the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

extension of the telluride erosion surface to washington state and its regional and tectonic significance abstract the wenatchee formation of oligocene age 34 m y rests on a profound unconformity that developed during an extended period of erosion and tectonic quiescence from perhaps 40 34 m y b p its occurrence strongly suggests that a stripped erosion surface in the northern cascade range previously considered of miocene age is of late eocene early oligocene age and that it is part of a more widespread surface in the pacific north west the late eocene early oligocene unconformity erosion surface is apparently of continental extent from british columbia to mexico where it is incised on crystalline basement rocks it now may be exhumed and exposed as a plateau or level crested divides of concordant elevation where it is developed on sedimentary rocks it may be found as a profound unconformity widespread eruptions of pyroclastic volcanic rocks covered much of the surface in oligocene time and tended to protect and preserve it the surface is particularly well documented in the san juan volcanic field of colorado there it was given the name telluride peneplain which is here broadened to the telluride erosion surface of continental extent a magmatic gap indicating a period of magmatic quiescence is generally associated with tectonic quiescence implied by the erosion surface but the timing varies regionally development of the telluride surface and associated magmatic gaps correlates with general plate reorganization during the eocene an erosion surface correlates with a global tectonic transition because erosion rates immediately dominate uplift rates when compressive deformation is slowed or shut off an analysis of the dynamic balance between erosion rates and plate convergence rates suggests that erosion may maintain a continental surface of low relief if the relative convergence rate falls to about 0 5 cm yr or less gaps in magmatic activity do not correlate as precisely with the 40 m y tectonic transition in part because of the possibility of time lags a great deal of scattered geologic evidence suggests a major global event at about 40 m y bp the timing does not always precisely correlate with the timing of events in western north america which suggests that the 40 m y tectonic transition was spread over several millions of years a most impressive global impact of the 40 m y transition is the largest drop in eustatic sealevel at about 30 m y bp the 40 m y tectonic transition profoundly altered geologic history including the development of continental landscapes global sea level and global climate classical stratigraphic studies remain the best way of documenting its effect on the continental surface the small remnant of oligocene rocks in central washington overlooked in early reconnaissance studies is of great geologic significance other undiscovered remnants probably exist in western north america

influence of climate change and uplift on colorado plateau paleotemperatures from carbonate clumped isotope thermometry the elevation history of earth s surface is key to understanding the geodynamic processes responsible for the rise of plateaus we investigate the timing of colorado plateau uplift by estimating depositional temperatures of tertiary lake sediments that blanket the plateau interior and adjacent lowlands using carbonate clumped isotope paleothermometry a measure of the temperature dependent enrichment of 13 c 18 o bonds in carbonates comparison of modern and ancient samples deposited near sea level provides an opportunity to quantify the influence of climate and therefore assess the contribution of changes in elevation to the variations of surface temperature on the plateau analysis of modern lake calcite from 350 to 3300 m elevation in the southwestern united states reveals a lake water carbonate temperature lct lapse rate of 4 2 0 6 c km analysis of miocene deposits from 88 to 1900 m elevation in the colorado river drainage suggests that the ancient lct lapse rate was 4 1 0 7 c km and temperatures were 7 7 2 0 c warmer at any one elevation than predicted by the modern trend the inferred cooling is plausible in light of pliocene temperature estimates off the coast of california and the consistency of lapse rates through time supports the interpretation that there has been little or no elevation change for any of the samples since 6 ma together with previous paleorelief estimates from apatite u th he data from the grand canyon our results suggest most or all of the plateau s lithospheric buoyancy was acquired 80 60 ma and do not support explanations that ascribe most plateau uplift to oligocene or younger disposal of either the farallon or north american mantle lithosphere

trace elements and u pb ages in petrified wood as indicators of paleo hydrologic events abstract subsurface fluid systems are important for chemical weathering ore formation and thermal evolution of the crust changes in the dynamics and distribution of subsurface fluid flow systems are controlled by changes in global and regional terrestrial climate tectonics and elevation this paper concerns the dating of changes in ancient subsurface hydrologic systems however direct dating of water rock interaction is challenging because of the lack of appropriate materials to date and the more open and complex nature of subsurface flow regimes here we explore the prospects of using u pb dating of petrified silicified wood as a means of quantifying continental paleo hydrology oxidizing fluids often of meteoric origin tend to leach and mobilize u from the country rock but when such waters contact organic rich material u can become reduced and immobilized resulting in u rich silicified wood we present in situ laser ablation icpms analyses of u pb isotopes and trace elements in petrified wood from the upper triassic chinle formation 225 208 myr in the petrified forest national park in arizona usa allowing us to establish a generalized workflow for making meaningful paleo hydrologic interpretations of the u pb systematics of silicified wood wood characterized by brownish colors and preservation of cellular structure have low fe contents and positive ce anomalies indicating silicification in reducing environments and isolation in relatively reduced conditions after silicification resulting in closed system behavior of u and pb wood characterized by vivid colors orange red etc and little to no preservation of cellular structure are much higher in fe and exhibit negative ce anomalies indicating influence by more oxidized fluids the brownish samples yield u pb ages clustered between 250 and 200 ma with a peak coinciding with the time of deposition 220 ma which indicates that fossilization largely took place almost immediately after deposition and that u pb in quartz faithfully retains the time of such fossilization in contrast the orangish reddish whitish samples yield younger u pb ages defining distinct errorchron ages which reflect subsequent generations of quartz crystallization scatter associated with errorchrons are likely due to local mm to cm scale u or pb mobility or variable initial pb composition distinct younger age peaks appear to correlate with the timing of regional unconformities associated with tectonic or epeirogenic uplift we suggest that uplift and exhumation may initiate the onset of oxidizing fluid systems resulting in leaching and transport of u from the surroundings followed by subsequent generations of quartz precipitation in summary u pb dating of petrified wood or silicified organic material has high potential for dating paleo hydrologic events however due to complexities in terrestrial hydrologic systems interpretations of u pb systematics must be informed by accompanying geochemical and textural observations

6 ma age of carving westernmost grand canyon reconciling geologic data with combined aft u th he and 4he 3he thermochronologic data conflicting hypotheses about the timing of carving of the grand canyon involve either a 70 ma old or 6 ma young grand canyon this paper evaluates the controversial westernmost segment of the grand canyon where the following lines of published evidence firmly favor a young canyon 1 north derived paleocene hindu fanglomerate was deposited across the present track of the westernmost grand canyon which therefore was not present at 55 ma 2 the 19 ma separation point basalt is stranded between high relief side canyons feeding the main stem of the colorado river and was emplaced before these tributaries and the main canyon were incised 3 geomorphic constraints indicate that relief generation in tributaries and on plateaus adjacent to the westernmost grand canyon took place after 17 ma 4 the late miocene pliocene muddy creek formation constraint shows that no river carrying far traveled materials exited at the mouth of the grand canyon until after 6 ma interpretations of previously published low temperature thermochronologic data conflict with these lines of evidence but are reconciled in this paper via the integration of three methods of analyses on the same sample apatite u th he ages ahe 4he 3he thermochronometry 4he 3he and apatite fission track ages and lengths aft hefty software was used to generate time temperature t t paths that predict all new and published 4he 3he ahe and aft data to within assumed uncertainties these t t paths show cooling from 100 c to 40 60 c in the laramide 70 50 ma long term residence at 40 60 c in the mid tertiary 50 10 ma and cooling to near surface temperatures after 10 ma and thus support young incision of the westernmost grand canyon a subset of ahe data when interpreted alone i e without 4he 3he or aft data are better predicted by t t paths that cool to surface temperatures during the laramide consistent with an old grand canyon however the combined aft ahe and 4he 3he analysis of a key sample from separation canyon can only be reconciled by a young canyon additional new aft 5 samples and ahe data 3 samples in several locations along the canyon corridor also support a young canyon this inconsistency which mimics the overall controversy of the age of the grand canyon is reconciled here by optimizing cooling paths so they are most consistent with multiple thermochronometers from the same rocks to do this we adjusted model parameters and uncertainties to account for uncertainty in the rate of radiation damage annealing in these apatites during sedimentary burial and the resulting variations in he retentivity in westernmost grand canyon peak burial conditions temperature and duration during the laramide were likely insufficient to fully anneal radiation damage that accumulated during prolonged near surface residence since the proterozoic we conclude that application of multiple thermochronometers from common rocks reconciles conflicting thermochronologic interpretations and the data presented here are best explained by a young westernmost grand canyon samples spread along the river corridor also suggest the possibility of variable mid tertiary thermal histories beneath north retreating cliffs

thrusting of the claron formation the bryce canyon region utah a thrust system at least 40 km long and with as much as 2 000 m of shortening has been mapped in the southern high plateaus of utah partly within bryce canyon national park seismic data clearly reveal that the thrusts are of thin skinned ramp flat style soling out in evaporite rich layers of the jurassic carmel formation mapping discloses that large portions of the thrusts terminate upward in fault propagated folds marked by vertical to overturned beds spaced cleavage reveals that some shortening is accommodated by pressure dissolution shallow drilling for coal provides data suggestive of more than one episode of thrusting an older event predates deposition of the upper cretaceous and paleocene canaan peak formation this deformation is presumably related in the broadest sense of the term to the sevier orogeny the younger thrusting is southvergent and involves rocks as young as the lower to middle eocene white limestone member of the claron formation a formation that is considered post orogenic by most geologists the upper age of the younger event is not constrained in time and its tectonic significance remains poorly understood

laboratory experiments on eddy generation by a buoyant coastal current flowing over variable bathymetry irminger rings are warm core eddies formed off the west coast of greenland recent studies suggest that these eddies which are implicated in the rapid springtime restratification of the labrador sea are formed by an internal instability of the west greenland current wgc triggered by bathymetric variations this study seeks to explore the effect of the magnitude and downstream length scale of bathymetric variations on the stability of a simple model of the wgc in a series of laboratory experiments in which a buoyant coastal current was allowed to flow over bathymetry consisting of piecewise constant slopes of varying magnitude the currents did not form eddies over gently sloping bathymetry and only formed eddies over steep bathymetry if the current width exceeded the width of the sloping bathymetry eddying currents were immediately stabilized if they flowed onto gently sloping topography bathymetric variations that persisted only a short distance downstream perturbed the flow locally but did not lead to eddy formation eddies formed only once the downstream length of the bathymetric variations exceeded a critical scale of about 8 rossby radii these results are consistent with the observed behavior of the wgc which begins to form irminger rings after entering a region where the continental slope abruptly steepens and becomes narrower than the wgc itself in a region spanning about 20 80 rossby radii of downstream distance

unroofing incision and uplift history of the southwestern colorado plateau from apatite u th he thermochronometry the source of buoyancy for the uplift of cratonic plateaus is a fundamental question in continental dynamics the 1 9 km uplift of the colorado plateau since the late cretaceous is a prime example of this problem we used apatite u th he thermochronometry 230 analyses 36 samples to provide the fi rst single system regional scale proxy for the unroofi ng history of the southwestern quadrant of the plateau the results confi rm overall southwest to northeast unroofi ng from plateau margin to plateau interior a single phase of unroofi ng along the plateau margin in late cretaceous to early tertiary sevierlaramide time contrasts with multiphase unroofi ng of the southwestern plateau interior in early and mid to late tertiary time the early cretaceous was characterized by northeastward tilting and regional erosion followed by aggradation of 1500 m of upper cretaceous sediments along the eroded plateau margin sevier laramide denudation affected the entire southwestern plateau was concentrated along the plateau margin and migrated from northwest to southeast following a period of relative stability of the landscape from ca 50 30 ma signifi cant unroofi ng of the southwestern plateau interior occurred between ca 28 and 16 ma additional denudation north of the grand canyon took place in latest tertiary time mid tertiary dates from the grand canyon basement at the bottom of the upper granite gorge limit signifi cant incision of the modern grand canyon below the kaibab surface to 1500 m difference in vertical structural position if these models are correct they indicate that a proto grand canyon of kilometer scale depth had incised post paleozoic strata by the early eocene evidence for kilometerscale mid tertiary relief in northeast fl owing drainages along the plateau margin as well as the mid tertiary episode of plateau interior unroofi ng imply that the southwestern plateau interior had attained substantial elevation by at least 25 20 ma if not much earlier these observations are inconsistent with any model calling for exclusively late tertiary uplift of the southwestern plateau sevier laramide plateau surface uplift and incision thus result from one or more processes that enhanced the buoyancy of the plateau lithosphere expanding the cordillera s orogenic highlands into its lowstanding cratonic foreland the onset of the laramide slab s demise at ca 40 ma and the major pulse of extension in the basin and range from ca 16 10 ma appear to have had little infl uence on the denudation history of the southwestern plateau in contrast the post laramide unroofi ng episodes may be explained by drainage adjustments induced by rift related lowering of regions adjacent to the plateau without the need to otherwise modify the plateau lithosphere our data do not preclude a large component of post early eocene elevation gain or the geodynamic mechanisms it may imply but they do point toward laramide age buoyancy sources as the initial cause of signifi cant surface uplift ending more than 500 m y of residence near sea level

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

paleocene lakes of the central rocky mountains western united states abstract lacustrine environments are well developed in the paleocene to early eocene sedimentary sequences in the intermontane basins of the central rocky mountains the available evidence points toward a widespread episode of lake formation in the middle late paleocene torrejonian tiffanian followed by a second major period during the latest paleocene to early eocene clarkforkian wasatchian the earlier period of lake formation is dominated by relatively shallow although often areally extensive freshwater lakes associated with fluvial and paludal environments in basins in northern wyoming and montana a high clastic input prevailed thich mudstone often structureless marks the deep water facies of these lakes with tabular sandstone lenticular carbonate and lignite occurring along the fluviodeltaic margins farther south and west in colorado and wyoming alkaline lakes dominated by carbonate deposition were prevalent the appearance and disappearance of standing water in both northern and southern basins may be more closely controlled by tectonics than by climate most evidence points toward prevailing humid sub tropical conditions until the very end of the paleocene

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

cenozoic paleogeography of the central mogollon rim southern colorado plateau region arizona revealed by tertiary gravel deposits oligocene to pleistocene lava flows and incised streams eocene to pleistocene paleogeography of the mogollon rim southern colorado plateau region in central arizona is interpreted by use of tertiary sedimentary deposits and mafic lavas that have been correlated on the bases of provenance of pebbles stratigraphic sequence geomorphic positions of deposits and flows published k ar ages tracing of lava flows to vents and lava textures and structures five categories of tertiary sedimentary deposits are 1 paleocene to oligocene fluvial and lacustrine deposits on the margin of the colorado plateau that were derived from local and distant southern sources 2 oligocene to miocene deposits in valleys at the base of the mogollon rim that were derived from local and distant southern sources 3 oligocene to pliocene locally derived gravel on pediments and in canyons along the rim 4 miocene locally derived lag gravel beneath sheet flows on the plateau and 5 miocene to pleistocene locally derived channel filling gravel on the plateau mafic lavas used are 1 upper oligocene to lower miocene basalt and minette in the transition zone 2 miocene pahoehoe sheet flows on the colorado plateau and draped over the mogollon rim 3 middle miocene to pleistocene cones and flows of basalt scattered across the region and 4 pliocene to pleistocene basalts of the mormon and san francisco volcanic fields basalt sheet flows in ten middle ca 12 ma and upper ca 6 ma miocene flow fields traced to 12 low profile vent structures cover nearly 30 of a 9000 km2 area of the central mogollon rim region and depict the paleogeography at the times of eruption principal conclusions are 1 the mogollon rim developed near its present position after precambrian rocks in central arizona had been unroofed during the paleocene to eocene epochs and before late oligocene to early miocene mafic volcanism 2 after erosional quiescence during the paleogene triassic strata were stripped off the southern colorado plateau in a miocene to pleistocene erosional sweep from the southwest to the northeast 3 incision of meanders on the mogollon slope occurred in the late pliocene to pleistocene epochs as a result of integration of the little colorado river with the colorado river in the grand canyon

a proposed laramide proto grand canyon abstract the absence of rim gravels north of grand canyon and of canaan peak type gravels south of grand canyon suggests that a paleocanyon which intersected the transport of these gravels north and south may have begun forming in the laramide in approximately the same position as today s central grand canyon this laramide age canyon is envisioned as having flowed generally from the sw to ne from the peach springs canyon area to mile 197 where it was captured by karst then along a n 60 e joint system to the kanab point area where it converged with drainage coming off the west side of the kaibab arch from there it flowed north along the west flank of the kaibab arch to paleogene lake claron the critical idea suggested by this proposed model is that the modern colorado river utilized laramide paleotopography in establishing its course through the central grand canyon with younger sections of the canyon integrating with it later in the middle to late miocene this paleocanyon route in association with headward erosion from the grand wash cliffs toward the kaibab arch after 16 17 ma helps account for the total volume of rock eroded from grand canyon which cannot be explained by present day incision rates

comment on apatite 4he 3he and u th he evidence for an ancient grand canyon flowers and farley reports 21 december 2012 p 1616 published online 29 november 2012 propose that the grand canyon is 70 million years old starkly contrasting models for the age of the grand canyon 70 versus 6 million years can be reconciled by a shallow paleocanyon that was carved in the eastern grand canyon 25 to 15 million years ago ma negating the proposed 70 ma and 55 ma paleocanyons cooling models and geologic data are most consistent with a 5 to 6 ma age for western grand canyon and marble canyon

synorogenic evolution of large scale drainage patterns isotope paleohydrology of sequential laramide basins in the past decade we and others have compiled an extensive dataset of o c and sr isotope stratigraphies from sedimentary basins throughout the paleogene north american cordillera in this study we present new results from the piceance creek basin of northwest colorado which record the evolving hydrology of the eocene green river lake system we then place the new data in the context of the broader cordilleran dataset and summarize implications for understanding the synorogenic evolution of large scale drainage patterns the combined data reflect 1 a period of throughgoing foreland rivers heading in the sevier fold and thrust belt and flowing east 2 ponding of freshwater lakes in the foredeep as laramide uplifts blocked drainage 3 hydrologic closure that led to both intensive evaporation in the terminal sink of the piceance creek basin and integration of catchments over length scales 1000 km 4 infilling of basin accommodation by southward migrating magmatism in distal catchments leading to the freshening and demise of intraforeland lakes that also stepped south over time

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

paleomagnetism and tectonic rotation of the lower miocene peach springs tuff colorado plateau arizona to barstow california we have determined remanent magnetization directions of the lower miocene peach springs tuff at 41 localities in western arizona and southeastern california an unusual northeast and shallow magnetization direction confirms the proposed geologic correlation of isolated outcrops of the tuff from the colorado plateau to barstow california a distance of 350 km the peach springs tuff was apparently emplaced as a single cooling unit about 18 or 19 ma and is now exposed in 4 tectonic provinces west of the plateau including the transition zone basin and range colorado river extensional corridor and central mojave desert strike slip zone as such the tuff is an ideal stratigraphic and structural marker for paleomagnetic assessment of regional variations in tectonic rotations about vertical axes from 4 sites on the stable colorado plateau we have determined a reference direction of remanent magnetization i 36 4 d 33 0 95 3 4 that we interpret as a representation of the ambient magnetic field at the time of eruption a steeper direction of magnetization i 54 8 d 22 5 95 2 3 was observed at kingman where the tuff is more than 100 m thick and similar directions were determined at 7 other thick exposures of the peach springs tuff the steeper component is presumably a later stage magnetization acquired after prolonged cooling of the ignimbrite when compared to the plateau reference direction tilt corrected directions from 3 of 6 sites in the central mojave strike slip zone show localized rotations up to 13 in the vicinity of strike slip faults the other three sites show no significant rotations with respect to the colorado plateau both clockwise and counterclockwise rotations were measured and no systematic regional pattern is evident our results do not support kinematic models which require consistent rotation of large regions to accommodate the cumulative displacement of major post middle miocene strike slip faults in the central mojave desert most of our sites in the transition zone and basin and range province have had no significant rotation although small counterclockwise rotation in the mccullough and new york mountains may be related to sinistral shear along en echelon faults southwest of the lake mead shear zone the larger rotations occur in the colorado river extensional corridor where 8 of 14 sites show rotations ranging from 37 clockwise to 51 counterclockwise these rotations occur in allochthonous tilt blocks which have been transported northeastward above the chemehuevi whipple mountains detachment fault upper plate blocks within 1 km of the exposed detachment unexpectedly show no significant rotation from this relation we infer that rotations are accommodated along numerous low angle faults at higher structural levels above the detachment surface

reverse drag revisited why footwall deformation may be the key to inferring listric fault geometry abstract although reverse drag the down warping of hanging wall strata toward a normal fault is widely accepted as an indicator of listric fault geometry previous studies have shown that similar folding may form in response to slip on faults of finite vertical extent with listric or planar geometry in this study we therefore seek more general criteria for inferring subsurface fault geometry from observations of near surface deformation by directly comparing patterns of displacement stress and strain around planar and listric faults as predicted by elastic boundary element models in agreement with previous work we find that models with finite planar planar detached and listric detached faults all develop hanging wall reverse drag folds all of these model geometries also predict a region of tension and elevated maximum coulomb stress in the hanging wall suggesting that the distribution and orientation of near surface joints and secondary faults may also be of limited utility in predicting subsurface fault geometry the most notable difference between the three models however is the magnitude of footwall uplift footwall uplift decreases slightly with introduction of a detachment and more significantly with the addition of a listric fault shape a parametric investigation of faults with constant slip ranging from nearly planar to strongly listric over depths from 1 to 15 km reveals that footwall fold width is sensitive to fault geometry while hanging wall fold width largely reflects fault depth application of a graphical approach based on these results as well as more complete inverse modeling illustrates how patterns of combined hanging wall and footwall deformation may be used to constrain subsurface fault geometry

provenance of quartzite clasts in the eocene oligocene sespe formation paleogeographic implications for southern california and the ancestral colorado river quartzite clasts in fluvial sespe formation conglomerates of the santa monica mountains were compared petrographically with potential source rocks in the basin and range and transition zone provinces to determine their provenance modal analysis indicates that sespe orthoquartzite clasts are mainly quartzofeldspathic and derived from source rocks with craton interior provenance such clasts were probably derived from stirling quartzite and wood canyon formation sources in the northern mojave desert the mazatzal peak quartzite of central arizona is the probable source of quartzolithic orthoquartzite clasts containing abundant detrital jasper grains sespe metaquartzite clasts are mainly from sources in the southern mojave desert but some may be from the yavapai terrane of central arizona the dual provenance of orthoquartzite clasts suggests that the sespe paleoriver was a bifurcating system tapping both mojave desert and sonora desert sources after compensating for neogene displacement along the san andreas fault a paleogeographic reconstruction shows that there is a general spatial coincidence between sespe and other cenozoic paleodelta deposits around los angeles that contain exotic clasts and the inferred location of the mouth of the colorado river assuming that it existed during the eocene thus the colorado river may be much older than previously thought as old as late paleocene and perhaps was once part of an ancient fluvial connection with the mojave desert interior

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

use of clumped isotope thermometry to constrain the crystallization temperature of diagenetic calcite we describe an approach to estimating the crystallization temperatures of diagenetic calcites using clumped isotope thermometry a paleothermometer based on the 13 c 18 o bond enrichment in carbonates application of this thermometer to calcified gastropod shells and calcite cements in an early eocene limestone from the colorado plateau reveals a record of calcite precipitation and replacement at temperatures varying from 14 to 123 c the early eocene host sediments were never deeply buried but they experienced a significant thermal pulse associated with the emplacement of a late miocene basalt flow the combination of independent constraints on thermal history with clumped isotope thermometry petrographic including cathodoluminescence observations and oxygen isotopic data provides an improved basis for estimation of the temperature and timing of diagenetic events and fluid sources the petrography and calcite 18 o values taken alone suggest that the aragonite to calcite transformation of gastropod shell material occurred simultaneously with early formation of cements and lithification of the matrix in the same sample however addition of clumped isotope thermometry demonstrates that this phase transformation of shell material occurred at temperatures of 94 123 c in a highly rock buffered microenvironment i e with the isotopic composition of fluid buffered by coexisting carbonate millions of years after lithification of the matrix and formation of initial low temperature 14 19 c calcite cements within shell body cavities clumped isotope temperatures in excess of reasonable earth surface conditions recorded by later formed cements demand that cement growth occurred in association with the lava emplacement our results illustrate the potential for clumped isotope thermometry to constrain conditions of diagenesis and guide interpretations that would not be possible on the basis of conventional stable isotopic and petrographic data alone and demonstrate how petrographic characterization of clumped isotope thermometry samples can benefit paleoclimate studies

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

impact of drainage integration on basin geomorphology and landform evolution a case study along the salt and verde rivers sonoran desert usa abstract the salt and verde rivers sonoran desert usa integrated multiple endorheic extensional basins near the start of the quaternary integration began via spillover on both rivers spillover resulted in headward erosion in upstream basins leading to basin wide incision and excavation of large volumes of sediment transported into downstream basins as a result the verde river aggraded to what is now the highest terrace in the lower verde river valley lvrv the lousley hills terrace evidence suggests deposition of the lousley hills terrace fill led to aggradation and backfilling of adjacent pediments ancestral salt river deposits asrd of the combined salt and verde rivers deposited an extensive alluvial fan in the higley basin raising local base level for over 2 million years eventually this led to aggradational piracy of the salt river 0 46 ma that integrated the higley and luke basins consequently this shortened and steepened the salt river resulting in 30 m of aggradation downstream and headward incision upstream headward incision abandoned the asrd and created the sawik stream terrace piedmont pediment and alluvial fan systems that were formerly adjusted to the asrd incised in response since abandonment the asrd floodplain has accumulated a sheet of silt and fine sand from aeolian and local fluvial processes sedimentary metamorphic and granitic pediments that developed in slowly aggrading endorheic basins display evidence of response to base level adjustments resulting from drainage integration processes classic ballena landforms eroding alluvial fans began to form in the lvrv only after drainage integration providing the first known maximum age for the ballena form drainage integration of the salt and verde rivers clearly demonstrates the impact of base level fluctuations on basin scale geomorphology however integration led to very different geomorphic responses in different extensional basins revealing the difficulty of a one size fits all conceptual model of geomorphic response drainage integration

an uplift history of the colorado plateau and its surroundings from inverse modeling of longitudinal river profiles 1 it is generally agreed that a region encompassing the colorado plateau has been uplifted by sub crustal processes admittance calculations tomographic studies and receiver function analyses suggest that dynamic support is generated by some combination of convective upwelling and lithospheric thickness changes notwithstanding advances in our understanding of present day setting uplift rate histories are poorly constrained and debated an improved history will aid discrimination between proposed models here we show that a regional uplift rate history can be obtained by inverting longitudinal river profiles we assume that the shape of a river profile is controlled by uplift rate and moderated by erosion in our model uplift rate is allowed to vary smoothly as a function of space and time upstream drainage area is invariant with time simultaneous inversion of river profiles from the colorado rio grande columbia and mississippi catchments shows that three phases of regional uplift occurred the first phase occurred between 80 and 50 myrs when 1 km of uplift was generated at a rate of 0 03 mm yr a second phase occurred between 35 and 15 myrs when 1 5 km of uplift was generated at a faster rate of 0 06 mm yr a final phase of uplift commenced 5 myrs ago these distinct phases of late cretaceous and oligocene uplift are corroborated by stratigraphic considerations by thermochronometric data and by stratigraphic evidence of periodic clastic efflux delivered into the gulf of mexico an episodic uplift history is consistent with staged removal of thick lithospheric mantle beneath a large region which is currently centered on yellowstone

cretaceous eocene laramide landscape development and oligocene pliocene drainage reorganization of transition zone and colorado plateau arizona landscape development of central and northern arizona can no longer be ascribed mainly to events of miocene and pliocene age new information on the age and distribution of older cenozoic deposits has led to the recognition of a regional cretaceous paleocene surface of erosion that conforms to major elements of the present topography and to the recognition that a formerly thick deposit of gravel accumulated on this regional surface of erosion these relations cast new light on the history of evolution of the landscape and indicate a much greater age for the main landscape elements and a more complicated and prolonged history of erosion and deposition than has been previously supposed the timing of events postulated for development of drainage on the colorado plateau can now be compared and partly reconciled with events recognized in the adjacent closely related mountain region transition zone of central arizona as a consequence of late cretaceous paleocene laramide compression central and northern arizona underwent at least 1200 m of uplift documented by paleochannels cut into erosionally truncated paleozoic strata on the hualapai plateau of the southwestern colorado plateau during this time a highly irregular erosion surface was developed on proterozoic rocks across the transition zone south of the mogollon rim the scarp of the mogollon rim was eroded to its present height 600 900 m and an extensive stripped surface was developed on resistant upper paleozoic strata north of the rim deposition of several hundred meters of paleocene eocene rim gravels derived from highlands south and west of the region followed covering much of the cretaceous paleocene erosion surface nearly complete burial of the rim is suggested by the distribution of remnants of the rim gravels across the erosional scarps and on high plateau areas north of the rim a second increment of uplift apparently occurring in late eocene time and apparently recorded by a series of fission track cooling ages from the marble and grand canyons is inferred to have been responsible for ending deposition of the rim gravels for initiating differential uplift of contemporaneous deposits canaan peak and claron formations to their positions in the high plateaus of central utah and for causing the drainage reorganization required to explain the extensive removal of rim gravels from much of the region a southerly flowing ancestral verde river related to the drainage reorganization removed much of the older gravel cover from the transition zone of central arizona resulting in a younger regional erosion surface having 600 900 m of relief a surface closely approximating the cretaceous paleocene erosion surface late oligocene and early miocene rocks locally rest unconformably on remnants of rim gravels in the transition zone indicating that the second episode of regional erosion had been completed by late oligocene time north of the mogollon rim a west flowing ancestral colorado river is inferred to have become established on the rim gravels draining the interior parts of the colorado plateau and transporting detritus off the plateau exhumation of the mogollon rim and development of 600 900 m of topographic relief in the transition zone by an ancestral verde river system suggests the potential for a comparable coeval entrenchment of an ancestral colorado river in paleozoic strata north of the mogollon rim regional extension and volcanic activity ensued in late oligocene to pliocene time the oligocene erosion surface in the extensional basins of central arizona became largely concealed by miocene and pliocene deposits as the neogene climate became drier in late miocene and pliocene time perennial streams appear to have been lacking transport of detritus appears to have been principally by flash flooding little or no detritus appears to have been removed from the region and much of the precipitation presumably moved by groundwater flow through the deposits of aggradation a coeval episode of aggradation in the grand canyon is suggested by deposits that appear to have once choked much of the canyon if this event parallels the episode of late miocene and pliocene aggradation recorded east south and west of the grand canyon the colorado river could have been incised to its present level by late miocene time a return to wetter conditions in late pliocene time presumably was responsible for renewed erosion and reexcavation of older drainages and basins an understanding of this tertiary structural erosional and depositional history can be important for the geological analysis of geophysical transects across the region

apatite 4he 3he and u th he evidence for an ancient grand canyon the grand canyon is one of the most dramatic features on earth yet when and why it was carved have been controversial topics for more than 150 years here we present apatite 4he 3he thermochronometry data from the grand canyon basement that tightly constrain the near surface cooling history associated with canyon incision 4he 3he spectra for eastern grand canyon apatites of differing he date radiation damage and u th zonation yield a self consistent cooling history that substantially validates the he diffusion kinetic model applied here similar data for the western grand canyon provide evidence that it was excavated to within a few hundred meters of modern depths by 70 million years ago ma in contrast to the conventional model in which the entire canyon was carved since 5 to 6 ma

a cenozoic uplift history of mexico and its surroundings from longitudinal river profiles geodynamic models of mantle convection predict that mexico and western north america share a history of dynamic support we calculate admittance between gravity and topography which indicates that the elastic thickness of the plate in mexico is 11 km and in western north america it is 12 km admittance at wavelengths 500 km in these regions suggests that topography is partly supported by subcrustal processes these results corroborate estimates of residual topography from isostatic calculations and suggest that the amount of north american topography supported by the mantle may exceed 1 km the cenozoic history of magmatism sedimentary flux thermochronometric denudation estimates and uplifted marine terraces imply that north american lithosphere was uplifted and eroded during the last 30 ma we jointly invert 533 mexican and north american longitudinal river profiles to reconstruct a continent scale rock uplift rate history uplift rate is permitted to vary in space and time erosional parameters are calibrated using incision rate data in southwest mexico and the colorado plateau calculated rock uplift rates were 0 15 0 2 mm yr between 25 and10 ma central mexico experienced the highest uplift rates central and southern mexico continued to uplift at 0 1 mm yr until recent times this uplift history is corroborated by independent constraints we predict clastic flux to the gulf of mexico and compare it to independent estimates we tentatively suggest that the loop between uplift erosion and deposition can be closed here mexico s staged uplift history suggests that its dynamic support has changed during the last 30 ma