T11C-1878
Using Crust-Mantle Coupling to Study the Behavior of Faults Near the Earth's Surface
Faults at plate boundaries accommodate the velocity gradient between adjacent plates, so their behavior is directly linked to the coupling between the crust, where these faults reside, and the mantle below. This coupling can be then exploited to study specific characteristics of faults. One example is fault strength: there is mounting evidence that at least the major faults have low effective friction. Other issues concerning fault strength are the possible variability of strength within a fault population, the strength and downward extension of both major and secondary faults below the upper crust, and the acceptable range of strength values. We approach these questions with a numerical model of the faults in California where we integrate detailed 3-D fault geometry into a global grid to obtain self-consistent boundary conditions around the area of interest. The computation is done with the 2-D finite element program SHELLS, using mantle circulation models as the driving mechanism. We then compare the results of the calculations with observed fault slip-rates, measured GPS velocities, and directions of maximum compression from the world stress map. The simulations yield low effective friction coefficients in the brittle crust for all faults, and indicate that major faults are the weakest of all and must continue downward into the ductile crust as zones of weakness.
T11C-1879
The Caribbean plate: Pushed, pulled or dragged?
Mechanical coupling between the lithosphere and the asthenosphere remains a controversial topic in the geosciences. Beneath the Caribbean plate, shear wave splitting measurements indicate EW strain in the asthenosphere, which was previously interpreted as: mantle flow significantly drives the Caribbean plate. Here, we constrain the average shear traction on the base of the Caribbean plate by balancing all torques. These torques result from body forces that act on the Caribbean (slab pull, ridge push, lateral density variations), from plate boundary friction and from basal shear tractions. We obtain a range of physically realistic torque solutions, which we examine further by computing the corresponding strain and rotations within the Caribbean plate for comparison with observations. Models that match the observations (1) have a near-zero basal shear traction (¡Ü 0.3 MPa), (2) have (lithosphere-)averaged fault friction ¡Ü 10 MPa, (3) have Cocos Ridge indenting into the overriding Panama plate, (4) require a net pull by the Caribbean slab. We conclude that the mechanical coupling of the Caribbean plate to the underlying asthenosphere is small.
T11C-1880
Phanerozoic burial and unroofing of the western Slave craton and Wopmay orogen from apatite (U-Th)/He thermochronometry: assessing links between surface and deep-seated geodynamic processes
Low temperature thermochronology of cratonic regions offers the potential for elucidating linkages between burial and unroofing patterns, surface uplift and subsidence, and lithosphere-asthenosphere interactions. The Slave craton contains >4.0 Ga rocks, is underlain by a cold, thick, chemically depleted lithospheric mantle root, was stabilized by late Archean time, and then surrounded by Proterozoic orogens. Despite its insulation from plate margin processes and apparent stability, both mafic dikes and kimberlites have repeatedly disrupted the Slave craton in the Proterozoic and Phanerozoic. Phanerozoic sedimentary xenoliths contained in kimberlites that range in age from ca. 610 to 0.45 Ma record the past extent of strata that were subsequently denuded, indicating that the Slave craton underwent a more dynamic history of burial and unroofing than widely recognized. These observations raise the question of whether cratonic deposition and denudation was influenced by Phanerozoic thermal and mechanical perturbations associated with kimberlite emplacement, changing mantle flow regimes, and/or far-field plate boundary processes. We present new apatite (U-Th)/He thermochronometry data for nine samples along an E-W transect from the interior of the Slave craton into the adjacent Paleoproterozoic Wopmay orogen to more comprehensively constrain the region's burial and unroofing history. The transect was designed both to address the issues outlined above and to specifically assess how the cratonic architecture across the Paleoproterozoic suture between these two terranes may have controlled the lithospheric response to Phanerozoic perturbations. All samples yielded Permian to mid-Triassic apatite (U-Th)/He dates. When combined with geologic and stratigraphic constraints, the results can be modeled as consistent with a history characterized by complete He loss from the apatites between the Devonian and Triassic. Assuming a surface temperature of 5°C and a 20°C/km geotherm, this corresponds to burial depths of >2.2 km during this time interval, followed by cooling and unroofing to near-surface conditions in the Late Jurassic, and lesser reheating or reburial during Cretaceous and Early Tertiary time. The broad uniformity of the apatite (U-Th)/He dates indicates that rocks across the more than 250 km covered by our sample transect underwent similar thermal histories. This pattern suggests this area behaved as a single, broadly coherent cratonic region since at least ca. 250 Ma. The results allow a preliminary assessment of potential relationships between the burial and unroofing history, tectonic events at the plate margins, and/or deep-seated geodynamic processes.
T11C-1881
Thinning and Localization of Deformation During Rayleigh-Taylor Instability and its Implication for Intracontinental Magmatism
Thinning of the lower lithosphere due to Rayleigh--Taylor instability can be a source for continental magmatism near active or recently active plate boundaries. We consider whether it is also plausible as a mechanism for intra-continental magmatism, several hundred kilometers from active subduction or rift zones. For depth varying viscosity, Rayleigh--Taylor instabilities with a shear-stress free top can grow more rapidly than we expect when wavelengths are greater than approximately three times layer thickness (> 3h) and localized thinning can occur far from the loci of downwelling. We perform 2D Rayleigh--Taylor experiments and find that the combination of a shear-stress free top and non-Newtonian flow splits the deformation field of a growing instability into two groups, largely dependent on how viscosity varies with depth and with a transition zone in between. For small variation with depth, with the e-folding depth scale as large as a third to a half of the thickness of the unstable layer, deformation concentrates at the ends of the layer in zones of localized upwelling and downwelling, and the middle part of the layer moves horizontally towards the downwelling as a coherent block undergoing minimal strain rate. When viscosity varies more rapidly with depth, the pattern of deformation is such that the similarities between upwelling and downwelling diverge. These zones develop with different growth rates so that the localized upwelling deformation is minimal, and thinning of the layer is instead distributed laterally over a wide zone. Between the regions of thickening and thinning, shear strain and vertical gradients in horizontal velocity prevent this area from ever moving as a coherent block. The rheological exponent in the relation of strain rate varying as a power of stress, n, controls the degree of localization, or width, of the downwelling and upwelling. In geologic settings where a shear-stress free top condition could be applicable, a mix of rheological properties could provide a mechanism for the narrow zones of thinning and upwelling, which would facilitate decompression related volcanism.
T11C-1882
Dynamic Uplift and Drainage of Africa
We show the longitudinal form of a river profile can be used to quantify long-term (1--10 Myr) regional uplift in areas of predicted dynamic support. We have developed a one-dimensional forward model that calculates a river's response to a set of defined parameters. The forward model is based on the simple premise: rivers respond to the first order control of uplift and over a long time-scale erosion can be approximated by diffusion and advection. This forward model has been extensively tested; it is numerically robust and compares well to real river profiles. The geologically more interesting inverse problem solves for uplift rate as a function of time and distance along the river profile. A search algorithm can be used to calculate the temporal and spatial uplift rate variation required to fit river profiles across Africa's long wavelength swells. Inverse modelling produces excellent fits to rivers which drain the Angolan swell where the uplift rate has been determined independently. It is now generally accepted that the 'basin and swell' topography of Africa is dynamically supported by circulation within the convecting mantle. Our approach should enable quantitative estimate of uplift rates across Africa's long wavelength swells. Better study of the spatial and temporal variation of uplift rates across Africa will improve out understanding of the controlling convective processes.
T11C-1883 INVITED
TOPO-EUROPE: Studying Continental Topography and Deep Earth – Surface Processes in 4D
Topography influences various aspects of society, not only in terms of the slow process of landscape evolution but also through climate (e.g. mountain building). Topographic evolution (changes in land, water and sea level) can seriously affect human life, as well as terrestrial geo-ecosystems. When fresh water or sea-water levels rise, or when land subsides, the risk of flooding increases. This directly affects local geo- ecosystems and human settlements. On the other hand, declining water levels and uplift may lead to a higher risk of erosion and even desertification. Similar examples could be given for groundwater, early life and climate change. Studying these aspects in an integrated way is essential to forward solid Earth Sciences in response to the needs of society (see http://www.yearofplanetearth.org/). To quantify topography evolution in space and time, understanding of the coupled deep Earth and surface processes is a requisite. The TOPO-EUROPE initiative of the International Lithophere Program (ILP) addresses the 4-D topography of the orogens and intra-plate regions of Europe through a multidisciplinary approach linking geology, geophysics, geodesy and geotechnology. TOPO-EUROPE integrates monitoring, imaging, reconstruction and modelling of the interplay between processes controlling continental topography and related natural hazards. Until now, research on neotectonics and related topography development of orogens and intra-plate regions has received little attention. TOPO-EUROPE initiates a number of novel studies on the quantification of rates of vertical motions, related tectonically controlled river evolution and land subsidence in carefully selected natural laboratories in Europe. From orogen through platform to continental margin, these natural laboratories include the Alps/Carpathians-Pannonian Basin System, the West and Central European Platform, the Apennines-Tyrrhenian-Maghrebian and the Aegean-Anatolian regions, the Iberian Peninsula and the Scandinavian Continental Margin. TOPO-EUROPE integrates European research facilities and know- how essential to advance the understanding of the role of topography in Earth System Dynamics. The principal objective of the network is twofold. Namely, to integrate national research programs into a common European network and, furthermore, to integrate activities among TOPO-EUROPE institutes and participants. Key objectives are to provide an interdisciplinary forum to share knowledge and information in the field of the neotectonic and topographic evolution of Europe, to promote and encourage multidisciplinary research on a truly European scale, to increase mobility of scientists and to train young scientists. An important step has been the selection in early 2008 by the European Science Foundation (ESF) of TOPO-EUROPE as one of its large scale European collaborative research initiatives (EUROCORES). In response to the ESF call for proposals, 42 outline proposals were submitted, resulting in 22 full proposals submitted for international peer-review. Out of these, ten collaborative research projects (CRP's) were selected for the ESF EUROCORES TOPO-EUROPE, with a total funding of 13 million Euro (M$ 18) and new research positions for more than 50 PhD students and post-doctoral researchers.
T11C-1884
Age-Depth Analysis of Heavily Sedimented Old Ocean Floor
Analysis of the relationship between ocean age and tectonic subsidence has long been used to study mantle processes beneath oceanic lithosphere. However, there is a lack of observations on old ocean floor adjoining continental margins because of the large and often uncertain sediment load. In order to address this problem, we have analysed a comprehensive database of seismic reflection and wide-angle profiles on ocean crust abutting the Atlantic continental margins. Where possible, we have made measurements of sea-floor depth, sediment thickness and crustal thickness. We have also used coincident reflection and wide-angle profiles to develop a method for estimating sediment thickness where data are presented only in two-way travel time. On each line, we have estimated the equivalent water-loaded depth to basement and compared these data to an average global age-depth curve. When there is good control on both sediment and crustal thickness, we are able to quantify residual depth and assess the validity of the thermal plate model near the continent-ocean boundary. We find that dynamic perturbations to the plate model adequately explain the observed subsidence and are highly correlated with the long-wavelength gravity anomalies, given an admittance of 25-30 mGal km-1. These observations can further be used to constrain theoretical models of dynamic topography calculated using seismic tomography. Where data are of poorer quality, we instead turn the approach around and use available observations to estimate crustal thickness or illuminate errors in the published interpretation.
T11C-1885
Geodynamical Models of the Rotation and Extension of Alcapa and Tisza Blocks in the Pannonian Basin of Central Europe
The two major crustal blocks of the Pannonian basin, Alcapa (Alpine-Carpathian-Pannonian) and Tisza, underwent a complex process of rotation and extension of variable magnitude during the Tertiary. The northward push of the Adriatic Block initiated the eastward displacement and rotation of both the Alcapa and Tisza blocks. Emplacement was accompanied by substantial strike-slip movements, together with shortening and possible extension across the Mid-Hungarian Line, which now separates the two domains. Anti-clockwise rotations of variable amplitude occurred during the Early Miocene in the Alcapa unit, and clockwise rotations of the Tisza block occurred between Late Cretaceous and Late Miocene. The opposite rotations of the two plates led to NW-SE convergence and NE-SW extension in the space between the two Intra-Carpathian terranes. Subsequently both domains underwent extension dominantly in the NE-SW direction. We have constructed geodynamical models of the rotation and extension of the two Pannonian blocks. We decompose this complex process into two stages. We aim to show how the two plates deformed under the influence of a NW push by the Adriatic block, a NE pull from a retreating subduction zone on the eastern Carpathians, and the internal buoyancy forces arising from crustal thickness variations. We consider only 2D aspects of the problem, using an idealised thin viscous sheet model of the continental lithosphere. The deformation of the lithosphere is described by a non-linear viscous constitutive relationship. Our approach is based on the finite element method, and we consider several distinct models of initial geometry, boundary conditions, and constitutive parameters. Rotation and distortion vary across both blocks, with clockwise rotation occurring in the Alcapa plate, and anticlockwise rotation in the Tisza block. For a fixed exponent in the non-linear stress vs strain-rate law, increasing the viscosity coefficients of the blocks relative to the surrounding domain has a distinct impact on the distribution of rotation and deformation within the two blocks.
T11C-1886
Surface Expression of Mantle Shortening at an Active Continental Margin
We use teleseismic P-wave advances to show the mantle lithosphere has thickened beneath the central South Island of New Zealand. About 100 km of shortening has been absorbed within the South Island in the last 10 my or so. A 3-D ray tracing model is implemented to model teleseismic rays from a variety of azimuths. Our best fitting shape for the thickened mantle is for a sub-vertical body directly beneath the crustal root of the Southern Alps that has a strike sub-parallel to them. Dimensions of the body are 125 km thick and ~ 100 km, the centre around 110 km deep and an average Vp contrast with the regular mantle of ~0.6 km/s. We interpret the higher Vp within the thickened mantle as being due to colder temperatures compared to regular asthenosphere. Colder mantle will also be more dense than the asthenosphere, and hence the thickened mantle will act like a load pulling down on the overlying crust. Indirect evidence for such a loading is the observation that the crustal root is about 17 km thick, yet the mean elevation of the alps is only about 1600 m – an elevation about half that predicted by Airy isostasy. A further measure of the imbalance and suppression of surface topography comes from a pervasive negative isostatic gravity anomaly of about -30 mgals. In central and western North Island of New Zealand a similar amount (70 -100 km) of shortening occurred from late Oligocene to late Miocene times. Evidence for shortening comes from a Miocene fold and thrust belt with associated foreland basin. Subsidence curves from the basin provide an estimate of total shortening. At 5 Ma compression ceased and the region underwent a regional (400 km in extent) and rapid (few my) rock uplift of 2.5 km (or surface uplift of about 1100 m). Widespread volcanism followed thereafter. Today there is evidence that the mantle lithosphere has either gone from, or been highly attenuated beneath, much of the central and western North Island. In contrast to central South Island, isostatic gravity anomalies are in the + 20-60 mgal range. We interpret both negative and positive dynamic topographies of the South and North Island as, respectively, an early and late response to the same process. i.e. uniform thickening, then rapid release of mantle lithosphere in the early stages of a developing continental margin.
T11C-1887
Surface and tectonic consequences of plume-lithosphere interactions in continents: insights from modelling approach based on realistic representation of lithosphere.
Plume-Lithosphere Interactions (PLI) are conditioned by plume dynamics but also by complex (visco-elasto- plastic) lithosphere rheology, structure and regional intraplate stress field. In continents, PLI are often identified within extensional or compression geodynamic contexts, near boundaries between younger plates (e.g., orogenic) and older stable plates (e.g., cratons), which represent important geometrical and thermo- rheological barriers that affect plume head emplacement (e.g., Archean West Africa, East Africa, Pannonian - Carpathian system). We address these problems by considering a free-surface thermo-dynamically coupled (continuous phase transforms) thermo-mechanical numerical model of PLI that treats stratified elasto-viscous-plastic (EVP) continental blocks of contrasting properties submitted to regional compression and extension. The results show that: (1) topographic response to PLI is highly different from the predictions of classical convection models, in particular, the commonly expected long-wavelength uplift is short-lived and is replaced by mainly bi-harmonic deformation of 'tectonic- style' (poly-phase basins and uplifts) characterized by two short wavelengths (50-100 and 200-400 km). (2) tectonic deformation due to far-field forces, such as folding, may interact with lithospheric response to PLI, sometimes in a very complex way; (3) in presence of intra-plate boundaries or blocks, plume head flattening is highly asymmetric and can be blocked from one side by older (and colder) lithospheric block, which leads to mechanical decoupling of crust from the mantle lithosphere and cab be accompanied by localized faulting at the margin; (4) the return flow from the plume head results in sub- vertical down-thrusting (delamination) of the lithosphere below the margin, producing vertical cold 'subduction like' boundary that can be traced down to the 400 km depth; (5) plume head flattening and migration towards the younger plate results in concurrent surface extension above the centre of the plume and in compression (pushing), down-thrusting and magmatic events at the cratonic margin (down-thrusting is also produced at the opposite border of the younger plate); these processes may result in continental growth at the 'craton side'; (5) the negative Rayleigh-Taylor instabilities in the lithosphere above the plume head provide a mechanism for crustal delamination. This study suggests also that the absence of magmatic events should not be always interpreted as evidence for the absence of plume event, as the hot source plume material stalls below Moho and forms a long-lasting (10-100 Myr) sub-Moho reservoir. This should induce strong crustal melting that may overprint deep geochemical signatures, since this melting yields light low-viscosity rapidly ascending magmas. Drip-like down-sagging of lithospheric mantle and crustal material inside the plume head may contaminate the latter and thus alter the geochemical signature of plume-related magmas.
T11C-1888 INVITED
High Resolution Modelling of Mantle Convective Flow Below the North American Plate.
Recent progress in joint seismic-geodynamic tomographic imaging of both thermal and compositional anomalies in Earth's mantle allows us to carry out new high-resolution calculations of the present-day mantle convective flow at all depths, from the lithosphere down to the core-mantle boundary. We are therefore able to delineate the detailed connections between fundamental geological and geophysical surface processes and the underlying mantle convection. We employ these convection calculations to consider North American continental dynamics, with a special focus on the detailed relationship between flow in upper mantle, especially in the asthenosphere, and the impact on present-day dynamic topography as well as its rate-of- change. The surface dynamics that we obtain show a clear and detailed connection to the mantle flow driven by the descent of the ancient Kula-Farallon plate system and a buoyant, actively ascending hot upwelling under the western US. Of particular importance is the relationship of the deep-seated upwelling under the Colorado Plateau as a driving force for current rifting in the Rio Grande River valley. This rifting and its temporal evolution bears a strong resemblance to the convection-induced rifting our convection model also predicts under the East African Rift system. The close similarity between these two rifts, in terms of asthenospheric flow dynamics, will be discussed.
T11C-1889 INVITED
High Resolution Modelling of Mantle Convective Flow Below the North American Plate.
Recent progress in joint seismic-geodynamic tomographic imaging of both thermal and compositional anomalies in Earth's mantle allows us to carry out new high-resolution calculations of the present-day mantle convective flow at all depths, from the lithosphere down to the core-mantle boundary. We are therefore able to delineate the detailed connections between fundamental geological and geophysical surface processes and the underlying mantle convection. We employ these convection calculations to consider North American continental dynamics, with a special focus on the detailed relationship between flow in upper mantle, especially in the asthenosphere, and the impact on present-day dynamic topography as well as its rate-of- change. The surface dynamics that we obtain show a clear and detailed connection to the mantle flow driven by the descent of the ancient Kula-Farallon plate system and a buoyant, actively ascending hot upwelling under the western US. Of particular importance is the relationship of the deep-seated upwelling under the Colorado Plateau as a driving force for current rifting in the Rio Grande River valley. This rifting and its temporal evolution bears a strong resemblance to the convection-induced rifting our convection model also predicts under the East African Rift system. The close similarity between these two rifts, in terms of asthenospheric flow dynamics, will be discussed.
T11C-1890
Lithosphere-mantle coupling: constraints from the World Stress Map
The goal of the present study lies in understanding lithospheric deformation in terms of mantle flow. We seek to quantify how tractions generated by density buoyancy driven mantle convection couple to the overlying plates, thereby producing lithospheric deformation. Previous studies have addressed this problem either by quantifying lithospheric stress field or by predicting plate motions. Here, we present results of a joint modeling of lithosphere dynamics and mantle convection. Our models are constrained by plate motions, the strain rate tensor field from the Global Strain Rate Model (GSRM), and stress orientations and styles from the World Stress Map (WSM). Our lithosphere model is a thin sheet model with lateral strength variations, whereas our convection model is a whole mantle model, with free slip boundary conditions, that incorporates 3-4 orders of lateral viscosity variations. We predict deviatoric stresses within the lithosphere arising from gravitational potential energy (GPE) differences as well as stresses associated with lithospheric basal tractions arising from density buoyancy driven mantle convection. Our earlier studies have shown that combined models with several orders of lateral viscosity variations within the lithosphere, caused by major geological features of the Earth, such as the continent-ocean divide, the presence of old cratons, and age differences in the oceanic lithosphere are able to match plate motions, toroidal-poloidal velocity ratio, as well as deformation indicators from GSRM. In the present study, we use the additional constraint of stresses from WSM. By using these different constraints simultaneously, we can eliminate a wide range of models and can delineate a small group of models that explain the observations.
T11C-1891
Pervasive Thermal and Rheological Discontinuities in Mantle Lithosphere: Effects on Mantle Flow and Intraplate Motions
Cold, strong and presumably stabilized cratonic lithosphere are often bordered by belts that show pervasively deformed, hot and weak lithosphere well into the 'stable' continental interior. Stark lateral discontinuities in the thermal and rheological structure of the upper mantle near these boundaries may be responsible for the intraplate tectonic activity. For example, thermal steps in the upper mantle-lithosphere can introduce flow instabilities and edge-driven convection. This study uses numerical geodynamic experiments to test the coupled response of mantle flow and overlying lithosphere to such first order discontinuities in upper mantle structure. A continental lithosphere is considered that is thin, hot and weak to one side and stepping to a typical thick, cool and strong cratonic lithosphere. We observe that mantle flow starts at the thermal and rheological discontinuity, but the flow consequently affects the transition and can erode the lithosphere base. This leads to thermal re-adjustment (in density and rheology) of the overlying lithosphere and resulting surface vertical motions and heat-flux variations. The study is inspired by SW Canada where according to geophysical probing the Cordillera, located at an active plate margin and considered as a hot orogen, transitions sharply to typical cratonic lithosphere. In particular, significant thinning of the thermal lithosphere has been documented from elevated surface heat- flow data. Based on the geodynamic models, we interpret that the response of the lithosphere--i.e., amplitude and wavelength of the surface motions--is controlled by the interplay of the sub-lithospheric mantle flow and the deduced rheological/thermal transition between the belt and craton. Ultimately, insights on the geodynamical response from this study may also help explain the long wavelength late- to post-Laramide denudation. This involved both the Foreland belt and easterly plains with 2-4 km of denudation that is understood as epi-orogenic (intraplate) uplift.
T11C-1892
The Geodynamic Origins of Western U.S. Topography: Correlations and Speculations.
The topography of the Earth's surface provides important information for regional and global geomorphic studies because it reflects the interplay between tectonic-associated processes of uplift and climate- associated processes of erosion. The topography of the actively deforming western U.S. Cordillera is characterized by high relief and regionally high elevation, typically exceeding 1.5 km. Intriguingly, much of the high elevation coincides with thin or attenuated continental crust, necessitating topographic support by anomalous buoyancy of the mantle - suggesting that geodynamics responsible for the topography in the Western U.S. has a significant component associated with deep sources in the upper mantle. In an attempt to sharpen our understanding of the underlying geodynamics, we present a quantitative comparison of three databases (topography, geoid and Lg Q) that provide information about the geodynamics of the western U.S. at different depth scales. The unprecedented breadth and density of the USArray allows us to resolve lateral variations of 1-Hz Lg (a prominent seismic phase on most short period, regional distance seismograms along continental paths, resulting from the superposition of trapped, crustal shear waves) attenuation to 0.5 degrees over much of the western United States. In the Western U.S., Lg Q ranges from 60 to 550 and shows striking correlation with regional geology and topography, with lowest Q associated with recent volcanic and geothermal activity, and active shear zones, and highest Q associated with older, stable crust, intrusives, and competent, topographically bounded blocks in the Basin and Range. Several areas - most notably Yellowstone, the margins of the Colorado Plateau, and the Aspen Anomaly - exhibit a strong correlation between the surface topography, long-wavelength geoid anomalies and Lg Q and provide compelling support for the notion that many of the topographic features that characterize the Western U.S. are associated with dynamic uplift from upper mantle processes.
T11C-1893
Evaluating Neogene Uplift and Denudational History of the Colorado Rockies Using River Profiles and Incision Records
The goal of the Colorado Rockies Experiment and Seismic Transects (CREST) is to image the velocity structure beneath the Rocky Mountains (2008-2009) and evaluate mantle to surface interconnections that may illuminate causes and timing of uplift of the Rockies. Existing mantle tomography shows a zone of low- density mantle, the Aspen Anomaly, that underlies the highest topography in Colorado. The tectonic geomorphic component of the project involves understanding incision patterns in time and space throughout the bedrock fluvial systems of western Rocky Mountains and eastern Colorado Plateau. The Colorado River and its tributaries drain the western slope of highest topography of the Colorado Rockies; The Green River drains the Wyoming Rockies and northern Colorado Plateau. Both cross highly variable substrates (Precambrian basement to Cretaceous Mancos shale) and active faults. Preliminary analysis of longitudinal profiles of the trunk rivers indicates that for a given drainage area, the Colorado generally has a higher steepness index (a measure of gradient normalized for upstream drainage area) than the Green. Localized reaches of high steepness index along the Green are interpreted to reflect resistant substrate. We suggest that these rivers, of similar stream power, are responding to different sustained forcings, wherein the Colorado River is responding to uplift above the Aspen Anomaly. We have compiled all known incision rates for the region for the last 10 Ma. The bedrock incision rate at a given reach is determined by dates on elevated straths where gravels are overlain by or inter-layered with basalt flows (Ar-Ar dates), ash layers (tephrochronology), or can be dated by cosmogenic burial ages. A suite of new samples have also been taken for undated reaches of the Colorado River, with plans for sampling the Green for comparison of incision rates. Available data show differential incision along both the Green and Colorado rivers. When combined with profile analysis that shows non-equilibrium profiles, we identify important transient knick-points, convexities, and over-steepened reaches that are interpreted to represent a combination of tectonic and geomorphic features. Regionally important knick-points in the upper Colorado River drainage occur at Black Canyon of the Gunnison and Gore Canyon of the Colorado. These features show crude spatial correlation to the edges of the hypothesized uplift regional above the Aspen Anomaly. Gradients in topographic roughness, lithospheric geoid anomaly, normalized river gradients, and incision rate through time in these areas may be associated with dynamic uplift associated with the Aspen anomaly. Combined data sets are needed to show interactions between climate forcings, base-level fall and drainage reorganization, and tectonic epeirogeny.
T11C-1894
Hypothesis for epeirogenic uplift above the Jemez lineament: Is Neogene doming recorded by river profiles and terraces?
Rivers in the Rio Grande drainage of southern Colorado and northern New Mexico drain the southern Rockies southwards through the Rio Grande rift and across the NE trending Jemez lineament. We test the hypothesis that Quaternary tectonism (both faulting and broad doming due to magmatism and mantle driven dynamic uplift) may be recorded by drainage patterns and river profiles. These effects are not easy to distinguish from those of base level fall, drainage reorganization, and climate changes, but a regional look at New Mexico's rivers through time may help distinguish tectonic from climatic and geomorphic forcings. Longitudinal profiles of major drainages in northern New Mexico were constructed from 7.5 minute topographic maps, and DEM analysis. Bedrock lithologies, geometry of elevated terrace, and positions of basalt flows were compiled for each river. There are a striking number of reaches that exhibit sharp knickpoints and/or convexities in the profile. Some of these convexities and knickpoints seem to be bedrock-controlled; that is, they exist at hard rock-soft rock contacts. However, some convexities are not controlled by bedrock (e.g. entirely in shale); and similarly, some hard rock areas show no convexity, suggesting that bedrock control cannot always be used to explain convexities. The Rio Grande exhibits a double concave profile suggesting ongoing adjustments to neotectonic and geomorphic forcings. In map pattern, DEM analysis suggests a regional spatial correlation between the appearance of multiple convexities in numerous drainages and the Jemez lineament, a northeast trending zone of Quaternary magmatism and tectonism. Slope-area analysis, combined with Hack index analysis and topographic roughness analysis show good correlations of topographic parameters: 1) high gradient reaches (normalized for discharge), 2) regions of highest topographic roughness, and 3) zones of lowest mantle velocity. Thus, we support and expand the hypothesis that convexities in drainages crossing the Jemez lineament are the result of the drainages" response to Neogene (last 6 Ma) epeirogenic warping. These further tests of the interactions of tectonics and river incision can lead to fundamental insights about neotectonics in the Rocky Mountains and Rio Grand rift as well as processes of river incision.
T11C-1895 INVITED
Comparison between spatial-temporal variations in paleoelevation and modern lithospheric structure of the Andean plateau
Modern lithospheric structure of active mountain belts in comparison to paleoelevation reconstructions provides insights into the geodynamic processes that lead to the rise of orogenic plateaus. High altitude stable isotope records of fossil mammal teeth and pedogenic carbonates, collected between 17°S and 24°S, are used to reconstruct spatial and temporal variations in isotopic composition of paleo-surface waters. Both Subandean and Altiplano records are compared to modern rainfall compositions to evaluate climate trends over time that are potentially related to changes in paleoelevation. Prior paleoelevation estimates from the northern Altiplano near Callapa (17.5°S, 3800 m) suggest rapid late Miocene surface uplift of ~2.5 ± 1 km between 10.3 and 6.4 Ma. Based on relatively positive δ18O values of sedimentary carbonates that range in age from 16.3 Ma to 10.3 Ma from sites in both the northern and southern Altiplano, we infer a regional middle Miocene elevation approximately 1.5 to 2.5 km lower than modern elevations. Further evaluation of a late Miocene site near Quehua (20.0°S, 3800 m) in the southern Altiplano will indicate whether there is similar evidence for late Miocene surface uplift that would support the inference that a vast region of lower lithosphere was removed. Present day lithospheric structure, imaged by seismological studies, indicate that the upper mantle beneath the Altiplano and Eastern Cordillera is heterogeneous with large north south variations. The northern Altiplano is underlain by low seismic velocities consistent with little or no lithosphere. The central Altiplano seismic images indicate high velocities consistent with lithospheric mantle. However, the upper mantle beneath the Altiplano-Eastern Cordillera boundary has low seismic velocities strongly suggestive of little if any lithospheric mantle. The southern Altiplano and northern Puna appear to have much less mantle lithosphere and major surface volcanism. Taken together the seismic images suggest more of a piecemeal, rather than wholesale, lithospheric removal process. Discrepancies between the scale of lower lithospheric removal suggested by geophysics as compared to paleoaltimetry studies can be resolved through better understanding of paleoelevation histories over a broader spatial and temporal extent and through higher resolution seismic experiments over poorly studied portions of the Andean lithosphere.
T11C-1896
Colorado River System of the Southwestern U.S.: Analysis of the Longitudinal Profile, Differential Incision, and Hypothesis for Dynamic Uplift and Rapid Incision in the Last 6 Ma
The Colorado River (CR) has a double concave-up longitudinal profile with a major knickpoint near Lee's Ferry, Arizona that separates the Lower and Upper CR basins. The knickpoint is proposed here to be a transient feature, as indicated by different incision rates above and below it, and by systematic convex profiles of tributaries below, but not above, the knickpoint. The Lower CR concave portion has evolved, and Grand Canyon has been incised, since 6 Ma due to drainage integration via lake spill-over and headward erosion interacting with tectonic forcings that involve dynamic uplift of the Colorado Plateau and accompanying differential incision due to faulting. Ongoing dynamic uplift of the edge of the Colorado Plateau is supported by mantle tomography and geodynamic modeling that suggest edge-driven mantle convection across a step in lithospheric thickness near the Plateau edge that produces a ~400 m high topographic welt and a 2-4 m geoid high. This model for dynamic surface uplift in the last 6 Ma contrasts with the notion of passive incision of Grand Canyon due solely to river integration and geomorphic response to base level fall. The Upper CR appears to have evolved somewhat separately. Slope/drainage area analysis shows low normalized gradients in the center of the Colorado Plateau and along the Green River. Steep knickzones in the Black Canyon of the Gunnison and Gore Canyon of the CR are interpreted to be transients based on differential incision across them at both long term (10 Ma) and short term (640 ka) timescales. Rapid exhumation began in the Upper CR at 6 Ma as constrained by AFT data in the MWX well and near the summit of 14,000 peaks of the Needle Mountains. This is not readily explained by climate change at ~3.5 Ma, nor by upstream propagation of incision driven by integration of the lower CR at 6 Ma. Instead, the onset of rapid incision and exhumation at 6 Ma in the Upper CR may be a response to epeirogenic uplift and formation of dynamic topography related to the Aspen mantle anomaly.
T11C-1897
A case of Distributed Continental Collision: Late Cretaceous Intraplate Shortening from Central Europe to North Africa
Intraplate thrusting and basin inversion affected west-central Europe in Late Cretaceous time. The timing of this event is fairly well constrained between c. 90 and 65 Ma. The dominantly NW-trending European intraplate structures were often interpreted to have been dextrally transpressive, reflecting a northward push induced by the early collision of the Adria microplate with Europe's southern margin. However, many fault kinematic and other structural data from central Europe indicate dip-slip contraction essentially perpendicular to the main faults, suggesting a push from the southwest. In addition, recent plate reconstructions of the Mediterranean around 85 Ma place Adria far to the southeast and roughly along strike of the central European intraplate structures. The early Alpine nappe stack on Adria's leading edge was still separated from Europe by subducting oceanic lithosphere and had entered a phase of extension after the first orogenic event. All this makes Alpine collision an unlikely cause for intraplate thrusting in Europe. Rather, the timing, kinematics and location of structures suggest that intraplate shortening in Europe was a direct effect of convergence with the Iberian and African plates, with stresses transmitted across the Azores-Gibraltar fracture zone. This hypothesis is supported by structures of Late Cretaceous age indicating SW-NE to S-N shortening in France, Spain (particularly the onset of convergence in the Pyrenees) and northwestern Africa. In contrast to other examples such as the Laramides, intraplate thrusting in this case was not a foreland phenomenon related to a coeval orogen. It does not reflect a transition from subduction to continental collision, but the beginning of convergence across two former transform boundaries. This system which included no strongly thickened and weakened crust was mostly governed by far-field stresses and therefore responded rapidly to plate reorganizations. Specifically, the onset of thrusting coincided with Africa's northeast turn after an extended period of SE-directed transform motion along Europe. Thrusting terminated in mid-Paleocene time when Africa-Europe convergence slowed drastically due to transtensional rifting in the North Atlantic (Nielsen et al., Nature 450, 2007).
T11C-1898
Temporal and Spatial Variations of Late Cretaceous-Paleogene Inversion in the Central European Basin System
Inversion of the Central European Basin system (CEBS) was commonly regarded as a single, if discontinuous, event spanning the Late Cretaceous to Early Tertiary. However, there are marked temporal and regional differences in the way structural inversion (i.e., with reactivation of faults) affected the basin. Paleogene inversion present in rifts from the British Isles to the Netherlands is weak in the Lower Saxony basin and apparently absent in the North East German basin (NEGB) further east. Inversion in the Lower Saxony basin (LSB) and in the NEGB occurred in Late Cretaceous time. The Dekorp Basin 9601 regional seismic section shows flat lying Paleocene strata on top of steeply dipping, folded and thrust-faulted Triassic to Upper Cretaceous strata. Thickness variations of the Lower Tertiary in the southern NEGB result from differential subsidence by salt withdrawal. Salt-cored anticlines subsided after Late Cretaceous inversion and formed up to 1400m deep basins predominantly filled with Eocene to Oligocene sediments. Depocentre shifts away from Late Cretaceous uplifts around 62 Ma (Nielsen et al., Nature 450, 2007) could not be observed because of low stratigraphic resolution, erosion and salt-tectonic overprinting. Fission track dating on the basement highs in central Germany supports the hypothesis of a short but intense phase of inversion only in Late Cretaceous time. Apart from the timing of inversion, the different parts of the CEBS differ in the magnitudes of uplift and horizontal shortening and in structural style. Late Cretaceous structural inversion in the western and central part of the basin system is tied to NW-SE trending Jurassic to Lower Cretaceous extensional basins. Further east, the shortening is not accommodated within the NEGB but concentrated in basement uplifts and associated footwall structures on the southern basin margin. It is therefore not inversion in a strict sense. Nevertheless, first results of structural balancing across the CEBS suggest that shortening and uplift attained a maximum here. This corroborates the importance of the Late Cretaceous event as an episode of increased far-field stress, different from the Paleocene inversion phase.
T11C-1899
North Atlantic Break-up: a Link to Pacific Subduction?
The Earth's spherical geometry and the interaction of lithospheric plates along their margins imply that tectonic processes can be globally interconnected. Obviously, the relative timing of events is central to unravelling cause and effect, but the network character of lithosphere interactions and the rapid communication by stress transfer through lithospheric plates means that the timing of widely separated events may be within the resolution of biostratigraphic or other means of dating. However, in addition to the timing issue, a physically based (i.e. dynamic) cause and effect relationship that is consistent with the respective modes of deformation of potentially related events or processes must exist, and this is where numerical modelling can be of guidance. The present contribution uses numerical modelling of stress transfer in an elastic sphere to investigate if a change in the interaction between the eastern Eurasian margin and subduction of the Pacific plate can be related to the occurrence of a sudden left-lateral translation through the N Atlantic and Arctic oceans at ~62 Ma. We use the age of the ocean floor to calculate the potential energy of the oceanic lithosphere for the source term in the stress equilibrium equations, and analyse the difference between solutions with a free or a connected east Eurasian margin.
T11C-1900
The mid-Danian First-order Sequence Boundary in Arctic-North Atlantic Sedimentary Basins (and Elsewhere?): Cause and Effect
A possible genetic link has recently been demonstrated between a number of plate-scale tectonic events that occurred simultaneously – in the mid-Danian – in the European-North Atlantic-Arctic area. These included North Atlantic volcanism, widespread lithofacies changes in the North Sea (i.e., the temporary termination of in-situ chalk production because of the closure of gateways), cessation of convergence between Africa and Europe, acceleration of separation between Canada and Greenland and, most importantly, because of the highly resolved timing constraints that were available, a fundamental change in the tectonic style of sedimentary basin inversion structures within intraplate Europe. What possibly linked all of these (and other) events was a sudden (mechanically as opposed to thermally driven) stress change within the Greenland- Europe plate that resulted from the formation of the present-day North Atlantic-Arctic plate boundary separating Greenland and Europe. Since the sedimentary record is probably the most sensitive recorder of (even rather) subtle changes of stress within lithospheric plates, the model, which basically involves the rapid reconfiguration of plate boundaries, implies a domino effect whereby major shifts in sedimentary patterns and lithofacies may be expected throughout adjacent regions (in this case, the North Atlantic-Arctic region) and, perhaps, globally. Recent compilations suggest that this is indeed the case for sedimentary basins surrounding the Arctic Ocean, in particular the Sverdrup and Svalbard basins. A lack of (very) precise stratigraphic correlation and (generally) poorly constrained tectonic reconstructions to some extent limits determining whether the mid-Danian event is subtly expressed further afield, such as in Tethyan basins in Asia and in the west Pacific.