shallow crustal structure beneath the juan de fuca ridge from 2 d seismic refraction tomography summary the formation of oceanic lithosphere along ocean ridges and the role that crustal magma chambers play in the accretionary process continue to be fundamental issues in plate tectonics to address these issues a multireceiver airgun oceanbottom seismograph refraction line was shot across the endeavour segment of the juan de fuca ridge near 48 n 129 w 2 d traveltime tomography applied to the data results in a three layer model for the upper crust layer 1 is 250 650m thick with u1 2 5 km s and vzvl 0 5 s layer 2 is 800 m thick u2 4 8 km s and vzuz 1 0 layer 1 and layer 2 probably represent the sequence of extrusives and the transition to layer 3 u3 5 8 km s l vzv3 0 5 s is associated with the extrusives dike complex transition an abrupt velocity transition between layer 1 and layer 2 may be a metamorphic front within the basalt pillows or it may be the depth to which surface fissures penetrate a low velocity anomaly velocities decreased by 0 45 km s exists beneath the ridge in layer 2 and upper layer 3 it is interpreted as a zone of increased fracture porosity and or permeability associated with axial hydrothermal circulation and correlates reasonably well with a sub axial reflector no evidence is found for the existence of a crustal magma chamber in the depth range of 1 5 3 5 km sub bottom however the velocity anomalies observed in layer 3 suggest that crustal temperatures in this layer are elevated by 150 250 c beneath and to the east of the ridge relative to temperatures west of the ridge

velocity structure in upper ocean crust at hole 504b from vertical seismic profiles hole 504b provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust providing a baseline for comparison with seismic measurements elsewhere we determine p and s velocities from vertical seismic profiles vsps obtained on ocean drilling program odp legs 111 and 148 four issues are considered the location of the seismic layer 2 3 boundary p to s wave conversion by scattering transverse isotropy and poisson s ratio as an indicator of lithology porosity and structure 1 in the p velocity profile the change in slope marking the layer 2 3 boundary coincides with the top of the sheeted dike unit seismic layer 2 is composed of the extrusives and the lithologic transition zone the layer in which flows and dikes interfinger 2 even in these normal incident vsps several second arrivals with velocities indicative of vertically polarized shear energy are observed p to s wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis 3 vertical velocities determined from the vsp differ by 10 from horizontal velocities obtained from the oblique seismic experiment ose on deep sea drilling project dsdp leg 92 the p wave velocity structure is determined by small and intermediate 1 cm pore structure with no measurable anisotropy the large scale well oriented vertical fractures which are formed tectonically do not have a detectable effect on compressional wave velocities 4 high poisson s ratio in the upper 300 m of basement coincides with an extrusive layer composed of pillows and thin flows low poisson s ratio at 850 1150 m below seafloor mbsf coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes relatively large aspect ratio cracks are required to produce such low values of poisson s ratio the cracks were likely created by hydraulic fracturing when hot dikes encountered low temperature seawater

tomographic image of the magma chamber at 12 50 n on the east pacific rise the east pacific rise at 12 50 n fig 1 is a fast spreading ridge with intense hydrothermal activity1 and ophiolite studies2and thermal modelling3 indicate that this is a likely setting for a magma chamber a recent seismic reflection experiment4 imaged the top of the magma chamber at this site at a depth of 1 4 km below the sea floor and found that it is continuous for tens of kilometres along the rise axis here we examine a large set of accurate p wave travel times from a detailed seismic refraction experiment at the same site5 the patterns observed in the travel times demonstrate that a zone of low seismic velocities exists beneath the rise axis throughout the region studied the best fitting two dimensional structure obtained from linear inversion of the travel times includes an axial low velocity zone magma chamber only 6 km wide in which velocities are depressed by more than 0 5 km s 1

a record from lake cardiel of climate change in southern south america southern south america the only continental land mass lying between 38 s and the antarctic circle presents a rare opportunity to reconstruct terrestrial records of palaeoclimate in an area that is subject to shifts of polar and mid latitude wind and pressure belts in the bulk of this region patagonia are many hydrographically closed lakes which despite the palaeoclimatic significance of the region and the proven utility of lake level records in reconstructing past climates1 2 have escaped detailed study using sedimentary and geomorphic evidence together with 14c dating we present here a detailed lake level record for lake cardiel fig 1 the highest level of the past 21 000 years occurred at about 9 780 bp reminiscent of tropical lakes rather than those in northern mid latitudes1 2 the lake cardiel record corroborates and clarifies the timing of the wetness record inferred from pollen studies in patagonia

an estimate of the conductivity structure for the vancouver island region from geomagnetic results abstract the depth to the highly conductive layer of the mantle below the vancouver island region is estimated using the results from a scaled laboratory analogue model a two dimensional numerical model and field observations at the centre of the island the responses of numerical models with depths to the mantle of 40 and 80 km bracket the field results the subduction regime in the vancouver island region suggests a dipping interface for the conducting layer the numerical model based on this type of tectonic structure has a conducting layer within the mantle that varies in depth from 40 km at the continental margin increasing under the island to 80 km at the shallow channel and decreasing again to 40 km near the mainland coast the response of this model is compatible with the field observations

glacier modeling and the climate of patagonia during the last glacial maximum ice cap modeling constrained by empirical studies provides an effective way of reconstructing past climates the former patagonian ice sheet is in a climatically significant location since it lies athwart the southern hemisphere westerlies and responds to the latitudinal migration of climatic belts during glacial cycles a numerical model of the patagonian ice cap for the last glacial maximum lgm is developed which is time dependent and driven by changing the mass balance altitude relationship it relies on a vertically integrated continuity model of ice mass solved over a finite difference grid the model is relatively insensitive to ice flow parameters but highly sensitive to mass balance the climatic input is adjusted to produce the best fit with the known limits of the ice cap at the lgm the ice cap extends 1800 km along the andes and has a volume of 440 000 km3 during the lgm the equilibrium line altitude ela was lower than at present by at least 560 m near latitude 40 s 160 m near latitude 50 s and 360 m near latitude 56 s the latitudinal variation in ela depression can be explained by an overall fall in temperature of about 3 0 c and the northward migration of precipitation belts by about 5 latitude annual precipitation totals may have decreased by about 0 7 m at latitude 50 s and increased by about 0 7 m at latitude 40 s the ela rises steeply by up to 4 m per kilometer from west to east as the westerlies cross the andes and this prevents ice growth to the east the limited decrease in temperature during the lgm could be related to the modest migration of the antarctic convergence between south america and the antarctic peninsula

comment climatic inferences from glacial and palaeoecological evidence at the last glacial termination southern south america r d mcculloch m j bentley r s purves n r j hulton d e sugden and c m clapperton 2000 journal of quaternary science 15 409 417

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

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

electromagnetic induction in new zealand analogue model and field results abstract the behaviour of electric and magnetic variations over north island new zealand is studied with the aid of a laboratory analogue model the source field frequencies used in the analogue modelling simulate naturally occurring geomagnetic variations of 5 120 min periods in phase and quadrature magnetic and electric fields for a selection of traverses for the modelled region of north island are presented since north island is of a relatively narrow cross section the field responses even for inland locations are expected to show strongly the effects of the surrounding ocean the irregular coastlines as well as the strait between north and south islands lead to coastal and inland field anomalies due to induced currents being deflected and channelled to produce localized current densities the comparison of model results with field station measurements obtained earlier individually by ingham and by midha for sites in the northeastern central and southern near cook strait regions of north island demonstrates the large role the ocean has in the observed field responses differences in the model and field results at some sites are expected and should reflect the effects of the local geology and the conductive substructure related to the complex tectonics of the region not simulated in the model

seismic ridge subduction and topography foreland deformation in the patagonian andes abstract the patagonian andes recorded several episodes of active ridge subduction in the last 80 million years an analysis of the spatial and temporal relation between the present segment of collision of the chile ridge and the digital topography of the foreland shows a correlation with the beginning of deformation and uplift in the inner sector of the patagonia fold and thrust belt several magmatic episodes related to the collision such as near trench magmatism adakite emplacement oib plateau basalts in the retroarc and the arc volcanic gap are associated with the uplift and deformation of the patagonian cordillera based on these correlations a collision of the aluk or phoenix farallon ridge during paleogene times south of 43 30 is identified changes in magmatic patterns molasses deposits deformation and uplift of the patagonian cordillera constrain the region affected by the collision similar evidence implies a third period of collision in the late cretaceous based on the occurrence of adakitic rocks arc magmatic gap and deformation along the southern patagonian andes this earliest hypothesized collision would require the existence of a new oceanic microplate between the pacific and the aluk plates during late cretaceous times present rapid isostatic rebound related to the continental ice cap retreat in the patagonian andes is restricted to the region south of chile triple junction 46 30 s the uplift rate here is more than two times more rapid than normal isostatic rebounds recorded in the northern hemisphere and requires an abnormally hot mantle with low viscosity this abnormal mantle may be a consequence of several episodes of ridge collision and development of asthenospheric windows that are inconsistent with periods of cold flat slab subduction proposed by some authors to explain the arc volcanic gaps

the glaciogenic origin of the pleistocene calcareous dust in argentina on the basis of field mineralogical textural and geochemical analyses calcareous dust occurs in argentina as layers and pockets closely associated with pleistocene deposits and periglacial features from southernmost patagonia to at least the mendoza precordillera and has been traditionally interpreted as a soil horizon resulting from postdepositional pedogenesis during interglacials detailed field and microscopic observations and sedimentological and geochemical analyses of more than 100 samples collected from lower to upper pleistocene deposits between 51 s and 33 s and from near sea level to 2800 m asl allow us to interpret the dust as synchronous with the host sediment all observations and analyses lead us to conclude that 1 the cryogenic morphology and the chemical signatures of the calcite component show that the dust is glaciogenic 2 the dust was carried by southeasterly antarctic winds and 3 it was deposited over most of southern and central argentina field observations geomorphic evidence and radiocarbon dates suggest that the dust was deposited during several pleistocene glacial episodes

the coast effect response in geomagnetic field measurements abstract two dimensional 2d numerical models of electromagnetic em induction for anomalous conductors in continental and island coastal regions are studied to determine constraints on ocean conductor separation distances that would permit to within an acceptable approximation a simple vector subtraction of the coast effect response the anomalous conductors are in the form of an upwelling or a depression in the asthenospheric conductive substratum at depth the coast effect response range yr is arbitrarily taken to be the distance from the ocean at which the b z b y n coast effect response is reduced to a value of 0 2 the response ranges are determined and presented empirically as a function of period for a range of conductive substratum depths island widths and ocean depths the response ranges are tested in 2d numerical models that include an anomalous conductor in the form of either an upwelling or a depression in the conductive substratum under the ocean and host earth the induction arrows for the anomalous conductor are shown to be in good agreement with the difference arrows obtained by subtracting the responses of a model without the anomalous conductor from the responses of the model that includes the anomalous conductor as an application difference arrows obtained by subtracting three dimensional laboratory analogue model coast effect responses from field site responses are employed in the interpretation of induction arrow responses available for a number of sites in the bohai bay region of china and the kii peninsula region of japan

geophysical evidence for the absence of a crustal magma chamber under the northern juan de fuca ridge a contrast with rose results deep tow reflection data seismic refraction data to ocean bottom seismometers and gravity interpretations of the northern juan de fuca ridge all suggest a very narrow or absent crustal magma chamber under this portion of the rise axis the seismic features that have been suggested as indicating crustal magma chambers on other parts of the east pacific rise such as travel time delays and reflections from the top of a shallow magma reservoir are not found on the northern part of this ridge these results indicate that at spreading rates of 3 cm yr crustal cooling probably by water penetration is sufficiently rapid to prevent the existence of a steady state crustal magma chamber and therefore crustal accretion may be episodic

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

late glacial history of lago argentino argentina and age of the puerto bandera moraines in the west central part of lago argentino the puerto bandera moraines are clearly detached from longer more prominent moraines of the last glaciation and from shorter and smaller neoglacial moraines scientists have long speculated about the age of the puerto bandera moraines detailed geomorphologic studies in the western area of lago argentino including stratigraphic profiles at bahia del quemado in the northern branch brazo norte indicate that the puerto bandera moraines were deposited by three pulses of ice each of the three pulses is represented by single moraine ridges and belts of tightly arranged ridges the timing of the three glacier advances was established by radiocarbon dating including data published by john mercer the oldest moraine system formed during the puerto bandera i substade was deposited ca 13 000 14c yr b p moraines of the puerto bandera ii substade were deposited ca 11 000 14c yr b p the youngest moraine system was deposited during a minor readvance shortly before 10 390 c14 yr b p and thus appears to have occurred some time during the european younger dryas interval after this third substade the ice tongues retreated into the interior branches of lago argentino and have remained there since evidence found at bahia del quemado together with data provided by other authors attests to a significant climatic change by the middle holocene which we believe occurred during the herminita advance the first holocene glacial readvance recognized within the area

rivera ocean seismic experiment rose overview the rivera ocean seismic experiment rose was designed as a combined sea and land seismic program to utilize both explosive and earthquakes to study a number of features of the structure and evolution of a mid ocean ridge a major oceanic fracture zone and the transition region between ocean and continent the primary region selected for the experiment included the rivera fracture zone the crest and eastern flank of the east pacific north of the rivera and adjacent areas of baja california and mainland mexico the experiment included 1 study of the east pacific rise south of the orozco fracture zone primarily using ocean bottom recording and explosive sources 2 a seismicity program at the orozco and 3 a land based program of recording natural events along the coastal region of mexico a considerable amount of useful data was obtained in each of the three subprograms in the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle structure and dynamics of the east pacific rise seismicity of the orozco fracture zone and partitioning of energy transmission between the ocean volume and the crust lithosphere on land the fortuitousmore occurrence of the petatlan m7 6 earthquake of march 14 1979 permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event which provide new insight into the filling of a major seismic gap in the region this overview describes the scientific rationale and the design of the experiments along with some general results less

fluctuations of outlet and valley glaciers in the southern andes argentina during the past 13 000 years abstract in the southern argentine andes ten advances of valley glaciers were used to reconstruct the late glacial and holocene glacier history the accumulation areas of these glaciers lie in the precordillera and are thus independent of fluctuations of the south patagonian icefield like the viedma outlet glacier the valley glaciers advanced three times during late glacial time 14 000 10 000 yr b p the youngest advance correlates with the younger dryas stade based on two minimum ams 14 c dates of 9588 and 9482 yr b p the second oldest advance occurred before 11 800 yr b p during the first half of the holocene ca 10 000 5000 yr b p advances culminated about 8500 8000 7500 and 5800 5500 yr b p during the second half of the holocene advances occurred between ca 4500 and 4200 yr b p as well as between 3600 and 3300 yr b p in the rio condor valley three subsequent advances have been identified

a laboratory electromagnetic model study of the juan de fuca plate region the behaviour of the magnetic field variations over the juan de fuca plate region is studied using a scaled laboratory analogue model the model includes a simulation of the complex juan de fuca plate subducting the vancouver island region the subducting plate is modelled with a profile of increasing inclination from east to west horizontal offshore dipping at 10 under vancouver island and bending further under georgia strait to subduct the continent at 30 for the b c region and 45 for the washington oregon region the strike of the bending plate follows the general strike of the continental coastline with an abrupt change in direction 42 in the puget sound area the model substructure simulates a subducting plate overplated by a sediment layer several kilometres thick and underlain by a 30 km thick highly conducting upper asthenosphere the model source frequencies used simulate periods 5 120 min in the geophysical scale in phase and quadrature hx hy and hz magnetic field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for e and h polarizations electric field of the source approximately parallel and perpendicular respectively to the west coast of vancouver island the fields for three regions of the model over vancouver island over the olympic peninsula and over a linear portion of the u s coastline are examined in detail the general conclusion is that the effect of the dipping subducting plates is to significantly attenuate at short periods the maxima in the anomalies at the coastlines underlain by the 10 dipping plate while leading to anomalous vertical and horizontal fields over ranges as large as 500 km inland over a wide period range anomalous fields are observed over the offshore and inland knee bends of the subducting plates at all periods for both e and h polarizations for locations on land the in phase induction arrows point seaward and perpendicular to the strikes of the dipping plates for all periods while the quadrature arrows at short periods point landward and rotate to point seaward for periods greater than 20 min

a finite element computation of seismic diffraction about the rise axis magma chamber in a previous study models of the crust and magma chamber at 12 50 n on the east pacific rise were derived with a two dimensional ray tracing analysis of rise normal refraction lines the best model featured a 4 km wide axial magma chamber but the data contained diffraction effects that were not modeled by geometric ray theory this raised questions as to whether this model would reproduce the waveform data and whether diffraction might be obscuring a much larger magma chamber we have tested the model with a finite element calculation of synthetic seismograms this method incorporates full wave effects including diffraction the synthetics accurately reproduce the data and exhibit diffraction phenomena similar to those in the data the previous travel time analysis was largely successful and was only slightly compromised by diffraction diffraction in rise normal refraction experiments is therefore not obscuring a large low velocity magma chamber

early formation of gnammas weathering pits in a recently glaciated area of torres del paine southern patagonia chile abstract incipient gnammas weathering pits were identified and measured in the frances valley of torres del paine southern patagonia the gnammas were located on the granite blocks of a lateral moraine deposited during a late holocene neoglacial advance the gnammas were measured for maximum and minimum depth length and width in addition to other observations based on these measurements the surface area and the volume of the gnammas were calculated most of the gnammas have

em induction in the vancouver island region 3d numerical analogue model and field site results electromagnetic induction in the vancouver island region for a uniform inducing source field for 300 s period is investigated with the aid of three dimensional 3 d numerical and analogue model results and field site measurements the thin sheet numerical model based on the subducting juan de fuca plate analogue model ofdossoet al consists of a 5km thick non uniform thin sheet comprising the lateral conductivity contrasts arising from the land the varying depth ocean and the sediment underlain by a four layer conductive structure the four layer conductive structure beneath the non uniform thin sheet simulates the effect of the juan de fuca plate subducting vancouver island to examine the effects of the ocean channel depth between vancouver island and the british columbia canada mainland numerical results were obtained for two channel depths 0 and 600 m the results indicate that the channel plays an important role in the geomagnetic response in the central and inner coastal regions of vancouver island the general agreement of the 3 d numerical model induction arrows with the analogue model and field site induction arrows for 300 s supports the premise of a layered conductive substructure dipping at a small angle at most beneath vancouver island

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

climatic inferences from glacial and palaeoecological evidence at the last glacial termination southern south america there is uncertainty about the interhemispheric timing of climatic changes during the last glacial interglacial transition different hypotheses relying on different lines of evidence point variously to the northern hemisphere leading the southern hemisphere and vice versa or to synchrony between the hemispheres southern south america is well placed to test the various alternatives using both glacial and palaeoecological evidence we argue here from a synthesis of key proxy records that there was a sudden rise in temperature that initiated deglaciation sychronously over 16 of latitude at 14 600 14 300 14 c yr bp 17 500 17 150 cal yr there was a second step of warming in the chilean lake district at 13 000 12 700 14 cy r bp 15 650 15 350 cal yr which saw temperatures rise to close to modern values a third warming step particularly clear in the south occurred at ca 10 000 14 c yr bp 11 400 cal yr the latter achieving holocene levels of warmth following the initial warming there was a lagged response in precipitation as the westerlies after a delay of ca 1600 yr migrated from their northern glacial location to their present latitude which was attained by 12 300 14 cy r bp 14 300 cal yr the latitudinal contrasts in the timing of maximum precipitation are reflected in regional contrasts in vegetation change and in glacier behaviour the large scale of a 80 km glacier advance in the strait of magellan at 12 700 10 300 14 c yr bp 15 350 12 250 cal yr which spans the antarctic cold reversal and the younger dryas was influenced by the return of the westerlies to southern latitudes the delay in the migration of the westerlies coincides with the heinrich 1 iceberg event in the north atlantic the suppressed global thermohaline circulation at the time may have affected sea surface temperatures in the south pacific and the return of the westerlies to their present southerly latitude only followed ocean reorganisation to its present interglacial mode copyright 2000 john wiley sons ltd

evolution of the ocean crust results from recent seismic experiments we present results from recent analyses of seismic refraction and sea floor microseismicity studies in the pacific and atlantic oceans which lend support to the hypothesis that processes responsible for the construction of ophiolite suites are similar to phenomena extant at midocean ridges seismicity at fast spreading ridges is characterized by very low magnitude 0 1 and shallow 2 3 km microearthquakes and long term oscillations or harmonic tremor a detailed seismic refraction experiment on a fast spreading portion of the east pacific rise supports the hypothesis of the existence of a crustal magma chamber analyses of these data indicate that the chamber is largely unperturbed by the presence of conjugate spreading centres and the inverted deltashaped zone of partial melt is characterized by a half width in excess of 6 km finally a new approach for obtaining upper crustal velocities when applied to data characterized by the presence of shear waves provides several counter examples to the hypothesis that the shallow crust evolves with time

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

p wave velocity structure of the earth s crust beneath vancouver island refraction data were recorded from three shot points out to a maximum distance of 330 km as part of the 1980 vancouver island seismic project visp80 these vertical component data are partially reversed and so can be interpreted in terms of two dimensional structures by iterative modeling of p wave travel times and amplitudes the structure of the upper crust is the best constrained part of the model it consists generally of a gradually increasing velocity from 5 3 km s at the surface to 6 4 km s at 2 km depth to 6 75 km s at 15 5 km depth where the velocity increases sharply to 7 km s below 20 km depth the model becomes speculative because the data provide only indirect constraints on velocities at these depths an interpretation that fits the observed times and amplitudes has a low velocity zone in the lower crust and a moho at 37 km depth the only significant departure from this general structure is beneath the central part of vancouver island where the 15 5 km boundary in the model atta

the east pacific rise in cross section a seismic model in 1982 we undertook a seismic refraction experiment known as the magma expedition to examine the detailed structure of the east pacific rise near 12 50 n this segment of the rise where the full spreading rate is about 110 mm yr is near the projected trace of the o gorman fracture zone and is the site of an overlapping spreading center as well as black smoker hydrothermal activity in this paper we describe the analysis of a subset of the travel time data collected during the magma expedition namely the data from profiles which were oriented normal to the rise axis these profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross sectional structure of the rise we have modeled these data using a two dimensional ray tracing program we have found that the seismic velocities in the young oceanic crust are rather high with velocity gradients of 4 0 5 5 s 1 in the uppermost crust the highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0 1 ma this decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads the decrease in velocity does not appear to penetrate deeper than about 0 5 km and may reverse itself as hydrothermal alteration fills the pores and cracks the fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits a result consistent with the high temperatures and discrete nature of the vents our best model includes a magma chamber some 4 km wide and extending from the moho to within about 1 1 km of the seafloor this magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the east pacific rise these discrepancies probably arise because the magma chamber under the east pacific rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis

seismic propagation across the east pacific rise finite difference experiments and implications for seismic tomography we use a full waveform acousto elastic finite difference technique to investigate seismic propagation across the east pacific rise at 9 30 n for a two dimensional velocity model based on that proposed by vera and others 1990 the primary feature of the model is an upper crustal low velocity region corresponding to the axial magma chamber which includes a small magma body located 1 6 km beneath the seafloor at the rise axis the high velocity gradients in this region result in a complex pattern of propagation which includes considerable scattering of energy above and below the magma chamber a qualitative comparison of finite difference seismograms with data collected by receivers located 9 km and 20 km off axis during a tomography experiment at 9 30 n shows generally good agreement for paths that cross the rise axis the first arrival in the finite difference solutions diffracts above the magma chamber this phase has a very low amplitude and at larger offsets falls below the ambient noise levels observed during the tomography experiment in such cases the first arrival with significant energy is a diffraction from below the magma chamber a high amplitude moho turning pmp phase which results from the large velocity change across the moho beneath the rise axis is apparent in both the finite difference solutions and the observations ray theoretical calculations of the paths of the diffracted arrivals are very unstable and for the diffractions above the magma chamber no solution can be found with a single precision algorithm synthetic delay time inversions using an approximate ray tracing algorithm demonstrate the importance of ensuring that picked arrival times are assigned to paths that pass to the correct side of the magma body synthetic inversions of spectral estimates of t show that q 1 models are compromised not only if the ray paths are incorrect but also if t estimates include significant contributions from more than one phase deterministic scattering from the magma chamber may contribute noticeably to spectral estimates of t but the results of the finite difference experiments imply that high levels of attenuation observed for phases passing below the magma chamber are predominantly the result of intrinsic attenuation

tomographic image of the axial low velocity zone at 12 50 n on the east pacific rise the 1982 magma seismic refraction experiment yielded a large set of accurate p wave travel times corrected for bathymetry and anisotropy which sample the structure of the east pacific rise epr at 12 50 n the arrivals were recorded using ocean bottom seismographs at three sites on axis and at 7 km and 16 km east of the axis we invert 2320 travel times for the twodimensional crustal seismic velocity structure across the epr axis assuming that the velocity structure is invariant along strike the travel times provide strong evidence for compressional velocity anisotropy in the upper crust corresponding to 10 faster velocities for propagation parallel to the axis than perpendicular to the axis the travel tunes used for the tomography are corrected for the effects of this azimuthal anisotropy our preferred model contains only the structure clearly required by the data structure which is stable under excessive smoothing and achieves a variance reduction of 81 relative to the laterally homogeneous starting model we resolve a substantial zone beneath the rise axis in which the velocity is reduced by 0 4 to 0 7 km s this low velocity zone lvz is about 7 km wide and extends from a depth of about 1 5 2 0 km down to moho at a depth of 5 5 km the lvz is slightly asymmetric extending 1 km further to the east than to the west of the axis in the shallow 1 0 km depth crust a pattern of velocity variations is imaged in which velocities are high at the spreading axis decrease between 3 km and 7 km east of the axis and then increase again between 10 and 15 km east of the axis we investigate the resolution of the inversion using an impulse response method the lvz and off axis upper crustal variations are well resolved in addition the travel time data indicate that an axial high velocity anomaly less than 2 km wide exists in the upper crust but is not resolved by the inversion the small velocity reductions of the lvz are consistent with hot rock containing only a small quantity of melt these results combined with the multichannel seismic reflection lines and expanded spread profiles from the northern epr suggest that the zone of high melt fraction under the spreading center is confined to a narrow thin lens capping a broad zone of hot plutonic rocks the upper crustal velocity reduction within 1 km of the axis reflects near axial thickening of the extrusive layer and the later reduction probably reflects porosity increases due to near axial tectonism the upper crustal velocity increase beyond 15 km off axis is attributed to porosity decreases associated with hydrothermal alteration

glacier dynamics during a phase of late quaternary warming in patagonia reconstructed from sediment landform associations abstract the geomorphological record in glaciated landscapes can provide important information for the study of the response of glaciers to rapid climate change this study presents a new reconstruction of the glacial history of the northern monte san lorenzo ice cap southern south america during a period of accelerated warming and deglaciation following the antarctic cold reversal 14 5 12 8 ka we present a detailed geomorphological map of the valleys to the north of monte san lorenzo sediment landform assemblages identified include lateral and terminal moraine ridges flutes deltas ice contact fans palaeoshorelines kame terraces and outwash plains we map 14 primary ice limits 7 of which are newly identified and 7 of which are from previous studies mapped here in greater detail we devise landsystem models to formalise and document spatial and temporal changes in glacial processes and environments our new geomorphological mapping and landsystem reconstructions provide an important insight into the response of temperate patagonian glaciers to rapidly warming climate

electromagnetic induction in the british isles region analogue model and field station results the results of a laboratory electromagnetic analogue model study which employs a horizontal inducing field over a simple model of the british isles region delineate the location and frequency dependence of the major coast effect induction anomalies of the scotland region contours of amplitudes amplitude ratios and in phase and quadrature parts of the model field measurements are presented the model vertical magnetic fields for two orthogonal source field polarizations and field station values for two hypothetical events for corresponding polarizations are compared while major discrepancies occur between model and field hz amplitudes the hz gradients across scotland which can be attributed to the coast effect are comparable in value although sometimes reversed in sign superimposed on this coast effect the field data indicate the existence of current concentrations associated with the great glen and the southern uplands faults and possibly also of currents within the scottish mainland near the east and west coasts

radiocarbon chronology of the late glacial puerto bandera moraines southern patagonian icefield argentina abstract we report radiocarbon dates that constrain the timing of the deposition of the late glacial puerto bandera moraine system alongside the western reaches of lago argentino adjacent to the southern patagonian icefield close maximum limiting radiocarbon ages n 11 for glacier advance into the outer moraines with a mean value of 11 100 60 14 c yrs bp 12 990 80 cal yrs bp were obtained from wood in deformation soft till exposed beneath flow and lodgment till in bahia del quemado on the northeast side of brazo norte north branch of western lago argentino other exposures of this basal deformation till in bahia del quemado reveal incorporated clasts of peat along with larger inclusions of deformed glaciofluvial and lacustrine deposits radiocarbon dates of wood included in these reworked peat clasts range from 11 450 45 14 c yrs bp to 13 450 150 14 c yrs bp 13 315 60 to 16 440 340 cal yrs bp the implication is that during this interval glacier fronts were situated inboard of the puerto bandera moraines with the peat clasts and larger proglacial deposits being eroded and then included in the basal till during the puerto bandera advance minimum limiting radiocarbon ages for ice retreat come from basal peat in cores sampled in spillways and depressions generated during abandonment of the puerto bandera moraines glacier recession and subsequent plant colonization were initiated close behind different frontal sectors of these moraines prior to 10 750 75 14 c yrs bp 12 660 70 cal yrs bp east of brazo rico 10 550 55 14 c yrs bp 12 490 80 cal yrs bp in peninsula avellaneda and 10 400 50 14 c yrs bp 12 280 110 cal yrs bp in bahia catalana in addition a radiocarbon date indicates that by 10 350 45 14 c yrs bp 12 220 110 cal yrs bp the brazo norte lobe or former upsala glacier had receded well up the northern branch of lago argentino to a position behind the herminita moraines furthermore glacier termini had receded to just outboard of the outer holocene moraines at lago frias and lago pearson anita prior to 10 400 40 14 c yrs bp 12 270 100 cal yrs bp and 9040 45 14 c yrs bp 10 210 50 cal yrs bp respectively the most extensive recession registered during the early holocene was in agassiz este valley where the upsala glacier had pulled back behind the outer holocene moraine reaching close to the present day glacier terminus before 8290 40 14 c yrs bp 9300 80 cal yrs bp the radiocarbon dated fluctuations of the lago argentino glacier in late glacial time given here are in accord with changes in ocean mixed layer properties predominately temperature derived from the isotopic record given here of odp core 1233 taken a short distance off shore of the chilean lake district it also matches recently published chronologies of late glacial moraines in the southern alps of new zealand on the opposite side of the pacific ocean from lago argentino finally the timing of the late glacial reversal of the lago argentino glacier fits the most recent chronology for the culmination of the antarctic cold reversal acr in the deuterium record of the epica dome c ice core from high on the east antarctic plateau therefore we conclude that the climate signature of the acr was widespread in both the ocean and the atmosphere over at least the southern quarter of the globe

glacier expansion in central patagonia during the antarctic cold reversal followed by retreat and stabilisation during the younger dryas abstract the spatial temporal footprint of millennial scale climate events during the last glacial interglacial transition can yield insights into the underlying drivers of climate change but remains poorly resolved in patagonia here we assess the glacier response to abrupt cold events and palaeolake evolution using geomorphological mapping along with 10 b e ages and optically stimulated luminescence ages from near lago belgrano 47 9 s on the eastern side of monte san lorenzo the former belgrano glacier was sustained by a climatically sensitive ice cap making the site ideal for investigating the glacier response to abrupt cold reversals our data reveal an extensive re advance at 13 1 0 6 ka consistent with cooling and increased precipitation during the antarctic cold reversal acr subsequently ice retreated by at least 10 km and created an ice dammed proglacial lake in the belgrano valley rapid recession was punctuated by smaller advances still stands sufficient to maintain an ice dam for the palaeolake and deposit a lateral moraine dated at 12 4 0 3 ka during the younger dryas yd the final withdrawal of glaciers to the mountains allowed the palaeolake to drain and resulted in an atlantic pacific drainage reversal this marks the final separation of the patagonian ice sheet into the individual ice fields at the yd holocene transition our data demonstrate the dominant acr climate signal in central patagonia but also reveals a co occurrence of the northern hemisphere yd signal albeit of smaller magnitude the acr re advance was primarily climatically controlled but its relative magnitude was likely a consequence of ice divide migration and ice flow re routing during the break up of the patagonian ice sheet

a geoelectric section across ireland from magnetotelluric soundings abstract a magnetotelluric survey across ireland has been completed the 40 sites were positioned on a line normal to the predominant caledonian geological trend i e ne sw of the country the average site separation was 5 km and frequencies from 640 to 0 01 hz were sampled bostick niblett and occam inversions of the data suggest that the carboniferous limestone of the main part of the profile is underlain by ordovician volcanic rocks in a block and trough structure with a wavelength of 15 km this supports previous gravity and magnetic interpretations at greater depths which vary significantly along the traverse an anomalous mid lower crustal conductor is apparent this conductor is probably a complex manifestation of the caledonian tectonic zone known as the iapetus suture in this respect it is significant that comparable conducting zones of similar geometry have previously been found in the suture region in southern scotland and northern england

constraints on the structure of the east pacific rise from seismic refraction data university of washington seismic data taken during rose phase 1 have been used to study the structure of the east pacific rise in the vicinity of 12 n off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2 5 km a zone of low but uniform velocity gradient from 2 5 to about 5 5 km and a transitional zone about 1 km thick to mantle material there is also evidence for substantial anisotropy in mantle velocity gradients with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise under the rise axis the data allow at most a very narrow zone of low velocity less than about 2 km wide in the crust

magmatic evolution of the austral patagonian andes abstract new geological evidence and geochronological data obtained within the patagonian cordillera between lat 47 and 49 s allow the redefinition of an andean region with different and characteristic geological evolution compared to the adjacent segments an acidic middle jurassic volcanism covered a metamorphic basement formed by low grade metasediments and metabasites of upper paleozoic age dike swarms of basic alkaline rocks were emplaced since the cretaceous in a marginal basin developed on the jurassic sialic crust the outcrop density of such bodies seems to indicate an attenuation of the sialic crust towards the south where oceanic crust of upper jurassic lower cretaceous age floored the marginal basin the cretaceous volcanic activity is well developed in the adjacent sectors but almost absent in the studied segment several magmatic pulses lead to the final emplacement of the patagonian batholith which yields its peak activity around 98 m y and presents typical geochemical variations the upper cretaceous and cenozoic magmatism are mainly exposed to the east of the cordillera and are represented by a series of alkali plateau basalts different pulses of activity have been recorded during eocene upper miocene pliocene and even during quaternary times associated with deep crustal fractures and probable upper mantle origin as evidenced by the low 87sr 86sr ratios and the ultramafic and mafic xenoliths of the lavas granitic stocks in the andean domain are post orogenic in relation to a compressive tectonic phase during miocene times the upper cenozoic calc alkaline volcanism presents a gap in this segment of the andean belt probably related with the post miocene migration of the chile rise

seismic stratigraphy buried beach ridges and contourite drifts the late quaternary history of the closed lago cardiel basin argentina 49 s the results of a seismic stratigraphic analysis of a closed lake basin lago cardiel in southernmost south america are reported very few high resolution continental records spanning the late quaternary have been obtained from this region seismic sequence stratigraphic analysis allows a reconstruction of lake level variations two major hiatuses of unknown age occurred during the early evolution of the basin with the deposition of an alluvial fan in a restricted area in the intervening time period following the development of a relatively shallow lake during the late pleistocene and a short desiccation pulse around 11 220 14c yr bp a transgression of over 135 m occurred at the beginning of the holocene the transgression was associated with the formation of beach ridges preserved in the lake stratigraphy on the floor of the modern lago cardiel at four different elevations the preservation of largely unreworked beach ridges indicates a stepwise rise in the lake level there is no seismic evidence of a major lowering of the lake below modern level during the entire holocene deposition since the mid holocene is marked by strong lateral differences in sediment accumulation with a depocentre slightly to the north of the basin midpoint and a pronounced mounded distribution seismic reflection geometries as well as sedimentological characteristics indicate a lacustrine contourite drift covering an area of 80 100 km2 as lago cardiel is under the influence of westerly winds these most likely drove lake circulation the identification of drowned beach ridges and of contourite drifts illustrates that high resolution seismic stratigraphy is not only a powerful tool in reconstructing past lake level elevations for closed lake basins but it can also provide information about the rate of lake level changes and the presence and strength of lake currents

a geomagnetic study of the great glen fault summary an attempt is made to determine the range of two dimensional current models consistent with the measured magnetovariational response for periods from 5 30 min near the great glen fault in northern scotland all current models must be symmetric about the fault line but because of uncertainty about the magnitude of the ocean effect models ranging from a line current at 80 km depth to a uniform current sheet 60 km wide at 10 km depth are equally acceptable comparison with other geophysical studies of the same area suggests that a suitable conducting zone is unlikely to be present at shallow depths and interpretation in terms of a conducting zone in the 20 80 km depth range is favoured although no such zone has been resolved by the other studies

comment on the last glacial maximum and deglaciation in southern south america by n r j hulton r s purves r d mcculloch d e sugden m j bentley quaternary science reviews 21 2002 233 241

propagation of pl and implications for the structure of tibet pl is a long period 20 s or more wave train beginning just after the p arrival in seismograms and continuing until the s arrival or the rayleigh wave the wave train can be observed at epicentral distances of about 5 20 with instruments sensitive to these low frequencies pl propagates as a partially trapped p sv wave in the crust s wave energy is lost to the mantle during propagation making pl a leaky mode we study pl propagation for a variety of earth models using the synthetic seismogram algorithm wave number integration and find that the vertical travel time in the crust is the most important parameter controlling pl s oscillation period this period can vary by more than a factor of two between oceanic and continental paths p sv leaky mode propagation includes many different modes the low frequency motion termed pl is only the first or fundamental mode in this family a second higher frequency mode roughly equal in amplitude to the fundamental appears for models without a net positive velocity gradient in the crust we use these results to match an observed pl wave train whose propagation path consisted almost entirely of the tibetan plateau by considering first and second pl mode behavior we find that the tibetan crust is about 85 km thick with an uncertainty of about 20 km and possesses a significant 0 01 s 1 positive velocity gradient the presence of a large low velocity zone in the lower tibetan crust thus seems unlikely much higher frequency pl modes also appear in the green s functions for layer over half space models and appear to be responsible for the high frequency pg phase making pl and pg different members of the same type of leaky mode propagation

recent studies of time variations of natural electromagnetic fields in scotland abstract a series of geomagnetic induction studies has been undertaken in scotland since 1973 it includes the operation of two geomagnetic arrays one over northern scotland and the other over southern scotland subsequent individual station and small scale geomagnetic array studies and three sets of magnetotelluric soundings which traverse scotland from the lewisian foreland to the english border the problems associated with the interpretation of induction data from an island located in the subauroral region are discussed qualitatively and the manner in which both coast and source field effects can be accounted for is described the geomagnetic deep sounding data gds from all the observation sites have been collated and examples of hypothetical event contour maps and traverses across the great glen and of individual events from the northern array are presented they indicate that significant lateral variations in electrical conductivity structure within the crust and upper mantle are associated with the major geological faults in the region examples of the results of the magnetotelluric soundings are also presented together with an outline of the procedure used for one and two dimensional modelling models of the geo electric structure in both northern and southern scotland have been obtained these show distinctive features which are compatible with the qualitative interpretation of the magnetovariational data for example the major granitic blocks are highly resistive while regions of relatively low resistivity exist at upper crustal depths near the great glen highland boundary and southern uplands faults a zone of low resistivity exists at lower crust uppermost mantle depths throughout much of the region the lowest value occurring under the southern uplands

geomagnetic variation sounding of the asthenosphere beneath the juan de fuca ridge two ocean bottom magnetometers have been deployed at sites on either side of the juan de fuca ridge 20 and 40 km from the crest near latitude 47 n twelve days of data were recorded simultaneously at the two sites and these data have been used to determine the electrical conductivity structure beneath the region the vertical field variations at both stations are of very low amplitude in the period range 0 25 to 8 hours with no detectable phase reversal across the ridge electric fields in the ocean layer have been estimated from the attenuation of sea floor horizontal fields with respect to victoria and the resulting impedance spectra inverted to find resistivity as a function of depth using the technique of oldenburg 1979 resistivity models which fit the data within the specified bounds are characterized by a comparatively resistive crust and upper lithosphere overlying a low resistivity zone between depths of 55 and 75 km which in turn appears to be underlain by a higher resistivity section the average resistivities are used to estimate temperature and melt fraction on the basis of the effective medium theory of shankland and waff 1976 for a basalt melt fraction within an olivine matrix for the low resistivity zone and depending on the water content of the melt 0 to 10 the estimated temperature range is 1400 c to 1250 c and the corresponding melt fraction 0 05 to 0 09 the data suggest that the melt fraction decreases below 80 km the conductivity profile from magnetotelluric data on 72 ma ocean floor in the north central pacific filloux 1977 has similar characteristics to the 1 ma old conductivity structure near the juan de fuca ridge but with the low resistivity layer at depths of 140 to 220 km in both cases the resistivity minimum should be equated with the region of maximum partial melt within the asthenosphere rather than with the base of the lithosphere the increase in depth to this zone with increasing lithosphere age is in general agreement with plate tectonic concepts

the glacial geomorphology and pleistocene history of south america between 38 s and 56 s abstract this paper presents new mapping of the glacial geomorphology of southern south america between latitudes 38 s and 56 s approximately the area covered by the former patagonian ice sheets glacial geomorphological features including glacial lineations moraines meltwater channels trimlines sandur and cirques were mapped from remotely sensed images landsat 7 etm pan sharpened landsat 7 and aster the landform record indicates that the patagonian ice sheets consisted of 66 main outlet glaciers together with numerous local cirque glaciers and independent ice domes in the surrounding mountains in the northern part of the mapped area in the chilean lake district 38 42 s large piedmont glaciers developed on the western side of the andes and the maximum positions of these outlet glaciers are in general marked by arcuate terminal moraines to the east of the andes between 38 s and 42 s outlet glaciers were more restricted in extent and formed alpine style valley glaciers along the eastern flank of the andes south of 45 s a series of large fast flowing outlet glaciers drained the ice sheet the location of these outlet glaciers was topographically controlled and there was limited scope for interactions between individual lobes west of the andes at this latitude there is geomorphological evidence for an independent ice cap close to sea level on the taitao peninsula the age of this ice cap is unclear but it may represent evidence of glacier growth during the antarctic cold reversal and or younger dryas chronozone maximum glacier positions are difficult to determine along much of the western side of the andes south of 42 s because of the limited land there and it is assumed that most of these glaciers had marine termini in the south east of the mapped area in the fuegan andes cordillera darwin the landform record provides evidence of ice sheet initiation by adding published dates for glacier advances from the literature we present maps of pre last glacial maximum lgm glacier extent lgm extent and the positions of other large mapped moraines younger than lgm in age a number of large moraines occur within the known lgm limits the age of these moraines is unknown but since many of them lie well outside the established maximum neoglacial positions the possibility that they reflect a return to glacial climates during the younger dryas chronozone or antarctic cold reversal cannot be discounted

an analogue model study of electromagnetic induction in the tasmania region abstract laboratory analogue model magnetic measurements are carried out for a model of the region including tasmania bass strait with its highly conductive deep sedimentary basins and the south coast of mainland australia the model source frequencies used simulate naturally occurring geomagnetic variations of periods 5 120 min in phase and quadrature magnetic hx hy and hz field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for e polarization electric field of the source in the n s direction and for h polarization electric field of the source in the e w direction large anomalous in phase and quadrature model magnetic fields are observed over bass strait and the coastal regions at short periods for both e and h polarization but with increasing period the field anomalies decrease more rapidly for e polarization than for h polarization the difference in response with polarization for the bass strait region is attributed to current induced in the deep ocean for all periods being channelled through bass strait for h polarization but not for e polarization the persistent large coastal field anomalies elsewhere for h polarization can be accounted for by the coastal current concentrations due to currents induced in the deep ocean for all periods deflected to the south and to the north by the shelving sea floor and channelled through bass strait and around the southern coast of tasmania the phenomena of current deflection and channelling for h polarization for the geometry of the southern australia coastline and associated ocean bathymetry is particularly effective in producing field anomalies for a large period range the coastal horizontal hx and hy field anomalies present for e polarization at short periods and for h polarization at all periods do not extend far inland and thus for inland station sites somewhat removed from the coast should not present serious problems for magnetic soundings in field work the sharp vertical field hz gradient over tasmania at short periods which is predominantly in the e w direction for e polarization and the n s direction for h polarization is strongly frequency dependent becoming almost undetectable at 60 min the behaviour of the hz field gradients however are very similar from traverse to traverse over inland tasmania and thus the effects of the ocean should not present too serious a problem in the interpretation of field station studies the discrepancies between model and field station results should be useful in mapping geological boundaries in the region

analogue model study of electromagnetic induction in the newfoundland region abstract the behaviour of magnetic field variations over the newfoundland canada region is studied using a scaled laboratory analogue model the model source frequencies used simulate periods of 100 s 300 s 900 s and 1800 s in the geophysical scale contour views of in phase and quadrature phase parts of the three magnetic field components for two orthogonal source field polarizations for 100 s and 1800 s period variations are shown large anomalous in phase and quadrature model magnetic fields are observed over the strait of belle isle and cabot strait for all four periods studied with the anomalous fields reaching maxima when the skin depth is of the order of six times the depth of the strait the cape effect that is the large field enhancements due to deflection of induced current at capes is observed for all four periods enhanced fields are observed at the continental shelf edge for 1800 s variations but not for 100 s variations a general shift in the anomalous field from the in phase to the quadrature part for all three magnetic components is found for increasing period the magnitudes and directions of inter station induction arrows and single station induction arrows are studied the two types of induction arrows differ when an anomalous horizontal magnetic field is present this is especially evident near the coastline for a period of 100 s for all periods studied the in phase induction arrows point towards the nearest and largest current concentrations in traversing the coastline the single station quadrature arrows reverse direction for the 100 s period but not for the 1800 s period the model results indicate that there is no preference for using either single station or inter station induction arrows with respect to minimizing the effects due to coastal variations for inland locations further the model results show that for periods studied 100 s 1800 s the coastal effects would not produce serious perturbations to the field measured over crustal anomalies within the central regions of newfoundland

asynchronous glacier dynamics during the antarctic cold reversal in central patagonia abstract we present 14 new 10be cosmogenic nuclide exposure ages quantifying asynchronous readvances during the antarctic cold reversal from glaciers in the baker valley region of central patagonia we constrain glacier and ice dammed palaeolake dynamics using a landsystems approach concentrating on outlet glaciers from the eastern northern patagonian icefield npi and monte san lorenzo msl soler glacier npi produced lateral moraines above lago bertrand from 15 1 0 7 to 14 0 0 6 ka when it dammed the drainage of lago general carrera buenos aires through rio baker at a bedrock pinning point at this time soler glacier terminated into the 400 m deseado level of the ice dammed palaeolake later calluqueo glacier msl deposited subaerial and subaqueous moraines in the salto valley near cochrane at 13 0 0 6 ka these moraines were deposited in an ice dammed palaeolake unified through the baker valley lago chalenko 350 m asl the salto valley glaciolacustrine landsystem includes subaqueous morainal banks ice scoured bedrock glacial diamicton plastered onto valley sides perched delta terraces kame terraces ice contact fans palaeoshorelines and subaerial push and lateral moraines boulders from the subaqueous salto moraine became exposed at 12 1 0 6 years indicating palaeolake drainage these data show an asynchronous advance of outlet glaciers from the northern patagonian icefield and monte san lorenzo during the antarctic cold reversal these advances occurred during a period of regional climatic cooling but differential moraine extent and timing of advance was controlled by topography and calving processes

electromagnetic induction in the san juan bay region of vancouver island abstract the response of the electric and magnetic field variations over the san juan bay region of vancouver island is studied using a scaled laboratory analogue model the laboratory frequencies simulate periods of 10 and 100 s in the geophysical problem the model results indicate that for both e and h polarization of the source field induced current in the ocean is deflected around cape flattery and channelled into the juan de fuca strait with increasing period the proportion of current channelled into the strait decreases sharply induced current in the strait is also funnelled into san juan bay a finger shaped bay ca 4 km wide and 7 5 km long for both polarizations of the source field the effect of the bay on the field response is confined to the local region within approximately 6 km of the centre of the bay good agreement between field station and analogue model hz results was obtained for the san juan bay the behaviour of the parkinson arrow for these two stations is examined with the aid of the analogue model results

the structure of young oceanic crust near a very fast spreading ridge we have modeled the structure of the young oceanic crust near the fastest spreading ridge ever to be profiled using seismic refraction techniques the data were originally collected during the capricorn expedition of the scripps institute of oceanography in 1953 one of the split leg profiles from that expedition c22 was conducted near the east pacific rise at 15 s where the half spreading rate is over 80 mm yr by digitizing the data and modeling the two splits of c22 using synthetic seismograms we have found that the crust is normal in thickness 5 to 7 km and in velocity structure an upper crustal transition zone is present on both legs and would appear to be a regular feature of young oceanic crust it is probably too deep 1 5 to 2 km beneath the seafloor to be correlated with the boundary between pillow basalts and sheeted dikes instead we attribute this transition zone to changes in crustal porosity and or hydrous minerals in the oceanic crust from the appearance of wide angle moho reflections near the rise axis we suggest that the very large crustal magma chamber that might be expected under this fast spreading ridge is not present this presumably results from the cooling of the crust by pervasive hydrothermal circulation

an analogue model study of electromagnetic induction for cape and bay coastlines abstract the electric and magnetic field variations across bay and cape coastlines are determined from scaled laboratory analogue models the effect on the field response of such features along an otherwise straight coastline is limited to a distance approximately equal to the dimensions diameter of the bay or cape of the irregularity for the e polarization of the source field the perturbations in hz and hy are more confined mainly to the immediate region of the bay or cape the coast effect hz hy for this polarization and variations with a 4 2 min period is approximately a factor of two larger for the cape than for the bay while the straight coastline hz hy is roughly mid way between the cape and bay values for h polarization the coast effect is again approximately a factor of two larger for the cape than for the bay but both values are larger than the value for a straight coastline the response of the various field components to the bay and cape features is decreased at longer periods and as a measure the coast effect is approximately a factor of 1 2 greater for the cape than for the bay for a 42 min period as compared with the factor of 2 for a 4 2 min period the bay and cape deflect the induced currents flowing in the ocean and modify the currents diffusing into the land this study demonstrates that data from coastal sites may be unreliable for correcting magnetic surveys or for analyses of short period geomagnetic source field variations

fluctuations of the south patagonian ice field during the last glaciation and the holocene mapping along a transect from the southeastern margin of the south patagonian ice field in torres del paine national park chile to the limits of fresh moraines of the last glacial cycle indentified eight glacier advances the four younger ones have been dated by dendrochronology tephrochronology and radiocarbon dating although the bases of 10 m deep bogs were sampled close limiting radiocarbon dates were not obtained because bog formation in this rain shadow area appears not to have commenced until ca 12000 yr ago the outermost little ice age moraine formed during the seventeenth century and three inner ones were deposited around ad 1805 1845 and after 1890 densely vegetated older moraines contiguous with little ice age deposits are possibly of late holocene age tephra from the eruption of reclus volcano at ca 11 880 yr bp was incorporated by a readvance that deposited large multiple moraines 10 16 km from the modern ice front the oldest basal peat found inside the moraine has been dated to ca 9200 yr bp these bracketing dates indicate that some eastern outlet glaciers of the ice field advanced at a time when some western tidewater outlet glaciers terminated inside their modern limits this questions the view of j h mercer and other that patagonian glaciers did not readvance during the late glacial interval a stadial event also occurred when the glaciers were some 18 20 km from their modern positions and is closely dated to ca 11880 yr bp because reclus pumice flushed down glacier forms thick upper beds in outwash deltas deposited in proglacial lakes the four older moraines pre date the late glacial eruption of reclus but are not dated closely comparison of their spatial extent with well dated moraines in the chilean lakes region suggests that they may mark advances culminating at ca 14000 yr bp ca 20000 yr bp and earlier

laboratory electromagnetic model results for the emslab region apparent resistivities and vertical to horizontal magnetic field ratios for the lincoln line traverse in the emslab project region are studied using a scaled laboratory analogue model the model includes a simulation of the juan de fuca plate subducting the canadian u s pacific northwest region in the model the subducting plate is taken to be horizontal offshore dipping at 10 beneath the coast then inland bending farther to dip at 45 in the washington oregon region and at 30 in the british columbia region the model results show that for inland sites the apparent resistivity for the telluric field component parallel to the coast and thus also parallel to the strike of the dipping conductive substructure depends strongly on period while the resistivity for the telluric field perpendicular to the coast is largely a galvanic effect and changes less with period the amplitude of the vertical to horizontal magnetic field ratio along the lincoln line shows a saddle shaped response with the coastal maximum followed by a second maximum inland due to the dipping substructure further rather than decreasing rapidly with distance inland as expected for a simple coast effect the field ratios retain significant values large distances inland for periods of 3 120 min model results for 120 min period are compared with one example of field results for the emslab project region

the vancouver island seismic project a co crust onshore offshore study of a convergent margin the seismic structure of the british columbia continental margin has been investigated using four reversed refraction profiles the profile across strike extended 350 km from the volcanic arc on the continent to the deep ocean of the juan de fuca plate the three profiles along strike were located on vancouver island on the continental shelf and in the deep ocean on the juan de fuca plate interpretation of the profile along vancouver island yields a well constrained model for the upper crust with velocity increasing from 5 3 km s at the surface to 6 4 km s at 2 km depth to 6 75 km s at 15 5 km depth where the velocity increases sharply to 7 km s the velocity structure of the deep crust and the crustal thickness are poorly constrained four possible velocity functions based on ambiguous first arrivals and or secondary phases interpreted as moho reflections are presented the preferred one includes a deep crustal low velocity zone with a crustal thickness of 37 km models with a constant 7 1 km

an analogue model study of em induction for an island near bay and cape coastlines abstract laboratory electric and magnetic model measurements were carried out to examine how the addition of a nearby elliptical island to an otherwise complex continental coastline involving bays and capes affects electromagnetic induction in coastal regions irregularities in a coastline in the form of bays and capes lead to significant spatial field variations in coastal regions due to variations in density of induced current flowing in the ocean caused by current deflection or channelling by the coastal irregularities the addition of a nearby island which for the appropriate field polarization can result in current channelling through the strait greatly influences these field spatial variations for e polarization the case of the model electric field of the inducing source parallel to the strait and for short period 4 2 min studied here variations the effect on the fields of the model elliptical island in the shallow coastal ocean at points on the nearby continental coastline was to increase by factors as large as two to three the h y amplitudes at the centres of the bay and the cape and the e x amplitude at each bay cape and straight coastline in the region of the strait as compared with the values for coastline models without an offshore island for h polarization the case of the electric field of the source perpendicular to the strait the effect of the island was to increase by a factor of two to three the h z coastline values at the edge of the bay and to decrease by a similar factor the value at the edge of the cape further the spatial variations in the fields for both polarizations over the island and over the strait were strongly influenced by current channelling through the strait thus any interpretation of field station measurements on such an island on the sea floor in the strait or on the continent in coastal areas near an island should take into account the effects due to current concentrations in the strait particularly at short periods

seismic reflections from subducting lithosphere data from two short reflection lines obtained using explosives and an airgun source together with offshore and onshore refraction profiles and a gravity interpretation provide evidence of coherent reflections between the subducting oceanic juan de fuca and overriding continental america plates

conductivity structure of oceanic upper mantle beneath the pacific plate summary ocean bottom magnetotelluric responses from three ages on the pacific plate have been inverted to reveal the presence of a conductive zone whose depth is a function of lithospheric age the conductivity achieves its maximum value 0 1 sm 1 at depths near 70 120 and 180 km for lithospheric ages of 1 30 and 72 myr respectively the existence of this conductive structure has firmly been established by constructing a number of different acceptable models and also by obtaining unique averages over depth of the conductivity an explanation in terms of partial melting in the upper mantle has produced lower bounds for the volume fraction of melt which must exist in order to produce the high conductivity beneath jdf t 1 myr the lower bound lies in the range 0 04 0 12 beneath cal t 30 myr it lies in the range 0 02 0 10 and beneath ncp t 72 myr it lies in the range 0 008 0 02 the distributions of partial melt show that the major portion of melt beneath jdf lies between 40 and 90 km and that melting has ceased by depths of about 170 km at all three sites geotherms have been computed and they are qualitatively similar to those predicted from a convective system where the temperature increases rapidly through a boundary layer to achieve a maximum value and then possibly decreases with depth the depth at which the maximum temperature is reached is about 125 km for cal and about 175 km for ncp and the maximum temperatures are estimated to be 1400 1500 c lastly as an independent check upon the partial melt explanation the location and magnitude of the seismic low velocity zone has been correlated positively with the computed distribution of partial melt at all three ocean bottom sites

fast flowing outlet glaciers of the last glacial maximum patagonian icefield abstract glacial geomorphology around the northern patagonian icefield indicates that a number of fast flowing outlet glaciers the continuation of ice streams further upglacier drained the icefield during the last glacial maximum these topographically controlled fast flowing glaciers may have dictated the overall pattern of last glacial maximum ice discharge lowered the ice surface profile and forced the ice divide westward the influence of the fast flowing outlet glaciers on icefield behavior also helps to explain why the configuration of the patagonian icefield at the last glacial maximum is not accurately represented in existing numerical ice sheet models fast flowing outlet glaciers would have strongly influenced ice discharge patterns and therefore partially decoupled the icefield from climatically induced changes in thickness and extent

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

compressional and shear wave structure of the east pacific rise at 11 20 n constraints from three component ocean bottom seismometer data we report the analysis of seismograms from two orthogonal refraction lines in the rose area of the east pacific rise at 11 20 n recorded by a three component ocean bottom seismometer obs converted shear waves are clearly observed on the horizontal seismometers for both lines including paths crossing the rise axis neither a travel time delay nor measurable attenuation is observed for shear or compressional waves which have passed beneath the rise crest at depths down to 4 km on the basis of these observations we conclude that no large crustal magma chamber is present beneath the rise at this latitude we invert the travel times of p and converted s waves to infer the crustal structure in the vicinity of the rise and we use p wave amplitudes to constrain further the range of solutions the p wave velocity distribution does not differ markedly from that of normal young oceanic crust but the shear velocity distribution results in a poisson s ratio 0 32 that may be more typical of highly fractured crust in rise axis regions a thin 80 m layer of sediment is inferred to be present beneath the obs on the basis of the measured particle motions the observation of an sp phase and the reverberatory character of the seismograms a combined consideration of the results of this experiment and other observations of rise crossing crustal phases elsewhere in the rose area leads us to conclude that any magma within the crust beneath the rise axis of this region must be confined to narrow dikes or pipes or to small isolated bodies with a vertical thickness of less than about 1 km

inversion of ocean bottom magnetotelluric data revisited three ocean bottom magnetotelluric data sets from sites on the pacific plate are reinterpreted the initial analysis found a correlation between the lithospheric age and the depth to a conductive zone beneath each site that work also suggested that the resistivity increased below the conductor this analysis which includes new methods for constructing one dimensional conductivity models shows that the postulated increase in resistivity is not demanded by the data it also reveals an unexpectedly large nonuniqueness inherent in the interpretation of these data the previously reported trends with lithospheric age still exist but they are not as strong as initially believed finally it is shown rigorously that the different age sites are distinct in that no one dimensional model can account for all three data sets

comment on cosmogenic nuclide chronology of pre last glacial maximum moraines at lago buenos aires 46 s argentina quaternary research 63 3 2005 301 315

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

lake level changes in central patagonia argentina crossing environmental thresholds for lateglacial and holocene human occupation the role and extent of climate as a cause of the expansion and decline of human cultures is still debatable it is clear however that human environment interactions are enhanced and interplay more closely in climatically sensitive areas such as around hydrologically closed basins lago cardiel is located at 498 s in the very arid rain shadow east of the andes providing an exceptionally receptive system to changes in hydrological balance results of a geophysical survey combined with sedimentological and geochemical studies provide a continuous lateglacial holo cene record of substantial water level changes these variations combined with archaeological results from the catchment area offer a unique possibility to explore the pattern of peopling within this remote area of the globe and its possible relation to climate change human occupation in patagonia is well documented towards the andes throughout the entire holocene archaeological data from the lago cardiel basin however show an apparent lack of human activity during the first part of this period which coincides with well constrained high lake levels our results show an intriguing coincidence between low lake level and increasing human occupation suggesting that the lago cardiel basin has focused human use during intervals with relatively lower effective moisture such as during the late pleistocene but its evidence may have been submerged this interpretation is confirmed by archae ological remains from lago strobel another perennial lake with a comparable catchment located in the same climatic region and thus sharing the same climatic history as lago cardiel copyright 2009 john wiley sons ltd

an analogue model study of em induction in the north china korea coastal region abstract the behaviour of the magnetic field variations over the north china korea coastal region is studied with the aid of a scaled laboratory analogue model the model source frequencies simulate naturally occurring geomagnetic variations of 3 60 min periods in phase and quadrature magnetic bx by and bz field measurements for the modelled region are presented for e and b polarizations large anomalous in phase and quadrature model magnetic fields are observed over the korea japan strait for e polarization and over the bohai strait for b polarization due to current channelling through the straits large responses of the peninsulas in the shallow coastal areas occur at short periods but decrease abruptly with increasing period in general the effects of peninsulas straits bays and irregularities in the coastlines play an important role in the magnetic field responses both on shore and off shore for this complex north china korea coastal region model and field site induction arrows are compared for three sites west of bohai bay

a geomagnetic variation study of scotland summary data from eighteen cough reitzel magnetometers and four fluxgate magnetometers which were operated in north scotland are presented and discussed the coverage given by this set of instruments was not dense enough to resolve satisfactorily the complex induction anomalies in this area but some of the major features seen are described the features observed cannot be accounted for either by oceanic induction effects or by source field effects the great glen shows up as a major conductivity feature other effects are also observed some apparently associated with the highly resistive granites found in this area

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

comparison of analogue model and field station em responses on southern vancouver island abstract measurements of geomagnetic induction responses for a laboratory analogue model that includes a simulation of the subducting juan de fuca plate are compared with those obtained at three sites on vancouver island good agreement between model and field responses at the central vancouver island site is observed over the period range 3 60 min while at the east and west coastal sites good agreement is achieved only for periods greater than 20 min at shorter periods departures of the observed responses from those of the model are possibly the result of upper crust inhomogeneities not replicated in the simulation and the complex strait with its numerous small islands and its irregular coastlines inadequately simulated in the course model nevertheless the analogue model results are consistent with the premise of a juan de fuca plate underlain by the conductive asthenosphere subducting at a shallow depth beneath vancouver island