textures of syntaxial quartz veins synthesized by hydrothermal experiments syntaxial quartz veins were synthesized by hydrothermal flow through experiments using rock blocks metachert sandstone and granite containing slits based on analyses of vein textures using birefringence imaging microscopy we identified two stages of crystal growth during stage 1 quartz grain growth occurs without an increase in grain width during stage 2 quartz grains develop facets and grow preferentially parallel to the c axis orientation and the aspect ratio of quartz grains shifts toward 2 9 competitive growth occurs significantly at stage 2 and the transition from stage 1 2 occurs at a critical distance from the vein wall being approximately equal to the host rock grain size crystal growth in the slits produce various textures controlled by the ratio of slit aperture h to host rock grain size d in high h d cracks elongate blocky texture develops by grain impingement during stage 2 whereas in low h d cracks crystals that bridge the crack form without competitive growth by grain impingement at stage 1 heterogeneous structures of fracture porosity are produced during syntaxial vein formation due to the anisotropy in the growth rate of quartz such incompletely sealed cracks may act as important fluid pathways and as weak planes in the upper crust

quartz vein fabrics coupled to elevated fluid pressures in the stawell gold deposit south eastern australia shear and extensional veins formed during the reactivation of the magdala shear system at stawell in western victoria australia contribute to the formation of the auriferous central and basalt contact lodes within this shear system is a range of fault rocks accompanied by steep dipping 65 quartz rich laminated shear veins and relatively flat lying extensional veins both vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant deviatoric stresses the macro and microstructures suggest that the dilatancy that produced mineralized veins formed under conditions of overpressure generated by fluid infiltration late in a tectonic regime a new microfabric analysis technique is used to investigate the quartz rich veins which allows rapid integration of the microstructure with the crystallographic preferred orientations cpos both the shear and extensional quartz veins have a random cpo with 120 dihedral angles between the quartz quartz grains which is typical of a metamorphic equilibrium microfabric the microstructures indicate that the quartz has undergone extensive grain adjustment in the solid state with grain shape and size affected by interfacial solution pressure solution effects these features are consistent with inferences from experimental rock deformation studies where grain boundary migration is enhanced in a water rich environment the onset of solution transfer processes pressure solution developed as the quartz microfabric stabilized and continued to modify the cpo and microstructure significantly it is concluded that grain growth and pressure solution are coupled diffusive mass transfer processes related to fluctuations in pore fluid pressures in a region undergoing deformation at near lithostatic pressures

distribution of shortening between the indian and australian plates in the central indian ocean we analyze a single channel seismic scs reflection profile that completely crosses the zone of deformed oceanic lithosphere in the central indian ocean at 78 8 e by summing the apparent shortening on all seismically resolvable faults throws 10 m we find that 11 2 2 km of shortening has occurred at this longitude during the past 7 5 m y this estimate together with the 27 f 5 km of shortening previously estimated from a multichannel seismic mcs profile farther east at 81 5 e are consistent with the west to east increase in shortening predicted by euler poles which treat the indian and australian plates as separate tectonic units our result therefore provides direct evidence from the deformation itself that the compression of oceanic lithosphere in the central indian ocean originally regarded as intraplate is better described as constituting part of a broad boundary zone between distinct indian and australian plates we also examine the size statistics of faults revealed in scs and mcs profiles running nearly normal to the deformation trends in the longitude band 78 8 e 81 5 e the n s extent of the deformation does not change appreciably over these longitudes we find that the average fault spacing remains constant at about 7 km whereas the mean throw increases systematically from west to east ff 74 m to n 177 m basically the contribution of small faults those with throws of lo 50 m decreases systematically across the deforming region i e with increasing amount of shortening this suggests that the deformation occurs by reactivation of a select fault population that these faults continue to add displacement with time and that relatively few new faults are initiated we also infer from the fault size statistics that the contribution to the deformation of faults below the resolution of the seismic methods 10 m for scs and 50 m for mcs is likely to be quite limited

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

location of the africa australia india triple junction and motion between the australian and indian plates results from an aeromagnetic investigation of the central indian and carlsberg ridges accepted 1994 june 7 received 1994 may 23 in original form 1993 may 24 summary prior studies have proposed and examined the hypothesis that india and australia are separate rigid plates separated by a wide near equatorial e w striking plate boundary attempts to place narrow limits on the location of the africa australiaindia triple junction have been hindered however by the lack of useful magnetic profiles crossing the eastern carlsberg ridge and northern central indian ridge herein we present near ridge portions of new profiles from an aeromagnetic survey of the carlsberg ridge east of 66 e and of the central indian ridge north of 19 s these new data are used to estimate 35 new spreading rates averaged from the middle of chron 2a 3 03 ma to the present all other plate motion data along the central indian and carlsberg ridges are also analysed to investigate the present kinematics of the indian ocean especially the motion and boundary between the indian and australian plates unlike prior efforts we objectively estimate uncertainties in the strikes of transform faults along the carlsberg and central indian ridges

evidence for motion between nubia and somalia along the southwest indian ridge the east african rift marks the northern boundary of the nubian west african and somalian east african plates and has formed by horizontal stretching due to the separation of these plates1 south of 20 s any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes although the boundary must continue until it intersects another plate boundary the nearest such boundary is that of the antarctic plate marked by the southwest indian ridge but previous analyses of plate motion data have indicated no significant difference between nubia antarctica and somalia antarctica motion2 3 here we show using a large compilation of plate motion data that nubia antarctica motion does differ from somalia antarctica motion and we determine a relative angular velocity of the two plates that has compact confidence limits our analysis places the pole of rotation near to the southern limit of african seismicity implying that the southern part of the nubian somalian plate boundary is a diffuse zone of convergence up to 2 mm yr 1 whereas up to 6 mm yr 1 of separation is accommodated across the east african rift about half the separation rate of the slowest mid ocean ridge

arsenic mobility in groundwater surface water systems in carbonate rich pleistocene glacial drift aquifers michigan abstract within the lower peninsula of michigan groundwaters from the marshall formation mississippian contain as derived from as rich pyrites often exceeding the world heath organization drinking water limit of 10 g l many michigan watersheds established on top of pleistocene glacial drift derived from erosion of the underlying marshall formation also have waters with elevated as the huron river watershed in southeastern lower michigan is a well characterized hydrogeochemical system of glacial drift deposits proximate to the marshall fm subcrop which hosts carbonate rich groundwaters streams and wetlands fens and well developed soil profiles aqueous and solid phase geochemistry was determined for soils soil waters surface waters streams and fens and groundwaters from glacial drift aquifers to better understand the hydrogeologic and chemical controls on as mobility soil profiles established on the glacial drift exhibit enrichment in both fe and as in the oxyhydroxide rich zone of accumulation the amounts of fe and as present as oxyhydroxides are comparable to those reported from bulk marshall fm core samples by previous workers however the as host in core samples is largely unaltered pyrite and arsenopyrite this suggests that the transformation of fe sulfides to fe oxyhydroxides largely retains as and fe at the oxidative weathering site groundwaters have the highest as values of all the waters sampled and many were at or above the world health limit most groundwaters are anaerobic within the zones of fe3 and as v reduction although reduction of fe iii oxyhydroxides is the probable source of as there is no correlation between as and fe concentrations the as fe mole ratios in drift groundwaters are about an order of magnitude greater than those in soil profiles suggesting that as is more mobile than fe this is consistent with the dominance of as iii in these groundwaters and with the partitioning of fe2 into carbonate cements soil waters have very low as and fe contents consistent with the stability of oxyhydroxides under oxidizing vadose conditions when co2 charged groundwaters discharge in streams and fens dissolved as is effectively removed by adsorption onto fe oxides or carbonate marls although fe does not display conservative behavior with as in groundwaters a strong positive correlation exists between as and sr concentrations as water rock interactions proceed the as fe and sr ca ratios would be expected to increase because both as and sr behave as incompatible elements comparisons with groundwater chemistries from other drift hosted aquifers proximate to the marshall sandstone are consistent with these relations thus the sr content of carbonate rich groundwaters may provide useful constraints on the occurrence origin and evolution of dissolved as in such systems

pacific antarctic australia motion and the formation of the macquarie plate summary magnetic anomaly and fracture zone data on the southeast indian ridge seir are analysed in order to constrain the kinematic history of the macquarie plate the region of the australian plate roughly east of 145 e and south of 52 s finite rotations for australia antarctic motion are determined for nine chrons 2ay 3ay 5o 6o 8o 10o 12o 13o and 17o using data limited to the region between 88 e and 139 e these rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the seir an analysis of the synthetic flowlines shows that the macquarie plate region has behaved as an independent rigid plate for roughly the last 6 myr finite rotations for macquarie antarctic motion are determined for chrons 2ay and 3ay these rotations are summed with australia antarctic rotations to determine macquarie australia rotations we find that the best fit macquarie australia rotation poles lie within the zone of diffuse intraplate seismicity in the south tasman sea separating the macquarie plate from the main part of the australian plate motion of the macquarie plate relative to the pacific plate for chrons 2ay and 3ay is determined by summing macquarie antarctic and antarctic pacific rotations the pacific macquarie rotations predict a smaller rate of convergence perpendicular to the hjort trench than the pacific australia rotations the onset of the deformation of the south tasman sea and the development of the macquarie plate appears to have been triggered by the subduction of young buoyant oceanic crust near the hjort trench and coincided with a clockwise change in pacific australia motion around 6 ma the revised pacific australia rotations also have implications for the tectonics of the alpine fault zone of new zealand we find that changes in relative displacement along the alpine fault have been small over the last 20 myr the average rate of convergence over the last 6 myr is about 40 per cent smaller than in previous models

the plate tectonic approximation plate nonrigidity diffuse plate boundaries and global plate reconstructions the central assumption of plate tectonics that plate interiors are rigid remains a useful but uncertain approximation strain rates of stable plate interiors are bounded between 10 i12 10 i11 year i1 and 4 10 i10 year i1 the narrowness of all plate boundaries the other main assumption of plate tectonics as originally conceived is contradicted by many observations both in the continents and in the oceans some diffuse plate boundaries in both continents and oceans exceed dimensions of 1000 km on a side diffuse plate boundaries cover 15 of earth s surface the maximum speed of relative plate motion across any one diffuse plate boundary ranges from 2t o 15 mm year which is faster than some upper bounds on intraplate motion across stable plate interiors 2m m year i1 strain rates in diffuse plate boundaries can be as high as 10 i8 year i1 25 times higher than the upper bound on strain rates of stable plate interiors but 600 times lower than the lowest strain rates across typical narrow plate boundaries the poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary which is a consequence of the strong coupling across the boundary

kinematic constraints on distributed lithospheric deformation in the equatorial indian ocean from present motion between the australian and indian plates from an expanded accurate and up to date data set comprising 110 spreading rates 46 transform azimuths and 151 earthquake slip vectors from the indian ocean and gulf of aden we determine a new rigid plate model describing the motion since 3 ma between india and australia the euler vector 0 313 m y about 5 s 78 e lies near the middle of the equatorial diffuse plate boundary dividing the indian from the australian plate and predicts a rate of north south shortening along 85 e of 4 3 mm yr only 30 as fast as predicted by our prior model the new model also predicts north south extension of 6 2 mm yr at 68 e along the western segment of the diffuse plate boundary where our prior model predicted north south contraction using data only along the carlsberg and central indian ridges and no other plate boundaries we show that plate motion data cannot be fit by a single euler vector however the data are well fit by two euler vectors when an east west striking india australia plate boundary is assumed to intersect the central indian ridge near the equator the best location along the central indian ridge for this triple junction is 6 s 3 s with 95 confidence limits of 9 s 4 n just west of a region of intense seismicity the sense of deformation as recorded in earthquake focal mechanisms reverse faults mapped with reflection seismic data and undulations in basement topography surface gravity and the geoid agrees well with the north south extension predicted in the western part and the north south shortening predicted in the eastern part of the india australia boundary the predicted rate of north south shortening between 79 e and 86 e is consistent with the rate of shortening inferred from observed faulting and folding shortening taken up by faulting is 6 to 100 times greater than that taken up by the spectacular basement folds little of the rapid north south shortening predicted east of 86 e is taken up by crustal thickening instead lithosphere is transported northeastward toward the sumatra trench through strike slip faulting and possible clockwise rotation the failure of the convergent segment of the diffuse plate boundary to form a subducting trench provides some new observations that must be satisfied by models for the initiation of subduction a model of separate rigid indian and australian plates divided by a diffuse plate boundary appears to be valid and useful because the predictions of the model agree with independent data and because the predicted velocity of relative motion is statistically significant and comparable with that across other plate boundaries

a source sink model of the generation of plate tectonics from non newtonian mantle flow a model of mantle convection which generates plate tectonics requires strain rate or stress dependent rheology in order to produce strong platelike flows with weak margins as well as strike slip deformation and plate spin i e toroidal motion here we employ a simple model of source sink driven surface flow to determine the form of such a rheology that is appropriate for earth s present day plate motions in this model lithospheric motion is treated as shallow layer flow driven by sources and sinks which correspond to spreading centers and subduction zones respectively the source sink field is derived from the horizontal divergence of plate velocities and thus directly prescribes poloidal motion the toroidal flow field is solved through the non newtonian stokes equation for shallow layer tangential flow on the surface of a sphere two plate motion models are used to derive the source sink field the first is an idealized square plate which is used to explore the basic aspects of the model the second is an analytically continuous model of earth s present day plates bercovici and wessel 1994 as originally implied in the simpler cartesian version of this model bercovici 1993 the classical power law rheologies do not generate platelike flows as well as the hypothetical whitehead gans stick slip rheology which incorporates a simple self lubrication mechanism for the idealized plate geometry the power law rheologies yield much more diffuse strike slip shear i e radial vorticity than the stick slip rheology for the present day plate geometry the power law rheologies fail to reproduce the original proportion of left and right lateral strike slip shear whereas the stick slip rheology gives almost exactly the right proportion none of the fluid rheologies examined however produce more than approximately 60 of the original maximum shear for either plate model the viscosity fields produced by the power law rheologies are diffuse and the viscosity lows over strike slip shear zones or pseudo margins are not as small as over the prescribed convergent divergent margins in contrast the stick slip rheology generates very platelike viscosity fields with sharp gradients at the plate boundaries and margins with almost uniformly low viscosity quantitative comparisons with the toroidal poloidal kinetic energy partitioning and vorticity fields of the original plate model are also examined and the stick slip rheology is generally found to yield the most favorable comparisons power law rheologies with high viscosity contrasts however lead to almost equally favorable comparisons though these also yield the least platelike viscosity fields this implies that the magnitude of toroidal flow and platelike strength distributions are not necessarily related arid thus may present independent constraints on the determination of a self consistent plate mantle rheology the results of this study however predict that if such a rheology can indeed be uniquely determined it is likely to be in the class of stick slip self lubricating rheologies

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

experimental tests of simple models for the dynamics of diffuse oceanic plate boundaries summary diffuse plate boundaries which are zones of deformation hundreds to thousands of kilometres wide occur in both continental and oceanic lithosphere here we build on our prior work in which we use analytical and numerical models to investigate the dynamics of diffuse oceanic plate boundaries assuming that the vertically averaged rheology of deforming oceanic lithosphere is characterized by a power law rheology that is n where 1 n is strain rate and is deviatoric stress a major conclusion of this prior work is that the pole of relative rotation of plates adjoining a diffuse oceanic plate boundary is predicted to be confined to the diffuse oceanic plate boundary especially if n 3 here we use laboratory experiments to test the general validity of this conclusion in these analogue experiments deforming oceanic lithosphere is modelled alternately with a newtonian fluid n 1 and a power law fluid n 6 10 the experimental results are consistent with the analytical and numerical models in particular the experimental results confirm that for a given applied force the pole of rotation is more strongly confined to the laboratory analogue of a diffuse oceanic plate boundary for a power law fluid than it is for a newtonian fluid for forces applied far enough from the centre of the analogue diffuse plate boundary however results for newtonian and power law fluids are similar

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

plate motions and crustal deformation over the past 30 years the plate tectonic paradigm has revolutionized the earth sciences through its powerful yet elegant quantitative treatment of large scale crustal deformation and through its conceptually integrative role within the earth sciences this review summarizes several important developments in plate tectonic research for the period 1991 to early 1994 with emphasis placed on publications that have either significantly improved our understanding of plate motions and their relation to crustal deformation or have raised significant new questions revisions to the geomagnetic reversal time scale cenozoic plate motions microplate kinematics oblique convergence and deformation of forearcs and kinematic information derived from detailed bathymetric and side scan sonar studies are featured

indian ocean floor deformation induced by the reunion plume rather than the tibetan plateau the central indian ocean is considered the archetypal diffuse oceanic plate boundary data from seismic stratigraphy and deep sea drilling indicate that the contractional deformation of the indian ocean lithosphere commenced at 15 4 13 9 ma but experienced a sharp increase at 8 7 5 ma this has been maintained through to the present day with over 80 of the shortening accrued over the past 8 myr here we build on previous efforts to refine the form timing and magnitude of the regional plate motion changes by mitigating the noise in reconstructed indian and capricorn plate motions relative to somalia our noise mitigated reconstructions tightly constrain the significant speed up of the capricorn plate over the past 8 myr and demonstrate that the history of the indian ocean floor deformation cannot be explained without this plate motion change we propose that the capricorn plate motion change is driven by an increase in the eastward directed asthenospheric flow associated with the adjacent reunion plume and quantitatively demonstrate the viability of this hypothesis our inference is supported by volcanic age distributions along the reunion hotspot track the anomalously high residual bathymetry of the central indian ridge full waveform seismic tomography of the underlying asthenosphere and geochemical observations from the central indian ridge these findings challenge the commonly accepted link between the deformation of the indian ocean floor and the tibetan plateau s orogenic evolution and demonstrate that temporal variations in upwelling mantle flow can drive major tectonic events at the earth s surface deformation of the indian ocean floor over the past 8 million years was caused by a change in plate motions linked to flow of the reunion mantle plume according to analyses of forces upon plates

eocene 46 44 ma onset of australia pacific plate motion in the southwest pacific inferred from stratigraphy in new caledonia and new zealand

deformation of indian ocean lithosphere evidence for a highly nonlinear rheological law the width of diffuse oceanic plate boundaries is determined by the rheology of oceanic lithosphere here we apply thin viscous sheet models which have been successfully applied to deformation in several continental deforming zones to investigate the deformation of oceanic lithosphere in the diffuse oceanic plate boundaries between the india capricorn and australia plates we apply kinematic boundary conditions based on the current motion between these plates we neglect buoyancy forces due to plate thinning or thickening and assume that the thin viscous sheet has the same depth integrated nonlinear viscosity coefficient everywhere our initial models have only one adjustable parameter n the power law exponent with n 1 3 10 30 and 100 the predicted width of the deforming zone decreases with increasing n with n 30 explaining the observations this n value is higher than has been estimated for continental lithosphere and suggests that more of the strength of oceanic lithosphere lies in layers deforming by faulting or by dislocation glide than for continental lithosphere to obtain a stress field that better fits the distribution and type of earthquake focal mechanisms in the diffuse oceanic plate boundary we add a second adjustable parameter representing the effect of slab pull stretching the oceanic plate near the sumatra trench we show that an average velocity increment on this boundary segment of 5 mm a 1 relative to the average velocity of the india and australia plates fits the observed distribution of fault types better than velocities of 3 3 mm a 1 or 10 mm a 1

cretaceous to present kinematics of the indian african and seychelles plates an iterative inverse model of seafloor spreading data from the mascarene and madagascar basins and the flanks of the carlsberg ridge describes a continuous history of indian african plate divergence since 84 ma visual fit modelling of conjugate magnetic anomaly data from near the seychelles platform and laxmi ridge documents rapid rotation of a seychelles plate about a nearby euler pole in palaeocene times as the euler pole migrated during this rotation the amirante trench on the western side of the plate accommodated first convergence and later divergence with the african plate the unusual present day morphology of the amirante trench and neighbouring amirante banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase the model fits fracture zone trends in the north arabian and east somali basins suggesting that they formed in india africa plate divergence seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71 69 ma and that upon arrival of the deccan re union plume and an increase to faster plate divergence rates in the period 69 65 ma segments of the ridge lengthened and propagated ridge propagation into the indian continental margin led first to the formation of the laxmi basin which accompanied extensive volcanism onshore at the deccan traps and offshore at the saurashtra high and somnath ridge a second propagation episode initiated the ancestral carlsberg ridge at which seychelles india and india africa plate motions were accommodated with the completion of this propagation the plate boundaries in the mascarene basin were abandoned seafloor spreading between this time and the present has been accommodated solely at the carlsberg ridge

monsoon speeds up indian plate motion article i nfo short term plate motion variations on the order of a few myr are a powerful probe into the nature of plate boundary forces as mantle related buoyancies evolve on longer time scales new reconstructions of the ocean floor spreading record reveal an increasing number of such variations but the dynamic mechanisms producing them are still unclear here we show quantitatively that climate changes may impact the short term evolution of plate motion by linking explicitly the observed counter clockwise rotation of the indian plate since 10 ma to increased erosion and reduced elevation along the eastern himalayas due to temporal variations in monsoon intensity by assimilating observations into empirical relations for the competing contributions of erosion and mountain building we estimate the first order decrease in elevation along the eastern himalayas since initial strengthening of the monsoon furthermore we show with global geodynamic models of the coupled mantle lithosphere system that the inferred reduction in elevation is consistent with the indian plate motion record over the same period of time and that lowered gravitational potential energy in the eastern himalayas following stronger erosion is a key factor to foster plate convergence in this region our study implicates lateral variations in plate coupling and their temporal changes as an efficient source to induce an uncommon form of plate motion where the euler pole falls within its associated plate

a little spin in the indian ocean plate circuit abstract divergence of the australian and east antarctic plates is well understood from the late jurassic onset of half graben development on the australian continental shelf and from post mid eocene chron 20 45 ma seafloor spreading isochrons further offshore relative plate motion between these times is less confidently interpretable from magnetic reversal anomalies landwards of isochron 20 and localised evidence for mid to late cretaceous subsidence and growth strata from the continental shelf and rise south of australia a new test of this history examines it within the post 34y 84 ma indian ocean plate circuit built using seafloor spreading data from the wharton and central indian ocean basins the australian antarctic jurassic onset rift system is interpreted to have been abandoned before 34y because motion in the circuit is inconsistent with an active plate boundary during 34y 26y 84 58 ma starting 26y the model depicts plate divergence distances and azimuths that after 25y 57 ma can be independently confirmed by reassessment of the pre chron 20 magnetic reversal anomaly pattern previous studies have identified evidence for mantle exhumation and focused magmatism in basement marginwards of these anomalies these processes are not directly or confidently dated but mantle exhumation is inconsistent with the circuit model s fast plate divergence at 26y 25y hence plate motion during the immediate build up to post 57 ma seafloor spreading may have been accommodated by focused magmatism whilst mantle exhumation may mark the conclusion of the jurassic onset rift phase during a slower pre 84 ma period of plate divergence using the new model to make tectonic reconstructions results in a large overlap between tasmania and victoria land that can be explained with reference to eocene strike slip faulting and transtension in recently discovered subglacial basins of wilkes and george v lands and terre adelie

intraplate shortening in the central indian ocean determined from a 2100 km long north south deep seismic reflection profile although intraplate deformation is commonly observed on the continents there are only a few places in the oceans where deformation is distributed over a broad area rather than being localized at a single plate boundary the central indian ocean has long been recognized as a site of active widespread compressive deformation of the oceanic lithosphere reflecting an exceptionally high stress level however the strain pattern derived from indirect and incomplete observations remains poorly known a more complete data set was obtained during the phedre cruise 1991 by means of deep seismic reflection profiling one 2100 km long profile runs along the 81 e meridian from lat 14 s to the coast of sri lanka and provides us with the first long cross section through the region of observed crustal level faults fault analysis offers new estimates of the amount of shortening which was obtained across individual faults by measuring the maximum vertical uplift of sedimentary reflectors at the base of the sedimentary cover and converting it to horizontal throw under various assumptions of the fault geometry the contribution of the seismically resolvable faults amounts to 22 37 km of shortening or 2 5 4 3 this is two to three times greater than previous estimates extrapolated from local measurements on shorter seismic reflection profiles

topography boundary forces and the indo australian intraplate stress field the relative contribution of topographic e g ridge push continental margins and elevated continental crust and plate boundary e g subduction and collisional forces to the intraplate stress field in the indo australian plate iap is evaluated through a finite element analysis two important aspects of the iap intraplate stress field are highlighted in the present study 1 if substantial focusing of the ridge push torque occurs along the collisional boundaries i e himalaya new guinea and new zealand many of the first order features of the observed stress field can be explained without appealing to either subduction or basal drag forces and 2 it is possible to fit the observed shmax maximum horizontal stress orientation and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes e g tens of megapascals averaged over the thickness of the lithosphere throughout the plate this study therefore presents a plausible alternative to previous studies of the iap intraplate stress field which predicted very large tectonic stress magnitudes hundreds of megapascals in some parts of the plate in addition topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental india and australia and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions a central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin we note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field and this nonuniqueness continues to make modeling the iap stress field problematic while our study is an important step forward in understanding the sources of the iap intraplate stress field a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin

mineral distribution within polymineralic veins in the sanbagawa belt japan implications for mass transfer during vein formation pelitic schists of the sanbagawa metamorphic belt contain several types of polymineralic veins that formed during the late stages of exhumation the vein mineral assemblages are quartz albite k feldspar chlorite calcite type i ii and quartz albite calcite type iii type i and ii veins contain quartz and albite with stretched crystal and elongate blocky textures respectively the mineral species within type i veins vary with compositional bands within the host rocks type iii veins are characterized by euhedral to subhedral quartz grains with concentric zoning and a homogeneous distribution along the vein length the vein textures vary depending on the crack aperture during multiple crack seal events 0 08 mm for type i and 0 5 10 mm for type iii type ii veins show intermediate features between type i and iii veins in terms of mineral distribution weak dependence on the host rock composition and apparent crack aperture less than 1 15 mm these observations suggest a transition in the dominant transport mechanism of vein components with increasing crack aperture from diffusion from host rocks to fluid advection along cracks

the motion and boundary between the capricorn and australian plates the motions between the somalian antarctic and australian plates the three plates believed to meet at the rodrigues triple junction in the indian ocean are inconsistent with the assumption that all three plates are rigid the discrepancy is best explained if the australian plate contains two component plates thus the traditionally defined indo australian plate consists of three component plates and multiple diffuse plate boundaries the pattern of present deformation indicates that the boundaries between the three component plates are two unconnected zones accommodating divergence and a larger zone which we interpret as three diffuse convergent plate boundaries and a diffuse triple junction

changes in plate boundary kinematics punctuated or smoothly varying evidence from the mid cenozoic transition from lithospheric extension to shortening in new zealand abstract the marine magnetic anomaly record and plate kinematics derived from that data provide evidence of numerous cases of significant changes in plate motions and plate interactions however in most cases the temporal resolution provided by the marine record does not discriminate between punctuated abrupt or smooth variations in plate motions during a 10 million year period 30 ma to 20 ma the euler poles stage poles describing australia pacific relative plate motion migrated more than 20 in latitude the new zealand segment of the plate boundary is close to the location of the euler poles and their migration making the tectonics of its plate boundary structures particularly sensitive to the history of plate kinematics based on the rapid southward migration of the euler stage poles from north of new zealand to south of the landmass there is an expected rapid migrating transition from an extensional stage pole north of the site to transpressional stage pole south of the site plate boundary system the geologic signature of this rapid change in plate boundary kinematics is preserved in the stratigraphic record of a suite of basin deposits that span the latitudinal sweep of the migrating euler poles analyses of these deposits indicate that the timing of the transition from extensional to transpressional tectonics shows a continuous and systematic southward sweep indicating that the changes in australia pacific plate motions during the 30 ma 20 ma interval were smoothly continuous and not punctuated

slow downs and speed ups of india eurasia convergence since 20 ma data noise uncertainties and dynamic implications india somalia and north america eurasia relative motions since early miocene 20 ma have been recently reconstructed at unprecedented temporal resolution o 1 myr from magnetic surveys of the carlsberg and northern mid atlantic ridges these new datasets revamped interest in the convergence of india relative to eurasia which is obtained from the india somalia nubia north america eurasia plate circuit unless finite rotations are arbitrarily smoothed through time however the reconstructed kinematics i e stage euler vectors appear to be surprisingly unusual over the past 20 myr in fact the euler pole for the india eurasia rigid motion scattered erratically over a broad region while the associated angular velocity underwent sudden increases and decreases consequently convergence across the himalayan front featured significant speed ups as well as slow downs with almost no consistent trend arguably this pattern arises from the presence of data noise which biases kinematic reconstructions particularly at high temporal resolution the rapid and important india eurasia platemotion changes reconstructed since early miocene are likely to be of apparent nature because they cannot result even from the most optimistic estimates of torques associated for instance with the descent of the indian slab into earth s mantle our previous work aimed at reducing noise in finiterotation datasets via an expanded bayesian formulation which offers several advantages over arbitrary smoothing methods here we build on this advance and revise the india eurasia kinematics since 20 ma accounting also for three alternative histories of rifting in africa we find that india eurasia kinematics are simpler and most importantly geodynamically plausible upon noise reduction convergence across the himalayan front overall decreased until 10 ma but then systematically increased albeit moderately towards the present day we test with global dynamic models of the coupled mantle lithosphere system how a previously proposed uplift of the tibetan plateau by 1 2 km as well as the break up of the indo australian plate into the india and australia units impact the kinematic record from 20 to 10 ma our modelling results indicate that the convergence slow down until 10 ma resulted chiefly from the indo australian break up however the tectonic scenario yielding the best fit to kinematic reconstructions is indeed one where the tibetan mean elevation increased by 1 2 km

the malpelo plate hypothesis and implications for nonclosure of the cocos nazca pacific plate motion circuit using global multiresolution topography we estimate new transform fault azimuths along the cocos nazca plate boundary and show that the direction of relative plate motion is 3 3 1 8 95 confidence limits clockwise of prior estimates the new direction of cocos nazca plate motion is moreover 4 9 2 7 95 confidence limits clockwise of the azimuth of the panama transform fault we infer that the plate east of the panama transform fault is not the nazca plate but instead is a microplate that we term the malpelo plate with the improved transform fault data the nonclosure of the nazca cocos pacific plate motion circuit is reduced from 15 0 mm a 1 3 8 mm a 1 to 11 6 mm a 1 3 8 mm a 1 95 confidence limits the nonclosure seems too large to be due entirely to horizontal thermal contraction of oceanic lithosphere and suggests that one or more additional plate boundaries remain to be discovered

analytic models for the dynamics of diffuse oceanic plate boundaries summary some plates deform across zones that are many hundreds to thousands of kilometres wide much broader than traditional boundaries such as mid ocean ridges deep sea trenches and oceanic transform faults across which most deformation is concentrated in a zone just a few kilometres wide these wide zones of deformation commonly referred to as diffuse plate boundaries occur in both continental and oceanic lithosphere composite plates are composed of two or more rigid or nearly rigid component plates separated by one or more diffuse plate boundaries royer gordon 1997 such complete diffuse boundaries are terminated at both ends by triple junctions although there are other diffuse boundaries that transform into narrow boundaries here we consider the dynamics of complete diffuse oceanic plate boundaries by constructing simple analytical models on a flat earth and on a spherical earth assuming that the viscous force resisting deformation is described by either a linear newtonian law or a high exponent power law we investigate the observed tendency for the pole of relative motion between component plates separated by a diffuse plate boundary to lie within the diffuse boundary itself we show that this tendency is due to geometrical effects that make it unlikely that the total torque acting between plates at a diffuse boundary could be oriented such that relative rotation occurs between the plates about a pole lying outside the boundary this is demonstrated for both flat and spherical earth cases assuming that resistance to strain along the diffuse boundary increases linearly with stress newtonian rheology we further show that the pole of rotation is even more likely to lie in the diffuse plate boundary if the viscous force resisting deformation is described by a high exponent power law rather than a newtonian law

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

global tectonics and space geodesy much of the success of plate tectonics can be attributed to the near rigidity of tectonic plates and the availability of data that describe the rates and directions of motion across narrow plate boundaries m 1 to 60 kilometers wide nonetheless many plate boundaries in both continental and oceanic lithosphere are not narrow but are hundreds to thousands of kilometers wide wide plate boundary zones cover m 15 percent of earth s surface area space geodesy which includes very long baseline radio interferometry satellite laser ranging and the global positioning system is providing the accurate long distance measurements needed to estimate the present motion across and within wide plate boundary zones space geodetic data show that plate velocities averaged over years are remarkably similar to velocities averaged over millions of years