hazards posed by active major faults along the golmud lhasa railway route tibetan plateau china abstract the high number of active faults in the vicinity of the planned 1142 km long golmud lhasa railway line across the tibetan plateau poses a great hazard to the railway this is not only from periodic fault offsets measured in meters and the effects from large to great earthquakes but also from significant ground displacement from annual fault controlled freeze thaw features in the areas of permafrost twenty seven major active faults in eight seismic zones were revealed by detailed mapping along the railway route most of the faults have strike slip movement with those in the northern plateau being sinistral and those in the southern part dextral although some normal faults are associated with graben and pull apart basins many faults have produced earthquakes in the ms 6 0 8 0 range and the high slip rates of 4 15 mm year found on others indicate a potentially similar range the ms 8 1 central kunlun earthquake of 2001 had an associated left lateral displacement of 7 14 m near the epicenter and a 4 5 m offset where the railway line was cut such earthquakes which can cause serious damage to the railway and associated facilities occur relatively frequently tectonic creep along the faults between earthquakes also causes problems and has destroyed a section of highway adjacent to the railway line the tibetan plateau has an average elevation of 4500 m and the higher northern part has permafrost with the usual problems of ground deformation from freeze thaw effects and mass movement and the need to thermally isolate structures active faults in this region affect groundwater flow and thus control unusual permafrost features linear zones of ground deformation from freezing and thawing and extensive broad belts of ground fractures occur along faults to damage structures most importantly the faults were found to control zones of pingos many of these ice cored domes rise in the late fall and winter only to be drained in the spring by linear alignments of springs some pingos occasionally migrate along the faults with time the resulting deformation has bent pipelines and ruined roadways along the route overcoming these hazards to provide for the safety of passengers and maintain the integrity of the railway is a great challenge building in regions of severe climate and permafrost is always difficult and the problems associated with the many active faults here make for unusual conditions no other large construction will be subjected to such extreme effects from direct and indirect fault movement and the engineering geology community will learn much from the experience gained here

gravity anomalies flexure and the elastic thickness structure of the india eurasia collisional system wehaveusedbouguergravityanomalyandtopographydatatodeterminetheequivalentelasticthicknessofthelithosphere te in the region of the india eurasia collisional system comparison of observed and modelled gravity anomalies along 1dimensional profiles suggest there are significant variations in te along strike of the himalaya foreland estimates decrease from a high of 70 km in the central region to 30 50 km in the east and west we have verified these inferences of spatial variations using a 2 dimensional non spectral interative flexure and gravity anomaly modelling technique the himalaya foreland forms a high te 40bteb100 km rigid block with a well defined edge as shown by the localisation of faulting and deformation along its northern margin other highteblocks occur to the north beneath the qaidam and sichuan basins the tibetan plateau forms a low te 0bteb20 km weak region that extends from the central part of the plateau into south western china tectonic styles in the india eurasia collisional system therefore involve both drigidt and dnon rigidt blocks where high te rigid blocks are present the stylesdominatedbyunderthrustingofthemorerigidblock wherethecollisionalzoneisnotconstrainedbyrigidblocks however the style appears to be dominated by lower crustal flow and a more continuous style of deformation d 2005 elsevier b v all rights reserved

role of a low viscosity zone in stabilizing plate tectonics implications for comparative terrestrial planetology 1 earth s near surface layer its lithosphere is broken into quasi rigid plates that form the upper thermal boundary layer for mantle convection since the discovery of plate tectonics it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone lvz over which the plates glide easily the lvz or asthenosphere concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the earth s gravity field here we show in both two dimensional 2 d cartesian and 3 d spherical earth models that combining a pronounced lvz and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection with 30 toroidal surface motion for the 3 d case recycling of water into the upper mantle at subduction zones is a plausible cause of earth s lvz whereas venus is dry and lacks both an lvz and plate tectonics

response to the discussion by van vliet lanoe et al of the paper features caused by ground ice growth and decay in late pleistocene fluvial deposits paris basin france bertran et al 2018 geomorphology 310 84 101 abstract the response by van vliet lanoe et al to our paper on potential thermokarst features in pleistocene alluvial deposits from the paris basin france presents inconsistencies that we consider here successively these are 1 the map of the maximum extent of pleistocene permafrost in france 2 the genesis of liquefaction and fluidization structures in periglacial environments 3 the origin of thermokarst lakes and recumbent folds 4 the depositional context of sandy units and 5 the age of the studied deposits all structures result from the interplay between 1 the growth and degradation of ice wedges which are responsible for the development of a mound like topography badland thermokarst reliefs on the edge of the alluvial terrace and from the initiation of ponding elsewhere on the terrace 2 the degradation of lithalsa that developed later in the lacustrine deposits the sequence was dated confidently to the late weichselian

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

lithospheric flexure modelling seaward of the chile trench implications for oceanic plate weakening in the trench outer rise region summary the chilean subduction zone presents a unique opportunity to study trench outer rise deformation of the subducting oceanic lithosphere at different thermal ages the shape of the outer rise for plate ages ranging from 0 to 50 ma is predicted by using an elastic plate model with variable elastic thickness te x as a function of the distance measured from the trench axis in addition the uncertainties of our results are estimated by performing a monte carlo type analysis and we considered explicitly the sediment loading effect on the lithospheric flexure in regions where the trench is heavily sedimented the results show a systematic reduction in te of up to 50 per cent or reduction in the flexural rigidity d of up to 90 per cent from the peak of the outer rise to the trench axis the reduction in te and d observed in most of the bathymetric profiles is coincident with i high plate curvatures 5 10 7 m 1 ii strong bending moments 10 16 n m iii pervasive fracturing and faulting of the oceanic basement as imaged by high resolution bathymetry data and iv reduction in crustal and mantle seismic velocities the reduction in flexural rigidity towards the trench suggests a weakening of the oceanic lithosphere and is interpreted to be caused partially by fracturing and a likely increase in fluid pore pressure in general our estimates do not show consistent increases in elastic thickness as a function of plate age this result suggests that either te is independent of plate age or te depends strongly on other factors these factors could include lithospheric weakening due to hydro fracturing and the loading history of the plate prior to subduction

frostquakes in new england frostquakes are generated by tensional fracturing of the ground due to relatively rapid freezing they are often accompanied by explosive noise and may reach intensity v effects yet they are only felt in neighborhoods and are not recorded on seismic networks frostquakes may crack macadam concrete slabs shallow soil pipe and some foundation materials along the line of the fracture they occur in the early hours of the morning in mid winter during major cold snaps the ones investigated occurred in glacial sand and silt and none were found on till or bedrock the recognition of frostquakes is important in correcting the records for fault caused earthquakes frostquakes in the historic record complicate the difficulty of predicting large earthquakes from small ones

orogenic pulses in the alberta rocky mountains radiometric dating of major faults and comparison with the regional tectono stratigraphic record radiometric ages from regionally distributed thrust fault gouge show that the eastward propagation of the southern rocky mountain fold and thrust belt in canada occurred in four orogenic pulses that correlate with tectonic events of the cordilleran interior and with depositional patterns in the adjacent foreland in the main ranges the pyramid 163 0 ma simpson pass 161 7 ma and johnson creek 145 7 ma thrusts were related to the initiation of thin skinned deformation from jurassic terrane accretion and were partly contemporaneous with development of the first clastic wedge in the foreland basin in the front ranges the emplacement of the greenock thrust 103 1 ma and broadview snake indian thrust 99 2 ma was contemporaneous with development of cenomanian deltaic deposits in the immediate foreland three thrusts in the front ranges rocky pass 74 8 ma sulfur mountain 75 6 ma and clearwater 74 2 ma thrusts define a campanian phase of tectonic loading that led to the last major transgression in the southern portion of the alberta foreland along the eastern margin of the front ranges the mcconnell thrust 54 0 ma together with the muskeg 52 4 ma brule 53 9 ma and nikanassin 52 1 ma thrusts in the foothills recorded the last phase of regional contraction the late jurassic mid cretaceous late cretaceous and early eocene deformation pulses are separated by relatively long periods 40 m y 20 m y and 10 m y of tectonic quiescence these spatially and temporally restricted fault motion pulses contrast with gradual forward fault propagation while regional eastward progression of deformation is preserved

beginning of foreland subsidence in the columbian sevier belts southern canada and northwest montana subsidence analysis and geometry of jurassic cretaceous foreland strata in northwestern montana and southern alberta and british columbia suggest that loading by the fold thrust belt in canada began as much as 40 m y earlier than in montana in canada early foreland basin deposits are late jurassic age thicken rapidly westward and are restricted to a narrow belt within 30 km of the thrust belt in western montana contemporaneous deposits are widespread and do not increase markedly in thickness toward the thrust belt the unconformity overlying these deposits also changes from canada where it is angular to a disconformity in western montana near great falls between these two areas foreland geometry is transitional over a distance of 250 km beyond the transition zone early foreland basin geometries are broadly consistent showing late jurassic foreland subsidence in southern canada and early cretaceous initial subsidence in the united states

flexural rigidity of the basin and range colorado plateau rocky mountain transition from coherence analysis of gravity and topography stochastic inversion for flexural loads and flexural rigidity of the continental elastic layer can be accomplished most effectively by using the coherence of gravity and topography however the spatial resolution of coherence analysis has been limited by use of two dimensional periodogram spectra from very large 105 km 2 windows that generally include multiple tectonic features using a two dimensional spectral estimator based on the maximum entropy method the spatial resolution of flexural properties can be enhanced by a factor of 4 or more enabling more detailed analysis at the scale of individual tectonic features this new approach is used to map the spatial variation of flexural rigidity along the basin and range transition to the colorado plateau and middle rocky mountains physiographic provinces large variations in flexural isostatic re sponse are found with rigidities ranging from as low as 8 7x102o n m elastic thickness t 4 6 km in the basin and range to as high as 4 1x1024 n m t 77 km in the middle rocky mountains these results compare favorably with independent determinations of flexural rigidity in the region areas of low flexural rigidity correlate strongly with areas of high surface heat flow as is expected from the contingence of flexural rigidity on a temperature dependent flow law also late cenozoic normal faults with large displacements are found primarily in areas of low flexural rigidity while deformation fronts of mesozoic tertiary overthrusts occur 0 to 100 km east of the low rigidity region the highest flexural rigidity is found within the archean wyoming craton where evidence suggests that deeply rooted cratonic lithosphere may play a role in deter mining the distribution of tectonism at the surface

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

dynamic versus flexural controls of late cretaceous western interior basin usa the united states cretaceous western interior basin has long been considered a foreland basin driven by the sevier thrust and associated basin sediment loads however flexural studies demonstrate that this effect exists only within a narrow band in front of the thrust belt most of the basin appears to be due to mantle flow induced dynamic subsidence associated with farallon plate subduction here we show how the components of evolving long wavelength dynamic subsidence and flexural subsidence created the accommodation space and controlled the stratigraphy across the western united states based on a correlated stratigraphic section across central utah and colorado these backstripped subsidence data reveal the dynamic topography driven nature of the western interior basin the results seem to support that the depocenters track the trough of dynamic subsidence with ca 18 myr cycles through time and space and the stratigraphic patterns of large scale progradation eastward migration of depocenter and regional clinoform like downlap are related with the dynamic subsidence interpretation of these data also provides more insights into the repeated ca 2 to 6 myr cycles of thrust induced subsidence in front of the thrust belt which control the local eastward progradation of the sand bodies from the thrust belt the dynamic flexural subsidence and eustatic sea level changes interacted and controlled the timing and distribution of unconformities our work shows how the stratigraphy precisely records the timing patterns and position of dynamic versus flexural subsidences and that combination of such data leads to important geophysical discoveries and supplies strict constraints for geodynamic modeling

late cretaceous subsidence in wyoming quantifying the dynamic component the late cretaceous western interior basin of north america is generally considered to have been a retroarc foreland basin flexural backstripping of the stratigraphic record from 97 2 ma to 73 4 ma along a section perpendicular to the wyoming idaho salient of the sevier belt clearly demonstrates that there were components of subsidence in addition to those driven by the thrust and associated sediment loads the simulation demonstrates that the flexural foredeep is only 180 120 km wide and shows a forebulge located near the subsequent laramide rock springs uplift foredeep strata between the rock springs uplift on the east and the thrust front to the west were mostly involved in the eastward overlapping thrust belt or deeply truncated by inferred rebound after 78 5 ma the difference between the observed decompacted cumulative subsidence and the amount of subsidence that can be explained by simple flexural loading the residual subsidence increases from 800 m in eastern wyoming to 1 8 km near the thrust belt the westward thickening upper cretaceous sediment wedge filled the accommodation space generated by downward tilting to the west of the north american cratonic margin by underplating and mantle flow associated with the shallowly subducted farallon plate

flexural versus dynamic processes of subsidence in the north american cordillera foreland basin 1 dynamic subsidence related to subduction is an important process in retroarc foreland basin systems dynamic subsidence in the north american retroarc foreland has been proposed as dominant in the late cretaceous however questions remain about the nature of the subcrustal load and the basin response to such processes we present new isopach data using 130 data points covering a large portion of the u s and a revised flexural analysis of the sevier foreland higher rigidity associated with the wyoming craton can best model flexural subsidence directly prior to 81 ma we constrain the transition from flexural to nonflexural subsidence to 81 ma and correlate this with large late cretaceous progradation the locus of subsidence and the location of the subducted shatsky oceanic plateau suggest that dynamic loading from viscous flow above the subducting plate is the mechanism driving dynamic subsidence in the retroarc foreland basin of the north american cordillera

coal quality coal rank variation and its relation to reconstructed overburden upper cretaceous and tertiary plains coals alberta canada near surface coals in the alberta plains occur in upper cretaceous and tertiary sediments the former represented by the belly river group horseshoe canyon formation and wapiti formation and the latter by the scollard member of the paskapoo formation statistical techniques were used to document both the distribution of and the interrelationships among the components of proximate and ultimate analysis and calorific value determinations the coals discussed range in rank from subbituminous c to high volatile bituminous c these coals are variable in ash content and low in sulfur content the relationships among calorific value dry basis in kj kg and ash dry basis in and calorific value moist mineral matter free basis in kj kg and equilibrium moisture meq in were determined to be cvd 29 677 315 17 ashd and cvmmmf 31 816 442 05 meq respectively the maximum depth of coal seam burial dob in m was reconstructed on the basis of previously published graphs relating the equilibrium moisture loss to depth of coal seam burial using the relationship log10meq 1 865 0 000416 dob a least squares regression analysis of calorific value mmmf basis in k kg on the calculated depth of burial dob in m yielded the equation cvmmmf 14 748 6 25 dob indicating a coalification gradient determined on the reconstructed overburden of 6 25 kj kg m 0 82 btu lb ft of depth or overburden near surface coals in the alberta plains increase in rank toward the west southwest toward the foothills and mountains region because of progressively greater amounts of overburden that existed in that direction during the tertiary erosion has since removed between 900 and 1 900 m 3 000 and 6 200 ft of sediment with the greatest amount of removal occurring in a west southwest direction

lithospheric 3 d flexure modelling of the oceanic plate seaward of the trench using variable elastic thickness summary whendescribingthemechanicalbehaviourofthelithospheremodelledasathinplate themost important parameter corresponds to itsflexural rigidity which is commonly expressed through the effective elastic thickness te this parameter is a measure of the stiffness of the plate and defines the maximum magnitude and wavelength of those surface loads that can be supported without suffering unelastic deformation realistic 3 d models of the flexural response of the lithosphere near the trench are scarce because of the mathematical and computational complexity we present a method for determining the flexure of the lithosphere caused by the combined effect of 3 d seamount loading and bending of the lithosphere near the trench our method consists on solving numerically the flexure equations of the reissner mindlin thin plate theory including variable thickness using the finite element method with mesh adaptation the method was applied to study the flexure of the oceanic nazca lithosphere beneath the o higgins seamount group which lies 70km seaward of the chile trench the results show that an elastic thickness te of 5km under the seamounts a te of 15km far from the trench and a te of 13km near the trench can explain both the down deflection of the oceanic moho and bending of the oceanic lithosphere observed in seismic and gravity profiles in order to study the impact of high trench curvature on the morphology of the outer rise we apply the same methodology to study and model the flexure of the lithosphere in the arica bend region 14 s 23 s results indicate that the te values are overestimated if the 3 d trench curvature is not included in the modelling

jurassic onset of foreland basin deposition in northwestern montana usa implications for along strike synchroneity of cordilleran orogenic activity stratigraphic provenance and subsidence analyses suggest that by the middle to late jurassic a foreland basin system was active in northwestern montana united states u pb ages of detrital zircons and detrital modes of sandstones indicate provenance from accreted terranes and deformed miogeoclinal rocks to the west subsidence commenced ca 170 ma and followed a sigmoidal pattern characteristic of foreland basin systems thin jurassic deposits of the ellis group and morrison formation accumulated in a backbulge depozone a regional unconformity and or paleosol zone separates the morrison from early cretaceous foredeep deposits of the kootenai formation the model presented here is consistent with regional deformation events registered in hinterland regions and challenges previous interpretations of a strongly diachronous onset of cordilleran foreland basin deposition from northwestern montana to southern canada

burial history and thermal maturity rocky mountain front ranges foothills and foreland east central british columbia and adjacent alberta canada the regional pattern of maturation of cretaceous strata in the study area was determined from vitrinite reflectance measurements maturation increases from west to east across the foothills to a maximum near the eastern limit of foothills deformation and decreases farther east maturation along the eastern limit of deformation also decreases northward significantly reflectance measurements from carboniferous strata exposed in the front ranges are much lower than values from the lower cretaceous near the eastern limit of deformation modeling using burial history curves indicates the regional maturation pattern largely reflects variations in the depth and or duration of burial beneath maastrichtian eocene foredeep deposits however differential vertical movements associa ed with the peace river arch embayment in the carboniferous triassic early cretaceous and maastrichtian eocene had an important effect on the maturation pattern determined and estimated maturation levels for reservoir strata are consistent with the known occurrences of gas fields and oil pools except along the relatively unexplored western margin of the study area there moderate maturation levels indicate a potential for wet gas or oil preservation in shallow structures containing triassic and lower carboniferous carbonates in the south in the north structures in the western foothills deforming triassic strata with lower levels of maturation are breached

evolution of the northern cordilleran foreland basin during the middle cretaceous a detailed study of the middle late cenomanian belle fourche formation in southern alberta helps to elucidate the evolution of the cordilleran foreland basin during the middle cretaceous by combining isopach data from the study area with previously published research on middle cretaceous stratigraphy in adjacent areas of northern alberta and montana it is possible to defi ne the position of the proximal foredeep forebulge and backbulge depozones from late albian to cenomanian time the foredeep probably lay to the west of the present fold thrust belt in western montana from the late albian to middle cenomanian with the forebulge extending northwestward through the northwest corner of wyoming and curving northeastward through the present day location of calgary thick dunvegan deposits middle cenomanian of northwestern alberta are interpreted as foredeep deposits palinspastic reconstruction of the southern rocky mountains of alberta is consistent with a location of the foredeep within the area of the present fold thrust belt at the end of the middle cenomanian the forebulge retreated westward within alberta defi ning a nearly straight northwest southeast trend through the present position of the foothills and front ranges of alberta and northwestern montana the change in trend of the foreland basin at the end of the middle cenomanian may refl ect the change of convergence vectors along the western margin of north america during the middle cretaceous

flexure of the lithosphere and the geodynamical evolution of non cylindrical rifted passive margins results from a numerical model incorporating variable elastic thickness surface processes and 3d thermal subsidence we present a new numerical model to calculate the surface deflection of a two dimensional yet variable thickness thin elastic plate the model is based on a multi grid finite difference solution of the fourth order differential equation that incorporates the terms arising from the non uniform thickness assumption the model has been developed to calculate the flexural response of the continental lithosphere subjected to an arbitrary instantaneous stretching the flexural model is coupled to a a finite element three dimensional thermal model incorporating the conduction advection and production terms that allows the computation of the thermal subsidence resulting from the stretching induced perturbation of the isotherms assuming that the effective elastic thickness is controlled by the depth to a given isotherm and b a finite difference surface process model that assumes that transport is linearly proportional to slope leading to a second order diffusion type partial differential equation the model also incorporates the effect of sediment compaction we present a series of simple benchmarks that demonstrate the accuracy of the model we also present results of simple 2d and 3d stretching experiments highlighting the importance of 3d flexural effects and the assumed variable elastic thickness on the development of a passive margin and its thermal evolution finally we perform a numerical experiment based on a stretching geometry derived from the present day geometry of the western africatransform margin to predict sediment accumulation patterns and a stratigraphic architecture which we can compare to observations

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

an iterative spectral solution method for thin elastic plate flexure with variable rigidity s u m m a r y thin plate flexure theory provides an accurate model for the response of the lithosphere to vertical loads on horizontal length scales ranging from tens to hundreds of kilometres examples include flexure at seamounts fracture zones sedimentary basins and subduction zones when applying this theory to real world situations most studies assume a locally uniform plate thickness to enable simple fourier transform solutions however in cases where the amplitude of the flexure is prominent such as subduction zones or there are rapid variations in seafloor age such as fracture zones these models are inadequate here we present a computationally efficient algorithm for solving the thin plate flexure equation for non uniform plate thickness and arbitrary vertical load the iterative scheme takes advantage of the 2 d fast fourier transform to perform calculations in both the spatial and spectral domains resulting in an accurate and computationally efficient solution we illustrate the accuracy of the method through comparisons with known analytic solutions finally we present results from three simple models demonstrating the differences in trench outer rise flexure when 2 d variations in plate rigidity and applied bending moment are taken into account although we focus our analysis on ocean trench flexure the method is applicable to other 2 d flexure problems having spatial rigidity variations such as seamount loading of a thermally eroded lithosphere or flexure across the continental oceanic crust boundary

a finite difference technique to incorporate spatial variations in rigidity and planar faults into 3 d models for lithospheric flexure summary we present a finite difference formulation for 3 d elastic flexure of the lithosphere which is solved by a direct matrix method to incorporate the effect of spatial variations in rigidity additional terms for the bi harmonic 3 d flexure equation have been derived from a variational displacement formulation as used in finite element methods additionally planar faults are treated as discontinuities these are implemented by an additional degree of freedom for fault heave and a coupled continuum equation for zero differential tilting across the fault the 3 d finite difference results have been tested for line loads point loads and disc loads by analytical solutions and for spatial variation in effective elastic thickness eet by 2 d finite difference solutions fault related flexure patterns are compared to the 2 d analytical broken plate model developed by vening meinesz 1950 we subsequently apply the 3 d fault model to investigate fault controlled 3 d basement geometries in lake tanganyika east africa we show that our model is capable of predicting 3 d basement geometries characteristically observed in rifted basins the modelling results indicate that fault controlled upper crustal flexure patterns are associated with low values for eet a comparison with regional scale model studies showing a superposition of high eet flexure effects supports a multilayered rheological control on continental rifting

two phase stratigraphic model of foreland basin sequences application of flexural models to nonmarine foreland basin evolution indicates that two different stratigraphic styles of basin fill may develop over time basin subsidence is most rapid during times of thrust load emplacement associated sedimentation is coarse grained immediately adjacent to the thrust front and grades rapidly into fine grained deposits that cover most of the basin the distal part of the basin may also contain deposits derived from streams that flow from beyond the basin toward the thrust belt subsequent removal of the thrust load by erosion and other processes results in flexural rebound of the thrust belt and adjacent foreland basin during this postorogenic phase of adjustment a regional unconformity develops in the proximal part of the foreland basin proximal deposits along with thrust derived sediment are redeposited in the distal foreland basin and beyond this two phase model of foreland sedimentation predicts that coarsening upward sequences in the proximal and distal parts of the basin have reciprocal significance the proximal sequence represents thrust belt advance whereas the distal sequence represents thrust belt cessation

exploiting radiation damage control on apatite u th he dates in cratonic regions abstract apatites from four pairs of samples of precambrian basement from the western canadian shield were analyzed by u th he thermochronometry to test for the influence of radiation damage on apatite u th he dates in this cratonic region recent studies have demonstrated that the accumulation of radiation damage increases the apatite he retentivity so that apatites with a span of effective u concentrations eu that experienced the same thermal history may be characterized by a range of closure temperatures in this investigation each sample pair consisted of a mafic dike cross cutting felsic gneisses from a single outcrop or nearby outcrops that contained apatites with a span of eu the apatites yielded u th he dates from 846 to 123 ma and were positively correlated with eu within each sample pair these results can be explained using a model that tracks the evolution of he mobility in response to the accumulation of radiation damage when coupled with regional geological constraints the data appear to require partial to complete he loss due to burial and reheating in phanerozoic time new apatite fission track dates and length data were obtained for five of these samples the apatite fission track dates are proterozoic regardless of apatite eu thermal history simulations indicate that the apatite fission track data are compatible with the u th he results although the thermal histories are not identical in detail and the fission track results alone do not require phanerozoic heating together the data are consistent with burial of this region by 1 km of phanerozoic strata that were subsequently denuded thus pointing toward significant phanerozoic deposition in the north american cratonic interior hundreds of kilometers east of where previously documented the results suggest that exploiting radiation damage control on apatite u th he dates through investigation of surface sample apatites with a span of closure temperatures can impose tighter restrictions on thermal histories in low relief cratonic landscapes than was previously possible

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

the cypress hills formation upper eocene to miocene a semi arid braidplain deposit resulting from intrusive uplift the upper eocene miocene cypress hills formation of the cypress hills plateau in southwestern saskatchewan and southeastern alberta is interpreted as a braidplain deposit the regional paleoslope dipped to the northeast and the east west distribution of outcrop exposes facies representing lateral variation across the slope of the braidplain overall the average clast size of the deposits decreases from west to east with western area sediments dominated by boulder sized gravels deposited on longitudinal bars the eastern outcrop area contains deposits of braided channels cut into and interbedded with finer interchannel material including lacustrine marlstones silcretes and debris flow deposits the latter commonly containing abundant fossils the gravels of the cypress hills formation are multicyclic they were originally derived from the western ranges of the rocky mountains during laramide orogenesis and later shed farther into the basin during rebound due to unloading of the laramide thrusts by

couette and poiseuille flows in a low viscosity asthenosphere effects of internal heating rate rayleigh number and plate representation abstract mantle convection models with a low viscosity asthenosphere and high viscosity surface plates have been shown to produce very large aspect ratio convection cells like those inferred to exist in earth s mantle and to exhibit two asthenospheric flow regimes when the surface plate is highly mobile the plate velocity exceeds the flow velocities in the asthenosphere and the plate drives a couette type flow in the asthenospheric channel for sluggish plates the flow velocities in the asthenosphere exceed the plate velocity and the asthenospheric flow is more poiseuille like it has been shown that under certain circumstances flows become increasingly couette like as the aspect ratio of the plate is increased in numerical simulations these models also show an increase in the average surface heat flux with aspect ratio which is counterintuitive as one would expect that large aspect ratio models would result in older and colder oceanic lithosphere previous investigations have used single internal heating rates and rayleigh numbers and a plate formulation that did not preclude significant deformation within the plate in this paper we investigate the conditions necessary for couette and poiseuille asthenospheric flows and for surface heat flux to increase with plate aspect ratio by varying the internal heating rate the rayleigh number and the representation of surface plates in 2d mantle convection models plates are represented as a high viscosity layer with 1 a free slip top surface boundary condition and 2 a force balance boundary condition that imposes a constant surface velocity within the plate we find that for models with a free slip surface boundary condition the internal heating rate and rayleigh number do not strongly affect the dominance of couette or poiseuille flows in the asthenosphere but the increase in surface heat flux with model aspect ratio in the poiseuille asthenospheric flow regime increases with internal heating rate for models using the force balance representation of surface plates the flow regime in the asthenosphere is found to be almost independent of the plate aspect ratio and spatially averaged surface heat flux decreases with increasing aspect ratio the dependence of heat flux on plate aspect ratio is an important consideration in earth s thermal history while the preponderance of poiseuille or couette flows in the asthenosphere may be important for understanding the mobility of surface plates and asthenospheric seismic anisotropy

the modern foreland basin system adjacent to the central andes regional variations in sediment thickness internal structures average elevation and bouguer gravity define a four component foreland basin system adjacent to the central andes in the most proximal part of the foreland basin system the eastern subandean zone and westernmost chaco plain 1 3 km of cenozoic deposits overlies active folds and thrusts of the frontal andean orogenic wedge these wedge top deposits pass cratonward into a foredeep depozone containing a 3 4 km thick sedimentary prism that tapers toward and locally pinches out against a broad wavelength forebulge in the central eastern chaco plain the forebulge is underlain by precambrian mesozoic rocks and is largely covered by a thin veneer of quaternary alluvium east of the forebulge a thin 0 5 km saucer shaped accumulation of sediment beneath the pantanal wetland represents a back bulge depozone ancient counterparts of these four depozones can be identified in the central andes suggesting that modern basin architecture is the result of continuous eastward migration of the coupled orogenic wedge and foreland basin system since the late cretaceous paleocene

isomerization and aromatization of hydrocarbons and the paleothermometry and burial history of alberta foreland basin the hydrocarbon fractions extracted from 34 cretaceous shales from a cross section of the alberta basin have been analyzed using gas chromatography mass spectrometry particular attention has been paid to the extents to which three temperature dependent geochemical reactions have occurred isomerization of steranes at c 20 isomerization of hopanes at c 22 and aromatization of c ring monoaromatic steroid hydrocarbons estimates of the kinetic constants of these reactions have been published previously therefore constraints may be put on the maximum temperature experienced by the sediments and the time for which this temperature existed the subsidence history of the alberta basin is simulated using a lithospheric flexural model in which the lithosphere is downwarped by the loading of overthrusts in the adjacent cordilleran orogene when overthrusting ceases erosion of the overthrusted region lightens the load and the lithosphere rebounds causing uplift and erosion of the foreland basin major overthrusting and therefore deep burial occurred during the laramide orogeny of late cretaceous to mid tertiary time the extent of chemical reactions suggests that the present maturity of the cretaceous shales is almost entirely a reflection of burial and heating during the laramide orogeny in a manner that is consistent with the lithospheric flexural model for the basin the chemical reactions suggest that the paleogeothermal gradient increased systematically away from the foothills of the rocky mountains along the profile studied that a similar trend exists today suggests this situation has persisted at least since the early stages of the laramide orogeny this variation is best explained by advective heat transfer water entering the basin in the faulted and topographically higher foothills is heated at depth thereby depressing the geothermal gradient this hot water then migrates updip and away from the foothills to enhance the geothermal gradients in the edmonton area and beyond the route of the migrating hot water is similar to that taken by hydrocarbons from devonian and cretaceous source rocks in the deep basin updip to the oil and tar sand accumulations of alberta