lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

stress distribution and plate boundary force associated with collision mountain ranges abstract tectonic stresses associated with collision mountain ranges are studied by finite element modelling large horizontal deviatoric tension is produced in the strong elastic core of the upper crust by the topographical loading and the associated crustal root equally significant is the effect of the dense downbulge of cool lithospheric mantle produced by plate convergence as recently observed beneath the alpine region this dense slab with its associated downflexure produces compression this compression is 25 or less of the theoretical value computed from the density moment function because of the finite width of the slab the actual state of upper crustal stress is a superimposition of root produced tension and slab produced compression an asymmetrically located deep slab produces large compressions at one edge of a mountain belt co existing with large tensions towards the other side of the range leading to mountain building at one edge and extensional tectonics elsewhere the stresses produced by the dense slab support and accentuate the crustal root in opposition to the self inflicted root erosion when the upper crust is thrust faulted the sinking slab produces a collision slab pull force of about 1 2 10 12 n m depending on slab size which is an essential factor with ridge push in initiating and developing mountain ranges surface downflexure produced by slab downpull may give rise to wide borderland sedimentary basins such as the po basin

thermal regime rheology and seismicity in central japan abstract in the present study geothermal models of the crust for a number of individual structural units of honshu island japan are constructed the depth distribution of crustal seismicity is used as a tool to assess the reliability of the calculated thermal models it is demonstrated that where reliable information is available on cutoffs in seismicity at certain depths the temperature at those depths can be estimated from theological considerations to the accuracy of not worse than from geothermal modelling such temperatures can then be used as reference values in checking the validity of a geothermal model for a region this approach may be especially useful in constructing geothermal models for tectonically disturbed areas with non steady state thermal conditions

vertical averages of rheology of the continental lithosphere relation to thin sheet parameters abstract we calculate vertically averaged rheologies for continental lithosphere subject to biaxial states of stress corresponding to extensional compressional and strike slip regimes we assume that brittle deformation occurs by byerlee s law and that ductile deformation occurs by steady state creep mechanisms under these conditions the vertically averaged rheology may be approximated by a single power law over the range of strain rates from 10 13 to 10 17 s 1 this is true not only when either brittle or ductile behavior dominates the strength of the lithospere but also when the contributions of brittle and ductile behavior are comparable the value of the power law exponent n and the strength of the lithosphere depend on the depth and stress difference at the brittle ductile transition the temperature at the moho and the geothermal gradient in the range of vertically integrated stress between 1012 and 1013 n m 1n reaches a minimum of between 5 and 10 under other circumstancesn increases rapidly as brittle behavior dominates over ductile behavior the strength of the lithosphere depends most strongly on the thermal state of the upper mantle but also is influenced by the stress supported in brittle portions of the lithosphere

flow laws for rocks containing two non linear viscous phases a phenomenological approach abstract mylonitic rocks and rock analogue materials reveal two basic types of structure 1 a load bearing framework lbf of strong phase contains isolated pockets of weak phase 2 interconnected layers of weak phase iwl separate boudins and clasts of strong phase aggregates with the lbf microstructure are characterized by nearly uniform strain rate stress is concentrated in the load bearing framework in aggregates with an iwl microstructure strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase the degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases based on these observations the rheology of two phase rock is modelled with separate functions for lbf and iwl microstructures a new flow law is derived for rock with iwl structure in which two phases undergo dislocation creep the flow law expresses composite creep strength in terms of temperature bulk strain rate and the volume proportions and creep parameters of the minerals in the rock strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate composite strengths predicted with the iwl flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates a hypothesis of viscous strain energy minimization is used to determine the relative stability of the lbf and iwl microstructures during steady state creep the iwl microstructure is predicted to be stable over a broad range of two phase compositions and mineral strength contrasts whereas the lbf microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts the iwl flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

lithospheric extension near lake mead nevada a model for ductile flow in the lower crust small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions in this study we examine the boundary between the unextended colorado plateau and a strongly extended domain in the basin and range province in the lake mead nevada region bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required finite element modeling of newtonian flow and power law creep shows that flow over the length scale of the eastern basin and range 500 km or more corresponding to upper crustal extension by a factor of 1 4 3 over 10 m y requires effective viscosities less than 10 18 10 20 pa s for ductile channels 10 25 km thick flow over shorter length scales 150 km may be accommodated with effective viscosities as high as 10 21 pa s modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory derived power law creep rates the longer scale flow may require elevated crustal temperatures more than 700 c depending on the composition and material properties assumed under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep this work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions and thereby explaining the relative uniformity in gravity and topography between such regions

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

the effects of pressure and porosity on the micromechanics of the brittle ductile transition in quartzite room temperature experiments on a quartzite with 7 porosity show that with increasing pressure there is a transition from brittle faulting to ductile cataclastic flow at about 600 mpa in contrast experiments on nonporous quartzite show only faulting up to 1000 mpa the transition to cataclastic flow in the porous quartzite is due to the changing influence of the pores microstructural observations and elastic theory suggest that the transition occurs at the pressure where the differential stress producing a compressive stress concentration sufficient to initiate pore collapse becomes less than that producing a tensile stress concentration sufficient to nucleate axial microcracks at the tops and bottoms of pores cataclastic flow of the porous quartzite is only transient deformation remains distributed and accompanied by work hardening only as long as pore collapse continues to 16 strain at higher strains microcracking produces a net dilatancy which causes strain weakening that leads to faulting at 26 strain at a confining pressure of 750 mpa the faults are stable whereas at 1000 mpa they are unstable the results indicate that porous rocks may undergo limited ductile deformation at shallow levels in the crust but higher strains will cause deformation to localize

recent large fold nucleation in the upper crust insight from gravity magnetic magnetotelluric and seismicity data sierra de los filabres sierra de las estancias internal zones betic cordillera abstract rheological heterogeneities in the upper crust have a close relationship with the fold position where rigid bodies could constitute initial perturbations that allow the nucleation of folds consequently establish the position and geometry of anomalous rocks located in the upper crust by geophysical studies help to understand the folded structure observed on surface new geological observations in the field along with gravity magnetic magnetotelluric and seismicity data reveal the subsurface structure in the sierra de los filabres sierra de las estancias folded region part of the alpine belt in southern spain the geometry of the upper crust is determined by geological field data 2d gravity models 2d magnetic models and 2d mt resistivity model while seismicity evidences the location of the deep active structures these results allow us to propose that a basic rock body at 4 to 9 km depth has determined the nucleation and development of the sierra de los filabres kilometric antiform n vergent large late folds are subjected to a variable present day stress field earthquake focal mechanisms suggest the presence in depth of a regional nw se compressive stress field however most of the seismogenetic structures do not extend up to the surface where nw se and wnw ese outcropping active normal faults are observed thus indicating a ne sw extension in the upper crust simultaneous to orthogonal nw se compression related to reverse faults and minor folds developed in the eastern almanzora corridor and in the nearby huercal overa basin the recent and active tectonic studies of cordilleras hinterland subjected to late folding greatly benefits from the integration of surface observations together with geophysical data

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

pleistocene lake outburst floods and fan formation along the eastern sierra nevada california implications for the interpretation of intermontane lacustrine records berin d i owen l a finkel r c clemmens s 2006 pleistocene lake outburst floods and fan formation along the eastern sierra nevada california implications for the interpretation of intermontane lacustrine records quaternary science reviews 25 2729 2748 sponsorship national science foundation grant ear 0207245

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

integrated lithospheric modeling combining thermal gravity and local isostasy analysis application to the ne spanish geotransect a two dimensional algorithm to determine the steady state thermal structure of the lithosphere that integrates thermal gravity and local isostasy analyses is presented gravity analyses together with seismic data are used to constrain spatial variations in density and crustal structure while absolute elevation is used to determine the lithospheric mantle thickness the calculation is performed using a finite element technique that links the different physical equations the program optionally calculates the temperature at any material boundary and with given rheological parameters the strength distribution and the total lithospheric strength in selected columns we apply the algorithm to the northeastern spanish geotransect which extends from the pyrenees to the balearic promontory and along which a strong variation in crustal and lithospheric thickness is evident we assess the use of two different inferred density models for the lithospheric mantle the first assumes a linear decrease in density with increasing temperature using the asthenospheric density as a reference the second model assumes a constant density for the whole lithospheric mantle although conceptually the two hypotheses differ substantially the results obtained do not show significant differences lithospheric thicknesses of 120 130 km below the pyrenees 60 65 km in the valencia trough and 65 75 km below the balearic promontory are deduced in all cases the mean lithospheric mantle density has to be 40 60 kg m 3 higher than the asthenospheric density the algorithm is shown to be a powerful tool in lithospheric thermal modeling especially in areas where surface heat flow is poorly constrained because of the temperature density elevation relationship

two types of brittle fracture of silicate rocks under confining pressure and their implications in the earth s crust abstract four silicate rocks granite gabbro dunite and eclogite were triaxially compressed up to a confining pressure of 3 gpa at room temperature and were found to exhibit brittle fracture behaviour from the measurements of the compressive and frictional strengths and the activity of acoustic emission ae and the observations of the microstructure of fractured specimens the pattern of fracture was found to change when the compressive strength became equal to the frictional strength two types of fracture mechanism low pressure and high pressure types were inferred the low pressure type fracture exhibits features of brittle fracture equivalent to those recorded by previous workers the features of high pressure type fracture are similar to those observed between brittle fracture and ductile creep in high temperature triaxial experiments microcracks are not concentrated close to the main fault and the main fault is sharp and oriented at approximately 45 to the stress direction this suggests that the high pressure type fracture might correspond to the transitional fracture type between the brittle and ductile regimes at room temperature for granite and gabbro samples of a few centimeters size high pressure type fracturing begins at 0 8 gpa confining pressure while for dunite and eclogite it begins at 1 0 and 2 0 gpa respectively the size effect on compressive strength under confining pressure was estimated based on the suggestion that the uniaxial strength decreases with increasing specimen size and ceases to decrease for specimens 1 m this estimation suggests that the compressive strength for a granite specimen a few meters in size would become equal to the frictional strength at confining pressures as low as 60 mpa the significance of this is that it may be necessary to model earthquakes using the high pressure type fracturing because in the absence of pore pressure 60 mpa confining pressure corresponds to a depth of 3 km faulting processes in the crust including both the initial faulting and the movement of existing faults were inferred using the experimental results and the estimated size effect on strength

thermal regime and post orogenic extension in collision belts in order to assess the conditions leading to post orogenic extension the temperature and strength of the lithosphere during continental collision are calculated the calculations assume that heat is transported by conduction and either that the lithosphere is homogeneously thickened or that only the crust is thickened during the collision despite increased crustal thickness and heat production the temperature does not increase if the rate of shortening is moderate 5 10 16s 1 leading to 100 increase in crustal thickness in 40 myears the temperature distribution is used to determine a rheological profile that is compared with the stress induced by compensated topography the calculations show that for homogeneous lithospheric thickening the strength of the lithosphere increases and is higher than the tensile stress except in the shallow crust the total strength of stable continental lithosphere at the end of shortening is on the order of 1013 n m 1 it is larger by one order than the force induced by compensated topography 1012 n m 1 it appears that homogeneous lithospheric thickening would not lead to post orogenic extension unless the initial conditions are very special with the lithosphere hotter than normal for crustal thickening only the strength of the lithosphere decreases slightly and is on the same order as the tensile stress extension does not necessarily follow from thickening of the crust only but it could take place for a relatively wide range of initial conditions with initial surface heat flow 60 mw m 2 or higher alternatively an event such as rapid removal of the mantle lithosphere by delamination or small scale convection would increase the tensile stress heat rapidly the lithosphere reduce its strength and always trigger extension

flow laws of multiphase rocks calculated from experimental data on the constituent phases abstract an attempt has been made to develop expressions for the flow laws of polyphase rocks in terms of the volume fractions and flow laws of their constituent phases the flow strengths predicted by the proposed model for granite diorite diabase and lherzolite are in good agreement with previously determined experimental values the study allows one to calculate to the first approximation the flow laws of a large number of rocks at geologic conditions based on the experimentally determined flow laws of a relatively small number of monomineralic aggregates

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

the origin of sedimentary basins a status report from the task force of the international lithosphere program abstract better insights into the mechanisms of basin formation have a direct bearing on the predictive power of forward models for basin fill models that link flexure normal faulting and thrusting on a subbasin scale provide important constraints on the tectonic control of the sedimentary record enabling a quantitative comparsion with eustatic contributions to the basin architecture thus of special importance is the need to link processes with different spatial and temporal scales involved in the evolution of extensional and compressional basins by numerical modelling a new generation of basin modelling concepts and techniques is one of the main objectives of the task force origin of sedimentary basins of the international lithosphere program the focus of this work is on the quantification of the effects of the thermomechanical structure of the lithosphere and its interplay with regional and local stress changes other important developments include the step towards three dimensional modelling and the full integration of rapidly expanding fission track and other data sets to constrain vertical motions in basins as natural laboratories for the interactive testing and development of new basin modelling techniques a number of basins with high quality data from industry government and academia have been selected on the basis of specific testable problems

rheology of some continental lower crustal rocks abstract to gain insight into the rheology of selected feldspar bearing rocks from the continental lower crust clc three granulites from exposed lower crustal terrains and a microgabbro have been deformed to ca 10 strain at confining pressures of 0 8 1 0 gpa temperatures of 600 900 c and constant strain rates of 10 4 10 7 s 1 the most extensive work was done on the mafic pikwitonei and felsic adirondack granulites the pikwitonei mg hornblende plag an70 2 px has only a limited t range of apparent steady state behavior and strain softens at t 800 c at 10 5 10 6s 1 apparently associated with developing shear zones this strain softening is pressure insensitive from pc 0 8 1 0 gpa at t 850 c 10 6s 1 even though the dominant strain accommodation mechanisms change from semibrittle cataclastic to crystal plastic including dynamic recrystallization with increasing pressure plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs kapuskasing plag cpx gt granulite deformed at 800 c and at 850 c 10 6 s 1 has deformed in the steady state at stress levels only slightly greater than the pikwitonei the stillwater microgabbro and adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady state behavior over the entire range of conditions investigated for the adirondack 2 felds 2 px gt qtz granulite quartz 21 appears to control the creep rate at these low strains and steady state mechanical data fit well a creep power law with a 8 10 3 mpa ns 1 qc 243 kj mol and n 3 1 extrapolation of this flow law and of preliminary power law creep relations for the pikwitonei and microgabbro to a range of physical conditions assumed to encompass those at the continental moho supports geological and geophysical inferences that a mechanical discontinuity occurs at this transition zone with the lower crust generally being the weaker material in addition the stain softening exhibited by the pikwitonei may bear directly on the development of detachment zones at mid to lower crustal depths during prograde metamorphism

critical stress difference fault orientation and slip direction in anisotropic rocks under non andersonian stress systems abstract the coulomb navier failure criterion is applied to geological faulting in the general three dimensional case of rocks containing arbitrarily oriented strength anisotropies and subject to non andersonian stress systems i e with none of the principal stresses acting in a vertical direction general expressions for the critical stress difference necessary to cause failure as a function of depth are given in terms of material parameters pore fluid pressure orientation of the stress field and orientation of the strength anisotropy the range of angles between a plane of anisotropy and the maximum principal stress direction for which slip occurs along the pre existing anisotropy rather than along a new fault is calculated as a function of depth for different stress regimes when the stress field is non andersonian and or strength anisotropies not containing the intermediate stress direction occur in the rock faulting generally is oblique slip a kinematic classification of faulting is given on the basis of the angle between the strike direction and the slip direction on the fault plane triangular diagrams analogous to those used in petrology are introduced to describe i faulting in isotropic rock subject to arbitrarily oriented stress fields and ii faulting in anisotropic rock when one principal stress direction is vertical the type of faulting as a function of stress field and anisotropy orientation can be read off directly from these diagrams

a geodynamic model for extension in the shuswap core complex southeastern canadian cordillera the shuswap core complex records mesozoic and paleocene crustal shortening with superimposed eocene extension during the shortening phase the crust was thickened and slowly uplifted within at most 20 ma of the climax of compression the crustal welt was rapidly uplifted and tectonically denuded shortening of the continental crust may be balanced in the underlying lithosphere by thickening or underplating forming in either case a lithospheric root beneath the crustal welt we consider a model whereby tertiary crustal extension in the shuswap core complex and possibly also in other segments of the north american cordillera is accounted for by the detachment of this root both the time frame and the stress field resulting from the detachment are compatible with geological and geophysical observations far field stresses played an accommodating role but the potential energy required for extension of the shuswap complex was induced by lithospheric thickening

experimental investigation into the microstructural and mechanical evolution of phyllosilicate bearing fault rock under conditions favouring pressure solution abstract mature crustal fault zones are known to be zones of persistent weakness this weakness is believed to result from microstructural modifications during deformation such as grain size reduction and foliation development around the brittle ductile transition phyllosilicates are expected to have a significant effect on fault strength in particular under conditions favouring pressure solution to study such effects we performed rotary shear experiments on brine saturated halite kaolinite mixtures aimed at investigating the relation between microstructural and mechanical evolution in a system where pressure solution and cataclasis dominate the results show significant strain weakening and a transition with progressive strain towards more rate sensitive and less normal stress sensitive behaviour this was accompanied by a microstructural evolution from a purely cataclastic microstructure to a mylonitic microstructure consisting of elongate asymmetric clasts in a fine grained foliated matrix the results demonstrate that strain weakening and the development of a typical mylonitic microstructure can occur as a consequence of grain size reduction by cataclasis and a transition to pressure solution accommodated deformation even in the absence of dislocation creep the data raise questions regarding the reliability of microstructures as rheology indicators as well as on the use of low strain monomineralic flow laws for modelling crustal dynamics

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

lithospheric scale structures from the perspective of analogue continental collision abstract analogue models were employed to investigate continental collision addressing the roles of 1 a suture zone separating different crustal blocks 2 mid crustal weak layers and 3 mantle strengths these models confirmed that low amplitude lithospheric and crustal buckling is the primary response to shortening with a wavelength mainly controlled by the strong upper crust and or lithospheric mantle with increasing shortening bivergent thrusts formed across the brittle layers equivalent to upper crust at inflection points of the buckles while ductile layers lower crust and mantle continued being folded the resulting geometries displayed pop ups and pop downs above the boundaries sutures between blocks further shortening led to wider thrust bounded deformation zones belts in front of the stronger blocks identified as effective indenters during shortening upward extrusion into belts allowed exhumation of deep material weak layers enhanced strain localisation these modelling results have similarity in large scale structures of modern orogens and are relevant to the understanding of natural collisional processes

archean oceanic flake tectonics assuming that the archean mantle was hotter and the oceanic crust thicker than at present we estimate that the archean oceanic lithosphere had a sandwich rheology with the lower crust behaving as a soft ductile layer accordingly the archean oceanic lithosphere had a rheological structure similar to present day continental lithosphere but unlike present day oceanic lithosphere the thickness of the ductile layer is a decreasing function of age while its strength is an increasing function of age the buoyancy problem in archean subduction could be overcome by delamination of oceanic upper crust but mass balance calculation indicates that most archean oceanic crust must have been recycled where the right dynamic conditions existed for delamination oceanic flake tectonics may have been an important process leading to the preservation of oceanic upper crust in greenstone belts that are characteristic of archean terrains

strain accumulation in western washington the juan de fuca plate is subducted beneath the north american plate off the coast of washington at a rate of about 40 mm yr n68 e the average principal strain rates extension reckoned positive measured in northwestern washington are as follows olympic peninsula 25 km south of port angeles from 1982 through 1990 e 1 0 011 0 027 strain yr n31 e 6 6 and e 2 0 092 0 028 strain yr n59 w 6 6 and near seattle from 1972 through 1985 e 1 0 027 0 019 strain yr n22 e 6 4 and e 2 0 036 0 013 strain yr n68 w 6 4 both strain measurements are consistent with uniaxial contraction in the direction of plate convergence uplift rates inferred from tide gage recordings are about 4 mm yr on the pacific coast and near 0 mm yr farther inland near seattle these deformation rates are consistent with a model of the cascadia subduction zone in which the plate interface beneath the continental slope and outer continental shelf is locked but free to slip farther landward the limited downdip extent of the locked segment of the plate interface is consistent with a shallow depth 20 km of the isotherm 450 c that defines the brittle ductile transition small thrust events diagnostic of seismic subduction should then occur only offshore and at shallow depths the principal strain rates measured from 1972 through 1983 in the back arc region near richland washington are e 1 0 016 0 013 strain yr n03 w 34 and e 2 0 024 0 013 strain yr n87 e 34

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

continental thermal and tectonic regimes during the archaean metamorphic pressures estimated from mineral assemblages in archaean high grade terrains may be used to constrain geothermal regimes in the archaean continental crust in two ways first they may be combined with temperature estimates to yield bounds on the heat flux during the thickening events that produced the metamorphism the pressure temperature data do not require any greater supply of heat to the base of the continental crust in the archaean than is supplied at the present day second the pressures alone may be used to infer the height of archaean mountains these mountains appear to have had elevations comparable with those of present day mountain belts it seems probable that the elevation of present day belts is limited by the creep strength of the lithosphere and in view of the strong temperature dependence of creep in earth materials this second observation also implies that archaean continental thermal regimes were similar to present day ones these observations together with the common

on the stability of isostatically compensated mountain belts the formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence the evolution of potential energy stored in an orogen is dependent on 1 the density structure 2 the thermal evolution and 3 the way convergent deformation is partitioned between crust and mantle lithosphere it is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension or collapse may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened e g england 1987 in this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the brace goetze model for the rheology of the lithosphere the reference lithosphere defined to be in potential energy and isostatic balance with the mid ocean ridges changes with different thermal parameters of the lithosphere the instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts measured relative to the reference state with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range it is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates greater than a few times 10 16 s 1 and leading to significant finite extension providing the moho temperatures exceed about 650 700 c a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust consequently processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse the calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening i e fl fc 2 where fl and fc are the lithospheric and crustal thickening factors

critical stress difference and orientation of faults in rocks with strength anisotropies the two dimensional case abstract equations are derived for the critical stress difference on thrust normal and strike slip faults with finite cohesive strength both in homogeneous isotropic rock and along pre existing strength anisotropies with different cohesion and coefficient of friction subject to the limitation that the plane of anisotropy contains the intermediate axis of stress the range of orientations for which sliding occurs along pre existing planes of weakness rather than along a new fault is given as a function of material parameters and critical stress difference for the intact rock given the principal stress directions the equations allow the direct calculation of the stress and orientation conditions for the three faulting regimes

implications of melting for stabilisation of the lithosphere and heat loss in the archaean abstract the increased depth and volume of melting induced in a higher temperature archaean mantle controls the stability of the lithosphere heat loss rates and the thickness of the oceanic crust the relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions the density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200 c or more hotter than modern upper mantle if archaean continental crust formed initially above oceanic lithosphere the compositional density gradients may be sufficient to preserve a thick archaean continental lithosphere within which the archaean age diamonds are preserved the amount of heat advected by melts at mid ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases archaean tectonics could have been dominated by spreading rates 2 3 times greater than now and with mantle temperatures between ca 1600 c and 1800 c at the depth of the solidus mid ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form protected from copious volcanism and high mantle temperatures

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

lithospheric thermal regime under the qinling orogenic belt and the weihe basin a transect across the yangtze and the north china cratons in central china abstract the thermal regime of the lithosphere could be a clue for the processes of geodynamics based on the yuquan yulin seismic reflection transect along the sichuan basin yangtze craton qinling orogenic belt qob weihe basin ordos basin north china craton we modified crustal and mantle lithospheric structures to fit the observed data such as elevation gravity geoid height and heat flow then we studied thermal structure lithospheric thermal thickness and rheological structure of this area the results suggest that the moho temperature along the transect ranges from 580 to 740 c the thickness of the thermal lithosphere varies from 70 to 190 km the rheological strength is characterized by the rheological characteristics of weak crust strong mantle in the sichuan basin the south qob and the ordos basin and strong crust weak mantle in the north qob and weihe basin the sichuan basin has the lowest heat flow the thickest lithosphere and the strongest lithospheric strength while the north qob and weihe basin have the highest heat flow the thinnest lithosphere and the weakest lithosphere the thermal regime of the sichuan and ordos basins is affected by the stability of the craton the thermal regime of the qob and weihe basin respond to the thermal effect of extensional rifting tectonics the thermal regime along the transect indicates that this area is mainly affected by the convergence caused by the continental collision of the indian plate

lithosphere strength inferred from fracture strength of rocks at high confining pressures and temperatures abstract a model is proposed for the strength of the crust or lithosphere in which the peak strength is in the brittle regime but not on the brittle ductile transition the model is based on experimental evidence which suggests that the fracture mechanism changes from ordinary brittle fracture to a high pressure type when the compressive strength equals the frictional strength the model assumes that a high pressure type of fracture is dominant in the earth s lower crust except where very high pore pressures exist it takes account of the scale effect on the strength of rocks the strength of the high pressure regime is estimated as a function of depth or pressure and temperature using experimental results derived at high confining pressures and temperatures in the upper part of the brittle zone where the frictional strength is lower than the compressive strength byerlee s friction law is used in the ductile regime the steady state creep law is used two cases of lithospheric strength dry and wet hydrostatic water pressure respectively are evaluated for thrust strike slip and normal faulting mechanisms with the exception of a wet lithosphere and a normal faulting mechanism the strength between the frictional and creep regimes is approximately constant with depth although the peak strength is in this part

claritas rise mars pre tharsis magmatism claritas rise is a prominent ancient noachian center of tectonism identified through investigation of comprehensive paleotectonic information of the western hemisphere of mars this center is interpreted to be the result of magmatic driven activity including uplift and associated tectonism as well as possible hydrothermal activity coupled with its ancient stratigraphy high density of impact craters and complex structure a possible magnetic signature may indicate that it formed during an ancient period of mars evolution such as when the dynamo was in operation as tharsis lacks magnetic signatures claritas rise may pre date the development of tharsis or mark incipient development since some of the crustal materials underlying tharsis and older parts of the magmatic complex respectively could have been highly resurfaced destroying any remanent magnetism here we detail the significant characteristics of the claritas rise and present a case for why it should be targeted by the mars odyssey mars reconnaissance orbiter and mars express spacecrafts as well as be considered as a prime target for future tier scalable robotic reconnaissance