

zircon and apatite u th he evidence for paleogene and neogene extension in the southern snake range nevada usa despite decades of study the timing rates and magnitude of extension in the basin and range are poorly quantified in some areas this study integrates new zircon and apatite u th he analyses zrnhe and aphe with published thermochronologic data to quantify these extensional parameters in the southern snake range ssr of east central nevada the new zrnhe dates range from 40 7 4 9 ma in the western ssr to 21 0 3 3 ma near the present day trace of the southern snake range decollement ssrd and the aphe dates range from 15 1 2 4 ma in the central ssr to 13 6 0 7 ma closest to the ssrd trace these new and previously published low temperature thermochronologic cooling ages were inverted for the extensional history of the ssr using a bayesian monte carlo method incorporating pecube the posterior extensional histories indicate three significant pulses of extension occurred during the paleogene and neogene 1 50 45 to 38 ma eocene 2 33 30 to 23 ma oligocene and 3 23 20 to 10 8 ma miocene modeled rates of extension were low at 0 5 mm a 1 however more rapid rates possibly occurred during the eocene and the miocene based on posterior histories net cumulative extension from posterior histories is 19 8 to 34 9 km with a mean of 29 7 km about 10 18 km of extension occurred during the eocene and oligocene model results indicate no relationship between extension and magmatism in the ssr our new model results and interpretations also indicate extensional collapse of the nevadaplano initiated prior to 17 ma

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

40ar 39ar mineral age record of polyphase tectonothermal evolution in the southern mauritanide orogen southeastern senegal 40 ar 39 ar incremental release mineral ages have been determined for several tectonostratigraphic units in southernmost portions of the maiuritanide orogen in senegal here middle proterozoic crystalline basement rocks of the west african craton are unconformably overlain by an upper proterozoic ca 1100 ma continental sedimentary succession madina kouta group transitional into volcanic and intercalated deep marine sediments tennesse group western portions of the volcano sedimentary unit were variably deformed and metamorphosed prior to deposition of an unconformably overlying upper precambrian lower cambrian sedimentary succession mali group muscovite within cleavage domains in the low grade metamorphic terrane records 40 ar 39 ar plateau dates of ca 655 665 ma and are interpreted to closely date the pan african i phase of orogenic activity the mali group itself was locally folded during a second phase of pan african tectonothermal activity which also affected a western calc alkaline volcanic plutonic sequence niokolo koba group igneous hornblende from several granites within this sequence displays internally discordant 40 ar 39 ar release spectra which record variably developed plateaus of ca 640 660 ma these are interpreted to approximately date initial postmagmatic cooling through 500 c and are consistent with a previously published rb sr whole rock iso chron crystallization age of 683 18 ma for one of the plutons associated calc alkaline volcanic and volcaniclastic rocks are more penetratively deformed muscovite that is aligned along a regional cleavage records 40 ar 39 ar plateau ages of ca 550 560 ma which are interpreted to closely date the pan african ii orogenesis widespread deposition of a molassic sequence youkoukoun formation occurred in the middle cambrian early ordovician this was disconformably overlain by upper ordovician upper devonian sedimentary rocks bove basin succession which are locally folded more significant late paleozoic tectonothermal activity is recorded in north westernmost portions of the study area where the calc alkaline upper proterozoic granites locally display penetrative mylonitic fabrics development of these fabrics is interpreted to be closely dated by the 40 ar 39 ar 270 to 280 ma plateau ages recorded by dynamically recrystallized muscovite developed along fluxion foliations these results allow demarcation of a late paleozoic tectonic front and permit correlations with genetically related events in southeastern portions of the appalachian orogen

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

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

prevolcanic extensional seaman breakaway fault and its geologic implications for eastern nevada and western utah stratigraphic and structural relations indicate that a large displacement east dipping normal fault the seaman fault is concealed under southern white river valley nevada the seaman fault forms the breakaway of an eastward rooting pre middle oligocene pre 30 6 ma extensional detachment system and may connect northward to the breakaway of the oligocene extensional system exposed in the snake schell creek and egan ranges we propose that this regional breakaway zone bounds a single eastward rooting oligocene extensional belt the snake stampede system that affected much of eastern nevada and western utah the southward migration with time of tertiary volcanism in the great basin caused varying temporal relations between volcanism and deformation in the snake stampede system approximately synchronous extension is prevolcanic to the south but syn to postvolcanic to the north this pattern conflicts with widely accepted temporal and genetic links between volcanism and crustal extension basin and range crustal extension probably neither triggered nor was triggered by magmatism but instead volcanism and extension operated somewhat independently

a tibetan model for the early tertiary western united states a comparative study of the geology of the early tertiary western united states and the modern tibetan and turkish iranian plateaux indicates that these three regions situated in the hinterland of major orogenic belts share a common taphrogenic evolutionary path both the tibetan and turkish iranian plateaux have high mean elevations above sea level 5 km and 2 0 2 5 km respectively have experienced fast uplift and widespread unroofing during the last 10 8 ma and are undergoing extension nearly perpendicular to the plate convergence direction successive accretionary and collisional events accompanied by significant shortening and underplating during the formation of these plateaux resulted in the generation of overthickened continental crust 50 km that is currently in the process of extending a two stage magmatism characterized by widespread calc alkaline volcanism followed and or overlapped by bimodal basaltic to rhyolitic volcanism occurs synchronously with crustal extension and thinning the plateaux in general have an internal closed drainage system and are drained by antecedent rivers near their edges the early tertiary western us cordillera displayed similar geological and geomorphic features to those observed in the tibetan and turkish iranian plateaux in the aftermath of a prolonged mesozoic orogeny and underwent an orogenic collapse starting around mid tertiary time a n trending crustal welt with a maximum thickness of 60 km and an elevation around 3 km formed a highland with alpine flora in the hinterland of the cordilleran orogenic belt and separated internal drainage basins and saline lakes on the east from major flood plains of antecedent river systems on the west this highland subsequently became the locus of crustal extension metamorphic core complex formation and wide spread calcalkaline volcanism 9ignimbrite flare up9 this early stage of orogenic collapse was followed by the basin and range extension characterized by block faulting and bimodal volcanism starting around 18 16 ma the basin and range extension and associated subsidence resulted in development of the great basin with a mean elevation of 1 5 km above sea level and a crustal thickness of 30 km similarities in the post orogenic evolutionary paths of these three regions indicate that continental lithosphere reacts to overthickening in a consistent way through large scale crustal remobilization and extensional collapse regardless of the mode and nature of its orogenic build up

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

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

a field based estimate of the zircon fission track closure temperature abstract a field based estimate of the zircon fission track closure temperature is proposed from 13 zircon fission track cooling ages of the gold butte block se nevada using previously published and new thermochronological data the thermal history of the gold butte block is re evaluated resulting in a geothermal gradient estimate of 20 3 c km and a slow cooling rate of 0 55 0 02 c m y from the late jurassic to the early miocene before onset of rapid miocene cooling the rapidly cooled part of the gold butte block provides a slip rate of 10 5 km m y of the hanging wall that caused the miocene exhumation of this crustal section assuming these thermal conditions are correct the closure temperature of the zircon fission track system is determined at 205 18 c which is in good agreement with previous field based estimates because of the variation in zircon fission track annealing behavior with radiation damage field studies are a suitable way to calibrate the closure temperature of the zircon fission track system

u pb zircon and titanite systematics of the fish canyon tuff an assessment of high precision u pb geochronology and its application to young volcanic rocks a large data set of single and multi grain zircon and titanite analyses from a sample of the oligocene fish canyon tuff fct a voluminous ash flow from the san juan mountains of colorado and widely used 40ar 39ar geochronological standard has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution u pb zircon method twenty three single zircon grains and seven small multi grain fractions of the fct yield an inverse variance weighted mean 206pb 238u date of 28 402 0 023 ma 2 mswd 0 93 and a slightly older weighted mean 207pb 235u date of 28 529 0 030 ma mswd 0 74 which are insensitive to common pb corrections initial 230th disequilibria calculated from a newly measured th u 2 2 for fct pumice shards indicate th deficiency for these zircons its correction brings the weighted mean 206pb 238u date to 28 478 0 024 mswd 0 97 minimizing the discordance of the fct zircon analyses these calculations culminate in concordia ages ludwig 1998 for the crystallization of the fct zircons of 28 476 0 029 ma mswd 1 50 without propagating decay constant errors or 28 498 0 035 ma mswd 1 03 with propagated decay constant errors these results are in close agreement with a previous single grain 206pb 238u zircon date of 28 41 0 05 ma for the fct oberli et al 1990 however they do not corroborate the discordia lower intercept date of 27 52 0 09 put forth by lanphere and baadsgard 2001 the 28 476 0 064 ma concordia age for fct zircons including systematic decay constant errors and uncertainty in pb u tracer calibration is substantially older than all published 40ar 39ar sanidine ages which range from 27 5 to 28 05 ma lanphere and baadsgaard 1997 renne et al 1998 a lengthy period 400 k y of zircon residence in the magma chamber before eruption is commonly called upon to explain the age discrepancy however fct titanite which may remain open to pb diffusive exchange until quenching upon eruption yield a minimum 230th disequilibrium corrected weighted mean 206pb 238u age of 28 395 0 049 ma 0 078 with tracer uncertainty weakening residence time arguments and supporting recent calls for re evaluation of the 40k decay constant and the accepted ages for commonly used 40ar 39ar mineral standards the fct zircon concordia age also provides an empirical constraint on the decay constants of the u isotopes indicating the relative accuracy of the currently accepted decay constants to within the original counting statistic errors of jaffey et al 1971

cenozoic extension and tilting recorded in upper cretaceous and tertiary rocks at the hall molybdenum deposit northern san antonio mountains nevada structural and paleomagnetic data from the 66 to 70 ma hall stock and its environs suggest that extension related tilting has continued in the northern san antonio mountains of nevada since at least 17 18 ma evidence includes both surface and subsurface mapping and paleomagnetic data the stock and its related molybdenum deposit most likely magnetized during a reversed polarity chron have been significantly tilted to the east as have nearby 16 to 29 ma volcanic rocks the data do not preclude the possibility of counterclockwise crustal rotations related to dextral motion of the san andreas and related faults similar horizontal tilt axes for the stock n30 w and the volcanic rocks n n20 w and similar tilt magnitudes among all units however suggest that much or all of the tilting in the district occurred between late oligocene and latest miocene time during west southwest east northeast pre basin range extension maximum pre oligocene post cretaceous tilt is clearly

post laramide removal of the farallon slab western united states i propose that post laramide removal of the subhorizontally subducting farallon slab occurred by buckling downward along an approximately east northeast trending axis this process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the united states and propagation of the separated edges toward the central axis of downwelling accompanied by aesthenosphic upwelling behind the trailing edges initial buckling probably began near 50 ma and slab removal was complete by 20 ma this model is based primarily on the space time evolution of the ignimbrite flare up a major mid tertiary igneous event of mantle origin which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central nevada from the north and southeast post laramide uplift extension establishment of the cascadia subduction zone and active magmatism may be consequences of lithosphere scale modifications caused by the laramide removal of the slab and the resulting asthenospheric upwelling

a precise 40ar 39ar reference age for the durango apatite u th he and fission track dating standard the distinctive durango apatite is widely distributed as a reference material for mineralogy geochemistry and thermochronology the apatite occurs within the cerro de mercado open pit iron mine located in the northern portion of durango city mexico it is largely confined to vugs and open fractures within the lower portion of the ore deposit apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the chupaderos caldera complex this activity occurred between eruptions of two major ignimbrites from the caldera complex four single crystal sanidine anorthoclase 40 ar 39 ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31 44 0 18 ma 2 for the apatite itself four additional multi crystal measurements are in general agreement with this age also in agreement are a set of 24 u th sm he ages recently determined directly on the apatite which have a mean of 31 02 1 01 ma 1 with a standard error s e of 0 22 ma

the influence of natural radiation damage on helium diffusion kinetics in apatite abstract stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4he and proton induced 3he reveal a range in closure temperature tc from 50 to 115 c for a cooling rate of 10 c myr there is no correlation between helium diffusion and apatite chemistry including f cl ratio but the closure temperature is positively correlated with the radiogenic 4he concentration 4he in each sample we argue that 4he is a proxy for a sample s natural exposure to actinide radioactivity below the closure temperature and that helium diffusion in apatite is impeded by radiation induced damage to the apatite structure the kinetics must therefore be an evolving function of time measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample the effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity including grain size our diffusion data are well described by a previously proposed quantitative model that consists of two arrhenius relations one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage traps the unknown parameters in this trapping model were determined from the diffusion experiments and allow us to develop a tentative mathematical function that relates diffusivity to temperature and 4he by inserting this function into a 4he production diffusion model we have explored how these results affect the interpretation of apatite u th he thermochronometry the model predicts that the effective 4he closure temperature of apatite will vary with cooling rate and effective u concentration eu and may differ from the commonly assumed tc of 70 c by up to 15 c the 4he partial retention zone will look similar to previous expectations but its depth will depend on accumulation time and on eu most notably samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected these predictions are consistent with recent observations of unexpected apatite u th he ages in some settings

improved measurement of fission track annealing in apatite using c axis projection apatite fission track length data are used extensively for thermal history inversion however several studies have documented instances of poor reproducibility of length data we address this problem by using c axis projection to normalize track lengths for crystallographic angle in the extensive laboratory annealing data set acquired by barbarand et al 2003a 2003b a new simplification reduces the c axis projection model from six to four fitted parameters normalizing for track angle using c axis projection improves every aspect of length measurement reproducibility examined it accelerates convergence of mean length in single analyses increases consistency among replicate measurements by a single analyst enhances consistency of measurements of the same mounts by different analysts and improves the match between analyses conducted with and without cf irradiation c axis projection is also shown to enhance the thermal sensitivity of length data based on these results we assert that c axis projection is a good means of compensating for observer bias although it does not overcome differences caused by experimental error

effects of inherited cores and magmatic overgrowths on zircon u th he ages and age eu trends from greater himalayan sequence rocks mount everest region tibet previous constraints on the timing and rate of exhumation of the footwall of the south tibetan detachment system stds north of mount everest suggest rapid miocene cooling from 700 c to 120 c between 14 17 ma however 25 new single grain zircon he ages from leucogranites intruding greater himalayan sequence rocks in the footwall of the stds are between 9 9 and 15 ma with weighted mean ages between 10 and 12 ma zircon grains exhibit a positive correlation between age and effective uranium eu laser ablation zircon u pb geochronology detailed sem observations and laser ablation depth profiling of these zircons reveal low eu 0 5 2 5 ga inherited cores overgrown by high eu 17 22 ma rims this intragranular zonation produces ages as much as 32 too young when a standard alpha ejection correction assuming uniform eu distribution is applied modeling of the effects of varying rim thickness and rim eu concentration on the bulk grain eu and alpha ejection correction suggests that zonation also exerts the primary control on the form of the age eu correlation observed application of grain specific zonation dependent age corrections to our data yields zircon he ages between 14 and 17 ma in agreement with aft and 40ar 39ar ages growth of magmatic rims followed by cooling to 120 c within 1 6 million years supports rapid tectonic exhumation associated with slip along the stds in the miocene this study highlights the importance of characterizing parent nuclide zonation in zircon he studies which seek to understand the timing of exhumation along exhumed crustal blocks

late proterozoic grenville deformation in central new mexico determined from single crystal muscovite 40ar 39ar age spectra abstract 40 ar 39 ar age spectrum analyses on deformed and undeformed single crystal muscovites from the manzano mountains in central new mexico provide insights on the argon systematics of deformed muscovite the post 1 4 ga cooling history of the region and the timing of low temperature deformation microstructures in narrow east side up shear zones in and around the 1427 ma priest pluton indicate post pluton greenschist facies 350 c deformation single crystal 40 ar 39 ar analysis of three samples containing undeformed muscovite yield plateau spectra with ages of 1390 3 1395 2 and 1401 3 ma suggests that by this time the pluton had cooled below 350 400 c single crystal muscovites from within the shear zones have markedly different age spectra and reveal well defined age gradients from 1 1 to 1 4 ga the age gradients are interpreted as resulting from deformation induced argon loss or reduction of the effective radius for argon diffusion thereby decreasing the closure temperature of the originally pristine magmatic muscovites these data and previously published results suggest a long period of crustal stability ca 1 4 to 1 1 ga in which average cooling rates were probably about 1 c ma the 1 1 ga ages in the deformed muscovite age spectra are interpreted as indicating that grenvillian tectonism to the southeast affected central new mexico it can not be unambiguously determined if the consequences of the grenville orogeny in new mexico was uplift and erosion cooling a discrete thermal pulse reheating or if the shear zones record grenville age contractual deformation

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

displacement rates of normal faults previous estimates of displacement rates on individual faults have been limited to neotectonic faults and averaged over time intervals of about 200 kyr or less1 2 3 4 5 these estimates have been highly variable which has led to a belief that longer term displacement rates on individual faults are likely to be variable as well here we report estimates of long term normal fault displacement rates averaged over time intervals ranging from 1 to 40 myr and based on observed decreases in displacement of progressively younger horizons intersected by syn sedimentary faults we find that displacement rates are remarkably stable over these longer time periods and within a given fault system the rates are strongly dependent on the relative size of the fault as measured by cumulative vertical displacement taken together these results indicate that faults become large relative to nearby faults by having higher displacement rates even when small rather than as a consequence of having been active for longer our analyses also show that high regional strain rates tend to be accommodated by high fault displacement rates rather than high fault densities

synconvergent surface breaking normal faults of late cretaceous age within the sevier hinterland east central nevada the hinterland of the sevier orogenic belt of western north america is widely interpreted as a cretaceous to paleogene orogenic plateau although evidence for mid crustal extension of late cretaceous age within the sevier hinterland is widespread coeval surface breaking normal fault systems have not been documented new 1 12 000 scale mapping within the type section of the latest cretaceous to eocene sheep pass formation of east central nevada suggests that deposition occurred in response to normal fault movement recording up to 4 km of late cretaceous and paleogene stratigraphic throw intrabasinal normal faulting caused lateral thickness variations within the basal sheep pass formation although upper members are largely unaffected an extensional basin setting best explains the fanning of bedding dips the deposition of megabreccia and the presence of syndepositional normal faults within the sheep pass formation deposition of the basal member of the sheep pass formation is bracketed between ca 81 3 3 7 ma and 66 1 5 4 ma based on the u th he cooling ages of detrital zircons and on a u pb carbonate age derived from the overlying lacustrine limestone member these new data provide the strongest evidence to date for the existence of late cretaceous surface breaking normal faults in the sevier hinterland normal faulting was coeval with mid crustal hinterland extension and with continued contraction within the sevier foreland to the east

metallogeny of the zoujiashan uranium deposit in the mesozoic xiangshan volcanic intrusive complex southeast china insights from chemical compositions of hydrothermal apatite and metal elements of individual fluid inclusions abstract the zoujiashan u deposit in the mesozoic xiangshan volcanic intrusive complex is one of the largest volcanic related type u deposits in china chemical composition of hydrothermal apatite and metal contents in individual fluid inclusions of quartz and fluorite are investigated in order to understand the precipitation mechanism of u and to further constrain the origin of u and ore forming fluids of the zoujiashan deposit illitization hematitization and u mineralization successively occurred within rhyolite and porphyritic lava three types of hydrothermal apatite are identified apatite type 1 ap1 and apatite type 2 ap2 are both euhedral subhedral crystals found in hematitization zone while only ap2 shows evident irregular chemical zoning along its rims apatite type 3 ap3 occurs as anhedral aggregates closely associated with u minerals in u ore veins implying synchronous precipitation of ap3 and u ores comparable chemical compositions of ap1 and cores of ap2 including similar high cl content indicate both of them were precipitated from a cl rich fluid that induced hematitization the elevated f contents of ap3 and rims of ap2 implies that a f rich fluid was involved and altered ap2 during the main stage of u deposition ap1 and cores of ap2 have lower mn contents than ap3 and rims of ap2 indicating an oxidized character for the cl rich fluid and a reducing character for the f rich fluid metal contents of fluid inclusions in fluorite and quartz occurring in u ores suggest that two end members of fluids including a u rich and a u poor were involved in the main stage of u mineralization mixing between these two fluids led to the deposition of u and th minerals through oxidation reduction reactions and destabilization of th f complexes respectively the u rich fluid is depleted in th and sr compared with the u poor fluid based on comparison of chemical compositions of apatite and fluid inclusions we suggest that the cl rich fluid precipitating ap1 and ap2 corresponds to the u rich end member fluid while the f rich fluid precipitating ap3 represents a mixture between the two end members the high u 12 7 58 5 ppm in the u rich end member fluid is in agreement with a relatively high oxygen fugacity and a surface derived origin the u poor end member fluid has lower rb cs than the u rich end member and the unaltered host rocks indicating the u poor fluid was derived from a mantle wedge that experienced addition of cs by fluids generated from the devolatilization of the subducted paleo pacific slab

trace element and sr nd isotope geochemistry of fluorite from the xiangshan uranium deposit southeast china the trace element and sr nd isotope geochemistry of fluorite have been investigated in the xiangshan volcanic rock hosted uranium deposit in southeast china detailed sr nd isotope data of the hydrothermal fluorite closely associated with the uranium mineralization suggest that ore forming fluids were a combination of mantle derived fluid and fluids equilibrated with basement metamorphic rocks this model is consistent with previously published h o isotope data and is incompatible with meteoric water as the only ore forming fluid trace element geochemical profiles across the uranium orebody can be interpreted to reflect reaction between the host rocks and a mantle derived fluid containing high u and th with low th u ratios the mantle derived fluid is considered to have been u f and carbonate rich mixing with shallowly circulated meteoric water within the fractures of the host volcanic rocks caused dilution and cooling of the ore forming fluid and subsequent precipitation of fluorite and uranium ore