relief inversion triggered by subduction of an active spreading ridge evidence from glacial morphology in central patagonia this study examines the response of the central patagonian cordillera to the subduction of the active south chile ridge and the development of the asthenospheric slab window beneath patagonia since the pliocene we investigated the eastern part of the cordillera where the glacio volcanic piedmont is topographically inverted glacial morphology and associated deposits capping the top of these piedmonts reveal that during the pliocene after 3 ma glaciers were flowing to the east over a regional surface connecting the western cordillera and the palaeo piedmont n160 trending normal faulting along the western edge of the inverted plateau disrupted the geomorphic surface after 3 ma at a regional scale extensional tectonics is responsible for a relief inversion with a minimum value of 800 m the timing of extensional tectonics is contemporaneous with the subduction of the spreading axis and the development of the slab window beneath central patagonia

late cenozoic transpressional ductile deformation north of the nazca south america antarctica triple junction abstract the southern andes plate boundary zone records a protracted history of bulk transpressional deformation during the cenozoic which has been causally related to either oblique subduction or ridge collision however few structural and chronological studies of regional deformation are available to support one hypothesis or the other we address along and across strike variations in the nature and timing of plate boundary deformation to better understand the cenozoic tectonics of the southern andes two east west structural transects were mapped at puyuhuapi and aysen immediately north of the nazca south america antarctica triple junction at puyuhuapi 44 s north south striking high angle contractional and strike slip ductile shear zones developed from plutons coexist with moderately dipping dextral oblique shear zones in the wallrocks in aysen 45 46 top to the southwest oblique thrusting predominates to the west of the cenozoic magmatic arc whereas dextral strike slip shear zones develop within it new 40ar 39ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike slip oblique slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen similar ages were obtained on both high strain pelitic schists with dextral strike slip kinematics 4 4 0 3 ma laser on muscovite biotite aggregates aysen transect 45 s and on mylonitic plutonic rocks with contractional deformation 3 8 0 2 to 4 2 0 2 ma fine grained recrystallized biotite puyuhuapi transect oblique slip dextral reverse kinematics of uncertain age is documented at the canal costa shear zone 45 s and at the queulat shear zone at 44 s published dates for the undeformed protholiths suggest both shear zones are likely late miocene or pliocene coeval with contractional and strike slip shear zones farther north coeval strike slip oblique slip and contractional deformation on ductile shear zones of the southern andes suggest different degrees of along and across strike deformation partitioning of bulk transpressional deformation the long term dextral transpressional regime appears to be driven by oblique subduction the short term deformation is in turn controlled by ridge collision from 6 ma to present day this is indicated by most deformation ages and by a southward increase in the contractional component of deformation oblique slip to contractional shear zones at both western and eastern margins of the miocene belt of the patagonian batholith define a large scale pop up structure by which deeper levels of the crust have been differentially exhumed since the pliocene at a rate in excess of 1 7 mm year

geochronology stratigraphy and geochemistry of cindery tuff in pliocene hominid bearing sediments of the middle awash ethiopia cindery tuff is a subalkaline rhyolitic air fall deposit that was probably produced by a mixed magma eruption it is a distinctive datable regional isochronous marker bed within the pliocene sediments of the middle awash district and is stratigraphically situated between two new fossil hominid discoveries based on 40ar 39ar analyses of plagioclase rhyolitic glass and basaltic glass as well as fission track analyses of zircons we estimate its age to be 3 8 4 0 myr this implies that associated hominid skull fragments are at least 3 9 myr old

annual layers revealed by gpr in the subsurface of a prograding coastal barrier southwest washington u s a abstract the southwest washington coastline has experienced extremely high rates of progradation during the late holocene subsurface stratigraphy preserved because of progradation and interpreted using ground penetrating radar gpr has previously been used successfully to document coastal response to prehistoric storm and earthquake events new gpr data collected at ocean shores washington suggest that the historic stratigraphy of the coastal barrier in this area represents a higher resolution record of coastal behavior than previously thought gpr records for this location at 200 mhz reveal a series of gently sloping seaward dipping reflections with slopes similar to the modern beach and spacings on the order of 20 45 cm field evidence and model results suggest that thin 1 10 cm possibly magnetite rich heavy mineral lags or low porosity layers left by winter storms and separated by thick 20 40 cm summer progradational sequences are responsible for generating the gpr reflections these results indicate that a record of annual progradation is preserved in the subsurface of the prograding barrier and can be quantified using gpr such records of annual coastal behavior where available will be invaluable in understanding past coastal response to climatic and tectonic forcing

volcanic record of the subduction of an active spreading ridge taitao peninsula southern chile during the pliocene the taitao peninsula was the locus of the subduction of the chile active spreading ridge beneath the south american margin new field data allow us to distinguish within the taitao ophiolite a genuine ophiolitic body of undetermined age and two volcanic and volcaniclastic units one unit shows evidence for deposition in a shallow water environment and it unconformably overlies the chilean continental basement its established pliocene age demonstrates that it was deposited during the subduction of the chile ridge lavas from both units include enriched mid oceanic ridge basalts morbs together with nb depleted basalts andesites and dacites la nb ratios vary randomly from 1 to 4 in both units and are positively correlated with sio 2 content these chemical characteristics are consistent with upper crustal contamination of morb type basalts by the chilean crust as already suggested from isotopic data we consider these volcanic suites to be the result of eruptions of morb magmas that originated from the downgoing shallowly buried active spreading center and were contaminated during their rise through the chilean continental basement

global vegetation dynamics and latitudinal temperature gradients during the mid to late miocene 15 97 5 33 ma a 617 site palaeobotanical dataset for the mid to late miocene is presented this dataset is internally consistent and provides a comprehensive overview of vegetational change from 15 97 to 5 33 ma the palaeobotanical dataset has been translated into the biome4 classification scheme to enable direct comparison with climate model outputs the vegetation change throughout the langhian seravallian tortonian and messinian is discussed the data shows that the langhian which includes the end of the mid miocene climatic optimum represents a world significantly warmer than today the high northern latitudes were characterised by cool temperate forests the mid latitudes by warm temperate mixed forests the tropics by tropical evergreen broadleaf forests and antarctica by tundra shrub vegetation cool temperate mixed forest existed during the seravallian in the high northern latitudes a reduction in warm temperate mixed forests in the mid latitudes and a loss of tundra on antarctica was noted tortonian vegetation distribution indicates that further cooling had occurred since the seravallian the major changes in vegetation include the first evidence for cold taiga forest in the high northern latitudes and a further reduction of warm temperate mixed forests by the messinian this cooling trend had eliminated warm temperate mixed forests from the western usa and australia and had formed mid latitude deserts despite the cooling trend throughout the mid to late miocene the vegetation distribution of all four reconstructed stages reflect the vegetation of a world warmer than the pre industrial conditions the latitudinal distribution of bioclimatic zones suggests that the latitudinal temperature gradient for the langhian would have been significantly shallower than at present and has gradually but asymmetrically become more modern towards the end of the miocene first the southern hemisphere distribution of bioclimatic zones became more modern probably due to the climatic effects of a fully glaciated antarctica the northern hemisphere bioclimatic zone gradient continued to be shallower than modern throughout the miocene and slowly became more modern by the messinian

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

gpr in sediments advice on data collection basic processing and interpretation a good practice guide within sedimentological studies ground penetrating radar gpr is being used with increasing frequency because it yields images of the shallow subsurface that cannot be achieved by any other non destructive method the purpose of this paper is to provide an introduction to the collection processing and interpretation of gpr data so that future sedimentary studies can be improved with gpr equipment now being lightweight robust and portable proper data collection and survey design methods need to be followed in order to acquire high resolution subsurface digital data various factors are discussed including reflection profiling velocity soundings test surveys topography logistics data quality and extreme environments basic data processing and visualization are then reviewed followed by a discussion on gpr interpretation strategies including a background to radar stratigraphy for the sedimentary geologist or geomorphologist gpr offers unique data of the shallow subsurface including stratigraphy geometry architecture and structure

the influence of tectonically derived relief and climate on the extent of the last glaciation east of the patagonian ice fields argentina chile abstract two large ice fields between 46 30 and 51 30 s cover the patagonian andes the north and south patagonian ice fields are separated by the transandine depth line at 47 45 to 48 15 s canal and rio baker run through this depression the two ice fields are generally considered relics of a continuous ice cap which covered the entire patagonian andes from 39 to 52 s and extended far into the eastern foreland of the andes this assumption is not correct for the 200 km long section of the andes between lago pueyrredon lago cochrane in chile 47 15 s and lago san martin lago o higgins in chile 48 45 s the lack of a continuous ice cap extending far into the east is caused by the transandine depth line playing a crucial role in the fluvial erosion and the glacial scouring of this tectonic zone this depression formed a river system e g rio baker rio bravo and rio mayer that drains towards the west reconstruction of the maximum glacial advance of the last ice age shows that the eastern outlet glaciers of the two ice fields between lago san martin and lago pueyrredon did not drain towards the east but rather followed the general gradient of the transandine depth line in this area the eastern flank of the andes between monte san lorenzo 3770 m and sa de sangra 2155 m supported valley glaciers which were independent of the expanding ice fields only a few valley glaciers advanced towards the patagonian meseta the terminal moraines of these glaciers were erroneously interpreted as the eastern edge of a continuous ice cap north of 47 30 s the outlet glaciers of the npi advanced 200 km during the lgm and the late glacial advances nearly reached to 71 w in contrast south of 49 s glacier expansion was comparatively less the lgm is situated only 85 115 km east of the present margins of the large outlet glaciers o higgins viedma and upsala and no late glacial advance reached 72 w these considerable differences of glacier expansion were influenced by the northward migration of the westerly precipitation belt during glacial cycles there is tentative evidence that the glaciers advanced three times in the period from 14 000 to 9 500 14c years bp

accommodation space controls on incised valley sediment accumulation rates during the holocene marine transgression 0 11 ka in grays harbor a large meso tidal estuary washington usa abstract holocene deposits 0 11 ka that were drill cored 275 samples from 17 sites in the meso tidal grays harbor estuary were analyzed for 1 sediment texture and source components and 2 calibrated radiocarbon age these data were used with basin 10 m contour interval volumes to establish relations between rates of increasing basin accommodation space and sediment volume accumulation rates of increasing accommodation space 0 3 2 1 106 m3 yr 1 are based on interval volumes and a sea level curve 0 11 5 ka for the grays harbor basin total sediment accumulation rates 0 41 1 4 106 m3 yr 1 are computed for the same 10 m depth intervals 0 60 m depth msl and corresponding 10 m interval sedimentation durations 0 7 5 0 103 ka from a sediment level curve the rates of basin increasing accommodation space and total sediment accumulation are positively correlated r2 0 81 but sediment accumulation lagged behind increasing accommodation space 23 37 during early and middle holocene time 7 10 ka slowing rates of sea level rise after 8 ka permitted combined river and littoral inputs of sediment to reduce basin subtidal water depths 10 m to intertidal water depths 1 m the ratios between sand and mud components in the analyzed holocene deposits are negatively correlated to basin water depth shallowing tidal flats promoted the export of river mud out of the estuary and the import of littoral sand into the central estuary areas variable rates of increasing basin accommodation space 1 directly controlled the total sediment accumulation rate in the evolving basin and 2 indirectly controlled the relative proportions of mud and littoral sand accumulation due to changing water depths in the basin the late holocene accumulation of littoral sand 52 of total sediment volume is used to compute the net littoral sand sink 100 106 m3 in the grays harbor basin from a potential 1 0 m rise of future sea level following predicted global warming the contribution of littoral sand from adjacent barrier sand spits 20 km in length to fill the increasing accommodation space in the grays harbor estuary could result in 0 5 1 0 km of shoreline retreat following a 1 0 m rise in sea level

geological manifestations of ridge collision evidence from the golfo de penas taitao basin southern chile recent geological and geophysical studies in the southern andes adjacent to the intersection of the chile rise with the peru chile trench ant naz sam triple junction have revealed a number of features and a neogene geologic history that are unique along the pacific margin of south america this history includes 1 development of a tertiary quaternary marine basin with up to 3 km of sediment infill golfo de penas taitao basin gtb 2 disruption of the region by a series of faults with both normal and strike slip movements and 3 localization of silicic near trench volcanism and epizonal plutonism and related hydrothermal activity the northern portion of the gtb began to subside in the late miocene possibly earlier and has subsequently been deformed uplifted and exposed gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today golfo de penas subsidence and uplift have thus occurred diachronously in the region although it is unclear when subsidence began in the golfo de penas tectonic disruption of the region is likely related to the liquine ofqui fault lof a major ns trending crustal shear zone that curves westward and terminates in the golfo de penas the lof has both down to the west and right lateral offset and separates the main andean cordillera on the east from a large crustal block the chiloe block on the west we hypothesize that the gtb has developed as a pull apart basin in response to northward movement of the chiloe block along the lof we propose a dynamic model whereby a stress gradient that decreases longitudinally away from the chile rise peru chile trench intersection is set up because the youngest most buoyant oceanic lithosphere is being subducted at the triple junction the chile rise is viewed as a type of indenter which is acting to drive the chiloe block northward in front of the northward migrating triple junction this model explains the unique set of geologic features found in the region and suggests that ridge trench interactions may be an important factor in orogenesis at active continental margins

episodic development of a convergent margin inferred from structures and processes along the southern chile margin seismic reflection data acquired in the vicinity of isla mocha across the southern coast of chile image structures formed along the continental margin and reveal an episodic history of accretion nonaccretion and possibly erosion structures formed at the toe of the continental slope suggest frontal accretion of to 1 km of trench fill seismic images also reveal that a small accretionary wedge 20 30 km wide abuts the truncated continental metamorphic basement that extends seaward from beneath the shelf the small size of the accretionary wedge on three profiles examined here is not consistent with a long history of accretion with the current deformational style as current rates of frontal accretion could have accumulated all of the existing accretionary wedge in less than 1 2 m y this is a small fraction of convergence history along this margin and the current accretionary mode has not been consistently maintained in the past the isla mocha region is located between the temperate climate of central chile and the glacial climate of southern chile and climatic conditions in this region have likely fluctuated sufficiently to cause significant variation in trench sediment supply accretionary and nonaccretionary or erosional episodes are probably linked to temporal variations in trench sediment thickness as suggested by observations along the chile margin currently thick trench sediment correlates with accretion along the southern chile margin and thin trench sediment correlates with nonaccretion tectonic erosion as near the chile ridge and from the juan fernandez ridge to northern chile the isla mocha region also lies 900 1000 km north of the chile triple junction and the chile ridge lies approximately 2000 km to the west and has not yet collided and affected the margin near isla mocha this part of the precollision zone provides an excellent reference to examine the effects of chile ridge collision in the development of the chile margin the most apparent effect of subduction of the buoyant young crust of the chile ridge is a shallow trench that is nearly devoid of sediment consequently the triple junction is undergoing nonaccretion or erosion and the accretionary complex near the triple junction remains smaller than to the north or south because the current phase of rapid accretion elsewhere in the trench has bypassed the triple junction region the interplay of subduction zone processes such as trench sedimentation and ridge collision has resulted in an episodic development of the margin and produced a discontinuous record of convergence history within the accretionary wedge

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

ground penetrating radar antennae frequencies and transmitter powers compared for penetration depth resolution and reflection continuity1 twelve ground penetrating radar gpr experiments were conducted on the modern wave influenced william river delta on the southern shore of lake athabasca in northern saskatchewan canada the delta is a well sorted quartzoserich clean sand dominated water saturated geomorphic feature which provided an ideal site to test gpr penetration depths resolution and continuity of reflections were compared for different antennae frequencies 25 50 100 200 mhz and transmitter powers pulser voltage 400 v 1000 v the data show significant variations in vertical resolution from 0 15 m to 0 76 m 200 25 mhz depth of penetration from 14 m 28 m 200 25 mhz and continuity of reflections increasing the transmitter power from 400 v to 1000 v increases the depth of penetration by 5 to 14 and improves the continuity of reflections with little effect on the resolution

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

mid latitude glacial erosion hotspot related to equatorial shifts in southern westerlies glaciation has affected the shape of mountain ranges and has induced a global increase in erosion rates during the past 2 m y the observed increase in erosion rates appears to vary with latitude reaching a maximum at mid latitudes that is particularly well defined in the southern hemisphere although it is likely that climate played an important role the processes responsible for such latitudinal distribution of erosion are unclear here we exploit the meridional extent of the patagonian andes and identify an erosion hotspot at 44 s using a glacial erosion model and formally inverting the available thermochronometric and geobarometric data we show that this hotspot coincides with the location of maximum precipitation that follows the southern hemisphere westerlies during glacial periods we propose that the increased precipitation rates at 44 s led to greater ice sliding velocities and faster glacial erosion our results imply that the migration of the westerly wind belt toward the equator since 2 3 ma may have played an important role in determining the distribution of mountain erosion in the southern hemisphere

accommodation space in a high wave energy inner shelf during the holocene marine transgression correlation of onshore and offshore inner shelf deposits 0 12 ka in the columbia river littoral cell system washington and oregon usa abstract the columbia river littoral cell crlc a high wave energy littoral system extends 160 km alongshore generally north of the large columbia river and 10 15 km in across shelf distance from paleo beach backshores to about 50 m present water depths onshore drill holes 19 in number and 5 35 m in subsurface depth and offshore vibracores 33 in number and 1 5 m in subsurface depth constrain inner shelf sand grain sizes sample means 0 13 0 25 mm and heavy mineral source indicators 90 holocene columbia river sand of the inner shelf facies 90 fine sand stratigraphic correlation of the transgressive ravinement surface in onshore drill holes and in offshore seismic reflection profiles provide age constraints 0 12 ka on post ravinement inner shelf deposits using paleo sea level curves and radiocarbon dates post ravinement deposit thickness 1 50 m and long term sedimentation rates 0 4 4 4 m ka 1 are positively correlated to the cross shelf gradients 0 36 0 63 of the transgressive ravinement surface the total post ravinement fill volume of fine littoral sand 2 48 1010 m3 in the inner shelf represents about 2 07 106 m3 year 1 fine sand accumulation rate during the last 12 ka or about one third of the estimated middle to late holocene columbia river bedload or sand discharge 5 6 106 m3 year 1 to the littoral zone the fine sand accumulation in the inner shelf represents post ravinement accommodation space resulting from 1 geometry and depth of the transgressive ravinement surface 2 post ravinement sea level rise and 3 fine sand dispersal in the inner shelf by combined high wave energy and geostrophic flow down welling drift currents during major winter storms

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

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

the chronology of the last glacial maximum and deglacial events in central argentine patagonia this paper evaluates the chronology of the last glacial cycle and deglaciation in the lago pueyrredon valley of central patagonia 47 5 s argentina the valley was a major outlet of the former patagonian ice sheet and the moraines that record its fluctuations are an important proxy record of climate change in southern south america such moraines are well preserved in the lago pueyrredon valley owing in part to the semi arid environment east of the mountain front here we provide the first direct chronology for the age of the rio blanco moraine system by utilizing cosmogenic nuclide surface exposure ages boulders on the moraines give 10be exposure ages that indicate the last glacial maximum lgm maximum extent occurred by 27 25 ka subsequent advances occurred at 23 22 ka 20 18 ka and ca 18 17 ka initial deglaciation began after ca 18 17 ka and was interrupted as evidenced by the lago columna moraines up valley subsequently the outlet glaciers occupying both the lago pueyrredon basin chilean name lago cochrane and the lago buenos aires basin chilean name lago general carrera to the north rapidly retreated more than 80 km at around 16 5 15 ka the timing of the lgm maximum extent and the onset of deglaciation occurred broadly synchronously throughout patagonia deglaciation resulted in a series of interconnected glacier dammed lakes in the region that initially drained toward the atlantic ocean and later drained to the pacific ocean as a consequence of disintegrating ice in the andes

folding thrusting and development of push up structures during the miocene tectonic inversion of the austral basin southern patagonian andes 50 s abstract rift and post rift sections of the austral basin in the southern patagonian fold and thrust belt were examined in outcrop and in 2d seismic reflection sections to evaluate geometric and kinematic aspects of the extension and subsequent inversion the syn rift section is composed of dacitic lava flows of the el quemado complex jurassic the lowermost unit of the basin the coastal sandstones of the springhill formation tithonian berriasian and marine shales of the rio mayer formation berriasian albian are thought to be sag units however field data suggest that these units are also part of the syn rift successions as evidenced by extensional growth strata in the lower levels of the rio mayer formation a dacitic flow that is texturally and compositionally similar to those of the syn rift phase and that is termed here the rio guanaco dacite is interfingered with the basal strata of the rio mayer formation and has a u pb zircon crystallization age of c 141 ma berriasian the outcropping section was inverted by compression in the early miocene according to structural and isotopic data of adjacent areas producing a broad anticline with inverted and fossil extensional faults as well as newly generated thrusts at the seismic scale most of the extensional master faults present negligible reverse slip and shortening was accommodated in hanging wall push up structures this situation is interpreted as a result of the competent metamorphic basement and volcanic syn rift section inducing frictional lock up of the master faults

latest pleistocene and holocene 2 16ka sedimentation in the columbia river estuary oregon usa abstract a deep borehole drilled at warrenton oregon in the ancestral valley of the columbia river represents a geologic record that extends from 2 ka to 16 ka prior to the onset of the holocene marine transgression at 16 ka the incised columbia river valley was cut to 112 m below present sea level at this location the onset of estuarine circulation in the columbia river estuary occurred at 11 5 ka as determined by the first appearance of brackish water diatoms in sediments from borehole cores at a depth of 70 m grain size and heavy mineral analyses indicate that the columbia river tidal basin served initially 16 11 ka as a bedload bypassing conduit to the continental shelf and or the astoria canyon with the ongoing holocene transgression the columbia river tidal basin became more efficient as a river sediment sink between 11 5 and 9 0 ka after 9 0 ka the filling tidal basin again served as a sediment bypassing conduit source of sand to the coastline and continental shelf heavy mineral analyses indicate that between 16 and 11 ka the columbia river tidal basin was dominated by sediment from the metamorphic interior basins of the columbia drainage basin the dominant sediment source changed from the metamorphic interior basins of the columbia drainage basin to the cascade volcanic arc between 11 5 and 9 0 ka after 9 0 ka the tidal basin was dominated by cascade volcanic arc derived sediments the total volume of holocene sediment primarily bedload which has accumulated in the lower columbia river valley is 73 km 3 this compares to only 13 km 3 accumulation during the last 5 ka the tidal basin accumulation rate gradually increased from 0 6 million m 3 yr to slightly over 18 million m 3 yr between the depths of 112 m and 30 m 16 to 9 ka above the 30 m depth corresponding to 8 2 ka the rate of basin volume fill dramatically decreased to just over 4 million m 3 yr such a large decrease in sediment accumulation rate suggests that after 9 ka sediments were bypassing the nearly full tidal basin to the beaches and inner shelf the rates of bedload bypassing the lower columbia river valley substantially greater than 2 4 million m 3 yr supported the shoreface progradation in the littoral cell during the late holocene

tidal marsh stratigraphy sea level change and large earthquakes i a 5000 year record in washington u s a abstract many of the estuaries of the pacific northwest of the u s a and canada contain stratigraphic sequences typified by alternating peat mud couplets recent studies in this region interpret such couplets as the product of repeated large magnitude s or 9 earthquakes on the cascadia subduction zone the resultant pattern of land level movements is described by a model the earthquake deformation cycle of coseismic land subsidence followed by land uplift during interseismic strain accumulation however peat mud couplets similar to those recorded in the pacific northwest are found on other less tectonically active temperate latitude coasts such as northwest europe and the atlantic coast of the u s a where they have been interpreted as the product of non seismic coastal processes in this paper we apply the methods and scientific framework common to sea level investigations in northwest europe to a sequence of peat mud couplets recorded in the lower johns river an estuary in southern washington to provide a test of the earthquake deformation cycle stratigraphic investigations of the intertidal sediments along the lower johns river using lithological pollen diatom and foraminiferal data show evidence for eight coastal submergence events during the last 5000 years to evaluate the earthquake deformation cycle we assess the lateral extent of peat mud couplets the synchroneity of submergence the presence of tsunami deposits accompanying submergence and the suddenness and amount of submergence each submergence is shown to be accompanied by changes in coastal sedimentation broadly commensurate with those predicted by the earthquake deformation cycle demonstrating the continued intermittent seismic activity of the cascadia subduction zone throughout the mid and late holocene quantitative analyses of contemporary and fossil biostratigraphic data using twinspan and detrended correspondence analysis enable us to estimate the magnitude of submergence accompanying each peat mud couplet one event was accompanied by submergence of about 1 5 m or more four events by intermediate submergence of about 1 0 5 m and a further three events by submergence of

mesozoic cenozoic evolution of the southern patagonian andes fold and thrust belt 47 48 s influence of the rocas verdes basin inversion and onset of patagonian glaciations abstract the southern patagonian andes spa between 47 48 s are characterized by relatively high elevations 1500 4000 m and a 200 km wide largely exhumed basement domain along the backbone of the cordillera bounded by a narrow 30 km wide fold and thrust belt to the east the tectonic evolution of the spa includes widespread jurassic extension during the opening of the austral magallanes basin and the rocas verdes back arc oceanic basin rvb followed by cretaceous to cenozoic shortening although the main phases of deformation that affected this andean segment have been the focus of previous studies its finite geometry and step by step structural and kinematic evolution have not been quantitatively determined in this work we present kinematically restored cross sections showing the variable geometry trough time of the eastern basement domain and the fold and thrust belt a regional westward dipping listric decollement interpreted as formed during the jurassic extensional event was reactivated during the andean deformation constituting the main detachment surface of basement structures and the fold and thrust belt west verging faults systems are considered a consequence of inversion of north south trending extensional features as passive roof high angle west vergent faults using regional data such as field relations unconformities and existing ages to differentiate deformational stages three main phases of contractional deformation are proposed for the area starting with an aptian cenomanian phase related to inversion of the extensional depocenter in inmediate response to the closure of northernmost rvb a second miocene phase responsible for most of the shortening occurred between 18 ma and 10 ma finally the onset of glaciations and glacial erosion might have triggered out of sequence thrusting in the interior of the orogen after 7 ma

glacial interglacial trench supply variation spreading ridge subduction and feedback controls on the andean margin development at the chile triple junction area 45 48 s during the chile triple junction ctj cruise march april 1997 em12 bathymetry and seismic reflection data were collected in the vicinity of the chile triple junction 45 480s where an active spreading ridge is being subducted beneath the andean continental margin results show a continental margin development shaped by tectonic processes spanning a spectrum from subduction erosion to subduction accretion the andean continental margin and the chile trench exhibit a strong segmentation which reflects the slab segmentation and the chile triple junction migration three segments were identified along the andean continental margin the presubduction the synsubduction and the postsubduction segments from north to south both climate induced variations of the sediment supply to the trench and the tectonic reorganization at the nazca antarctica plate boundary involving postsubduction ridge jump are the two main factors that control the tectonic regime of this continental margin along the survey area we infer the succession of two different periods during the last glacial interglacial cycle a glacial period with ice rafted detrital discharges restricted to the shoreline area and low river output and a warmer period during which the andean ice cap retreat allowed the andes to be drained off during these warm periods rapid increase in trench deposition caused the margin to switch from subductionerosion or nonaccretion to subduction accretion 1 along the presubduction segment after the last deglaciation and 2 along the postsubduction segment after the interglacial episode at 130 117 ka conversely a nonaccretion or subduction erosion mode characterized the presubduction and postsubduction segments during glacial maximums the major effects of subduction of the buoyant chile ridge include a shallow trench which diverts trench sediment supply and tectonic instabilities at the nazca antarctica plate boundary we suggest that a postsubduction westward jump of the chile ridge occurred during the past 780 kyr it produced slab fragmentation and individualization of an ephemeral microplate north of the taitao fracture zone the chonos microplate in 780 kyr two episodes of subduction accretion separated by an episode of subduction erosion occurred in relation with the chonos microplate individualization and subduction the current northward migration of the triple junction along the chonos microplate south america plate boundary introduces a sharp change in the tectonic mode from subduction erosion to the north to subduction accretion to the south the data collected along the taitao ridge have revealed the complex three dimensional structure of an accretionary wedge which includes a midslope thrust sheet exhibiting the characteristics of an ophiolite the taitao ridge ophiolite no connection exists between the taitao ridge ophiolite and the bahia barrientos ophiolite cropping out onland in the taitao peninsula

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

composition age and depositional rates of shoreface deposits under barriers and beach plains of the columbia river littoral cell usa abstract the columbia river littoral cell crlc consists of four subcells totaling 160 km in length that are unique in the west coast of the united states in that they contain prograded barriers and beach plains reaching 0 5 3 km in width fig 1 the prograded beach deposits 1 5 ka in age overlie shoreface deposits 1 8 ka in age as identified in 18 ground penetrating radar profiles and sampled from 24 boreholes two competing hypotheses were initially proposed to account for the origins of these unique progradative shorelines 1 cross shore feeding by onshore wave transport of pre holocene sand from the submerged shelf and 2 longshore dispersal of nearshore sand that was supplied to the littoral system by bedload sediment discharge from the columbia river during the holocene the crlc sand forming the shoreface deposits is fine diameter 0 2 0 02 mm and rich in lithic fragments 20 40 by volume gravel and shell lag layers are uncommon in most of the crlc shoreface deposits but they show greater abundance locally near ravinement surfaces tidal inlets and in the clatsop subcell located south of the columbia river mouth gravel and granule layers increase upsection in barriers south of the columbia river and downsection in barriers at the northern end of the littoral system these trends suggest different mechanisms of shoreface sediment feeding within the four subcells however borehole samples from all four subcells show the same sand provenance i e post glacial columbia river sand which is identified by high ratios of hypersthene augite in heavy mineral fractions selected shoreface sections were dated 0 5 8 ka by ams radiocarbon analysis of articulated shell and wood fragments recovered from auger flights 3 22 m depth subsurface relatively young shoreface deposition 2 5 ka at 6 5 m elevation ngvd88 in the clatsop subcell south of the columbia river shows a net southward beach transport that fed shoreface and beachface progradation into deeper water older and deeper shoreface deposition 4 4 ka at 7 1 m elevation in the long beach subcell north of the columbia river was a result of the filling of innermost shelf accommodation space prior to beachface progradation the total volume of shoreface sand deposited under the barrier spits and beach plains of the crlc is estimated to be 6 7 km3 deposited since 6 8 ka there was a net northward transport of littoral sand 1 106 m3 year 1 along the nearshore and inner shelf subsequently some of this sand was transported onshore to feed beaches of the northernmost subcells columbia river sand was also the source for the formation of the offshore shelf wedge above the transgressive ravinement surface and for the filling of major tidal basins located north of the columbia river in summary the unique progradational history of the crlc barriers and beach plains derives from the combination of 1 longshore dispersal of fine sand discharged from the columbia river during holocene time and 2 across shore feeding of beaches at the northern end of the littoral system from fine sand carried north along the nearshore and the inner shelf

mesozoic cenozoic denudation history of the patagonian andes southern chile and its correlation to different subduction processes fission track ft analysis is applied to assess the mesozoic and cenozoic thermal and denudational history of the patagonian andes between 44 and 51 s and the geologic and geomorphic response of late cenozoic subduction of the active chile rise mid oceanic spreading center on the overriding plate seventy two ft ages from 43 samples are presented zircon ft ages indicate fast post intrusion cooling of cretaceous parts of the patagonian batholith and previously unreported miocene magmatic rocks south of 48 s metamorphic basement rocks to the east of the batholith are constrained as having been deposited and metamorphosed in the early carboniferous and late permian apatite ft data reveal initiation of accelerated cooling and denudation at ca 30 ma at the western margin of southern continental south america followed by an up to 200 km eastward migration of the locus of maximum denudation that ceased at ca 12 8 ma at the position of the present day main topographic divide this migration is proposed to be related to either coeval eastward migration of the retroarc deformation the effects of subduction erosion in the overriding plate at the peru chile trench or less likely shallowing of the angle of subduction east of the divide 3 km of denudation has occurred since the late cretaceous enhanced denudation is interpreted to be the result of increased tectonic uplift driven by a large increase in convergence rates at ca 28 26 ma that triggered orographically enhanced precipitation on the west side of the patagonian andes allowing increased erosion by fluvial incision and mass transport processes the actual process of spreading center subduction had remarkably little influence on denudation in the upper plate and indeed coincides with a slowdown in denudation

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

coseismic coastal subsidence associated with unusually wide rupture of prehistoric earthquakes on the kamchatka subduction zone a record in buried erosional scarps and tsunami deposits abstract the prograding strand plain of avachinsky bay kamchatka russia along the highly active kamchatka subduction zone exhibits geological evidence buried erosional scarps for coseismic subsidence only three times in the last four millennia the last event about 1200 years ago this same coast has a historical record since a d 1737 of five subduction zone earthquakes with large tsunami runup 5 m the last of which was the 1952 mw 9 kamchatka earthquake and a geological record of more than 30 large tsunamis in the last 4000 years this rarity of buried scarps relative to large earthquakes contrasts with the cascadia strand plain in sw washington state where most or all large events are represented by buried scarps a strong factor in the amplitude and sign of coseismic deformation is distance from the seaward edge of a subduction zone the trench the avachinsky bay coastline is 180 200 km from the trench with 25 slab dip requiring unusually wide ruptures to generate significant coseismic subsidence this coastal zone is undergoing net subsidence approximately equivalent to the total of the three coseismic subsidence events generating a sequence of beach ridges that increase in elevation seaward each of the three unusual coseismic subsidence events comprises a an earthquake whose deformation field caused b onshore coseismic subsidence thus local sea level rise and c sufficient deformation offshore to produce a large tsunami a b c followed by d a period of coastal erosion and shoreline retreat leaving e an erosional beach scarp that was f subsequently buried once progradation resumed we identified dated and correlated the scarps and tsunami deposits from these events with several field methods including trenching tephrostratigraphy and ground penetrating radar the scarps were correlated over an alongshore distance of 50 70 km the most recent event event 1 occurred 800 cal ad 1100 1250 14 years bp event 2 600 cal bc 2400 2450 14 years bp and event 3 1700 cal bc 3300 3500 14 years bp we developed methods for quantifying subsidence coastal erosion and tsunami size for each of these events all three retain evidence of 0 4 1 2 m of coseismic subsidence coastal erosion in the case of event 1 averaged more than 100 m all three event tsunamis were amongst the largest in the last 4000 years

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

latest miocene benthic δ 18 o changes global ice volume sea level and the messinian salinity crisis oxygen isotope evidence indicates high but variable 18o values in benthic foraminiferal calcite during the latest miocene and earliest pliocene these high values may represent increases in global ice volume and associated sea level fall the 18o record resembles glacial interglacial cycles but with only one third the amplitude of the late pleistocene signal this variability may reflect instability in the antarctic ice sheet and palaeomagnetic correlation points to an isotopic event coinciding with the isolation and desiccation of the mediterranean basin during the latest messinian

tectonics in the eastern central patagonian cordillera 45 30 47 30 s a subhorizontal angular erosional unconformity that separates subhorizontal upper palaeocene lower eocene fluvial deposits of the ligorio marquez formation from folded and faulted lower cretaceous tuffs and sedimentary rocks in the area south of chile chico in the northern patagonian fold and thrust belt of southern chile 46 45 s indicates tectonism prior to the previously recognized post early miocene contractional event similar observations between latitudes 45 30 and 47 30 s in argentina support the existence of a pre palaeocene eocene contractional tectonic phase and locally of a late early cretaceous event this indicates that the northern patagonian fold and thrust belt includes structures earlier than previously thought probably masked by the later neogene ones the timing of this earlier tectonism is not precisely known beyond being pre upper palaeocene lower eocene and post barremian one possibility is that it may be the earlier stages of a progressive deformation that in its later stages is represented by the post lower miocene tectonism previously documented in the region in this case it may represent the latest cretaceous palaeocene major contractional tectonism and uplift well documented along the southernmost patagonian cordillera south of latitude 51 s the upper palaeocene lower eocene ligorio marquez formation may represent the first accumulation in a foreland wedge top basin developed east of the orogenic front subsequently and probably during the middle miocene these beds were folded and faulted during the tectonism that generated the foreland basin where the fluvial deposits of the galera formation accumulated different speculative scenarios for the origin of this compression can be invoked depending on its age

subduction erosion related to spreading ridge subduction taitao peninsula chile margin triple junction area during the pliocene subduction of the chile ridge beneath the south american margin was coeval with the emplacement of the cabo raper pluton located at the seaward edge of the taitao peninsula the chemical characteristics of the cabo raper pluton combined with the available tectonic data allow us to reconstruct the paleogeometry of the chile margin 3 4 2 m y ago when compared to the modern configuration the volume of material removed by subduction erosion can be estimated quantitatively from 3 4 2 to 1 5 1 6 ma 625 km 3 of rock were removed along each kilometre of margin along the taitao peninsula transect this leads to a conservative subduction erosion rate of 231 443 km 3 km 1 m y 1 significantly higher than those calculated along the japan and peru convergent margins it is proposed that subduction of the chile ridge contributes fluids in addition to the subducted sediment resulting in a higher rate of subduction erosion

pliocene near trench magmatism in southern chile a possible manifestation of ridge collision eight shallow level intrusions and an ophiolite complex have recently been discovered in a remote region along the pacific coast of southern chile known as the taitao peninsula the magmatic rocks are found only 10 to 15 km from the buried extension of the peru chile trench the ophiolite body includes serpentinized ultramaflc rocks gabbro dikes and an interbedded sequence of volcanic and marine sedimentary rocks more regionally distributed granodiorite plutons and porphyritic stocks and sills intrude pre late jurassic basement as well as tertiary marine sequences of the south american forearc k ar ages of 3 5 to 4 0 ma of the ophiolite and other intrusions suggest that these rocks represent a short lived pulse of pliocene magmatism directly seaward of the taitao peninsula a segment of the chile rise bounded by the tres monies and taitao fracture zones collided with the chilean forearc and was subducted about 2 5 to 4 0 ma because of this good spatial and temporal correlation the magmatic activity of the taitao peninsula is likely to be a direct result of ridge subduction

simple models for late holocene and present day patagonian glacier fluctuations and predictions of a geodetically detectable isostatic response summary the late holocene glacial moraine chronology in the southernmost south american andes includes four 14c dated neoglacial advances and retreats these are used as proxy information to characterize mass fluctuation of the patagonian icefields during the last 5000 years modelled ice loads force a phase lagged viscoelastic gravitational deformation of the solid earth the ancient glacier fluctuations may therefore drive present day crustal motion even in the absence of present day ice mass imbalance numerical models show that such rates of present day uplift and subsidence are larger than those driven by the viscous memory of late pleistocene deglaciation both spherical and flat earth models are employed the latter being used to study exhaustively the evects of glacial load history on the predicted vertical crustal velocity recent assessment of net mass balance from 1944 to 1985 indicates that the southern patagonian icefield has significantly deteriorated due to snout retreat and thinning volume loss rates are estimated at about 3 4 9 3 km3 yr 1 the predicted vertical isostatic response to this recession and to the modelled holocene neoglaciations is at a marginally detectable level 1m m yr 1 if the mantle asthenosphere beneath patagonia has a viscosity of about 1021 pa s however for reduced mantle viscosities the younger holocene glacial load histories predict larger signatures in fact if the viscosity is about 2 1020 pa s or lower then geodetically detectable vertical motion may be driven by a regional little ice age lia 1400 1750 ad glacier advance and subsequent 20th century retreat although this value for mantle viscosity is lower than thought typical of continental shield mantle 1021 pa s it is consistent with inversions for post seismic relaxation time constants in island arc environments and in regions with significant neogene continental tectonism in the viscosity regime of 5 1018 2 1019 pa s the predicted rates of vertical crustal motion are similar to those presently occurring in fennoscandia and hudson bay 5 10 mm yr 1 geodetic data may be sensitive to the time integrated growth and subsequent retreat of patagonian glacier ice mass over the last 600 years

magellan strait part of a neogene rift system the magellan strait joins the atlantic and pacific oceans separating tierra del fuego from southernmost continental south america the strait cuts both the andean cordillera and the magellan foreland basin other arms of the sea intrude the axial zone of the magellan basin these depressions have long been interpreted as glacial valleys on the basis of landsat images digital topography and field data we interpret the depressions as rifts or half rifts in general active rifts developed within foreland basins are unusual but in patagonia they are consistent with regional deformation and its plate tectonic setting during the neogene

timing and origin of deformation along the patagonian fold and thrust belt the andean orogeny in the patagonian cordillera of southern south america reflects the consequences of the mesozoic and cenozoic subduction of an oceanic plate beneath the south american continental margin the geological evolution of the region has been influenced by the eocene collision and subduction of the farallon aluk ridge and the miocene recent subduction of the chile ridge another aspect of plate interaction during this period was two intervals of rapid plate convergence one at 50 42 ma and the other at 25 10 ma between the south american and the oceanic plates it has been proposed that the collision of the chile ridge with the trench was responsible for the development at least in part of the patagonian fold and thrust belt this belt extends for more than 1000 km along the eastern foothills of the southern andes between 46 and 54 s along the southwestern rim of the austral basin the interpretation of a link between subduction of the ridge and formation of the fold and thrust belt is based on assumed time coincidences between contractional tectonism and the collision of ridge segments during middle and late miocene times the main tertiary contractional events in the patagonian fold and thrust belt took place during latest cretaceous palaeocene eocene and during miocene times although the timing of deformation is still poorly constrained the evidence currently available suggests that there is little or no relationship between the timing of the fold and thrust belt and the collision of ridge segments most if not all of the contractional tectonism pre dated the latest episodes of ridge collision collision of a ridge crest with the continental margin has been active for the past 14 to 15 million years contrary to the suggestion of a relationship between ridge subduction and compression the main result of this collision has been fast uplift and extensional tectonism the initiation of the patagonian fold and thrust belt in latest cretaceous or early tertiary times coincided with a fundamental change in the tectonic evolution of the austral basin throughout the cretaceous most of this basin subsided as a broad backarc continental shelf only in latest cretaceous times and coinciding with the initiation of the fold and thrust belt the basin underwent a transition to a retro arc foreland basin this change to an asymmetrically subsiding foreland basin with an associated foreland fold and thrust belt was related to uplift of the andean orogenic belt in the west

melting of a subducting oceanic crust from u th disequilibria in austral andean lavas understanding crustal genesis at convergent plate boundaries is important for determining mass transfer between different geochemical reservoirs in the earth s mantle and for deciphering the long term growth of the continental crust most arc magmas are thought to be generated from fluid induced melting of the mantle wedge above slabs of subducting oceanic crust1 such magmas frequently display 238u enrichments or radioactive equilibrium2 3 between 238u and its radiogenic product 230th but where a young and hot oceanic crust is being subducted it may itself partially melt and produce calc alkaline andesites and dacites termed adakites4 here we report a uniform excess of 230th over 238u but variable th isotope ratios in young adakites from the andean austral volcanic zone south of the triple junction where the chile ridge subducts beneath south america we show that these results are compatible with the adakites having been formed by approximately 20 equilibrium melting due to amphibole decomposition in a heterogeneous5 oceanic crust moreover both the degree of melting of the oceanic crust and its thermal structure appear to be uniform under most of the andean austral volcanic zone such partial melting of subducted oceanic slabs may have occurred throughout the earth s history where young oceanic plates were subducted

archaeological evidence for a former bay at seaside oregon cultural shell middens a common feature of coastal landscapes preserve a record of past human use of coastal resources and often provide important paleoenvironmental information molluscan remains from two shell midden sites located 0 7 km from the modern pacific coast at seaside oregon suggest the former presence of a small sheltered bay at this locality the modern sand beaches in the seaside area stretching for 30 km southward from the mouth of the columbia river are exposed to high energy surf where razor clams siliqua patula are today the only common bivalve by contrast over 90 of molluscan remains by weight recovered from the archaeological sites represent species that favor sheltered water geologic studies in the seaside area have shown that the beach prograded some 2 km over the last 4 millennia the midden evidence suggests that the progradation was accompanied by the infilling of a former bay

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

jurassic to miocene k ar dates from eastern central patagonian cordillera plutons chile 45 48 s thirty nine k ar and one ar ar radiometric dates from the eastern central meso cenozoic patagonian batholith and eastern satellite plutons of the aysen region of southern chile between latitudes 45 and 48 s combined with previous dating of seven plutons have yielded six age groups 1 middle to late jurassic 2 early cretaceous 3 mid cretaceous 4 late cretaceous 5 oligocene and 6 miocene in general the cretaceous and younger ages correspond to previous reported ages for other parts of the main batholith but for the satellite plutons to the east show a wider age spectrum than the previously accepted late miocene dates these results indicate a relatively continuous late jurassic to mid cretaceous plutonism known to have been coeval with volcanic activity followed by intermittent magmatism biotite k ar dates of c 143 151 and 106 109 ma from cataclastic granitoids may be marking the time of deformation a review of all radiometric data on magmatic rocks from the region between 45 and 48 s in chile shows a gap in palaeocene ages that may correlate with a period of low angle flat slab subduction between 65 50 ma

andean foothills structures in northern magallanes basin argentina the foothills of the northern segment of the magallanes basin between 47 and 49 s latitudes record the development of a complex fold and thrust belt in the patagonian cordillera during the late miocene upper paleozoic sedimentary and metasedimentary rocks and thick sequences of lower cretaceous deposits with well defined source and reservoir rocks are deformed by the andean events the structure is characterized by a triangle zone and a series of related underthrusts which define a nonemergent backthrust system at the mountain front differences in the degree of deformation from south to north reveal the evolution of the triangle zone the stratigraphy of the molasse deposits constrains the beginning of the deformation at least to the early tertiary although the final structure was developed during the late miocene these deformed deposits are covered by extensive late miocene horizontal retro arc basalts analysis of the evolving subduction zone located west of and beneath the study area shows that the formation of a subsequent volcanic gap in the magmatic arc and the development of extensive retro arc basalts are related to the collision of a segment of the chile ridge during the late miocene the final deformation and uplift of the patagonian cordillera fold and thrust belt at these latitudes are closely linked to this collision

dating of late holocene beach shoreline positions by regional correlation of coseismic retreat events in the columbia river littoral cell usa abstract beaches and barrier spits of the columbia river littoral cell crlc 160 km in length experienced substantial progradation 0 5 2 5 km in width during the late holocene the accreted beach plains and barriers preserved evidence of episodic catastrophic retreat that was forced by coseismic subsidence the abrupt subsidence events 0 5 1 5 m submergence are associated with slip on the underlying cascadia megathrust with a mean recurrence interval of 500 yr ground penetrating radar gpr profiles taken across the barriers and beach plains show high angle beach retreat scarps 3 15 m vertical relief and foredune ridge slump scarps 5 15 m vertical relief the upper shoreface retreat features are correlated throughout the littoral cell by radiocarbon age position relative to abandoned foredune ridges and distinct sequences of large and small scarps particularly large retreat scarps 10 15 m in height are associated with the first oldest and fourth oldest prehistoric scarps a and d respectively a maximum of 10 retreat scarps in the long beach subcell date from 216 211 calrcybp to 4756 114 calrcybp the youngest seven scarps a g are correlated between all four subcells showing a regional response to the coseismic strain cycles backshore disturbance features 1 3 m vertical relief are observed in the gpr profiles but they are not identified here due to their discontinuities between adjacent profiles at 5 km longshore spacing the seven regional scarps a g are directly correlated to seven coseismic subsidence events reported to date between 0 3 and 3 2 ka the correlation of scarps to earthquake events provides high resolution proxy ages for shoreline positions in late holocene time the proxy shoreline positions are dated at 0 3 1 1 1 3 1 7 2 5 2 8 and 3 2 ka based on buried wetland sequences from adjacent tidal basins the paleoshoreline dating demonstrates that the onset of net shoreface progradation between 4 7 and 0 3 ka was delayed with increasing distance 0 100 km from the columbia river sand source the history of beach progradation reflects the sequential filling of bay and shoreface accommodation spaces located down drift of the columbia river mouth these late holocene shoreline changes portend future redistributions of existing sand within the littoral system as the limited sand reserves continue to migrate away from the columbia river mouth source

large scale coastal change in the columbia river littoral cell an overview abstract this overview introduces large scale coastal change in the columbia river littoral cell crlc covering 165 km of the southwest washington and northwest oregon coasts the littoral cell is made up of wide low sloping dissipative beaches broad coastal dunes and barrier plains three large estuaries and is bounded by rocky headlands the beaches and inner shelf are composed of fine grained sand from the columbia river and are exposed to a high energy winter wave climate throughout the holocene the crlc has undergone large fluctuations in shoreline change trends responding to a variety of coastal change drivers including changing rates of sea level rise infrequent yet catastrophic co seismic subsidence events a large regional sediment supply inter annual climatic fluctuations el nino cycles seasonally varying wave climate and numerous anthropogenic influences human influences on the crlc include construction of over 200 dams in the columbia river drainage basin dredging of navigation channels removing sand to upland sites and offshore deep water sites and construction of large inlet jetties at the entrances to the columbia river and grays harbor the construction of these massive entrance jetties at the end of the 19th century has been the dominant driver of coastal change through most of the littoral cell over the last hundred years presently some beaches in the littoral cell are eroding in response to nearshore sediment deficits resulting from a ebb jets of the confined entrances pushing the previously large shallow ebb tidal deltas offshore into deeper water and b waves dispersing the nearshore delta flanks initially onshore and then alongshore away from the jetties this overview describes 1 the motivation for developing a system wide understanding of sediment dynamics in the littoral cell at multiple time and space scales 2 the formation and approach of the southwest washington coastal erosion study and 3 an introduction to the papers in this special issue

morphodynamics of prograding beaches a synthesis of seasonal to century scale observations of the columbia river littoral cell abstract findings from nearly two decades of research focused on the columbia river littoral cell crlc a set of rapidly prograding coastal barriers and strand plains in the u s pacific northwest are synthesized to investigate the morphodynamics associated with prograding beaches due to a large sediment supply from the columbia river the crlc is the only extensive stretch of shoreline on the u s west coast to have advanced significantly seaward during the late holocene since the last cascadia subduction zone csz earthquake in 1700 with associated co seismic subsidence and tsunami much of the crlc has prograded hundreds of meters however the rates of progradation and the processes most responsible for sediment accumulation vary depending on time scale and the morphological unit in question remarkably the 20th and early 21st century shoreline change rates were more than double the late prehistoric rates that include recovery from the last major csz event most likely due to an increase in sediment supply resulting from inlet jetty construction in some locations detailed beach morphology monitoring reveals that at interannual to decadal scale the upper shoreface aggraded about 2 cm yr subtidal sandbars migrated offshore and decayed while intertidal bars migrated onshore and welded to the shoreline the shoreline prograded about 4 m yr and 1 to 2 new foredune ridges were generated a detailed meso scale sediment budget analysis in one location within the littoral cell shows that approximately 100 m3 m yr accumulated between 12 m seaward limit of data and 9 m crest of landward most foredune gradients in alongshore sediment transport net onshore directed cross shore sediment transport within the surf zone and cross shore feeding from a shoreface out of equilibrium with forcing conditions are each partially responsible for the significant rates of sediment supplied to the beaches and dunes of the crlc during the observational period direct observations of beach progradation at seasonal to decadal scale are put in context of measured or inferred changes over time scales of decades to centuries

seismic responses to ridge transform subduction chile triple junction the first local seismic network established in the remote region bordering the chile triple junction monitored earthquakes at a rate of three per day with magnitudes m 0 to m 4 far smaller than any recorded previously from that region focal mechanisms were obtained by the adaptation of the relative amplitude method to local earthquakes epicentral locations define a lineament corresponding to the active segment of a subducted transform fault and events corresponding to the predicted position of a subducted ridge show extensional mechanisms with the exclusion of thrusting and trench parallel mechanisms these data suggest that oceanic spreading and seismic activity continue in the subducted ridge transform system extension in the slab influences the overlying crust giving rise to dextral transtension over the past 10 m y the locus of ridge subduction has migrated northward we postulate that the pattern through time of extension and basin development in the overlying crust has mirrored this

sedimentology pollen spores and diatoms of a 148 m deep miocene drill hole from oku lake east central nigeria abstract the oku lake series in nigeria to the ne of makurdi comprise about 148 m of continental deposits resting on folded cretaceous marine sediments of the benue trough an oblique core 192m long includes four different lithologic units from bottom to top 1 clays and sandstones weathered at the top 2 grey to black massive and laminated muds with alternating oxidized levels 3 green to white muds covered with 4 reddish mixed clay and sand these units correpond to 4 environmental stages for the oku lake a a shallow water lake subaerially exposed during the development of a thick paleosol b an open lacustrine environment influenced by low episodic water levels c littoral margin and flats d an emergent phase the palynological sequence which appears continuous through the black muds is interpreted as a transition from an open lacustrine environment to a thicker boggy forested area and then to a poorly drained grassland vegetation the diatom floras show parallel trends and indicate that the age could range from late miocene to early pliocene however on the basis of the climatic interpretation proposed a late miocene age is adopted

evidence for eight great earthquake subsidence events detected with ground penetrating radar willapa barrier washington a new approach to detect holocene subduction zone earthquakes combines the results from ground penetrating radar gpr vibracores and accelerator mass spectrometry ams dates from a barrier spit located west of willapa bay southwest washington gpr data show a 10 m thick facies of beach sand within which we identify and vibracores confirm beach parallel wave eroded buried scarps mantled with multiple beds of magnetite the eight gpr detected buried scarps are interpreted to be eroded by minor transgressions caused by instantaneous barrier subsidence during earthquakes associated with the juan de fuca plate subducting under the north american plate of these scarps four have been ams dated at 300 1110 2540 and 4250 radiocarbon yr b p no datable material has yet been found for the other four radar detected scarps but we interpolate and extrapolate dates of 1800 3400 5000 and 5800 yr b p

seismic noise tomography in the chile ridge subduction region summary we used cross correlation of ambient seismic noise recorded in the chile triple junction ctj region to estimate interstation surface wave time domain green s functions and then inverted traveltimes to obtain crustal surface wave velocity models interstation distances within the chile ridge subduction project crsp temporary seismic network ranged from 40 to 100 km we selected 365 d and cross correlated and stacked 24 hr of vertical component data at 38 stations pairs resulting in nominally 703 traveltimes along assumed straight interstation paths velocities in 2 d cells of 30 km 30 km were calculated using a linear least squares inversion of the rayleigh wave group velocity traveltimes furthermore we performed a rayleigh wave group velocity dispersion analysis to estimate the sensitivity of different period waves at depth and to calculate a 3 d shear velocity model of the patagonian crust the process was applied to cross correlation pairs determined in two period bands 5 10 s corresponding to shallow crustal velocities down to approximately 10 km depth and 10 20 s for velocities down to around 20 km our results show that cell velocities correlate well with known geological features we find high crustal velocities where the patagonian batholith outcrops or is likely present at depth and low velocities correlate with the active volcanic arc of the southern volcanic zone and the subducted chile ridge in taitao peninsula where thermal activity of hot springs is present high velocities in the mountainous portions of the southeastern study area appear to correlate with outcropping older metamorphic units low velocity in the east correlate with sequences of volcaniclastic deposits

petrogenesis of the eocene and mio pliocene alkaline basaltic magmatism in meseta chile chico southern patagonia chile evidence for the participation of two slab windows the meseta chile chico mcc 46 48s is the westernmost exposure of eocene lower basaltic sequence lbs 55 40 ma k ar ages and mio pliocene upper basaltic sequence ubs 16 4 ma k ar ages flood basalt volcanism in patagonia the mcc is located south of the lago general carrera buenos aires lgcba southeast from the present day chile triple junction ctj east of the actual volcanic gap between southern south volcanic zone and austral volcanic zone ssvz and avz respectively and just above the inferred location of the south chile ridge segment subducted at 6 ma scr 1 erupted products consist of mainly ne normative olivine basalt with minor hy normative tholeiites basalt trachybasalt and basanite mcc lavas are alkaline 42 7 53 1 wt sio2 3 8 wt na2o k2o and relatively primitive ni 133 360 ppm cr 161 193 ppm co 35 72 ppm 4 16 5 mgo wt they have a marked oib like signature as shown by their isotopic compositions 87 sr 86 sro 0 70311 0 70414 and qnd 4 7 5 1 and their incompatible trace elements ratios ba la 10 20 la nb 0 46 1 09 ce pb 15 52 27 5 sr lab25 reflecting deep mantle origin ubs primitive lavas have characteristics similar to those of the eocene lbs basalts while ubs intermediate lavas show geochemical imprints la nbn1 sr lan25 low ce pb nb u compatible with contamination by arc slab derived and or crustal components we propose that the genesis and extrusion of

spatial distribution of cones and satellite detected lineaments in the pali aike volcanic field southernmost patagonia insights into the tectonic setting of a neogene rift system abstract the relationships between the distribution and morphometric features of eruptive structures scoria and spatter cones maar tuff rings and the fracture network were investigated in the pliocene quaternary pali aike volcanic field southernmost patagonia argentina chile the alkali basaltic basanitic magmas which erupted in this area have nearly primary magma compositions and often bear mantle xenoliths hence magma ascent from deep seated reservoirs was probably very fast with no significant stagnation at crustal levels field surveys and satellite image analysis led to the identification of up to 467 eruptive structures and four main nw se ne sw e w and n s fracture systems the spatial distribution of eruptive cones and fractures was investigated through the computation of power law exponents df for self similar clustering the self similarity of cones and fractures was defined between lower and upper cut offs which were in turn related to the thickness of the fractured mechanical layer the fractal character of cones and fracture distribution clustering in the pali aike volcanic field area was thus correlated with crustal thickness the self similarity of fractures was used to establish the relative chronology of the detected fracture systems the self similar clustering exponent is highest in the e w and nw se fracture systems df 1 78 and 1 77 respectively and lowest in the n s system df 1 65 the self similar clustering of eruptive structures is well defined df 1 45 the intense volcano tectonic activity in the pali aike area marks a major pliocene quaternary phase in the development of the magellan neogene rift system

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

stratigraphic and structural evolution of the tertiary cosmelli basin and its relationship to the chile triple junction at the taitao peninsula in southern chile 46 5 s an active mid ocean ridge is being subducted under the south american continent continental crust south of the current triple junction has experienced subduction of three ridge segments since the mid miocene a series of mid late tertiary sedimentary basins lies inboard of the triple junction the cosmelli basin lies in the region which has experienced the earlier ridge subduction events within the cosmelli basin fill abrupt facies dislocations give clear evidence of marked base level changes four sequence boundaries are delineated with an upward increase in degree of facies dislocation which is believed to reflect increasing magnitudes of base level fall the lower part of the basin fill is folded and then thrust eastward as a series of imbricate slices while the overlying greater thickness of fluvial sediments is only gently tilted westwards this geometry indicates that the early basin fill was deforming due to contractional tectonics while the later basin fill was being deposited these processes controlled accommodation space and thus stratigraphic architecture during much of the basin life this complex basin history plus the presence of basaltic sheet intrusions may reflect the underlying slab history as successive ridge segments were subducted

patagonian broken foreland and related synorogenic rifting the origin of the chubut group basin abstract the central patagonia is characterized by prominent continental deposits that belong to the cretaceous chubut group basin whose tectonic setting remains controversial it has been interpreted as a foreland basin an extensional basin and even as a sag stage of a rift event related deposits outcrop as part of the patagonian broken foreland east of the patagonian andes particulary the san bernardo fold and thrust belt between 42 and 48 s as the main sector of the broken foreland constitutes a nnw trending intraplate belt located 450 km away from the trench coincidentally at these latitudes 1200 km east of the trench intraplate deformation has caused tectonic inversion of mesozoic basins presently located at the atlantic offshore area additionally at the san jorge gulf area this orogenic system interferes with a conspicuous e w extensional system transversally disposed to the andean strike mainly active between the late early cretaceous and late paleocene we carried out an integrated analysis from structural 2 d and 3 d seismic data at the san bernardo ftb and the adjacent rio mayo basin studying sections of the chubut group basin to unravel the different evolutionary stages thus surface and subsurface evidence of syn compressional deposition indicate the development of a broken foreland basin related to the formation of the san bernardo ftb suggesting an initial growth during the late early cretaceous and probably up to the late paleocene our results imply that the cretaceous compression acted in concert with transversal foreland extension hence we propose a syncontractional rift reactivation of a potential lithospheric anisotropy triggered by regional contraction finally the origin of compression during this time is discussed in relation to multiple processes that go from high convergence between plates and trenchward motion of south america collision of mid ocean ridges to potential dip changes in the subducted slab as suggested by arc expansion

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

seismic anisotropy above and below the subducting nazca lithosphere in southern south america 1 the goal of this study is to better constrain anisotropy and mantle flow above and below the nazca slab from 28 s to 42 s through modeling of shear wave splitting in local s sks and skks sk k s phases comparisons of local s splitting times and path lengths in the slab mantle wedge and upper plate indicate that splitting times for arc and back arc stations are consistent with anisotropy in the mantle wedge but long slab paths to fore arc stations imply that slab anisotropy is also significant sk k s shear wave splitting observations and models for sub slab anisotropy show that significant anisotropy is present below the slab and that the orientation of sub slab anisotropy sometimes differs from anisotropy above the slab anisotropy both above the slab and below the slab in the south american subduction zone is consistent with mantle flow that is driven by a combination of entrainment with downgoing slab motion and flow complexity related to variations in slab shape and slab rollback

new geochemical constraints for the origin of ridge subduction related plutonic and volcanic suites from the chile triple junction taitao peninsula and site 862 leg odp141 on the taitao ridge abstract several features of neogene and quaternary magmatism in the region south of the present day chile triple junction ctj at 46 12 s are directly related to the migration of the triple junction due to the obliquity of the ridge orientation with respect to the subduction front the triple junction migrated from south to north during the last 14 ma the taitao peninsula the westernmost promontory of the chile coast and the taitao ridge a submarine promontory north of the taitao peninsula provide the most complete collection of ridge subduction related magmatic products in the region the emplacement of near trench volcanics the intrusion of a variety of plutonic rocks and the related hydrothermal activity at these two sites have been interpreted as resulting from magma interactions between subducted ridge segments of the chile spreading centre and the continental crust we present new field observations and geochemical data that help to better constrain the problem of the sources and evolution of the taitao magmas the new geochemical data were obtained on samples collected from the taitao peninsula during a field expedition in 1995 and from samples of the taitao ridge during leg odp141 site 862 which have been re sampled in 1996 by one of us selected major and trace element compositions of 20 volcanic rocks from the taitao ridge are discussed together with 53 analyses from different rock types from the taitao peninsula including 24 unpublished analyses nd and sr isotopic compositions were obtained from 5 whole rocks and separated minerals of the taitao peninsula together with the oxygen isotope composition of four separated clinopyroxenes six main magmatic types are identified 1 n type morb 2 e type morb 3 lree depleted morb showing some trace element features typical of arc basalts 4 moderately nb depleted e morb 5 calc alkaline andesites dacites and rhyolites and 6 andesites and dacites with adakitic signature chemical similarities exist between some forearc magmas of the taitao ridge and the taitao peninsula and magmas emplaced at the chile active spreading ridge one important result based on isotope data is that the lavas emplaced over the continental crust taitao peninsula did not originate from melting of continental crust nor from extensive assimilation of such a crust by mantle derived magmas the likely source of these basalts could be the hot convective oceanic mantle of the southern chile spreading ridge buried at moderate depth 10 30 km

sea level rise and anthropogenic activities recorded in the late pleistocene holocene sedimentary infill of the guadiana estuary sw iberia abstract this study reviews data on sea level rise during the last 13000 yr cal bp 13 kyr as recorded in the estuarine sediments of the guadiana river se portugal sw spain we combined new data from a 63 m long borehole drilled through the entire postglacial sedimentary sequence with information on five previously studied cores by integrating sedimentological geochemical and palaeontological proxies we were able to make a palaeoenvironmental reconstruction of the guadiana terminal palaeovalley during the last 13 kyr and propose a curve of sea level rise for the sw iberian atlantic margin our foraminifera based palaeoecological reconstruction anchored to a 14 c age model reveals rapid sea level rise from 13 kyr interrupted during the younger dryas and resuming ca 11 5 kyr the pace of marine transgression slackened ca 7 5 kyr and since then has progressed upwards at a rate of 1 2 mm yr 1 holocene anthropocene sediments from two boreholes were also analysed to assess the timing levels and sources of trace metals produced by acid mine drainage from the iberian pyrite belt study of metal aluminium ratios through the profiles allowed background metal concentrations to be estimated from lithostratigraphic units older than ca 5 kyr i e unaffected by anthropogenic activities human activities are especially evident from 4 5 kyr the beginning of the copper age with anthropogenic sources of metal fluxes prevailing over natural sources especially pb co ni and mn and to a lesser extent zn cu and ni mining activities became particularly intensive between the late bronze age and the roman period 3 1 5 kyr when the highest metal enrichment factors were recorded ef pb 2 ef cd 10 ef cr 2 ef cu 3 ef zn 1 4 this study reveals the utility of postglacial sedimentary records for reconstructing historical changes in regional water sediment quality and separating natural and anthropogenic sources of geochemical contaminants

heat flow and lithospheric thickness analysis in the patagonian asthenospheric windows southern south america abstract the lithosphere asthenosphere boundary lab is a first order discontinuity essential to understand the earth composition and evolution however its detection has been critical and several regions still lack of coverage southern patagonia in southern most south america is one such area which has been affected by the subduction of a seismic oceanic ridge south chile ridge and formation of an extensive slab window since 12 ma we calculate the lab position by defining the thermal lithospheric thickness of the southernmost patagonia using the thermal conductivity equation to estimate the surface heat flow we used data from seventy five hydrocarbon wells of two from the most productive petroleum basins in argentina the golfo de san jorge and magallanes austral basins our results show the highest heat flow values in the southernmost region over the austral basin and core of the slab window 70 90 mw m2 these values are twice as hot as global average to the north over de golfo de san jorge basin and northern margin of the slab window heat flows are normal 50 60 mw m2 these thermal contrasts from south to north agree with the kinematic reconstructions of the chile ridge and northward widening of the slab window these heat flows values evidence an attenuated lithospheres which thickens eastward from between 25 km to the west trench region to 50 km to the east foreland region

exploring linkages between coastal progradation rates and the el nino southern oscillation southwest washington usa 1 climate oscillations such as the el nino southern oscillation enso affect storm tracks wave climate precipitation and sea level in the u s pacific northwest the impacts of these changes on coastal behavior have not been investigated in detail beyond the study of recent el nino events largely because existing historical records of coastal behavior are not of sufficient resolution to study annual responses to climatic forcing we compare a newly developed annual record of coastal progradation for a location on the washington coast generated using high resolution subsurface ground penetrating radar gpr with enso indices this analysis reveals higher rates of seaward coastal growth following the warm el nino enso phase and lower rates of coastal growth following the cold la nina enso phase the observed relationship between enso and progradation although weak is hypothesized to result from differences in sediment transport patterns and beach recovery rates following el nino and la nina events

paleoceanographic implications of miocene deep sea hiatuses miocene paleoceanographic evolution exhibits major changes resulting from the opening and closing of passages the subsequent changes in oceanic circulation and development of major antarctic glaciation the consequences and timing of these events can be observed in variations in the distribution of deep sea hiatuses sedimentation patterns and biogeographic distribution of planktic organisms the opening of the drake passage in the latest oligocene to early miocene 25 20 ma resulted in the establishment of the deep circumpolar current which led to thermal isolation of antarctica and increased global cooling this development was associated with a major turnover in planktic organisms resulting in the evolution of neogene assemblages and the eventual extinction of paleogene assemblages the erosive patterns of two widespread hiatuses ph 23 0 22 5 ma and nh 1 20 18 ma indicate that a deep circumequatorial circulation existed at this time characterized by a broad band of carbonate ooze deposition siliceous sedimentation was restricted to the north atlantic and a narrow band around antarctica a major reorganization in deep sea sedimentation and hiatus distribution patterns occurred near the early middle miocene boundary apparently resulting from changes in oceanic circulation beginning at this time deep sea erosion occurred throughout the caribbean hiatus nh 2 16 15 ma suggesting disruption of the deep circumequatorial circulation and northward deflection of deep currents and or intensification of the gulf stream sediment distribution patterns changed dramatically with the sudden appearance of siliceous ooze deposition in the marginal and east equatorial north pacific by 16 0 to 15 5 ma coincident with the decline of siliceous sedimentation in the north atlantic this silica switch may have been caused by the introduction of norwegian overflow water into the north atlantic acting as a barrier to outcropping of silica rich antarctic bottom water the main aspects of the present oceanic circulation system and sediment distribution pattern were established by 13 5 to 12 5 ma hiatus nh 3 coincident with the establishment of a major east antarctic ice cap antarctic glaciation resulted in a broadening belt of siliceous ooze deposition around antarctica increased siliceous sedimentation in the marginal and east equatorial north pacific and indian oceans and further northward restriction of siliceous sediments in the north atlantic periodic cool climatic events were accompanied by lower eustatic sea levels and widespread deep sea erosion at 12 to 11 ma nh 4 10 to 9 ma nh 5 7 5 to 6 2 ma nh 6 and 5 2 to 4 7 ma nh 7

effects of magnetite on high frequency ground penetrating radar large concentrations of magnetite in sedimentary deposits and soils with igneous parent material have been reported to affect geophysical sensor performance we have undertaken the first systematic experimental effort to understand the effects of magnetite for ground penetrating radar gpr characterization of the shallow subsurface laboratory experiments were conducted to study how homogeneous magnetite sand mixtures and magnetite concentrated in layers affect the propagation behavior velocity attenuation of high frequency gpr waves and the reflection characteristics of a buried target important observations were that magnetite had a strong effect on signal velocity and reflection at magnitudes comparable to what has been observed in small scale laboratory experiments that measured electromagnetic properties of magnetite silica mixtures magnetite also altered signal attenuation and affected the reflection characteristics of buried targets our results indicated important implications for several fields including land mine detection martian exploration engineering and moisture mapping using satellite remote sensing and radiometers

historical evolution of the columbia river littoral cell abstract this paper details the historical coastal evolution of the columbia river littoral cell in the pacific northwest of the united states geological data from a d 1700 and records leading up to the late 1800s provide insights to the natural system dynamics prior to significant human intervention most notably jetty construction between 1885 and 1917 all reliable surveys charts and aerial photos are used to quantify decadal scale changes at the three estuary entrances and four sub cells of the littoral cell shoreline bathymetric and topographic change over three historical intervals 1870s 1920s 1920s 1950s and 1950s 1990s are integrated to provide an understanding of sediment sharing relationships among the littoral cell components regional morphological change data are developed for alongshore segments of approximately 5 km enabling comparisons of shoreline change to upper shoreface and barrier volume change within common compartments the construction of entrance jetties at the columbia river 1885 1917 and grays harbor 1898 1916 has profoundly affected the evolution of the littoral cell and has accentuated the morphological coupling between the inlets ebb tidal deltas shorefaces and barriers the jetties induced erosion of the inlets and offshore migration of ebb tidal deltas the change in boundary conditions at the entrances enabled waves to rework the flanks of ebb tidal deltas and supply enormous quantities of sand to the adjacent coasts over several decades the initial sand pulses have been dispersed alongshore up to tens of kilometers from the estuary entrances winter waves and coastal currents produce net northward sediment transport across the shoreface while summer conditions tend to induce onshore sediment transport and accumulation of the upper shoreface and barriers at relatively high rates historical shoreline progradation rates since jetty construction are approximately double the late prehistoric rates between 1700 and the 1870s erosion rates of the mid to lower shoreface to the south of the jettied estuary entrances have typically been greater than the accumulation rates of the upper shoreface and barrier suggesting that the lower shoreface has been an important source of littoral sediments over decadal and longer time scales until recent decades sediment supply from the ebb tidal delta flanks and lower shoreface has largely masked the decline in columbia river sediment supply resulting from flow regulation and dredging disposal practices with the contemporary onset and expansion of coastal erosion adjacent to the jettied estuary entrances proper management of dredged sediment is imperative to mitigate the effects of a declining sediment budget

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

late miocene earliest pliocene glaciation in southern argentina implications for global ice sheet history abstract in south america at latitude 47 s till was deposited sometime between 7 m y and 4 6 m y ago at a time when the local climate was colder than today s during this same interval glaciers extended to sea level in southeast alaska and widespread cooling of the ocean surface in middle latitudes worldwide marine regression and change in the oxygen isotopic composition of ocean water occurred from the last three events major late miocene expansion of the antarctic ice sheet has been inferred on the assumption that the history of north atlantic ice rafting precludes the existence of northern hemisphere ice sheets until 3 m y ago this is disputed first because precipitation in antarctica would probably have decreased at temperatures below today s second because the antarctic ice sheet cannot expand appreciably until buildup of northern hemisphere ice sheets has lowered sea level third because virtually no late miocene sediments are present at the labrador sea dsdp sites that are critical to the reconstruction of north atlantic ice rafting history and fourth because the scale of late miocene glaciation in alaska is at least permissive for simultaneous buildup of ice at similar latitudes further east if global ice sheet volume at the end of the miocene was greater than it is today by an amount that would have lowered sea level by several tens of meters the excess ice did not accumulate in antarctica but in the northern hemisphere chiefly in north america away from the atlantic coast

neogene uplift of central eastern patagonia dynamic response to active spreading ridge subduction the chile triple junction ctj episodically migrated northward during the past 14 ma from 54 s to its present day position at 46 30 s as different almost trench parallel spreading segments entered in subduction this migration resulted in the opening of an asthenospheric window below patagonia inducing a disturbance in the regional mantle convection on the overriding plate the middle miocene time corresponds to a major change in the central patagonian basin dynamics with a transition from subsidence to generalized uplift the detailed mapping and the morphological study of post middle miocene terraces evidence regional scale trench parallel tilt histories that differ depending on latitude south of 46 30 s the slopes of the fluvial terraces indicate a change from northward to southward tilt during late miocene terraces younger than the early pliocene do not show any trench parallel tilt north of 46 30 s in contrast only northward tilt active until the pleistocene is recorded we use a semianalytical model of dynamic topography which suggests that the northward migration of the chile triple junction should be accompanied by a dynamic uplift of the central patagonian basin uplift would cause both north directed and south directed tilt north and south of the triple junction respectively with values of 0 1 0 2 500 km east of the trench tilt measured in the southeastern part of the central patagonian basin 0 1 is comparable to values predicted using the semianalytical model the dynamic topography associated to the chile ridge subduction may have exerted a major control on the neogene dynamics of patagonia

late holocene shoreline responses to competing shelf bay and beach accommodation spaces under conditions of relative sea level change and the potential for future catastrophic beach retreat in the columbia river littoral cell washington and oregon usa abstract the columbia river littoral cell crlc 160 km in length provides opportunities to compare competing accommodation space relations under different conditions of relative sea level change the crlc system includes abundant littoral sand supply from the large columbia river late holocene prograded beach plains and barrier spits 0 5 5 km in width two large marine dominated estuaries willapa bay and grays harbor and a high wave energy inner shelf littoral sand accumulation rates in prograded beach plains and barrier deposits are based on paleo shoreline positions that are dated by great earthquake catastrophic beach retreat scarps n 10 from 0 3 to 5 0 ka the retreat scarp timelines are mapped in across shore gpr transects n 79 yielding timeline bounded polygons n 247 the polygons are evaluated for littoral sand volume and bounding ages yielding volume accretion rates m3 ka 1 which are summed for the four crlc subcells clatsop plains 27 4 106 m3 ka 1 long beach 19 8 106 m3 ka 1 grayland plains 15 9 106 m3 ka 1 north beaches 11 7 106 m3 ka 1 major submarine sinks of littoral sand including the inner shelf and large marine dominated estuaries are evaluated for increased littoral sand accommodation space that could result from potential future sea level rise of 1 2 and 3 m during the next century or two the estimated beach and nearshore sand erosion needed to fill the increased submarine accommodation space from a 2 0 m rise in relative sea level would result in averaged beach retreat values of 0 7 km clatsop plains 1 2 km long beach and 1 3 km grayland plains about 30 50 larger than previous estimates based on the brunn method these catastrophic shoreline retreat distances 0 7 1 3 km represent 25 50 of the present widths of prograded barrier spits and beach plains they serve as warnings about future catastrophic beach erosion resulting from potential future slr in other similar barrier spit and beach plain shorelines worldwide

primary and diagenetic carbonates in the anoxic sediments of lake bosumtwi ghana cores of organic rich muds from the tropical meteorite crater lake bosumtwi ghana contain laminae of authigenic calcite polyhedra and aragonite needles as well as scattered diagenetic calcite mg calcite spherulites and aggregates of dolomite crystals their respective origins are traced by analyses of oxygen and carbon stable isotopes anoxic dolomite and mg calcite formed in pore waters reflecting highest degrees of evaporative concentration and high rates of methanogenesis carbon dioxide reduction must have been the dominant methanogenic pathway producing diagenetic carbonates with delta sup 13 c values up to 27 pdb this example shows the inherent problems of interpreting bulk samples with progressive mineralogical sequences of calcite to mg calcite to aragonite to dolomite merely in terms of mg sup 2 ca sup 2 in lake waters it provides a model for the interpretation of ancient lacustrine carbonates from organic rich environments

regional mid pleistocene glaciation in central patagonia abstract southern south america contains a glacial geomorphological record that spans the past million years and has the potential to provide palaeoclimate information for several glacial periods in earth s history in central patagonia two major outlet glaciers of the former patagonian ice sheet carved deep basins 50 km wide and extending over 100 km into the andean plain east of the mountain front a succession of nested glacial moraines offers the possibility of determining when the ice lobes advanced and whether such advances occurred synchronously the existing chronology which was obtained using different methods in each valley indicates the penultimate moraines differ in age by a full glacial cycle here we test this hypothesis further using a uniform methodology that combines cosmogenic nuclide ages from moraine boulders moraine cobbles and outwash cobbles 10 be concentrations in eighteen outwash cobbles from the moreno outwash terrace in the lago buenos aires valley yield surface exposure ages of 169 269 ka we find 10 be inheritance is low and therefore use the oldest surface cobbles to date the deposit at 260 270 ka which is indistinguishable from the age obtained in the neighbouring lago pueyrredon valley this suggests a regionally significant glaciation during marine isotope stage 8 and broad interhemispheric synchrony of glacial maxima during the mid to late pleistocene finally we find the dated outwash terrace is 70 100 ka older than the associated moraines on the basis of geomorphological observations we suggest this difference can be explained by exhumation of moraine boulders

time and size of a giant earthquake in cascadia inferred from japanese tsunami records of january 1700 geological evidence shows that great earthquakes have occurred in the recent prehistoric past in the cascadia subduction zone off the pacific coast of north america the most recent event or series of events is dated at about 300 years ago1 4 but the precise date and magnitude have not been determined geological investigations have not been able to distinguish a single giant earthquake from a series of great earthquakes occurring over a timespan of a decade or two4 although this information is important for the assessment of future hazard5 we have found several tsunami records in japan from ad 1700 with no indication of a local cause historical earthquake records and palaeoseismic evidence indicate the absence of a large earthquake in 1700 in south america alaska or kamchatka leaving cascadia as the most likely source of this tsunami the estimated time of the earthquake is the evening about 21 00 local time of 26 january 1700 the magnitude is estimated as 9 from the tsunami heights in which case the earthquake ruptured the entire length of the cascadia subduction zone2 these estimates are consistent with native american legends that an earthquake occurred on a winter night6

modeling the effects of wave climate and sediment supply variability on large scale shoreline change abstract the application of an integrated data analysis and modeling scheme reveals that decadal scale shoreline evolution along a u s pacific northwest littoral cell is highly dependent on both sediment supply and wave climate variability in particular accurate estimates of columbia river sediment supply and sediment feeding from the lower shoreface are critical components of balancing the barrier beach sediment budget and are therefore essential to making sensible shoreline change hindcasts and forecasts a simple deterministic one line shoreline change model applied in a quasi probabilistic manner enables evaluation of the influence of sediment supply and wave climate variability through simulation of historical shoreline change through iteration a range of realistic scenarios are developed to constrain decadal scale shoreline change predictions modeled shoreline changes are significantly sensitive to directional changes in the incident waves and therefore sensitive to the occurrence of interannual climatic fluctuations such as major el nino events a predicted increase in the intensity of the east pacific wave climate 1 0 m increase in significant wave height in 20 yr affects forecast shoreline positions only when this increase occurs during the winter storm season however the effect of this increase in storm power during any given year is small relative to the impact of major el nino events the model has significant skill in decadal scale hindcasts suggesting that alongshore gradients in sediment transport dominate coastal change at this scale at this site however both data and model results suggest that net onshore feeding from the lower shoreface is responsible for approximately 20 of the decadal scale coastal change field measurements and poor model skill at annual scale indicate that cross shore processes likely dominate coastal change at shorter time scales

cenozoic crustal thickening wrenching and rifting in the foothills of the southernmost andes abstract the southernmost andes form an orocline between the patagonian cordillera trending n s and the fueguian cordilleras trending e w on the foreland side is the magellan basin the area has a history of paleozoic compression triassic to early cretaceous rifting and late cretaceous to quaternary compression in response to changing plate tectonics major structures of late cretaceous and cenozoic age vary along the strike in the patagonian cordillera and foothills folds and thrusts trend nnw slightly oblique to the orogen whereas strike slip faults are parallel to the orogen and right lateral in the fueguian cordilleras and foothills folds and thrusts trend ese slightly oblique to the orogen whereas strike slip faults are parallel to the orogen and left lateral in the axial zone of the magellan basin folds and thrusts are parallel to the orogen and rifts are sub perpendicular to it to a first approximation the pattern of structures has mirror symmetry about the axis of the magellan basin in detail however wrenching appears to be more prevalent in the fueguian cordillera and foothills than it is in the patagonian cordillera and foothills minor faults of cenozoic age are common in the foothills from a kinematic analysis of fault slip data 1 shortening and stretching directions are mostly sub horizontal 2 shortening directions vary in trend from ene in the patagonian foothills to ne in the fueguian foothills and 3 stretching directions are sub parallel to traces of major thrusts in the fueguian cordillera and foothills strike slip faulting is prevalent in the patagonian foothills crustal thickening is prevalent over strike slip faulting the kinematics reflect a combination of thrusting and wrenching and they are consistent with the major structures to investigate the origin of the cenozoic structures we used analogue models on a fully lithospheric scale where an oceanic plate subducted beneath a continental corner the corner was an area of transition from frontal subduction to transcurrent motion the boundary conditions may not have been fully realistic but the experiments did account for the major elements of the structural pattern in southernmost south america including rifts that are perpendicular to the orogen and counterclockwise block rotations

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

interaction between the chile ridge and chile trench geophysical and geothermal evidence geophysical and geothermal data are examined from the three southernmost sections of the chile ridge starting at 44 s and continuing south to the triple junction of the nazca antarctic and south america plates at 47 s these sections represent three progressively younger stages in a ridge trench collision event corresponding to 3 m y before the collision 1 m y before the collision and culminating in an ongoing collision at the triple junction magnetic and bathymetric data across the chile rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench in the collision zone a normal looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope there is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the pacific farallon spreading center collided with north america in contrast the overriding plate is greatly affected by the ridge collision the landward trench slope steepens and narrows as the collision zone is approached at the southern end of the collision zone a ridge associated with the taitao fracture zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope geothermal measurements were made along three transects of the margin corresponding to the time 3 m y before the collision during the collision and 6 m y after the collision the heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge values as high as 345 mw m2 were recorded on the lower trench slope a prominent bottom simulating reflector bsr observed over a wide area of the landward trench slope north of the triple junction and more locally south of the triple junction is used to expand the grid of heat flow observations excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the bsr the heat flow measurements compare favorably with a theoretical model assuming conductive heat flow we estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion periodic ridge collisions in the past may account for the apparent truncation of the andean forearc region

neogene to quaternary tectonic evolution of the patagonian andes at the latitude of the chile triple junction abstract we conducted a preliminary structural and tectonic analysis of the region north and south of the lake general carrera buenos aires lgcba southern patagonian andes chile and argentina in order to study the effects of the subduction of the active south chile spreading ridge scr on the development of the tectonic front of the patagonian andes at the latitude of the chile triple junction ctj here one segment of the scr entered the trench at 6 ma and the corresponding slab window is now located below the study area we emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments a to d trending mainly n170 and right laterally offset this geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction the four segments of the tectonic front are separated and locally deformed by n170 oriented more recent sinistral strike slip fault zones kinematic analysis of minor fault and striaes confirms the abundance of strike slip faults and suggests rotation of the stress tensor after at least 9 ma a consequence of the subduction of the scr beneath the south american plate at the latitude of the lgcba is a two step history 1 pre ridge subduction oblique to the western margin of the south america plate showing contractional regime with its main axis trending ne sw responsible for dextral transpressive deformation and 2 post ridge subduction more orthogonal convergent regime recent uplift occurred later starting at 3 5 ma and could still be active neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial lacustrine terraces anomalies in stream development can be found although evidences for recent to active faulting can be shown effects of syn glacial and post glacial related processes cannot be ruled out the evolution of this region during the late cenozoic can be divided into three distinct periods 1 20 15 ma deposition of foreland continental detrital sediments rio zeballos group followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10 9 ma 2 9 4 ma erosion peneplanation and emplacement of voluminous plateau basalts coeval with fluvial glacial sedimentation and 3 3 0 ma renewed orogenic activity uplift and disruption of former erosional surfaces in relation to activation of strike slip fault zones and north trending thrusts onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the chile ridge at 14 15 ma in southernmost patagonia inducing a different stress regime in the frontal arc region