extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

new inferences of mantle viscosity from joint inversion of long wavelength mantle convection and post glacial rebound data we perform joint inversions for mantle viscosity of geophysical observables associated with both mantle convection and glacial isostatic adjustment gia our data include coefficients of the non hydrostatic geoid up to degree 8 and decay times associated with post glacial relative sea level rsl variations in hudson bay and fennoscandia we find that both data sets may be reconciled using a single profile of mantle viscosity which is characterized by a significant increase with depth across this region this result weakens previous arguments that the mantle rheology has significant transient effects the viscosity profiles we obtain are found to reconcile the observed free air gravity anomaly over hudson bay

s wave velocity structure of the canadian upper mantle abstract we investigate mantle structure beneath the canadian landmass using 500 surface wave waveforms recorded at stations of the canadian national seismic network and global seismic network the data are processed using the method of partitioned waveform inversion and afford a comprehensive sampling of s velocity structure beneath the north american craton and adjacent orogenic belts our study thereby extends results from previous regional s wave studies north of the coterminous us and provides a characterization of mantle structure beneath canada which is complementary to the body wave model of grand j geophys res 99 1994 11591 the resulting 3d velocity model is well resolved to a depth of 400 km over most of the region and displays two prominent large scale anomalies a high velocity structure beneath the canadian shield and platform associated with the cratonic keel and pronounced low velocities beneath the cordillera the contrast between these features is strong up to 10 and at more southerly latitudes is sharp with the transition occurring over distances of 600 km or less laterally coherent high velocities persist to depths of ca 250 km beneath the north american craton which we interpret to represent the base of the continental keel smaller scale variations are observed within the cratonic mantle but in general show little correlation with surface geological provinces moderately low velocities beneath the st lawrence valley region and labrador may be related to intracontinental volcanism and rifting respectively

lithospheric deformation during the early stages of continental collision numerical experiments and comparison with south island new zealand 1 the nature of lithospheric deformation during continental plate collision still remains unresolved while it has often been proposed that the mantle lithosphere is accommodated by distributed thickening of a viscous root an alternate hypothesis suggests that significant portions of convergent mantle lithosphere essentially undergo underthrusting or subduction to further consider this issue we model the thermochemical evolution of the lithosphere mantle system using arbitrary lagrangian eulerian finite element techniques we incorporate a mix of viscous thermally activated power law creep and plastic frictional coulomb with strain softening rheologies in the numerical experiments to treat disparate composition in the crust and mantle a range of rheological and mechanical parameters is explored to determine controls on the style of lithospheric deformation the models suggest that during the initial stages of plate collision the mantle lithosphere is characterized by plate like behavior and underthrusting subduction of the upper region in conjunction with distributed thickening and rayleigh taylor type viscous instability of the lower portion depending on the material rheology temperature regime and imposed convergence velocity the deforming mantle lithosphere demonstrates various combinations of these end member behavioral modes the modeling results are interpreted in the context of observed lithospheric deformation across south island new zealand a combined style of underthrusting and distributed thickening is consistent with the observed crustal structure in the young collisional orogen as well as seismic imaging of the geometry of the underlying lithosphere

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

depth migration of deep seismic reflection profiles crustal thickness variations in alberta variations in crustal thickness provide important clues as to the formation of the crust present day isostatic equilibrium and crustal stress the map of the topography of the mohorovicic discontinuity in alberta is revised using 1900 km of reanalysed seismic reflection profiles acquired as part of the lithoprobe alberta basement transect time sections were depth migrated using a parallelized algorithm that accounts for steeply dipping structures the migration process employed a geologically consistent velocity model of the metamorphic crust derived from earlier refraction experiments and constrained by compilations of high pressure rock velocity measurements we found that knowledge of the velocities in the sedimentary column strongly influenced the quality of the migration calculations the mohorovicic discontinuity is generally distinguished in these profiles on the basis of sharp changes in reflectivity at depths of 35 48 km sharp reflections from this boundary are rare a number of geologic fe

broadside wide angle seismic studies and three dimensional structure of the crust in the southeast canadian cordillera broadside or fan recordings of a lithoprobe seismic refraction wide angle reflection experiment in the southeastern canadian cordillera show several features further illuminating the crustal structure beyond that previously derived from score 90 southern cordillera refraction experiment of 1990 in line data analysis of a nearly in line profile centred on castlegar british columbia shows lower velocities in the upper crust associated with the purcell anticlinorium as well as velocity variations that may have some association with the purcell fault zone the depth to moho is almost 38 km somewhat deeper and on trend with the structure that has been established farther north the broadside records show high signal to noise ratio pmp arrivals i e reflections from the bottom of the crust these pmp fan picks were analysed in regions away from in line profiles providing further measurements of the depth to moho in the southeastern cordillera the analysis of the broadside records combined with t

surface wave images of western canada lithospheric variations across the cordillera craton boundary two station surface wave analysis was used to measure rayleigh wave phase velocities between 105 station pairs in western canada straddling the boundary between the tectonically active cordillera

electromagnetic images of crustal structures in southern and central canadian cordillera data from more than 400 magnetotelluric soundings made since the early 1980 s in the canadian cordillera over a 300 000 km2 area between 49 and 53 5 n are used to image qualitatively regional three dimensional crustal variation in electrical conductivity by means of phase maps phase frequency sections and maps of resistivity at depth two hundred of the soundings were acquired as part of lithoprobe southern canadian cordillera transect activities and their locations were coordinated with the seismic reflection and refraction experiments the lower crust has a generally pervasive low resistivity 1 100 m throughout the cordillera west of the foreland belt within this canadian cordilleran regional conductor the magnetotelluric data reveal both two dimensional structures with highest conductivities along the coast belt and omineca belt and three dimensional variation along geological strike this conductor mapped over a volume in excess of 106 km3 is most probably caused by fluids saline wat

inversion of three dimensional wide angle seismic data from the southwestern canadian cordillera seismic refraction wide angle reflection data were recorded on a triangular array in southwestern british columbia centered on the boundary between the coast belt to the southwest and the intermontane belt to the northeast the experiment part of the lithoprobe southern cordillera transect enabled determination of the three dimensional 3 d velocity structure of the crust and upper mantle an algorithm for the inversion of wide angle seismic data to determine 3 d velocity structure and depth to reflecting interfaces is developed the algorithm is based on existing procedures for the inversion and forward modeling of first arrival travel times and forward modeling of reflection travel times including 1 forward modeling using a 3 d finite difference algorithm and 2 a simple velocity model parameterization for the inversion which eliminates the need to solve a large system of equations the existing procedure is extended to allow 1 the inversion of reflection times to solve for depth to a reflecting interface and or velocity structure 2 the inversion of first arrival travel times to solve for depth to a refracting interface and 3 layer stripping application of the algorithm to southern cordillera data uses pg to constrain upper crustal velocity structure pmp to constrain lower crustal velocity structure and depth to moho and pn to constrain upper mantle velocities and depth to moho the 3 d velocity model for the southwestern canadian cordillera is characterized by 1 significant lateral velocity variations at all depths that do not in general correlate with surface geological features or gravity data 2 a relatively high velocity middle and lower crust in the southwestern part of the study area which correlates with a strong relative gravity high and outlines the eastern extent of lower wrangellia an accreted terrane forming the insular belt to the west 3 a narrow zone of slower velocity in the lower crust and change in crustal thickness associated with the fraser fault system lending additional support to the view that it is a crustal penetrating fault 4 an average upper mantle velocity of 7 85 km s and 5 a depth to moho of 33 36 km in the intermontane belt and 36 38 km throughout most of the coast belt decreasing in the west to 33 km near the insular coast contact horizontal velocity structure slices and an interpreted cross section based on these and other results show the complexity of crustal structure in the region

small scale convection at the edge of the colorado plateau implications for topography magmatism and evolution of proterozoic lithosphere the colorado plateau of the southwestern united states is characterized by a bowl shaped high elevation late neogene quaternary magmatism at its edge large gradients in seismic wave velocity across its margins and relatively low lithospheric seismic wave velocities we explain these observations by edge driven convection following rehydration of colorado plateau lithosphere a rapidly emplaced cenozoic step in lithosphere thickness between the colorado plateau and adjacent extended rio grande rift and basin and range province causes small scale convection in the asthenosphere a lithospheric drip below the plateau is removing lithosphere material from the edge that is heated and metasomatized resulting in magmatism edgedriven convection also drives margin uplift giving the plateau its characteristic bowl shape the edge driven convection model shows good consistency with features resolved by seismic tomography

time dependent surface topography in a coupled crust mantle convection model summary recent geodynamic research has shown that convective flow in the mantle may have an important role in the development of long wavelength surface topography this flow induced dynamic topography is usually derived from mantle convection models by computing the vertical component of hydrodynamic stress at the top of the model and assuming the stress is compensated by deflection of the surface however these models have generally ignored the presence of an overlying buoyant crust and its deformational response to the mantle flow we consider the effects of horizontal convective forcing on the crust and investigate how this crust lithosphere deformation interacts with the vertical component of mantle flow induced subsidence uplift at the surface in particular we test the response of various rheologies of the crust and mantle lithosphere to an episode of mantle downwelling the evolution of crustal thickness and topography is tracked using thermomechanical numerical models of the crust mantle system with a free surface upper boundary a strong crust c 2 5 10 25 pa s does not experience significant internal deformation and subsides above a mantle downwelling for a weaker crust c 10 23 pa s descending mantle flow initially induces subsidence but there is an inversion to surface uplift as crustal thickening induced by convergent mantle flow overcomes the dynamic subsidence the presence of a strong mantle lithosphere however may effectively shield a weak crust from deformation imposed by the underlying horizontal mantle convective stresses with temperature dependent rheologies in the model the interplay of the vertical horizontal mantle forcings with the thermal evolution of the crust results in a highly time dependent signal of topography subsequent to crustal thickening and uplift the system may undergo rapid topographic subsidence and crustal thinning by localized channel flow in the hot and weak lower crust these results suggest an alternative interpretation for the development of mantle flow induced topography in certain tectonic environments

plateau uplift in western canada caused by lithospheric delamination along a craton edge mantle convection helps create continental plateaux seismic imaging of the mantle beneath the canadian cordillera an ancient plateau suggests the plateau formed when upwelling mantle caused a block of lithosphere to detach

heat flow in the western mediterranean thermal anomalies on the margins the seafloor and the transfer zones the western mediterranean basin has been formed by miocene back arc extension and is underlain by a thin and young lithosphere this young lithosphere is warm as testified by an overall elevated offshore heat flow heat flow within the western mediterranean is however highly variable and existing data are unevenly distributed and poorly studied in the central part of the liguro provencal and algero balearic basins this central part is floored by a young oceanic crust bordered by different continental margins cut by transform faults and filled by up to 8 km of sediments we present a total of 148 new heat flow data collected during the medsalt and westmedflux cruises in 2015 and 2016 and aligned along seven regional profiles that show an important heat flow variability on the basin scale but also locally on the margins a new heat flow map for the western mediterranean outlines the following regional features 1 a higher average heat flow in the algero balearic basin compared to the liguro provencal basin 94 13 mw m2 and 78 16 mw m2 respectively and 2 a regional thermal asymmetry in both basins but with opposed heat flow trends up to 20 of this heat flow difference can be explained by sediment blanketing but age and heterogeneity of ocean crust due to an asymmetric and polyphased opening of the basins are believed to have given the major thermal imprint estimates of the age of the oceanic crust based on the new heat flow suggest a considerably younger west algerian basin 16 23 ma compared to the east algerian basin and the west sardinia oceanic floor 31 37 ma on the margins and ocean continent transitions of the western mediterranean the new heat flow data point out the existence of two types of local anomalies length scale 5 30 km 1 locally increased heat flow up to 153 mw m2 on the gulf of lion margin results from thermal refraction of large salt diapirs and 2 the co existing of both low 110 mw m2 heat flow areas on the south balearic margin suggests a heat redistribution system we suspect the lateral heat advection is resulting from a regional fluid circulation in the sediments associated to the widespread plio pleistocene volcanism on the south balearic margin

two stage subduction history under north america inferred from multiple frequency tomography eastward subduction of oceanic tectonic plates has shaped the geologic history of western north america over the past 150 million years the mountain building and volcanism that brought forth the spectacular landscapes of the west are credited to the vast ancient farallon plate which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle here we use finite frequency travel time and amplitude measurements of teleseismic p waves in seven frequency bands to obtain a high resolution tomographic image to approx1 800 km depth we discover several large previously unknown pieces of the plate which show that two distinct stages of whole mantle subduction are present under north america the currently active one descends from the pacific northwest coast to 1 500 km depth beneath the great plains whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent we argue that the separation between them is linked to the laramide era 70 50 myr ago a time of unusual volcanism and mountain building far inland generally explained by an episode of extremely flat subduction

small scale convection at a continental back arc to craton transition application to the southern canadian cordillera a step in the depth of the lithosphere base associated with lateral variations in the upper mantle temperature structure can trigger mantle flow that is referred to as edge driven convection this paper aims at outlining the implications of such edge driven flow at a lateral temperature transition from a hot and thin to a cold and thick lithosphere of a continental back arc this configuration finds application in the southern canadian cordillera where a hot and thin back arc is adjacent to the cold and thick north american craton a series of geodynamical models tested the thermodynamical behavior of the lithosphere and upper mantle induced by a step in lithosphere thickness the mantle flow patterns thickness and heat flow evolution of the lithosphere and surface topography are examined we find that the lateral temperature transition shifts cratonward due to the vigorous edge driven mantle flow that erodes the craton edge unless the craton has a distinct high viscosity mantle lithosphere the mantle lithosphere viscosity structure determines the impact of edge driven flow on crustal deformation and surface heat flow a dry olivine rheology for the craton prevents the edge from migrating and supports a persistent surface heat flow contrast these phenomena are well illustrated at the transition from the hot canadian cordillera to craton that is supported by a rheological change and that coincides with a lateral change in surface heat flow fast seismic wave velocities observed in the upper mantle cratonward of the step can be explained as downwellings induced by the edge driven flow

body wave tomography of western canada abstract in this study we have produced p and s wave velocity models for western canada using 23 420 delay times measured on vertical component seismograms and 15 805 delay times measured on transverse component seismograms respectively from a range of permanent and temporary networks resolution is best in southwestern british columbia and along the canoe northwestern alberta southern yukon and northwest territories and batholiths northwestern bc arrays where the station density is the highest and fair elsewhere we focus our attention on two distinct features 1 the transition from phanerozoic to cratonic mantle in northwestern canada and 2 the complex tectonic environment at the northern terminus of the cascadia subduction zone where the plate boundary changes from convergent to transform we find that the main transition from phanerozoic to cratonic mantle in northwestern canada occurs at the cordilleran deformation front and represents a sharp jump in seismic velocity from 2 to 2 over a distance of 50 km in northern cascadia we have imaged and characterized the signature of the subducting juan de fuca plate and observed evidence of subduction beyond the northern edge of the slab we also demonstrate that the anahim hotspot track is underlain by a 2 low velocity zone possibly extending to 400 km beneath nazko cone that appears to be the source of volcanism in this area consequently we associate the source of magmatism in this area to a mantle scale rather than lithospheric scale process

tectonic and eustatic control on albian shallowing viking and paddy formations in the western canada foreland basin late albian shallowing in the viking and paddy formations is investigated using a dynamic forward model on two cross sections through the western canada foreland basin the model incorporates flexural loading of the evolving cordilleran fold and thrust belt sea level changes cratonic tilt and in plane stress fluctuations the stratigraphic observations are explained either by a rapid eustatic sea level drop or by eastward tilting of the north american craton during late albian times in both cases southward increases in the vertical growth rates of the adjacent accretionary wedge are implied differences between the predicted stratigraphic patterns in the upper albian formations of the western and southern parts of the western canada foreland basin demonstrate the complications arising from local tectonics in determining eustatic sea level fluctuations from sequence stratigraphy the models also show that changes both in the magnitudes of intraplate stress and in continental tilt forces can produce intrabasinal highs like those underlying the upper albian formations in the basin furthermore sediment loading is demonstrated to affect the stratigraphic patterns in relation to the sedimentation rate and the rate of change of accumulation space

a simple parameterization of strain localization in the ductile regime due to grain size reduction a case study for olivine we propose a simple parameterization of the transition between dislocation creep and grain size sensitive creep under conditions characteristic of the lithospheric mantle and derived from the results of laboratory experiments on olivine rich rocks through numerical modeling and linear stability analysis we determine the conditions under which this transition takes place and potentially leads to strain localization we pay particular attention to the effect of cooling rate and strain rate which are likely to be dominant parameters in actively deforming tectonic areas we conclude that at constant temperature strain localization can only take place if the rheology of the material is nonlinearly related to grain size that strain localization is facilitated by syndeformation cooling that there is only a narrow region in the strain rate versus cooling rate parameter space where localization is likely to take place and that grain growth inhibits strain localization at fast cooling rates but may lead to grain growth localization at low cooling rates we draw attention to the potential consequences of our analysis of strain localization for the style of plate motions at the earth s surface

timescales of migmatization melt crystallization and cooling in a cordilleran gneiss dome valhalla complex southeastern british columbia 1 integration of in situ and conventional ion microprobe u th pb zircon and monazite geochronology with 40ar 39ar hornblende biotite and muscovite thermochronology provides a temporal record of metamorphism partial melting fluids and cooling of exhumed midcrustal to lower crustal rocks the valhalla complex southeastern british columbia is one of a series of elongate gneiss domes within the omineca belt of the canadian cordillera we have performed u th pb and 40ar 39ar age measurements on migmatitic gneisses from a range of structural levels and settings within the valhalla complex in an effort to better document the timing of migmatization and exhumation stromatic migmatites and leucosomes crystallized in boudin necks yield concordant u pb zircon ages that cluster near 60 ma and th pb monazite ages of circa 60 50 ma in situ analyses of monazite in samples that contain 60 50 ma however the combined data also suggest that melt crystallization circa 60 ma in the valhalla complex occurred rapidly and before the other domes within the omineca belt cooling at 49 ma throughout the omineca is associated with regional extension and exhumation of migmatites in the domes the presence of abundant migmatites and the similarity in timing of melt crystallization and cooling in domes along the 400 km long omineca belt is consistent with localization of exhumation at the boundary between the flowing crust of the hinterland and the rigid colder crust of the rocky mountain foreland

mantle flow and the evolution of the lithosphere abstract the evolution of the lithosphere is mainly controlled by time dependent forces due to 1 plate tectonic processes and 2 sublithospheric mantle flow plate tectonic processes like continental collision may provide strong thermal disturbances and after completion may trigger secondary convection beneath the lithosphere without such mantle flows lateral variations of temperature and associated variations of lithospheric thickness hl or seismic velocities will equilibrate after time scales which are considerably shorter than the geologic ages of several provinces in europe we will discuss the role of sublithospheric mantle flows on the evolution of the lithosphere and the asthenosphere and compare the results with observations from the european lithosphere steady state convection models with a rheology based on laboratory data on lherzolite show that there exists a simple relationship between mantle heat flow and hl however steady state may be reached only after transition times of the order of 1 ga during such times hl shows strong lateral variations if such variations were inherited from plate tectonic events the role of sublithospheric convection would be to prolong their lifetime considerably at higher heat flows sublithospheric mantle convection becomes strongly time dependent with a time scale of 25 50 ma on this time scale variations of hl are small altogether we observe no strong direct correlation between upwelling mantle flows and thin lithosphere in some cases the correlation is even reversed on the other hand the presence or absence of a partially molten asthenosphere may be strongly affected by sublithospheric convection and may show strong lateral variations as well these variations in space and time can be correlated with the laterally heterogeneous lvz with seismic tomography data and with intraplate volcanism in europe mantle convection mantle diapirs and variations in thickness of the lithosphere may exert forces on the lithosphere and crust they will lead to bending of the lithosphere changes in surface topography and thinning or thickening of the crust induced by lateral flows within the lower crust depending on the time scale of loading and unloading of the lithosphere and the relaxation time associated with lower crustal flows characteristic relations between moho depth and surface topography are predicted comparison with observed correlations between moho depth surface topography and lithospheric thickness of european crustal age provinces show some trends in agreement with the proposed crustal deformation model

a dynamical model for generating sharp seismic velocity contrasts underneath continents application to the sorgenfrei tornquist zone new seismic velocity models based on teleseismic traveltime tomography show a sharp lithospheric boundary at the sorgenfrei tornquist zone stz between 100 and 250 km depth with p waves about 4 faster and s waves 6 faster within the cratonic lithosphere to the north experiments and thermodynamic calculations indicate that seismic velocity differences in the shallow mantle down to the transition zone must be mostly of thermal origin as typical mantle rocks are characterized by similar velocities based on composition alone we propose a dynamical model of convection in the upper mantle that is consistent with rheological data and that satisfies the seismic observations by maintaining an abrupt lateral temperature contrast over hundreds of myrs a step like increase in lithospheric thickness from 100 to 250 km is assumed to have formed in a triassic rifting event at the stz around 220 ma and is subsequently exposed to active convection below a lithosphere that is distinct from the mantle in terms of temperature and composition remains stable against convective erosion heat advection to different depth beneath the thin and the thick lithosphere leads to a maximum horizontal contrast of 500 c at 150 km depth over a lateral transition distance of 100 km sufficient to generate 5 and 8 in maximum p and s wave velocity perturbation respectively a purely conductive model under the same conditions yields only vp 1 and vs 2 while a lithospheric evolution simulation without a compositional effect on the rheology leads to significant thermo mechanical erosion of the lithosphere giving vp 2 and vs 4

the reflection moho beneath the southern canadian cordillera the transition from the crust to the mantle beneath the canadian portion of the north american cordillera varies in depth geometry and tectonic age across the orogen these variations are rarely spatially related to the positions of morphologic or tectonic belts based on surface geology nor to nearly 25 km of structural relief identified in outcrop and on seismic reflection data the moho in this region is thus interpreted to be a long lived feature perhaps as old as proterozoic in the eastern part of the cordillera that probably has been active as a structural boundary during periods of crustal contraction and subsequent crustal stretching recognition of the moho and lower crust as a zone of localized tectonic activity provides a partial explanation for the problem of where regional detachments that underlie the foreland thrust and fold belt go as they project westward to deep structural levels beneath the interior of the orogen they likely project to the base of the crust where they flatten and

utopia basin mars characterization of topography and morphology and assessment of the origin and evolution of basin internal structure recently obtained mars orbiter laser altimeter mola topography has permitted a new assessment of the morphology structure and history of utopia planitia the new topographic data convincingly demonstrate that the utopia region is an impact basin as originally proposed by mcgill 1989 whose major topographic expression is a circular 1 3 km deep depression 3200 km in diameter utopia basin is the largest easily recognizable impact structure in the northern hemisphere of mars and is the only portion of the northern lowlands that exhibits a distinct large scale impact signature its presence there and its ancient age verify that a significant part of the northern lowlands dates back to the noachian period using slope maps derived from gridded topography we have mapped a series of linear slope anomalies within the basin and have analyzed their origin these features can be classified into two groups one oriented dominantly radial to the basin and the other oriented dominantly circumferential the circumferential features are tens of kilometers wide and hundreds of kilometers long and appear to remain relatively constant with respect to elevation we explore end member hypotheses 1 that the features formed by the direct action of water ice or 2 that they are tectonic wrinkle ridges we interpret the majority of these features to be due to tectonic deformation primarily wrinkle ridge formation modified by later sedimentation associated with the emplacement of the outflow channels and the formation of the vastitas borealis formation on the basis of the characteristics of these features and their association with polygonal terrain thumbprint terrain smoothness of units and evidence of sedimentary deposits we find that our results support earlier hypotheses proposing that a significant volume of water occupied utopia basin during the late hesperian period of outflow channel formation utopia basin differs significantly in depth and degradation state from the deeper and fresher appearing hellas basin although the two are both noachian in age a significant part of this difference seems reasonably interpreted to be the result of the extensive sedimentary and volcanic infilling history of the northern lowlands

anomalous topography in the western atlantic caused by edge driven convection 1 the western atlantic region contains a long wavelength intraplate topography anomaly that is defined by the ne sw trending bermuda rise and two adjacent topography lows using numerical experiments we test the hypothesis that the anomalous topography may be the surface response to edge driven convection a primary edge driven convection cell and secondary flow circulation develops at a modeled continent ocean plate margin and induces subsidence at the continent ocean margin an off shore peak plateau of high topography on the ocean plate and distal ocean plate subsidence unlike hot spots the edge driven convection cell and associated topography migrate with moving surface plates the flow cell and wavelength of topography is broadened with continent ward motion of the lithosphere relative to the mantle whereas a migration in the ocean ward direction suppresses the formation of the edge driven convection cell and surface topography the wavelength of observed anomalous topography in the western atlantic and estimates of plate motions relative to a fixed hot spot reference frame are consistent with the former

lithospheric memory state of stress and rheology neotectonic controls on europe s intraplate continental topography abstract to date research on neotectonics and related continental topography development has mostly focused on active plate boundaries characterized by generally high deformation rates the intraplate sedimentary basins and rifts of the northern alpine foreland are associated with a much higher level of neotectonic activity than hitherto assumed seismicity and stress indicator data combined with geodetic and geomorphologic observations demonstrate that europe s intraplate lithosphere is being actively deformed this has major implications for the assessment of its natural hazards and environmental degradation the lithosphere of the northern alpine foreland has undergone a polyphase evolution with an intense interplay between upper mantle thermal perturbations and stress induced intraplate deformation that points to the importance of lithospheric folding of the thermally weakened lithosphere in this paper we address relationships between deeper lithospheric processes neotectonics and surface processes in the northern alpine foreland with special emphasis on tectonically induced topography the objectives are to quantify the effects of ongoing alpine collision and atlantic ridge push on the intraplate deformation in europe and its impact on topography evolution and related natural hazards this paper reviews the four dimensional topographic evolution of the european lithosphere through a multi disciplinary approach linking geology geophysics and geotechnology until now research on neotectonics and related topography development of intraplate regions has received little attention our study examines a number of selected natural laboratories in continental europe from orogen through platform to continental margin these natural laboratories include the carpathians pannonian system the northwest european platform iberia and the atlantic continental margin we focus on lithosphere memory and neotectonics with special attention to the thermo mechanical structure of the lithosphere mechanisms of large scale intraplate deformation late neogene anomalies in subsidence and uplift and links with surface processes and topography evolution

density temperature and rheological models for the southeastern canadian cordillera implications for its geodynamic evolution two dimensional density temperature and rheological models are constructed for a 350 km northeast trending transect of the southeastern canadian cordillera all models highlight several major physical differences between foreland and hinterland lithosphere significant features of the density model are the presence of an anomalously low density 3 10 103 kg m 3 layer with a maximum thickness of 12 km beneath the moho in the hinterland the similar densities of the monashee terrane and the cratonic crust of the foreland and an increase in crustal thickness beneath the southern rocky mountain trench the temperature model shows steeper gradients and higher moho temperatures beneath the hinterland than beneath the foreland in the rheological model the hinterland is characterized by a thin brittle upper crust beneath which the entire lithosphere is hot weak and ductile in contrast the foreland is composed of a thick brittle upper crust with an additional brittle zone in the upper mantle the

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

thinning of continental backarc lithosphere by flow induced gravitational instability abstract many continental backarcs have thin 60 km lithosphere for 100s of km behind the volcanic arc even where there has been no significant extension one mechanism to produce thin backarc lithosphere is through gravitational thinning of normal thickness lithosphere by subduction related mantle flow the stability of backarc mantle lithosphere is examined using thermal mechanical models of subduction of an oceanic plate beneath continental lithosphere with an initial thickness of 120 km and a thermal structure similar to average phanerozoic continental lithosphere subduction induced mantle flow shears the base of the backarc lithosphere producing lateral density perturbations owing to the non newtonian lithosphere rheology shearing also reduces the effective viscosity of the lowermost lithosphere enabling the density perturbations to become gravitationally unstable the perturbations develop into rapidly growing downwellings which result in removal of lower backarc lithosphere on timescales of 5 10 ma conductive heating and shearing of the remaining lithosphere results in a second more muted phase of gravitational instability and thinning lower lithosphere instability is enhanced by higher subduction rates weaker intrinsic rheology higher compositional density and hotter initial thermal structure the numerical model results are in good agreement with a buoyancy stability analysis which includes the vertical gradients in temperature and strain rate through the lithosphere as both rheology and density depend on lithosphere composition significant thinning may be restricted to continental mantle lithosphere that is fertile and contains a small amount of water to produce a final thickness comparable to that observed without lithospheric contraction or extension it is necessary to have a weak lithosphere rheology an initial thermal structure hotter than average phanerozoic continental geotherms or a combination of the two the region of thin lithosphere at the northern cascadia backarc coincides with terranes that were accreted to the north american craton the known fertile composition of the backarc lithosphere may allow it to be thinned while the drier more refractory craton lithosphere is resistant to thinning

small scale convection and divergent plate boundaries this paper explores the suggestion that observations of large igneous crustal thickness at rifted volcanic margins may in part be explained by small scale convection in the upper mantle this may increase the delivery of magma to the overlying lithosphere without the need for anomalously high mantle temperatures this concept is quantitatively assessed by numerical modeling of the flow caused by divergent plate motions in a viscous temperature and pressure dependent nonlinear fluid significant time dependent small scale convection is generated at the lower model viscosities and or higher temperatures and or sharper rift geometries these models can provide the required thick igneous crust observed at volcanic margins however they also give excessive variations in the thickness of igneous crust within the ocean basin and are therefore unacceptable an additional factor we have explored is the recent suggestion that the oceanic mantle may become dehydrated when melt is generated and removed this increases its viscosity by 2 to 3 orders of magnitude in the melting zone above 80 km the high viscosity lid stabilizes the flow and provides a more uniform oceanic crustal thickness while at the same time allowing for vigorous small scale convection during the early history of the rift and the delivery of thick igneous crust against the margin this factor coupled to the models for flow described above allow the main features of volcanic margins to be simulated besides the thick igneous crust predicted at the margins these models suggest that time dependent small scale convection below the margin may persist for tens of million years following the onset of seafloor spreading and that there may be coupling between this flow at the margin and that at the ridge axis the periodicity of this time dependent convection is primarily dictated by the model viscosities the models suggest that episodicities of tens of million years are reasonable these episodicities may be reflected in the record of vertical motions at rifted margins

late paleogene extensional collapse of the cordilleran foreland fold and thrust belt the cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle eocene to early miocene ca 49 20 ma episode of crustal extension the sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern canada to central utah extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy structural relief and sole faults of preexisting thrust fold structures the sole faults dip 3 6 west above an undeformed precambrian crystalline basement and accommodated tectonic transport of a thick up to 20 km eastward tapering hanging wall during regimes of crustal shortening and extension the chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces dated movement on two extensionally reactivated thrusts the lewis thrust of northwest montana and southeast british columbia and the medicine butte thrust of southwest wyoming and northeast utah suggests that the hiatus between the end of crustal shortening in the early or early middle eocene and the start of extension in the early middle eocene was brief lateral spreading and extensional basin formation in the cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late paleogene cordilleran orogenic wedge is accomplished by applying the extensional wedge coulomb critical taper model the rapid drop in north america pacific plate convergence rate and or steepening of the subducted oceanic slab at ca 50 ma resulted in a large reduction in east west horizontal compressive stress in the cordillera as a result the cordilleran orogenic wedge was left unsupported and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca 20 ma subsequently crustal extension and magmatism during the basin and range event ca 17 0 ma overprinted much of this earlier phase of extension

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

strong lateral strength contrasts in the mantle lithosphere of continents a case study from the hot sw canadian cordillera abstract this study aims at quantifying the 3 d variability in lithosphere strength of the south eastern canadian cordillera and adjacent craton to the east strength is calculated in a forward manner starting from rheological laws of brittle and ductile deformation the work flow calculates a temperature model based on a multi layer compositional model and subsequently estimates the strength distribution from both the compositional and temperature models the temperature modeling involves numerical inversion and cubic spline algorithms which enable to impose boundary conditions that account better for strong lateral variations in lithosphere thickness and lateral heat flow this addresses the lithosphere structure between the canadian cordillera and craton the canadian cordillera is marked by a hot and thin lithosphere of 60 km thickness that stands in contrast to a cold and thick craton of 170 km thickness to the east which consequently results in pronounced changes in bulk lithosphere strength the high surface heat flow of the cordillera interior and its contrast with the foreland basin can be reproduced by temperature models that combine elevated mantle heat flow due to the thin lithosphere and higher crustal heat production from magmatic intrusions a series of rheological models which examines the role of different temperature input models composition and strain rate shows that the first order strength pattern is persistent for the hot canadian cordillera strength resides for 80 in the upper crust and with integrated lithosphere strength of 2 0 3 5 106 mpa m for the cold craton the upper mantle provides 75 of the integrated strength with values of 4 0 8 0 106 mpa m for the crust and 20 65 106 mpa m for the lithosphere effective elastic thickness is estimated between 5 and 15 km for the cordillera and 40 80 km for the craton this illustrates the cordillera to craton transition as a prominent rheological feature for upper mantle flow dynamics and spatial seismicity trends