the lost inca plateau cause of flat subduction beneath peru abstract since flat subduction of the nazca plate beneath peru was first recognized in the 1970s and 1980s a satisfactory explanation has eluded researchers we present evidence that a lost oceanic plateau inca plateau has subducted beneath northern peru and propose that the combined buoyancy of inca plateau and nazca ridge in southern peru supports a 1500 km long segment of the downgoing slab and shuts off arc volcanism this conclusion is based on an analysis of the seismicity of the subducting nazca plate the structure and geochemistry of the marquesas plateau as well as tectonic reconstructions of the pacific farallon spreading center 34 to 43 ma these restore three sub parallel pacific oceanic plateaus the austral tuamotu and marquesas to two farallon plate counterparts the iquique and nazca ridges inca plateau is apparently the sixth and missing piece in an ensemble of v shaped hotspot tracks formed at on axis positions we argue the mirror image of the inca plateau the marquesas plateau is an ancient edifice overprinted by recent volcanism in disagreement with the widely accepted young

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

location location location the variable lifespan of the laramide orogeny the laramide orogeny had a spatially variable lifespan which we explain using a geodynamic model that incorporates onset and demise of at slab subduction laramide shortening and attendant uplift began in southeast california usa at ca 90 ma swept to the northeast to arrive in the black hills of south dakota usa at ca 60 ma and concluded in south dakotawithin 10 m y during subsequent slab rollback the areal extent of laramide deformation decreased as the eastern edge of active deformation retreated to the southwest rapidly from ca 55 to 45 ma and more slowly from ca 45 to 40 ma with deformation ultimately ceasing in the southwestern part of the orogen at ca 30 ma geodynamic modeling of this process suggests that changes in the strength of the north america plate and densifcation of the farallon plate played important roles in controlling the areal extent of the laramide orogen and hence the lifespan of the orogenic event at any particular location in western north america

long term sea level fluctuations driven by ocean basin dynamics earth9s long term sea level history is characterized by widespread continental flooding in the cretaceous period 145 to 65 million years ago followed by gradual regression of inland seas however published estimates of the late cretaceous sea level high differ by half an order of magnitude from 40 to 250 meters above the present level the low estimate is based on the stratigraphy of the new jersey margin by assimilating marine geophysical data into reconstructions of ancient ocean basins we model a late cretaceous sea level that is 170 85 to 270 meters higher than it is today we use a mantle convection model to suggest that new jersey subsided by 105 to 180 meters in the past 70 million years because of north america9s westward passage over the subducted farallon plate this mechanism reconciles new jersey margin based sea level estimates with ocean basin reconstructions

diagnosing an ancient shallow angle subduction event from cenozoic depositional and deformational records in the central andes of southern peru abstract the spectrum of responses exhibited by upper plates to changes in the subduction zone geometry remains poorly constrained in the paleogene the central andes in southern peru potentially experienced an episode of flat or shallow angle subduction presented here are new geochronologic thermochronologic basin subsidence backstripping analysis and 2d flexural modeling results these are synthesized with existing records on the temporal variation of the volcanic arc position basin fill chronostratigraphy deformation style and location and geophysical constraints on lithospheric architecture to investigate how this segment of the andes developed during a proposed ancient modification to the subduction zone dip basin chronostratigraphy confirms persistent contribution from volcanic arc products during the eocene miocene arguing against complete cessation of arc magmatism sediment accumulation was pulsed and generally more rapid during the late eocene oligocene event the subsidence curve is generally concave up atypical for a foreland basin situated within the compressional central andes the tectonic subsidence magnitude observed from backstripping analysis is at least 1 km less than the magnitude of subsidence predicted by 2d flexural model results this suggests a component of basin uplift potentially as a dynamic response to subduction shallowing may have opposed flexural subsidence that was driven by crustal thickening and topographic loading new geo and thermochronologic results constrain the timing of late triassic pluton emplacement and initial unroofing that predates the proposed timing of flat or shallow angle subduction this new timing suggests initial deformation with mixed thin and thick skinned deformation style in the eastern cordillera may be linked to reactivation of inherited structures rather than inboard deformation advance as a response to subduction flattening the lack of basement cored uplifts preservation of mantle lithosphere beneath this segment of the central andes and volcanic arc patterns argue against the occurrence of a flat slab event instead these results point to a shallow angle subduction event and highlight the diverse behaviors of cordilleran margins affected by variable subduction zone geometry and upper plates with inherited crustal heterogeneities

simultaneous inversion of mantle properties and initial conditions using an adjoint of mantle convection through the assimilation of present day mantle seismic structure adjoint methods can be used to constrain the structure of the mantle at earlier times i e mantle initial conditions however the application to geophysical problems is restricted through both the high computational expense from repeated iteration between forward and adjoint models and the need to know mantle properties such as viscosity and the absolute magnitude of temperature or density a priori we propose that an optimal first guess to the initial condition can be obtained through a simple backward integration sbi of the governing equations thus lessening the computational expense given a model with known mantle properties we show that a solution based on an sbi generated first guess has smaller residuals than arbitrary guesses mantle viscosity and the effective rayleigh number are crucial for mantle convection models neither of which is exactly known we place additional constraints on these basic mantle properties when the convection induced dynamic topography on earth s surface is considered within an adjoint inverse method besides assimilating present day seismic structure as a constraint we use dynamic topography and its rate of change in an inverse method that allows simultaneous inversion of the absolute upper and lower mantle viscosities scaling between seismic velocity and thermal anomalies and initial condition the theory is derived from the governing equations of mantle convection and validated by synthetic experiments for both one layer viscosity and two layer viscosity regionally bounded spherical shells for the one layer model at any instant of time the magnitude of dynamic topography is controlled by the temperature scaling while the rate of change of topography is controlled by the absolute value of viscosity for the two layer case the rate of change of topography constrains upper mantle viscosity while the magnitude of dynamic topography determines the temperature scaling lower mantle viscosity when upper mantle lower mantle density anomaly dominates the flow field this two stage scheme minimizes the tradeoff between temperature and lower mantle viscosity for both cases we show that the theory can constrain mantle properties with errors arising through the adjoint recovery of the initial condition for the two layer model this error is manifest as a tradeoff between the temperature scaling and lower mantle viscosity

the unusual 3d interplay of basement fault reactivation and fault propagation fold development a case study of the laramide age stillwell anticline west texas usa abstract subsurface fault geometries have a systematic influence on folds formed above those faults we use the extraordinarily well exposed fold geometries of the laramide age stillwell anticline in west texas usa to develop a strain predictive model of fault propagation fold formation the anticline is a 10 km long nw trending ne vergent asymmetric fold system with an axis that displays a map view left stepping en echelon pattern we integrated field observations geologic and structural data cross sections and 2d kinematic modeling to establish an unusual 3d two stage model of contractional fold formation including 1 reverse reactivation of a pre existing nw striking sw dipping left stepping en echelon normal fault system in paleozoic basement rocks to generate monoclinal flexures in overlying layered cretaceous carbonate rocks and 2 the formation of a subsequent flat ramp fault system that propagated horizontally along a mechanically weak clay rich cretaceous unit before ramping up at the hinge of the pre existing monocline system strain is focused within the forelimb of the system in front of the propagating fault tip and is accommodated by a combination of interlayer slip flat ramp faulting and fracturing proximal to planes of slip this strain predictive model can be applied to similar less well exposed contractional systems worldwide and provides a new unusual example of laramide age contractional deformation

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

the rupture extent of low frequency earthquakes near parkfield ca the low frequency earthquakes lfes that constitute tectonic tremor are often inferred to be slow to have durations of 0 2 to 0 5 s a factor of 10 to 100 longer than those of typical m w 1 2 earthquakes here we examine lfes near parkfield ca in order to assess several proposed explanations for lfes long durations we determine lfe rupture areas and location distributions using a new approach similar to directivity analysis where we examine how signals coming from various locations within lfes finite rupture extents create differences in the apparent source time functions recorded at various stations we use synthetic ruptures to determine how much the lfe signals recorded at each station would be modified by spatial variations of the source station travel time within the rupture area given various possible rupture diameters and then compare those synthetics with the data our synthetics show that the methodology can identify inter station variations created by heterogeneous slip distributions or complex rupture edges and thus lets us estimate lfe rupture extents for unilateral or bilateral ruptures to obtain robust estimates of the sources similarity across stations we stack signals from thousands of lfes using an empirical green s function approach to isolate the lfes apparent source time functions from the path effects our analysis of lfes in parkfield implies that lfes apparent source time functions are similar across stations at frequencies up to 8 to 16 hz depending on the family the inter station coherence observed at these relatively high frequencies or short wave lengths down to 0 2 to 0 5 km suggest that lfes in each of the 7 families examined occur on asperities they are clustered in patches with sub 1 km diameters the individual lfes rupture diameters are estimated to be smaller than 1 1 km for all families and smaller than 0 5 km and 1 km for the two shallowest families which were previously found to have 0 2 s durations coupling the diameters with the durations suggests that it is possible to model these m w 1 2 lfes with earthquake like rupture speeds around 70 of the shear wave speed however that rupture speed matches the data only at the edge of our uncertainty estimates for the family with highest coherence the data for that family are better matched if lfes have rupture velocities smaller than 40 of the shear wave speed or if lfes have different rupture dynamics they could have long rise times contain composite sub ruptures or have slip distributions that persist from event to event

mechanisms of decadal north atlantic climate variability and implications for the recent cold anomaly decadal sea surface temperature sst fluctuations in the north atlantic ocean influence climate over adjacent land areas and are a major source of skill in climate predictions however the mechanisms underlying decadal sst variability remain to be fully understood this study isolates the mechanisms driving north atlantic sst variability on decadal time scales using low frequency component analysis which identifies the spatial and temporal structure of low frequency variability based on observations large ensemble historical simulations and pre industrial control simulations we identify a decadal mode of atmosphere ocean variability in the north atlantic with a dominant time scale of 13 18 years large scale atmospheric circulation anomalies drive sst anomalies both through contemporaneous air sea heat fluxes and through delayed ocean circulation changes the latter involving both the meridional overturning circulation and the horizontal gyre circulation the decadal sst anomalies alter the atmospheric meridional temperature gradient leading to a reversal of the initial atmospheric circulation anomaly the time scale of variability is consistent with westward propagation of baroclinic rossby waves across the subtropical north atlantic the temporal development and spatial pattern of observed decadal sst variability are consistent with the recent observed cooling in the subpolar north atlantic this suggests that the recent cold anomaly in the subpolar north atlantic is in part a result of decadal sst variability

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

far traveled latest cretaceous paleocene conglomerates of the southern rocky mountains usa record of transient laramide tectonism a solitary thin widespread conglomerate is found in latest cretaceous paleocene sections in each of these laramide basins of the southern rockies the table cliff northwestern kaiparowits plateau and uinta basin utah the piceance creek and denver basins colorado and the san juan basin new mexico these units are unusual compared to typical syntectonic conglomerates in that they occur as a thin interval underlain by a disconformity are far traveled across most of the length of their basins generally reflect a change in dispersal direction relative to underlying deposits and were deposited during a period of reduced subsidence all units were deposited well after local initiation of the laramide orogeny but within 8 m y of each other however each conglomerate unit was separately derived from individual source areas and is not part of a once continuous gravel sheet similar widespread conglomerate units of this age are apparently absent in other rocky mountain laramide basins previous studies postulate the shallow subduction of an oceanic plateau the conjugate shatsky rise beneath the u s cordillera during the time interval of conglomerate deposition the distribution timing and dispersion directions of the far traveled conglomerates are grossly consistent with vertical motions and surface tilting along the transport path of the oceanic plateau an exact match of conglomerate timing differs from previously proposed models but uncertainties in timing of deposits and the lack of constraints on the shape and structure of the subducted plateau preclude our ability to further refine the comparison elsewise local tectonic activity climate change and sea level fluctuations fail to explain the observed spatiotemporal pattern hence we suggest that these deposits record a transient mantle derived tectonic effect traversing the rocky mountain region and demonstrate the utility of stratigraphy as a record of subtle tectonic events

major late cretaceous cooling events in the eastern peninsular ranges california and their implications for cordilleran tectonics fission track ages from the peninsular ranges batholith in the san jacinto mountains near palm springs california indicate two major late cretaceous cooling events concordant sphene and zircon fission track ages of plutonic rocks tonalite and low k granodiorite range from ca 85 ma to 98 ma with a mean of 92 ma the concordance of these ages over 2 5 km of exposed vertical section coupled with previous u pb zircon dating 94 93 ma implies major cooling from crystallization temperatures to below 235 c between 94 ma and ca 92 ma cooling is attributed to initial intrusion of the tonalite and granodiorite into an evolving brittle ductile normal fault zone detachment followed by 10 14 5 km of exhumation during continued crustal extension along the santa rosa mylonite belt field relationships microstructural fabrics and amphibole thermobarometry from the mylonite belt support the notion that extensional faulting was preceded by west directed thrusting this sequence of tectonic thickening followed by extension is similar to other areas in the southwestern cordillera and may represent the creation and failure of an unstable regional crustal welt in late cretaceous time a second event is evident from concordant fission track ages and indistinguishable tracklength distributions from apatite at all elevations these data indicate very rapid cooling of the tonalitic plutons through the apatite partial annealing zone 130 70 c between ca 74 ma and 80 ma mean age of 76 ma the occurrence of late cretaceous age regional unconformities throughout baja and southern california suggests that this cooling event was the result of surface uplift and erosion by using a reasonable range of geothermal gradients 20 40 c km the cooling data imply a minimum of 4 5 km of exhumation in conjunction with the ca 76 ma event surface uplift coincided with several important changes in the north american and farallon or kula plate motions these include an increase in convergence rate an increase in the absolute velocity of the north american plate and a change in the direction of convergence between the plates the changes in plate motions are thought to have initiated low angle or flat slab subduction that in turn was responsible for the laramide deformation of the cordilleran foreland surface uplift erosion and associated cooling during exhumation of the eastern peninsular ranges are considered to be an initial expression of laramide tectonism phenomena documented here are consistent with uplift brought on by tectonic erosion of the north american lithospheric mantle due to low angle subduction and by subsequent underplating of lower crustal rocks

experimental and numerical investigations of the seismic response of a rock soil mixture deposit slope inertial instability induced slope failure is one of the most destructive seismic hazards encountered in china depositional slopes are widely distributed across the south western part of china to study the seismic performance of rock soil mixture deposit rsmd slopes in detail physical and mechanical tests of air dried rsmd and a 1 50 geometric scale generalised rsmd slope centrifugal shaking table modelling tests were conducted the acceleration response slope deformation and instability mode of the air dried rsmd slope under the continuous action of four earthquakes with different intensities are analysed besides the dynamic finite difference numerical simulations were performed with flac to simulate the centrifuge model test the numerical simulation results were compared with the test results in terms of the horizontal acceleration response time history of crest settlements and the slope failure characteristics the results show that the horizontal peak ground acceleration pga amplification factors of the rsmd slope reflect typical elevation and surface amplification effects in addition the spectral distribution of the seismic wave has changed significantly with propagation along the slope elevation the energy of input seismic wave in a frequency range close to the natural frequency of an rsmd slope is significantly amplified during the test it was observed that the failure of an air dried rsmd slope occurred when the pga reached 0 216 g and the settlement at the crest of the slope was obvious the failure mode was dominated by shallow collapse

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

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

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

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

middle to late cenozoic cooling and high topography in the central rocky mountains constraints from clumped isotope geochemistry abstract the timing and mechanism of the formation of the high elevations of the central rocky mountains rockies in the western interior of north america remain controversial we examine the middle and late cenozoic paleorelief between the great plains and the central rockies using clumped isotope geothermometry of carbonate cements of fluvial and eolian sandstones our petrographic observations suggest that the studied carbonate cements were formed primarily during eodiagenesis in near surface conditions and retain primary paleoclimate and paleoelevation signals carbonate clumped isotope temperatures decrease 10 c and calculated water o 18 values decrease 4 from 35 ma to 5 ma in both the central rockies and the western great plains following middle and late cenozoic global cooling trend a persistent longitudinal gradient in temperature and o 18 value existed between the central rockies and the great plains during the oligocene and miocene with temperatures 4 8 c higher and calculated water o 18 values 3 6 higher in the great plains these observations suggest the mean elevation of the central rockies was 1 km higher than the western great plains during the middle and late cenozoic similar to the present day regional relief when placed in the context of other paleoaltimetry studies and geological observations our findings support the hypothesis that the high mean topography of the rockies was developed during the late eocene possibly related to isostatic adjustment or dynamic uplift caused by foundering of lower mantle lithosphere or the farallon slab or both

restoration of laramide right lateral strike slip in northern new mexico by using proterozoic piercing points tectonic implications from the proterozoic to the cenozoic early proterozoic rocks in northern new mexico can be used to document 100 170 km of right lateral slip along a network of north striking laramide late cretaceous early tertiary faults that formed the precursor of the rio grande rift piercing lines are defined by the intersection of a subborizontal 4 kbar isobaric surface with steeply dipping stratigraphic markers and regional structures restoration of slip provides new insight into the nature of proterozoic orogenic and crustal province boundaries the yavapai crustal province 1 75 1 72 ga juvenile volcanogenic rocks is imbricated with overlying hondo group sedimentary cover in an originally east trending ductile thrust belt that formed near the southern margin of the yavapai province a series of offset magnetic anomalies jemez lineament is interpreted to mark the thrust buried boundary between older yavapai province and younger mazatzal province crust our microplate tectonic model suggests that laramide strike slip movement new mexico was kinematically linked with coeval extensional collapse arizona and foreland shortening wyoming during decoupling and northward movement of the rigid colorado plateau relative to the undeformed midcontinent crustal scale strike slip faults in new mexico were reactivated during later tertiary extension and continue to localize high heat flow in the rio grande rift and northward into colorado

subduction zone evolution and low viscosity wedges and channels dehydration of subducting lithosphere likely transports fluid into the mantle wedge where the viscosity is decreased such a decrease in viscosity could form a low viscosity wedge lvw or a low viscosity channel lvc on top of the subducting slab using numerical models we investigate the influence of low viscosity wedges and channels on subduction zone structure slab dip changes substantially with the viscosity reduction within the lvws and lvcs for models with or without trench rollback overthickening of slabs is greatly reduced bylvws or lvcs two divergent evolutionary pathways have been found depending on the maximum depth extentofthelvwandwedgeviscosity assumingaviscositycontrastof0 1withbackgroundasthenosphere modelswithalvwthat extendsdown to 400km depthshow asteeply dipping slab while models with anlvw that extends to muchshallower depth such as 200 km can produce slabs that are flat lying beneath the overriding plate there is a narrow range of mantle viscosities that produces flatslabs 5to10 10 19 pas andthe slab flatteningprocessisenhanced bytrenchrollback slabcanbedecoupled fromthe overriding platewithalvcifthe thicknessisatleastafew10s ofkm the viscosityreduction isatleastafactor oftwoandthedepthextentofthe lvc is several hundred km these models have important implications for the geochemical and spatial evolution of volcanic arcs and the state of stress within the overriding plate the models explain the poor correlation between traditional geodynamic controls subductingplateageandconvergencerates onslabdip wepredictthatwhenvolcanicarcschangetheirdistancefromthetrench they could be preceded by changes in arc chemistry we predict that there could be a larger volatile input into the wedge when arcs migrate toward the trench and visa versa the transition of a subduction zone into the flat lying regime could be preceded by changes in the volatile budget such that the dehydration front moves to shallower depths our flat slab models shed some light on puzzling flat subductionsystems likeincentralmexico wherethereisnodeformationwithintheoverridingplateabovetheflatsegment thelack

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

influence of attenuated lithosphere and sediment loading on flexure of the deep water magallanes retroarc foreland basin southern andes flexural subsidence in foreland basins is controlled by applied loads such as topography water sediment and subcrustal forces and the mechanical properties of the lithosphere we investigate the controls on subsidence observed within the upper cretaceous magallanes retroarc foreland basin of southern south america to evaluate the impact of lateral variations in flexural rigidity due to late jurassic extension conventional elastic models cannot explain the observed basin deflection and thick accumulation of deep water cenomanian turonian basin strata however models in which the lithosphere has been previously thinned and deflects under topographic and sedimentary loads successfully reproduce regional subsidence patterns results satisfy paleobathymetric observations in the magallanes basin and suggest that lithospheric thinning is necessary to produce both long wavelength and deep subsidence during late cretaceous basin evolution results indicate that elastic thickness decreases westward from 45 25 km in the distal foreland to 37 15 km beneath the foredeep these findings are consistent with a westward reduction in crustal thickness associated with the jurassic extensional history of the patagonian lithosphere our results also show that sediment loading exerts an important control on regional deflection patterns and promotes a wider region of subsidence and reduced forebulge uplift we propose that lateral variations in mechanical properties and large sediment loads restrict depocenter migration and may cause the foredeep to remain fixed for prolonged periods of time these findings confirm that loading of thinned lithosphere imposes different mechanical controls on the flexural profile and have potential implications for other retroarc foreland basins characterized by earlier extensional histories

franciscan complex calera limestones accreted remnants of farallon plate oceanic plateaus the calera limestone part of the franciscan complex of northern california may have formed in a palaeoenvironment similar to hess and shatsky rises of the present north west pacific1 we report here new palaeomagnetic results palaeontological data and recent plate motion models that reinforce this assertion the calera limestone may have formed on farallon plate plateaus north of the pacific farallon spreading centre as a counterpart to hess or shatsky rises in one model2 the plateaus were formed by hotspots close to the farallon pacific ridge axis on accretion to north america plateau dissection in the late cretaceous to eocene 50 70 myr could explain the occurrence of large volumes of pillow basalt and exotic blocks of limestone in the franciscan complex partial subduction of the plateaus could have contributed to laramide 70 40 myr compressional events3

large scale rotations of the chortis block honduras at the southern termination of the laramide flat slab abstract paleogeographic reconstructions place the chortis block adjacent to southern mexico in the late cretaceous and earlier time forming the tectonically disturbed southwestern tip of the north american plate this study assesses the relative motion between chortis and north america during the laramide orogeny later development of the north america caribbean plate boundaries we carried out paleomagnetic analysis of rotations and latitudinal displacement using a total of 90 sites from the atima limestone and the valle de angeles formation redbeds sampled at seven localities in honduras two secondary but ancient magnetizations post date late cretaceous folding at most localities the oldest magnetization is a chrm of high laboratory unblocking temperature in the redbeds 650 c and a moderate temperature in the limestones 500 600 c this component is wnw directed at five localities varying somewhat in declination having a consistent moderate positive inclination mean i 39 7 95 6 8 n 53 sites the age of the chrm is bracketed between 85 and 63 ma based on youngest zircon age and the age of a cross cutting dike an intermediate unblocking temperature component present in both units dec 146 2 inc 28 5 k 13 1 95 6 8 n 37 sites is interpreted as an overprint the chrm indicates an average counterclockwise rotation of 55 4 5 7 with respect to the expected direction the younger overprint was acquired between 50 and 15 ma its mean is also discordant with the expected north america direction implying some neogene rotation of chortis abundant late albian turonian zircons in valle de angeles redbeds explicitly require extending the laramide arc along western mexico into chortis a reconstruction of chortis south of mexico that takes into account 55 degrees of rotation shows a gap between both continental blocks at about 70 ma we propose a model where this gap formed by back arc extension creating the late cretaceous chontal basin at the southern termination of the laramide flat slab

mountain building and mantle dynamics 1 mountain building at convergent margins requires tectonic forces that can overcome frictional resistance along large scale thrust faults and support the gravitational potential energy stored within the thickened crust of the orogen a general dynamic model for this process is still lacking here we propose that mountain belts can be classified between two end members first those of slab pull type where subduction is mainly confined to the upper mantle and rollback trench motion lead to moderately thick crustal stacks such as in the mediterranean second those of slab suction type where whole mantle convection cells conveyor belts lead to the more extreme expressions of orogeny such as the largely thickened crust and high plateaus of present day tibet and the altiplano for the slab suction type deep mantle convection produces the unique conditions to drag plates toward each other irrespective of their nature and other boundary conditions we support this hypothesis by analyzing the orogenic volcanic and convective history associated with the tertiary formation of the andes after 40 ma and himalayas after collision at 55 ma based on mantle circulation modeling and tectonic reconstructions we surmise that the forces necessary to sustain slab suction mountain building in those orogens derive after transient slab ponding from the mantle drag induced upon slab penetration into the lower mantle and from an associated surge of mantle upwelling beneath africa this process started at 65 55 ma for tibet himalaya when the tethyan slab penetrated into the lower mantle and 10 myr later in the andes when the nazca slab did this surge of mantle convection drags plates against each other generating the necessary compressional forces to create and sustain these two orogenic belts if our model is correct the available geological records of orogeny can be used to decipher time dependent mantle convection with implications for the supercontinental cycle

lithospheric radial anisotropy beneath the gulf of mexico the lithosphere asthenosphere boundary lab where a layer of low viscosity asthenosphere decouples with the upper plate motion plays an essential role in plate tectonics most dynamic modeling assumes that the shear velocity can be used as a surrogate for viscosity which provides key information about mantle flow here we derive a shear velocity model for the lab structure beneath the gulf of mexico allowing a detailed comparison with that beneath the pacific pac and atlantic atl our study takes advantage of the usarray data from the march 25th 2013 guatemala earthquake at a depth of 200 km such data is unique in that we can observe a direct upward traveling lid arrival which remains the first arrival ahead of the triplications beyond 18 this extra feature in conjunction with upper mantle triplication sampling allows good depth control of the lab and a new upper mantle seismic model atm a modification of atl to be developed atm has a prominent low velocity zone similar to the structure beneath the western atlantic the model contains strong radial anisotropy in the lid where v sh is about 6 faster than v sv this anisotropic feature ends at the bottom of the lithosphere at about the depth of 175 km in contrast to the pacific where it extends to over 300 km another important feature of atm is the weaker velocity gradient from the depth of 175 to 350 km compared to pacific models which may be related to differences in mantle flow

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

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

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

sea level and vertical motion of continents from dynamic earth models since the late cretaceous dynamic earth models are used to better understand the impact of mantle dynamics on the vertical motion of continents and regional and global sea level change since the late cretaceous a hybrid approach combines inverse and forward models of mantle convection and accounts for the principal contributors to long term sea level change the evolving distribution of ocean floor age dynamic topography in oceanic and continental regions and the geoid we infer the relative importance of dynamic versus other factors of sea level change determine time dependent patterns of dynamic subsidence and uplift of continents and derive a sea level curve we find that both dynamic factors and the evolving distribution of sea floor age are important in controlling sea level we track the movement of continents over large scale dynamic topography by consistently mapping between mantle and plate frames of reference and we find that this movement results in dynamic subsidence and uplift of continents the amplitude of dynamic topography in continental regions is larger than global sea level in several regions and periods so that it has controlled regional sea level in north and south america and australia since the late cretaceous northern africa and arabia since the late eocene and southeast asia in the oligocene miocene eastern and southern africa have experienced dynamic uplift over the last 20 to 30 m y whereas siberia and australia have experienced cenozoic tilting the dominant factor controlling global sea level is a changing oceanic lithosphere production that has resulted in a large amplitude sea level fall since the late cretaceous with dynamic topography offsetting this fall