uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

evidence for relative motions between the indian and australian plates during the last 20 m y from plate tectonic reconstructions implications for the deformation of the indo australian plate we use plate tectonic reconstructions to establish whether motions between india and australia occurred since chron 18 43 ma we test the africa antarctica australia india plate circuit closure at chrons 5 10 ma 6 21 ma and 13 36 ma using a compilation of magnetic anomalies and fracture zone traces from the southeast southwest central indian and the carlsberg ridges additional reconstructions at chrons 23 55 ma and 26 61 ma are used to estimate the overall motion between india and australia relative motions between the indian and australian plates are estimated using the plate circuit india africa australia a new statistical approach based on spherical regression analyses is used to assess the uncertainty of the best fitting finite rotations from the uncertainties in the data the uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary to the distribution and estimated errors of the data points along it our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path results for chron 5 are remarkably consistent with present day kinematics in the indian ocean except that the arabian and indian plates are found to be separate plates comparisons of the motions between the indian and african plates across the carlsberg ridge with that between the australian and african plates across the central indian ridge evidence a significant counterclockwise rotation of the australian plate relative to the indian plate about a pole located in the central indian basin the determinations are consistent for chrons 26 13 6 and 5 determination at chron 23 is different but questionable due to the small number of available data we propose two alternative solutions that both predict convergence within the wharton and central indian basins and extension in the vicinity of the chagos laccadive ridge the first solution assumes that all the deformation in the equatorial indian ocean started 7 ma ago as found by ocean drilling program leg 116 hence all the determinations at different times represent the total motion between india and australia the averaged india australia euler vector chrons 5 6 13 and 26 11 1 s 78 0 3 54 lies within the central indian basin and yields a n s contraction of 46 52 km at 85 and 80 63 km at 90 however the difference of the india australia euler vectors at chrons 5 and 6 suggests that the india australia convergence started between 10 and 21 ma following the continent continent collision of india with asia in the early miocene the second averaged solution chrons 6 13 and 26 5 2 s 74 3 e 5 93 predicts a total n s contraction of 123 73 km at 85 e and 178 91 km at 90 e both models are compatible with the deformation pattern observed in the equatorial indian ocean

mathematical modeling of graded river profiles numerical modeling of the longitudinal profiles of rivers at grade is accomplished using the basic equations of open channel flow sediment transport equations and empirical relations for downstream variation in flow discharge sediment discharge sediment caliber and channel width only in some cases are the computed stream profiles fit exactly by any one of the commonly supported mathematical function analogs to graded profile form exponential logarithmic or power function but in most cases any of these functions can provide a fit with a degree of error smaller than would be noted in treating field data profiles dominated by spatial change in fluid and sediment discharge are distinctly power functions while profiles dominated by sediment size reduction are not necessarily exponential in form other important controls on profile shape are the degree of downstream width change in response to increasing discharge and the general range of sediment size a dynamic model of a river s approach to grade

paleoclimatic evidence for cenozoic migration of alaskan terranes chronostratigraphic and paleoclimatic comparisons of microfossils from deep sea cores from samples of an exploratory drill hole and from dredged rock of the gulf of alaska with coeval microfossil assemblages on the north american continent provide constraints on the northward migration of the yakutat block the prince william terrane and the pacific plate during tertiary time the comparative paleolatitudes of microfauna and flora provide three main constraints 1 the prince william terrane was in its present position with respect to north america at high latitudes 50 5 n by middle eocene time 40 42 ma consistent with models derived from paleomagnetic data 2 the adjacent yakutat block was 30 5 south of its present position in early eocene 50 ma 20 5 south in middle eocene 40 44 ma and 15 5 south in late eocene time 37 40 ma thus requiring a northward motion of about 30 since 50 ma moreover the yakutat block was at least 10 south of the prince william terrane during eocene time these data are consistent with migration of the yakutat block with the pacific and kula plates for at least the last 50 ma 3 site 192 on the pacific plate was at about 15 5 n latitude in the late cretaceous 68 ma at 30 5 n in early eocene 50 ma at 40 5 n in middle eocene 40 44 ma at 45 5 n in late eocene 37 40 ma and north of 50 5 n in latest eocene to early oligocene time 34 37 ma these paleolatitudes based on planktonic foraminiferal assemblages indicate northward drift consistent with the north america pacific plate reconstructions from about 68 ma to 40 ma engebretson 1982 however from cretaceous to early eocene time faunal data indicate significantly lower latitudinal positions and from oligocwne to early miocene time significantly higher latitudinal positions these discrepancies can be explained by the northward expansion of tropical faunas during the globally warm early tertiary and southward expansion of cold subarctic faunas as a result of global cooling during oligocene time

experiments on downstream fining of gravel ii wide and sandy runs this is the second of two papers about experiments on downstream fining of gravel here we briefly summarize the complete series of runs described in the first paper where the influence of sediment feed rate was evaluated in a narrow channel we then present results for variation in two more conditions sand content and channel width the experiments were carried out in a long flume with an effective 40 m long test section with two different widths of 0 3 and 2 7 m in addition two sediment mixtures were used with different sand contents i e 33 and 55 with the remainder being gravel the following results were obtained the increase in channel width allowed the formation of transverse topography enhanced the development of sediment patches and increased the fining rate of gravel for id d5 d0 and finer percentiles increasing sand content rendered the long profile of the bed much more concave caused marked patch development and most notably increased the fining rate for all percentiles of gravel sized bed material

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

spatial and temporal analysis of hillslope channel coupling and implications for the longitudinal profile in a dryland basin author s michaelides katerina hollings rory singer michael bliss nichols mary h nearing mark a

aridity is expressed in river topography globally it has long been suggested that climate shapes land surface topography through interactions between rainfall runoff and erosion in drainage basins1 4 the longitudinal profile of a river elevation versus distance downstream is a key morphological attribute that reflects the history of drainage basin evolution so its form should be diagnostic of the regional expression of climate and its interaction with the land surface5 9 however both detecting climatic signatures in longitudinal profiles and deciphering the climatic mechanisms of their development have been challenging owing to the lack of relevant global data and to the variable effects of tectonics lithology land surface properties and human activities10 11 here we present a global dataset of 333 502 river longitudinal profiles and use it to explore differences in overall profile shape concavity across climate zones we show that river profiles are systematically straighter with increasing aridity through simple numerical modelling we demonstrate that these global patterns in longitudinal profile shape can be explained by hydrological controls that reflect rainfall runoff regimes in different climate zones the most important of these is the downstream rate of change in streamflow independent of the area of the drainage basin our results illustrate that river topography expresses a signature of aridity suggesting that climate is a first order control on the evolution of the drainage basin a global dataset of river longitudinal profiles shows that river profiles become straighter with increasing aridity and numerical modelling suggests that this can be explained by rainfall runoff regimes in different climate zones

how is topographic simplicity maintained in ephemeral dryland channels the lack of topographic complexity in ephemeral dryland channels despite large variations in hydraulic controls and sediment texture presents an unexplained paradox that limits understanding of their long term evolution in dryland basins spatially and temporally discontinuous channel flow transports and sorts sediment along the bed intermittently and irregularly the cumulative effect of these processes counterintuitively produces simple topography manifest in straight longitudinal profiles and symmetrical cross sections in contrast with perennial channels this paper presents numerical modeling experiments based on field measurements to investigate dryland channel topographic development through the responses of bed material flux and net sediment storage to variations in channel hydrology we show that spatially variable flow creates and subsequently destroys incipient topography along ephemeral reaches and that large flood events above a threshold overcome hydraulic and grain size controls to dampen fluctuations in longitudinal sediment flux through a smoothing of the incipient channel bar forms the results provide a physical explanation for emergent topographic simplicity in ephemeral dryland channels despite higher variability in streamflow and sedimentary characteristics compared to perennial systems

sediment transport by runoff on debris mantled dryland hillslopes 1 hillslopes supply sediment to river channels and therefore impact drainage basin functioning and evolution the relationship between hillslope attributes and sediment flux forms the basis of geomorphic transport laws used to model the long term topographic evolution of drainage basins but their specific interactions during individual storm events are not well understood runoff driven erosion of coarse particles prevalent in dryland environments presents a particular set of conditions for sediment transport that is poorly resolved in current models in order to address this gap we developed a particle based force balance model for sheetwash sediment transport on coarse debris mantled hillslopes within a rainfall runoff model we use the model to examine how the interplay between hillslope attributes gradient length and grain size distribution and runoff characteristics affects sediment transport grain size changes on the hillslope and sediment supply to the slope base the relationship between sediment flux and hillslope gradient was found to transition from linear above a threshold to sigmoidal depending on hillslope length initial grain sizes and runoff characteristics grain sizes supplied to the slope base vary in a complex manner with hillslope attributes but an overall coarsening of the hillslopes is found to occur with increasing gradient corroborating previous findings from field measurements intense short duration storms result in within hillslope sediment redistribution and equifinality in sediment supply for different hillslope characteristics which explain the lack of field evidence for any systematic relationships our model findings provide insights into hillslope responses to climatic forcing and have theoretical implications for modeling hillslope evolution in dry lands

the bed morphology of upland single thread channels in semi arid environments evidence of repeating bedforms and their wider implications for gravel bed rivers single thread gravel bed streams of moderate slope in the northern negev are characterized by three channel units bars exhibit steeper than average slopes and poorly sorted mixtures of small medium cobbles and coarse very coarse pebbles flats are associated with more gentle slopes and well sorted medium fine pebbles and granules and transitional units have intermediate slopes and grain size in general all three units are planar span the full channel width and have well defined boundaries bars and flats are more common than the transitional units and alternate downstream for distances of several hundred metres forming sequences that are reminiscent of the riffle pool structure commonly observed in humid temperate gravel bed rivers a notable contrast is the absence of significant bed relief bars lack crests and flats lack depressions the relative lack of bed relief in bar flat sequences is attributed to the high rate of sediment supply from the sparsely vegetated hillslopes which promotes the infilling of depressions and to the erosion of crests under conditions of intense transport this reduction of bed relief lowers channel roughness which in turn increases flow velocity and therefore the ability of the channel to transmit the large sediment loads it receives although our analyses pertain to a semi arid river system the results have wider implications for understanding the adjustment of channel bedform to high sediment loads in other fluvial environments copyright 2012 john wiley sons ltd

preliminary work on an early warning and rapid response program for moderate earthquakes warning of imminent ground shaking due to a large earthquake would be useful to a variety of agencies this kind of ground motion prediction is possible in southern california for events with magnitude less than 6 where path effects dominate the 28 june 1991 sierra madre earthquake is presented as a test case for this concept a single station inversion of the record from the pasadena station 20 km sw of the epicenter produces reasonable source parameters for the event with these source parameters and a library of green s functions calculated for an average southern california crustal model ground motions can be predicted throughout the region in particular since the peak displacement for the sierra madre event occurs at pasadena before ground motion begins at a station near the san andreas fault in san bernardino ground motions near the san andreas fault can be calculated before the seismic energy has propagated into the area considering this scenario in the reverse direction records from a station near an earthquake on the san andreas fault could be used to predict ground motions in the metropolitan los angeles area broadband high dynamic range seismic instruments produce high quality records for events over a wide magnitude range thus the development of a warning system can be approached in stages starting with small events with path effects determined by modeling moderate size events work can begin on developing distributed fault models to predict ground motions of great earthquakes

revised history of early tertiary plate motion in the south west pacific reanalysis of early tertiary magnetic anomalies on the pacific plate south of the campbell plateau indicates that in early tertiary time there was a previously unrecognized triple junction of the pacific the antarctic and a third plate now beneath the bellingshausen sea corresponding revised parameters for pacific antarctica motion do not require early tertiary displacements within antarctica or new zealand but suggest a change in direction of the pacific plate over the hotspots of approximately the same sense and timing as the hawaiian emperor bend

coarse bed material patch evolution in low order ephemeral channels abstract in river channel beds composed of a wide range of grain sizes the bed material is often arranged in discrete patches discernable by relative texture these bed material patches are the primary source of entrainable coarse sediment within the channel system and their composition and size have been found to influence the composition and rate of sediment transport twelve coarse gravel cobble sediment patches distributed throughout the channel network within a 4 53 ha watershed in southeastern arizona were monitored for 2 years changes in patch area and grain size were measured and painted patch grains were monitored to confirm that patch grains were mobilized during flow individual coarse bed material patches exhibited variable persistence during flows with return frequencies ranging from approximately 1 year to 4 6 years while no patch fully dispersed during the study period two new patches formed most coarse patches remained relatively stable in area and grain size distribution despite the entrainment of patch grains as lost grains were sufficiently replaced with sediment from upstream because of the grain replacement process and the effect of other sediment supply dynamics the changes in patch area and grain size distribution display a complex relationship with the magnitude of predicted grain mobilization within each patch results indicate that relative stability varies from patch to patch influenced by the balance of patch grains transported out of the patch and the deposition of new grains into the patch predictive models of coarse sediment transport and yield that assume the channel bed is a fixed source of sediment supply may not adequately capture the sediment dynamics within patchy channel beds and should be used with caution when applied to these environments due to the possibility of patch instability as documented in this study

a high resolution model for eurasia north america plate kinematics since 20 ma summary we derive the first chronologically detailed model of eurasia north america plate motion since 20 ma from ship and airplane surveys of the well expressed magnetic lineations along this slowly spreading plate boundary including previously unavailable dense russian magnetic data from the southern reykjanes ridge and northern mid atlantic ridge near the charlie gibbs fracture zone from more than 7000 crossings of 21 magnetic anomalies from anomaly 1n 0 78 ma to anomaly 6n 19 7 ma we estimate best fitting finite rotations and realistic uncertainties linear regressions of total opening distances versus their reversal ages at different locations along the plate boundary show that reversal boundaries are shifted systematically outwards from the spreading axis with respect to their idealized locations with the outward shift ranging from more than 5 km between iceland and the charlie gibbs fracture zone to 2 km elsewhere this outward displacement which is a consequence of the finite zone of seafloor accretion degrades estimates of the underlying plate motion and is thus removed for the ensuing kinematic analysis the corrected plate motion rotations reveal surprising previously unrecognized features in the relative motions of these two plates within the uncertainties motion was steady from 20 to 8 ma around a pole that was located 600 km north of the present pole with seafloor spreading rates that changed by no more than 5 per cent 1 mm yr 1 along the reykjanes ridge during this period seafloor spreading rates decreased abruptly by 20 2 per cent at 7 5 6 5 ma coinciding with rapid southward migration of the pole of rotation and a5 10 counter clockwise change in the plate slip direction eurasia north america plate motion since 6 7 ma has remained remarkably steady with an apparently stationary axis of rotation and upper limit of 2 per cent on any variations in the rate of angular rotation during this period based on the good agreement between seismotectonic constraints on present deformation in northeast asia and directions of motion that are predicted by our 6 7 ma to present pole we hypothesize that motion has remained steady to the present and attempt to test this hypothesis with published gps estimates for eurasia north america motion we find however that gps estimates that are tied to recent versions of the international geodetic reference frame and rely principally on station velocities from europe give implausible estimates of recent motion with the most recently published gps model predicting convergence along the southern gakkel ridge and in the laptev sea where seafloor spreading occurs an alternative gps estimate that is not tied to the international terrestrial reference frame and employs gps station velocities from northeastern asia is marginally consistent with our 6 7 0 ma motion estimate

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

errors in minimum plate velocity determined from paleomagnetic data in prior studies paleomagnetic data have been used to determine ancient minimum velocities of the continents the greatest minimum velocities 50 km m y exceed the present day absolute velocities of the major continents this result is important because the observation that present day oceanic plates move faster than present day continental plates can be treated as a major constraint on plate dynamics however these prior studies are limited because they ignore known errors in the paleomagnetic data in this paper we present a technique for incorporating these errors and using them to estimate 95 confidence limits on the minimum velocities this new technique was applied to pennsylvanian to cretaceous paleomagnetic data from north america and to the ancient geometry of laurasia when errors are considered minimum velocities are 10 to 50 km m y lower than when errors are ignored additional uncertainties owing to a hypothesized clockwise rotation of the colorado plateau a source of many of the data are considered and found to alter the minimum velocities only negligibly allowing for uncertainties in the pole positions uncertainties in ages and the uncertainty due to possible rotation of the colorado plateau we estimate that laurasia moved at least 48 km m y from early jurassic to early cretaceous time and at least 43 km m y from pennsylvanian through late triassic time there is only a 5 risk that the true minimum velocities are lower than these lower bounds these velocities are less than the peak velocities over short intervals found in prior analyses but exceed the average velocities previously found for triassic and jurassic time these newly estimated velocities are still much higher than the present velocities of the continents and as in the prior studies imply that the slow motion characteristic of major continents at present is not a fundamental attribute of plate motions

australian antarctic breakup and seafloor spreading balancing geological and geophysical constraints abstract the motion of diverging tectonic plates is typically constrained by geophysical data from preserved ocean crust however constraining plate motions during continental rifting and the breakup process relies on balancing evidence from a diverse range of geological and geophysical observations often subject to differing interpretations reconstructing the evolution of rifting and breakup between australia and antarctica epitomizes the challenges involved in creating detailed models of pangea breakup in this example differing degrees of emphasis on and alternative interpretations of offshore geophysical data in particular magnetic anomalies and seismic reflection profiles and onshore geological data lead to starkly contrasting views of how the continents were configured at the onset of mesozoic rifting here we critically review reconstructions of rifting and breakup in the light of all available geological and geophysical data including magnetic anomalies fracture zones conjugate crustal domains amounts of continental extension continental geology plate boundary locations break up ages and stratigraphy we identify the most viable plate tectonic reconstructions both with and without the input of the oldest more controversial magnetic anomaly interpretations and discuss implications for reconstructions of other margin pairs our analysis highlights key discrepancies between reconstructions based solely on geological piercing points and those based on a range of constraints these insights provide a powerful framework for reducing the range of viable models for australian antarctic rifting and provide key lessons for future efforts aimed at constraining pre and syn rift plate tectonic reconstructions

morpho textural implications to bedload flux and texture in the sand gravel ephemeral poveda gully abstract we report on channel morpho texture and bedload transport in a natural steep sand gravel ephemeral channel draining the small poveda gully watershed in the mining area of the alto tajo natural park spain first ever continuous bedload flux and texture monitoring in a transitional sand gravel environment was undertaken by two independent reid type slot samplers morphological changes in the feeder reach have been quantified by tls terrestrial laser scanning and sfm structure from motion technologies we identified a pattern in channel bed morphology and texture morpho texture when the channel is incised its texture is coarser otherwise sand filling occurs these changes determine bedload flux and texture sand fill brings rise to high fluxes and fine grained bedload whereas incision caused by evacuation of sand leaves a sandy gravel surface with lower bedload fluxes and coarser texture a video camera recording during events allowed identification of dramatic changes in bedload flux and texture owing to the appearance and erosion of bars supplying field evidence to explain the difficulty in the prediction of bedload flux at short time intervals

characteristics of gravel bars in ephemeral streams abstract bars are major sediment storage places within channels and much is known about their character from proglacial and temperate regions little is know however about bar characteristics from ephemeral streams this study describes the morphology and composition of simple and compound gravel bars in nahal zin israel an ephemeral desert stream a hierarchy of microscale mesoscale and macroscale structures and bed forms was identified in nahal zin that is similar to the hierarchy described for perennial streams while the hierarchy is similar between arid and perennial streams bar composition patterns are distinct compound bars and unit bars in humid areas are known to have armored surfaces composed of structured gravels whereas the bar surfaces of both unit bars and compound bars in nahal zin were found to lack surface structures and to contain a large amount of sand in ephemeral streams both accreted and dissected compound bars do not display any downstream fining in contrast downstream fining along bars in humid areas is well documented the flashy nature of floods high sediment supply high sediment transport rates and large amount of sand associated with the bars are likely reasons the surfaces were not armored in nahal zin erosion and accretion dominate the bar edges and accretion alone dominates the platform part of the bars in summary sediments in ephemeral streams of the nahal zin type are well mixed unsorted and unlikely to leave a well defined stratigraphic record these characteristics are distinct from their counterparts in proglacial and temperate regions and may be useful when investigating paleoenvironmental conditions

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

pacific plate reconstructions and uncertainties abstract the pacific plate is positioned into the hotspot reference frame first directly relative to the pacific hotspots and then indirectly relative to the african hotspots by using seafloor spreading data the reconstructions are repeated for 10 15 37 48 and 61 ma the errors associated with each link for both approaches are expressed by covariance matrices which can be combined to yield the overall error in the case of the relative plate motion circuit assuming the hotspots are fixed and that the reconstructions record the true relative positions of the plates the reconstructions should be equivalent within their uncertainties in fact however there is agreement within the uncertainties only for 10 ma with the misfit increasing with age for the earlier reconstructions these misfits can he accounted for by three alternative explanations large hotspot location uncertainties a shift in time of the pacific group of hotspots relative to the african group or an additional plate boundary in the relative plate motion circuit which we assume to be in antarctica by examining each of these possibilities in turn we conclude that the uncertainties in individual hotspot locations required to bring the reconstructions into agreement are unreasonably large and that the data available at present do not allow us to distinguish which of the remaining two possibilities may be the primary source of reconstruction errors

high resolution inversion for south atlantic plate kinematics using joint altimeter and magnetic anomaly data we present an inversion for plate kinematics that solves for finite rotation parameters using fracture zone fz and magnetic anomaly location data jointly we define misfit functions that incorporate properties unique to each data type in particular the fz misfit function does not depend upon alignment of conjugate fz traces in the same way as magnetic lineations under the finite rotations this property is useful for fz locations in which the signals on conjugate sides of the ridge may include systematic differences or where data from one side of the ridge are sparse or missing formal error bounds estimated for the pole parameters show that the magnetic and fz data are complementary in their information content error bounds computed for the joint inversion are substantially smaller than for either the fz or magnetics data used separately indicating that simultaneous use of the data in an inversion is crucial we apply this method to seasat altimeter data and magnetic anomaly picks in the south atlantic we solve for finite poles corresponding to magnetic anomalies 5 6 8 13 21 22 25 30 33 33r and 34 these define a smooth path over the past 84 m y indicating that the plate motions have not been as erratic as found previously

a practical method for analysis of river waves and for kinematic wave routing in natural channel networks the behaviour of river waves is described using a simplified dimensionless form of the momentum equation in conjunction with the continuity equation three dimensionless parameters were derived based on a quantitative linear analysis these parameters which depend on the froude number of the steady uniform flow and the geometric characteristics of the river permit quantification of the influence of inertia and pressure in the momentum equation it was found that dynamic and diffusion waves occur mainly on gentle channel slopes and the transition between them is characterized by the froude number on the other hand the kinematic wave has a wide range of applications if the channel slope is greater than 1 the kinematic wave is particularly suitable for describing the hydraulics of flow since slopes in natural channel networks are often greater than 1 an analytical solution of the linearized kinematic wave equation with lateral inflow uniformly distributed along the channel is desirable and was therefore derived the analytical solution was then implemented in a channel routing module of an existing simple rainfall runoff model the results obtained using the analytical solution compared well with those obtained from a non linear kinematic wave model copyright 2000 john wiley sons ltd

impact of coarse sediment supply from hillslopes to the channel in runoff dominated dryland fluvial systems sediment supply from hillslopes to channels is an important control on basin functioning and evolution however current theoretical frameworks do not adequately consider processes of runoff driven hillslope sediment supply which affect river channels spatially and temporally mountainous dryland basins exhibit an important manifestation of these processes because their debris mantled hillslopes produce coarse sediment and because rainfall is delivered as infrequent high intensity short duration rainstorms this paper combines field measurements and modeling to explore runoff driven coarse sediment supply from hillslopes to the channel and assesses a range of plausible storms on the longitudinal patterns of sediment load and its caliber over a dryland basin reach our results show that modeled sediment load and its grain size distribution are determined by the nonlinear interaction between rainfall characteristics and hillslope attributes resulting in longitudinal fluctuations in sediment supply the relative magnitude and location of which varies between storms results suggest that long hillslopes are most sensitive to rainfall and they exhibit large variations in supplied sediment load and grain size for different runoff characteristics short and steep hillslopes are less sensitive to rainfall variations as gradient effects dominate over the role of length in modulating runoff accumulation furthermore the signal of the median fraction d50 of modeled sediment supplied by the hillslope is preserved in the coarse fraction of the measured in channel grain sizes d90 finally we propose a simple index which provides new insights into the effectiveness of different rainstorms in terms of the impact of hillslope sediment supply on the channel

tectonomagmatic evolution of cenozoic extension in the north american cordillera the spatial and temporal distributions of cenozoic extension and magmatism in the cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt 100 km pre extension and followed the onset of intermediate and silicic magmatism by no more than a few million years extension began in early eocene time in southern british columbia northern washington idaho and montana farther s extension began at about the eocene oligocene boundary in the great basin and slightly later in the mojave sonora desert region the intervening area at the latitude of las vegas remained quiescent until mid miocene time compositional and isotopic characteristics of most pre miocene magmas are consistent with their containing major components of melted continental crust in mid miocene time two major changes occurred widening of the area of extension and the widespread appearance of basaltic magmas the area affected by extension from southwestern montana to the lake mead region widened to several hundred kilometres by this time extension in southern british columbia northern washington and northern montana had ceased probably before the end of the eocene and extension s of lake mead except in the gulf of california had waned regions affected by the broader belt of extension during late miocene pliocene and quaternary time experienced basaltic magmatism which began along a central rift zone in the northern part of the region and which had within a few million years spread to include most of the region later basaltic activity has tended to concentrate in restricted zones especially near the margins of the extended area we recognize a correlation between the amount of earlier crustal thickening and cenozoic extension and between the length of time after shortening but before extension and the degree to which a given region was intruded by late cretaceous plutons the localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal mechanical model developed in a companion paper sonder et al this raises the possibility that stresses inherent in the north american plate dominated over plate interaction forces as controls of the cenozoic tectonomagmatic evolution of the north american cordillera especially in its earlier stages

crustal reflections and the nature of regional p coda abstract f k spectra for regional p coda are presented for events recorded at the scandinavian noress finesa and arcess arrays and the new england nynex array the spectra are dominated by on azimuth energy with apparent velocities between pn or faster and lg following this analysis reflection coefficients calculated with an efficient boundary integral scheme are used to study the role irregular interfaces play in the creation of regional p coda we find that observed crustal scattering in these regions is strikingly consistent with p p and p sv scattering from the two dimensional 2d irregular moho and even more consistent with scattering from a 2d irregular near surface interface

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

linking the late cretaceous to paleogene pacific plate and the atlantic bordering continents using plate circuits and paleomagnetic data 1 late cretaceous to paleogene paleomagnetic data from the pacific plate the emperor seamounts can be compared with data from the atlantic bordering continents through the use of plate circuit reconstructions here we summarize the uncertainties in all data sets and present formal tests we report agreement between late cretaceous pacific paleomagnetic data and predictions based on the estimates of non pacific pole positions from synthetic apparent polar wander paths this congruency points to the veracity of the plate circuits and the accuracy of the paleomagnetic estimates in contrast to the agreement seen for the late cretaceous small discrepancies are observed in the comparisons of the pacific paleogene data and predictions from synthetic apparent polar wander paths such a disparity in a younger time interval is unexpected given the agreement of the late cretaceous data the possibility that minor temporally variable nondipole field components contribute to the discrepancy cannot be completely discounted however an alternative and more straightforward explanation is suggested by further comparisons of the mean non pacific paleomagnetic data and the highest quality poles that contribute to the means in particular we note that 1 the pacific paleogene data are in full agreement with coeval poles from north america meeting strict reliability criteria and 2 the non pacific paleogene poles of synthetic apparent polar wander paths are dominated by results from the north atlantic igneous province naip but taken as a whole the naip data fail a paleomagnetic reversal test hence minor discrepancies between paleocene paleomagnetic data from the pacific and atlantic hemispheres may point to limitations of the latter which incorporate a relatively large number of older lower quality data these findings call for renewed data collections utilizing comprehensive rock magnetic and paleomagnetic demagnetization procedures to improve resolution of paleocene non pacific data the uncertainties of the pacific paleolatitude data and the non pacific reference poles are larger than the differences related to the use of the two alternative plate circuits through east to west antarctica and through australia to the lord howe rise that link the atlantic and pacific hemispheres while no preference can thus be given to either plate circuit their overall consistency with paleomagnetic data suggests that they can be used to investigate long term motion of the pacific plate since late cretaceous times

constraints on indian plate motion since 20 ma from dense russian magnetic data implications for indian plate dynamics we use more than 230 000 km of russian marine magnetic and bathymetric data from the carlsberg and northern central indian ridges comprising one of the most geographically extensive dense shipboard surveys anywhere in the ocean basins to describe in detail seafloor spreading since 20 ma along the trailing edge of the indian plate india somalia plate rotations for 1 myr intervals over the past 20 myr are derived from inversions of more than 6600 crossings of 20 magnetic reversals and 1400 crossings of fracture zones that offset these two ridges statistical analysis of the numerous data indicates that outward displacement of reversal boundaries due to finite seafloor emplacement widths and correlated noise for anomaly crossings from individual spreading segments constitute two distinct sources of systematic bias in the locations of magnetic anomaly crossings contrary to the often made assumption that random gaussian distributed noise dominates the error budget seafloor spreading rates slowed gradually by 30 from 20 ma to 10 1 ma about a relatively stationary pole of rotation from 11 ma to 9 ma the rotation axis migrated several angular degrees toward the plate boundary modestly increasing the spreading gradient along the plate boundary india somalia kinematic data for times since 9 ma are consistent with remarkably steady motion with no evidence for a change in either the rotation pole or rate of angular opening within the few percent precision of our data the timing and nature of changes in india somalia motion since 20 ma closely resemble those for the capricorn somalia plate pair indicating that india and capricorn plate motions are strongly coupled we speculate that the slowdown in seafloor spreading at the trailing edges of the indo capricorn composite plate from 20 ma to 10 1 ma resulted from the increasing amount of work that was needed to build topography in the himalayan collisional zone the transition to stable india somalia and capricorn somalia seafloor spreading at 10 9 ma corresponds well with the onset at 8 ma of folding and faulting across an equatorial plate boundary separating the indian and capricorn plates suggesting that the latter may have played a fundamental role in restoring equilibrium between the torques that were driving and resisting the northward motions of the indian and capricorn plates

event detection and location performance of the finesa array in finland abstract the experimental seismic array finesa in finland is designed to monitor weak seismic events at regional and teleseismic distances the array geometry currently comprises 15 short period vertical seismometers in three concentric rings a b and c rings with a diameter of the outer ring of about 2 km in late 1989 the data acquisition system of the array was completely modernized signals are now transferred continuously via high speed telephone lines to the processing centers at the institute of seismology in helsinki and norsar in norway therefore allowing automatic real time processing of the recorded data in this paper the detection performance of the array in the current configuration has been evaluated the results are encouraging during a 2 week test period finesa detected at least one p and one s phase for 84 per cent of the events reported in the regional bulletin of the university of helsinki and 99 per cent of the events in the weekly teleseismic bulletins many additional events at both distance ranges were also found the estimated phase velocities obtained by the broadband frequency wave number analysis confidently identify the phase type teleseismic pgional pgional s however the resolution of the analysis is not sufficient to separate pg from pn and lg from sn the estimated backazimuths are reliable for phase association the standard deviation of the estimates being 7 for regional p phases 6 for regional s phases and 23 for teleseismic p phases finally preliminary results from finesa9s on line event location capability showed that the average error in the location estimates is 21 per cent of the true epicentral distance the greatest error sources are uncertainty in the estimated azimuths and occasional misidentification of secondary phases lg sn and rg the error could be reduced by constructing a regional correction term for the azimuth estimates and tuning the phase identification algorithms for finesa the characteristics of the rg phase need to be especially considered

the impact of the streamflow hydrograph on sediment supply from terrace erosion abstract sediment supply from banks and terraces has important implications for grain size distributions in alluvial rivers and by extension for aquatic habitat as well as for the delivery of floodplain stored nutrients and contaminants to the aquatic environment the interactions between streamflow hydrographs and lateral channel boundary failure control the sediment supply from banks and terraces however the relationships between variable flow and discrete sediment supply from catastrophic erosion of lateral boundaries and subsequent mass sediment flux in rivers are not well characterised by existing methods and models that focus only on one of several relevant interrelated processes in order to improve predictive capability of catastrophic sediment supply from lateral boundaries we adopt a new approach to modelling the process interactions between stream hydrology erosion of banks terraces and the corresponding discrete supply of sediment to channels we develop a modelling framework for terrace channel coupling that combines existing theories of flow through porous media bank stability and fractional sediment flux we demonstrate the utility of this modelling approach by assessing hydrologically driven erosion evolution of grain size in the channel and fine sediment flux from a study site along the yuba river in california over individual flood hydrographs and over decadal historical flow series we quantify the supply of sediment eroded from a contaminated nineteenth century fan terrace of hydraulic gold mining tailings intersecting the yuba and find that a threshold for erosion exists at a stage in the channel in excess of 8 m producing episodic sediment concentrations in excess of 300 mg l 1 the modelling produced erosion and fine sediment pulses from each of three major floods in the past several decades until the flow drops below 500 m3 s 1 and a bed armor layer forms while no sediment was generated from the terrace during smaller floods we further assess the impact on terrace erosion of various river management scenarios with distinct hydrograph shapes and find increased erosion potential when the terrace contains antecedent moisture or the flood time series is run over an extended duration sensitivity analysis demonstrated that elevated antecedent moisture within the lateral boundary and increased hydrograph rising time each reduce bank stability and thus increase volumes of failed material we also show that fluctuations in the hydrograph typically associated with hydroelectricity generation result in a more stable terrace than those of a longer duration because there is less time for hyporheic stream water to infiltrate the lateral boundary this study demonstrates that changes in hydrograph shape as a consequence of climatic forcing or anthropogenic dam releases may have considerable impacts upon sediment delivery and associated contaminants from banks and terraces to the downstream environment

oscillations in arid alluvial channel geometry arid alluvial channels on piedmonts and valley floors often exhibit an oscillating pattern of narrow deeply incised reaches and wide shallow reaches with a characteristic wavelength how do these oscillations develop and what controls their wavelengths to address these questions we developed a two dimensional numerical model that couples erosion and deposition in a channel bed with cross sectional widening and narrowing this model is inherently unstable over a range of spatial scales dependent on the channel width depth and slope in the initial phase of model evolution wider than average channel reaches become zones of distributary flow that aggrade lose stream power and further widen in a positive feedback simultaneously narrower than average reaches incise gain stream power and further narrow in the second stage of model evolution this instability is balanced by the diffusive nature of longitudinal profile evolution and solitary topographic waves propagate in the upstream direction with a characteristic wavelength and amplitude the model predicts a specific quantitative relationship between the oscillation wavelength and channel width depth and slope that is verified by a database of channel geometries in southern arizona

integrated array and three component processing using a seismic microarray abstract a microarray as defined in this paper is modeled on a sub geometry of the noress array and comprises a 3 component center seismometer surrounded by 3 closely spaced vertical component sensors deployed over a typical aperture of 0 3 km analysis of five days of continuous data has shown that such a system combines the benefits of array and 3 component processing in providing reliable automatic detection phase identification and azimuth estimation of weak seismic events at local and regional distances using vertical components only broadband f k array analysis enables correct phase identification p type or s type phase in 95 of the cases and gives s wave azimuths with a root mean square rms deviation of 13 9 degrees from the estimates of the full noress array it is particularly significant that the small array eliminates the need for introducing particle motion models which creates ambiguities in 3 component analysis of secondary phases when interfering sh and sv phases occur p phase azimuths are estimated using integrated array and 3 component f k analysis and have an rms deviation relative to noress of only 9 6 degrees compared to the full noress array the p wave detection capability is reduced especially for epicentral distances greater than 500 km the s phase detection capability is enhanced by incoherent beamforming of the horizontal channels and approaches that of noress at all distances a considerable reduction in the detector false alarm rate is achieved by imposing constraints on the estimates of apparent velocity obtained from the f k analysis before accepting a detected phase

evidence of past climatic change in the namib desert abstract a preliminary survey of some areas of the central and northern namib desert was undertaken in order to identify geomorphic and sedimentary features that could potentially provide information on past climatic change in the region wherever possible samples were collected for radiocarbon dating the visits to the skeleton coast area were especially revealing practically every river valley in this tract shows a sequence of terraces and deposits that would warrant a detailed study in all about 120 new radiocarbon dates were obtained for the region these have supplied a wealth of individual results and collectively have enabled the construction of a tentative chronological sequence of the changes in climate for the past 40 000 yr three periods of increased aridity in the hinterland but with decreasing intensity are postulated during the last glacial maximum the early holocene and between a d 1200 and a d 1600 the inter pleniglacial up to 23 000 yr b p the late glacial and the period between about 4000 yr b p and 1200 yr b p on the other hand were more humid than today also with decreasing intensity other indicators of past climate were also identified but since they lie beyond the range of radiocarbon dating they cannot as yet be placed in a chronological context of significance is the fact that convincing evidence was obtained for climatic change within the last millennium and especially the rainfall is found to have been increasing since the 16th century

redistribution and export of contaminated sediment within eastern fukushima prefecture due to typhoon flooding tropical cyclones expose river basins to heavy rainfall and flooding and cause substantial soil erosion and sediment transport there is heightened interest in the effects of typhoon floods on river basins in northeast japan as the migration of radiocaesium bearing soils contaminated by the fukushima daiichi nuclear power plant fdnpp accident will affect future radiation levels the five main catchments surrounding fdnpp are the odaka ukedo maeda kuma and tomioka basins but little quantitative modelling has been undertaken to identify the sediment redistribution patterns and controlling processes across these basins here we address this issue and report catchment scale modelling of the five basins using the getflows simulation code the three dimensional 3d models of the basins incorporated details of the geology soil type land cover and used data from meteorological records as inputs the simulation results were checked against field monitoring data for water flow rates suspended sediment concentrations and accumulated sediment erosion and deposition the results show that the majority of annual sediment migration in the basins occurs over storm periods thus making typhoons the main vectors for redistribution the ukedo and tomioka basins are the most important basins in the region in terms of overall sediment transport followed by the other three basins each with similar discharge amounts erosion is strongly correlated with the underlying geology and the surface topography in the study area a low permeability pliocene dainenji formation in the coastal area causes high surface water flow rates and soil erosion conversely erosion is lower in an area with high permeability granite basement rocks between the hatagawa and futaba faults in the centre of the study area land cover is also a factor controlling differences in erosion and transport rates between forested areas in the west of the study area and predominantly agricultural areas towards the east the largest sediment depositions occur in the ogaki and takigawa dams at the confluence of the takase and ukedo rivers and at the ukedo river mouth having clarified the sediment redistribution patterns and controlling processes these results can assist the ongoing task of monitoring radioactive caesium redistribution within fukushima prefecture and contribute to the design and implementation of measures to protect health and the environment copyright 2016 john wiley sons ltd

a generalized exner equation for sediment mass balance 1 the advance of morphodynamics research into new areas has led to a proliferation of forms of sediment mass balance equation without a general equation it is often difficult to know what these problem specific versions of sediment mass balance leave out to address this we derive a general form of the standard exner equation for sediment mass balance that includes effects of tectonic uplift and subsidence soil formation and creep compaction and chemical precipitation and dissolution the complete equation 17 allows for independent evolution of two critical interfaces that between bedrock and sediment or soil and that between sediment and flow by eliminating terms from the general equation it is straightforward to derive mass balance equations applicable to a wide range of problems such as short term bed evolution basin evolution bedrock uplift and soil formation and carbonate precipitation and transport dropping terms makes explicit what is not being considered in a given problem and can be done by inspection or by a formal scaling analysis of the terms scaling analysis leads directly to dimensionless numbers that measure the relative importance of terms in the equation for example the relative influence of spatial versus temporal changes in sediment load on bed evolution combining scaling analysis with time averaging shows how the relative importance of terms in the equation can change with timescale for example the term representing bed evolution due to temporal change in sediment load tends to zero as timescale increases

evidence for normal and inhomogeneous lowermost mantle and core mantle boundary structure under the arctic and northern canada summary seismic p waves from a total of about 200 nuclear explosions in the usa the former ussr and china observed at 10 arrays and four networks in europe canada and the usa are used to analyse the structure at the base of the mantle and the core mantle boundary cmb the simple waveforms and well controlled source parameters of nuclear explosions allow one to use the events as source arrays in addition to the usual receiver array configuration a new array technique double beamforming kruger et al 1993 integrating both concepts is applied which increases the slowness resolution considerably a total of 56 source receiver combinations i e reflection points in the lower mantle or on the cmb could be analysed in five regions anomalous arrivals pdp with slowness and arrival times between those of p and pcp are observed one of these five areas svalbard region shows short period pcp p amplitude ratios which are about three times higher than those predicted by standard earth models in the severnaya zemlya region where pdp and pcp precursors were observed previously kruger et al 1993 pcp shows azimuth deviations of up to 10 for some other regions deviations of the pcp waveform from the direct p waveform are also observed these anomalous phases and the pcp waveform distortions cannot be explained with standard 1 d earth models they are probably produced by inhomogeneities in the lowermost mantle the observed variations in the waveforms are strong indications of a laterally heterogeneous structure in two depth ranges the first is the cmb and its immediate vicinity of a few tens of kilometres the second region is the depth range between about 200 and 300 km above the cmb maps of the north pole region giving the distributions of inhomogeneities in the lower mantle and on the cmb are presented these maps show evidence of strong heterogeneity of the d boundary layer and possibly also of the cmb in the same area

investigation of azimuth residuals observed at stations of the gsett 3 alpha network the gsett 3 experiment was conducted for more than two years and the international data center idc reported more than 20 000 events for 1995 in the reviewed event bulletins reb we have used this data base to investigate the location bias of arrays and selected three component stations in the alpha network for which the recorded data is automatically processed to determine earthquake locations in near real time the alpha network which consists of 14 arrays provides estimates of event azimuths determined by f k processing we selected 19 three component stations that were operated all or most of 1995 and for which event azimuth was estimated by polarization analysis only events with epicentral distances of less than 100 were considered in order to exclude phases with steep incidence angles associated with unreliable azimuth estimates teleseismic p phases and the major regional and local phases were examined only data for which a signal to noise ratio was given were included in this analysis the signal to noise ratios were used as a weighting factor for the statistical parameters and the graphical representation of the azimuth residuals the preliminary analysis of azimuth residuals identified some arrays that give significantly erroneous azimuths in some cases the error is systematic in other cases it is random one array with systematic azimuthal bias is cmar this array shows positive residuals for event paths from the south while phases for events with paths from the north show negative residuals similar patterns which strongly suggest azimuthal bias are observed for pdar txar and spits in the case of mjar the large azimuthal errors observed particularly for regional and local distances are not a function of direction the scatter in the azimuth residuals is much larger for 3 c stations than for the arrays clear systematic errors were identified for schq and cpup which show constant offsets and at vnda which has negative residuals for events to the north and positive residuals for events from the south the most significant azimuth bias was observed for pdy where the azimuth residuals change from about 90 to more than 90 at backazimuths near 120 meaning that all rays appear from the northwest

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

tectonic history of the juan de fuca ridge over the last 40 million years interpretation of a new compilation of magnetic anomaly data for north of the mendocino fracture zone and younger than chron 15 39 ma indicates a transition from a pattern of stable transform faults to a pattern of persistent ridge propagation reorganizations of the propagation pattern occurred at 30 ma in response to a counterclockwise change in direction of relative motion and at 19 ma in response to a clock wise change in addition there are significant changes in the gradient of the spreading rate apparently without major changes in motion direction at 27 and 24 ma the change at 30 ma is apparently the result of the separation of the juan de fuca plate from the rest of the farallon plate along the pioneer fracture zone the pattern of absolute motion after 30 ma does not generally support menard s pivoting subduction model and suggests that juan de fuca plate motion is being significantly affected by forces transmitted across the mendocino transform fault

some geometrical aspects of uncertainties in combined plate reconstructions the uncertainties in both the position of a pole of rotation and the rotation angle describing a plate reconstruction depend not only on the number and quality of data used but also on two geometrical factors the length of plate boundary represented by the data and the distance from the best fit pole to the center of the data region we describe a simple geometrical method that can be used to calculate minimum uncertainties in reconstructions based on positions of magnetic anomalies and fracture zones with finite uncertainties uncertainties in pole positions and angles corresponding to 10 km uncertainty in individual data points can when combined yield uncertainties in reconstructed positions greatly in excess of 10 km per rotation for example an uncertainty of 10 km in anomaly 6 reconstructions in the south pacific southeast indian northwest indian and north atlantic oceans would result in uncertainty of up to 190 km in the reconstructed position of the pacific plate with respect to north america at 19 8 m y b p existing data show that the realistic uncertainties for this case are twice as large as the calculated minimum uncertainties

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

testing the fixed hotspot hypothesis using ar 40 ar 39 age progressions along seamount trails abstract hotspots and their associated intra plate volcanism producing seamount trails have become an accepted fact in geology from a conceptual theory the azimuths and age progressions of these seamount trails provide the only means to determine absolute plate motions with respect to an independent reference frame of fixed hotspots however the presumed fixity of hotspots is in disagreement with recent paleomagnetic studies and global circuit plate reconstructions for the hawaiian emperor seamount trail in this study we provide independent evidence suggesting that hotspots are not fixed relative to each other we use a straightforward test that compares the observed 40ar 39ar age progressions along pacific seamount trails 0 140 myr with the pacific plate velocities as predicted by their poles of plate rotation i e euler poles in most of these comparisons the age progressions were found incompatible with published euler poles or with a new set of euler poles as derived in this study using discrete seamount locations digitized from the bathymetry maps of smith and sandwell eos 77 1996 315 science 277 1997 1956 1921 we conclude that the relative motion between hotspots may be required to reconcile the observed age progressions with the predicted plate velocities from their modeled euler poles on average the pacific hotspots may show motion at 10 60 mm yr over the last 100 myr partly attributed to individual hotspot motion whereas systematic motion of these hotspots due to true polar wander may account for the remainder

the uncertainties of finite rotations in plate tectonics incomplete knowledge of the pattern of magnetic lineations and fossil transform faults represented by sea floor spreading data on two plates generated by the same spreading center leads to uncertainties in a reconstruction of the past relative configuration of the plates a reconstruction may be represented by the finite rotation that describes one configuration relative to another in this paper a method of reconstruction is provided that reflects the uncertainties in the data the method minimizes a weighted least squares measure of fit as a function of the rotation parameters for a given rotation the measure of fit represents the sum of squares of the weighted distances of fixed and rotated data points representing corresponding former plate margin segments from a common plate margin the common margin is estimated from the locations of both fixed and rotated data points the estimated margin consists of a set of great circle arcs it is shown how the method of reconstruction may be utilized to obtain an uncertainty region for the pole and angle of rotation that characterize the finite rotation these techniques are then used to study the uncertainties of reconstructions in the south pacific for the times of anomalies 13 and 18 the best fit reconstructions and the uncertainty region for the anomaly 18 pole are in good agreement with previous work however the uncertainties in the anomaly 13 pole were substantially underestimated by previous investigators

timing and tectonic setting of tertiary low angle normal faulting and associated magmatism in the southwestern united states major episodes of tertiary low angle normal faulting in the southwestern united states south of latitude 37 n moved northward with time occurring about 25 million years ago in southern arizona 20 million years ago in the mojave desert region and 10 to 15 million years ago in the las vegas area in most areas faulting and associated volcanism occurred when the mendocino triple junction lay to the west this correspondence probably resulted from the unstable configuration of the triple junction east west to northeast southwest horizontal stretching as plates pulled apart at the triple junction prevented development of a hole in the lithosphere at the triple junction the regional strain history predicted by this model is consistent with observed tertiary low angle faults strike slip faults and folds

automatic seismogram analysis statistical phase picking and locating methods using one station three component data abstract seismic events at local and regional distances are located using data from the vaasa vaf 63 0 n 22 7 e three component station in western finland the analysis is performed off line after pseudo on line detections the continuous data are first recorded on long disk loops these loops can cover 10 days of data the events are picked using murdock hutt detector and after that they are recorded as separate files for further analysis the direction of approach which estimates the polarization state of the signal is determined by the well established maximum likelihood method the difference between the arrival times of p and s phases is used to estimate the distance phases observed up to 185 km distance from the source are assumed to be pg and sg and beyond that pn and sn the onset times of the phases are estimated by a statistical data adaptive method the onsets calculated in this way are very accurate when compared with earlier methods like sta lta epicenter locations are similar to those of finsa network reported in helsinki bulletins the median of the location differences is about 50 km

relative motions of the pacific rivera north american and cocos plates since 0 78 ma we use magnetic anomaly and fracture zone crossings from 3 n 24 n along the east pacific rise to define the boundaries and relative velocities of the rigid pacific rivera north american and cocos plates since 0 78 ma and to test several hypotheses regarding the nature of deformation between these plates crossings of the 0 78 ma isochron from the pacific rivera rise north of 22 0 n are significantly better fit by the 0 0 78 ma pacific north america rotation than by a pacific rivera rotation thereby supporting a recently proposed model for accretion of the northern part of the rivera plate to north america a significant misfit of the pacific cocos rotation to crossings of anomaly ln from the east pacific rise north of 16 4 n suggests that seafloor north of 16 n moves relative to the rigid pacific or cocos plates we also find that nine crossings of the eastern rivera fracture zone and eight crossings of anomaly ln flanking the manzanillo spreading segment are fit well by a pacific rivera rotation the rivera cocos angular velocity derived from the best fitting pacific cocos and pacific rivera angular velocities predicts 19 3 mm yr 1 95 confidence limit of n s convergence near the center of the rivera cocos boundary nearby left lateral strike slip earthquakes with north trending fault planes suggest that convergence is accommodated via sinistral shear along a north trending zone the unambiguous kinematic and seismologic evidence for significant rivera cocos motion indicates that the absence of a well defined rivera cocos plate boundary cannot be interpreted as evidence that rivera cocos motion is slow or zero as is postulated in several previous studies the rivera cocos angular velocity predicts oblique rivera cocos convergence at the el gordo graben and beneath the manzanillo trough and southern colima graben any extension across these features since 0 78 ma thus cannot have resulted from divergence between the subducting rivera and cococs plates