space geodetic observations of nazca south america convergence across the central andes space geodetic data recorded rates and directions of motion across the convergent boundary zone between the oceanic nazca and continental south american plates in peru and bolivia roughly half of the overall convergence about 30 to 40 millimeters per year accumulated on the locked plate interface and can be released in future earthquakes about 10 to 15 millimeters per year of crustal shortening occurred inland at the sub andean foreland fold and thrust belt indicating that the andes are continuing to build little 5 to 10 millimeters per year along trench motion of coastal forearc slivers was observed despite the oblique convergence

active thrusting and folding along the northern tien shan and late cenozoic rotation of the tarim relative to dzungaria and kazakhstan we have studied geometries and rates of late cenozoic thrust faulting and folding along the northern piedmont of the tien shan mountain belt west of urumqi where the m 8 3 manas earthquake occurred on december 23 1906 the northern range of the tien shan rising above 5000 m overthrusts a flexural foredeep filled with up to 11 000 m of sediment of the dzungarian basement our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front within a 200 km long zone of neogene quaternary anticlines fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range in all the valleys the scarps offset vertically the highest terrace surface by the same amount 10 2 0 7 m inferring an early holocene age 10 2 kyr for this terrace which is continuous with the largest recent fans of the piedmont yields a rate of vertical throw of 1 0 0 3mm yr on the main active thrust at the surface a quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5 5 2 5 m 2 kyr a rather fresh surface scarp 0 8 0 15 m high that is unlikely to result from shallow earthquakes with 6 m 7 in the last 230 years is visible at the extremities of the main fold zone we associate this scarp with the 1906 manas earthquake and infer that a structure comprising a deep basement ramp under the range gently dipping flats in the foreland and shallow ramps responsible for the formation of the active fault propagation anticlines could have been activated by that earthquake if so the return period of a 1906 type event would be 850 380 years the small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth 2 3 is taken up by incremental folding near the surface comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising quaternary ranges such as the san gabriel mountains in california or the mont blanc aar massifs in the alps we estimate the finite cenozoic shortening of the folded dzungarian sediments to be of the order of 30 km and the cenozoic shortening rate to have been 3 1 5 mm yr assuming comparable shortening along the tarim piedmont and minor additional active thrusting within the mountain belt we infer the rate of shortening across the tien shan to be at least 6 3 mm yr at the longitude of manas 85 5 e a total shortening of 125 30 km is estimated from crustal thickening assuming local airy isostatic equilibrium under the same assumption serial n s sections imply that cenozoic shortening across the belt increases westwards to 203 50 km at the longitude of kashgar 76 e as reflected by the westward increase of the width of the belt this strain gradient implies a clockwise rotation of tarim relative to dzungaria and kazakhstan of 7 2 5 around a pole located near the eastern extremity of the tien shan west of hami 96 e 43 5 n comparable to that revealed by paleomagnetism between tarim and dzungaria 8 6 8 7 a 6 mm yr rate of shortening at the longitude of manas would imply a rate of rotation of 0 45 m y and would be consistent with a shortening rate of 12 mm yr north of kashgar taking such values to be representative of late cenozoic rates would place the onset of reactivation of the tien shan by the india asia collision in the early to middle miocene 16 22 9 m y in accord with the existence of particularly thick late neogene and quaternary deposits such reactivation would thus have started much later than the collision roughly at the time of the great mid miocene changes in tectonic regimes denudation and sedimentation rates observed in southeast asia the himalayas and the bay of bengal and of the correlative rapid change in seawater sr isotopic ratio 20 to 15 ma like these other changes the rise of the tien shan might be a distant consequence of the end of indochina s escape

geological setting of the early devonian rhynie cherts aberdeenshire scotland an early terrestrial hot spring system abstract the drilling of nine cored boreholes including a deep hole 233 m to a gabbroic basement in the vicinity of the rhynie chert locality has resulted in a major revision of the structure and stratigraphy of the area the main new structural element recognized is a low angle extensional fault system which defines the western edge of a half graben containing the early devonian succession the fault system was the main conduit for the fluids that fed the hot springs and a heat source to the southeast of rhynie is indicated rapid and unexpected lateral and vertical variations in lithology together with new lithological units have been identified the latter include a thick unit of intensely altered lapilli tuffs which are unique to the rhynie basin and to other devonian basins in ne scotland analysis of these lithologies in conjunction with the new structural model allows the succession at rhynie to be correlated with the succession in the rest of the basin and a model of basin evolution and hot spring development to be constructed siliceous sinters were deposited by multiple episodes of hot spring activity and are variably interbedded with shales sandstones and minor tuffs in a unit about 35 m thick sinter deposition was ultimately controlled by repeated subsidence along the basin margin geothermal activity was probably widespread in northern britain in the early devonian but the surface deposits are unlikely to be preserved the rhynie deposit has survived due to a fortuitous combination of circumstances

current deformation in the northern canadian cordillera inferred from gps measurements 1 continuous and campaign style global positioning system gps measurements provide new constraints on the first order current deformation pattern of the northern cordillera of nw canada and eastern alaska the yakutat block is currently colliding with north america in the corner of the gulf of alaska our data infer that relative yakutat north america motion is accommodated across the eastern boundary by right lateral motion 40 mm a mainly on the fairweather fault and minor shortening 6 mm a to the north collision is taken up by shortening 31 mm a mainly on the chugach st elias fault system with westward extrusion and possible counterclockwise rotation of the yakutat block and alaskan fore arc facilitated by 23 mm a right lateral motion that is shared by several faults the seismicity pattern indicates that plate boundary deformation is far reaching producing strain throughout eastern alaska yukon and western northwest territories our gps data confirm the transfer of strain from the yakutat collision zone and enable the extent of plate boundary deformation to be mapped thereby defining the western edge of stable north america gps sites in sw yukon show motion of 3 10 mm a to the ne confirming that strain is transferred at least 400 km from the yakutat collision continued transfer north toward the mackenzie delta and ne to the mackenzie mountains indicated by current seismicity is not yet well resolved by the campaign gps data which are hampered by coseismic and postseismic effects of the 2002 mw 7 9 earthquake on the denali fault in eastern alaska

an integrated crustal velocity field for the central andes 1 we present an integrated velocity field for the central andes derived from gps observations collected between january 1993 and march 2001 that eliminates the velocity bias between the south america nazca plate project snapp and central andes gps project cap velocity fields published by norabuena et al 1998 and bevis et al 1999 the reference frame is realized by minimizing the motion of eight continuous gps stations and one rover gps station located in the stable core of the south american plate the rms horizontal motion of these stations is just 1 1 mm yr the amplitude of these residual motions is roughly compatible with expected levels of measurement error in our new solution five of the six snapp stations located just outside the orogenic belt are effectively stationary and the velocities for adjacent cap and snapp stations now agree at a level consistent with their formal uncertainties

strain and rotation rate from gps in tibet anatolia and the altiplano 1 deformation measured by regional gps networks in continental plateaus reflects the geologic and tectonic variability of the plateaus for two collisional plateaus tibet and anatolia and one noncollisional the altiplano we analyze the regional strain and rotation rate by inverting gps velocities to calculate the full two dimensional velocity gradient tensor to test the method we use gridded velocities determined from an elastic block model for the eastern mediterranean middle east region and show that to a first order the deformation calculated directly from the gps vectors provides an accurate description of regional deformation patterns principal shortening and extension rate axes vertical axis rotation and two dimensional 2 d volume strain dilatation are very consistent with long term geological features over large areas indicating that the gps velocity fields reflect processes responsible for the recent geologic evolution of the plateaus differences between geological and gps descriptions of deformation can be attributed either to gps networks that are too sparse to capture local interseismic deformation or to permanent deformation that accrues during strong earthquakes the altiplano has higher internal shortening magnitudes than the other two plateaus and negative 2 d dilatation everywhere vertical axis rotation changes sign across the topographic symmetry axis and is due to distributed deformation throughout the plateau in contrast the collisional plateaus have large regions of quasi rigid body rotation bounded by strike slip faults with the opposite rotation sense from the rotating blocks tibet and anatolia are the mirror images of each other both have regions of positive dilatation on the outboard sides of the rotating blocks positive dilatation in the aegean correlates with a region of crustal thinning whereas that in eastern tibet and yunnan province in china is associated with an area of vertical uplift rollback of the hellenic trench clearly facilitates the rotation of anatolia rollback of the sumatra burma trench probably also enables rotation about the eastern syntaxis of tibet

continental scale erosion and transport laws a new approach to quantitatively investigate macroscale landscapes and associated sediment fluxes over the geological past although critical to a variety of issues in earth sciences paleotopography remains poorly constrained over the geological past indeed sediments preserve a record of the history of the earth surface but deconvolving these archives remains a challenge in the absence of a proper quantification of surface processes at the large spatial and temporal resolution imposed by these data to solve for this we propose a set of simple bedrock erosion and sediment transport laws that apply over large spatial 100 km and temporal 1 10 ma scales these laws are tested in light of physical experiments of landscape evolution under different tectonic and climatic forcings and are calibrated using present day large scale earth topography and sediment fluxes in rivers we subsequently implement these processes into a numerical code toposed that is able to predict the evolution of macroscale topography sediment fluxes paleogeographies and bedrock exhumation given a tectonic and climatic input scenario the results of such simulations can be directly compared to sedimentary or thermochronological data to test the plausibility of the input tectonics and predicted topography a series of tests on the sensitivity of such predictions to the uncertainties on input parameters shows that it should be possible from sedimentary data to invert for paleouplift rates and also for paleotopographies during periods covering major tectonic or climatic events although this code is meant to be refined in the future as we improve our understanding of surface processes at these macroscales toposed provides a powerful tool to put constraints on past geodynamic processes such as dynamic topography by extracting quantitative information on the evolution of the earth surface from sedimentary data

progress in reconstructing vegetation on the old red sandstone continent two emphanisporites producers from the lochkovian sequence of the welsh borderland small coalified fossils mesofossils have yielded new insights into vegetation of the old red sandstone continent in early devonian times particularly useful are those containing spores that can be placed in dispersed spore taxa although patinate and emphanoid spores have not hitherto been found in situ emphanisporites cf micrornatus richardson lister is described in a bifurcating cylindrical sporangium preserved as a cuticular sheath a terminal dehiscence feature is compared with that in horneophyton the sporangium is encased in amorphous detritus with some tubular fragments similar associations occur on other sporangia e g tortilicaulis and axes at this north brown clee hill locality and they are interpreted as remains of a microbial or fungal film fragmentary cuticles interpreted as isolated sporangial valves bear an undescribed species of emphanisporites with fine interdigitating proximal muri and laevigate distal surfaces referred to emphanisporites sp a richardson lister analysis of dispersed spore assemblages from the locality and others in the welsh borderland indicate that the two emphanoid taxa were not common components of the spore rain this evidence coupled with the dearth of mesofossils of the producers suggests that the plants grew at the upper reaches of the drainage basin of the river that deposited the sediment although the paucity of sporangia may also be attributed to their low fossilization potential

geomorphic evidence for post 10 ma uplift of the western flank of the central andes 18 30 22 s 1 the western andean mountain front forms the western edge of the central andean plateau between 18 5 and 22 s latitude the mountain front has 3000 m of relief over 50 km horizontal distance that has developed in the absence of major local neogene deformation models of the evolution of the plateau as well as paleoaltimetry estimates all call for continued large magnitude uplift of the plateau surface into the late miocene i e younger than 10 ma longitudinal river profiles from 20 catchments that drain the western andean mountain front contain several streams with knickpoint bounded segments that we use to reconstruct the history of post 10 ma surface uplift of the western flank of the central andean plateau the generation of knickpoints is attributed to tectonic processes and is not a consequence of base level change related to pacific ocean capture eustatic change or climate change as causes for creating the knickpoint bounded stream segments observed minor valley filling alluvial gravels intercalated with the 5 4 ma carcote ignimbrite suggest uplift related river incision was well under way by 5 4 ma the maximum age of river incision is provided by the regionally extensive approximately 10 ma el diablo altos de pica paleosurface the river profiles reveal that relative surface uplift of at least1 km occurred after 10 ma

tectonics climate and mountain topography 1 by regressing simple independent variables that describe climate and tectonic processes against measures of topography and relief of 69 mountain ranges worldwide we quantify the relative importance of these processes in shaping observed landscapes climate variables include latitude as a surrogate for mean annual temperature and insolation but most importantly for the likelihood of glaciation and mean annual precipitation to quantify tectonics we use shortening rates across each range as a measure of topography we use mean and maximum elevations and relief calculated over different length scales we show that the combination of climate negative correlation and tectonics positive correlation explain substantial fractions 25 but 50 of mean and maximum elevations of mountain ranges but that shortening rates account for smaller portions 25 of the variance in most measures of topography and relief i e with low correlations and large scatter relief is insensitive to mean annual precipitation but does depend on latitude especially for relief calculated over small 1 km length scales which we infer to reflect the importance of glacial erosion larger scale averaged over length scales of 10 km relief however correlates positively with tectonic shortening rate moreover the ratio between small scale and large scale relief as well as the relative relief the relief normalized by the mean elevation of the region varies most strongly with latitude strong positive correlation therefore the location of a mountain range on earth with its corresponding climatic conditions not just tectonic forcing appears to be a key factor in determining its shape and size in any case the combination of tectonics and climate as quantified here can account for approximately half of the variance in these measures of topography the failure of present day shortening rates to account for more than 25 of most measures of relief raises the question is active tectonics overrated in attempts to account for present day relief and exhumation rates of high terrain

an introduction to the rhynie chert the terrestrialization of life has profoundly affected the biosphere geosphere and atmosphere and the geological magazine has published key works charting the development of our understanding of this process integral to this understanding and featuring in one of the geological magazine publications is the devonian rhynie chert konservat lagerstatte located in aberdeenshire scotland here we provide a review of the work on this important early terrestrial deposit to date we begin by highlighting contributions of note in the geological magazine improving understanding of terrestrialization and palaeozoic terrestrial ecosystems we then introduce the rhynie chert the review highlights its geological setting the caledonian context of the rhynie basin and its nature at the time of deposition of the cherts which host its famous fossils there follows an introduction to the development of the half graben in which the cherts and host sediments were deposited the palaeoenvironment this represented and the taphonomy of the fossils themselves we subsequently provide an overview of the mineralization and geochemistry of the deposit and then the fossils found within the rhynie chert these include six plant genera which continue to provide significant insights into the evolution of life on land a range of different fungi with recent work starting to probe plant fungus interactions lichens amoebae and a range of unicellular eukaryotes and prokaryotes algae and cyanobacteria and finally a range of both aquatic and terrestrial arthropods through continued study coupled with methodological advances rhynie fossils will continue to provide unique insights into early life on land

40ar 39ar dating of hydrothermal activity biota and gold mineralization in the rhynie hot spring system aberdeenshire scotland this study presents a new high precision 40ar 39ar age for the devonian hot spring system at rhynie hydrothermal k feldspar sampled from two veins that represent feeder conduits and a hydrothermally altered andesite wall rock date the hydrothermal activity the fossilised biota and syn k feldspar gold mineralization at 403 9 2 1 ma 2 oxygen isotope data for the parent fluid 4 to 2 show that the k feldspar was precipitated from a dominantly meteoric fluid which mixed with magmatic fluids from a degassing magma chamber the 40ar 39ar age 403 9 2 1 ma 2 when recalculated 407 1 2 2 ma 2 with respect to the astronomically tuned age for fish canyon sanidine 28 201 0 023 ma 1 also provides a robust marker for the polygonalis emsiensis spore assemblage biozone within the pragian earliest emsian furthermore the age identifies the devonian pull apart volcano sedimentary basins of the british and irish caledonides and their root zones as specific targets for future gold exploration

fore arc motion and cocos ridge collision in central america 1 we present the first regional surface velocity field for central america showing crustal response to interaction of the cocos and caribbean plates elastic half space models for interseismic strain accumulation on the dipping subduction plate boundary fit the gps data well and show strain accumulation offshore and beneath the nicoya and osa peninsulas in costa rica but not in nicaragua since large subduction zone earthquakes occur in nicaragua we suggest that interseismic locking in nicaragua and some other parts of central america occurs but is mainly shallow 20 km depth too far offshore to be detected by our on land gps measurements our data also show significant trench parallel motion for most of the region generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates however trench parallel motion is also observed in central costa rica where plate convergence is normal to the trench and in the nicaraguan fore arc where trench parallel motion is fast up to 9 mm a 1 but mechanical coupling is low a finite element model of collision as opposed to subduction involving the aseismic cocos ridge also fits the gps surface velocity field most significantly reproducing the pattern of trench parallel motion we infer that buoyant thickened cns 2 cocos ridge crust resists normal subduction and instead acts as an indenter to the caribbean plate driving crustal shortening in southern costa rica and contributing to trench parallel fore arc motion in costa rica and perhaps nicaragua as a type of tectonic escape

on the strength of interplate coupling and the rate of back arc convergence in the central andes an analysis of the interseismic velocity field 1 we interpret the interseismic crustal velocity field of the central andes using a simple three plate model in which the andean mountain belt is treated as a rigid microplate located between the nazca and south american soam plates we assume that the euler vectors associated with these plates are strictly coaxial and that the surface velocity field can be decomposed into an elastic loading field driven by locking of the main plate boundary and a smaller contribution associated with back arc convergence we obtain our best fit to the geodetic velocities if the main plate boundary is fully 100 locked between depths of 10 50 km and 8 5 of nazca soam plate convergence is achieved in the back arc by underthrusting of the brazilian shield beneath the subandean zone

pliocene extensional tectonics in the eastern central patagonian cordillera geochronological constraints and new field evidence recent field work and review of radiometric data obtained from neogene lavas and plutonic rocks exposed in the eastern central patagonian cordillera 46 48os which overlie subducted segments of the south chile ridge suggest important late miocene to pleistocene morphological changes in relation to base level variations and or tectonic events we present new field observations from a region south of lago general carrerabuenos aires between the main cordillera and the meseta del lago buenos aires demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5 4 ma and after 3 ma we also show that the 12 ma basaltic flows of the meseta del lago buenos aires 2000 m a s l have been subjected to deep incision with younger lavas dated at 1 2 ma partially filling the valleys these incisions are thought to reflect progressive eastward tilting of the entire meseta our new observations together with additional features from central patagonia strongly suggest that tectonic events led to a regional widespread morphological change after 5 ma the coincidence in time and space between the subduction of segments of the south chile ridge at 6 and 3 ma causing opening of a slab window and strong base level variations in the studied area suggests a cause and effect relationship in central patagonia compressional tectonics ended well before extensional events reported here causes of uplift and further extension are probably completely disconnected the uplift is purely tectonic in origin and occurred prior to the subduction of the south chile ridge extension should be a consequence of this subduction

feedbacks among climate erosion and tectonics in a critical wedge orogen the interactions among climate erosion and tectonics have long been of interest to geologists but have not yet been united into a single theoretical framework in this study representations of orographic precipitation and fluvial erosion are combined with the concept of a critical wedge orogen the idealized framework captures the basic system dynamics it also allows for a formal analysis of the precipitation and tectonic interactions in terms of feedback factors and gains and so permits a quantitative comparison of their relative strengths the constraint of self similar growth in a critical wedge orogen acts as a tectonic governor whereby changes in orogen size are strongly damped we determine that this negative tectonic feedback is stronger than the precipitation feedback which may be negative or positive depending on whether precipitation increases or decreases with orogen size for an extreme positive feedback unconstrained runaway growth is possible ultimately leading to plateau formation when orographic precipitation leads to a significant rain shadow there is a strong partitioning of rock uplift rates that favors the wet windward flank of the orogen this flank also dominates the response of the whole orogen to changes in climate or tectonic forcing except in the case where a large fraction of the eroded material is recycled into the wedge finally it is demonstrated that the response time of the orogen depends on the feedbacks and is proportional to the gain of the system

the liquine ofqui fault zone a long lived intra arc fault system in southern chile abstract plate reconstructions for the cenozoic document relatively steady right oblique subduction of the farallon nazca plate beneath the chilean continental margin the kinematic signature recorded along the intra arc liquine ofqui fault zone lofz a major feature of the southern andes may help to understand the way in which the nazca south america slip vector has been partitioned into strain and displacement along and across the continental margin the lofz consists of two nne trending right stepping straight lineaments a strike slip duplex at the right step and curved features which splay off the straight lineaments toward the northwest the lofz runs mostly through heterogeneously deformed cenozoic plutonic rock of the north patagonian batholith and patchy metamorphic wall rock early cenozoic volcano sedimentary rocks and dyke swarms found in close spatial association with the strike slip duplex are believed to have developed in strike slip related basins quaternary volcanoes are aligned parallel to the lofz both ductile and brittle kinematic indicators within centimeter to meter wide high strain zones document late cenozoic dextral shear deformation contrasting left lateral deformation recorded on older and wider mylonitic zones suggests that the lofz may be a long lived shear zone that accommodated continental scale deformation arising from the farallon nazca south america plate convergence a block rotation pattern as indicated by paleomagnetic data is consistent with the geometry and cenozoic kinematics of the lofz

active faulting and heterogeneous deformation across a megathrust segment boundary from gps data south central chile 36 39 s 1 this study focuses on the present day deformation mechanisms of the south central chile margin at the transition zone between two megathrust earthquake segments defined from historical data the valdivia and concepcion sectors new gps data and finite element models with complex geometries constrained by geophysical data are presented to gain insight into forearc kinematics and to address the role of upper plate faults on contemporary deformation gps vectors are heterogeneously distributed in two domains that follow these two earthquake segments we find that models which simulate only interseismic locking on the plate interface fail to reproduce surface deformation in the entire study area in the concepcion domain models that include a crustal scale fault in the upper plate better reproduce the gps observations in the valdivia domain gps data show regional scale vertical axis rotations which could reflect postseismic deformation processes at the edge of the mw 9 5 earthquake that ruptured in 1960 and or activity of another crustal fault related to motion of a forearc sliver our study suggests that upper plate faults in addition to earthquake cycle transients may exert an important control on the surface velocity of subduction zone forearcs

distribution of the pacific north america motion in the queen charlotte islands s alaska plate boundary zone 1 we present gps data that constrain the distribution of the relative pacific north america motion across the queen charlotte islands alaska panhandle margin nw north america velocities from a network of 22 campaign and permanent sites indicate that the pacific north america transpressive motion is mostly accommodated along the locked queen charlotte fairweather fault a significant portion 6 7 mm yr of the relative plate motion is taken up by distributed dextral shear across a 200 km wide region of the margin two models have been proposed to describe how the pacific north america convergence is accommodated off the queen charlotte islands internal shortening vs underthrusting of the pacific plate although the gps data cannot discriminate between the models they provide strong constraints on the convergence distribution the significant non transient motion of gps sites along the central british columbia southern alaska margin has implications for seismic hazard and tectonic evolution models of the canadian cordillera

geomorphic limits to climate induced increases in topographic relief recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross disciplinary investigation and debate on the question of whether tectonics have driven long term climate change or vice versa it has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision because isostasy acts to compensate mean elevations the debate hinges on the question of whether climate change can significantly increase topographic relief or more precisely increase the volume of missing mass between summits and ridges here we show that in tectonically active mountain ranges geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change thus although climate change may cause significant increases in denudation rates potentially establishing an important feedback between surficial and crustal processes neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift

late quaternary depositional systems and sea level change santa monica and san pedro basins california continental borderland a suite of seismic reflection data that provides different degrees of resolution and penetration was used to map the depositional systems that have developed in santa monica and san pedro basins during the late quaternary submarine fan growth particularly at the mouths of hueneme and redondo canyons has been the dominant mode of basin filling mass movement processes ranging from creep to large scale catastrophic slumping have been important locally in general large scale fan growth fits normark s model in which the suprafan is the primary locus of coarse sediment deposition smaller scale morphologic and depositional patterns on the hueneme and redondo fans e g distributary channels and coarse sediment concentrations basinward of the inner suprafan suggest that a significant amount of coarse sediment presently bypasses the suprafans however long distance coarse sediment transport was particularly pronounced during late wisconsinan lowstand of sea level and resulted in progradation of lower mid fan and lower fan deposits as sea level rose rapidly between 18 000 y b p and 10 000 y b p shelves trapped much of the sediment supply consequently rates of basin sedimentation decreased markedly and the area of fan developmen was restricted as the rate of shelf sedimentation decreased concomitantly with a decrease in the rate of sea level rise 10 000 y b p sediment transport to the basins was renewed and modern depositional patterns were established fan development also has been influenced by large scale mass movement extensive mid fan overbank flow and the consequent development of sediment waves deflection of turbidity currents by bathymetric obstacles and local tectonism correlations between sand layer characteristics and high resolution echo types for fan and basin plain deposits show trends similar to some of those observed in deep sea depositional environments however because santa monica and san pedro basins are small enclosed tectonically active basins that receive sediment from relatively large drainage areas proximal to distal changes in sand layer thickness abundance and frequency are less pronounced than those observed in deep sea environments

primary siliceous rhizoliths from loop road hot springs north island new zealand abstract siliceous rhizoliths and rhizocretions which developed in diatomaceous sediments at loop road hot springs on the north island of new zealand started to form while the plants were alive or shortly after their demise the porous and solid laminae in the rhizoliths and rhizocretions incorporate well preserved filamentous microbes spores and diatoms the silica needed for the formation of these laminae came from the hydrothermal fluids that permeate through the sediments silica precipitation was triggered by degradation of the plant tissue which caused a local increase in co2 and a decrease in ph during the initial phase of rhizolith development the plant tissues acted as a template for silica precipitation the silica precipitated during these early phases provided the sites for later more extensive silica precipitation

a morphotectonic analysis of central patagonian cordillera negative inversion of the andean belt over a buried spreading center 1 we use digital elevation models and satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the central cordillera of patagonia between 46 s and 48 s in a region overlying subducted segments of the chile spreading ridge this region shows a number of peculiar features not expected in a normal subduction related belt located away from a buried spreading center first it is characterized by overall short wavelength topography and highly contrasted reliefs typical of deeply incised young mountain belts but surprisingly also by an average very low elevation of 900 m second the cordillera lacks central high reliefs instead there is a central depressed domain with average low slope gradients including the deepest point of the study area 300 m below sea level in the bottom of lake general carrera buenos aires this depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips to the east the central patagonian cordillera is now separated from the flat plains of the pampean domain representing its former piedmont and several elevated basaltic plateaus mesetas by 160 n 180 n trending morphotectonic corridors and the western border scarps of the mesetas these scarps correspond to neotectonic features controlled by recent vertical faults mostly active after 3 ma remnants of a former peneplain the avellanos surface are widespread within the entire cordilleran region from geological evidence this peneplain formed between 15 and 4 ma and was connected originally with the mesetas domain and now is dissected by these corridors it is shown that a negative inversion related to extensional and strike slip faulting occurred in the frontal region of the cordillera at 3 ma synchronously with the emplacement of magmatic products timing of the extensional deformation in central patagonia is concomitant with the migration at depth of different segments of the south chile ridge which entered the trench at 6 and 3 ma respectively and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions finally migration of segments of the chile spreading ridge since 6 ma triggered regional uplift of the cordillera followed by limited extension in relation with increased thermal weakening of the crust

first seismic record for intra arc strike slip tectonics along the liquine ofqui fault zone at the obliquely convergent plate margin of the southern andes a temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike slip liquine ofqui fault zone lofz in southern chile seventy five shallow crustal events with magnitudes up to m tief w 3 8 and depths shallower than 25 km were observed in an 11 month period mainly occurring in different clusters those clusters are spatially related to the lofz to the volcanoes chaiten michinmahuida and corcovado and to active faulting on secondary faults further activity along the lofz is indicated by individual events located in direct vicinity of the surface expression of the lofz focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike slip mechanisms indicating a ne sw direction of the p axis for the lofz at this latitude the seismic activity reveals the present day activity of the fault zone the recent m tief w 6 2 event near puerto aysen southern chile at 45 4 s on april 21 2007 shows that the lofz is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region

estimates of interseismic deformation in northeast india from gps measurements abstract estimates of interseismic deformation in northeastern india based on gps measurements at eight permanent stations 2003 2006 and six campaign sites 1997 2006 are presented here the euler pole of rotation of indian tectonic plate in itrf2000 determined from the present data set is located at 51 7 0 5 n 15 1 1 5 e with angular velocity of 0 469 0 01 myr 1 the results show that there is a statistically insignificant present day active deformation within the shillong plateau and in the foreland spur north of the plateau in the brahmaputra valley convergence rate of the northeastern gps sites with respect to the igs station lhasa along baselines that are normal to the himalayan arc in this region is 16 0 5 mm yr this represents the arc normal indo eurasian convergence rate across the northeastern boundary similar to arc normal convergence rates determined in central nepal along the himalayan arc however unlike central nepal in the arunachal himalaya the 16 mm yr shortening is distributed between the lesser as well as higher and tethyan himalayas baselines between sites on the indo burmese fold and thrust belt ibftb and shillong plateau indicate variations in the shortening rate from 1 5 mm yr on the tripura mizoram salient trs south of the plateau to 6 mm yr in the imphal recess ir to the east and 8 mm yr in naga salient ns to the northeast this suggests that the deformation in the ibftb is segmented into n s blocks along e w transverse zones exhibiting dextral slip between ns ir and sinistral slip between ir and trs baselines between the ibftb sites also show 10 0 6 mm yr convergence pointing to the existence of an active transverse zone between aizawl and imphal

a geothermally influenced wetland containing unconsolidated geochemical sediments hot spring waters flowing from porkchop geyser norris geyser basin yellowstone national park usa enter a shallow wetland basin and precipitate opal a silica particulate particulate formation by chemical rather than biochemical and colloidal mechanisms is suggested by floc and shard like particle morphologies comprising opal a silica nanospheres and microspheres of colloidal dimensions and precipitation from waters with opaque milky blue colouration indicative of aqueous silica sol conditions sediment accumulates in the wetland at a rate of ca 20 25 mm year is unconsolidated and massive to diffusely bedded to laminated post depositional features include soft sediment deformation and scouring and in drying conditions relatively deep desiccation establishment of geochemically dominated wetland sedimentation is favoured where alkali chloride hot spring fluids of circum neutral to basic ph and high silica concentration discharge to and cool to ca 35 c within topographic depressions that receive only small volumes of non hot spring water local wetland vegetation which is composed of hydrophytes halophytes and alkali tolerant species more typical of coastal wetlands colonizes the soft wetland substrate and may be relatively quickly buried by rapid sediment accumulation prior to the evolution of the diatom silica sink geothermal wetlands containing geochemically precipitated silica sediments may have been much more common and widespread rhizoliths chert nodules with organic cores scour fabrics soft sediment deformation desiccation cracks and massive to diffuse bedding preserved in palaeozoic geothermal environments may all be evidence of ancient unconsolidated geochemical sediments and geothermal wetland conditions

erosion and exhumation in accretionary orogens experimental and geological approaches 1 this study examines the role of erosion on exhumation and deformation history of accretionary orogens through experimental and geological methods fault propagation kinematics and exhumation rates in an eroded thrust wedge are analyzed through scaled sandbox simulations by testing different parameters friction rheologic layering and sediment input output ratio a model coulomb wedge is submitted to erosion under flux steady state conditions that is the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during shortening results show that the way of exhumation depends on the internal dynamics of thrust wedges and conversely on how this dynamics is modified by erosion in the eroded thrust wedges the diversity of exhumation patterns is controlled by the mode of fault propagation depending on the basal friction high or low the vertical component of exhumation is generally higher for the wedges with high basal friction than for low friction wedges the uplift of material occurs along a cluster of subvertical thrusts in the middle part of the eroded thrust wedge with low basal friction the material is exhumed along a series of inclined 20 50 thrusts in the rear of the high friction wedge the zone of maximum exhumation is generally localized in the middle part of the thrust wedge and migrates toward the backstop with continued shortening the vertical exhumation rate increases with time and the material accreted later is rapidly transferred to the main exhumation zone compared to the material accreted during the early stages the exhumation occurs at rates 2 times faster in the wedges with half the thickness than in full thickness wedges the total quantity of eroded material at the end of convergence constitutes 36 50 of initial model area for the thrust wedges with low basal friction and 20 40 for the high basal friction wedges the extent of basal underplating increases with total shortening the area of basal underplated material constitutes up to 30 and 40 of the eroded thrust wedge area for the models with low and high basal friction respectively presence of decollements in the accreted series allows underplating of thrust units developing an anticlinal stack whose growth and location is favored by erosion our results are compared to different present day active and ancient convergent orogens himalaya taiwan southern alps of new zealand and cascadia orogen allowing the different specific exhumation processes to be characterized

subaqueous silicification of the contents of small ponds in an early devonian hot spring complex rhynie scotland the early devonian rhynie and windyfield cherts of northeast scotland originated as siliceous sinters deposited by hot springs silicification affected both subaerial and subaqueous environments preserving a diverse terrestrial and freshwater biota cherts originally deposited in small shallow pools can be recognised on the basis of both texture and fossil content textures comprise clotted and microcoprolitic textures bacterial coatings on plant axes that can be stromatolitic and a variety of bacterial and fungal meshworks the crustacean lepidocaris the euthycarcinoid heterocrania the charophyte alga palaeonitella and chytrid fungi are typical elements of the aquatic biota observations of modern hot springs in yellowstone national park wyoming u s a demonstrate that shallow ponds and streams on low angle outwash areas and dormant vent orifices provide suitable environmental analogues textures comparable to those described from rhynie are recorded from yellowstone sinters but examples of the

a devonian auriferous hot spring system rhynie scotland the early devonian rhynie hot spring system is the oldest known and is of the low sulphidation type it extends for at least 1 5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments plant bearing sinters and andesitic lavas the age of sedimentation and hydrothermal activity has been determined by palynological pragian and radio metric 396 12 ma techniques the outlier is a half graben with a complex stepped western margin the devonian rocks show intense hydrothermal alteration along the fault zone the main alteration minerals are quartz k feldspar calcite hematite and illitic and chloritic clays multiple chert veining and brecciation are widely developed and geyserite and vent material are also present pyrite occurs in veins and all alteration facies sinters and altered rocks contain high concentrations of au as sb hg w and mo gold occurs in arsenian pyrite and as sub micron particles in oxidized rocks the fluid s responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water however incursions of high temperature 300 440 c magmatic fluids occurred with d 65 and 18 o around 8 5 34 s pyrite and initial 87 sr 86 sr ratios vein calcite lie mainly within the ranges 3 4 to 8 5 and 0 71138 to 0 71402 respectively these data indicate that late proterozoic dalradian metasediments are a likely source for s and sr but other sources are possible 13 c values for caliche and vein calcite imply derivation of carbon from non organic sources the rhynie cherts were deposited from a low salinity fluid of probable meteoric origin 18 o chert 13 1 to 16 5 which had interacted with the basement rocks and sediments high xe ar br cl and i cl ratios plant bearing chert yielded an 40 ar 39 ar ratio 292 1 0 6 significantly less than that of modem air and may be the first valid determination of a sample of ancient atmosphere

crustal motion in the southern andes 26 36 s do the andes behave like a microplate 1 a new global positioning system gps derived velocity field for the andes mountains 26 36 s allows analysis of instantaneous partitioning between elastic and anelastic deformation at the orogen s opposing sides adding an andes microplate to the traditional description of nazca south america plate convergence provides the kinematic framework for nearly complete explanation of the observed velocity field the results suggest the oceanic nazca boundary is fully locked while the continental backarc boundary creeps continuously at 4 5 mm yr the excellent fit of model to data 1 7 mm yr rms velocity misfit and the relative aseismicity of the upper crust in the interior andean region in comparison with its boundaries supports the notion that the mountains are not currently accruing significant permanent strains additionally the model implies permanent deformation is not accumulating throughout the backarc contractional wedge but rather that the deformation is accommodated only within a narrow deformational zone in the backarc

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

kinematic constraints on intra arc shear and strain partitioning in the southern andes between 38 s and 42 s latitude 1 on the basis of structural field work fault kinematic analysis and the analysis of digital imagery we describe the northern part of the southern andean intra arc liquine ofqui fault zone lofz as an sc like fault zone system accommodating part of the nazca south american plate convergence obliquity kinematic modeling suggests that the lofz accommodated 124 24 21 km of dextral displacement between 40 s and 42 s and 67 13 11 km between 38 s and 40 s since the pliocene associated vertical axis rotations are 31 4 clockwise and 9 1 counterclockwise along synthetic and antithetic faults respectively mean pliocene to recent shear rates along the lofz decrease northward from 32 6 mm yr to 13 3 mm yr compatible with partitioning of half of the convergence obliquity into the intra arc zone north of 40 s and complete partitioning to the south the displacement gradient along the intra arc zone results in margin parallel shortening of the fore arc

gps velocity field for the tien shan and surrounding regions 1 measurements at 400 campaign style gps points and another 14 continuously recording stations in central asia define variations in their velocities both along and across the kyrgyz and neighboring parts of tien shan they show that at the longitude of kyrgyzstan the tarim basin converges with eurasia at 20 2 mm yr nearly two thirds of the total convergence rate between india and eurasia at this longitude this high rate suggests that the tien shan has grown into a major mountain range only late in the evolution of the india eurasia collision most of the convergence between tarim and eurasia within the upper crust of the tien shan presumably occurs by slip on faults on the edges of and within the belt but 1 3 mm yr of convergence is absorbed farther north at the dzungarian alatau and at a lower rate with the kazakh platform to the west the tarim basin is thrust beneath the tien shan at 4 7 mm yr with respect to eurasia the ferghana valley rotates counterclockwise at 0 7 myr 1 about an axis at the southwest end of the valley thus gps data place a bound of 4 mm yr on the rate of crustal shortening across the chatkal and neighboring ranges on the northwest margin of the ferghana valley and they limit the present day slip rate on the right lateral talas ferghana fault to less than 2 mm yr gps measurements corroborate geologic evidence indicating that the northern margin of the pamir overthrusts the alay valley and require a rate of at least 10 and possibly 15 mm yr

active fragmentation of adria the north african promontory central mediterranean orogen global positioning system gps velocities indicate that adria no longer behaves as a rigid tectonic indenter into southern europe and is divided into northwestern and southeastern velocity domains differential motions are recognized in a velocity field determined from the peri tyrrhenian geodetic array ptga and international gps service igs sites in the circum tyrrhenian region of the central mediterranean and published gps velocities from the eastern adriatic coast in a fixed eurasian reference frame ptga and igs site velocities in sicily and southern italy are as much as 10 mm yr in a northward direction similar to gps velocities along the eastern coast of the adriatic sea in contrast velocities in northern italy are small or statistically insignificant and similar to velocities in sardinia and corsica outboard of western adria the tectonic boundary dividing adria is seismically active and passes around the southern and eastern margins of the tyrrhenian basin crosses central italy extends into the adriatic sea and follows the western margin of the dinaride tectonic belt north to the gulf of venice the eastern margin of adria is approximately located and follows the axis of the central dinaric alps southeast to the hellenic arc southeastern adria has a velocity related to northward motion of africa whereas northwestern adria has negligible differential motion in the eurasian frame and now is part of the alpine collage of southern europe

fluvial response to horizontal shortening and glaciations a study in the southern alps of new zealand 1 it has been postulated that a steady state between erosional and tectonic processes may develop in continental collision however it is not clear whether steady state conditions can be reached for all components of the landscape here we show using landscape evolution models and field evidence that a true geomorphic steady state may never be reached in the southern alps of new zealand the strong asymmetries in tectonic uplift and tectonic advection and the onset of glaciations constantly interact to prevent the landscape from reaching a topographic steady state evidence suggests that the first order geomorphology on the western side of the southern alps is controlled by orographic precipitation combined with extreme rates of tectonic uplift whereas the development of deep glacial valleys on the eastern side is initiated by differential uplift along large faults we also develop a first order equation governing the dynamics of the main divide to show that both tectonic advection and fluvial erosion efficiency control the position and the height of the main drainage divide using a two dimensional landscape evolution model we demonstrate that the transition from glacial to fluvial conditions at the end of the last glaciation led to substantial modifications of the landscape while the main trunk channels get slowly uplifted ridges are leveled down causing the relief to decrease hillslopes appear to be affected by fluvial processes which seem to be driven by incision of river tributaries this reduction of relief will probably never reach a steady state since warmer interglacial periods are substantially shorter than glacial periods

strain partitioning and fault slip rates in the northeastern caribbean from gps measurements 1 gps data collected in the dominican republic from 1994 to 2001 show that oblique convergence between the north american and caribbean plates is partitioned between 5 2 2 mm yr 1 of dip slip reverse motion on the north hispaniola thrust approximately n s and 12 8 2 5 mm yr 1 and 9 0 9 0 mm yr 1 of approximately e w left lateral strike slip motion on the septentrional and enriquillo faults 95 confidence the agreement between gps and paleoseimological slip rates on the septentrional fault together with the 770 960 years since the last major earthquake and the 800 1200 years maximum repeat time of major earthquakes may indicate that the septentrional fault is currently in the late phase of its rupture cycle

glacial hydrology and erosion patterns a mechanism for carving glacial valleys article i nfo article history most glacial erosion models assume that erosion rates are proportional to ice sliding velocity while recent studies have shown that water plays a major role in modulating sliding velocities the impact it might have on erosion rates is still unclear here we incorporate subglacial hydrology into a glacial erosion model that is based on a sliding rule our results explicitly highlight that adding subglacial hydrology has profound impacts on the temporal and spatial patterns of glacial erosion first it suggests that erosion might mainly occur during melting seasons when subglacial water pressure is large and effective pressure is low i e before the channels fully develop and reduce the water pressure at the ice bedrock interface which promotes sliding and erosion second the distribution of erosion is bimodal with a peak at the equilibrium line altitude and as we demonstrate enhanced erosion at much lower altitudes within the ablation area where water due to melting abounds this has important implications because it explains why glacial erosion can simultaneously

incipient axial collapse of the main cordillera and strain partitioning gradient between the central and patagonian andes lago laja chile 1 lago laja is a late quaternary volcanic dammed lake located near the drainage divide of the south central andes field observations lake reflection seismic profiles bathymetry and remote sensing data reveal an active fault system that runs parallel to the volcanic arc along the axis of the main cordillera the lago laja fault system llfs normal faults of this extensional system cut late pleistocene volcanics 2 7 mm yr since 7 1 ka the llfs accounts for 0 7 of arc normal extension at an average minimum rate of 1 2 mm yr and strain rate of 10 14 s 1 seismites and surface ruptures evidence m 6 paleoearthquakes the main cordillera at 37 s is a large scale pop up structure uplifted by thrusting along its foothills in this light we interpret extension in the axial and highest part of the andes as incipient synorogenic gravitational collapse in response to uplift and crustal thickening thermal weakening due to elevated heat flow and postglacial lithospheric rebound and unbending have probably contributed to the arc limited collapse and holocene acceleration of deformation rates the lack of significant strike slip offsets along the llfs as well as along both foothills thrust systems at 37 s contrasts with the intra arc dextral fault zone south of 38 s regional structural data indicates that north of 38 s diffusely distributed strain reflects low partitioning of oblique subduction while to the south deformation is localized in a discrete strike slip fault zone along the volcanic arc reflecting a higher degree of partitioning we relate this strain partitioning gradient to favorable fault orientations in the fore arc north of the arauco peninsula a major seismotectonic boundary

active tectonics of the adriatic region from gps and earthquake slip vectors 1 to investigate the kinematics of the adriatic region we integrate continuous and episodic gps measurements with mw 4 5 earthquake slip vectors selected from the regional centroid moment tensor catalogue coherent motion of gps sites in the po valley in apulia and in the hyblean plateau allows us to estimate geodetically constrained angular velocities for these regions the predictions of the gps inferred angular velocities are compared with the earthquake slip vectors showing that the seismically expressed deformation at the microplate boundaries is consistent with the observed geodetic motion the remarkable consistency between geodetic seismological and geological evidence of active tectonics suggests that active deformation in the central adriatic is controlled by the relative motion between the adria and apulia microplates the microplates angular rotation rates are then compared with the rotation rates calculated with a simple block model supporting the hypotheses 1 that apulia forms a single microplate with the ionian sea and possibly with the hyblean region and 2 that adria and apulia rotate in such a way as to accommodate the eurasia nubia relative motion we suggest that the present day microplate configuration follows a recent fragmentation of the adriatic promontory that during the neogene rigidly transferred the africa motion to the orogenic belts that now surround the adriatic region

the adriatic region an independent microplate within the africa eurasia collision zone 1 we use gps measurements and block modeling to investigate the present day deformation of the adriatic region whose kinematics within the nubia eurasia plate boundary zone is not well constrained and remains controversial block modeling allows us to compute rigid plate angular velocities while accounting for elastic strain accumulation along block bounding faults results suggest that the adriatic is a microplate adria and that the southern boundary with the nubia plate and the aegean domain may be located along the apulia escarpment and the kefallinia fault geodetic data alone cannot discriminate between a single block ap or a two blocks gdap description of adria but the gdap model predicts boundary slip rates that are in better agreement with observations from previous studies

current strain regime in the western alps from continuous global positioning system measurements 1996 2001 four to six years of continuous measurements at 10 permanent global positioning system sites in the western alps show horizontal residual velocities of 2 mm yr with respect to stable europe uncertainties range from 0 3 to 1 4 mm yr these velocities and the associated strain rate field indicate that the central part of the range is currently dominated by east west extension whereas the southern part shows north south to northwest southeast compression the geodetic and seismotectonic data are consistent with a model where strain is essentially controlled by the counterclockwise rotation of the adriatic microplate with respect to eurasia this rotation together with the arcuate shape of the contact between the adriatic microplate and the alps induces dextral shear kinematic boundary conditions across the western alps with an additional divergence component in their central part and in switzerland and a convergence component in their southern part

gps measurements of present day convergence across the nepal himalaya the high elevations of the himalaya and tibet result from the continuing collision between india and asia which started more than 60 million years ago1 4 from geological and seismic studies of the slip rate of faults in asia5 it is believed that approximately one third of the present day convergence rate between india and asia 58 4mmyr 1 is responsible for the shortening uplift and moderate seismicity of the himalaya great earthquakes also occur infrequently in this region releasing in minutes the elastic strain accumulated near the boundary zone over several centuries and accounting for most of the advance of the himalaya over the plains of india the recurrence time for these great earthquakes is determined by the rate of slip of india beneath tibet which has hitherto been estimated indirectly from global plate motions6 from the slip rates of faults in asia7 8 from seismic productivity9 and from the advance of sediments on the northern ganges plain10 here we report geodetic measurements using the global positioning system gps of the rate of contraction across the himalaya which we find to be 17 52 2 mm yr 1 from the form of the deformation field we estimate the rate of slip of india beneath tibet to be 20 5 2 mmyr 1 strain sufficient to drive one or more great himalayan earthquakes with slip similar to that accompanying the magnitude 8 1 bihar nepal 1934 earthquake may currently be available in western nepal

crustal motion in the zone of the 1960 chile earthquake detangling earthquake cycle deformation and forearc sliver translation temporary deformation in great earthquake cycles and permanent shear deformation associated with oblique plate convergence both provide critical clues for understanding geodynamics and earthquake hazard at subduction zones in the region affected by the mw 9 5 great chile earthquake of 1960 we have obtained gps observations that provide information on both types of deformation our velocity solutions for the first time span the entire latitudinal range of the 1960 earthquake the new observations revealed a pattern of opposing roughly arc normal motion of coastal and inland sites consistent with what was reported earlier for the northern part of this region this finding supports the model of prolonged postseismic deformation as a result of viscoelastic stress relaxation in the mantle the new observations also provide the first geodetic evidence for the dextral motion of an intravolcanic arc fault system and the consequent northward translation of a forearc sliver the sliver motion can be modeled using a rate of 6 5 mm a accommodating about 75 of the margin parallel component of nazca south america relative plate motion with the rate diminishing to the north furthermore the new gps observations show a southward decrease in margin normal velocities of the coastal area we prefer explaining the southward decrease in terms of changes in the width or frictional properties of the megathrust seismogenic zone because of the much younger age of the subducting plate and warmer thermal regime in the south the currently locked portion of the plate interface may be narrower using a three dimensional viscoelastic finite element model of postseismic and interseismic deformation following the 1960 earthquake we demonstrate that this explanation although not unique is consistent with the gps observations to the first order

climatic control of denudation in the deglaciated landscape of the washington cascades erosion in the washington cascades mountains was dominated by glacier activity until the end of the last glacial maximum when fluvial incision and mass wasting became the dominant drivers an analysis of millennial scale rates of denudation in the range suggest erosion is sensitive both to spatial variations in precipitation and inherited topography

epithermal sinters of paleozoic age in north queensland australia silica sinters deposited from hot springs have been found associated with volcanic rocks of late devonian to late carboniferous age in north queensland australia these deposits have many features in common with modern hot spring sinters such as those of the taupo volcanic zone in new zealand they occur associated with subaerial volcanic rocks and are locally found with air fall tuffs and fluvial and lake deposits in which hydrothermal eruption breccias are common the associated rocks are extensively hydrothermally altered and are cut by silica veins showing characteristic epithermal vein textures the silica veins contain low salinity fluid inclusions trapped at epithermal temperatures and there is evidence of boiling textures preserved in the sinters are identical to those found in modern sinters they include columnar structures similar to bacterial stromatolites described from yellowstone national park as well as striated surfaces apparently resulting from silica deposition on filamentous algae plant fossils including oxroadia gracilis are abundant criteria applied to identify these ancient sinters may be applied to other possible sinter deposits and may provide evidence indicating the level of exposure of the former geothermal system which may be an important guide in exploration for epithermal precious metal deposits

gps constraints on continental deformation in the africa arabia eurasia continental collision zone and implications for the dynamics of plate interactions 1 the gps derived velocity field 1988 2005 for the zone of interaction of the arabian african nubian somalian and eurasian plates indicates counterclockwise rotation of a broad area of the earth s surface including the arabian plate adjacent parts of the zagros and central iran turkey and the aegean peloponnesus relative to eurasia at rates in the range of 20 30 mm yr this relatively rapid motion occurs within the framework of the slow moving 5 mm yr relative motions eurasian nubian and somalian plates the circulatory pattern of motion increases in rate toward the hellenic trench system we develop an elastic block model to constrain present day plate motions relative euler vectors regional deformation within the interplate zone and slip rates for major faults substantial areas of continental lithosphere within the region of plate interaction show coherent motion with internal deformations below 1 2 mm yr including central and eastern anatolia turkey the southwestern aegean peloponnesus the lesser caucasus and central iran geodetic slip rates for major block bounding structures are mostly comparable to geologic rates estimated for the most recent geological period 3 5 myr we find that the convergence of arabia with eurasia is accommodated in large part by lateral transport within the interior part of the collision zone and lithospheric shortening along the caucasus and zagros mountain belts around the periphery of the collision zone in addition we find that the principal boundary between the westerly moving anatolian plate and arabia east anatolian fault is presently characterized by pure left lateral strike slip with no fault normal convergence this implies that extrusion is not presently inducing westward motion of anatolia on the basis of the observed kinematics we hypothesize that deformation in the africa arabia eurasia collision zone is driven in large part by rollback of the subducting african lithosphere beneath the hellenic and cyprus trenches aided by slab pull on the southeastern side of the subducting arabian plate along the makran subduction zone we further suggest that the separation of arabia from africa is a response to plate motions induced by active subduction

palaeoecology and plant succession in a borehole through the rhynie cherts lower old red sandstone scotland a cored borehole through the early devonian rhynie cherts at rhynie aberdeenshire ne scotland has revealed 53 chert beds in 35 41 m of core the cherts originated as sinters deposited by hot spring activity chert comprises 4 20 m of the cored succession with the thickest bed representing a single silicification event being 0 31 m thick and the thickest composite chert comprising six beds 0 76 m thick average chert bed thickness is 80 mm forty five plant bearing chert beds are interbedded with sandstones mudstones and shales the sediments were deposited on an alluvial plain with local lakes the area being periodically affected by hot spring activity plants initially colonized both subaerial sand and sinter surfaces rhynia gwynne vaughanii and horneophyton lignieri commonly form the basal parts of the profiles with subsequent colonization by other genera rhynia is commonly found in life position above originally sandy substrates and horneophyton above sinter surfaces the composition of the rhynie vegetation is compared with coeval assemblages and on the basis of current knowledge it is concluded that there is no unequivocal evidence that the plants were adapted to life in the stressed environments in the immediate vicinity of hot springs

constraints on inner forearc deformation from balanced cross sections fila costena thrust belt costa rica 1 the fila costena thrust belt in the forearc of costa rica is accommodating a significant portion of the convergence of the cocos plate and panama microplate geologic mapping of the thrust belt depicts a duplex with three horses that incorporate eocene limestones and oligocene to early miocene clastics inboard of the subducting cocos ridge axis by constructing a cross section at this location along a ne sw trending transect perpendicular to the thrust belt we constrain a shortening rate of approximately 40 mm a and propose that as much as 50 of the total plate convergence rate is taken up in the inner forearc the eocene limestones at the base of the thrust sheets pinch out in both directions away from the onland projection of the cocos ridge axis owing to decrease in slip on faults and a lateral ramp in the basal decollement the thrust belt terminates near the panama border at the onland projection of the subducting panama fracture zone these observations suggest that shortening is propagating to the east with the migration of the panama triple junction and the onset of shallow subduction of the thickened edge of the cocos plate the absence of similar features in the nicaraguan forearc where the subducting crust is older subducts more steeply and lacks incoming ridges and seamounts indicates that deformation of the forearc basin in costa rica reflects greater coupling between the converging plates inboard of the cocos ridge

alpine glacial topography and the rate of rock column uplift a global perspective abstract the present study investigates the influence of alpine glacial erosion on the morphology and relief distribution of mountain regions associated with varying rock column uplift rates we take a global approach and analyse the surface area distribution of all mountain regions affected by glacial erosion between latitudes 60 s and 60 n most surface area in glaciated mountain ranges is concentrated below the modern snowline as predicted by the glacial buzzsaw hypothesis above the modern snowline the analysed mountain regions all exhibit an exponential decay of surface area with height on the basis of this decay the analysed mountain regions fall within three distinct groups primarily reflecting variations in average values of rock column uplift rates mountain ranges affected by rapid rock column uplift display high above snowline relief and large decay lengths whereas inactive orogens have little relief above the modern snowline we find a surprisingly simple relation between hypsometry above the local snowline and regional tectonic activity