an 40ar 39ar thermochronology of the ofoten troms region implications for terrane amalgamation and extensional collapse of the northern scandinavian caledonides fifteen 40ar 39ar cooling ages are reported for metamorphic hornblende and muscovite from far traveled terranes constituting the ofoten nappe stack of northern norway eight cooling ages on hornblende range from 425 to 394 ma and seven muscovite ages from the same or nearby outcrops as the hornblendes range from 400 to 373 ma these data are compared with 40ar 39ar ages from over a large part of the northern caledonides to evaluate regional mineral cooling patterns results indicate that 1 scandian silurian devonian metamorphism was predominant 2 most of the nappes investigated contain some vestige of pre scandian tectonism and or metamorphism 3 hornblende and muscovite cooling ages are progressively younger to the west and south which suggests a hinged to the east mineral cooling pattern and 4 a late out of sequence thrust is the only disruption of this cooling pattern synmetamorphic amalgamation of the nappes resulted from scandian a type subduction the hinged to the east mineral cooling pattern implies isostatic adjustment and exhumation of the footwall of a west dipping crustal scale extensional fault located somewhere west of the present norwegian coast during late synorogenic gravitational collapse the late out of sequence fault formed contemporaneously with uplift in the hinterland implying a kinematic and temporal connection with east directed contractional faulting in the foreland

short paper detrital mineral ages from the southern uplands using 40ar 39ar laser probe detrital mica ages in the southern uplands range from 458 to 502 ma the evidence favours a source in a basement which was part of an arc complex to the north rather than from the dalradian block which also has mica ages of this range no grenville ages have yet been recorded detrital volcanic clasts dispersed from the south have cooling ages of 560 and 530 ma indicating that they did not come from a contemporary arc but an older arc terrane of uncertain affinities

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

calculating velocity fields across plate boundaries from observed shear rates summary a general horizontal velocity field can be expressed uniquely in terms of the velocities at points along a line and an integral function of the 2 d shear rate consequently the relative motion across a plate boundary can be calculated from observed shear rates by taking the velocities along a line within one of the plates to be zero

geodetic detection of active faults in s california a new analysis of velocities of geodetic mark ers straddling the san andreas fault system in southern california reveals that interseismic deformation is localized along a dozen sub parallel narrow belts of high shear strain rate that correlate well with active geologic fault segments andlocallywithconcentratedzonesofmicroseismicity high shearstrainrates 0 3 0 95strain year areobservednorth ward and southward of the san andreas fault s big bend whereas the big bend itself is characterized by a diuse low magnitudeshearstrainrate dilatationaldeformationisdif fuse and of relatively low magnitude 0 2 strain year with the highest contraction rates occurring in the ventura andlos angeles basins because no prior assumptions were made regarding the geology tectonics or seismicity of the region ouranalysisdemonstratesthatgeodeticobservations alone can be used to detect active fault segments

seismotectonics of northeastern united states and adjacent canada data for local earthquakes recorded by a network of stations in northeastern united states and adjacent canada were analyzed to study the seismicity the relationship between earthquakes and known faults the state of stress and crustal and upper mantle velocity structure in addition portable seismographs were deployed in the field to study aftershocks as a result accurate locations for about 364 local earthquakes 2 mb 5 and 22 focal mechanism solutions were determined a comparison of the spatial distribution of these events 1970 1979 with historical earthquakes 1534 1959 reveals that seismic activity in the northeast is relatively stationary in space those areas that have had little or no seismicity historically are relatively aseismic today whereas the historically active areas are also active today the instrumental locations historical seismicity and focal mechanism solutions show an internal consistency that help us distinguish two distinct seismogenic provinces 1 the adirondack western quebec province is a northwesterly trending zone of seismic activity about 200 km wide and at least 500 km long extending from the se adirondacks into western quebec canada thrust faulting on planes striking nnw to nw appears to predominate and the inferred axis of maximum horizontal compression is largely uniform and trends wsw nearly parallel to the calculated absolute plate motion of north america little or no seismicity is found where anorthosite outcrops at the surface correlations between gravity anomalies and earthquake locations suggest that seismic activity in this zone is localized to regions of steep ne or sw gradient in bouguer anomalies this zone does not appear to extend southeastward to boston as proposed by some workers 2 the appalachian province is a northeasterly trending zone of seismic activity extending from northern virginia to new brunswick canada highangle reverse or thrust faulting on n to ne trending planes appears to predominate the western margin of this province however appears to be relatively aseismic we attribute this relative lack of activity to one or more of the following the presence of igneous activity postdating rifting of africa from north america the occurrence of intense metamorphism during the acadian orogeny which may have annealed preexisting faults and the predominance of ductile as opposed to brittle deformation in the geologic past the inferred axis of maximum horizontal compression along the eastern margin of the appalachians is rather uniform and trends w wnw almost perpendicular to the magnetic lineations offshore we suggest that this w wnw compression reflects the gravitational force arising from horizontal density variations in the oceanic lithosphere as it cools and moves away from spreading centers

activity along the osning thrust in central europe during the lateglacial ice sheet and lithosphere interactions abstract the osning thrust is one of the major fault systems in central europe with a length of approximately 115 km it underwent a polyphase tectonic evolution in the mesozoic that ranged from extensional movements in the jurassic to reverse faulting and thrusting during inversion in the late cretaceous a series of complex metre scale faults and related fold structures are developed within the upper pleistocene alluvial aeolian complex of the upper senne 1 km away from the thrust the faults dip towards the north and show offsets in a range of several decimetres growth strata indicate a two fold evolution of the structures the faults began as normal faults and were later transformed into reverse faults which resulted in the formation of small scale inversion structures with a typical harpoon shape glaciotectonics can be ruled out as the driving mechanism for these structures because new optically stimulated luminescence osl ages imply that the sedimentary succession was deposited during the late pleistocene between 29 3 2 9 ka and 13 1 1 5 ka at which time the ice margin was much further to the northeast we believe the inversion structures were caused by movement on the osning thrust due to generation of a forebulge of the late pleistocene weichselian ice sheet this led to normal faulting as a consequence of extension in the forebulge area during glaciation the osl ages for the normal fault related growth strata are in a range of 16 13 ka later reverse movements occurred during deglaciation due to the n s directed compressional stress field in northern germany numerical simulations of the deglaciation seismicity point to seismic events with a thrust mechanism in the study area between 15 5 and 12 3 ka bp although normal faulting is also possible in this time period numerous soft sediment deformation structures imply that movement on the osning thrust caused earthquakes with a significant magnitude in the autumn of 1612 an earthquake took place in this area it is very likely that this event is related to background seismicity on this fault although movement due to the influence of the ongoing glacial rebound in fennoscandia is also possible

the somalia plate and the east african rift system present day kinematics summary the motion of the somalia plate relative to the nubia africa arabia and antarctica platcs is re evaluated using a new inversion method based on a monte carlo technique and a least absolute value misfit criterion a subset of the nuvel 1 data set with additional data along the levant fault and in the red sea is used the results confirm that the motion of arabia with respect to africa is significantly different from the motion relative to somalia it is further shown that the data along the sw indian ridge are compatible with a pole of relative motion between africa and somalia located close to the hypothetical diffuse triple junction between the ridge and thc east african rift the resulting africa somalia motion is then compatible with the geological structures and seismological data along the east african rift system assuming a separate somalia plate thus solves kinematic and geological problems around the afar triple junction and along the east african rift

the evolution of deformation and topography of high elevated plateaus 1 model numerical analysis and general results a temperature dependent viscoplastic flow model of continental lithosphere is used to investigate the evolution of deformation and topography of high elevated plateaus such plateaus are products of both continent continent collision tibetan plateau and ocean continent collision the altiplano and develop in the overriding continental plate in this study we emphasize the mechanically simpler case of oceanic collision because it does not involve mass transfer between the two plates the lithosphere is deformed in response to tectonic and buoyancy forces the tectonic forces arise from subduction of an oceanic plate or underthrusting of continental lithosphere that horizontally indents and vertically shears the overriding lithosphere the buoyancy forces arise in response to horizontal density variations and tend to relax existing topography or thick crust the time evolution of the deformation and topography is investigated using a finite element technique that solves for the flow field in the overriding lithosphere the model produces dynamically supported near trench topography and inland mountain topography that is isostatically supported by a thick crust a finite region of localized deformation thick crust and high topography develops only if the model includes a horizontal thermal perturbation or an initially thick crust however only thermally perturbed lithosphere generates a plateau topography the shape and size of the calculated plateau depend on the wavelength of the thermal perturbation grashof number and density contrast between the crust and mantle the time evolution of the deformation shows a significant change in the deformation pattern as the high elevated plateau evolves during early stages compressional deformation of the crust and mantle are localized in the thermally perturbed weak zone at later stages as the crust thickens buoyancy forces of larger magnitude resist further thickening of the crust and the locus of compressional crustal deformation migrates inland this migration does not affect the location of the mantle deformation which remains in the thermally weak region but it is accompanied by a significant shear deformation in the weak lower crust the separation of the crustal locus from the mantle locus of deformation emphasizes the importance of vertically dependent deformation in the formation of high elevated plateaus this demonstrates the limitations of models that ignore changes of deformation with depth such as plane stress or thin sheet models

early carboniferous unroofing in western norway a tale of alkali feldspar thermochronology abstract 40ar 39ar thermochronologic data from an extensional detachment zone in western norway document punctuated late paleozoic through mesozoic tectonothermal activity diffusion and thermal modeling of alkali feldspars sampled in a profile through the detachment replicate laboratory argon release patterns when feldspars comprise three or four diffusion domains with differing activation energies that represent high intermediate and low closure temperatures these domains correspond to distinct changes in late paleozoic cooling rates 1 slow cooling at 0 4 2 2 c m yr from ca 380 360 ma 2 rapid cooling at 15 c m yr in late devonian early carboniferous time 360 340 ma 3 slow cooling at 0 4 1 7 c m yr after ca 340 ma with partial argon loss in permian and late jurassic early cretaceous times the early carboniferous rapid cooling event recorded by the feldspars links the western norway margin to contemporaneous unconformities regional extension basin inversion and igneous activit

tectonic motion and deformation from satellite laser ranging to lageos satellite laser ranging slr to lageos acquired during the period 1978 1988 has been analyzed to yield estimates of tectonic motion for 22 laser tracking stations situated on seven major plates the analysis is based on the precise modeling of the orbit dynamics of lageos and includes the determination of other geodynamic and nonconservative force model parameters involved in the orbit determination problem at the centimeter level site velocities were recovered from station positions determined each calendar quarter using a network adjustment procedure which maintains the reference frame within the stable interior of tectonic plates the recovered motions indicate good agreement with existing estimates of stress fields particularly across northern europe the eastern pacific and in the basin and range province of north america a comparison of the intersite motions for stations centrally located on separate plates between slr and those implied by the nuvel 1 geologic motion model yield a strong positive correlation of 0 989 and a 3 7 scale difference which may be attributable to recent changes in relative velocities or geologic time scale uncertainties sites located at or within 250 km of convergent plate boundaries have additional components of motion indicating a need for further tectonic modeling beyond that provided by simple rigid plate motions for lines crossing the north atlantic the san andreas fault and within the basin and range province the geodesic rates determined by slr are in good agreement with those determined by very long baseline interferometry an alternative space geodetic technique

40 ar 39 ar ages of detrital white mica from upper austroalpine units in the eastern alps austria evidence for cadomian and contrasting variscan sources five detrital white mica concentrates from very low grade metaclastic sequences within pre variscan basement and post variscan cover units of the upper austroalpine nappe complex eastern alps have been dated with 40ar 39ar incremental heating techniques to constrain the age of tectonothermal events in their respective source areas two samples from early palaeozoic sandstone exposed within the same alpine nappe record slightly discordant age spectra the maximum age recorded in one is 562 2 0 7 ma whereas the other yielded a 40ar 39ar plateau age of 607 3 0 3 ma these results indicate a source area affected by cadomian tectonothermal activity three detrital muscovite concentrates from post variscan late carboniferous and permian cover sequences exposed within three different alpine nappes yielded 40ar 39ar plateau ages of 359 6 1 1 ma 310 5 1 2 ma and 303 3 0 2 ma the contrasting detrital white mica ages are interpreted to reflect different source areas detrital muscovite from a post variscan carboniferous molasse type sequence and from a permian verrucano type sequence record ages which indicate late variscan e g 330 300 ma metamorphic sources by contrast detrital white mica from another permian verrucano type sequence suggests a source area affected by early variscan e g 400 360 ma metamorphism these results help clarify palinspastic relationships and tectonic correlations between pre late carboniferous metamorphic basement sequences and carboniferous to permian cover sequences

stress observations and driving force models for the south american plate stress observations are compared to predictions of driving force models for the south american plate stress observations have the potential to give detailed information about the forces driving the plates in that sets of forces with the same net torque can produce quite different stress patterns for a two dimensional vertical section model of a plate there are three conditions for dynamical equilibrium the net forces and the net torque about any point must all be zero these conditions are used to estimate the magnitudes of forces as a function of age with ridge push and slab pull as active forces and plate drag and slab resistance as passive forces the results are consistent with earlier published models and indicate that the primary balance for an oceanic plate is between slab pull and the resisting drag forces we generalize the model for the possible forces driving the south american plate and compare with observed stress measurements and kinematics the south american plate has a small slab pull component amounting to about one fifth the ridge push force with the other components reasonably isolated geographically thus the plate balance is largely between ridge push and plate drag this gives an opportunity of resolving the effects of trench suction or collision at the subduction zones most remarkable is the nearly uniform e w orientation of the regional stress field shmax directions which is extensional in the high andes a second feature is the n s orientations in the amazon basin the two dimensional horizontal stress patterns are compared for successive models of combinations of driving forces models with ridge push slab pull at the scotia and caribbean arcs and trench suction near the west coast balanced by plate drag produce similar stress patterns to those observed the magnitude of a possible trench suction force is estimated using a model with an eddy between the nazca slab and the overlying south american plate

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

joint initiation in bedded clastic rocks abstract two general types of sedimentary structures serve as initiation points ip for cross fold joints in siltstone beds of the ithaca formation appalachian plateau new york planar bed boundaries and geometrically complex features such as bedforms trace fossils and soft sediment deformation structures the relative abundance of each ip type varies with the stratigraphic position of the jointed body when they are mechanically isolated by thicker shales siltstone bodies have a higher percentage of ip associated with complex features this association supports the hypothesis that sedimentary structures serve to concentrate stress thus becoming preferred sites for primary joint initiation secondary joint initiation occurs at planar bed boundaries through interaction with pre existing joints in adjacent siltstone beds that are mechanically coupled to form composite joints the propagation path of joints passing through planar bed boundaries of composite joints can be traced backward to primary ip at complex sedimentary structures modeling suggests that when beds are mechanically coupled the modest joint tip stress concentration across a shale layer e g

mantle upflow under north america and plate dynamics in the development of plate dynamics there are two fundamental questions first do the plates slide over a passive asthenosphere or are they driven by a convecting upper mantle second are all ocean ridges passive pull apart plate margins or do some lie above upcurrents in the mantle and in that sense become active margins the combination of stress orientations in the crust of north america with other geophysical and geological information favours an active mantle interpretation both for the north american plate and for the mantle upflow which lay beneath the east pacific rise in tertiary times and which now lies beneath the western united states

state of stress and intraplate earthquakes in the united states recently compiled data on the state of stress have been used to define stress provinces in the conterminous united states in which the orientation and relative magnitude of the horizontal principal stresses are fairly uniform the observed pattems of stress constrain mechanisms for generating intraplate lithospheric stresses coupled with new information on geologic structure and tectonism in seismically active areas of the midcontinent and east these data help to define some characteristics common to these areas and to identify key questions regarding why certain faults seem to be seismically active

state of stress and crustal deformation along weak transform faults the state of stress and style of crustal deformation along weak transform plate boundaries is discussed in the context of available data and simple analytical models appreciable evidence indicates that while the frictional strength of the upper crust is high in general accordance with simple faulting theory and laboratory derived coefficients of friction the frictional resistance to motion along transform plate boundaries is extremely low these conditions require that horizontal principal stresses must be oriented approximately parallel and perpendicular to the transform fault zone to minimize the shear stresses acting parallel to the transform along plate margins that must accommodate relative convergence transpressive margins a pattern of near fault normal compression and fault normal crustal shortening is expected along divergent plate margins transtensional margins extension is expected to occur perpendicular to the transform as the direction of minimum horizontal principal stress is expected to be nearly perpendicular to it these patterns of stress and deformation can be observed along a number of transform faults around the world

argon composition of metamorphic fluids implications for 40ar 39ar geochronology the concentration and isotopic composition of argon trapped in fluid inclusions within a mylonitic amphibolite were determined by combining electron microprobe sup 40 ar sup 39 ar laser microprobe and fluid inclusion analyses on the same samples in our study fluid in inclusions formed during ductile deformation was probably derived locally from the paleozoic caledonian sequences whereas brines emplaced in cross cutting fractures were derived from proterozoic footwall rocks several kilometers from the mylonite by utilizing sup 38 ar sub cl to correct for open quotes excess sup 40 ar close quotes we estimate a 370 ma closure age for the amphibole which is distinctly younger than the 440 480 ma plateau ages indicated by step heating spectra the open quotes excess sup 40 ar close quotes responsible for the anomalous plateaus probably resulted from diffusive re equilibration of the amphibole with the fluids present during ductile deformation of the mylonite the significantly younger cooling age determined by our method is concordant with ages determined from more recent geochronological studies in the region these ages indicate that the western gneiss terrain in our study area records deformation and cooling associated with siluro devonian tectonothermal metamorphism rather than an earlier caledonian tectonism as proposed in previous sup 40 ar sup 39 ar investigators 46 refs 8 figs

dislocation model for aseismic crustal deformation at hollister california geodetic observations at many active plate margins reveal relatively steady aseismic motion during the time between major earthquakes we model the aseismic motion by the relative motion between several blocks that suffer frictional resistance in the upper crust frictional drag is represented by uniform dislocation on a fault surface we assume that friction occurs only on the upper brittle zone of each block boundary and that frictionless sliding occurs below this zone using a bayesian inversion procedure we invert geodetic data to determine the parameters of the block and dislocation model parameters to be estimated include the two horizontal components of velocity for each block and the dislocation rate dip angle slip angle and width of each rectangular fault patch for hollister we assume five blocks and nine rectangular fault patches rates of length change on 92 lines observed by trilateration make up the data triangulation leveling very long baseline interferometry satellite ranging and global positioning system data can also be easily included in our model our inversion procedure uses prior estimates of all block and fault parameters based on geological and seismological data the block parameters are estimated from geological observations of fault displacement rates while the widths of the fault patches are estimated from earthquake hypocentral depths we choose prior estimates of the dislocation rate to match observed creep rates the estimated block motion is well resolved by the geodetic data and agrees well with the geological estimates this suggests that the block motion is steady on time scales from 10 years to 1 m y the net motion across the san andreas calaveras fault system is 38 3 mm yr oriented s38 e thus some additional displacement possibly offshore is needed to match the rate of 56 mm yr predicted by plate tectonic models the depth of the transition from frictional to free sliding varies considerably from 0 4 km on the southern calaveras to 11 km on the central san andreas fault two segments of the san andreas north of 36 50 and south of 36 40 have the greatest potential for moderate to large earthquakes

geodetic observations of interseismic strain segmentation at the sumatra subduction zone deformation above the sumatra subduction zone revealed by global positioning system gps geodetic surveys shows nearly complete coupling of the forearc to the subducting plate south of 0 5 s and half as much to the north the abrupt change in plate coupling coincides with the boundary between the rupture zones of the 1833 and 1861 mw 8 thrust earthquakes the rupture boundary appears as an abrupt change in strain accumulation well into the interseismic cycle suggesting that seismic segmentation is controlled by properties of the plate interface that persist through more than one earthquake cycle structural evidence indicates that differences in basal shear stress may be related to elevated pore pressure in the north

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

fold cleavage relationships as indicator for late variscan sinistral transpression at the rheno hercynian saxo thuringian boundary zone central european variscides abstract the boundary between the rheno hercynian and the saxo thuringian zones of the european variscides is characterized by a ne sw striking late orogenic fold and thrust belt affecting the intervening rheic suture classical models used the first order strike of this zone as an indicator for perpendicular plate convergence i e nw se we present structural data from both sides of the suture focusing on fold cleavage relationships the statistical analysis reveals an orientation maximum of the youngest cleavage that deviates from the strike of the fold and thrust belt by c 22 the presence of clockwise transection of the folds by the cleavage up to 16 indicates pervasive sinistral transpression three types of fold cleavage relationships are observed ne trending folds i with or ii without a transecting cleavage and iii non transected ene trending folds we explain the occurrence of different fold cleavage types by strain partitioning due to nnw convergence obliquely to pre existent ne trending mechanical anisotropies in terms of plate tectonics we propose that the classical boundary of the rheno hercynian and the saxo thuringian zone represents an initial transform plate boundary that was finally affected by sinistral transpression

river mixing rate residence time and subsidence rates from isotopic indicators eocene sandstones of the u s pacific northwest isotopic analyses of eocene sandstones from the tyee formation and related units in the u s pacific northwest provide evidence for short residence time but extensive mixing and indirectly slow subsidence rates within the major river feeder systems that drained the western flank of the north american cordillera bulk isotopic analyses of coarse white mica are remarkably consistent throughout the tyee formation mean k ar 67 3 ma 18o 9 5 of the oregon coast range other tertiary sandstones in the region have similar values despite small scatter about these mean values the 40ar 39ar ages of single grains of white mica from these units range from about 45 to 120 ma showing that long river systems thoroughly mixed and homogenized the transported sand grains in addition overlap of the youngest mica cooling ages and depositional age for the tyee formation show that homogenization of grains took place during a relatively short residence time in the river transport system 2 m y these results require that the sediments were multiply cycled within the active river system this suggests that average subsidence rates along the alluvial system were low and thus sediment could be recycled by temporary storage in and reintroduction from the river flood plain without spending significant time buried the results show that extensive sediment mixing can occur in large rivers on relatively short time scales and that isotopic analyses can provide indirect evidence of subsidence rates for basins no longer preserved

ductile and brittle extension in the southern lofoten archipelago north norway implications for differences in tectonic style along an ancient collisional margin the lofoten archipelago north norway occupies the most inter nal position of the caledonian belt in northern scandinavia and rocks and structures exposed there are crucial to understanding processes of how the baltic basement and its cover allochthons responded to continental lithospheric subduc tion and subsequent continental separation relatively little is published about the structural and metamorphic development and especially the timing of these events consequently it is unknown how features exposed on these spatially isolated islands relate to those of the adjacent mainland rocks in lofoten were affected by caledonian regional metamorphism and structures record tops east contraction and later extension related to late to post caledonian basement exhumation tops west extension is preferentially developed in meter to km scale ductile shear zones containing west dipping extensional shear bands west verging rootless folds and asymmetric feldspar porphyroclasts west plunging sinistral oblique elongation lineations in the mylonitic foliation are interpreted to indicate the line of transport mesoscopic backfolds are locally developed within the tops west shear zones and further document west directed transport of structurally higher rocks the ductile extensional shear zones locally are cross cut by low angle tops west cataclastic normal faults reflecting progressive unroofing of the shear zones to shallower crustal levels during late and post caledonian extensional events northeast striking high angle brittle normal faults truncate all other fabrics and structures and juxtapose structurally deep undeformed precambrian basement with the caledonian nappe sequence 40 ar 39 ar data indicate that ductile extension in lofoten likely initiated soon after the caledo nian metamorphic peak at 430 ma and continued episodically until at least permian time rocks passed through the brittle ductile transition rapidly at 275 ma recent paleomagnetic data also point to later phases of brittle faulting in lofoten one in the jurassic cretaceous and one related to tertiary opening of the norwegian sea the magnitude and style of late to post caledonian exten sion and basement exhumation in lofoten differs markedly from the formation of phenomenal devonian basins and exposures of caledonian ultra high pressure assemblages in the western gneiss region wgr of southern norway differ ences in the styles of extension between northern and southern norway are interpreted to reflect inherited differences in crustal architecture that may have been enhanced during the caledonian orogeny

contrasting provenance of triassic jurassic sediments in north sea rift a single zircon sims sm nd and trace element study abstract an ion microprobe sims u pb zircon study has been performed on a selection of five samples in order to identify the various provenances of sands sandstones from the lunde and statfjord formations the gullfaks oilfield the data are integrated with sr element analyses and new and published sm nd whole rock data which give consistent and additional information in this geologically complex area previous work has shown that the upper parts of the lunde formation have relatively low nd model ages typically 1 5 to 1 6 ga reflecting stable tectonic conditions the overlying raude and eirikson members of the statfjord formation record interfingering units recording relatively low 1 5 ga and substantially higher nd model ages this may reflect an early stage of rifting in the northern north sea rift zone while the upper nansen member represents a return to relatively low uniform nd ages and quiet tectonic conditions the present data reveal a more complex interplay between sediment provenance terranes than previously assumed the sample of the lunde fm is characterised by a high number of zircon age peaks at 0 39 to 1 75 ga it was transported from the south and has an ultimate provenance in the sw margin of norway after the mid devonian the eirikson 1 sample with t dm of 2 4 ga represents the income of units with substantially higher nd model ages a prominent peak of archaean zircons at 2 56 to 3 00 ga reflects a sediment source in archaean gneisses which were located in the north smaller peaks at 0 38 to 0 54 ga and 1 10 to 1 90 ga represent a separate provenance component in the east shetland platform the raude and eirikson 2 samples have t dm of 1 7 and 1 8 ga and show a cumulative zircon peak at 0 38 to 0 46 ga and a high number of small peaks in the age ranges 0 70 to 2 23 ga and 2 40 to 3 00 ga these units where transported from the south and have an ultimate provenance in the east shetland platform the sample of the nansen fm records a peak of zircon ages at 1 58 to 1 74 ga was derived from the south and represents re cycled devonian sediments from sw norway the four samples derived from the south may have experienced sediment redeposition of these the nansen sample is the best example of a sediment sequence which has been preserved and transported as an isolated unit through erosion sediment transport and re deposition there is a perfect match between the geochemistry of this sample and devonian sediments from sw norway suggesting subordinate contamination from other provenance terranes during sedimentation and sediment re deposition this suggests that provenance studies of devonian sediments from the north sea area are important for an understanding of the ultimate source areas and processes of sediment re deposition in the younger rift

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

a theory of intraplate tectonics the theory of plate tectonics assumes a rigid behavior of tectonic plates and therefore fails to account for observed intraplate deformation a new theory of intraplate tectonics is developed to calculate the first order intraplate deformation induced by horizontal displacement of deformable plate boundaries it is based on simple assumptions that link the well established directions of relative plate motion to the displacement and deformation fields within a plate interior adjacent to three types of deformable plate boundaries inward outward and tangential displaced boundaries the theory predicts the direction of intraplate displacement displacement rate strain and stress fields in terms of small circles great circles and 45 loxodromes around the pole of rotation of two adjacent plates the 45 loxodromes are two orthogonal sets of directions clockwise and counterclockwise that intersect both small and great circles at 45 the principal axis of the maximum horizontal stress follows small circles for inward displaced boundaries great circles for out ward displaced boundaries and loxodromes for tangential displaced boundaries the theoretical predictions are systematically compared with more than 4000 reliable observed directions of maximum horizontal stress provided by the world stress map project zoback 1992 the theory indicates that the first order intraplate deformation is predominantly induced by horizontal forces acting on plate boundaries and by buoyancy forces that arise from lateral density variations between mid ocean ridges and plate interiors ridge push the simplicity of the predictions and their good agreement with observations suggests that intraplate deformation should be investigated in the pole of rotation spherical coordinate system

tests of the rigid plate hypothesis and bounds on intraplate deformation using geodetic data from very long baseline interferometry the geodetic results of ma et al 1994 from very long baseline interferometry are used to assess rates of intraplate deformation we analyze the resulting set of velocities of 18 plate interior sites nine on the north american plate four western european sites on the eurasian plate three on the pacific plate and two on the australian plate we formulate three alternative error budgets based partly on the work of ryan et al 1993a an optimistic error budget a conservative error budget and an upper bound error budget all three budgets assume the existence of a previously unmodeled error that is inversely proportional to the time span of observations at a site use of the conservative error budget indicates no significant motion between sites on any one of the four plates use of the upper bound error budget designed to be precisely large enough to be consistent with rigid plates places a usefully small upper bound on how fast any site might be moving relative to the rest of its plate speeds faster than 2 mm yr are excluded at the 95 confidence level for seven of the 18 plate interior sites adjustments of the site velocities for motion due to postglacial rebound as predicted from the model of tushingham and peltier 1991 give an insignificant improvement to the fit of the horizontal velocities to the rigid plate model the velocities of six sites near the western margin of the interior of the north american plate were also analyzed use of the conservative error budget indicates that a site near fairbanks alaska has an upper bound of 3 mm yr on its speed a site in nome alaska has an upper bound on its speed of 8 mm yr and a site in penticton british columbia has an upper bound on its speed of 4 mm yr of three sites on the colorado plateau two move marginally significantly and one moves insignificantly relative to stable north america upper limits on their speeds relative to stable north america range from 4 to 5 mm yr we conclude that the data do not yet demonstrate significant velocities between sites in the stable interior of any one plate but do provide useful and small upper bounds of 2 mm yr or less on the speeds of the best observed intraplate sites six sites near the western margin of the north american interior have upper bound speeds of 3 to 8 mm yr

numerical modeling of intraplate deformation simple mechanical models of continental collision we study a model of continental collision in which one of the continents acts as a rigid die indenting the other plate which flows as an incompressible viscoplastic medium we consider two extreme cases of plane deformation 1 plane strain which corresponds to an infinitely thick lithospheric plate and 2 plane stress corresponding to a very thin plate deformation of the lithosphere a thick plate should be intermediate between those extremes we found that the flow in the plane strain case is quite similar to that obtained by slip line theory the plane stress results are quite different since in this case most of the plate shortening is taken up by the thickening of the lithosphere we also explored the role of boundary conditions on the flow in particular the role of the side walls containing the flow of the lithosphere in the case of a free lateral boundary the main feature is a flow of matter toward this free wall and a s like pattern for the horizontal stress field for a rigid wall on the other hand the plane strain and the plane stress results are quite different in the first case there is a large return flow on the sides of the punch the material being extruded along the only free surface available in the plane stress case the return flow disappears and the material displaced by the penetration of the die tends to thicken the plate the role of a nonlinear constitutive relation is studied for power law creep as the power of the flow limit increases the flow retains its general features but the deformation localizes creating sharper contrasts between high and low strain rate areas in plane stress the effect of nonlinearity is to increase the contrasts in vertical motion available data for asia are discussed in the light of the new results

equivalent strike slip earthquake cycles in half space and lithosphere asthenosphere earth models by virtue of the images used in the dislocation solution the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike slip fault in an earth model consisting of an elastic plate lithosphere overlying a viscoelastic half space asthenosphere can be duplicated by prescribed slip on a vertical fault embedded in an elastic half space for the case in which each earthquake ruptures the entire lithosphere thickness h the half space equivalent slip rate is as follows depth interval 0 h slip identical to that in lithosphere asthenosphere model i e abrupt coseismic slip and no subsequent slip depth interval 2n 1 h to 2n 1 h n 1 2 slip rate uniform in space and dependent upon time as fn t exp t a where fn is a n 1 degree polynomial in t a is twice the asthenosphere relaxation time and t is measured from the instant after the preceding earthquake the slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion for reasonable values of a slip rates below 5h do not vary much from that mean value and can be treated as constant thus the surface deformation due to the earthquake cycle in the lithosphere asthenosphere model can be calculated very simply from the half space model with time dependent slip in the two depth intervals h 3h and 3h 5h and uniform slip at a rate equal to the secular relative plate velocity below depth 5h inversion of 1973 1988 geodetic measurements of deformation across the segment of the san andreas fault in the transverse ranges north of los angeles for the half space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm yr below 30 km one equivalent lithosphere asthenosphere model would have a 30 km thick lithosphere and an asthenosphere relaxation time greater than 33 years but other models are possible

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

source of the lachlan fold belt flysch linked to convective removal of the lithospheric mantle and rapid exhumation of the delamerian ross fold belt during the cambrian ordovician the paleo pacific margin of gondwana underwent a rapid transition from a site of convergent deformation delamerian ross orogeny to one of sedimentation during which the thick turbiditic sequences of the adjacent basal lachlan fold belt were deposited laser probe 40 ar 39 ar data show that the synkinematic 523 486 ma mid crustal i and s type granites and metamorphic country rocks of the delamerian fold belt were rapidly cooled at 490 485 ma coincident with the intrusion of a suite of high level postkinematic a type granites and gabbros ca 497 481 ma 40 ar 39 ar data on detrital muscovites from the basal sections of the lachlan fold belt provide information on exhumation rather than cooling but yield an age pattern 507 480 ma identical to that of the delamerian fold belt demonstrating that this was the source of the sediments the combined data require that some 15 km of exhumation occurred very rapidly 5 15 mm yr 1 coincident as implied by overlapping ages with the cessation of convergent deformation and partial melting to form the postkinematic magmas this scenario implies a causative link which we infer to have been convective removal of lithospheric mantle following orogenic thickening the model is analogous to the tertiary uplift of the india asia orogen which provided the sediment source for the bengal fan our results suggest that similar processes were important in the evolution of mountain belts at least as far back as the early phanerozoic

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

plate motion and crustal deformation estimated with geodetic data from the global positioning system we use geodetic data taken over four years with the global positioning system gps to estimate 1 motion between six major plates and 2 motion relative to these plates of ten sites in plate boundary zones the degree of consistency between geodetic velocities and rigid plates requires the one dimensional standard errors in horizontal velocities to be 2 mm yr each of the 15 angular velocities describing motion between plate pairs that we estimate with gps differs insignificantly from the corresponding angular velocity in global plate motion model nuvel 1a which averages motion over the past 3 m y the motion of the pacific plate relative to both the eurasian and north american plates is observed to be faster than predicted by nuvel 1a supporting the inference from very long baseline interferometry vlbi that motion of the pacific plate has sped up over the past few m y the eurasia north america pole of rotation is estimated to be north of nuvel 1a consistent with the independent hypothesis that the pole has recently migrated northward across northeast asia to near the lena river delta victoria which lies above the main thrust at the cascadia subduction zone moves relative to the interior of the overriding plate at 30 of the velocity of the subducting plate reinforcing the conclusion that the thrust there is locked beneath the continental shelf and slope

an integrated global model of present day plate motions and plate boundary deformation summary in this paper we present a global model gsrm 1 of both horizontal velocities on the earth s surface and horizontal strain rates for almost all deforming plate boundary zones a model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field in our model we perform a least squares fit between model velocities and observed geodetic velocities as well as between model strain rates and observed geological strain rates model velocities and strain rates are interpolated over a spherical earth using bi cubic bessel splines we include 3000 geodetic velocities from 50 different mostly published studies geological strain rates are obtained for central asia only and they are inferred from quaternary fault slip rates for all areas where no geological information is included a priori constraints are placed on the style and direction but not magnitude of the model strain rate field these constraints are taken from a seismic strain rate field inferred from normalized focal mechanisms of shallow earthquakes we present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones generally the strain rate tensor field is consistent with geological and seismological data with the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs we find that in general there is a good agreement between the present day plate motions we obtain and longterm plate motions but a few significant differences exist the rotation rates for the indian arabian and nubian plates relative to eurasia are 30 13 and 50 per cent slower than the nuvel1a estimate respectively and the rotation rate for the nazca plate relative to south america is 17 per cent slower on the other hand caribbean north america motion is 76 per cent faster than the nuvel 1a estimate while crustal blocks in the india eurasia collision zone move significantly and self consistently with respect to bounding plates only a very small motion is predicted between the nubian and somalian plates by integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in for instance redefining a no net rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

40ar 39ar laser probe dating of detrital white micas from cretaceous sedimentary rocks of the eastern alps evidence for variscan high pressure metamorphism and implications for alpine orogeny the detritus of cretaceous synorogenic sandstones of the northern margin of the austroalpine microplate contains evidence for a high pressure metamorphic basement and obducted oceanic crust exposed in early alpine time 40 ar 39 ar laser probe data of detrital white micas give excellent plateau ages in a narrow range from 320 to 360 ma white micas cover the whole range from muscovites up to phengites 3 04 to 3 48 si per formula unit heavy mineral spectra contain chrome spinel glaucophane chloritoid epidote and garnet as well as zircon tourmaline and rutile glaucophane chloritoid and phengite correlate in their abundance these minerals also correlate positively with the stable minerals zircon tourmaline and rutile derived from continental rock and negatively with chrome spinel which represents oceanic crustal provenance we propose that both glaucophane and phengite come from the same source rock variscan high pressure metamorphic rocks of the austroalpine basement the early carboniferous age gives constraints for paleogeographic models concerning the collision between gondwana and laurussia furthermore detrital glaucophane in the early alpine sedimentary rocks cannot be used as proof for cretaceous subduction at the austroalpine penninic plate boundary

two stage erosion and deposition in a continental margin setting an 40ar 39ar laserprobe study of offshore detrital white micas in the norwegian sea detrital white micas from a restricted region of the norwegian sea have been analysed by the 40ar 39ar laserprobe examining the ages of individual white mica grains from offshore triassic jurassic and cretaceous sandstones reveals a narrow range of detrital white mica ages 369 424 ma moreover there is an inverse relationship between sediment age and detrital white mica age with the triassic sands containing a larger proportion of younger white micas and the cretaceous sands containing the oldest white micas in addition residence times of the detritus which is the age of the sediment subtracted from the age of the detrital white mica range from c 150 to c 300 ma for the triassic and cretaceous sands respectively the inverse age relationships combined with the large residence times for these stratigraphic levels can best be explained by an erosion deposition model involving at least two stages taking into account previous provenance studies in the lofoten region of the norwegian sea these new data suggest that detritus shed initially from the exhuming caledonian nappes of nw norway in late silurian early devonian times was deposited in intramontane basins that were themselves subsequently unroofed and re eroded from permo triassic times this caused the recycling of the caledonian detritus from the devono carboniferous basins and led to the inverted detrital age distribution ovserved in the triassic cretaceous sediments examined in this study while sedimentary recycling is not a surprising feature for mesozoic basins in the north and norwegian seas this study documents a previously untapped method 40ar 39ar laserprobe provenance studies by which to identify its effect and extent

active tectonics of the alpine himalayan belt between western turkey and pakistan summary over 80 new fault plane solutions combined with satellite imagery as well as both modern and historical observations of earthquake faulting are used to investigate the active tectonics of the middle east between western turkey and pakistan the deformation of the western part of this region is dominated by the movement of continental material laterally away from the lake van region in eastern turkey this movement helps to avoid crustal thickening in the van region and allows some of the shortening between arabia and eurasia to be taken up by the thrusting of continental material over oceanic type basement in the southern caspian mediterranean makran and black sea thus central turkey bounded by the north and east anatolian strike slip faults is moving west from the van region and overrides the eastern mediterranean at two intermediate depth seismic zones one extending between antalya bay and southern cyprus and the other further west in the hellenic trench the motion of northern iran eastwards from the van region is achieved mainly by a conjugate system of strike slip faults and leads to the low angle thrusting of iran over the southern caspian sea the seismicity of the caucasus shows predominantly shortening perpendicular to the regional strike but there is also some minor elongation along the strike of the belt as the causcasus overrides the caspian and black seas the deformation of the eastern part of this region is dominated by the shortening of iran against the stable borders of turkmenistan and afghanistan the north east direction of compression seen in zagros is also seen in north east iran and the kopet dag where the shortening is taken up by a combination of strike slip and thrust faulting large structural as well as palaeomagnetic rotations are likely to have occurred in ne iran as a result of this style of deformation north south strike slip faults in southern iran allow some movement of material away from the collision zone in ne iran towards the makran subduction zone where genuinely intermediate depth seismicity is seen within this broad deforming belt large areas such as central turkey nw iran azerbaijan central iran and the southern caspian appear to be almost aseismic and therefore to behave as relatively rigid blocks surrounded by active belts 200 300 km wide the motion of these blocks can usefully be described by poles of rotation the poles presented in this paper predict motions consistent with those observed and also predict the opening of the gulf of iskenderun ne of cyprus the change within the zagros mountains from strike slip faulting in the nw to intense thrusting in the se and the relatively feeble seismicity in se iran baluchistan this description also explains why the north south structures along the iran afghanistan border do not cut the east west ranges of the makran within the active belts surrounding the relatively aseismic blocks a continuum approach is needed for a description of the deformation even though motions at the surface may be concentrated on faults the evolution of fault systems within the active zones is controlled by geometric constraints such as the requirement that simultaneously active faults do not in general intersect many of the active processes discussed in this paper particularly large scale rotations and lateral movement along the regional strike are likely to have caused substantial complexities in older mountain belts and should be accounted for in any reconstructions of them

gps measurement of relative motion of the cocos and caribbean plates and strain accumulation across the middle america trench global positioning system gps measurements in 1988 and 1991 on cocos island cocos plate san andres island caribbean plate and liberia caribbean plate mainland costa rica provide an estimate of relative motion between the cocos and caribbean plates the data for cocos and san andres islands both located more than 400 km from the middle america trench define a velocity that is equivalent within two standard errors 7 mm yr rate 5 degrees azimuth to the nuvel 1 plate motion model the data for liberia 120 km from the trench define a velocity that is similar in azimuth but substantially different in rate from nuvel 1 the discrepancy can be explained with a simple model of elastic strain accumulation with a subduction zone that is locked to a relatively shallow 20 z 5 km depth

40ar 39ar mineral dates related to devonian extension in the southwestern scandinavian caledonides abstract middle devonian sedimentary basins exposed in west central norway formed during post caledonian extension they developed along west dipping low angle normal faults which effected ductile shearing of pre caledonian basement units of the western gneiss region wgr kinematic indicators suggest that the west vergent ductile deformation overprinted older east vergent caledonian fabric elements ductile extensional deformation led to development of a 2 km thick shear zone beneath the devonian basins five amphibole and nine muscovite concentrates from rocks within the shear zone and from unaffected gneisses of the western gneiss region have been analyzed with 40ar 39ar incremental release dating techniques in an attempt to calibrate the timing of extensional events well defined plateau ages are recorded by muscovite concentrates within the shear zone amphiboles display disturbed plateau ages clearly related to extraneous argon outside the shear zone muscovite records caledonian ages results suggest that a late tectonometamorphic event affected the rocks within the shear zone this event which not exceed ca 500 c completely rejuvenated muscovite argon systems and perturbed argon distribution within amphiboles this metamorphism is correlated with the extensional deformation which effected exhumation of the caledonian high pressure rocks and controlled formation of devonian basins

a central european variscide source for upper carboniferous sediments in sw england 40ar 39ar detrital white mica ages from the forest of dean basin 40 ar 39 ar detrital white mica ages from a westphalian c d sediment in the varisan foreland forest of dean basin sw england range from 309 12 to 371 2 ma combining these ages with chemical data from the detrital white micas constrain their provenance to two sources reworked westphalian a to c and more controversially the central european variscides taking into account the palaeogeography of the late devonian to late carboniferous the sediment derived from the central european variscides must have resided in a holding basin proximal to their source for 30 40 ma during the mid to late carboniferous sediment was transported longitudinally from east to west along the variscan front deposition and mixing of the two source components took place in the forest of dean basin at approximately 307 ma

north america pacific plate boundary an elastic plastic megashear evidence from very long baseline interferometry i investigate a number of lithospheric deformation models based upon the concept that the motions of points near the north america pacific plate boundary are a linear combination of north america and pacific velocities this concept entails a shear strain transition zone in the vicinity of the boundary the best of these models fit 95 of the variance in 139 very long baseline interferometry vlbi length and transverse velocity observations taken over 5 5 years and require only 90 of the relative plate motion predicted by rm2 residual misfits in the megashear models are largely explained by a 6 1 mm yr component of basin and range spreading perpendicular to the plate boundary instantaneous shear deformation associated with plate tectonics is apparently developing in a zone 450 km wide paralleling the san andreas fault some of this deformation will be recovered through elastic rebound the rest will be permanently set through plastic processes thus depending on the elastic plastic composition of the borderlands the width and pattern of strain accumulation in the megashear over geological time may or may not be significantly different than what is currently observed because vlbi data have been assembled for only a small fraction of the earthquake cycle plastic and elastic strains are indistinguishable other information will have to be brought to bear to discriminate elastic and plastic behaviors and reconcile instantaneous and geologic deformation near the north america pacific margin

a continuum model of continental deformation above subduction zones application to the andes and the aegean large scale continental tectonics of back arc extensional and andean type compressional environments are investigated by using the thin viscous sheet model to calculate the deformation within continental lithosphere that is subjected to horizontal forces on its plate boundaries and to basal drag from the asthenospheric flow beneath the shear tractions acting on the base of a deformable lithosphere are derived from a corner flow model that assumes a rigid subducting plate and a deformable overlying plate because the calculated shear tractions and the deformation within the overlying plate are interdependent the corner flow and the thin viscous sheet models are solved simultaneously we use a perturbation method to obtain analytical solutions for the velocity and strain rate fields within the overlying continental lithosphere the solutions depend on the angle of subduction the dimensionless thickness of the lithosphere and the ratio of asthenospheric to lithospheric viscosities which governs the viscous coupling between the asthenosphere and the lithosphere calculations are compared with observations from the andes and the aegean our results explain some of the features of the deformation in these regions that have heretofore not been explained by other models our model predicts that in a compressional environment a broad region of uplifted topography will tend to develop above a more steeply dipping slab 30o rather than above a shallower slab 10o 15o this is in accord with observations in the various segments of the central andes for an extensional environment the model predicts that a zone of compression can develop near the trench and that extensional strain rate can increase with distance from the trench as is observed in the aegean we also estimate the effective viscosities of 102o pa s for the asthenosphere 2x 1021 pa s for the aegean lithosphere and 1022 pa s for the andean lithosphere

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

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

geodesy by radio interferometry evidence for contemporary plate motion analysis of 211 very long baseline interferometry observing sessions carried out between november 1979 and august 1984 has yielded estimates of the distances between various radio telescopes located in north america and europe the average rate of change of the distances between four radio telescopes in north america haystack observatory massachusetts westford radio telescope massachusetts george r agassiz station texas and owens valley radio observatory california and one in europe onsala space observatory sweden obtained from the analysis of these data is 19 10 mm yr where the 68 confidence interval standard deviation is for the estimate of the rate of change of the haystack onsala baseline length the one determined most accurately from these data this estimate of the standard deviation is dominated by the effects of correlated systematic errors due mostly to errors in the model used for the atmospheric delay which we infer introduces errors in each baseline length estimate of 40 mm standard deviation and 60 days correlation time by contrast the statistical standard deviation is only 2 mm yr the estimated geologic rates of change of these baseline lengths averaged over 106 years are 15 to 17 3 mm yr for the various north american sites to onsala improvements in our model of the atmosphere and continued monitoring of the distances between north american and european telescopes will allow the uncertainty of the rate estimates to be reduced over the next few years to a value small compared to our estimated rate of change of these baseline lengths the use of multiple radio telescopes in europe will allow us also to separate possible local site motions from plate motions

high precision provenance determination using detrital zircon ages and petrography of quaternary sands on the eastern snake river plain idaho the big lost trough is an upper pliocene to holocene sedimentary basin containing volcanic sills in the northeastern snake river plain idaho the basin receives sediment primarily from basin and range fluvial systems of the big lost river little lost river and birch creek the big lost trough contains a 200 m thick succession of lacustrine fluvial eolian and playa sediments recording high frequency quaternary climatic fluctuations interbedded with basalt flows alternating deposition of clay rich lacustrine sediments and sandy fluvial and eolian sediments in the central part of the basin was in response to the interaction of fluvial and eolian systems with pleistocene lake terreton the source areas for modern sands from the fluvial systems can be differentiated by using both petrography and u pb age spectra from detrital zircon populations provenance data from subsurface sands indicate that the big lost trough was supplied with sand largely deposited by the big lost river with local redeposition by eolian processes similar to the modern depositional system provenance and stratigraphic data suggest that during pleistocene wet climate cycles the center of the basin was dominated by lacustrine sedimentation during dry climate cycles the base level dropped the big lost river prograded across the basin and the eolianmore system became active at least seven climate oscillations are recorded in strata deposited between approximately 140 and approximately 1250 ka less

40ar 39ar laser probe dating of detrital micas from the montgomery creek formation northern california clues to provenance tectonics and weathering processes generic provenance data from the montgomery creek formation northern california even in light of paleocurrent data and regional stratigraphic patterns do not uniquely establish its sediment source dating individual detrital mica grains mica with the sup 40 ar sup 39 ar laser probe shows that the source area for the upper montgomery creek included the idaho batholith unaltered muscovite grains yield data that provide limits on timing and rate of uplift of the batholith combined with paleocurrent data the inferred provenance constrains the timing of tectonic rotation in the southeastern klamath orogen dating variably altered biotite grains elucidates the age and environment of alteration and establishes a distinct source for some of the lower montgomery creek

timing structural assembly metamorphism and cooling of caledonian nappes in the ofoten efjorden arca north norway tectonic insights from u pb and40ar 39ar geochronology u pb and 40ar 39ar geochronology from part of the northern norwegian caledonides indicates a cooling and unroofing history different from that inferred for the caledonides in southwest norway in the ofoten efjorden area of north norway the narvik nappe complex nnc represents the basal portion of the composite caledonian allochthon and consists of amphibolite facies oceanic affinity metasedimentary rocks intruded by numerous pretectonic felsic dikes two of these dikes yield indistinguishable u pb zircon ages of 437 1 ma providing a maximum age for tectonic assembly metamorphism and deformation at this structural level u pb ages of metamorphic monazite 432 2 ma and zircon 431 7 0 5 ma from pelitic schist and migmatite yield robust estimates of the age of peak metamorphism combined with structural evidence for transport parallel stretching and subvertical attenuation of the rock mass at the time of metamorphism these ages suggest that local and or episodic extensional modification of the

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

paleogeographic and paleotectonic evolution of the himalayan range as reflected by detrital modes of tertiary sandstones and modern sands indus transect india and pakistan detrital modes of sandstones derived from the himalayan suture belt record the history of the mountain range since initial collision between india and asia which began in latest paleocene time tertiary clastic wedges deposited in fore arc foreland and remnant ocean basins and exposed along the indus transect from northernmost india to the arabian sea represent the best opportunity to study sedimentary responses to successive tectonic events during continental collision quartzose continental block and feldspatholithic magmatic arc sandstones were deposited respectively on the passive indian tethys himalayan succession and active asian indus group continental margins during late cretaceous paleocene time closure of the neotethys was marked by sudden arrival of volcanic and ophiolitic detritus on the passive continental margin of the indian plate during deposition of sediments dated at foraminiferal zones p6 pakistan to p8 india starting in early eocene time deposition of chulung la formation and murree supergroup volcanic and ophiolitic to metasedimentary detritus was accumulated in rapidly subsiding piggy back and foreland basins homogeneous petrographic composition within the eocene lower miocene murree supergroup with only slight progressive increase of detrital feldspars suggests erosion of largely supracrustal rocks involved in thrusting in the north in middle miocene time marked enrichment in medium to high grade metamorphic detritus in foreland sandstones siwalik group reflects rapid uplift of a warm wedge of indian crust which was carried southward along the main central thrust this major paleogeographic change was recorded also by quartzolithic remnant ocean turbidites which were fed great distances along transverse fracture zones and later accreted in the coastal makran subduction complex panjgur association and makran group recycled orogen detritus derived from the elevated himalayan chain is still accumulating today in the indus fan enrichment in feldspars with respect to ancient sandstones reflects deep erosion levels into mid crustal rocks along the core of the growing orogen

polyphase tectonothermal evolution of exotic caledonian nappes in troms norway evidence from 40ar39ar mineral ages abstract 40 ar 39 ar incremental release ages have been determined for hornblende and muscovite from several high level exotic nappes in northern troms county norway within the nordmannvik nappe and upper tectonic levels of the lyngen nappe complex balsfjord group both minerals record plateau and isotope correlation ages between c 427 and 417 ma these indicate relatively rapid post metamorphic cooling which is interpreted to have occurred during scandian translation of exotic tectonic elements to higher crustal levels along the baltoscandian margin muscovite within various structural levels of the tromso nappe complex uppermost allochthon yeilds plateau ages of 427 410 ma which also record post scandian metamorphic cooling however hornblende within the tromso nappe complex records significantly older isotopic correlation ages which range between c 481 ma and 432 ma the 40 ar 39 ar spectra display no evidence of partial scandian rejuvenation and the isotope correlation ages are therefore interpreted to date cooling following pre scandian tectonothermal activity these results indicate that early paleozoic tectonothermal events affected outboard exotic tectonic elements at approximately the same time as the early caledonian orogenesis previously described for nappe units with initial baltic palinspastic affinities e g seve nappe complex

stress perturbation associated with the amazonas and other ancient continental rifts the state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts specifically a dense rift pillow in the lower crust may modify substantially the regional stress field both shallow 2 0 2 6 km depth breakout data and deep 20 45 km depth crustal earthquake focal mechanisms indicate a n to nne maximum horizontal compression in the vicinity of the paleozoic amazonas rift in central brazil this compressive stress direction is nearly perpendicular to the rift structure and represents a 75 rotation relative to a regional e w compressive stress direction in the south american plate elastic two dimensional finite element models of the density structure associated with the amazonas rift as inferred from independent gravity modeling indicate that elastic support of this dense feature would generate horizontal rift normal compressional stresses between 60 and 120 mpa with values of 80 100 mpa probably most representative of the overall structure the observed 75 stress rotation constrains the ratio of the regional horizontal stress difference to the rift normal compressive stress to be between 0 25 and 1 0 suggesting that this rift normal stress may be from 1 to 4 times larger than the regional horizontal stress difference a general expression for the modification of the normalized local horizontal shear stress relative to the regional horizontal shear stress shows that the same ratio of the rift normal compression relative to the regional horizontal stress difference which controls the amount of stress rotation also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress the potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike slip stress regimes in the amazonas rift case because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift normal compression to be between 0 25 and 1 0 our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced for ratios 0 5 or enhanced for ratios 0 5 additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust faulting stress regime in the vicinity of the amazonas rift a rift normal stress associated with the seismically active new madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike slip faults parallel to the axis of the rift although this results in a 20 40 reduction in the local horizontal shear stress within the seismic zone sparse stress data in the vicinity of the seismically quiescent midcontinent rift of the central united states suggest a stress state similar to that of new madrid with the local horizontal shear stress potentially reduced by as much as 60 thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows the modeling and analysis here demonstrate that rift normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension

episodic rapid uplift in the himalaya revealed by 40ar 39ar analysis of detrital k feldspar and muscovite bengal fan detrital k feldspar and muscovite samples from ocean drilling program leg 116 cores have been dated by the sup 40 ar sup 39 ar technique and have depositional ages from 0 to 18 ma from 4 to 13 individual k feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels in every level at least one k feldspar and one muscovite yielded a minimum age identical within uncertainty to the age of deposition these results indicate that a significant portion of the material in the bengal fan is first cycle detritus derived from the himalaya therefore the substantial amount of sediment deposited in the distal fan in early to middle miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time moreover these data indicate that throughout the neogene some part of the himalayan orogen was undergoing rapid erosion 1 to 10 mm yr this erosion must have been less than or equal to uplift relative to sea level the lack of granulite facies rocks in the eastern himalaya and tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt these datamore are incompatible with tectonic models in which the himalaya and tibetan plateau are uplifted either uniformly over the past 40 m y or mostly within the past 2 to 5 m y less

seismic strain rates in regions of distributed continental deformation we compare the rates and configurations of seismic deformation determined from moment tensor mechanisms of earthquakes in five continental regions with the rates and configurations expected if the seismicity reflected the relative plate motions in the regions the five regions the aegean iran california western new guinea and central asia are characterized by high levels of seismicity distributed over hundreds to thousands of kilometers from the accepted plate boundaries the earthquake mechanisms are compiled primarily from the catalog of centroid moment tensor solutions which is nearly complete over the interval 1977 1987 for earthquakes greater than ms 5 the thickness of the seismogenic layer in each region is constrained by the results of modeling broadband body wave phases and of previous earthquake studies in the aegean california and western new guinea the observed configurations of deformation agree with a simple model of distributed accommodation of relative plate motion in iran and central asia the moment tensors indicate that there is appreciable strain parallel to the strikes of the belts we augment our 10 year estimates of seismic deformation with the available historical moment and magnitude estimates for the five regions and find that moderate and large earthquakes account for between 25 and 70 of the deformation of the seismogenic layer

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

flat plateau and impossible isochrons apparent 40ar 39ar geochronology in a high pressure terrain until recently most ages determined by the 40ar 39ar technique have been represented by the age spectrum and isochron early in the history of the technique a series of criteria were outlined in order to discriminate between spectra which were considered to reflect disturbed isotopic systems and spectra which were considered to reflect the accurate age of the rock or mineral with the advancement of 40ar 39ar instrumentation and the introduction of laser ablation techniques it is now possible to obtain an age from single grains and on a sub grain scale in this case there is no production of an age spectra or determination of a plateau age however isochrons are still readily produced and used in the discussion of ar isotope ratios and specifically excess argon here new data illustrate the possible drawback first of the age spectrum and plateau age in the absence of an independent geochronological constraint in high pressure terranes and second of the accompanying isochron which we demonstrate can be both incorrect and geologically meaningless

cretaceous tertiary geodynamics a north atlantic exercise summary new reconstructions are presented for the cretaceous early tertiary north atlantic using a combination of palaeomagnetic hotspot and magnetic anomaly data we utilize these reconstructions in an analysis of previously described misfits between the north atlantic plate elements at successive intervals during this time period we are able to achieve reasonable overlap between the hotspot and palaeomagnetic reconstructions between 40 and 95 ma and thus are able to support the idea that the indo atlantic hotspots are relatively stationary small but systematic discrepancies for this time interval can readily be modelled with a long term octopole non dipole field contribution g3 g30 g10 0 08 however hotspot and palaeomagnetic reconstructions for the early cretaceous north atlantic show substantial differences that cannot be explained by constant non dipole fields and we favour an explanation for these discrepancies in terms of true polar wander tpw triggered by mantle instabilities between 125 and 95 ma this constitutes the only identifiable event of significant tpw since the early cretaceous taken in the context of available geochronological and geological data and seismic tomography from the region the 95 40 ma reconstructions and their time consequent geological products are interpreted in terms of specific conditions of mantle crust coupling and global plate motions tectonic activity highlights from these reconstructions show uniform ne movement of the coupled north american greenland and eurasian plates from 95 to 80 ma a marked cusp in the paths for all three elements at 80 ma where the three plates simultaneously change direction and follow a uniform nw directed motion until c 20 ma when eurasia diverges ne away from the still nw moving greenland and north american elements positioning of the iceland plume beneath the spreading ridge at 20 ma may have increased upwelling below the ridge increased the ridge push and caused a ne shift in the absolute direction of eurasia

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

global positioning system constraints on fault slip rates in southern california and northern baja mexico we use global positioning system gps estimates of horizontal site velocity to constrain slip rates on faults comprising the pacific north america plate boundary in southern california and northern mexico we enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary we estimate a pacific north america relative plate motion rate of 49 3 mm yr one standard deviation consistent with nuvel 1a estimates we are able to resolve robust slip rate estimates for the southernmost san andreas san jacinto and elsinore faults 26 2 9 2 and 6 2 mm yr respectively and for the imperial and cerro prieto faults 35 2 and 42 1 mm yr respectively accounting for about 86 of the total plate motion the remaining 14 appears to be accommodated to the west of these fault systems probably via slip along the san clemente fault and or the san miguel vallecitos rose canyon and newport inglewood fault systems these results are highly consistent with paleoseismic estimates for slip rates implying that off fault strain accumulation within the deforming zone of the plate boundary is largely elastic we estimate that the seismically quiescent southernmost san andreas fault has incurred about 8 2 m of slip deficit over the last few hundred years presumably to be recovered during a future large earthquake

north atlantic sea floor spreading rates implications for the tertiary development of inversion structures of the norwegian greenland sea the tertiary development of the norwegian continental margin was dominated by the opening of the arctic north atlantic ocean the correct identification of magnetic anomalies and their ages and the analysis of spreading rates during the formation of this ocean are important in understanding the development of the region and specifically the history of its passive margins three ocean domains the aegir reykjanes and mohns regions were investigated in an effort to understand the lateral changes in structural development of the passive margin after continental break up spreading rates generally slowed down from 2 cm a 1 after early eocene initiation of sea floor spreading to values around 0 5 cm a 1 in oligocene time an increase in spreading rates to around 1 cm a 1 coincided with the positioning of the iceland hotspot under the north atlantic mid ocean ridge at the same time the european plate changed its absolute plate motion from a north directed drift to a motion more towards the east the location of inversion structures in the voring and faeroes basin rather than in the more basin is related to differences in spreading rates the mohns and the reykjanes ridges produced more ocean floor than the aegir kolbeinsey ridges asymmetric ocean floor formation in the aegir ridge led to differential stress at the base of the lithosphere which probably explains the absence of inversion features in the more basin less mantle drag furthermore upper plate margins such as the voring basin and possibly the faeroe basin have a lower compressional strength than lower plate margins such as the more basin and therefore preferentially developed inversion structures along the transform boundaries separating the domains additional stress probably built up along extension of the transform zones into the extended continental crust this additional stress probably also assisted initiation of the inversion structures in the voring basin and the faeroes area the amplification of the inversion structures in the voring basin and the faeroes basin was subsequently caused by a variety of processes related to sedimentation and uplift erosion

40 ar 39 ar dating of detrital micas from oligocene pleistocene sandstones of the olympic peninsula klamath mountains and northern california coast ranges provenance and paleodrainage patterns sandstones of the eocene miocene hoh assemblage of the olympic peninsula and late oligocene miocene weaverville formation klamath mountains were studied to determine if the hoh sandstones could be tectonically transported equivalents of the weaverville formation distinct hoh sandstone types exposed between la push and the hoh river include 1 highly brecciated and veined quartz poor mica poor volcaniclastic sandstone preserved in melange blocks and 2 relatively unveined and unbrecciated quartzose micaceous sandstone preserved in melange blocks and bounding turbidites 40ar 39ar laser heating analyses of single crystals of detrital muscovite grains from quartzose hoh sandstones yield late cretaceous early tertiary ages consistent with contributions to hoh detritus from the idaho batholith volcaniclastic melange block sandstones could be derived from older tertiary volcanic terranes of the northeast olympic peninsula or from mesozoic accretionary terranes analyses of muscovites from the fl

fault friction regional stress and crust mantle coupling in southern california from finite element models ongoing deformation of southern california is simulated in a set of 63 models in an attempt to empirically determine fault friction to a previous code for simulation of anelastic crustal strain we introduce a new finite element to represent a curved fault of arbitrary rheology we also add several selfconsistent patterns of horizontal drag on the base of the crust caused by independent upper mantle flow a grid of isoparametric finite elements represents the regional fault geometry and the known variations in elevation and heat flow plate tectonic velocity boundary conditions are applied solutions include the time averaged flow velocities and fault slip rates and vertically averaged stresses each model is scored by comparing its slip rate predictions with 56 rates from dated offset geologic features in membrane stress models the optimal fault friction is 0 4 0 6 frictional self heating of faults is not a major factor determining strength addition of crust mantle coupling improves results if convergence of north american and pacific lithospheres is assumed to occur under the transverse ranges among such models those with fault friction of about 0 3 score best by most norms of the prediction error these have over 80 of regional strain in faults and only a small amount of block deformation however one model with friction of 0 85 scores better by the percent of correct predictions norm this model has only half the regional strain as fault slip and matches slow rates well but underpredicts slip rates over 1 cm yr furthermore its predictions are degraded considerably if hydraulic lubrication of thrust faults is assumed the low friction models are unaffected we conclude that some source of variation in fault friction remains unmodeled but that at least the fastslipping faults in california have low friction similar to that of hydrated clay gouges appendix is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 001 2 50 payment must accompany order

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