past rates of accumulation in central west antarctica 1 the spatial variation in ice accumulation is critical to the form and stability of ice sheets in west antarctica knowledge of present ice accumulation rates is restricted to sparse measurements and interpolations across data free regions while information on former ice accumulation is restricted to a small number of ice cores and numerical modeling results here isochronous internal ice sheet layers recorded by airborne radio echo sounding and dated at their stratigraphic position at the byrd ice core site are used to reconstruct the spatial pattern of accumulation across the main west antarctic ice divide for the past 16 000 years mean accumulation calculated for the last 3100 years matches well to present day accumulation rates which is compatible with previous observations on siple dome accumulation between 6400 and 16 000 years ago is calculated to be around half that at present in accordance with recent ice sheet modeling results index terms 1223 geodesy and gravity ocean earth atmosphere interactions 3339 1827 hydrology glaciology 1863 9310 information related to geographic region antarctica 9604 information related to geologic time cenozoic 3349 meteorology and atmospheric dynamics polar meteorology citation siegert m j and a j payne 2004 past rates of accumulation in central west antarctica geophys res lett 31 l12403 doi 10 1029 2004gl020290

mesozoic plate motion history below the northeast pacific ocean from seismic images of the subducted farallon slab the high resolution seismic imaging of subducted oceanic slabs1 2 has become a powerful tool for reconstructing palaeogeography3 the images can now be interpreted quantitatively by comparison with models of the general circulation of the earth s mantle4 here we use a three dimensional spherical computer model of mantle convection5 6 to show that seismic images of the subducted farallon plate provide strong evidence for a mesozoic period of low angle subduction under north america such a period of low angle subduction has been invoked independently to explain rocky mountain uplift far inland from the plate boundary during the laramide orogeny7 the computer simulations also allow us to locate the largely unknown kula farallon spreading plate boundary the location of which is important for inferring the trajectories of suspect terrain across the pacific basin8

mesozoic aseismic ridges on the farallon plate and southward migration of shallow subduction during the laramide orogeny we propose that an aseismic ridge of late jurassic and early cretaceous age on the farallon plate was subducted beneath the unites states cordillera during the laramide orogeny the relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70 40 ma in the existing near trench magmatic arc and laramide uplift and faulting of crystalline basement 1000 1500 km inland from the trench the timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the laramide orogeny and the magmatic lull in the western cordillera because the north northeast trending aseismic ridge was moving northeast relative to the north american plate the point of collision of the aseismic ridge with north america and therefore the locus of shallow subduction migrated southward with time causing the well documented southward migration of the magmatic lull

tectonic history of sumba island indonesia since the late cretaceous and its rapid escape into the forearc in the miocene abstract the island of sumba lies within the sunda banda forearc and contains volcanic plutonic and volcaniclastic rocks that record a history of arc volcanism extending from late cretaceous 80 ma or earlier to oligocene 31 ma time during the late cretaceous sumba formed part of a great indonesian volcanic arc system near southeastern eurasia between late cretaceous and early miocene time sumba remained part of that arc system which ceased to be volcanically active by 31 ma during the late miocene 18 ma sumba as part of the relict arc system was situated near the present site of alor and wetar at 16 ma sumba was torn from the relict arc and began to move at a velocity of 50 mm yr in a west south westerly direction volcanism along the modern banda arc soon followed by 7 ma sumba finally came to rest after having moved 450 km into the forearc we interpret the rapid motion of sumba into the forearc as a discrete block to have been a response to the collision between australia and indonesia in the area of timor the savu basin possibly originated as an arc crestal rift which then opened behind sumba as the latter moved westward over the past 16 my more than 3 km of sediment have been deposited in the savu basin by erosion from the surrounding islands sumba stopped moving to the west south west at 7 ma after reaching approximately its present location since then the island has ceased to be a discrete tectonic fragment and has behaved as an integral part of the forearc during the past 7 my compressional tectonics within the forearc has led to elevation of the rocks exposed on sumba by as much as 4 km

provenance of synorogenic foreland basin strata in southwestern montana requires revision of existing models for laramide tectonism north american cordillera

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

variability in accumulation rates from gpr profiling on the west antarctic plateau isochronal layers in firn detected with ground penetrating radar gpr and dated using results from ice core analyses are used to calculate accumulation rates along a 100 km across flow profile in west antarctica accumulation rates are shown to be highly variable over short distances elevation measurements from global positioning system surveys show that accumulation rates derived from shallow horizons correlate well with surface undulations which implies that wind redistribution of snow is the leading cause of this variability temporal changes in accumulation rate over 25 185 year intervals are smoothed to along track length scales comparable to surface undulations in order to identify trends in accumulation that are likely related to changes in climate results show that accumulation rates along this profile have decreased in recent decades which is consistent with core derived time series of annual accumulation rates measured at the two ends of the radar profile these results suggest that temporal variability observed in accumulation rate records from ice cores and gpr profiles can be obscured by spatial influences although it is possible to resolve temporal signals if the effects of local topography and ice flow are quantified and removed

mantle plumes and continental tectonics abstract it is now recognised that plumes originate from a thermal boundary layer deep in the mantle a new plume consists of a large head with a diameter of approximately 800 to 1200 km and it is followed by a smaller tail which has a diameter of 100 to 200 km at the top of its ascent the head flattens to form a disk 2000 to 2500 km in diameter that may melt to produce a continental flood basalt or oceanic plateau the tail melts to produce chains of volcanic activity such as the hawaiian emperor chain the ascent of a new plume leads to surface uplift of 500 to 1000 m over a length scale of 2000 km this places the lithosphere under tension but the horizontal forces produced do not result in runaway extension unless the continental lithosphere is already anomalously hot and therefore weak if the lithosphere is cold and strong slow extension leading to a new spreading ridge may postdate plume arrival by 10 to 40 m y the time difference being the time taken to conduct heat from the plume to the overlying lithosphere plume heads may control the timing and position of new ocean ridges but are not the fundamental force that drives plate tectonics the heat from plume tails may weaken the overlying plate and pin the position of spreading the ascent of a mantle plume can also have an important influence on subduction large oceanic plateaus such as the ontong java plateau are more buoyant than normal oceanic crust because they have a thicker basaltic layer and because they are underlain by a 200 km thick layer of hot material from a mantle plume head as a consequence oceanic plateaus are more difficult to subduct than normal oceanic crust this can lead to jamming of plates at subduction zones resulting in a flip in the sense of subduction as appears to have happened in the caribbean region or to obduction of the plateau finally when a plume head is emplaced beneath continental crust heat released from the plume may lead to melting of the overlying crust the granites associated with many greenstone belts and intercontinental magmatic provinces such as the paleozoic lachlan fold belt of eastern australia and the middle proterozoic anorogenic granites of the central and southwestern united states may have formed in this way

the fold analysis challenge a virtual globe based educational resource abstract we present an undergraduate structural geology laboratory exercise using the google earth virtual globe with collada models optionally including an interactive stereographic projection and javascript controls the learning resource challenges students to identify bedding traces and estimate bedding orientation at several locations on a fold to fit the fold axis and axial plane to stereographic projection data and to fit a doubly plunging fold model to the large scale structure the chosen fold is the sheep mountain anticline a laramide uplift in the big horn basin of wyoming we take an education research based approach guiding students through three levels of difficulty the exercise aims to counter common student misconceptions and stumbling blocks regarding penetrative structures it can be used in preparation for an in person field trip for post trip reinforcement or as a virtual field experience in an online only course our kml scripts can be easily transferred to other fold structures around the globe

the ups and downs of north america evaluating the role of mantle dynamic topography since the mesozoic the driving force for transient vertical motions of earth s surface remains an outstanding question a main difficulty lies in the uncertain role of the underlying mantle especially during the geological past here i review previous studies on both observational constraints and physical mechanisms of north american topographic evolution since the mesozoic i first summarize the north american vertical motion history using proxies from structural geology geochronology sedimentary stratigraphy and geomorphology based on which i then discuss the published physical models overall there is a progressive consensus on the contribution of mantle dynamic topography due to buoyancy structures associated with the past subduction at the continental scale a largely west to east migrating deformation pattern suggests an eastward translation of mantle dynamic effects consistent with models involving an eastward subduction and sinking of former farallon slabs since the cretaceous among the existing models the inverse model based on an adjoint algorithm and time dependent data constraints provides the most extensive explanations for the temporal changes of north american topography since the mesozoic at regional scales debates still exist on the predicted surface subsidence and uplift within both the western and eastern united states where discrepancies are likely due to differences in model setup e g mantle dynamic properties and boundary conditions and the amount of time dependent observational constraints toward the development of the next generation predictive geodynamic models new research directions may include 1 development of enhanced data assimilation capabilities 2 exploration of multiscale and multiphysics processes and 3 cross disciplinary code coupling

airborne radar and ice core observations of annual snow accumulation over thwaites glacier west antarctica confirm the spatiotemporal variability of global and regional atmospheric models 1 we use an airborne radar method verified with ice core accumulation records to determine the spatiotemporal variations of snow accumulation over thwaites glacier west antarctica between 1980 and 2009 we also present a regional evaluation of modeled accumulation in antarctica comparisons between radar derived measurements and model outputs show that three global models capture the interannual variability well r 0 9 but a high resolution regional model racmo2 has better absolute accuracy and captures the observed spatial variability r 0 86 neither the measured nor modeled accumulation records over thwaites glacier show any trend since 1980 although an increase in accumulation may potentially accompany the observed warming in the region the projected trend is too small to detect over the 30 year record

a probabilistic automated isochrone picking routine to derive annual surface mass balance from radar echograms the surface mass balance smb of west antarctica is an important glaciological input to understanding polar climate and sea level rise but with historically poor in situ data coverage previous studies demonstrate the utility of frequency modulated continuous wave radar to image subsurface layering in ice sheets providing an additional source of data with which to estimate smb traditional methods however require time intensive manual oversight here we present a probabilistic fully automated approach to estimate annual smb and uncertainties from radar echograms using successive peak finding and weighted neighborhood search algorithms with the monte carlo simulations based on annual layer likelihood scores we apply this method to ground based and airborne radar in a 175 km transect of the west antarctic interior and compare the results to traditional manual methods and independent estimates from firn cores the method demonstrates an automated estimation of smb across a range of accumulation rates 100 450 mm water equivalent per year and layer gradients up to 2 m km based on likelihood weighted f scores automated layer picks have a success rate between 64 and 84 6 compared with manually picked layers for three validation sites dispersed across the region comparisons between the automated smb estimates and independent firn cores show a bias of 24 70 mm water equivalent per year 12 35 water equivalent of the in situ mean accumulation rate although individual core site biases differ this new approach permits the fully automated extraction of annual smb rates and should be broadly and readily applicable to previously collected and ongoing radar data sets across polar regions

break up and seafloor spreading domains in the ne atlantic an updated magnetic anomaly grid of the ne atlantic and an improved database of magnetic anomaly and fracture zone identifications allow the kinematic history of this region to be revisited at break up time continental rupture occurred parallel to the mesozoic rift axes in the south but obliquely to the previous rifting trend in the north probably due to the proximity of the iceland plume at 57 54 ma the new oceanic lithosphere age grid is based on 30 isochrons c from c24n old 53 93 ma to c1n old 0 78 ma and documents ridge reorganizations in the se lofoten basin the jan mayen fracture zone region in iceland and offshore faroe islands updated continent ocean boundaries including the jan mayen microcontinent and detailed kinematics of the eocene present greenland eurasia relative motions are included in this model variations in the subduction regime in the ne pacific could have caused the sudden northwards motion of greenland and subsequent eurekan deformation these events caused seafloor spreading changes in the neighbouring labrador sea and a decrease in spreading rates in the ne atlantic boundaries between major oceanic crustal domains were formed when the european plate changed its absolute motion direction probably caused by successive adjustments along its southern boundary supplementary material figures showing the long wavelength of the nag tec magnetic anomaly grid detailed magnetic anomalies and isochrons and a table documenting aeromagnetic surveys for nag tec magnetic compilation are available at https doi org 10 6084 m9 figshare c 3661925

reconstructing farallon plate subduction beneath north america back to the late cretaceous using an inverse mantle convection model that assimilates seismic structure and plate motions we reconstruct farallon plate subduction back to 100 million years ago models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy requiring that the farallon slab was flat lying in the late cretaceous consistent with geological reconstructions the simulation predicts that an extensive zone of shallow dipping subduction extended beyond the flat lying slab farther east and north by up to 1000 kilometers the limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten the results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized

subduction adjacent to deep continental roots enhanced negative pressure in the mantle wedge mountain building and continental motion abstract subduction near to a deep continental root is modeled with finite elements the root restricts asthenospheric corner flow which increases mantle wedge suction the suction pulls the continental root toward the subduction zone simultaneously promoting trenchward continental motion and compressive tectonics in the lithosphere between the root and the subduction zone in a 2 d model with a root that extends to 250 km depth and lies 300 km from the mantle wedge corner we find two fold increase in mantle wedge suction and nearly a four fold increase in continental compression to 5 10 12 n m compared to a rootless reference model in 3 d modeling we find that distance to the subduction zone is more important than root width in subduction strike direction or reasonable variations in sub lithospheric viscosity structure with the root enhanced suction effects diminishing rapidly as the wedge corner root distance increases beyond 400 km when finite width roots are wider than the distance to the mantle wedge corner and closer than 400 km compression of the continent between the root and the subduction zone is 50 that found in the infinite width root

little ice age cold interval in west antarctica evidence from borehole temperature at the west antarctic ice sheet wais divide 1 the largest climate anomaly of the last 1000 years in the northern hemisphere was the little ice age lia from 1400 1850 c e but little is known about the signature of this event in the southern hemisphere especially in antarctica we present temperature data from a 300 m borehole at the west antarctic ice sheet wais divide results show that wais divide was colder than the last 1000 year average from 1300 to 1800 c e the temperature in the time period 1400 1800 c e was on average 0 52 0 28 c colder than the last 100 year average this amplitude is about half of that seen at greenland summit grip this result is consistent with the idea that the lia was a global event probably caused by a change in solar and volcanic forcing and was not simply a seesaw type redistribution of heat between the hemispheres as would be predicted by some ocean circulation hypotheses the difference in the magnitude of the lia between greenland and west antarctica suggests that the feedbacks amplifying the radiative forcing may not operate in the same way in both regions

structures in the jackass lakes pluton host rock system central sierra nevada california and inferred mid cretaceous farallon north america plate kinematics

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

spatiotemporal variations of jurassic cretaceous magmatism in eastern asia tan lu fault to sw japan evidence for flat slab subduction and slab rollback here we examine spatiotemporal variations of jurassic cretaceous magmatism along a c 1000 km transect across eastern asia including sw japan the korean jiaodong and liaodong peninsulas and eastern jilin province integration of tectonic regime data with age data from igneous rocks in eastern asia from the tan lu fault to sw japan suggests a shallowing of the subduction angle and subsequent flat slab subduction during the jurassic and slab rollback during the early cretaceous the combination of a subducting plateau and root enhanced suction provides the best explanation for the flat slab subduction in the final stage albian of slab rollback the geotectonic setting changed from subduction accretion to a continental arc in the area close to the ancient trench i e the inner zone of sw japan

influence of persistent wind scour on the surface mass balance of antarctica in the antarctic interior assessments of surface mass balance may overestimate accumulation because high winds remove some of the annual snowfall geophysical observations reveal localized zones of persistent wind scour where little or no snow accumulates that are predicted to occur across approximately 5 of the antarctic surface accurate quantification of surface snow accumulation over antarctica is a key constraint for estimates of the antarctic mass balance as well as climatic interpretations of ice core records1 2 over antarctica near surface winds accelerate down relatively steep surface slopes eroding and sublimating the snow this wind scour results in numerous localized regions 200 km2 with reduced surface accumulation3 4 5 6 7 estimates of antarctic surface mass balance rely on sparse point measurements or coarse atmospheric models that do not capture these local processes and overestimate the net mass input in wind scour zones3 here we combine airborne radar observations of unconformable stratigraphic layers with lidar derived surface roughness measurements to identify extensive wind scour zones over dome a in the interior of east antarctica the scour zones are persistent because they are controlled by bedrock topography on the basis of our dome a observations we develop an empirical model to predict wind scour zones across the antarctic continent and find that these zones are predominantly located in east antarctica we estimate that 2 7 6 6 of the surface area of antarctica has persistent negative net accumulation due to wind scour which suggests that across the continent the snow mass input is overestimated by 11 36 5 gt yr 1 in present surface mass balance calculations

farallon plate dynamics prior to the laramide orogeny numerical models of flat subduction abstract the laramide orogeny 80 50 ma was an anomalous period of mountain building in the western united states that occurred more than 1000 km inboard of the farallon plate subduction margin it is widely believed that this orogeny is coincident with a period of flat subhorizontal subduction however the factors that caused the farallon plate to evolve from a normal steep geometry to flat subduction are not well understood three proposed factors are 1 a westward trenchward increase in north america motion 2 an increased slab suction force owing to the presence of thick colorado plateau lithosphere and 3 subduction of a low density oceanic plateau this study uses 2d upper mantle scale numerical models to investigate these factors the models show that trenchward continental motion is the primary control on subduction geometry with decreasing slab dip as velocity increases however this can only create low angle subduction as the farallon plate was old 100 myr and denser than the mantle a transition to flat subduction requires 1 subduction of a buoyant oceanic plateau that includes an 18 km thick crust that does not undergo metamorphic densification and an underlying depleted harzburgite layer and 2 a slab break off at the landward side of the plateau the break off removes the dense frontal slab and flat subduction develops as the buoyant plateau deflects the slab upward the slab suction force has only a minor effect on slab flattening but the thickness of the colorado plateau lithosphere controls the depth of the flat slab with a continental velocity of 4 cm yr and a 400 km wide oceanic plateau flat subduction develops within 15 ma after plateau subduction the flat slab underthrusts the continent at 200 km depth eventually extending 1500 km inboard of the trench

multi beam surveys of the michelson ridge guyots subduction or obduction abstract this is the first graphic presentation of the michelson ridge in its entirety from a total coverage multi beam survey the ridge splits the izu and bonin trenches is comprised of four guyots and is anomalous to the strike of all other ridges heretofore discovered in the pacific ocean the ridge is obducting offscraping onto the philippine plate instead of subducting

martian wrinkle ridge topography evidence for subsurface faults from mola mars orbiter laser altimeter mola topography shows distinct elevation offsets across wrinkle ridges that can be explained simply by subsurface thrust faults mola has provided the first reliable topographic data of wrinkle ridges to test models for their origin although previous work has shown that wrinkle ridges result from compressional folding and faulting of near surface units the role of thrust faulting and its depth penetration have been argued topographic profiles across wrinkle ridges in solis planum lunae planum tempe terra xanthe terra arcadia terra sirenum thaumasia planum arabia terra syrtis major and hesperia planum show characteristic features such as superposed hills crenulations and elevation offsets between the plains on either side of the ridge the characteristic elevation offsets between plains surfaces on either side of the ridges shown by mola are easily explained by subsurface thrust faults that underlie the ridges and produce the offset in solis dorsa wrinkle ridges are 10 20 km wide well resolved by the 300 m spaced mola elevations have a total relief of 80 250 m and have elevation offsets of 50 180 m well above the uncertainty in the mola elevations mola topography shows that the plains decrease in elevation toward the southeast and are deformed into folds or arches not visible in images that are parallel to and in between the ridges the elevations of the plains on either side of the ridges in solis and lunae plana are characteristically down to the southeast and east respectively suggesting that faults beneath the wrinkle ridges offset the plains this offset indicates underlying thrust faults that dip to the northwest and west respectively and systematically lower the southeast side of the plains the observation that the elevation offsets across the ridges are maintained to the next ridge implies that the thrust fault penetrates to depths of tens of kilometers and thus well into the mechanical lithosphere small foreland basement uplifts on earth in which slip on shallowly dipping thrust faults in the crystalline basement is accommodated by folding in the overlying sedimentary rocks appear to be broadly analogous to these groups of wrinkle ridges and suggest that they are examples of thick skinned compressional deformation on mars these observations are not consistent with models for the formation of wrinkle ridges that involve folding of near surface layers only or subsurface thrust faults that flatten into decollements at shallow depths

geodynamic models of terrane accretion testing the fate of island arcs oceanic plateaus and continental fragments in subduction zones 1 crustal growth at convergent margins can occur by the accretion of future allochthonous terranes fats such as island arcs oceanic plateaus submarine ridges and continental fragments using geodynamic numerical experiments we demonstrate how crustal properties of fats impact the amount of fat crust that is accreted or subducted the type of accretionary process and the style of deformation on the overriding plate our results show that 1 accretion of crustal units occurs when there is a weak detachment layer within the fat 2 the depth of detachment controls the amount of crust accreted onto the overriding plate and 3 lithospheric buoyancy does not prevent fat subduction during constant convergence island arcs oceanic plateaus and continental fragments will completely subduct despite having buoyant lithospheric densities if they have rheologically strong crusts weak basal layers representing pre existing weaknesses or detachment layers will either lead to underplating of faulted blocks of fat crust to the overriding plate or collision and suturing of an unbroken fat crust our experiments show that the weak ultramafic layer found at the base of island arcs and oceanic plateaus plays a significant role in terrane accretion the different types of accretionary processes also affect deformation and uplift patterns in the overriding plate trench migration and jumping and the dip of the plate interface the resulting accreted terranes produced from our numerical experiments resemble observed accreted terranes such as the wrangellia terrane and klamath mountain terranes in the north american cordilleran belt

southwestward weakening of wyoming lithosphere during the laramide orogeny the mechanism of laramide deformation in the central rocky mountains remains enigmatic it is generally agreed that the deformation resulted from low angle subduction of the farallon plate beneath the north american plate during the latest cretaceous early eocene however recent studies have suggested the importance of slab removal or slab rollback in causing this deformation here we infer wyoming lithosphere structure and surface deformation pattern by conducting 2 d flexural subsidence modeling in order to provide constraints on the mechanism of laramide deformation we assume that wyoming lithosphere behaved as an infinite elastic plate subject to tectonic loading of mountain ranges and conduct 2 d flexural subsidence modeling to major laramide basins to document lithospheric stiffness and mountain load height our results show that the stiffness of wyoming lithosphere varied slightly in each basin during the 30 myr duration of the laramide deformation and decreased from northeastern wyoming te 32 46 km to southwestern wyoming te 6 9 km our results also imply that the increase of equivalent load height of major laramide ranges accelerated during the early eocene we propose that the bending stresses induced by the topographic load of the sevier fold and thrust belt combined with crust mantle decoupling initiated by the overthickened sevier hinterland and the end loads due to the low angle subduction at the western edge of the thick wyoming craton have caused the southwestward decrease of lithospheric stiffness in wyoming moreover we attribute the accelerated load height gain during the early eocene to both dynamic and isostatic effects associated with slab rollback

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

dynamic topography and vertical motion of the u s rocky mountain region prior to and during the laramide orogeny dynamic topography of earth s surface occurs in response to hydrodynamic stresses due to mantle flow beneath a flexible lithosphere here we compare the predicted dynamic topography from an inverse convection model that includes flat slab subduction with the known geologic history of the western united states from late cretaceous through paleocene time to evaluate the validity of the model results downwarping behind the evolving cordilleran volcanic arc took place after its inception in late triassic time culminating in the formation of the western interior seaway in late cretaceous time subsidence behind arcs is a consistent prediction of most models of dynamic topography a more detailed convection model is evaluated here by comparing it to the geologic history of the central rocky mountain region from 100 to 50 ma the match of predicted and observed tectonic subsidence is good up until the time that local deformation by the laramide orogeny begins this is most likely due to local flexural effects of mountain building overwhelming regional dynamic effects maps for successive time intervals show that the model matches the geologic history quite well through time and space in particular the subduction of the previously postulated oceanic plateau the conjugate shatsky rise has a significant impact on surface movements 1 from 95 to 88 ma the locations of most significant regional unconformities match the eastward migration of a zone of high topography 2 from 90 to 85 ma the zone of maximum subsidence coincides with the motion of the leading edge of the conjugate shatsky rise 3 from 78 to 60 ma the site of initiation of laramide deformation migrates coincidently with the position of the center of the conjugate shatsky rise 4 from 70 to 60 ma the time transgressive deposition of thin fluvial conglomerate units in the southern part of the study area is generally coincident with surface uplift caused by the trailing part of the conjugate shatsky rise these results suggest that the inverse model approximates the vertical motion history quite well synchroneity of the position of the center of the passing conjugate shatsky rise the landward limit of the evolving cordilleran volcanic arc and the initiation of laramide deformation suggests that the farallon plate became coupled with the overlying north american plate as the subducted oceanic plateau passed beneath progressively enhanced mechanical coupling between the plates was likely the impetus for laramide shortening this comparison of model results with surface geologic history provides a means to validate but not verify predictions of dynamic mantle flow models

was the laramide orogeny related to subduction of an oceanic plateau numerous models have been presented to explain the late cretaceous early cenozoic laramide orogeny which affected the foreland region of the western cordillera within the us1 the most attractive models invoke low angle subduction2 4 which can develop for various reasons5 6 evidence from south america indicates that one such reason may be the subduction of buoyant ocean floor we here extend the low angle subduction models by attributing the shallowing of the subduction angle during the laramide orogeny to subduction of a large oceanic plateau of anomalously thick and buoyant oceanic crust pacific plate history indicates that a twin to the hess rise may represent the oceanic plateau that triggered laramide events

the diagenetic destruction of porosity in plutoniclastic petrofacies the miocene diligencia and eocene maniobra formations orocopia mountains southern california usa

are diamond bearing cretaceous kimberlites related to low angle subduction beneath western north america abstract diamond bearing cretaceous kimberlites of western north america were emplaced 1000 1500 km inboard of the farallon plate subduction margin and overlap with the development of the western interior seaway shut down of the sierra nevada arc and the laramide orogeny these events are consistent with a decrease in subduction angle along much of the margin which placed the subducted farallon plate in close proximity to the continental interior at the time of kimberlite magmatism our numerical models demonstrate that low angle subduction can result from high plate convergence velocities and enhanced westward motion of north america further rapid subduction allows hydrous minerals to remain stable within the cool interior of the subducting plate to more than 1200 km from the trench destabilization of these minerals provides a fluid source that can infiltrate the overlying material potentially triggering partial melting and kimberlite lamproite magmatism

intra oceanic subduction shaped the assembly of cordilleran north america the western quarter of north america consists of accreted terranes crustal blocks added over the past 200 million years but the reason for this is unclear the widely accepted explanation posits that the oceanic farallon plate acted as a conveyor belt sweeping terranes into the continental margin while subducting under it here we show that this hypothesis which fails to explain many terrane complexities is also inconsistent with new tomographic images of lower mantle slabs and with their locations relative to plate reconstructions we offer a reinterpretation of north american palaeogeography and test it quantitatively collision events are clearly recorded by slab geometry and can be time calibrated and reconciled with plate reconstructions and surface geology the seas west of cretaceous north america must have resembled today s western pacific strung with island arcs all proto pacific plates initially subducted into almost stationary intra oceanic trenches and accumulated below as massive vertical slab walls above the slabs long lived volcanic archipelagos and subduction complexes grew crustal accretion occurred when north america overrode the archipelagos causing major episodes of cordilleran mountain building a new explanation for the origin of the accreted terranes that form the mountainous cordillera of western north america is proposed and tested stationary intra oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface which were overridden by and accreted to north america during cretaceous times the generally accepted account of how the geology of western north america developed involves the oceanic farallon plate acting as a conveyor belt sweeping crustal terranes onto the continental margin as it moves beneath the north american plate this model fails to explain many aspects of this accreted terrane and here karin sigloch and mitchell mihalynuk present an alternative model based on new tomographic images of lower mantle slabs and modelling of deep mantle structure in the region they propose that during the cretaceous period there were a series of island arcs west of north america beneath which proto pacific plates subducted into almost stationary trenches long lived volcanic archipelagos and subduction complexes grew above the slabs and then accreted onto north america as it overrode the archipelagos causing episodes of cordilleran mountain building

late cretaceous early eocene laramide uplift exhumation and basin subsidence in wyoming crustal responses to flat slab subduction low angle subduction of the farallon oceanic plate during the late cretaceous early eocene is generally considered as the main driver forming the high rocky mountains in wyoming and nearby areas how the deformation was transferred from mantle to upper crust over the great duration of deformation 40 myr is still debated here we reconstruct basin subsidence and compile paleoelevation thermochronology and provenance data to assess the timing magnitude and rates of rock uplift during the laramide deformation we reconstruct rock uplift as the sum of surface uplift and erosion constrained by combining paleoelevation and exhumation with regional stratigraphic thickness and chronostratigraphic information the amount and rate of rock uplift of individual laramide ranges was less than 2 4 4 8 km 0 21 0 32 mm yr during the early maastrichtian paleocene stage 1 and increased to more than 3 km 0 38 0 60 mm yr during the late paleocene early eocene stage 2 our quantitative constraints reveal a two stage development of the laramide deformation in wyoming and an increase of rock uplift during stage 2 associated with enhanced intermontane basin subsidence exhumation and uplift during stage 1 is consistent with eastward migration of cordilleran deformation associated with low angle subduction whereas the change in exhumation during stage 2 seems to follow a southwestward trend which requires an alternative explanation we here suggest that the increase of rock uplift rate during the late paleocene early eocene and the southwestward younging trend of uplift may be a response to the rollback and associated retreating delamination of the farallon oceanic slab

global eocene tectonic unrest possible causes and effects around the north american plate abstract many of our planet s crises were the result of sudden changes in plate tectonic configuration or catastrophic outbursts of volcanism caused by mantle plume impingement at the base of the lithosphere at the paleocene eocene boundary and in the early eocene several mantle plumes continental collision and mid ocean ridge subduction triggered a series of changes in seafloor spreading dynamics we have constructed a detailed global model of oceanic lithosphere age and spreading rates for the 60 to 35 ma interval we revise evidence for changes in seafloor spreading direction in the north atlantic arctic and ne pacific oceans at least two periods of spreading rate highs which are separated by sharp value decrease occurred along the entire eastern north american plate boundary from c25 to c18 time c 57 to 40 ma the collision and incipient subduction of the early eocene siletzia oceanic lip may have caused the sharp decrease in spreading rate at c23 time in the labrador sea and north of charlie gibbs fracture zone the post c23 rapid farallon slab break off and subsequent upper mantle flow upwelling may have led to further variations in north atlantic spreading rates at c22 21 time eastward pacific subduction may have resumed at c 43 ma as indicated by a steady ne pacific seafloor spreading regime which resumed at or shortly after c21 the north atlantic realm shows a delayed response to tectonic events west of north america with an increase in spreading rate south of charlie gibbs fracture zone from c20 to c18 time followed by a steady decrease until the oligocene north american late paleocene early eocene kimberlite magma that erupted more than 1000 km from its western plate boundary constitutes additional evidence that tectonic stresses due to changes in the mantle lithosphere interactions may have affected the entire plate and therefore also its eastern boundaries

seasonal variations of glaciochemical isotopic and stratigraphic properties in siple dome antarctica surface snow six snow pit records recovered from siple dome west antarctica during 1994 are used to study seasonal variations in chemical major ion and h 202 isotopic deuterium and physical stratigraphic properties during the 1988 94 period comparison of d measurements and satellite derived brightness temperature for the siple dome area suggests that most seasonal sd maxima occur within 4 weeks of each 1 january several other chemical species h 2o2 non sea salt nss so 4 2 methanesulfonic acid and no3 show coeval peaks with sd together providing an accurate method for identifying summer accumulation sea salt derived species generally peak during winter spring but episodic input is noted throughout some years no reliable seasonal signal is identified in species with continental sources nssca 2 nss mg 2 nh 4 or nsscl visible strata such as large depth hoar layers 5 cm are associated with summer accumulation and its metamorphosis but smaller hoar layers and crusts are more difficult to interpret a multi parameter approach is found to provide the most accurate dating of these snow pit records and is used to determine annual layer thicknesses at each site significant spatial accumulation variability exists on an annual basis but mean accumulation in the sampled 10 km 2 grid for the 1988 94 period is fairly uniform

tectono magmatic evolution of late jurassic to early cretaceous granitoids in the west central lhasa subterrane tibet abstract there is ongoing debate as to the subduction direction of the bangong nujiang ocean during the mesozoic northward southward or bidirectional subduction arc related intermediate to felsic intrusions could mark the location of the subduction zone and more importantly elucidate the dominant geodynamic processes we report whole rock geochemical and zircon u pb and hf isotopic data for granitoids from the west central lhasa subterrane e80 to e86 all rocks show metaluminous to peraluminous calc alkaline signatures with strong depletion of nb ta and ti enrichment of large ion lithophile elements e g cs rb k a negative correlation between sio 2 and p 2 o 5 and a positive correlation between rb and th all these features are indicative of i type arc magmatism new zircon u pb results together with data from the literature indicate continuous magmatism from the late jurassic to the early cretaceous 160 to 130 ma zircon u pb ages for samples from the northern part of the west central lhasa subterrane e80 to e82 30 yielded formation ages of 165 to 150 ma whereas ages of 142 to 130 ma were obtained on samples from the south this suggests flat or low angle subduction of the bangong nujiang ocean consistent with a slight southward decrease in zircon ehf t values for late jurassic rocks considering the crustal shortening the distance from the bangong nujiang suture zone and a typical subduction zone melting depth of 100 km the subduction angle was less than 14 for late jurassic magmatism in the central lhasa interior consistent with flat or low angle subduction compared with late jurassic rocks main ehf t values of 16 to 7 early cretaceous rocks 145 to 130 ma show markedly higher ehf t values mainly 8 to 0 possibly indicating slab roll back likely caused by slab foundering or break off combined with previously published works on arc magmatism in the central lhasa and west part of the southern qiangtang subterranes our results support the bidirectional subduction of the bangong nujiang ocean along the bangong nujiang suture zone and indicates flat or low angle southward subduction 165 to 145 ma followed by slab roll back 145 to 130 ma

climate variability in west antarctica derived from annual accumulation rate records from itase firn ice cores thirteen annually resolved accumulation rate records covering the last 200 years from the pine island thwaites and ross drainage systems and the south pole are used to examine climate variability over west antarctica accumulation is controlled spatially by the topography of the ice sheet and temporally by changes in moisture transport and cyclonic activity a comparison of mean accumulation since 1970 at each site to the long term mean indicates an increase in accumulation for sites located in the western sector of the pine island thwaites drainage system accumulation is negatively associated with the southern oscillation index soi for sites near the ice divide and periods of sustained negative soi 1940 42 1991 95 correspond to above mean accumulation at most sites correlations of the accumulation rate records with sea level pressure slp and the soi suggest that accumulation near the ice divide and in the ross drainage system may be associated with the mid latitudes the post 1970 increase in accumulation coupled with strong slp accumulation rate correlations near the coast suggests recent intensification of cyclonic activity in the pine island thwaites drainage system

onset of deglacial warming in west antarctica driven by local orbital forcing an annually resolved ice core record from west antarctica indicates that warming driven by local insolation resulting from sea ice decline began in that region about 2 000 years before warming in east antarctica reconciling two alternative explanations for deglacial warming in the southern hemisphere

reconstructing the ancestral yellowstone plume from accreted seamounts and its relationship to flat slab subduction recent geodynamic analyses have emphasized the relationship between modern flat slab subduction zones and the overriding of buoyant oceanic crust although most models for the evolution of the late mesozoic cenozoic laramide orogeny in the southwestern united states involve flat slab subduction the mechanisms proposed are controversial an examination of the geological evolution of the 60 50 ma crescent terrane of the coast ranges indicates that it was formed in a shallowing upward loihi type oceanic setting culminating in the eruption of subaerial lavas plate reconstructions indicate that the crescent terrane was emplaced into ca 20 ma crust and the presence of subaerial lavas implies an uplift due to the plume of ca 4 2 km which we use to calculate a minimum buoyancy flux of 1 1 mg s 1 similar to that of the modern yellowstone plume published paleomagnetic data indicate that the crescent terrane was formed at a paleolatitude similar to that of the yellowstone plume the crescent seamount was accreted within 5 my of the cessation of plume magmatism plate reconstructions indicate that it would have originated about 750 km to the west of the north american plate margin if it developed above a fixed yellowstone plume and are therefore consistent with the recorded very short interval between its formation and tectonic emplacement we interpret the crescent terrane as due to the ancestral yellowstone plume such a plume would have generated an elongate swell and related plateau that would have been overridden by the north american margin taken together the relationship between flat slab and overriding of oceanic plateau in laramide times would have been analogous to the relationship between modern andean flat slab subduction zones and the juan fernandez and nazca oceanic plateaus d 2003 elsevier science b v all rights reserved

uplift rupture and rollback of the farallon slab reflected in volcanic perturbations along the yellowstone adakite hot spot track field geochemical and geochronological data show that the southern segment of the ancestral cascades arc advanced into the oregon back arc region from 30 to 20 ma we attribute this event to thermal uplift of the farallon slab by the yellowstone mantle plume with heat diffusion decompression and the release of volatiles promoting high k calc alkaline volcanism throughout the back arc region the greatest degree of heating is expressed at the surface by a broad ene trending zone of adakites and related rocks generated by melting of oceanic crust from the farallon slab a hiatus in eruptive activity began at ca 22 20 ma but ended abruptly at 16 7 ma with renewed volcanism from slab rupture occurring in two separate regions the eastern rupture resulted in the extrusion of steens basalt during the ascent and melting of a dry mantle plume source contaminated with depleted mantle the contemporaneous western rupture resulted in renewed subduction melting of a wet mantle source and the rejuvenation of high k calc alkaline volcanism near the nevada california border at 16 7 ma here the initiation of slab rollback is evident in the westward migration of arc volcanism at 7 8 km ma today the uplifted slab is largely missing beneath the oregon back arc region replaced instead by a seismic hole which is bound on the south by the adakite hotspot track we attribute slab destruction to thermal uplift and mechanical dislocation that culminated in rapid tearing of the slab from 17 15 ma and possible foundering and sinking of slab segments from 16 10 ma

bayesian glaciological modelling to quantify uncertainties in ice core chronologies abstract valuable information about the environment and climate of the past is preserved in ice cores which are drilled through ice sheets in polar and alpine regions a pivotal part of interpreting the information held within the cores is to build ice core chronologies i e to relate time to depth existing dating methods can be categorised as follows 1 layer counting using the seasonality in signals 2 glaciological modelling describing processes such as snow accumulation and plastic deformation of ice 3 comparison with other dated records or 4 any combination of these conventionally implementation of these approaches does not use statistical methods in order to quantify dating uncertainties in this paper we develop the approach of category 2 further first the sources of uncertainty involved in glaciological models are formalised feeding these into a statistical framework that we call bayesian glaciological modelling bgm allows us to demonstrate the effect that uncertainty in the glaciological model has on the chronology bgm may also include additional information to constrain the resulting chronology for example from layer counting or other dated records such as traces from volcanic eruptions our case study involves applying bgm to date an antarctic ice core a dyer plateau core working through this example allows us to emphasise the importance of properly assessing uncertain elements in order to arrive at accurate chronologies including valid dating uncertainties valid dating uncertainties in turn facilitate the interpretation of environmental and climatic conditions at the location of the ice core as well as the comparison and development of ice core chronologies from different locations

linking slab break off hellenic trench retreat and uplift of the central and eastern anatolian plateaus abstract the central and eastern anatolian plateaus are integral parts of the world s third largest orogenic plateau in the past decade geophysical surveys have provided insights into the crust lithosphere and mantle beneath eastern anatolia these observations are now accompanied by recent surveys in central anatolia and new data constraining the timing and magnitude of uplift along its northern and southern margins together with predictions from geodynamic models on the effects of various processes on surface deformation and uplift the observations can be integrated to identify probable mechanisms of anatolian plateau growth a changeover from shortening to extension along the southern margin of central anatolia that is coeval with the start of uplift can be most easily associated with oceanic slab break off and tearing this interpretation is supported by tomography deep seismicity or lack thereof and gravity data based on the timing of uplift geophysical and geochemical observations and model predictions slab break off likely occurred first beneath eastern anatolia in middle to late miocene time and propagated westward toward cyprus by the latest miocene alternatively the break off near cyprus could have occurred in late pliocene to early pleistocene time in association with collision of the eratosthenes seamount continental fragment with the subduction zone uplift at the northern margin of central anatolia appears to result from crustal shortening starting in the late miocene or early pliocene which has been linked to the broad restraining bend of the north anatolian fault the uplift history of the interior of central anatolia since the late miocene is unclear although shortening there appears to have ended by the late miocene followed by ne sw extension this change in the deformation style broadly coincides with faster retreat of the hellenic trench as well as uplift of the northern and southern margins of central anatolia these different events throughout the plateau may be linked as faster retreat of the hellenic trench has been predicted to occur after slab break off which could have induced extension of central anatolia and helped to form the north anatolian fault through accelerated westward movement of anatolia relative to eurasia correlative geochronologic evidence that we summarize here supports the hypothesis that the geodynamic activity throughout the aegean anatolian domain starting in latest miocene to early pliocene time defines a series of events that may all be linked to slab break off

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

impact of aseismic ridges on subduction systems insights from analog modeling the influence of aseismic ridges on subduction kinematics and dynamics and on the deformation of the overriding plate remains a topic of debate this study presents laboratory based geodynamic models that simulate the process of aseismic ridge subduction below an overriding plate the analog experiments show that depending on its size subduction of an aseismic ridge can impact on the kinematics of subduction overriding plate deformation and to a lower extent on the geometry of the slab and that of the trench specifically it can cause a reduction of the subducting plate and trench retreat velocities decreases overriding plate extension and even drives local forearc shortening for the widest ridge increases the slab dip angle next to the aseismic ridge but decreases the dip angle at the aseismic ridge in case the ridge is thick and wide and produces a local perturbation indentation of the trench geometry the magnitude of the described modifications relies to a comparable extent on aseismic ridge width and thickness the effects of aseismic ridge subduction observed in our models compare to those of the louisville seamount chain the cocos ridge and the d entrecasteaux ridge system in nature and provide an explanation for the local deformation observed at these subduction zones

a dipping thick segment of the farallon slab beneath central u s it has been hypothesized that much of the crustal deformation attributed to the laramide orogeny of the southwest north american cordillera was caused by dynamic effects induced by the flat subduction of a large oceanic plateau that was embedded within the farallon plate previous studies have identified within the north american mantle a seismic velocity anomaly that plausibly represents the remnants of the subducted plateau coupled plate kinematic and dynamic modeling of the anomaly as well as surface geological findings identify this anomaly as the subducted conjugate to the shatsky rise here we find clear evidence for a northeastward dipping 35 dip thick up to 400 km thick slab like seismic velocity anomaly within the top of the lower mantle below the central united states using a deep focus earthquake below spain we find that the observed seismic waveforms recorded with the dense usarray display multi pathing indicative of sharp surfaces plate tectonic reconstructions in which the anomaly is migrated back in time suggest strong coupling of the plateau thickened slab segment to the overriding north america plate as it was subducted in combination with the reconstructions we interpret the structure as arising from eastward dipping farallon subduction at the western margin of north america during the cretaceous in contrast with some recent interpretations it appears that the plateau area of the slab has been further thickened which might undergo a combination of pure shear bulk shortening during flat slab subduction and or by a folding instability during penetration into the lower mantle

rapid cenozoic subsidence in the gulf of mexico resulting from hess rise conjugate subduction enigmatic surface deflections occurred in north america starting from the cretaceous including the continental scale drainage reorganization and the long wavelength subsidence in the western interior seaway these surface undulations cannot be simply explained by sea level change or flexure loading coinciding with the large scale surface deflection the gulf of mexico gom has an immense paleocene sediment deposition probably caused by tectonic subsidence increasing evidence indicates a distinct seismic anomaly localized in the mantle below the gom with geodynamic models we show that the hess rise conjugate coincides with the position of the seismic anomaly the basalt eclogite transition in the hess conjugate can lead to a localized dynamic subsidence in the gom which is superimposed on the broad surface deflection caused by the farallon slab the hess conjugate transformed to eclogite could tilt the surface southward in the u s and help frame the gom as a main depocenter in the cenozoic

from extension to tectonic inversion mid cretaceous onset of andean type orogeny in the lhasa block and early topographic growth of tibet

high resolution ice cores from us itase west antarctica development and validation of chronologies and determination of precision and accuracy shallow ice cores were obtained from widely distributed sites across the west antarctic ice sheet as part of the united states portion of the international trans antarctic scientific expedition us itase program the us itase cores have been dated by annual layer counting primarily through the identification of summer peaks in non sea salt sulfate nssso4 2 concentration absolute dating accuracy of better than 2 years and relative dating accuracy better than 1 year is demonstrated by the identification of multiple volcanic marker horizons in each of the cores tambora indonesia 1815 being the most prominent independent validation is provided by the tracing of isochronal layers from site to site using high frequency ice penetrating radar observations and by the timing of mid winter warming events in stable isotope ratios which demonstrate significantly better than 1 year accuracy in the last 20 years dating precision to 1 month is demonstrated by the occurrence of summer nitrate peaks and stable isotope ratios in phase with nssso4 2 and winter time sea salt peaks out of phase with phase variation of 1 month dating precision and accuracy are uniform with depth for at least the last 100 years

in search for the missing arc root of the southern california batholith p t t evolution of upper mantle xenoliths of the colorado plateau transition zone abstract xenolith and seismic studies provide evidence for tectonic erosion and eastward displacement of lower crust subcontinental mantle lithosphere lc scml underlying the mojave desert region i e southern california batholith scb intensified traction associated with the late cretaceous flattening of the subducting farallon plate responsible for deforming the sw u s likely played a key role in bulldozing the tectonically eroded lc scml 500 km eastwards to underneath the colorado plateau transition zone cptz and further inboard the garnet clinopyroxenite xenoliths from two cptz localities chino valley and camp creek central arizona provide a rare glimpse of the material underlying the cptz thermodynamic modeling in addition to major and trace element thermobarometry suggests that the xenoliths experienced peak conditions of equilibration at 600 900 c and 12 28 kbar these peak conditions along with the composition of the xenoliths type b garnet and diopsidic clinopyroxene strongly suggest a continental arc residue arclogite rather than a lower plate subduction eclogite origin a bimodal zircon u pb age distribution with peaks at ca 75 and 150 ma and a jurassic sm nd garnet age 154 16 ma with initial end value of 8 overlaps eastern scb pluton ages and suggests a consanguineous relationship cenozoic zircon u pb ages ree geochemistry of zircon grains and partially re equilibrated sm nd garnet ages indicate that displaced arclogite remained at elevated pt conditions 700 c for 10s of myr following its dispersal until late oligocene entrainment in host latite with a 100 myr long thermal history overlapping that of the scb and the cptz these assemblages also contain evidence for late stage hydration e g secondary amphibole potentially driven by de watering of the laramide slab in light of these results we suggest that the cptz arclogite originates from beneath the eastern half of the scb where it began forming in late jurassic time as mafic keel to continental arc magmas the displacement and re affixation of the arclogites further inboard during the late cretaceous flat slab subduction might have contributed to the tectonic stability of the colorado plateau relative to adjacent geologic provinces through laramide time and likely preconditioned the region to cenozoic tectonism e g present day delamination beneath the plateau high magnitude extension and formation of metamorphic core complexes

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

plume related oceanic plateaus as a potential source of gold mineralization we propose that the formation of anomalously gold enriched provinces on opposite sides of the pacific basin at ca 120 ma can be traced back to the formation of a mantle plume related oceanic plateau in the permian fertile fragments of this oceanic plateau travelled across the developing pacific basin and arrived at the accretionary margin of the north china and american cratons respectively some time earlier than 120 ma formation of the oceanic plateau proximal to a midoceanic rise or triple junction implies a long cooling history and reduced buoyancy by the time it arrived at an active subduction zone consequently in this model at least a part of the plateau material could have subducted like normal oceanic crust metamorphic devolatilization caused the release of gold into the overlying crust thereby fertilizing the advective hydrothermal system which gave rise to among others gold provinces in the sierra nevada foothills california and jiaodong peninsula china this process could be a mechanism for the complete temporal and spatial decoupling of the initial mantle plume activity formation of a fertile primitive oceanic source rock and the establishment of a highly endowed gold province at an accretionary margin

the role of oceanic plateau subduction in the laramide orogeny the cause of the laramide phase of mountain building remains uncertain conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant hess or shatsky oceanic plateaux independent verification of these models has remained elusive because the putative ridges or plateaux are no longer at the earth s surface inverse convection models have identified two prominent seismic anomalies on the recovered farallon plate here we combine inverse convection models with reconstructions of plate motions to show that these seismic anomalies coincide palaeogeographically with the restored positions of the shatsky and hess conjugate plateaux as they subducted beneath north america specifically the distribution of laramide crustal shortening events tracked the passage of the shatsky conjugate beneath north america whereas the effects of the hess conjugate subduction were restricted to the northern mexico foreland belt we propose that continued subduction caused the oceanic crust to undergo the basalt eclogite phase transformation during which the shatsky conjugate lost its extra buoyancy and was effectively removed increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward followed by regional scale surface rebound we conclude that laramide uplift resulted from the removal rather than emplacement of the shatsky conjugate

nazca south america interactions and the late eocene late oligocene flat slab episode in the central andes 1 the most prominent features of the andean range are the altiplano and puna plateaus which were constructed by crustal shortening and uplift over the past 45 myr the early construction of these plateaus may have controlled subsequent growth of the orogen proposed models have suggested that an abrupt acceleration in relative motion between the nazca plate and the south american plate at 30 ma may have led to compression of the continent however the major plate motion change occurred at 25 23 ma and paleomagnetic rotations and crustal shortening of the andean forearc require that the arica bend formed prior to about 25 ma inferred history of flat slab subduction along the altiplano section of the andean margin and the structure of the adjacent south american cratonic shield combine to suggest an alternate scenario based partly upon geodynamic models of oceanic continental plate interactions in subduction zones we propose that central andean tectonism may have been controlled by two distinct regimes of subduction 1 oblique subduction along the central andean margin during the late eocene and oligocene accompanied by downdip alignment with the center of the amazonian shield flat slab activity in this phase of orogenesis may have been caused by a combination of cratonic root enhanced tectonics and oceanic plateau subduction and 2 an abrupt transition to trench normal subduction after 25 ma toward the more distal sao francisco craton was accompanied by a return to normal angle subduction similar interactions are hypothesized to have occurred during the laramide orogeny in western north america

eocene to quaternary mafic intermediate volcanism in san luis potosi central mexico the transition from farallon plate subduction to intra plate continental magmatism abstract the san luis potosi volcanic field slpvf of central mexico includes volcanic sequences of felsic intermediate and basic compositions that were erupted as discrete episodes from the eocene to the pleistocene volcanism was dominated by widespread and voluminous rhyolitic ignimbrites of the mid tertiary ignimbrite flare up however the complete volcanic history must consider basaltic and andesitic eocene pleistocene volcanic successions that provide key evidence for understanding the geochemical evolution of the volcanism in the slpvf during this time span five sequences are recognized according to their geochemical characteristics each comprising a volcano tectonic episode the first episode comprises basaltic andesites and andesites erupted during three intervals 45 42 ma 36 31 ma and 31 30 ma the oldest was derived from subduction magmatism whereas the youngest has an intra plate magmatic signature and this represents the transition from the end of a long lasting subduction regime of the farallon plate to the initiation of intra plate continental extension in the north american plate the second episode at 29 5 28 ma comprises a bimodal succession of high silica rhyolites and alkaline basalts hawaiites that are interpreted as magmatism generated in an intra plate continental extension regime during the basin and range faulting the third episode at 21 ma is characterized by trachybasalts and trachyandesites that represent mantle basaltic melts that were contaminated through assimilation of the lower crust during advanced stage of intra plate extension that started at oligocene the fourth episode includes 12 ma alkaline basalts and andesites that were erupted from fissures these mantle derived magmas evolved to andesites by crustal anatexis and crystal fractionation within a continued extensional intra plate regime lastly the fifth episode comprises 5 0 to 0 6 ma alkaline basalts basanites containing mantle xenoliths that were erupted from maars and tuff cones which are the youngest manifestations of mantle derived intra plate extensional events based upon this volcanic record the last subduction manifestations of the extinct farallon plate occurred at about 42 ma this was followed by a transition to intra plate magmatism between 42 and 31 ma and an extensional intra plate tectonic setting from 31 ma to almost present

continent wide drainage reorganization in north america driven by mantle flow abstract north america na experienced pronounced changes in continental scale drainage characterized by a reversal for much of the continental interior from north into the canadian arctic to south into the gulf of mexico gom from the mid cretaceous to the paleocene however the driving mechanism for these profound drainage reorganizations remain unexplained here we investigate the role of mantle flow on landscape evolution by coupling dynamic topography with surface processes this approach enables us to simulate catchment dynamics and the rearrangement of sediment transport in response to mantle flow we show that a west to east drainage reversal can be induced by the na overriding the subducted farallon plate moreover augmented dynamic subsidence caused by a basalt to eclogite transformation of an oceanic plateau within the farallon slab depressed the gom region and expanded the integrated drainage to the gom since the early paleocene for the first time we show that dynamic topography can explain the north to south continental scale drainage reorganization in north america

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research