roller bearing tectonic evolution of the juan fernandez microplate a combined gloria sidescan and hydrosweep multi narrow beam sonar survey shows striking evidence for fan shaped spreading systems bounding pseudofaults and compression ridges associated with 90 of clockwise rotation of the juan fernandez microplate in the past 4 myr a quantitative analysis of this evolution suggests that the microplate has rotated as a rigid roller bearing for most of its history driven at its edges by shear between the surrounding major plates the remarkable similarity between the juan fernandez and easter microplates despite their different tectonic settings suggests that this is a general model for microplate tectonics

the late cretaceous to recent tectonic history of the pacific ocean basin abstract a vast ocean basin has spanned the region between the americas asia and australasia for well over 100 myr represented today by the pacific ocean its evolution includes a number of plate fragmentation and plate capture events such as the formation of the vancouver nazca and cocos plates from the break up of the farallon plate and the incorporation of the bellingshausen kula and aluk phoenix plates which have been studied individually but never been synthesised into one coherent model of ocean basin evolution previous regional tectonic models of the pacific typically restrict their scope to either the north or south pacific and global kinematic models fail to incorporate some of the complexities in the pacific plate evolution e g the independent motion of the bellingshausen and aluk plates thereby limiting their usefulness for understanding tectonic events and processes occurring in the pacific ocean perimeter we derive relative plate motions with 95 uncertainties for the pacific farallon vancouver kula pacific bellingshausen pacific and early pacific west antarctic spreading systems based on recent data including marine gravity anomalies well constrained fracture zone traces and a large compilation of magnetic anomaly identifications we find our well constrained relative plate motions result in a good match to the fracture zone traces and magnetic anomaly identifications in both the north and south pacific in conjunction with recently published and well constrained relative plate motions for other pacific spreading systems e g aluk west antarctic pacific cocos recent pacific west antarctic spreading we explore variations in the age of the oceanic crust seafloor spreading rates and crustal accretion and find considerable refinements have been made in the central and southern pacific asymmetries in crustal accretion within the overall pacific basin where both flanks of the spreading system are preserved have typically deviated less than 5 from symmetry and large variations in crustal accretion along the southern east pacific rise i e pacific nazca farallon spreading appear to be unique to this spreading corridor through a relative plate motion circuit we explore the implied convergence history along the north and south america s where we find that the inclusion of small tectonic plate fragments such as the aluk plate are critical for reconciling the history of convergence with onshore geological evidence

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

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

subduction of the south chile active spreading ridge a 17 ma to 3 ma magmatic record in central patagonia western edge of meseta del lago buenos aires argentina abstract the chile triple junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent the mlba plateau meseta del lago buenos aires is one of the neogene alkali basaltic plateaus located in the back arc region of the andean cordillera at the latitude of the current chile triple junction the genesis of mlba can be related with successive opening of slabs windows beneath patagonia within the subducting nazca plate itself and between the nazca and antarctic plates detailed 40ar 39ar dating and geochemical analysis of bimodal magmatism from the western flank of the mlba show major changes in the back arc magmatism which occurred between 14 5 ma and 12 5 ma with the transition from calc alkaline lavas cerro plomo to alkaline lavas mlba in relation with slab window opening in a second step at 4 3 ma alkaline felsic intrusions were emplaced in the western flank of the mlba coevally with the mlba basalts with which they are genetically related these late oib like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting nazca plate during the opening of the south chile spreading ridge related slab window these basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones the close association of bimodal magmatism with extensional tectonic features in the western mlba is a strong support to the model of patagonian collapse event proposed to have taken place between 5 and 3 ma as a consequence of the presence of the asthenospheric window scr 1 segment of south chile ridge below the mlba area

a model for the plate tectonic evolution of the east central pacific based on seasat investigations abstract a complete seasat dataset is used to investigate the small scale bathymetry of a 15 by 15 area of the east central pacific northeast of pitcairn island which was involved in the miocene ridge jump and reorientation two fracture zones fz1 and fz2 are mapped along 450 km at an azimuth of n95 e while the signature of the austral fracture zone disappears east of 135 w several seamounts are discovered or confirmed along the oeno henderson ducie lineament and the degree of isostatic compensation of a number of islands at the eastern end of the tuamotu group is studied islands of the northern branch have practically no signatures in the geoid while the southern ones must have formed on lithosphere at least 10 ma old on the basis of these constraints we present a model of the evolution of spreading since 40 ma b p in which the deactivation of the austral fracture zone and the transfer of its offset to fz1 in the late oligocene is interpreted as due to rift propagation triggered by a northern tuamotu hotspot we propose that the oeno ducie chain is the surface expression of a southern tuamotu hotspot deviated 15 in azimuth by the presence of a young and fresh fracture zone thus our model predicts that hotspots and ridge systems can have a moderate influence on each other s kinematics and surface expressions which does not however extend to permanent trapping

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

southeast pacific tectonic evolution from early oligocene to present plate tectonic reconstructions of the nazca antarctic and pacific plates are presented from late oligocene to present these reconstructions document major plate boundary reorganizations in the southeast pacific at dirons 6c 24 ma 6 o 20 ma and 5a 12 ma and a smaller reorganization at chron 3 o 5 ma during the chron 6 o reorganization it appears that a ridge propagated into crust north of the northernmost pacific antarctic ridge between the chiloe fracture zone fz of the chile ridge and agassiz fz of the pacific nazca ridge which resulted in a northward jump of the pacific antarctic nazca pac ant naz mid ocean triple junction during the chron 5a reorganization the chile ridge propagated northward from the valdivia fz system to the challenger fz through lithosphere formed roughly 5 myr earlier at the pacific nazca ridge during this reorganization a short lived microplate the friday microplate existed at the pac ant naz triple junction the pac ant naz triple junction jumped northward 500 km as a result of this reorganization from a location along the valdivia fz to a location along the challenger fz the chron 5a reorganization also included a change in spreading direction of the chile and pacific antarctic ridges the reorganization at chron 3 o initiated the formation of the juan fernandez and easter microplates along the east pacific rise the manner of plate boundary reorganization at chron 6 o and chron 5a and possibly today at the juan fernandez microplate included a sequence of rift propagation transfer of lithosphere from one plate to another microplate formation and microplate abandonment and resulted in northward migration of the pac ant naz triple junction the associated microplate differs from previously studied microplates in that there is no failed ridge

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

eocene propagating rifts in the southwest pacific and their conjugate features on the nazca plate we have mapped a 1200 km long nw se trending lineament in seasat and geosat radar altimeter data crossing a remote portion of the southwest pacific basin this lineament runs obliquely both to the fracture zones fzs and magnetic lineations between the austral and agassiz fzs by examining shipboard magnetics gravity and bathymetric profiles in this region and in the conjugate regions of the nazca plate we identify this feature as the site of a series of small ridge jumps highly asymmetrical spreading and small propagating rifts during the mid to late eocene chron c21 to chron c13 between the austral and agassiz fzs there is roughly 250 to 350 km of missing pacific plate crust which must have been transferred to the nazca plate by the propagating rifts and small ridge jumps on the nazca plate the conjugate area to the gravity lineament is now being subducted beneath the chile trench north of the challenger fz this area corresponds to a region of undecipherable magnetic anomalies south of the challenger fz there is evidence for a small 50 km ridge jump offshore of the chile trench this small ridge jump by itself is not sufficient to account for the missing crust on the pacific plate and we conclude that there must have been repeated small ridge jumps at the same site one consequence of these ridge jumps is that crust on the nazca plate entering the trench is not as old as would be predicted by simply mirror imaging the anomaly pattern on the pacific plate

edge driven microplate kinematics it is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate s motion relative to the bounding major plates are frequently located close to its margins with those plates close to the tips of propagating rifts we propose a class of edge driven block models to illustrate how slip across the microplate margins block rotation and propagation of rifting may be related to the relative motion of the plates on either side an important feature of these edge driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates according to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time pseudofault geometries and matching rotations of the easter microplate show that for most of its 5 m y history block rotation could be driven by the drag of the nazca and pacific plates on the microplate s edges rather than by a shear flow of mantle underneath

reconciling late neogene pacific absolute and relative plate motion changes new models of pacific absolute plate motion relative to hot spots and models of relative plate motion involving the pacific plate all agree there was a significant change in the late neogene chron 3a 5 89 ma reflecting a more northerly absolute motion than previously determined as pacific absolute plate motion became slightly more northerly left stepping transform segments came under compression some left stepping segments became microplates with clockwise rotation others show clear evidence of compressional deformation conversely right stepping transforms came under tension and many developed intratransform spreading centers or show similar evidence for transform magmatism several large left stepping transform offsets represent portions of the nazca plate protruding into the pacific and as such act as obstacles to the more northerly pacific absolute plate motion we suggest these obstructions act to enhance the generally tensile equatorial pacific stress regime caused by distant slab pull as a consequence the greater french polynesia region has experienced diffuse volcanism that increased considerably following the chron 3a plate motion change we propose that since the hawaii emperor bend time the distant pacific slab pull and the friction between the large buoyant ontong java plateau and the northern margin of the australia plate have produced episodes of increased tensile stresses in the equatorial pacific we believe these stress excursions are responsible for much of the intraplate volcanism observed in a wide triangular region from samoa to the east pacific and pacific antarctic rises

the development of two oceanic microplates easter and juan fernandez microplates east pacific rise in this paper we synthesize and discuss the results of recent detailed geophysical surveys and reconstructions of two active mid ocean ridge microplates on the east pacific rise we quantify their development in terms of the principal dimensions of the microplates and their boundaries and the temporal changes in these dimensions the results are discussed in the framework of the roller bearing microplate model of schouten and others which postulates that microplates are driven by shear couples imposed on them by the motions of their bounding major plates we test the specific predictions of this model for the geometry of the pseudofaults traces of the propagating tips of the microplates spreading boundaries and the histories of rotation of the microplates both microplates are currently rotating at the rates predicted by the model on the assumption of complete coupling no slip or deformation between the microplates and the major plates at instantaneous points of contact near the propagating tips of the spreading boundaries juan fernandez microplate was also rotating at the predicted rate just before 2 ma when it was driven by the nazca pacific couple rather than the nazca antarctic couple that currently applies however earlier rotations were slower than predicted implying some degree of decoupling or deformation at earlier times the microplates appear to have grown from moderate sized c 100 km probably left stepping offsets of the east pacific rise since about 5 ma the nature of these offsets is currently uncertain but they may have been propagating rifts which changed from shearing by bookshelf faulting to rigid block microplate rotation on reaching a critical size once rigid block rotation began the microplates grew by seafloor spreading accompanied by propagation of their bounding spreading centres the effects of the neighbouring easter mantle plume and pacific nazca antarctic triple junction can be discerned in the bathymetry and ridge propagation rates but do not appear to have had a major influence on the microplates development

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

placing bounds on lithospheric deformation in the central pacific ocean abstract lineations in the gravity field in the central pacific ocean have been explained by models of both small scale convection and lithospheric boudinage the boudinage model requires deformation of the plate and we have investigated bounds on the amount of deformation we calculate separations between a pair of fracture zones on the pacific plate which occur in an area where the lineations are well developed and compare these values to the separation of the corresponding fracture zone pair on the nazca plate where no lineations have been observed satellite and surface ship data was used to locate fracture zones and small circles were fitted to sections of picks which were chosen around their points of intersection with magnetic isochrons great circle distances from these small circles to stage poles were calculated and the difference between these distances gave the separations between the fracture zone pairs we found that between anomalies 13 and 7 the difference in fracture zone spacing on the pacific plate compared to the nazca plate appears to increase as the age of oceanic lithosphere decreases the maximum difference calculated was 97 km about 4 of the distance between the fracture zones and we attribute this difference to the re orientation of the ancient pacific farallon spreading centre between anomalies 7 and 5 the main lithospheric stretching models require the fracture zone separation to increase with age but this is not seen in our results

comprehensive sonar imaging of the easter microplate recent gloria and seamarc ii sidescan sonar surveys have imaged virtually the whole of the easter microplate and revealed unprecedented detail and complexity in its tectonic fabric and history there is clear evidence of rapid microplate rotation of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries

geophysics of the pitman fracture zone and pacific antarctic plate motions during the cenozoic multibeam bathymetry and magnetometer data from the pitman fracture zone fz permit construction of a plate motion history for the south pacific over the past 65 million years reconstructions show that motion between the antarctic and bellingshausen plates was smaller than previously hypothesized and ended earlier at chron c27 61 million years ago the fixed hot spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early tertiary either between east antarctica and west antarctica or between the north and south pacific a plate reorganization at chron c27 initiated the pitman fz and may have been responsible for the other right stepping fracture zones along the ridge an abrupt 8 clockwise rotation in the abyssal hill fabric along the pitman flowline near the young end of chron c3a 5 9 million years ago dates the major change in pacific antarctic relative motion in the late neogene

tectonic evolution of the north patagonian andes 41 44 s through recognition of syntectonic strata abstract the north patagonian fold thrust belt 41 44 s is characterized by a low topography reduced crustal thickness and a broad lateral development determined by a broken foreland system in the retroarc zone this particular structural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment here new field and seismic evidence of syntectonic strata constrain the timing of the main deformational stages evaluating the prevailing crustal regime for the different mountain domains through time growth strata and progressive unconformities controlled by extensional or compressive structures were recognized in volcanic and sedimentary rocks from the cordilleran to the extra andean domain these data were used to construct a balanced cross section whose deep structure was investigated through a thermomechanical model that characterizes the upper plate rheology our results indicate two main compressive stages interrupted by an extensional relaxation period the first contractional stage in the mid cretaceous inverted jurassic lower cretaceous half graben systems reactivating the western canadon asfalto rift border 500 km away from the trench at a time of arc foreland expansion for this stage available thermochronological data reveal forearc cooling episodes and global tectonic reconstructions indicate mid ocean ridge collisions against the western edge of an upper plate with rapid trenchward displacement widespread synextensional volcanism is recognized throughout the paleogene during plate reorganization retroarc paleocene eocene flare up activity is interpreted as product of a slab rollback and fore to retroarc oligocene slab asthenospheric derived products as an expression of enhanced extension the second stage of mountain growth occurred in miocene time associated with nazca plate subduction reaching nearly the same amplitude than the first compressive stage extensional weakening of the upper plate predating the described contractional stages appears as a necessary condition for abnormal lateral propagation of deformation

submarine weathering of silicate minerals and the extent of pore water freshening at active continental margins in order to investigate how submarine weathering processes may affect the water balance of sediments at convergent plate margins six sediment cores were retrieved off central chile at water depth between 800 and 4000 m the sediment solid phase was analyzed for its major element composition and the pore fluids were analyzed for dissolved sulfate sulfide total alkalinity major cations chloride bromide iodide hydrocarbons as well as the carbon isotopic composition of methane because of negligible weathering on land surface sediments off central chile are rich in reactive silicate minerals and have a bulk composition similar to volcanic rocks in the adjacent andes deep sourced fluxes of alkalinity cations and chloride indicate that silicate minerals are subject to weathering in the forearc during burial comparison of deep sourced signals with data from nearby ocean drilling program sites reveals two different types of weathering processes in shallow tens of meters methanic sediments of slope basins with high organic carbon burial rates reactive silicate minerals undergo incongruent dissolution through reaction with co2 from methanogenesis at greater burial depth hundreds of meters silicate weathering is dominated by authigenic smectite formation this process is accompanied by uptake of water into the clay interlayers thus leading to elevated salinities in the surrounding pore water deep seated smectite formation is more widespread than shallow silicate dissolution as it is independent from the availability of co2 from methanogenesis although solute transport is not focused enough to form cold seeps in the proper sense tectonically induced diffuse fluid flow transfers the deep seated signal of smectite formation into the shallow sediments the temperature controlled conversion of smectite to illite is considered the most important dehydration process in marine forearc environments depth of kilometers however in agreement with other studies at active margins e g aleutians cascadia nankai trough and despite ubiquitous evidence for smectite formation little evidence for seafloor seepage of dehydration fluids could be found off central chile we argue that the circular process of pore water uptake during smectite formation and release upon illitization implies a balanced freshwater budget and therefore a rather limited potential for net pore water freshening on a margin wide scale according to this rationale pore water freshening at seafloor seeps preferentially occurs at lower latitudes central america barbados mediterranean ridge where terrestrial weathering is more intense thus leading to external i e detrital smectite and thus freshwater inputs to the subduction system

the foundation seamounts tectonic setting of a newly discovered seamount chain in the south pacific abstract large areas of the south pacific remain unsurveyed because they are so remote a major volcanic chain the foundation seamounts lies between 33 s and 39 s and between 111 w and 125 w poorly defined on conventional bathymetric charts it becomes very apparent when combining an analysis of conventional geophysical data with the satellite altimeter data the length of the seamount chain is 1350 km and the volcanoes are clustered within a 180 km wide band trending approximately n70w which is the direction of absolute motion of the pacific plate over a fixed reference system some of these volcanoes are so shallow that they may have emerged above the sea surface at an earlier time no dates are known for the seamounts surrounding the foundation seamounts chain the tectonic setting is that of the pacific plate at the time when the strike of the spreading ridge changed from a northwesterly direction to the present northeasterly one this episode of spreading reorganization involved the existence of a short lived microplate west of the pacific antarctic ridge a captured segment of nazca plate accounts for the large discrepancies in the magnetic anomalies offsets along the agassiz fracture zone large propagators like the adventure and kurchatov troughs are described the newly discovered western segment of the chile fracture zone is the pacific plate match of the chile fracture zone on the nazca plate

comparative mass balance of volcanic edifices at the southern volcanic zone of the andes between 33 s and 46 s volcano edifice volume calculations are presented for 65 volcanoes of the 1400 km long chilean southern volcanic zone svz as a basic step in subduction zone mass budgets volume calculations are performed in a geographical information system that integrates digital elevation models based on of shuttle radar topography mission as well as aster gdem topographic data landsat satellite images and geological maps the method of volume calculation is straightforward for isolated morphologically well defined stratovolcanoes uncertainties increase for volcanic edifices that formed on pre existing rugged terrain for multi phase eruptive centers as well as for eroded edifices a revised segmentation of the arc is used to describe the spatial volume distribution of extruded magma along the svz and to discuss controlling tectonic factors peak volumes between arc and back arc are offset by 400 km the total volcanic extrusion is in the range of 10 13 km3 km ma major differences between the svz and the central american subduction system are notable with regard to volcano density and maximum volumes

coastal deformation and great subduction earthquakes isla santa maria chile 37 s isla santa maria at the active margin of south central chile is the result of earthquake related uplift and deformation in the forearc since at least late pleistocene time field mapping dating of key depositional horizons and analysis of seismic refl ection profi les reveal ongoing deformation in this sector of the chilean forearc the 30 km 2 island is located 12 km above the interplate seismogenic zone and 75 km landward of the trench it is situated near the southern termination of the concepcion earthquake rupture segment where charles darwin measured 3 m of coseismic uplift during a m 8 megathrust earthquake in 1835 permanent postearthquake deformation from this earthquake and an earlier event in 1751 is registered by emerged landward tilted abrasion surfaces uplift at 2 m k y and tilting at 0 025 k y of the island have been fairly constant throughout the late quaternary and have resulted in emergence of the island above sea level 31 k y ago the island is composed of a late pleistocene upper tilted surface with two asymmetric tilt domains and holocene lowlands characterized by uplifted and tilted strandlines industry offshore seismic refl ection profi les covering an area of 1800 km 2 and crustal seismicity reveal active reversefault cored anticlines surrounding isla santa maria the principal fault apparently roots in the plate interface thrust these reverse faults in the upper plate result from inversion of late cretaceous to early pliocene normal faults and rift structure of the arauco forearc basin positive inversion of these inherited structures started between 3 6 and 2 5 ma and resulted in continuous shortening rates of 0 8 mm yr the seismic refl ection profi les show that the asymmetric tilt domains and progressive syntectonic sedimentation are linked to the position of the island in the forelimbs of two converging anticlines whereas their backlimbs have been removed by cliff retreat the 2 m uplift contour of the 1835 earthquake is parallel to the strike of active faults and antiforms in the arauco concepcion region the close relation among the asymmetric uplift and tilt of the island modern deformation patterns and reverse faults rooted in the plate interface suggests that slip on the plate interface thrust infl uences localizes and segments surface deformation during large interplate earthquakes furthermore the link between positive inversion of pre existing structures uplift and tilt patterns in the forearc emphasizes the importance of inherited structural fabrics in guiding plate boundary deformation

geochemical and structural studies of the lamont seamounts seamounts as indicators of mantle processes abstract a chain of seamounts located just west of the east pacific rise epr axis at 09 55 n herein called the lamont seamounts have been investigated using sea marc i and sea beam sonar surveys alvin submersible dives deep sea camera tows and dredging the most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors the distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts ring fractures could be important in focusing post caldera collapse eruptions around summit craters and building summit plateaus structural and morphological data and the petrology of recovered lavas suggest that calderas in the lamont seamounts probably formed by collapse associated with magma withdrawal into large primary vertical magma conduit systems rather than by repeated inflation deflation cycles associated with steady state shallow level magma chambers calderas in the lamont seamounts have evolved through coalescence of adjoining craters and the plan view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time compared to lavas from the adjacent epr the majority of lamont seamount and cone lavas are more primitive depleted in incompatible elements and compositionally heterogeneous variations in light rare earth elements relative to major elements e g tio 2 na 2 o na 2 o cao and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single n type morb source rather it appears that lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the lamont seamounts are separate from those along the epr in particular seamount magmas appear to reflect near primary melting events in the mantle whereas epr lavas are the products of shallow level crystallization and mixing in crustal magma chambers absence of long lived magma chambers within the lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain furthermore because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the lamont chain as a whole we propose that the scale of mantle chemical heterogeneities is less than 5 km

a history of the easter microplate 5 25 ma to present gloria and other geophysical data are used to interpret the morphology and tectonic pattern of the easter microplate we model magnetic anomaly data and calculate spreading rates for use in euler pole calculations we calculate three plate closure and best fit poles for nazca pacific nazca easter and pacific easter plate motions and compare the results with earthquake focal mechanisms and seismicity we use the three plate closure poles to rotate digital gloria and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate s east rift first started to propagate northward at around 5 25 ma the microplate has evolved in two stages the first is from 5 25 ma to around 2 5 ma and involved continuous northward propagation of the east rift while the west rift became segmented along new transform faults as it adjusted to keep the overall nazca pacific velocities constant the transpressive northern boundary migrated with the east rift tip while the transtensional southern boundary remained more or less still at some time after 2 5 ma the east rift ceased northward propagation and the southwest rift began to open up along the transtensional pacific easter transform a convergent northern boundary formed involving southward thrusting of the nazca plate over the microplate no further nazca plate lithosphere was transferred from north of the microplate to the microplate interior and the pito deep region opened by pervasively rifting apart preexisting nazca plate lithosphere

sub slab mantle anisotropy beneath south central chile knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones by analysing shear wave splitting in both teleseismic and local arrivals we can determine the relative contribution from different parts of the subduction zone to the total observed sks splitting providing us with a depth constraint on anisotropy we use this methodology to determine the location orientation and strength of seismic anisotropy in the south central chile subduction zone data come from the tipteq network deployed on the forearc during 2004 2005 we obtain 110 teleseismic sks and 116 local good quality shear wave splitting measurements sks average delay times are 1 3 s and local s delay times are only 0 2 s weak shear wave splitting from local phases is consistent with a shape preferred orientation spo source in the upper crust we infer that the bulk of shear wave splitting is sourced either within or below the subducting nazca slab sks splitting measurements exhibit an average north easterly fast direction with a strong degree of variation further investigation suggests a relationship between the measurement s fast direction and the incoming ray s back azimuth finite element geodynamic modelling is used to investigate the strain rate field and predicted lpo characteristics in the subduction zone these models highlight a thick region of high strain rate and strong s wave anisotropy with plunging olivine a axes in the sub slab asthenosphere we forward model the sub slab sourced splitting with a strongly anisotropic layer of thick asthenosphere comprising an olivine a axis oriented parallel to the direction of subduction the subducting lithosphere is not thick enough to cause 1 2 s of splitting therefore our results and subsequent models show that the nazca slab is entraining the underlying asthenosphere its flow causes it to be strongly anisotropic our observation has important implications for the controlling factors on sub slab mantle flow and the movement of asthenospheric material within the earth

intratransform origins of mid ocean ridge microplates we present evidence that the easter and juan fernandez microplates originated by rift propagation from within transform fault zones at locations far from ridge transform intersections an intratransform spreading center may have served as the focal point for each new propagation both events appear coincident with a circum pacific plate reorganization dated at 5 6 ma which may have been related to a change in pacific absolute plate motion and a counterclockwise rotation of pacific nazca relative plate motion this intratransform model may be applicable to the generation of other microplates and may thus serve as a general model for microplate origins

birth of an intraoceanic spreading center the cocos nazca spreading center is one of the few examples of the formation of a spreading center by splitting of oceanic lithosphere it was created when the farallon plate broke up in the early miocene following the collision of the pacific farallon spreading center with the north american continent much of the ancient farallon plate corresponding to the area of opening is lost to subduction beneath central america and south america but new data from the conjugate area on the pacific plate allow the first detailed reconstruction of the break up process the opening began after chron 7 25 ma at a location of focused crustal extension caused by overlapping spreading centers that had evolved in response to a slight reorientation of a pacific farallon ridge segment beginning at chron 6b 22 7 ma eastward progressing seafloor spreading started along an axis that most likely migrated toward the region of weak lithosphere created by the galapagos hotspot by chron 6 19 5 ma plate splitting from the spreading center to the trench was complete allowing the fully detached cocos and nazca plates to move independently this kinematic change resulted in a significant ridge jump of the newly established pacific nazca spreading center a change in plate motion direction of the nazca plate by 20 clockwise and a large increase in pacific cocos plate velocity in the middle miocene

gully formation in the mcmurdo dry valleys antarctica multiple sources of water temporal sequence and relative importance in gully erosion and deposition processes

microplate and shear zone models for oceanic spreading center reorganizations the reorganization of oceanic spreading centers is often accomplished through the process of rift propagation we first examine general rift propagation geometries for the simple two plate case in which the propagating rift instantaneously reaches full spreading rate while the other rift dies instantaneously such that the propagating and dying rifts never overlap a velocity space representation provides a framework for describing the geometry of rift propagation and its various tectonic elements and shows how both the failed rift and isochrons between the two rifts are reoriented as a consequence of purely rigid plate tectonics we then discuss the more complex geometries that result when both growing and dying rifts spread simultaneously during propagation such that a technically independent overlap region like the easter or juan fernandez plates along the east pacific rise is produced we examine the effect of the propagation rate and rise time of spreading on the evolution of overlap regions between dual oceanic spreading centers the tectonics and evolution of such regions can be modeled by assuming that they act either as rigid microplates or as zones of distributed shear rigid plate models are suggested by the reasonable success of euler vectors derived from inversions of plate motion data spreading rates transform azimuths and slip vectors both in describing available data and in predicting subsequently acquired data on the other hand recent structural data showing fabric oblique to the surrounding spreading centers might suggest shearing rather than rigid behavior in the interior of the easter microplate we use schematic evolutionary simulations for an easter style geometry to examine whether plate motion data and orientations of preexisting and present structures can be used to discriminate between rigid and shear models in a microplate model rigid plate tectonics requires that relative motion occur only along the boundaries of the overlap region while structures within the overlap zone rotate rigidly in contrast in a shear model relative motions at the boundaries differ from those predicted by rigid plate tectonics and the angular relationships of structures within the overlap region are altered by internal deformation thus inversion of relative motion data yields different results for rigid and shear behavior similarly under favorable circumstances structural trends could be diagnostic of the different cases we find that much of the easter structural data suggestive of shearing are also consistent with rigid plate tectonics we conclude that microplate models offer useful testable hypotheses for describing overlap systems and that shearing need be invoked only to explain features not describable by rigid plate tectonics furthermore the approaches used here to study current overlap regions should be useful in the analysis of similar regions formed during past oceanic plate boundary reorganizations

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

on the stability of triple junctions and its relation to episodicity in spreading a convenient representation of triple junctions that involve only ridges r and transform faults f is proposed this representation combines in a simple way information from geographic and velocity spaces the velocity triangle provides the budget of lithospheric surface change which directly results from interactions of the three plates a discussion of the relative positions of the triple junction with respect to the velocity triangle demonstrates that in general there are several triple junction configurations that are compatible with a given triangle this discussion stresses the importance of oblique and asymmetric spreading pairs of triple junction configurations that are of particular interest are rrr rrf and rrr ffr when the triple junction lies outside of the triangle at least one of the ridges will shorten leading to yet another type of potentially unstable configuration the present configurations of the bouvet galapagos and indian ocean triple junctions are reviewed a method is proposed to reconstruct past configurations of the indian ocean junction at the time of anomalies 23 and 28 the main parameters that influence triple junction evolution are the lengths of transform faults the availability of magma and related connectivity of magma chambers and the also related spreading velocities this study confirms an earlier suggestion that activity at constructive plate boundaries occurs in two preferred modes the effusive and tectonic modes corresponding here to rrr and to rrf rff configurations respectively these modes apparently alternate in episodes of typically 1 ma duration the significance of this time constant and consequences for lithospheric mechanics are briefly discussed

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

geomorphology and structural segmentation of the crest of the southern pacific antarctic east pacific rise geomorphology of the boundary between pacific and antarctic plates was mapped with a sea beam multibeam echosounder and a seamarc ii bathymetric side scan sonar from the southern end of juan fernandez microplate at 35 s to heezen transform at 56 s there are six spreading center systems separated by two large left stepping nontransform offsets at 36 5 s and 41 5 s two right stepping transform fault systems and a left stepping hybrid structure with equally long strike slip and nontransform offsets axial rift zones spreading axes are generally within 1 of normal to the relative motion predicted by current plate rotation models most axial rift zones crop out along the crests of typical east pacific rise epr axial ridges which commonly have flat rather than humped long profiles about 2 of this fast spreading 84 100 mm yr rise crest has an axial rift valley instead of an axial ridge the longest rift valley segment is midway between menard and vacquier transforms near 51 s where an anomalously deep rise crest may mark a zone of below average mantle upwelling nontransform offsets structurally similar to those on the tropical epr but mostly formed by differential asymmetric spreading subdivide the four longest spreading center systems on the northern transform free two thirds of the rise net rift propagation at migrating offsets has been into the faster pacific plate this causes right steps to migrate north left steps to migrate south and crust to be transferred to the east flank this pattern has prevailed for several million years judging from the oblique fracture zones pseudofaults mapped on the rise flanks with older marine data and geosat altimetry on this rise crest the difference in stress on two plates with very different velocities may control the direction of offset migration

comparisons of gravity anomalies at pseudofaults fracture zones and nontransform discontinuities from fast to slow spreading areas published mechanisms for rift tip propagation at spreading centers include extensional deformation and an initial period of slow spreading we investigate whether the gravity signal and inferred crustal structure at pseudofaults formed in medium to superfast spreading environments resemble the gravity signal at fracture zones or nontransform discontinuities formed in slow spreading environments we find that altimetry based gravity anomalies on the mathematician bauer easter juan fernandez and northern chile ridge pseudofaults located in 75 150 mm yr full rate seafloor spreading environments are similar in amplitude and form to atlantic fracture zones with 20 30 mm yr spreading rates a 5 15 mgal positive mantle bouguer anomaly is observed on the pseudofault bounding the eastern juan fernandez microplate comparable to those at some similar age offset nontransform discontinuities in slow spreading environments our results suggest that the deeps associated with active propagating rift tips result from both a dynamic mantle component and anomalous crust the latter of which remains frozen at pseudofaults we predict that any pseudofaults with age offsets more than 1 m y and not coincident with hotspot volcanism will be associated with thin and possibly unusually dense crust even in superfast seafloor spreading environments

variations in axial morphology segmentation and seafloor roughness along the pacific antarctic ridge between 56 s and 66 s the spreading rate at the pacific antarctic ridge par increases rapidly from 54 mm yr near pitman fracture zone fz up to 76 mm yr near udintsev fz resulting in three domains of axial morphology an axial valley south of pitman fz an axial high north of saint exupery fz and in between the transitional domain extends over 650 km it comprises sections of ridge with an axial valley or an axial high and generally displays a very low cross sectional relief it is also characterized by two propagating rifts two domains of different seafloor roughness appear south of udintsev fz east of 157 w these two domains are separated by a 1000 km v shaped boundary west of 157 w the boundary approximately coincides with chron 3a or chron 4 the southward migration of the transitional area during the last 35 myr explains the v shaped boundary 1 increases in spreading rate above a threshold value produced changes in axial morphology and 2 in the transition zone rotations of the spreading direction were accommodated by the plate boundary either by rift propagation or by transitions from fracture zones to non transform discontinuities leaving trails on the seafloor that presently delineate the v seafloor roughness variations are not controlled by exactly the same spreading rate dependence as changes in axial morphology the transition from rough to smooth seems to have occurred everywhere for spreading rates greater than 50 mm yr except in a domain presently centered on saint exupery fz where it occurred for spreading rates 60 to 65 mm yr independent results from melting model calculations of major elements vlastelicet al 2000 indicate that the upper mantle temperature is likely to be cooler between antipodes and la rose fzs the combination of these two results reveals the existence of a 700 km long segmentation of the upper mantle with a cool area centered on saint exupery fz

mineral deposits and cu zn as dispersion contamination in stream sediments from the semiarid coquimbo region chile this paper presents cu zn as geochemical data from stream sediment surveys carried out in the three main watersheds of the coquimbo region of chile this mountainous semiarid realm occupies an area of 40 656 km2 between 29 and 32 s given that the area has a long historical record of mining activities important environmental disturbances were expected however despite the detection of three major geochemical anomalies for cu zn or as only one can be unmistakably linked to the development of mining metal recovery procedures andacollo panulcillo an investigation of the other two anomalies elqui and hurtado reveals three major causes that fully or partially account for them 1 the type of ore deposit and associated hydrothermal alteration 2 the regional structural setting intensity of fracturing and 3 climate landscape cu au as epithermal deposits prospects along the so called el indio belt are here regarded as the sources of both the elqui and hurtado anomalies the strong advanced argillic alteration present in some of the epithermal deposits prospects of the el indio belt may have induced the loss of the buffering capacity of rocks and therefore favoured metal dispersion during later oxidation leaching of sulphides this applies to the elqui and hurtado anomalies conversely given that the potassic propylitic and phyllic alterations do not affect the buffering capacity of rocks only minor metal dispersion is observed in relation to the los pelambres porphyry copper deposit besides the epithermal belt is located within a highly fractured andean domain 3 000 4 000 m of altitude which may have conditioned the fast unroofing of ore deposits contributed to enhanced circulation of meteoric waters and eventually to strong oxidation and leaching of metals metal dispersion is aggravated during rainy years in response to strong el nino episodes

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

tectonic history and new isochron chart of the south pacific we have developed an internally consistent isochron chart and a tectonic history of the south pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data highly accurate vertical deflection profiles 1 2 rad derived from 22 repeat cycles of geosat altimetry reveal subtle lineations in the gravity field associated with the south pacific fracture zones these fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas the high density of geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map the tectonic fabric was then used to derive new poles of rotation for 12 selected times in the late cretaceous and cenozoic from our reconstructions we estimated the former location of the spreading centers in order to derive a new set of isochrons interpreted unes of equal age on the ocean floor we believe that the use of new geosat altimeter data in combination with a multi plate reconstruction has led to an improvement in our understanding of south pacific tectonics

thermal and dynamic modeling of deep subduction of a spreading center implications for the fate of the subducted chile rise southern chile our objective is to determine whether the chile rise spreading center currently subducting south of the chile triple junction in southern chile continues or ceases to separate after subduction a two dimensional kinematic thermal model is used to predict the temperature history of the subduction zone during spreading center subduction density anomalies calculated from the temperature fields of the thermal model are then used to define body forces that drive the motion of a dynamic viscous fluid model of spreading center subduction viscous models that include only local in plane driving forces predict that the chile rise spreading center ceases to separate after ridge subduction if global in plane plate forces are also included in the viscous model continuing separation of the subducted chile rise is predicted but at a greatly diminished rate fully continuing separation of the subducted chile rise only occurs if the subducted nazca and antarctic plates remain rigid allowing the lateral transmission of global plate forces from along strike however this situation is unlikely as young subducted plates are believed to fragment continuing separation but at a greatly diminished rate is believed to be the most plausible hypothesis for spreading center subduction in southern chile dynamic viscous modeling of subduction in the central andes suggests a viscosity of 2 5 x 10 20 pa s for upper mantle above the 400 km phase transition

the chile ridge a tectonic framework a new chile ridge tectonic framework is developed based on satellite altimetry data shipboard geophysical data and primarily 38 500 km of magnetic data gathered on a joint u s chilean aeromagnetic survey eighteen active transforms with fossil fracture zones fzs including two complex systems the chile fz and valdivia fz systems have been mapped between the northern end of the antarctic nazca plate boundary chile ridge at 35 s and the chile margin triple junction at 47 s chile ridge spreading rates from 23 ma to present have been determined and show slowdowns in spreading rates that correspond to times of ridge trench collisions the valdivia fz system previously mapped as two fzs with an uncharted seismically active region between them is now recognized to be a multiple offset fz system composed of six fzs separated by short ridge segments 22 to 27 km long at chron 5a 12 ma the chile ridge propagated from the valdivia fz system northward into the nazca plate through crust formed 5 myr earlier at the pacific nazca ridge evidence for this propagation event includes the friday and crusoe troughs located at discontinuities in the magnetic anomaly sequence and interpreted as pseudofaults this propagation event led to the formation of the friday microplate which resulted in the transferal of crust from the nazca plate to the antarctic plate and in a 500 km northward stepwise migration of the pacific antarctic nazca triple junction rift propagation microplate formation microplate extinction and stepwise triple junction migration are found to occur during large scale plate motion changes and plate boundary changes in the southeast pacific

mantle subducting slab structure in the region of the 2010 m8 8 maule earthquake 30 40 s chile we present a new tomographic model of the mantle in the area of the 2010 m8 8 maule earthquake and surrounding regions increased ray coverage provided by the aftershock data allows us to image the detailed subducting slab structure in the mantle from the region of flat slab subduction north of the maule rupture to the area of overlapping rupture between the 1960 m9 5 and the 2010 m8 8 events to the south we have combined teleseismic primary and depth phase arrivals with available local arrivals to better constrain the teleseismic earthquake locations in the region which we use to conduct nested regional global tomography the new model reveals the detailed structure of the flat slab and its transition to a more moderately dipping slab in the maule region south of the maule region a steeply dipping relic slab is imaged from 200 to 1000 km depth that is distinct from the moderately dipping slab above it and from the more northerly slab at similar depths we interpret the images as revealing both horizontal and vertical tearing of the slab at 38 s to explain the imaged pattern of slab anomalies in the southern portion of the model in contrast the transition from a horizontal to moderately subducting slab in the northern portion of the model is imaged as a continuous slab bend we speculate that the tearing was most likely facilitated by a fracture zone in the downgoing plate or alternatively by a continental scale terrane boundary in the overriding plate

the mathematician paleoplate the pacific seafloor is littered with small fragments of lithosphere captured from adjacent plates by past plate boundary reorganizations one of the clearest examples of such a reorganization is documented in the mathematician seamounts region where a distinctive geomorphology and well developed magnetic anomalies are present this reorganization involved a short lived microplate between the failing mathematician ridge and a new propagating spreading center the east pacific rise it produced a transfer of a fragment of lithosphere from the farallon to the pacific plate and also created a number of landforms and magnetic patterns within and on the margins of the captured fragment these make up the mathematician paleoplate in many cases two sides of a microplate are active spreading ridges a microplate evolves into a paleoplate when dual spreading ceases and full spreading resumes at the prevailing spreading ridge we define a paleoplate as the area of the seafloor from the axis of a failed rift to the boundary of resumed full spreading it includes a fragment of captured lithosphere and the lithosphere slowly accreted to it during the period of dual spreading prior to complete abandonment of the failed rift the mathematician paleoplate has the following boundaries and components from west to east the axis of the mathematician failed rift the fragment of captured farallon plate a complex rift initiation site at the moctezuma trough a zone of slow spreading and an as yet ill defined eastern boundary where dual spreading stopped and full spreading resumed the northern boundary of the paleoplate is the rivera fracture zone its southeastern boundary a now inactive transform fault the west o gorman fracture zone in this case as well as in other more poorly documented ones relict landforms and magnetic patterns are carried on the aging lithosphere away from the spreading ridge recording a former geometry

geochemical modeling of coal mine drainage summit county ohio geochemical modeling was used to investigate downstream changes in coal mine drainage at silver creek metro park summit county ohio a simple mixing model identified the components that are undergoing conservative transport cl po43 ca2 k mg2 and na and those undergoing reactive transport do hco3 so42 fe2 mn2 and si fe2 is removed by precipitation of amorphous iron hydroxide mn2 are removed along with fe2 by adsorption onto surfaces of iron hydroxides do increases downstream due to absorption from the atmosphere the hco3 concentration increases downstream as a result of oxidation of organic material the rate of fe2 removal from the mine drainage was estimated from the linear relationship between fe 2 concentration and downstream distance to be 0 126 mg s results of this study can be used to improve the design of aerobic wetlands used to treat acid mine drainage