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

mountain building in the central andes the central andes is the middle part of the andean chain between about 13 s and 27 s characterized by the parallel running high mountain chains the western and eastern cordilleras at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km the altiplano puna from the examination of geophysical and geological data in this area including earthquakes deformation gravity anomaly volcanism uplift history and plate motion we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the central andes we propose that the present cycle of mountain building has continued in the cenozoic with the most active phase since the miocene and that the present subduction angle 30 is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the miocene because of the young and buoyant slab involved this situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab addition of volcanic materials was not restricted to the western edge where active volcanoes of the western cordillera exist but extended to the western and central portion of the altiplano puna the western half of the central andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there in the eastern edge the thermal effect is small and the crust is strongly pushed by the westward moving south american plate this caused the shortening of crustal blocks due to reverse faulting and folding in the eastern cordillera and amazonian foreland the magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west these two processes are superposed between the two mountain chains and form high plateaus there the altiplano of bolivia and peru and the puna of argentina this interpretation is supported by the observation that 1 neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the altiplano and the western cordillera 2 no significant reverse fault systems are observed on the altiplano 3 neogene volcanic rocks and volcanic centers since the miocene are not restricted to the western cordillera but are widely distributed over most of the altiplano 4 most of the altiplano is in a zone of high heat flow values 5 thick paleozoic rocks are strongly folded and faulted in the eastern cordillera with little volcanism and no large scale plutonism in the cenozoic age 6 crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the eastern cordillera and amazonian foreland and 7 the crustal thickness suddenly decreases at the junction of the eastern cordillera and the amazon basin exactly at the place of reverse earthquakes

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

relationship between slab dip and topography segmentation in an oblique subduction zone insights from numerical modeling slab dip controls the state of stress in an overriding plate and affects mountain building analog and numerical models have shown variations in tectonic regime induced by slab folding over the 660 km depth discontinuity zone in orthogonal convergence here using a three dimensional model of oblique subduction 30 and accounting for free top surfaces we show how slab folding generates an along strike slab dip segmentation inducing variations in topography of the overriding plate when the subducting plate begins to curve forward the elevation height rises inland and varies along the trench from 5 km to 2 km the andes are a suitable natural zone to compare our results with because of its linear margin and well constrained plates kinematics thus we provide a new explanation to the general decrease in elevation from the central to southern andes which still remains to be combined with other 3 d mechanisms to explain the actual andean topography

review of erosion dynamics along the major n s climatic gradient in chile and perspectives abstract chile is an elongated country running in a north south direction for more than 30 along a subduction zone its climate is progressively wetter and colder from north to south this particular geography has been used positively by a growing number of studies to better understand the relationships between erosion processes and climate land use slope tectonics volcanism etc here we review the erosion rates factors and dynamics over millennial to daily periods reported in the literature in addition 21 new catchment mean erosion rates suspended sediment and 10be are provided and previous suspended sediment derived erosion rates are updated a total of 485 local and catchment mean erosion rates are reported erosion rates vary between some of the smallest values on earth 10 5 mm a to moderate values 0 5 mm a compared to other active ranges this review highlights strong limitations concerning the quantification of local erosion factors because of uncertainties in sampling point location slope and rainfall data for the mean erosion rates e for the millennial and decennial catchments a model of the form e s 1 s 0 6 2 r with 0 3 0 8 accounts for 40 to 70 of the erosion variance confirming a primary role of slope s compared to precipitation rate r over this time scale over the long term this review points to the long 5 to 10 ma response time of rivers to surface uplift in north central arid chile over millennia data provide evidence for the progressive contribution of extreme erosion events to millennial averages for drier climates as well as the link between glacier erosion and glacier sliding velocity in this period of time a discrepancy exists between the long term offshore sedimentological record and continental decennial or millennial erosion data for which no single explanation appears still little information is available concerning the magnitude of variation of millennial erosion rates over centuries data show the variable role of groundwater in the dynamics of suspended load and document a decrease in erosion over hundreds of years probably associated with historical harvesting

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

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 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

un coupled thrust belt foreland deformation in the northern patagonian andes new insights from the esquel gastre sector 41 30 43 s the patagonian andes represent a unique natural laboratory to study surface deformation in relation to deep slab dynamics in the sector comprised between latitudes 41 30 and 43 s new apatite u th he ages indicate a markedly different unroofing pattern between the broken foreland area characterized by late cretaceous to paleogene exhumation and the adjacent andean sector to the west which is dominated by miocene pliocene exhumation these unroofing stages can be confidently ascribed to inversion tectonics involving reverse fault related uplift and concomitant erosion late cretaceous paleogene shortening and exhumation are well known to have affected also the thrust belt sector of the study area during a prolonged stage of flat slab subduction therefore the different ages of near surface unroofing documented in this study suggest coupling of the deformation between the thrust belt and its foreland during periods of flat slab subduction e g during late cretaceous paleogene times and dominant uncoupling during periods of steep slab subduction and rollback even when these are associated with high convergence rates i e 4 cm year as those documented in miocene times for the patagonian andes

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

models of active glacial isostasy roofing warm subduction case of the south patagonian ice field 1 modern geodetic techniques such as precise global positioning system gps and high resolution space gravity mapping gravity recovery and climate experiment grace make it possible to measure the present day rate of viscoelastic gravitational earth response to present and past glacier mass changes the andes of patagonia contain glacial environments of dramatic mass change these mass load changes occur near a tectonically active boundary between the antarctic and south american plates the mechanical strength of the continental side of this boundary is influenced by neogene ridge subduction and by the subduction of a youthful oceanic slab a ridge of young volcanos parallels the pacific coastline release of volatiles such as water at depth along this ridge creates a unique rheological environment to assess the influence of this rheological ridge structure on the observational land uplift rate we apply a two dimensional viscoelastic earth model a numerical study is presented which examines the sensitivity of the glacial loading unloading response to the complex structure at depth related to the subducting slab the viscous wedge between slab and continental lithosphere and the increase of elastic thickness from oceanic to continental lithosphere a key feature revealed by our numerical experiments is a continuum flow wherein the slab subdues the material transport toward oceanic mantle and crust the restricted flow is sensitive to the details of slab mechanical strength and penetration into the upper mantle the reduced viscosity within the mantle wedge however enhances the load induced material transport everywhere within the asthenosphere

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

the sedimentary record of glacial retreat marinelli fjord patagonia regional correlations and climate ties abstract the inland passage of chile comprises the most continuous latitudinal transect of ice fields ice caps and alpine glaciers in the southern hemisphere providing an excellent natural laboratory for the study of glacial response to climate change during the holocene here we report results from a marine geological investigation of marinelli fjord tierra del fuego 54 25 s 69 35 w and ainsworth fjord which merges with marinelli fjord data acquired during july 2005 onboard the rv ib nathaniel b palmer include high resolution seismic data multibeam swath bathymetry data and sediment cores post cruise analyses include detailed seismic facies and lithofacies analysis and radiocarbon dating the results indicate that a major transition in the glacimarine record from ice proximal facies to ice distal facies occurred in the outer basins of the fjord at 15 5 kyr to 12 5 kyr the ice proximal facies consists of pebbly mud with abundant laminations and is characterized by high average sedimentation rates 28 mm yr in contrast an ice distal facies is dominantly bioturbated mud with low average sedimentation rates 0 56 mm yr this transition marks the retreat of marinelli glacier from the outer basins with no evidence for glacial re advance subsequently during the mid to late holocene ainsworth glacier retreated and exposed ainsworth fjord our results are consistent with the climatic trends for southern south america where the final deglaciation of the region ended at 12 5 ka during the holocene stable ice conditions persisted until the mid 1960s when marinelli glacier began rapidly retreating to its modern day position

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

retroarc deformation and exhumation near the end of the andes southern patagonia abstract the southern patagonian andes constitute the narrow high latitude end of the andean orogen belt in south america where inherited basin paleogeography subduction processes retroarc crustal thickening and late cenozoic glaciation have collectively influenced their unusual tectonic and physiographic evolution new zircon and apatite u th he thermochronology from the patagonian andes between 50 30 and 51 30 s suggest concentrated exhumation across the retroarc leeward side of the orogen since early miocene time zircon u th he zhe ages range from 44 to 10 ma oldest ages are recorded in the patagonian batholith and along the far eastern frontal foreland monocline regionally uniform zhe ages between 22 and 18 ma located across a 75 km wide zone of the patagonian retroarc thrust belt indicate widespread early miocene cooling through the zhe partial retention zone mesozoic sedimentary and volcanic rocks in this region have been exhumed from at least 5 to 6 km depths early miocene denudation of the thrust belt deformation and increased foreland sedimentation rates coincided with opening of the scotia sea suggesting a causal response of the foreland to changes in plate dynamics the apatite u th he ahe ages from a similar region range from 11 to 3 ma the youngest ages 6 3 ma are spatially clustered within the more deeply exhumed central thrust domain we interpret these ahe ages to record 1 2 km of erosional denudation associated with late cenozoic glaciation and fluvioglacial processes in patagonia beginning ca 7 ma zircon results indicate that since ca 22 ma long term exhumation rates have been highest across the western and central thrust domains 0 22 0 37 mm yr 1 and significantly lower along the eastern thrust front 0 10 0 17 mm yr 1 since 7 ma apatite results from these same regions suggest comparable and slightly higher denudation rates 0 14 0 46 mm yr 1 particularly within the eastern thrust domain consistent with efficient erosional processes acting in the retroarc region these results from the patagonian retroarc region particularly the predominance of

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

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

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

tectonic control on rock uplift exhumation and topography above an oceanic ridge collision southern patagonian andes 47 s chile

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

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

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

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

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

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

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

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

can glacial erosion limit the extent of glaciation kaplan m r hein a s hubbard a 2009 can glacial erosion limit the extent of glaciation geomorphology 103 2 172 179

tectonic implications of a paleomagnetic study of the sarmiento ophiolitic complex southern chile abstract a paleomagnetic study was carried out on the late jurassic sarmiento ophiolitic complex soc exposed in the magallanes fold and thrust belt in the southern patagonian andes southern chile this complex mainly consisting of a thick succession of pillow lavas sheeted dikes and gabbros is a seafloor remnant of the late jurassic to early cretaceous rocas verdes basin that developed along the south western margin of south america stepwise thermal and alternating field demagnetization permitted the isolation of a post folding characteristic remanence apparently carried by fine grain sd magnetite both in the pillow lavas and dikes the mean in situ direction for the soc is dec 286 9 inc 58 5 95 6 9 n 11 sites rock magnetic properties petrography and whole rock k ar ages in the same rocks are interpreted as evidence of correlation between remanence acquisition and a greenschist facies metamorphic overprint that must have occurred during latest stages or after closure and tectonic inversion of the basin in the late cretaceous the mean remanence direction is anomalous relative to the expected late cretaceous direction from stable south america particularly a declination anomaly over 50 is suggestively similar to paleomagnetically interpreted counter clockwise rotations found in thrust slices of the jurassic el quemado fm located over 100 km north of the study area in argentina nevertheless a significant ccw rotation of the whole soc is difficult to reconcile with geologic evidence and paleogeographic models that suggest a narrow back arc basin sub parallel to the continental margin a rigid body 30 westward tilting of the soc block around a horizontal axis trending nnw is considered a much simpler explanation being consistent with geologic evidence this may have occurred as a consequence of inverse reactivation of old normal faults which limit both the soc exposures and the cordillera sarmiento to the east the age of tilting is unknown but it must postdate remanence acquisition in the late cretaceous two major orogenic events of the southern patagonian andes in the eocene ca 42 ma and middle miocene ca 12 ma respectively could have caused the proposed tilting

beef veins and thrust detachments in early cretaceous source rocks foothills of magallanes austral basin southern chile and argentina structural evidence for fluid overpressure during hydrocarbon maturation abstract we describe 1 bedding parallel veins of fibrous calcite beef and 2 thrust detachments which we believe provide good evidence for fluid overpressure in source rocks for petroleum our examples are from the surface or subsurface of the magallanes austral basin which lies at the southern tip of south america there the best source rocks for petroleum are of early cretaceous age in the central parts of the basin these source rocks have become overmature but at the eastern edge onshore and offshore they are today either immature or in the oil window in tierra del fuego the foothills of the andes consist mainly of sedimentary rocks which have undergone thin skinned thrusting in the vicuna area chile early cretaceous source rocks have reached the surface above thrust detachments which are visible on seismic data and well data at the surface we have found calcite beef containing hydrocarbons solid and or fluid in the rio jackson and vicuna formations which have reached the wet gas window in the rio gallegos area argentina the source rocks have not reached the surface but seismic and well data provide good evidence for thin skinned thrusting above flat lying detachments in early cretaceous source rock where it is in the early oil window in contrast there is little or no deformation where the source rock is still immature thus the deformation front coincides with the maturity front next to the central parts of the basin where the source rocks have reached the surface within the andes proper they have undergone low grade metamorphism within these source rocks we have found beef veins but of quartz not calcite to the east within the foreland basin seismic and well data provide evidence for a few compressional structures including thin skinned detachments in the deeply buried source rock finally in the northern part of the basin santa cruz province argentina where it is shallower the source rocks have reached the surface in the foothills above a series of back thrusts at lago san martin the source rocks have reached the oil window and they again contain calcite beef in conclusion where we have examined early cretaceous source rocks at the surface they contain either calcite beef if they have reached the late oil window or wet gas window or quartz beef if they are overmature independent evidence for overpressure in the form of source rock detachments comes from subsurface data especially at the southern end of the basin where the source rocks are not overmature and deformation is relatively intense thus we argue that hydrocarbon generation has led to overpressure as a result of chemical compaction and load transfer or volume changes or both

continental response to active ridge subduction 1 apatite fission track ages from a 2000 m elevation transect from the patagonian fold and thrust belt 47 5 s allow us to quantify the denudational and orographic response of the upper plate to active ridge subduction accelerated cooling started at 17 ma predating the onset of ridge collision 14 10 ma and was followed by reheating between 10 and 6 ma thermal modeling favors reheating on the order of 60 c at 28 c ma due to east migration of a slab window after the ridge trench collision final rapid cooling since 4 ma of 18 c ma geothermal gradient of 14 c km correlates with the presence of an orographic barrier and 1 km rock uplift in this region between 17 1 and 6 3 ma increased precipitation and erosion since 4 ma caused asymmetric exhumation with 3 4 km on the leeside repeated crustal unroofing in response to active ridge subduction can explain the positive gravity anomaly south of the chile triple junction

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

sunda java trench kinematics slab window formation and overriding plate deformation since the cretaceous the kinematics and time dependence of back arc extension or compression is one of the most poorly understood aspects of plate tectonics and has nearly exclusively been studied from snapshots of present day observations here we combine absolute and relative plate motions with reconstructions of now subducted ocean floor to analyse subduction kinematics and upper plate strain from geological observations since 80 ma along the 3200 km long sunda java trench one of the largest subduction systems on earth combining plate motions and slab geometries enables us to reconstruct a time dependent slab window beneath sundaland formed through wharton spreading ridge subduction we find that upper plate advance and retreat is the main influence on upper plate strain but subduction of large bathymetric ridges and slab window effects also play a significant and at times dominant role compression in the sundaland back arc region can be linked to advance of the upper plate extension of the sundaland backarc region correlates with two patterns of upper plate motion a retreat of the upper plate and b advance of the upper plate combined with more rapid advance of the sundaland margin due to hinge rollback subduction of large bathymetric ridges causes compression in the upper plate especially wharton ridge subduction underneath sumatra over the period 15 0 ma our reconstructions unravel the evolving geometry of a slab window underlying the java south sumatra region and we propose that decreased mantle wedge viscosities associated with this slab window exacerbated palaeogene extension in the java sea region via active rifting and enabled sumatran continental extension to continue at 50 35 ma when upper plate advance would otherwise have led to compression

geometry and evolution of the frontal part of the magallanes foreland thrust and fold belt vicuna area tierra del fuego southern chile the magallanes foreland thrust and fold belt is a thin skinned foreland thrust and fold belt of paleocene to oligocene age that deforms upper jurassic through tertiary volcanic volcaniclastic and siliciclastic strata of the magallanes basin southern andean cordillera chile this paper is a detailed description and analysis of the geology and structural evolution of the thrust front vicuna area of southern tierra del fuego reflection seismic and well data together with 1 50 000 scale geological mapping have been used in the analysis in the southern part of the vicuna area two different thrust systems have been found an upper imbricate fan that deforms upper jurassic and cretaceous strata and a younger lower duplex composed of cretaceous and probably upper jurassic rocks the imbricate fan is characterized by fault propagation folding in which listric thrust faults merge downward into a sole thrust that probably is located within the upper jurassic stratigraphy this fan system generates an intercutaneous wedge with a well developed triangle zone at its leading edge the sole thrust of the upper imbricates forms the roof thrust of the underlying duplex in the northern part of the vicuna area the syntectonic sedimentary wedge of the foredeep consists of late cretaceous through tertiary siliciclastics that have been deformed and uplifted by passive back thrusting at the triangle zone the structural style in the foreland region shows three main subhorizontal detachment levels located within the sedimentary wedge as a result of the progressive transfer of slip from the thrust belt to the foreland minor blind thrusts produce stacked pop up and triangle structures that result in complex geometries in the cores of anticlines a forward breaking sequence of thrusting is interpreted during deformation the active foredeep wedge migrated at least 10 km northward balanced geological cross sections indicate approximately 60 30 km shortening for this part of the magallanes thrust belt

magmatic evidence for neogene lithospheric evolution of the central andean flat slab between 30 s and 32 s abstract geochemical data from andean miocene magmas erupted through the now volcanically inactive flat slab between 30 s to 32 s coupled with geological and geophysical data illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone pb sr and nd isotopic and trace element data show that main cordilleran precordilleran and pampean magmas were contaminated in both the mantle and crust and that the nature of the contaminants varied in time and space reflecting tectonic events contaminants included both enriched high lil element high sr and pb and low nd isotopic ratios and depleted low lile element low sr and pb and high nd isotopic ratios types in the western region main cordilleran earlier miocene lavas had contaminants with less enriched signatures than later miocene lavas progressive enrichment is attributed to a increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge and b contamination in a thickening main cordilleran lower crust whose composition was progressively enriched this enrichment occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging heating and deformation of crust from beneath the shortening precordillera thrust belt to the east further east magmas erupted through back arc crust have more depleted signatures those erupted in the central part through the evolving precordilleran thrust belt were contaminated by an older thinner grenville 1100 ma basement whose depleted signature is unique among central andean terranes late miocene pocho lavas erupted further east in conjunction with uplift of the sierras pampeanas show enrichment through time arguably these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west and progressively incorporated into the convecting asthenosphere first order mass balance calculations comparing the lithospheric geometry proposed for 20 ma with that today suggests that 60 of the lithosphere within 600 km of the trench at 20 ma has been lost this lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab and thickening and shortening of the crust continental lithosphere recycled into the asthenosphere could contribute to the sources of the enriched em1 and em2 mantle components in oceanic island basalts

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

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

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

magmatic tectonic effects of high thermal regime at the site of active ridge subduction the chile triple junction model abstract high thermal gradients are expected to be found at sites of subduction of very young oceanic lithosphere and more particularly at ridge trench trench rtt triple junctions where active oceanic spreading ridges enter a subduction zone active tectonics associated with the emplacement of two main types of volcanic products 1 morb type magmas and 2 calc alkaline acidic magmas in the forearc also characterize these plate junction domains in this context morb type magmas are generally thought to derive from the buried active spreading center subducted at shallow depths whereas the origin of calc alkaline acidic magmas is more problematic one of the best constrained examples of ridge trench interaction is the chile triple junction ctj located southwest of the south american plate at 46 12 s where the active chile spreading center enters the subduction zone in this area there is a clear correlation between the emplacement of magmatic products and the migration of the triple junction along the active margin the ctj lava population is bimodal with mafic to intermediate lavas 48 56 sio 2 and acidic lavas ranging from dacites to rhyolites 66 73 sio 2 previous models have shown that partial melting of oceanic crust plus 10 20 of sediments leaving an amphibole and plagioclase rich residue is the only process that may account for the genesis of acidic magmas due to special plate geometry in the ctj area a given section of the margin may be successively affected by the passage of several ridge segments we emphasize that repeated passages will lead to the development of very high thermal gradients allowing melting of rocks of oceanic origin at temperatures of 800 900 c and low pressures corresponding to depths of 10 20 km depth only in addition the structure of the ctj forearc domain is dominated by horizontal displacements and tilting of crustal blocks along a network of strike slip faults the occurrence of such a deformed domain implies that an important tectonic coupling may exist between the upper and the lower plates leading to the partitioning of the continental lithosphere and to the tectonic underplating of very young oceanic lithosphere below the continental wedge we assume that in the case of the ctj the uncommon situation of three successive ridge segments entering the trench at 2 3 ma intervals only resulted in a strong and finally long lived thermal anomaly this anomaly caused remelting of underplated portions of very young still hot oceanic lithosphere only particular geometrical rtt configurations are able to produce such features these include linear continental margin short ridge segments slightly oblique to the trench and short transform faults finally the ctj example shows that a possible scenario for the origin of calc alkaline acidic rocks in the near trench region involves coeval tectonic coupling and repeated passage of thermal anomalies due to successive subduction of short ridge segments therefore the local abundance of calc alkaline acidic rocks associated with morb type lavas in ancient series could be the tracer of plate tectonic configurations involving the subduction of short ridge segments in a relatively short duration

formation of early archean granite greenstone terranes from a globally chondritic mantle insights from igneous rocks of the pilbara craton western australia abstract the continental crust grows via juvenile additions from the mantle however the timing of initial continent stabilisation and the rate of subsequent continental growth during the first billion years of earth history is widely debated in part due to uncertainty over the composition of the mantle source of new crust well preserved archean granite greenstone terranes as present within the pilbara craton western australia provide insights into the sources of felsic magmas and the processes of continental growth and evolution in the distant geological past at the regional scale here we present zircon u pb o and hf isotope data from ancient gneissic and granitic rocks of the pilbara craton to decipher magma sources and the timing and processes of craton growth there is no evidence for depleted mantle compositions and the simplest interpretation is that the crust of the pilbara craton was generated from mantle with a chondritic hf isotope composition our results indicate crustal addition at 3 59 ga represented by emplacement of gabbroic to anorthositic rocks we suggest that the formation of these igneous rocks and the foundering of the complementary residues triggered extensive melting of hot upwelling mantle leading to the subsequent accumulation of the 12 km thick greenstone belt eruptive sequences from 3 53 ga with emplacement of coeval felsic magmas at depth this process shaped the initial crustal configuration of the proto craton which subsequently underwent gravitationally driven overturn and reworking to generate stable cratonic continental crust with the distinctive dome and keel architecture the zircon hf and o isotope signatures of the pilbara igneous rocks from 3 59 3 4 ga do not support remelting of an ancient 3 8 ga basement and reinforce the overwhelmingly chondritic to near chondritic zircon hf isotope composition of eoarchean meta igneous rocks from a number of different archean cratons a corollary of this remarkable global consistency is that a significant volume of the mantle maintained a chondritic composition for the lu hf system from the formation of the earth into the paleoarchean up to 3 6 3 5 ga as would be the case if stabilised volumes of felsic continental crust prior to 3 5 ga were relatively small one implication is that the common assumption of a linear evolution of depleted mantle from 4 5 ga to the present day is inappropriate for determining the timing and volume of continental crust extraction in the archean the nearly identical early evolution of the pilbara and kaapvaal cratons suggests a common process to generate archean granite greenstone terranes that does not require extensive reworking of ancient crust but rather involves juvenile crustal addition above persistent zones of upwelling chondritic mantle

crustal collapse in the andean backarc since 2 ma tromen volcanic plateau southern central andes 36 40 37 30 s abstract analysis of seismic lines and gravity data shows the existence of quaternary extensional depocenters beneath thick covers of

hydrocarbon exploration potential within intraplate shear related depocenters deseado and san julian basins southern argentina the deseado and san julian late paleozoic mesozoic rift basins occupy sizable parts of central patagonia and offshore southern argentina respectively the basins record a similar linked tectonostratigraphic evolution since their inception in the late paleozoic both evolved as continental intracratonic rifts developed above a basement comprising devonian low grade metasediments overlying older continental crust in response to regional tectonic extension that preceded late disaggregation of gondwana this late permian rifting was followed by continuous filling of the basin during triassic thermal subsidence jurassic extension produced a framework of grabens and half grabens filled with mafic and felsic proximal volcanics and volcaniclastics the east west orientation of these jurassic rift features in sharp contrast to the north south trend of the permian triassic rift systems suggests that the jurassic depocenters shifted in response to oblique extension development of a transtensional pull apart basin in the upper valanginian aptian a strong compressive event marked by a prominent angular unconformity affected the deseado and san julian basins wrenching associated with this compressive event produced transpressive structures in both basins a period of relative tectonic stability was observed during the upper cretaceous and tertiary the only interruption being a mild transpressive reactivation along major preexisting faults during the miocene andean orogeny the first marine incursion occurred in the paleocene as the seas transgressed over an extensive peneplained terrain the deseado and san julian basins remain virtually unexplored only three wells having been drilled oil seeps in neocomian strata in the deseado basin indicate that this frontier basin has petroleum potential source quality intervals were encountered in the neocomian bajo grande formation of the san julian basin begin page 1796 distal lacustrine facies of the neocomian la matilde formation are expected to be potential source rocks in the deseado basin the permian triassic section is also thought to contain well developed source rocks however the section has yet to be penetrated reservoir quality rocks occur in clastics of the upper cretaceous neocomian triassic and permian formations possible play types include paleozoic reservoirs in positive transpressive features lateral updip pinch outs of neocomian sandstones and fractured jurassic volcanics adjacent to fault zones

tectonic events reflected by palaeocurrents zircon geochronology and palaeobotany in the sierra baguales of chilean patagonia abstract the sierra baguales situated north of the torres del paine national park in the magallanes region of southern chile shows a well exposed stratigraphic sequence ranging from the late cretaceous to late pliocene which presents a unique opportunity to study the evolution of sedimentological styles and trends palaeoclimate changes and tectonic events during this period the depositional environment changed from a continental slope and shelf during the cenomanian campanian tres pasos formation to deltaic between the campanian maastrichtian dorotea formation and estuarine in the lutetian bartonian man aike formation during the rupelian a continental environment with meandering rivers and overbank marshes was established rio leona formation this area was flooded in the early burdigalian estancia 25 de mayo formation during the patagonian transgression but emerged again during the late burdigalian santa cruz formation measured palaeocurrent directions in this mesozoic cenozoic succession indicate source areas situated between the northeast and east southeast during the late cretaceous east southeast during the middle eocene and southwest during the early oligocene to early miocene this is confirmed by detrital zircon age populations in the different units which can be linked to probable sources of similar ages in these areas the east southeastern provenance is here identified as the antarctic peninsula or its northeastern extension which is postulated to have been attached to fuegian patagonia during the eocene the southwestern and western sources were exhumed during gradual uplift of the southern patagonian andes coinciding with a change from marine to continental conditions in the magallanes austral basin as well as a decrease in mean annual temperature and precipitation indicated by fossil leaves in the rio leona formation the rain shadow to the east of the andes thus started to develop here during the late eocene early oligocene 34 ma long before the quechua phase of andean tectonics 19 18 ma that is generally invoked for its evolution at lower latitudes

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

supergene oxidation of epithermal gold silver mineralization in the deseado massif patagonia argentina response to subduction of the chile ridge radiometric ages for supergene alunite and jarosite effectively date the oxidation of former concentrations of pyrite and any associated sulfide minerals these k bearing sulfate minerals formed under low ph conditions are uncommon supergene products in low sulfidation epithermal deposits because of the general paucity of pyrite for acid generation for this reason the age of supergene oxidation locally to depths of 200 m or more in the epithermal au ag deposits of the deseado massif located in the extra andean foreland of patagonia southern argentina has remained unknown although theoretically the oxidation could have taken place anytime between the late jurassic when the au ag mineralization formed and pleistocene k ar ages for alunite and jarosite from two widely separated and unusually pyritic ag bearing hydrothermal breccias lejano and libanesa show it to have been mid miocene 13 8 1 8 ma this is the time when the deseado massif underwent appreciable regional scale tectonic uplift and valley incision following 140 myr during which the region was topographically subdued and the site of either fluvio lacustrine or shallow marine sedimentation the uplift combined with increasing aridity due to the orographic rain shadow caused by growth of the patagonian andes to the west and enhanced by global cooling would have depressed regional groundwater tables thereby promoting the supergene sulfide oxidation the mid miocene uplift appears to have been triggered by development of a slab tear and slab window beneath the deseado massif during early stages of subduction of the chile oceanic ridge spreading center at the pacific margin supergene sulfide oxidation in both the deseado massif and atacama desert of northern chile was the result of cenozoic uplift during progressive aridification although the causes of these phenomena were radically different however when the supergene oxidation was taking place in the deseado massif up to 30 myr of supergene activity in the atacama desert were coming to an end because of the onset of hyperaridity

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

constraints on deformation of the southern andes since the cretaceous from anisotropy of magnetic susceptibility the southernmost segment of the andean cordillera underwent a complex deformation history characterized by alternation of contractional extensional and strike slip tectonics key elements of southern andean deformation that remain poorly constrained include the origin of the orogenic bend known as the patagonian orocline here renamed as patagonian arc and the exhumation mechanism of an upper amphibolite facies metamorphic complex currently exposed in cordillera darwin here we present results of anisotropy of magnetic susceptibility ams from 22 sites in upper cretaceous to upper eocene sedimentary rocks within the internal structural domain of the magallanes fold and thrust belt in tierra del fuego argentina ams parameters from most sites reveal a weak tectonic overprint of the original magnetic fabric which was likely acquired upon layer parallel shortening soon after sedimentation magnetic lineation from 17 sites is interpreted to have formed during compressive tectonic phases associated to a continuous n s contraction our data combined with the existing ams database from adjacent areas show that the early cretaceous late oligocene tectonic phases in the southern andes yielded continuous contraction variable from e w in the patagonian andes to n s in the fuegian andes which defined a radial strain field a direct implication is that the exhumation of the cordillera darwin metamorphic complex occurred under compressive rather than extensional or strike slip tectonics as alternatively proposed if we agree with recent works considering the curved magallanes fold and thrust belt as a primary arc i e no relative vertical axis rotation of the limbs occurs during its formation then other mechanisms different from oroclinal bending should be invoked to explain the documented radial strain field we tentatively propose a kinematic model in which reactivation of variably oriented jurassic faults at the south american continental margin controlled the late cretaceous to cenozoic evolution of the magallanes fold and thrust belt yielding the observed deformation pattern

post tectonic uplift of the cordillera darwin orogenic core complex evidence from fission track geochronology and closing temperature time relationships the cordillera darwin orogenic core complex is a structural and topographic culmination in the southern andes which underwent mid cretaceous penetrative deformation and metamorphism associated with the deformation and incipient obduction of a partly ophiolitic back are basin rocas verdes block along its southern boundary whereas structural evidence suggests uplift of the rocas verdes terrane relative to the cordillera darwin during this event metamorphic assemblages reflect a greater net uplift of the opposite sense new fission track ages together with existing k ar and rb sr ages were used to estimate cooling and uplift histories and their relationships to foreland sedimentation and present day topographic relief the results suggest that whereas initiation of uplift in the cordillera darwin and rocas verdes blocks may have accompanied mid cretaceous deformation the major rapid uplift around 0 5 1 5 mm y occurred after the deformation also uplift occurred earlier in the rocas verdes block than in the cordillera darwin block the latter having experienced a greater net uplift since the mid cretaceous the major post tectonic uplift of the cordillera darwin occurred at a slower rate 0 05 0 20 mm y and coincides with the deposition of thick flysch and conglomerates in the foreland basin