the dynamics of motion of the south american plate observations of the intraplate stress field as compiled and evaluated in the context of the world stress map project can be compared with stress modelling results to increase our understanding of the dynamics of motion of the lithospheric plates in this paper we investigate the driving and resistive plate tectonic forces acting on the south american plate in order to establish a detailed basis for future stress modelling our basic assumption is that the plate is in mechanical equilibrium that is the net torque of the forces acting on the plate vanishes the complexity of the forces exerted along the margins of the south american plate requires the dynamical analysis to be performed step wise incorporating the forces which are expected to be of major importance a first order model is defined the effect of additional plate boundary forces is subsequently examined in terms of superpositions onto the first order model with the first order model the relation between three resistive forces and shear stress on the base of the lithosphere is derived the ridge push force driving the motion of the south american plate can be quantified independently and is included in the calculations as a known quantity solutions are obtained using four different determinations of the euler vector of absolute motion of the south american plate which is taken to define the orientation of the basal shear stress the results demonstrate that the concept of a basal shear stress actively driving plate motion cannot be discarded on the basis of a dynamical analysis further with the assumption that basal drag is entirely concentrated below the continental pan of the lithosphere estimates of the maximum amount of continental basal shear resistance are derived evaluation of an alternative model for the south american plate excluding the northernmost pan of the andes demonstrates the significance of the resistive force exerted on the south american plate due to convergence with the caribbean plate it is to be expected that comparison of theoretical distributions of intraplate stress computed on the basis of the present dynamical analysis with observations of the actual state of stress will further constrain the range of feasible force models

contribution of gravitational potential energy differences to the global stress field summary modelling the lithospheric stress field has proved to be an efficient means of determining the role of lithospheric versus sublithospheric buoyancies and also of constraining the driving forces behind plate tectonics both these sources of buoyancies are important in generating the lithospheric stress field however these sources and the contribution that they make are dependent on a number of variables such as the role of lateral strength variation in the lithosphere the reference level for computing the gravitational potential energy per unit area gpe of the lithosphere and even the definition of deviatoric stress for the mantle contribution much depends on the mantle convection model including the role of lateral and radial viscosity variations the spatial distribution of density buoyancies and the resolution of the convection model gpe differences are influenced by both lithosphere density buoyancies and by radial basal tractions that produce dynamic topography the global lithospheric stress field can thus be divided into 1 stresses associated with gpe differences including the contribution from radial basal tractions and 2 stresses associated with the contribution of horizontal basal tractions in this paper we investigate only the contribution of gpe differences both with and without the inferred contribution of radial basal tractions we use the crust 2 0 model to compute gpe values and show that these gpe differences are not sufficient alone to match all the directions and relative magnitudes of principal strain rate axes as inferred from the comparison of our depth integrated deviatoric stress tensor field with the velocity gradient tensor field within the earth s plate boundary zones we argue that gpe differences calibrate the absolute magnitudes of depth integrated deviatoric stresses within the lithosphere shortcomings of this contribution in matching the stress indicators within the plate boundary zones can be corrected by considering the contribution from horizontal tractions associated with density buoyancy driven mantle convection deviatoric stress magnitudes arising from gpe differences are in the range of 1 4 tn m 1 a part of which is contributed by dynamic topography the egm96 geoid data set is also used as a rough proxy for gpe values in the lithosphere however gpe differences from the geoid fail to yield depth integrated deviatoric stresses that can provide a good match to the deformation indicators gpe values inferred from the geoid have significant shortcomings when used on a global scale due to the role of dynamically support of topography another important factor in estimating the depth integrated deviatoric stresses is the use of the correct level of reference in calculating gpe we also elucidate the importance of understanding the reference pressure for calculating deviatoric stress and show that overestimates of deviatoric stress may result from either simplified 2 d approximations of the thin sheet equations or the assumption that the mean stress is equal to the vertical stress

fission track evidence of the cenozoic thermal history of the idaho batholith abstract apatite and zircon fission track data are combined with published k ar biotite dates from the idaho batholith to outline the cenozoic thermal history of central idaho apatite apparent ages reveal that parts of the batholith have been at temperatures below 100 c for over 50 ma suggesting that much of the batholith has remained at shallow crustal levels throughout cenozoic time eocene plutonism and attendant hydrothermal circulation resulted in thermal resetting of apatite and zircon dates from near the plutons concordant zircon and biotite dates indicate that hydrothermal systems associated with eocene plutonism operated at shallow crustal levels so that cooling occurred rapidly as geothermal activity waned the calculated depth to the 105 c apatite annealing isotherm suggests that as much as 3 km of erosion has occurred in deep canyons during the last 10 ma presumably associated with final erosion of the batholith following the establishment of drainage at low elevation along the snake river plain

style and intensity of late cenozoic deformation in the nagoorin basin eastern queensland australia and implications for the pattern of strain in an intraplate setting eastern australia was affected by late cenozoic intraplate deformation in response to far field stress transmitted from the plate boundaries but little is known about the intensity and pattern of this deformation we used recently surveyed two dimensional seismic reflection lines and aeromagnetic data and data from the recently released australian stress map to investigate the structure of the nagoorin basin in eastern queensland the western margin of the nagoorin beds was displaced by the boynedale fault which is a nnw striking sw dipping oblique strike slip reverse fault with a vertical throw of c 900 m and c 16 km sinistral displacement a significant part of this large sinistral displacement is interpreted to have occurred prior to late cenozoic time several low angle 30 thin skinned thrusts with a flat ramp geometry also displaced the nagoorin beds which are interpreted to have developed along detachment surfaces in oil shales and claystone the boynedale fault is a segment within longer nnw striking faults that include the north pine and west ipswich fault systems in eastern queensland these nnw striking faults are potentially active and may accommodate neotectonic thrust movement in response to the present day ne sw orientation of s results of this study in conjunction with previous information on sedimentary basins in eastern australia indicate that cenozoic contractional deformation is stronger at the continental margins possibly due to the presence of pre existing rift related structures

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

relationship of the trans challis fault system in central idaho to eocene and basin and range extensions the trans challis fault system tcfs is a new structural province in central idaho mapped during the challis cusmap program conducted by the u s geological survey the tcfs contains numerous northeast trending faults several eruptive centers for the challis volcanics and many precious metal deposits major movement occurred in the fault system during eocene extension that affected the area from the snake river plain north into british columbia the eastern snake river plain is the southern boundary of eocene extension and in idaho the lewis and clark line is the northern boundary the eastern snake river plain was welded together by passage of the plain over the yellowstone hotspot younger basin and range faults were then able to extend across the plain and now terminate at the next major eocene crustal break north of the plain the tcfs the tcfs is an extensional feature equal in importance to the lewis and clark zone and the eastern snake river plain and should be considered in tertiary reconstructions for the pacific northwest

kinematics and timing of three superposed extensional systems east central idaho evidence for an eocene tectonic transition cenozoic crustal extension in east central idaho began about 50 ma and continues at present three distinct episodes characterize one of the longest intervals of cenozoic extension yet documented in the north america cordillera crosscutting relationships between ne striking normal faults and volcanic rocks regional dike trends and slickenline data indicate nw se extension during peak eocene volcanism about 49 48 ma episode 1 ne striking normal faults with at most a few kilometers of offset formed in an intraarc setting during rapid ne subduction of oceanic plates under the pacific northwest north to nnw striking and west dipping normal faults with offsets up to 10 15 km formed during a younger middle eocene to oligocene basin forming event episode 2 this newly documented episode was the most important extensional event in east central idaho and began during the waning phases of challis volcanism wsw ene to sw ne extension during episode 2 was nearly perpendicular to the extension direction during episode 1 and perpendicular to the grain of the idaho montana fold and thrust belt the flip in extension direction between episode 1 and episode 2 is tightly constrained by 40ar 39ar age determinations to have taken place at the end of eocene challis magmatism about 46 48 ma i infer that plate boundary forces controlled the geometry of normal faults and dikes during episode 1 whereas internal stresses within previously thickened crust drove major sw to wsw directed extension during episode 2 a drop in convergence rates between the north american and farallon plates between 59 ma and 42 ma stock and molnar 1988 may coincide with the onset of gravitational spreading during episode 2 and may also explain the abrupt end of eocene magmatism in the pacific northwest miocene and younger sw dipping basin and range faults episode 3 extended the region in a ne sw direction although faults formed during episode 2 and episode 3 are not parallel slickenlines indicate only small changes in slip vector trends suggesting little rotation of the extension direction in east central idaho since 46 ma

the world stress map database release 2016 crustal stress pattern across scales abstract knowledge of the present day crustal in situ stress field is a key for the understanding of geodynamic processes such as global plate tectonics and earthquakes it is also essential for the management of geo reservoirs and underground storage sites for energy and waste since 1986 the world stress map wsm project has systematically compiled the orientation of maximum horizontal stress shmax for the 30th anniversary of the project the wsm database has been updated significantly with 42 870 data records which is double the amount of data in comparison to the database release in 2008 the update focuses on areas with previously sparse data coverage to resolve the stress pattern on different spatial scales in this paper we present details of the new wsm database release 2016 and an analysis of global and regional stress pattern with the higher data density we can now resolve stress pattern heterogeneities from plate wide to local scales in particular we show two examples of 40 60 shmax rotations within 70 km these rotations can be used as proxies to better understand the relative importance of plate boundary forces that control the long wave length pattern in comparison to regional and local controls of the crustal stress state in the new wsm project phase iv that started in 2017 we will continue to further refine the information on the shmax orientation and the stress regime however we will also focus on the compilation of stress magnitude data as this information is essential for the calibration of geomechanical numerical models this enables us to derive a 3 d continuous description of the stress tensor from point wise and incomplete stress tensor information provided with the wsm database such forward models are required for safety aspects of anthropogenic activities in the underground and for a better understanding of tectonic processes such as the earthquake cycle

tertiary calc alkaline magmatism associated with lithospheric extension in the pacific northwest the long standing assumption that calc alkaline magmatism requires contemporaneous subduction is critically reviewed two of many widely spread fields of tertiary calc alkaline magmatism in the pacific northwest one eocene and one miocene provide clear examples of the association between calc alkaline magmatism and lithospheric extension we suggest that the calc alkaline magmatism of these fields is better explained by decompression melting of the subcontinental lithospheric crust and mantle enriched by previous subduction events than by contemporaneous eocene subduction this model proposes that extension will cause calc alkaline dominated magmatism in areas of continental crust with a history of previous subduction as along the western margin of north america currently popular models for the tectonic magmatic evolution of the basin and range province in which subduction driven calc alkaline magmatism is followed by extension related bimodal magmatism may need modification much more of this magmatism may result from extension in the pacific northwest we suggest that subduction beneath the idaho batholith ceased at the end of the cretaceous blocked by the accreted terranes to be reactivated in its present position beneath the cascades during the eocene the various isolated eocene volcanic fields spread across the pacific northwest were due to lithospheric extension not to the presence of a subducting plate immediately below

paleo climate of the boise area idaho from the last glacial maximum to the present based on groundwater δ2h and δ18o compositions abstract a 30 ka paleo climate record of the boise area idaho usa has been delineated using groundwater stable isotopic compositions groundwater ages are modern cold batholith 5 15 ka thermal batholith 10 20 ka frontal fault and 20 30 ka snake river plain thermal the stable isotopic composition of groundwaters have been used as a surrogate for the stable isotopic composition of precipitation using 2h and 18o compositions local groundwater lines lgwl s were defined for each system each lgwl has been evaluated with defined slopes of 6 94 and 8 respectively and resulting deuterium excess values d were found for each groundwater system for each slope time dependent changes in moisture source humidity and temperature and boise area recharge temperatures calculated from stable isotopic data and the deuterium excess factors agree with previous paleo climate studies results indicate that from the last glacial maximum to the present time the humidity over the ocean moisture source increased by 9 sea surface temperature at the moisture source increased 6 7 c and local boise temperature increased by 4 5 c a greater increase of temperature at the moisture source as compared to the boise area may impart be due to a shift in the moisture source area

episodic intraplate deformation of stable continental margins evidence from late neogene and quaternary marine terraces cape liptrap southeastern australia the waratah fault is a northeast trending high angle reverse fault in the late paleozoic lachlan fold belt at cape liptrap on the southeastern australian coast it is susceptible to reactivation in the modern intraplate stress field in southeast australia and exhibits late pliocene to late pleistocene reactivation radiocarbon optically stimulated luminescence osl and cosmogenic radionuclide crn dating of marine terraces on cape liptrap are used to constrain rates of displacement across the reactivated waratah fault six marine terraces numbered qt6 tt1 youngest to oldest are well developed at cape liptrap with altitudes ranging from 1 5 m to 170 m amsl respectively on the lowest terrace qt6 barnacles in wave cut notches 1 5 m amsl yielded a radiocarbon age of 6090 5880 cal bp and reflect the local mid holocene sea level highstand qt5 yielded four osl ages from scattered locations around the cape ranging from 80 ka to 130 ka it formed during the last interglacial sea level highstand mis 5e at 125 ka inner edge elevations approximate paleo high tide line for qt5 occur at distinctly different elevations on opposite sides of the waratah fault offsets of the inner edges across the fault range from 1 3 m to 5 1 m with displacement rates ranging from 0 01 mm a to 0 04 mm a the most extensive terrace tt4 yielded four early pleistocene cosmogenic radionuclide crn ages two apparent burial ages of 0 858 ma 0 16 ma and 1 25 ma 0 265 ma and two apparent exposure ages of 1 071 ma 0 071 ma 10be and 0 798 ma 0 066 ma 26al allowing for muonic production effects from insufficient burial depths the depth corrected crn burial ages are 1 8 ma 0 56 ma and 2 52 ma 0 88 ma or late pliocene a late pliocene age is our preferred age offsets of tt4 across the waratah fault range from a minimum of 20 m for terrace surface treads to a maximum of 70 m for terrace bedrock straths calculated displacement rates for tt4 range from 0 01 mm a to 0 04 mm a using a late pliocene age 2 ma identical to the rates calculated for the last interglacial terrace qt5 this indicates that deformation at cape liptrap has been ongoing at similar time averaged rates at least since the late pliocene the upper terraces in the sequence tt3 110 m amsl tt2 140 m and tt1 180 m are undated but most likely correlate to sea level highstands in the neogene terraces tt1 tt4 show an increasing northward tilt with age the waratah fault forms a prominent structural boundary in the lachlan fold belt discernible from airborne magnetic and bouger gravity anomalies seismicity and deformation are episodic episodic movement on the waratah fault may be coincident with sea level highstands since the late pliocene possibly from increased loading and elevated pore pressure within the fault zone this suggests that intervals between major seismic events could be on the order of 100 ka

the tectonic stress field evolution of india since the oligocene abstract a multitude of observations suggest neotectonic deformation in and around india but its causes and history are unknown we use a 2 dimensional finite element model with heterogeneous elastic strengths in continental regions to model the regional stress field orientation and relative magnitudes since the oligocene the large scale geological structure of india is embedded in our model by using published outlines of cratons fold belts and basins associated with estimates of their relative strengths enabling the modelling of stress field deflections along interfaces between relatively strong and weak tectonic elements through time at 33 ma a roughly nnw sse oriented band of relatively high maximum horizontal compressive stress s hmax straddled india s west coast while india s east coast and the adjacent wharton basin were characterized by relatively low intraplate stresses between 20 ma and the present growing collisional boundary forces combined with maturing mid ocean ridge flanks result in the establishment of an arcuate belt with anomalously high intraplate stress that stretches from india to the wharton basin intersecting the continental shelf roughly orthogonally and crossing the 85 east and ninetyeast ridges this results in a compressive tectonic regime favouring folding and inversion northeast of the godavari graben on india s east coast as observed in seismic reflection data whereas no tectonic reactivation is observed on the continental margin further north closer to the mahanadi graben or further south our stress models account for these differences via spatial variations in modelled horizontal stress magnitudes and intersection angles between margin paralleling pre existing basement structures and the evolving neogene stress field the models further account for fracture zone strike slip reactivation offshore sumatra and lithospheric folding along india s west and southeast coast and can be used to estimate the onset of these deformation episodes to at least the oligocene and miocene respectively

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

structural sequences and styles of subsidence in the michigan basin subsidence in the michigan basin produced 5 km of sedimentation over a period of more than 200 m y during paleozoic time utilizing well log correlations and constrained by compaction corrections and estimates of paleobathymetry we recognize four different styles of subsidence in the basin trough shaped regional tilting narrow basin centered and broad basin centered subsidence began as a trough shaped northerly extension of the illinois basin during late cambrian to early ordovician time this was followed by narrow basin centered subsidence in early to middle ordovician time basin centered subsidence ceased for 30 m y during long wavelength 1000 km eastward tilting in middle to late ordovician time a pattern incompatible with thermal contraction subsidence models basin centered subsidence resumed in silurian time but with a broader distribution a second episode of narrow basin centered subsidence occurred in latest silurian through middle devonian time and was replaced by broad basin centered subsidence at the end of middle devonian time the geometry of upper devonian and younger paleozoic deposits suggests another eastward tilting event but results remain inconclusive due to erosion of strata and uncertainties in their paleobathymetry in addition to these subsidence patterns two distinct unconformity styles are present basin wide and marginal erosion there is no evidence for significant basin centered unconformities as predicted by purely thermal mechanisms a history of episodic subsidence reactivations is interpreted as the result of a stress induced crustal weakening mechanism for the narrow basin centered subsidence whereas broad basin centered subsidence is interpreted as thermal contraction related to lower crustal attenuation during the narrow subsidence episodes recently proposed dynamic topography related to initiation of ordovician subduction provides a driving mechanism for long wavelength eastward tilting together with a temporal correlation to appalachian tectonism these mechanisms provide a plate tectonic framework for the history of the michigan basin

stress triggering of conjugate normal faulting late aftershocks of the 1983 ms 7 3 borah peak idaho earthquake the 1984 devil canyon sequence was a late aftershock sequence of the 28 october 1983 ms 7 3 borah peak idaho earthquake the sequence began on 22 august 1984 with the ml 5 8 devil canyon earthquake which nucleated at a depth of 12 8 0 7 km between the surface traces of two normal faults the challis segment of the lost river fault and the lone pine fault two hundred thirty seven aftershocks were recorded by a temporary array during a 3 week period their focal mechanisms and hypocenter distribution define a cross sectional v pattern whose base corresponds to the ml 5 8 event whose tips correspond to the exposed fault traces and whose sides define two planar fault zones oriented n25 w 75 sw challis fault segment and n39 w 58 ne lone pine fault this pattern describes a graben bounded by conjugate normal faults temporal aspects of the devil canyon sequence provide strong evidence that slip on conjugate normal faults occurs sequentially aftershocks occurred primarily along the challis segment until the occurrence of the 8 september 1984 ml 5 0 earthquake along the lone pine fault after which aftershocks primarily occurred along this fault these observations are consistent with worldwide seismologic and geologic observations and with physical andmore numerical models of conjugate normal faulting aftershocks of the devil canyon sequence occurred immediately northwest of the ml 5 8 devils canyon earthquake which itself was immediately northwest of the thousand springs segment of the lost river fault the fault that slipped in association with the ms 7 3 borah peak earthquake coulomb failure stress analysis indicates that stress increases resulting from both the borah peak mainshock and devil canyon ml 5 8 earthquake were sufficient to induce failure on the lone pine fault these space time patterns suggest that conjugate normal faults may transfer stress or accommodate stress changes at the terminations of major normal faults in the basin and range province less

analysis of the south american intraplate stress field the first order south american intraplate stress field was modeled through a finite element analysis to evaluate the relative contribution of plate boundary forces and intraplate stress sources the finite element mesh consisted of 3100 nodes in a network of 5993 equal area triangular elements which provided a spatial resolution of about 1 at the equator an important aspect of our modeling is the inclusion of topographic forces due to the cooling oceanic lithosphere along the mid atlantic ridge e g ridge push the continental margins along the east coast of brazil and argentina and the elevated continental crust e g the andean cordillera predicted intraplate stresses for two representations of the western collisional boundary forces are evaluated pinned collisional boundaries and applied collisional boundary forces constraint for the modeling was provided by information about the orientation of the maximum horizontal compressive stress shmax provided by 217 stress indicators from the world stress map project as well as by shmax magnitude estimates and torque information from previous investigations our modeling results demonstrate that the first order features of the observed stress field can be explained with simple tectonic models which balance the torque acting on the plate either with a fixed western margin or drag forces applied along the base of the plate the predicted intraplate stress field is characterized by a nearly uniform e w shmax orientation throughout most regions of the plate with stress magnitudes generally less than 20 mpa averaged over a 100 km thick lithosphere significant perturbation of this regional stress field occurs in the western part of the plate in response to forces associated with the high topography of the andes although the magnitude of the collisional boundary forces acting along the western margin remains poorly constrained we estimate a plausible upper bound on the force per unit length acting along the peru chile trench to be about 2 5 1012 n m 1 while some of our models are consistent with a driving basal drag to balance the torques acting on the plate the magnitude of the drag torque is small compared to the contribution from other sources of stress such as the ridge push force

localized rotation during paleogene extension in east central idaho paleomagnetic and geologic evidence we collected paleomagnetic data from 137 sites in middle eocene volcanic rocks of the northeastern basin and range province in order to characterize the three dimensional kinematics of continental extension in the lost river and lemhi ranges idaho and to test previous block rotation models statistically significant flattening of directions at three of 10 localities reflects either insufficient averaging of paleosecular variation or erroneous tilt corrections five of the seven remaining locality means are statistically indistinguishable from the middle eocene reference direction this indicates that vertical axis rotation did not accompany post 50 ma normal faulting discordant declinations from two localities in the northern lost river range indicate 30 40 of counterclockwise rotation of a large domain of nnw striking volcanic rocks inclination flattening at one of the two rotated localities is moderate but statistically significant f f 9 9 6 5 in total the paleomagnetic data conflict with earlier predictions of uniform clockwise rotation in response to right lateral transtension adjacent to the eastern snake river plain there is a close correspondence between counterclockwise rotated localities and domains of nnw striking volcanic rocks and between unrotated localities and domains of north striking volcanic rocks new mapping shows that counterclockwise rotated localities are all in the hanging wall of a major west dipping low angle normal fault the donkey fault named for the first time this fault loses displacement southwards was active in eocene oligocene time and is probably a tilted high angle fault the spatial distribution of rotated and unrotated localities is best explained by counterclockwise rotation of the hanging wall of the donkey fault during opening of a southward tapering half graben according to this hanging wall rotation model the footwall of the donkey fault remained fixed relative to its rotating hanging wall our proposed kinematic model is generally applicable to extending continental lithosphere and provides a mechanism for coeval crustal thinning and vertical axis rotation thus vertical axis rotation may occur in the absence of strike slip faults in extensional orogens

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

paleoclimatic investigations using isotopic signatures of the late pleistocene holocene groundwater of the stratified aquifers in kuwait abstract the study attempts to determine paleo recharge sources for two major aquifer units of kuwait namely kuwait group aquifer kga and dammam formation dfm were collected and analyzed for major ions and isotopes the obtained results of 14c ages indicated that the groundwater samples ranged from 31 9 ky to 3 9 ky in dfm and 23 3 ky to 0 8 ky in kga the variations in 18o signatures with respect to age reflected that there were two major sources of rainwater such as enriched southern indian ocean monsoon and the depleted northwesterly mediterranean vapor source the samples found to show variations during the three marine isotopic stages mis 1 3 heinrich events bolling allerod and dryas time periods in both aquifers a shift in the intertropical convergence zone itcz was witnessed in the groundwater of both aquifers from 7 8 ky to 4 9 ky the variation in the holocene climate is mainly due to the influence of tropospheric easterly jet governed by the swing in itcz as a result of holocene volcanism the mean 18o values of dfm samples 3 81 reflect depletion than kga samples 2 31 and the d excess values were 0 95 and 2 05 respectively thus it could be inferred that the dfm was predominantly recharged during late pleistocene period and it was in a relatively cooler climate than recharge environment of kga the study infers that kga was recharged between 26on and 26 8on latitudes and that of the dfm was noted to be recharged south west of kuwait in two different patches between 25 8on and 26 2on and 26 7on 27 4on latitudes during late pleistocene the kga was concluded to be recharged between 25 5on and 27 5on latitudes south of kuwait during holocene

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

ridge forces absolute plate motions and the intraplate stress field torque poles are calculated for a variety of possible forces acting on the plates including ridge push slab pull and collisional resistance these torque poles are then compared to the directions of absolute plate motions there is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the north american south american pacific cocos and eurasian plates simple slab pull torques correlate well with absolute motion azimuths for the pacific nazca and cocos plates and moderately well with the absolute motion azimuth of the indoaustralian plate collisional resistance torque poles correlate with the absolute motion azimuth of the eurasian plate only the correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves torque poles for various forces are also compared with several long wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions in general ridge torque directions agree well with the orientations of maximum horizontal stresses for stable north america western europe and south america and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag collisional resistance forces can also explain the alignment of stresses in western europe for the indoaustralian plate the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate other processes in addition to collisional resistance must be important for the indo australian plate ridge push forces may account for a significant portion of the long wavelength features of the intraplate stress field especially away from continental collisions such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism as previously suggested slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates

timing of extension in the pioneer metamorphic core complex with implications for the spatial temporal pattern of cenozoic extension and exhumation in the northern u s cordillera 1 the pioneer core complex pcc in central idaho lies along a transition between early eocene and ca 40 ma core complexes to the north and south respectively thus the age of extensional development of the pcc is important in understanding the spatial temporal patterns of core complex development in the north american cordillera new results including structural observations and u pb zircon shrimp and icpms geochronology constrain the early extensional history of the footwall for the first time high temperature strain with a top wnw shear sense is pervasive throughout metamorphic rocks of the northwestern footwall an isoclinally folded dike yields a crystallization age of 48 47 ma whereas a crosscutting dike yielded an age of 46 ma metamorphic rocks are also intruded by the 50 48 ma pioneer intrusive suite pis a w dipping granodiorite sheet displaying a magmatic fabric northwest trending lineations are locally visible and also defined by anisotropy of magnetic susceptibility indicating that during emplacement the pis was undergoing similarly oriented extensional strain as the enclosing metamorphic rocks therefore wnw directed extension spanning this structural section occurred between 50 and 46 ma following emplacement of crosscutting 46 ma dikes deformation was partitioned into the wnw directed wildhorse detachment motion on the detachment occurred between 38 and 33 ma as documented by previous 40ar 39ar thermochronology it is not clear however whether extension was continuous through the interval between these two time periods although early eocene extension in the pcc was synchronous with extension in core complexes to the north rates of footwall exhumation in central idaho were much lower this southward slowing is compatible with n s differences in inferred subduction zone geometry kinematics and in the internal character of the orogenic wedge

source parameters of seven large australian earthquakes determined by body waveform inversion summary we determine source mechanisms and centroid depths for the seven largest earthquakes in and near the australian continent in the past 20yr using teleseismic body waves six of the earthquakes were thrusts and one was strike slip all five earthquakes that occurred beneath the continent were shallower than 10 km the ranges of centroid depths are constrained to be 1 5 2 4 0 3 5 11 and 6 10km and indicate nearly pure thrust faulting the three shallowest events were associated with surface faulting while the published first motion solutions for these three events included large amounts of strike slip our thrust mechanisms are more consistent with displacements observed at the surface of the earth the range of possible centroid depths for a strike slip earthquake beneath the continental shelf off the northwest coast is 23 29 km and that for a thrust earthquake in oceanic lithosphere beneath the tasman sea is 22 26 km below sea level the pressure axis is consistently near horizontal and in the western half of the continent five earthquakes indicate that the azimuth of the maximum compressive stress is e ene azimuthal range of 66 91 on the basis of one earthquake we conclude that the p axis is oriented nnw beneath the tasman sea

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

on the forces driving plate tectonics inferences from absolute plate velocities and intraplate stress summary absolute plate motions and intraplate stress both serve as tests of models for the forces acting on plate boundaries plate velocities relative to a presumedly fixed underlying mantle are calculated from the hypothesis that no net torque is exerted on the lithosphere intraplate stress is calculated by solving the equilibrium equations for thin elastic shells in the membrane state of stress the absolute velocity fields predicted from a wide assortment of physical and geological models are all very similar while the global pattern of absolute velocity is probably close to those predicted by such models the absolute motions do not therefore provide a strong test of the driving mechanism comparison of predicted intraplate stress with the long wavelength features of the global stress field however as determined by in situ measurements earthquake mechanisms and stressinduced geological structures does prove to be a powerful test of possible driving forces all absolute velocity models have several interesting properties lithosphere in the equatorial half of the earth is moving significantly faster than lithosphere in the polar half some connection to the earth s rotation is implied since this statement is demonstrably untrue for co ordinate poles much different from the geographic pole subducted slabs are characterized by a slow horizontal translation perpendicular to strike that is independent of the plate convergence rate confounding attempts to explain the dip angles of benioff zones in terms of a uniform vertical sinking and a variable absolute velocity for the overthrust plate ridges must migrate at a wide range of velocities relative to their underlying source of new lithosphere such rapid migration may be a necessary but is not a sufficient condition for ridge jumps force models considered in velocity and stress calculations include driving forces at spreading centres and subduction zones and various parameterizations of drag at the base of the lithosphere from the rms absolute velocities of individual plates there is a weak indication that pull by subducted lithosphere at trenches is an important driving force and that drag may be greater beneath continental than oceanic lithosphere the predicted intraplate deviatoric stress cannot match the well determined stress fields in north america and europe unless the driving force exerted at ridges is at least comparable in magnitude to other forces in the system the mid plate stresses are very sensitive to the nature of drag at the base of the lithosphere and thus measured stresses may ultimately provide a sensitive test of absolute plate velocities

paleogeographic evolution of the united states pacific northwest during paleogene time paleogeographic reconstructions for oregon and washington during paleogene time illustrate a major transition from a dominantly compressional prior to middle eocene time to an extensional tectonic regime this transition resulted in the development of three phases of paleogene basin evolution in the united states pacific northwest during the initial phase basins formed along the continental margin during collision of oceanic islands sediments in these basins were derived from nearby orogenic highlands the second phase of basin development began in middle eocene time and consisted of rapid subsidence of individual basins that formed within a broad forearc region nonmarine basins that formed during this phase were caused by extension possibly associated with transcurrent faulting rapid sedimentation in both marine and nonmarine basins during this time consisted dominantly of sandstone derived from cretaceous plutonic sources far to the east the final stage of basin development was the modification

statistical trends in the intraplate stress field the world stress map wsm database contains thousands of intraplate stress indicators with the potential to provide important constraint for arguments about the relationship between tectonic stresses and both the kinematics and dynamics of plate motion previous studies which relied almost exclusively on visual inspection of the data established the existence of broad regions of uniform maximum horizontal compressive stress orientation shmax and stress regimes on a regional scale in the present study we present a statistical analysis of the wsm stress indicators with the aim of quantifying trends in both the shmax orientations and stress regimes the analysis was carried out within 5 5 bins which provide a resolution of several hundred kilometers only the 4537 high quality wsm indicators with rating of a to c were used in the analysis we present results for two types of analysis on the information contained within the bins first we evaluate the spatial distribution of the average stress regime normal strike slip or thrust second we apply the rayleigh test a standard statistical method in the analysis of directional data to the distribution of shmax orientations to test the null hypothesis that the orientations are random an important aspect of our study is the quantification of the conclusions drawn from visual inspection of the world stress map our results indicate that broad regions of uniform shmax orientations exist in most continental regions at high confidence levels 90 and 95 and are less robust in the slowest moving continental plates we also quantify the predominance of strike slip and compressional stress regimes in continental regions importantly our analysis provides information about trends in the shmax orientations in regions where large amounts of scatter in the directional data prevented conclusions being drawn from visual inspection of the data for example in western north america and continental australia furthermore we find a strong correlation between average shmax orientations and both the ridge push torque and the absolute plate velocity azimuths our observation that a greater number of shmax orientations correlate with the ridge push torque directions is further evidence that the intraplate stress field is strongly influenced by the ridge push force

the african plate a history of oceanic crust accretion and subduction since the jurassic we present a model for the jurassic to present evolution of plate boundaries and oceanic crust of the african plate based on updated interpretation of magnetic gravity and other geological and geophysical data sets location of continent ocean boundaries and age and geometry of old oceanic crust jurassic and cretaceous are updated in the light of new data and models of passive margin formation a new set of oceanic palaeo age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the african plate palaeo stresses and may have influenced the evolution of intracontinental sedimentary basins as a case study we compute a simple model of palaeo stress for the late cretaceous time in order to assess how ridge push slab pull and horizontal mantle drag might have influenced the continental african plate we note that the changes in length of various plate boundaries especially trenches do not correlate well with absolute plate motion but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity

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

on the gravitational potential of the earth s lithosphere the mean potential energy of the lithosphere is useful for defining the tectonic reference state trs of the earth and can be used to constrain the ambient state of stress in the plates in the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the trs would remain undeformed thus the difference between the potential energy of a lithospheric column and the trs determines whether the column is in an extensional joeutral or compressional state of stress we evaluate and intraplate variations about this mean using a simple first order lithospheric density model this model assumed that the continental geotherm is linear and density variations below a depth of 125 km have negligible influence on and is consistent with observed geoid anomalies across continental margins is estimated to be 2 379 1014 n m 1 which is equivalent to the potential energy of both near sea level continental lithosphere 160 to 220 m for an assumed crustal density c in the range 2800 2700 kg m 3 and cooling oceanic lithosphere at a depth of 4 3 km with the exception of eurasia which has anomalously high mean potential energy 2 383 1014 n m 1 the mean potential energies of the continental plates are nearly identical to the global mean the mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere both the global and plate mean potential energies are relatively insensitive to a wide range in c the potential of the mid ocean ridges 2 391 1014 n m 1 is greater than the global mean which is consistent with the divergent nature of the ridges elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to suggesting that in the absence of external forces continental regions will be in a slightly extensional state of stress the importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the south american nazca indo australian and pacific plates

regional stress field of the indian plate we have calculated the regional stress field in the indian plate implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure the high level of the calculated stress field order of a few kbar and the dominance of compression in the plate are consequences of the unique dynamic situation of the present day indian plate the calculated stress field explains the concentration of intraplate deformation in the region of the ninetyeast ridge and yields insight into the variations in stress directions in the australian continent

localization of intraplate deformation through fluid assisted faulting in the lower crust the flinders ranges south australia abstract in this paper we present a hypothesis for localized intraplate deformation in the continental crust of south central australia that involves fluid assisted reactivation of faults in the mid to lower crust using data from a temporary seismometer deployment in the flinders ranges we show that earthquakes relocated in a 3d velocity model cluster in elongated low vp vs anomalies that extend to depths exceeding 20 km and are aligned with the axis of the flinders ranges in the northern flinders ranges these low vp vs anomalies can be interpreted as fractured neoproterozoic to cambrian sediments that separate two cratonic blocks the gawler craton to the west and the curnamona province in the east previous studies of helium isotopes in springs to the north of the area provide evidence of mantle derived fluids that may influence faulting at depth our focal mechanism and stress inversion results show a regionally compressive stress field that provides no evidence for stress concentration we also argue that mechanisms for localized faulting such as thermal weakening and isostatic rebound also fail to account for the occurrence of earthquakes at mid to lower crustal depth in this area of high heat flow and that the focused seismicity can only be explained by high pore fluid pressure in the lower crust

quantitative resolution of the debate over antiquity of the central australian landscape implications for the tectonic and geomorphic stability of cratonic interiors we report the first measure of long 100 myr and short 1 myr term denudation rates from key geologically stable landforms in the davenport range central australia these landforms have previously been assigned a cambrian age which arguably places them amongst the oldest persistent landforms on the continent if not on earth our results from combined apatite fission track thermochronology and in situ cosmogenic radionuclide analyses using 10 be and 26 al show that while average exhumation rates are low the denudation history for this cratonic region is incompatible with extreme sub aerial longevity and long term tectonic and geomorphic stability our revised model for the landscape evolution of this region is consistent with one of maximum burial prior to and during the mesozoic followed by a phase of kilometre scale exhumation that was largely complete by the beginning of the cainozoic we suggest that a similar process of burial and exhumation has probably been responsible for the sub aerial preservation of seemingly ancient landforms elsewhere in australia

cause and evolution of intraplate orogeny in australia abstractintraplate orogeny remains an enigmatic process in terms of plate tectonics regional continental stress elds agree well with stress orientations predicted by the main forces driv ing and resisting plate motion however regional continental stress elds fail to explain major localized compressive plate deformation in continental interiors far away from plate bound aries global or plate wide intraplate stress models typically assume a laterally homogeneous plate rheology and are con ned to modeling the present without addressing the antiquity of the current stress eld an understanding of intraplate stress and deformation requires the linking of spatial variations in continental rheology with time dependent plate geometry and driving forces here we demonstrate how the complex interplay between juxtaposed weak and strong geological provinces and changes in far eld plate boundary forces has caused intra plate orogenesis and tectonic reactivation in southeastern australia during the tertiary our ndings are contrary to the opinion that continental interiors are insensitive to compression at plate margins and help to explain the mechanisms causing intraplate orogenesis keywords

india asia collision and the cenozoic slowdown of the indian plate implications for the forces driving plate motions the plate motion of india changed dramatically between 50 and 35 ma with the rate of convergence between india and asia dropping from 15 to 4 cm yr this change is coincident with the onset of the india asia collision and with a rearrangement of plate boundaries in the indian ocean on the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate we perform force balance calculations for the precollision and postcollision configurations we show that the observed euler poles for the indian plate are well explained in terms of their locations and magnitudes if 1 the resistive force induced by mountain building in the himalaya tibet area is 5 6 10 12 n m 2 the net force exerted upon the indian plate by subduction zones is similar in magnitude to the ridge push force 2 5 10 12 n m and 3 basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame the third point implies an asthenospheric viscosity of 2 5 10 19 pa s assuming a thickness of 100 150 km synthetic euler poles show that crustal thickening in the tibetan plateau was the dominant cause of the cenozoic slowdown of the indian plate

the origins of the intraplate stress field in continental australia the ridge push force acting on the indo australian plate exerts a significant torque 8 5 1025n m about a pole at 30 3 n 34 5 e the angular difference between this torque pole and the observed pole of rotation for the plate 19 2 n 35 6 e is less than 12 and suggests that the ridge push force plays an important role in the dynamics of the indo australian plate we have used an elastic finite element analysis to study the predicted intraplate stress field in continental australia for four models which employ different boundary conditions to balance the ridge push torque acting on the plate the modeling indicates that a number of important features of the observed stress field within the australian continent can be explained in terms of balancing the ridge push torque with resistance imposed along the himalaya papua new guinea and new zealand collisional boundaries segments these features include n s to ne sw oriented compression in the northern australia and e w oriented compression in southern australia our analysis also shows that subduction processes along the northern and eastern boundaries provide only second order controls on the intraplate stress field in continental australia

neoarchean and paleoproterozoic crystalline basement rocks of north central idaho constraints on the formation of western laurentia zircon u pb analyses of 18 orthogneisses from the clearwater and priest river complexes of northern idaho have identified important exposures of neoarchean and paleoproterozoic basement rocks in northwest laurentia all samples have ages that fall into two tightly defined age ranges in the neoarchean from 2 67 to 2 65 ga and in the paleoproterozoic from 1 88 to 1 84 ga the neoarchean orthogneisses show no evidence of older components whereas some of the paleoproterozoic orthogneisses have xenocrystic zircon cores with ages overlapping with those of the neoarchean gneisses collectively these two packages of rocks represent the two main periods of crust formation in northwest laurentia one wholly juvenile in the neoarchean and the other a mixture of juvenile paleoproterozoic and inherited neoarchean components based on existing data the ages of orthogneisses in the clearwater and priest river complexes are identical and probably represent one continuous crustal block we refer to this basement domain as the clearwater block the paleoproterozoic 1 86 ga magmatism described here is distributed throughout the northwest laurentian margin and does not coincide with the proposed trend of the great falls tectonic zone therefore a model of a single linear arc preserved within the great falls tectonic zone is inadequate in describing the majority of the known 1 86 ga crust in the region

synextensional magmatism leading to crustal flow in the albion raft river grouse creek metamorphic core complex northeastern basin and range 1 this study addresses the origin of granitic magmas in the albion raft river grouse creek arg metamorphic core complex and environs and how these inform us about the deep crustal processes leading to crustal flow and the formation of the arg shrimp rg u pb zircon ages whole rock geochemical data major and trace element data as well as sr and nd isotopes and zircon geochemistry in situ o isotope hf isotope and trace element compositions from eocene to oligocene magmas now exposed at three structural levels of the arg show that the 41 32 ma emigrant pass and the 32 25 ma cassia plutonic complexes have a common origin sharing a deep crustal hot zone that remained above solidus temperatures for at least 16 myr this magmatism is part of the protracted magmatism that swept southward across the western u s between 42 and 21 ma inferred to be the result of foundering of the shallow farallon slab isotopic modeling of geochemical data from these magmas suggests that between 41 and 32 ma the influx of mantle derived basalt into the lower crust triggered large scale hybrid magmatism generating calc alkaline magmas that erupted and intruded the upper crust and significantly weakened the lower and middle crust between 32 and 25 ma this hot zone incorporated large amounts of continental crustal melts resulting in greater mobility of the lower and middle crust driving middle crustal flow and the formation of granitic plutons that rose to shallower levels of the crust forming the granite cored gneiss domes of the arg

topo oz insights into the various modes of intraplate deformation in the australian continent abstract as the fastest lowest flattest and amongst the most arid of continents australia preserves a unique geomorphic record of intraplate tectonic activity evidencing at least three distinct modes of surface deformation since its rapid northward drift commenced around 43 million years ago at long wavelengths several 1000s km systematic variations in the extent of neogene marine inundation imply the continent has tilted north down southwest up at intermediate wavelengths several 100s km several undulations of 100 200 m amplitude have developed on the 1 10 myr timescale at still shorter wavelengths several 10s km fault related motion has produced local relief at rates of up to 100 m myr over several million years the long wavelength north down tilting can be related to a dynamic topographic effect associated with australia s northward drift from the geoid low dynamic topography low now south of the continent to the geoid high dynamic topography low centred above the south east asian and melanesian subduction zones the short wavelength fault related deformation is attributed in time to plate wide increases in compressional stress levels as the result of distant plate boundary interactions and in space in part to variations in the thermal structure of the australian lithosphere at the intermediate wavelengths transient low amplitude undulations can be ascribed to either lithospheric buckling or the development of instabilities in the thermal boundary layer beneath the lithosphere in the latter case topographic asymmetries suggest the australian lithosphere is moving north with respect to the mantle beneath providing a unique attribution to the progressive alignment of seismic anisotropy and absolute plate motion observed near the base of the australian lithosphere

tectonic stresses in the lithosphere various types of observables earthquake focal mechanisms in situ measurements and geological deformations give information about the large scale lithospheric stress field the latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates this approach ignores the role of mass heterogeneities within the lithosphere here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude the presence of internal sources makes the problem three dimensional we show however that it can be reduced to a two dimensional plane stress formulation whereby the edge forces are expressed by the non hydrostatic stresses and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress for the oceanic lithosphere we derive a compression that increases with age the comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag for the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans

north american plate dynamics deformation within the north american plate in response to various tectonic processes is modeled using an elastic finite element analysis a coarse grid contains 328 elements and 190 nodes while a fine grid contains 718 elements and 396 nodes for a spatial resolution of 2 3 abundant information about the present day state of intraplate stress constrains the modeling the dominant pattern that must be fit by all acceptable models is an ene trend for the maximum compressive stress for most of the plate east of the rocky mountains the tectonic processes considered in the modeling include ridge forces associated with the normal thermal evolution of oceanic lithosphere shear and normal stresses transmitted across transforms normal stresses transmitted across convergent boundaries stresses due to horizontal density contrasts within the continent and shear tractions applied along the base of the plate model stresses are calculated with respect to a lithostatic reference stress state distributed ridge forces of magnitude 2 1012 n m predict deviatoric stresses of the order of 20 40 mpa that are capable of accounting for the dominant observed ene stress trend driving drag models also fit the trend but are not preferred because of a predicted tenfold increase in compressive stress magnitudes from east to west across the plate assuming that ridge forces account for the dominant ene stress trend bounds may be placed on other possible forces for example shear stresses transmitted across transform boundaries along the san andreas and caribbean are small of the order of 5 10 mpa also compressive stresses of the order of 5 10 mpa transmitted across the major transforms improve the fit to the data compressive stresses across convergent margins along the aleutians and the middle america trench are important