on the antarctic peninsula batholith the plutonic rocks of the antarctic peninsula magmatic arc form one of the major batholiths of the circum pacific rim the antarctic peninsula batholith is a 1350 km long by 210 km wide structure which was emplaced over the period 240 to 10 ma with a cretaceous peak of activity that started at 142 ma and waned during the late cretaceous early jurassic and late jurassic early cretaceous gaps in intrusive activity probably correspond to episodes of arc compression in a northern zone of the antarctic peninsula the batholith intrudes palaeozoic mesozoic low grade meta sedimentary rocks and in a central zone it intrudes schists and ortho and paragneisses which have late proterozoic nd model ages and were deformed during triassic to early jurassic compression in a southern zone the oldest exposed rocks are permian sedimentary rocks and deformed jurassic volcanic and sedimentary rocks all these pre batholith rocks formed a belt of relatively immature crust along the gondwana margin with few exceptions jurassic plutons crop out only within the central zone many are peraluminous having s like mineralogies and relatively high 87sr 86sri they are considered to consist largely of partial melts of upper crust schists and gneisses and components of mafic magmas that caused the partial fusion by contrast early cretaceous plutons crop out along the length of the batholith few magma compositions appear to have been affected by upper crust the bulk being compositionally independent of the type of country rock they intrude they are dominated by metaluminous calcic si oversaturated 1 type granitoid rocks with relatively low 87sr 86sri intermediate silicic compositions 5 mgo we interpret these to represent partial melts of basic to intermediate igneous locally garnet bearing lower crust contemporaneous mafic magmas e g syn plutonic dykes form a more alkaline si saturated series having higher 143nd 144nd at the same87sr 86sr than the intermediate silicic series to which they are not petrogenetically related the change from limited partial fusion of upper crust in jurassic times to widespread partial fusion of lower crust in early cretaceous times is considered to be a result of an increasing volume of basaltic intrusion into the crust with time

late jurassic bimodal magmatism in the northern sea floor remnant of the rocas verdes basin southern patagonian andes magmatic and detrital zircon ages from the rocas verdes basin a tectonically juxtaposed remnant of sea floor in the magallanes fold and thrust belt southern patagonia south america indicates that a rifting phase of the rocas verdes basin occurred between 152 and 142 ma and was accompanied by bimodal magmatism a dacite dyke cross cutting pillow basalt successions and a plagiogranite dyke in mixed mafic felsic terranes of the basal sarmiento ophiolite complex contain 150 ma zircon crystals indicating that mafic submarine volcanism had started prior to or during the late jurassic 10 15 ma earlier than previously thought the silicic pyroclastic rocks of the tobifera formation with two samples dated at 148 and 142 ma were heralded by synrift sedimentation along fault bounded grabens within palaeozoic metasediments no evidence for an active volcanic arc during the early formation of the rocas verdes basin was detected in detrital zircon grains of the lower sedimentary member of the tobifera formation a minimum of 25 ma of continuous sedimentation in the rocas verdes basin is suggested by detrital zircon grains in the upper member of the zapata formation the rocas verdes basin was rimmed on the western side by an incipient and subaerial magmatic arc only in its later evolution

tonalite granodiorite suites as cotectic systems a review of experimental studies with applications to granitoid petrogenesis abstract granodiorites and tonalites granotonalites in this review paper form large batholiths at active continental margins and collisional orogens although the relation with subduction is obvious the mechanisms of magma generation remain among the most debated topics in earth sciences today the most outstanding features of these calc alkaline batholiths are summarized in this review the problem has been addressed by laboratory experiments along decades dispersed data from experimental studies are compiled here in order to give phase equilibria constraints of general application it is apparent from chemical data that major element compositional trends follow cotectic patterns whose composition is largely dependent on intensive variables the comparison between experiments and batholith chemical trends produces an empirical thermodynamic framework useful to understand in a first approach the origin and evolution of magmas forming granite batholiths a new projection scheme of rock compositions onto the pseudoternary plane defined by the fe mg anorthite orthoclase is given this can be taken as first approach to phase equilibria model of batholiths it has been revealed useful to distinguish processes controlled by increasing melting or decreasing crystallization magma temperature from processes controlled by assimilation or magma mixing the available experimental data although scarce can be used to tentatively depict phase equilibria in the projected pseudoternary f an or diagram although important limitations are imposed by the projective procedure phase equilibria relations in equilibrium with liquids can be drafted in the diagram the stability fields of mineral in areas close to the liquid are defined according to reported phase assemblages in the experimental studies of reference the diagram is robust to identify cotectic and non cotectic relations phase equilibrium constraints and rock textures have revealed that one of the most important parameter controlling phase equilibria in granite magmas is the initial water content typically calc alkaline granitoids from qtz diorite to granotonalite and granite have crystallized from strongly undersaturated water magmas low initial water contents w i in the range 1 2 wt h 2 o may account for residual saturated melt fractions as low as 0 1 0 2 for plutons emplaced at 400 mpa the water content of these residual melt fractions is enough to stabilize hydrous minerals namely hbl and bt as predicted by experiments the initial water content is a primary feature of the magma and it is directly related to the melting reaction and more precisely to the temperature of magma generation the study of compositional trends supports magma fractionation by crystal liquid separation as a relevant role as a first approach the pressure composition relations in the melts can be seen as a response to the composition of the solid saturating assemblage and particularly related to the ratio grt pl in the solid residue

pacific subduction coeval with the karoo mantle plume the early jurasssic subcordilleran belt of northwestern patagonia the early mesozoic magmatism of southwestern gondwana is reviewed in the light of new u pb shrimp zircon ages 181 2 ma 181 3 ma 185 2 ma and 182 2 ma that establish an early jurassic age for the granites of the subcordilleran plutonic belt in northwestern argentine patagonia new geochemical and isotopic data confirm that this belt represents an early subduction related magmatic arc along the proto pacific margin of gondwana thus subduction was synchronous with the initial phase of chon aike rhyolite volcanism ascribed to the thermal effects of the karoo mantle plume and heralding rifting of this part of the supercontinent overall there is clear evidence that successive episodes of calc alkaline arc magmatism from late triassic times until establishment of the andean patagonian batholith in the late jurassic involved westerly migration and clockwise rotation of the arc this indicates a changing geodynamic regime during gondwana break up and suggests differential rollback of the subducted slab with accretion of new crustal material and or asymmetrical scissor like opening of back arc basins this almost certainly entailed dextral displacement of continental domains in patagonia

the evolution of the patagonian ice sheet from 35 ka to the present day patice abstract we present patice a gis database of patagonian glacial geomorphology and recalibrated chronological data patice includes 58 823 landforms and 1 669 geochronological ages and extends from 38 s to 55 s in southern south america we use these data to generate new empirical reconstructions of the patagonian ice sheet pis and subsequent ice masses and ice dammed palaeolakes at 35 ka 30 ka 25 ka 20 ka 15 ka 13 ka synchronous with the antarctic cold reversal 10 ka 5 ka 0 2 ka and 2011 ad at 35 ka the pis covered of 492 6 x103 km2 had a sea level equivalent of 1 496 mm was 350 km wide and 2090 km long and was grounded on the pacific continental shelf edge outlet glacier lobes remained topographically confined and the largest generated the suites of subglacial streamlined bedforms characteristic of ice streams the pis reached its maximum extent by 33 28 ka from 38 s to 48 s and earlier around 47 ka from 48 s southwards net retreat from maximum positions began by 25 ka with ice marginal stabilisation then at 21 18 ka which was then followed by rapid irreversible deglaciation by 15 ka the pis had separated into disparate ice masses draining into large ice dammed lakes along the eastern margin which strongly influenced rates of recession glacial readvances or stabilisations occurred at least at 14 13 ka 11 ka 6 5 ka 2 1 ka and 0 5 0 2 ka we suggest that 20th century glacial recession a 1 is occurring faster than at any time documented during the holocene

rocas verdes ophiolites southernmost south america remnants of progressive stages of development of oceanic type crust in a continental margin back arc basin

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

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

tectonic development of the north patagonian andes and their related miocene foreland basin 41 30 43 s 1 the northern patagonian andes have been constructed through multiple mechanisms that range from tectonic inversion of extensional structures of early to middle jurassic age in the main andes to oligocene in the precordilleran region these have acted during two distinctive orogenic stages first in late early cretaceous and later in miocene times late oligocene extension separates these two contractional periods and is recorded by half grabens developed in the retroarc region the last contractional stage coexists with an eastward foreland expansion of the late miocene arc whose roots are presently exposed as minor granitic stocks and volcanic piles subordinately in the main andes east of the present arc as a consequence of this orogenic stage a foreland basin has developed having progressed from 18 ma in the main north patagonian andes where the mountain front was flooded by a marine transgression corresponding to the base of the nirihuau formation to 11 ma in the foreland area cannibalization of this foreland basin occurred initially in the hinterland and then progressed to the foreland zone blind structures formed a broken foreland at the frontal zone inferred from growth strata geometries during pliocene to quaternary times most of the contractional deformation was dissipated in the orogenic wedge at the time when the arc front retracted to its present position

pyroxene megacrysts in proterozoic anorthosites implications for tectonic setting magma source and magmatic processes at the moho abstract proterozoic anorthosites from the 1630 1650 ma mealy mountains intrusive suite grenville province canada the 1289 1363 ma nain plutonic suite nain churchill provinces canada and the 920 949 ma rogaland anorthosite province sveconorwegian province norway all entrain comagmatic cumulate high alumina orthopyroxene megacrysts haoms the orthopyroxene megacrysts range in size from 0 2 to 1 m and all contain exsolution lamellae of plagioclase that indicate the incorporation of an excess ca al component inherited from the host magma at pressures in excess of 10 kbar at or near moho depths 30 40 km suites of haoms from each intrusion display a large range in 147sm 144nd 0 10 to 0 34 making them amenable for precise age dating with the sm nd system sm nd isochrons for haoms give ages of 1765 12 ma mealy mountains 1041 17 ma rogaland and 1444 100 ma nain all of them older by about 80 to 120 m y than the respective 1630 1650 920 949 and 1289 1363 ma crystallization ages of their host anorthosites internal mineral sm nd isochrons between plagioclase exsolution lamellae and the orthopyroxene host for haoms from the rogaland and nain complexes yield ages of 968 43 and 1347 6 ma respectively identical within error to the ages of the anorthosites themselves this age concordance establishes that decompression exsolution in the haom was coincident with magmatic emplacement of the anorthosites 100 m y after haoms crystallization at the moho correspondence of pb isotope ages 206pb 204pb vs 207pb 204pb with sm nd ages and other strong lines of evidence indicate that the older megacryst ages represent true crystallization ages and not the effects of time integrated mixing processes in the magmas nd isotopic evolution curves afc mixing calculations and the age relations between the hoams and their anorthosite hosts show that the haoms are much less contaminated with crustal components and are an older part of the same magmatic system from which the anorthosites are derived modeling of these anorthositic magmas with melts indicates that their ultramafic cumulates would have sunk in the magma and been sequestered at the moho where they may have sunk deeper into the mantle resulting in large scale compositional differentiation the haoms thus represent a rare example of part of a cumulate assemblage that was carried to the upper crust during anorthosite emplacement and together with the anorthosites illustrate the dramatic influence that magma ponding and differentiation at the moho has on residual magmas traveling towards the surface the new geochronologic and isotopic data indicate that the magmas were derived by melting of the mantle forming magmatic systems that could have been long lived e g 80 100 m y a geologic setting that would fit these temporal constraints is a long lived andean type margin

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

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

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

geochemical evidence for expansion of marine euxinia during an early silurian llandovery wenlock boundary mass extinction abstract repeated biotic crises have become the hallmark for the silurian with the three most significant marine turnover events being related to dramatic global environmental perturbations causal mechanisms linking these marine extinction events with positive carbon isotope 13 c excursions paleoceanographic change and climate remain poorly constrained here we examine the positive 13 c excursion across the llandovery wenlock boundary and the associated ireviken extinction event this positive 13 c excursion has been interpreted to reflect a major change in global oceanographic climate state and enhanced organic carbon burial new geochemical data from two paleocean basins have been analyzed to determine local and global redox conditions using carbon and sulfur isotopes 34 s and iodine i ca mg proxies the high resolution 13 c and 34 s data from both sections show positive excursions indicative of global perturbations to each of these elemental cycles with minimal temporal offsets between the two systems numerical box modeling of 13 c and 34 s data indicates that these isotopic shifts can be generated by significant increases in the burial of organic carbon and pyrite which are most likely due to enhanced burial under euxinic anoxic and sulfidic conditions independently i ca mg values point to locally anoxic bottom waters in the distal and deeper basinal setting in nevada before during and after the ireviken positive 13 c excursion i ca mg values in the proximal shelf setting in tennessee show relatively oxic waters during the onset of peak 13 c values after which bottom water oxygen concentrations dropped throughout the remainder of the excursion this multiproxy paleoredox dataset provides the first direct evidence for local and global expansion of reducing marine conditions coincident with the silurian biotic event and positive 13 c excursion integration of these geochemical data for local and global scale changes in marine redox conditions with the paleontological data and evidence for eustatic sea level rise points toward a shoaling of anoxic and or euxinic waters onto the shelf as a driver for the ireviken extinction event

new isotopic ages and the timing of orogenic events in the cordillera darwin southernmost chilean andes abstract the cordillera darwin a structural culmination in the andes of tierra del fuego exposes an orogenic core zone that has undergone polyphase deformation and metamorphism some of the classic problems of orogenic zones have remained unanswered in the cordillera darwin the age of deformed plutonic rocks the distinction of structurally reactivated basement and metamorphosed cover rocks and the timing of orogenic events this study addresses and partially answers these questions a well constrained rb sr isochron age of157 8m y and an initial 87 sr 86 sr ratio of 0 7087 obtained from a pre tectonic granitic suite suggest a genetic relation between this suite and upper jurassic silicic volcanic rocks in the cover sequence tobifera formation and also suggest involvement of continental crust in formation of these magmas a poorly constrained rb sr isochron age of240 40m y obtained from supposed basement schists is consistent with field relations in the area which suggest a late paleozoic early mesozoic metamorphism for these pre late jurassic rocks however because of scatter in the data and the uncertainties involved in dating metasedimentary rocks the significance of the isotopic age is dubious compilation of previously published ages in the area 9 with new mineral ages reported here indicate that early andean orogenic events occurred between 100 and 84 m y ago and that subduction related magmatism has contributed probably discontinuously to the crustal evolution of the region throughout the mesozoic

cretaceous diapiric plutonism in the southern cordillera chile the castores and probably the santa rosa plutons of north west isla navarino southern chile have been emplaced by in situ diapirism into metasedimentary rocks of the upper jurassic lower cretaceous yaghan formation for the former this model is consistent with the concentric foliation paralleling the margin of the pluton and the foliation and stratification planes in the metamorphic aureole only the southern part of the santa rosa pluton is preserved and it has some structures similar to those of the castores pluton which can also be interpreted as produced by an inflating diapir the main intrusive rocks of these plutons are quartz monzodiorites and quartz diorites with synmagmatic foliation they were preceded by minor bodies of hornblende gabbros and followed by dykes and small bodies of non foliated granodiorites non foliated to weakly foliated granodiorites forming the centre of the castores pluton probably represent a younger intrusive pulse twelve k ar mineral dates from 10 specimens of plutonic rocks interpreted as near crystallization ages span the period 80 90 ma these dates do not show the sequence of intrusion of the different rock types which may suggest that all of them were intruded and cooled in a short period of time the timing of emplacement of these plutons in relation to tectonism is difficult to determine however a post tectonic emplacement for at least the castores pluton is proposed

the patagonian batholith s of tierra del fuego chile timing and tectonic implications reconnaissance field studies on the patagonian batholith exposed s of tierra del fuego together with 49 new k ar and rb sr mineral dates render new insights into two main aspects of the late mesozoic geology the geochronology and composition of the batholith and time relations between the emplacement of the plutons and deformation the envelope of the batholith in this area is mainly composed of late jurassic early cretaceous sedimentary and volcanic rocks formed in an island arc marginal basin system three main plutonic groups have been recognized which have yielded mineral dates interpreted as crystallization ages ranging from 141 to 34 ma the earliest is a gabbroic assemblage including gabbros diorites and hornblendites exposed as relicts within later granitoids and with dates ranging from 141 to 103 ma the canal beagle plutonic group is composed of granitoids characterized by a penetrative syn magmatic foliation and with dates of 113 to 81 ma the seno ano nuevo plutonic group is formed of granitoids with dates of 60 to 34 ma and is mainly exposed in the outer islands indicating a local oceanwards shift of plutonism with time lower cretaceous island arc volcanism was contemporaneous with the intrusion of the gabbroic assemblage and in part at least of the canal beagle plutonic group folding of the marginal basin infill which has been related to the closure of the marginal basin occurred from about 100 to 93 ma ago the gabbroic assemblage was apparently mainly pre tectonic whereas the seno ano nuevo plutonic group was clearly intruded after this deformational event a post tectonic or possibly late tectonic emplacement for the youngest plutons of the canal beagle plutonic group and a pre or syn tectonic intrusion for the oldest is inferred

origins of large volume rhyolitic volcanism in the antarctic peninsula and patagonia by crustal melting voluminous rhyolitic volcanism along the palaeo pacific margin of gondwana was marked by three principal episodes of magmatism the first of these v1 is essentially coincident with the main episode of karoo ferrar magmatism at 184 ma a younger v2 episode occurred at 168 ma and a third episode v3 occurred in the interval 157 153 ma we evaluate the origin of v1 and v2 rhyolites from the antarctic peninsula using major and trace element and isotopic sr nd o data an isotopically uniform 87sr 86sri 0 707 endi 3 andesite dacite magma was generated as a result of anatexis of grenvillian age hydrous mafic lower crust linked to earlier arc related underplating the lower crustal partial melts would have mixed with fractionated components of the mafic underplate followed by subsequent storage and homogenization early jurassic v1 rocks of the southern antarctic peninsula are interpreted as melts of upper crustal paragneiss which have mixed with the isotopically uniform magma in upper crustal magma chambers the v2 rhyolites are the result of assimilation fractional crystallization of the isotopically uniform magma this occurred in upper crustal magma chambers involving assimilants with similar isotopic composition to that of the magma a continental margin setting was crucial in developing hydrous readily fusible lower crust lower crustal anatexis was in response to mafic underplating associated with the discovery shona bouvet group of plumes thought to be responsible for the karoo magmatic province the progression old to young of volcanism from ne to sw in patagonia and south to north in the antarctic peninsula is consistent with migration away from the mantle plumes towards the proto pacific margin of gondwana during rifting and break up

a 100 m y long window onto mass flow processes in the patagonian mesozoic subduction zone diego de almagro island chile diego de almagro island was formed by the subduction and accretion of several seafloor derived tectonic slices with very heterogeneous ages and pressure temperature time p t t paths the highest element of the pile the lazaro unit evidences subduction in the high p granulite field 1 3 gpa 750 c at ca 163 ma below it a thin tectonic sliver the garnet amphibolite unit preserves eclogite facies remnants 570 c and 1 7 gpa formed at ca 131 ma in situ u pb zircon rim ages peak assemblages were nearly fully amphibolitized during decompression down to 1 2 gpa and 600 c at 125 120 ma rb sr multimineral dating the underlying blueschist unit has 50 m y younger metamorphic ages and exhibits slightly cooler peak burial conditions 520 c 1 7 gpa ca 80 ma in situ white mica ar ar ages and multimineral rb sr dating and is devoid of amphibolitization the mylonites from the sinistral strike slip seno arcabuz shear zone bounding diego de almagro island to the east also exhibit amphibolite facies 620 c and 0 9 gpa deformation at ca 117 ma multimineral rb sr ages in situ white mica ar ar dating and multimineral rb sr dating of low t mylonites 450 c along the base of the lazaro unit reveal partial resetting of high t assemblages during tectonic displacement between 115 and 72 ma and exhumation of the slice stack detrital zircon u th pb ages indicate that the material accreted on diego de almagro island has been mostly recycled from a permian triassic accretionary wedge madre de dios accretionary complex exposed along the subduction buttress geological and geochronological constraints suggest that the rocks of the seno arcabuz shear zone and the lazaro unit were tectonically eroded from the buttress while the underlying garnet amphibolite and blueschist units instead derive from the subducted oceanic basin with increasingly younger maximum depositional ages the very long residence time of the rocks 90 m y for the lazaro unit along the hanging wall of the subduction interface recorded long term cooling along the patagonian subduction zone during the mesozoic diego de almagro island therefore represents a unique window onto long term tectonic processes such as subduction interface down stepping tectonic erosion and episodic underplating near the base of an accretionary wedge 40 50 km

cretaceous 100 ma high silica granites in the gajin area central tibet petrogenesis and implications for collision between the lhasa and qiangtang terranes abstract high silica granites that occur as isolated plutons are widely distributed in collisional orogenic belts and are considered a hallmark of continental collision processes the late mesozoic collision between the lhasa and qiangtang terranes along the bangong nujiang suture zone was key event in the evolution of the tibetan plateau however the timing of this collision remains debated here we report the ages and geochemical characteristics of granites from the gajin area in the north central lhasa terrane central tibet zircon u pb dating reveals that the gajin granites formed at 100 ma these granites exhibit uniformly high sio2 contents 74 9 78 4 wt are weakly peraluminous compositions and have high k2o moderate na2o and al2o3 and low fe2o3t mgo and cao contents they are characterized by enrichments in rb cs and u and by depletions in zr hf nb and ti these characteristics in combination with high rb sr ratios and pronounced negative eu anomalies indicate that the gajin granites are highly fractionated rocks the granites have enriched whole rock nd end t 4 3 to 2 9 and relatively depleted and slightly variable zircon hf ehf t 1 0 to 5 5 isotope compositions the gajin granites were probably generated by high degrees of crystal fractionations of magma produced by partial melting of mature crustal sources the cretaceous magmatic rocks in the north central lhasa terrane record a magmatic lull between 100 and 85 ma which corresponds to a transition in both geochemical composition e g hf nd isotopes and rock association at 100 ma combining the above characteristics with the development of a regional angular unconformity around the time of the formation of the granites we suggest that the initial continental collision between the lhasa and qiangtang terranes probably occurred between 110 and 90 ma therefore the 100 ma high silica granites in the gajin area were formed during the continuing continental convergence that followed the initial collision between the lhasa and qiangtang terranes the generation of discrete and monotonous high silica granites may constitute evidence for a significant tectonothermal transition in evolution of the continental crust

evolution of deep water stratigraphic architecture magallanes basin chile abstract the 4000 m thick and 20 myr deep water sedimentary fill of the upper cretaceous magallanes basin was deposited in three major phases each with contrasting stratigraphic architecture 1 the oldest deep water formation punta barrosa formation comprises tabular to slightly lenticular packages of interbedded sandy turbidites slurry flow deposits and siltstone that are interpreted to record lobe deposition in an unconfined to weakly ponded setting 2 the overlying 2500 m thick and shale dominated cerro toro formation includes a succession of stacked conglomeratic and sandstone channel fill deposits with associated finer grained overbank deposits interpreted to record deposition in a foredeep axial channel levee system 3 the final phase of deep water sedimentation is characterized by sandstone rich successions of highly variable thickness and cross sectional geometry and mudstone rich mass transport deposits mtds that are interpreted to record deposition at the base of slope and lower slope segments of a prograding delta fed slope system the deep water formations are capped by shallow marine and deltaic deposits of the dorotea formation these architectural changes are associated with the combined influences of tectonically driven changes and intrinsic evolution including 1 the variability of amount and type of source material 2 variations in basin shape through time and 3 evolution of the fill as a function of prograding systems filling the deep water accommodation while the expression of these controls in the stratigraphic architecture of other deep water successions might differ in detail the controls themselves are common to all deep water basins information about source material and basin shape is contained within the detrital record and when integrated and analyzed within the context of stratigraphic patterns attains a more robust linkage of processes to products than stratigraphic characterization alone

geochronology and geochemistry of the northern scotia sea a revised interpretation of the north and west scotia ridge junction abstract understanding the tectonic evolution of the scotia sea is critical to interpreting how ocean gateways developed during the cenozoic and their influence on ocean circulation patterns and water exchange between the atlantic and southern oceans we examine the geochronology and detrital age history of lithologies from the prominent submerged barker plateau of the north scotia ridge metasedimentary rocks of the north scotia ridge share a strong geological affinity with the fuegian andes and south georgia indicating a common geological history and no direct affinity to the antarctic peninsula the detrital zircon geochronology indicates that deposition was likely to have taken place during the mid late cretaceous a tonalite intrusion from the barker plateau has been dated at 49 6 0 3 ma and indicates that magmatism of the patagonian fuegian batholith continued into the eocene this was coincident with the very early stages of drake passage opening the expansion of the proto scotia sea and reorganization of the fuegian andes the west scotia ridge is an extinct spreading center that shaped the scotia sea and consists of seven spreading segments separated by prominent transform faults spreading was active from 30 6 ma and ceased with activity on the w7 segment at the junction with the north scotia ridge reinterpretation of the gravity and magnetic anomalies indicate that the architecture of the w7 spreading segment is distinct to the other segments of the west scotia ridge basaltic lava samples from the eastern flank of the w7 segment have been dated as early mid cretaceous in age 137 93 ma and have a prominent arc geochemical signature indicating that seafloor spreading did not occur on the w7 segment instead the w7 segment is likely to represent a downfaulted block of the north scotia ridge of the fuegian andes continental margin arc or is potentially related to the putative cretaceous central scotia sea

crustal growth during back arc closure cretaceous exhumation history of cordillera darwin southern patagonia the cordillera darwin metamorphic complex is unique in the andes in exposing kyanite staurolite schist north of the beagle channel in southern patagonia garnet in amphibolite facies pelitic schists from bahia pia has patchy textures whereby some grains consist of clear grossular rich garnet with fine grained s1 inclusion trails truncated by regions of turbid spessartine pyrope rich garnet with biotite muscovite plagioclase and quartz inclusions micron scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress s2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix pseudosection modelling in na2o cao k2o feo mgo al2o3 sio2 h2o tio2 fe2o3 nckfmashto is used to infer a p t path dominated by decompression from 12 to 9 kbar at t 620 c coupled with garnet mode decreasing from 5 to 1 u th pb in situ dating of s2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72 6 1 1 ma contact aureoles developed adjacent to late granite intrusions include sillimanite bearing migmatites formed at p 6 kbar after 72 ma metamorphism of southern cordillera darwin induced by continental underthrusting beneath the arc related to closure of the rocas verdes back arc basin was terminated by thrusting controlled exhumation with the rocks at p 9 kbar by c 73 ma and 6 kbar by c 70 ma

the south patagonian batholith 150 my of granite magmatism on a plate margin a new database of 70 u pb zircon ages mostly determined by shrimp indicates that the south patagonian batholith resulted from the amalgamation of subduction related plutons from the late jurassic to the neogene construction of the batholith began with a voluminous previously undetected late jurassic bimodal body mainly composed of leucogranite with some gabbro emplaced along its present eastern margin within a restricted time span 157 to 145 ma this episode is at least in part coeval with voluminous rhyolitic ignimbrites of the tobifera formation deposited in the deep rocas verdes basin east of the batholith this was the last of several southwestward migrating silicic volcanic episodes in patagonia that commenced in an early jurassic extensional tectonic regime the quasi oceanic mafic floor of the basin was also contemporaneous with this late jurassic batholithic event as indicated by mutually cross cutting field relationships changes in subduction parameters then triggered the generation of earliest cretaceous plutons cretaceous 1 144 137 ma west of the late jurassic ones a westward shift that culminated at 136 127 ma cretaceous 2 along the present western margin of the batholith most mid to late cretaceous cretaceous 3 126 75 ma and paleogene 67 40 ma granitoids are represented by geographically restricted plutons mainly emplaced between the previously established margins of the batholith and mostly in the far south no associated volcanic rocks of similar age are known at present in this area during the final neogene stage of plutonism 25 15 ma a recurrence of coeval volcanism is recognized within and east of the batholith typical endt values for the granitoids vary from strongly negative 5 in the late jurassic to progressively higher values for cretaceous 1 4 cretaceous 2 0 7 cretaceous 3 2 and the paleogene 5 followed by lower and more variable ones in the neogene 1 to 5 these variations may reflect different modes of pluton emplacement large crustal magma chambers developed in the early stages late jurassic to cretaceous 1 leading to widespread emplacement of plutons with a crustal signature whereas the cretaceous 2 cretaceous 3 and palaeogene parts of the batholith resulted from incremental assembly of small plutons generated at greater depths and with higher endt this does not in itself justify the idea of a reduction in crustal character due to progressive exhaustion of fusible material in the crust through which the magmas passed

ice thickness of the northern half of the patagonia icefields of south america from high resolution airborne gravity surveys author s gourlet p rignot e rivera a casassa g abstract 2015 american geophysical union all rights reserved we employ high resolution high precision helicopter borne gravity observations of the northern npi and southern patagonia icefields spi south america to infer ice thickness and bed topography using a three dimensional model constrained by fjord and lake bathymetry and a land ice mask the results reveal thicker ice than the reflecting horizon of radar sounders with 1 km deep ice for glaciar san rafael and colonia npi and 1 5 km deep ice for glaciar occidental spi these bedrock troughs channelize fast motion of ice from the plateaus combining ice motion and thickness we calculate balance accumulation levels of 3 6 m yr water equivalent for the plateaus bed elevation remains below sea lake level 15 20 km inland for jorge montt and o higgins which favors retreat and is at sea level for san rafael which halted its retreat the results demonstrate the utility of airborne gravity surveys for providing critical data on ice volume bed elevation and balance accumulation of temperate ice masses

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

interpretation of emplacement units in recent surge deposits on lipari italy abstract the gabellotto fiume bianco tephra on lipari consists of surge and fall deposits of rhyolitic composition the sequence contains excellent exposures of large scale wavy strata that consist of paired bedsets with a coarse pumiceous lower part that grades upward into an overlying fine ash layer the contacts between the upper ash layer and the pumice of the next higher bedset are sharp size analyses of samples from this deposit are polymodal and contain various proportions of several distinct subpopulations each subpopulation can be ascribed to particles transported by a specific mechanism fall traction intermittent saltation saltation or suspension particles carried by several mechanisms may be deposited together in a bed as a model for emplacement each bedset is considered to have resulted from the passage of a single surge or surge lobe pumice clasts that settle to the base of the surge cloud are carried for a short distance in a traction carpet to form the dominant component of the coarse grained lower part of the bedset as the velocity at a point wanes during the passage of a surge the mean size of particles carried by the cloud decreases until the trailing edge of the surge cloud leaves a capping layer of fine ash with only sparse pumice clasts

influence of attenuated lithosphere and sediment loading on flexure of the deep water magallanes retroarc foreland basin southern andes flexural subsidence in foreland basins is controlled by applied loads such as topography water sediment and subcrustal forces and the mechanical properties of the lithosphere we investigate the controls on subsidence observed within the upper cretaceous magallanes retroarc foreland basin of southern south america to evaluate the impact of lateral variations in flexural rigidity due to late jurassic extension conventional elastic models cannot explain the observed basin deflection and thick accumulation of deep water cenomanian turonian basin strata however models in which the lithosphere has been previously thinned and deflects under topographic and sedimentary loads successfully reproduce regional subsidence patterns results satisfy paleobathymetric observations in the magallanes basin and suggest that lithospheric thinning is necessary to produce both long wavelength and deep subsidence during late cretaceous basin evolution results indicate that elastic thickness decreases westward from 45 25 km in the distal foreland to 37 15 km beneath the foredeep these findings are consistent with a westward reduction in crustal thickness associated with the jurassic extensional history of the patagonian lithosphere our results also show that sediment loading exerts an important control on regional deflection patterns and promotes a wider region of subsidence and reduced forebulge uplift we propose that lateral variations in mechanical properties and large sediment loads restrict depocenter migration and may cause the foredeep to remain fixed for prolonged periods of time these findings confirm that loading of thinned lithosphere imposes different mechanical controls on the flexural profile and have potential implications for other retroarc foreland basins characterized by earlier extensional histories

back arc extension in the southern andes a review and critical reappraisal the interpretation that the mafic rocas verdes green rocks complex of the southern andes represents part of the uplifted floor of a late jurassic to early cretaceous back arc basin has proved particularly useful in understanding the geological evolution of the southern andes the north scotia ridge and the antarctic peninsula clear field evidence of the back arc setting of the 9rocas verdes gabbro sheeted dyke pillow lava ophiolitic assemblages has encouraged fruitful petrological and geochemical comparison with mid ocean ridge and marginal basin basalts other onshore ophiolite complexes and archaean greenstone belts uncertainty still surrounds estimates of the original width and depth of the basin as well as the proportion of new mafic crust compared with relict sialic crust in the basin floor these questions are unresolved owing mainly to the considerable lower cretaceous turbiditic basin infill and the effects of mid cretaceous compressional deformation while the field relations clearly indicate that the rocas verdes basin is not an older piece of ocean floor trapped behind a volcanic arc it is not yet clear whether the basin is directly subduction related or falls in the category of back arc leaky transforms like the proto gulf of california or apparent rip off features like the andaman sea

production of jurassic rhyolite by anatexis of the lower crust of patagonia abstract the mid jurassic marifil and chon aike volcanic rocks of eastern patagonia are part of one of the largest silicic igneous provinces known rb sr geochronology indicates eruptive ages of 175 190 ma for the marifil complex mostly toarcian aalenian the majority of the rocks are isotopically uniform with initial 87sr 86sr 0 7067 0 0003 and endt 4 2 primary magmas of andesitic composition were generated by partial melting of mafic grenvillian lower crust indentified by depleted mantle model ages of 1150 1600 ma lower crustal pyroxene granulite xenoliths can be modelled as residual jurassic source although they may alternatively be co genetic cumulates the dacite rhyolite suite formed by crystal liquid fractionation processes from the primary andesites involving multistage crystallization and re melting during magma ascent with the later stages of evolution being explicable by fractional crystallization of plagioclase amphibole and accessory minerals the province represents large scale lower crustal reworking associated with the unique tectonic and igneous environment of gondwana break up

jurassic extensional tectono magmatism and associated mineralization of the el faldeo polymetallic district chilean patagonia geochemical and isotopic evidence of crustal contribution the el faldeo is the southernmost andean mineral district and consists of paleozoic polymetamorphic basement unconformably overlain by a 200 m thick homoclinal sequence of sedimentary and felsic volcanic beds of the ibanez formation in addition two epizonal intrusive units and related subvolcanic porphyries sills and dikes which intrude both the metamorphic basement and the ibanez formation are recognized the quebrada colorada granodiorite qcg and the cerro esmeralda tonalite cet the polymetallic mineralization was formed in four stages and includes pyrite arsenopyrite gold silver sphalerite galena and chalcopyrite hosted in felsic tuffs of the ibanez formation hydrothermal breccias and epizonal intrusions field observations and geochronological data u pb ar ar and fission track on cet rocks suggest 1 the development of subsiding pull apart basins and coeval formation of spaces for magmatic emplacement and mineralization in an extensional tectonic regime in mid jurassic time 2 a tectonic exhumation of the rocks of the district probably triggered by the chile ridge trench interactions that started in late miocene time sr nd isotopic data from cet samples and the pb isotope characteristics of galenas from the second and third mineralization stages are indicative of crustal contribution to the origin of the magmas and galena mineralization rocks of the paleozoic metamorphic basement are prime candidates for the crustal source components of the analyzed galenas

petrogenesis of the puerto eden igneous and metamorphic complex magallanes chile late jurassic syn deformational anatexis of metapelites and granitoid magma genesis a suite of schists gneisses migmatites and biotite granitoids from the puerto eden igneous and metamorphic complex peimc and biotite hornblende granitoids of the south patagonian batholith southern chile has been studied for that purpose the chemistry of minerals and the bulk rock composition of major and trace elements including rb sr and sm nd isotopes were determined mineralogical observations and geothermobarometric calculations indicate high temperature and low pressure conditions ca 600 700 c and 3 to 4 5 kbar for an event of metamorphism and partial melting of metapelites in late jurassic times previously determined by shrimp u pb zircon ages structures in schists gneisses migmatites and mylonites indicate non coaxial deformation flow during and after peak metamorphic and anatectic conditions andalusite schists and sillimanite gneisses yield initial 87sr 86sr ratios of up to 0 7134 and end150 values as low as 7 6 contemporaneous biotite granitoids and a coarse grained orthogneiss have initial 87sr 86sr ratios between 0 7073 and 0 7089 and end150 values in the range 7 6 to 4 4 this indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source thus a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids the peimc is considered as a segment of an evolving kilometre sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in late jurassic times a biotite hornblende granodiorite and a muscovite garnet leucogranite show initial 87sr 86sr ratios of 0 7048 and 0 7061 and end100 values of 2 6 and 1 8 respectively and are thus probably related to early cretaceous magmas not involved in the anatexis of the metasedimentary rocks

paleomagnetic evidence for a very large counterclockwise rotation of the madre de dios archipelago southern chile a paleomagnetic study of late paleozoic exotic units exposed in the madre de dios archipelago and diego de almagro island southern chile is reported these units correspond to the tarlton limestone and the metabasaltic pillow lavas of the denaro complex nineteen sites were drilled 14 in limestone and five in lavas standard demagnetization procedures show that the primary remanence is not present in the investigated rocks preventing the determination of the original latitudinal position of these rocks however a post tectonic secondary magnetic component was isolated in eight sites at the madre de dios archipelago in both limestone and basaltic pillow lavas rock magnetic experiments and microscopic observations indicated that the carrier of this remanence is secondary titanomagnetite in the limestone and titanomagnetite titanomaghemite in the lavas the most likely event to produce this secondary magnetization is the nearby intrusion of the south patagonian batholith in the early cretaceous mean direction from eight sites shows a very large 117 7 29 9 counterclockwise rotation of the study area with negligible paleolatitude anomaly this rotation is only constrained as post early cretaceous restoration of the structures to their pre rotation position indicates that accretion of the exotic block took place from the northwest and not from the south as previously proposed this is more consistent with a low latitude origin for the limestone and the general paleogeographic framework different tectonic mechanisms and ages for this large rotation are possible

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

provenance of eastern magallanes foreland basin sediments heavy mineral analysis reveals paleogene tectonic unroofing of the fuegian andes hinterland abstract documenting variations in the composition of foreland basin detrital sediments is a useful tool for reconstructing the kinematic history of associated orogenic systems herein we report the results of our sem eds heavy mineral analysis of the late cretaceous to early miocene eastern magallanes basin of southernmost argentina our data indicate that campanian to middle eocene sediments had a mafic ophiolitic provenance which we interpret as being derived from the patagonian fuegian magmatic arc and the mafic floor of the preceding rocas verdes marginal basin upper middle eocene to lower miocene heavy minerals on the other hand indicate a metamorphic metasedimentary provenance which we interpret as being derived from the cordillera darwin metamorphic complex together these data indicate an abrupt shift in sediment provenance in middle to late eocene time thereby corroborating recent interpretations of the basin s detrital zircon geochrononology and providing further support for temporal and possibly genetic relationships between development of the patagonian orocline the opening of drake passage and the oi 1 glaciation of antarctica

integrated stratigraphy and paleontology of the lower miocene monte leon formation southeastern patagonia argentina unraveling paleoenvironmental changes and factors controlling sedimentation abstract marine sedimentary rocks of the lower miocene monte leon formation of southeastern patagonia austral basin argentina are important geological archives for better understanding regional tectonics paleoenvironments oceanography and climate in this paper we describe assemblages of invertebrates palynomorphs foraminifera ostracods and calcareous nannofossils in a stratigraphical and sedimentological framework which suggest deposition during a transgressive regressive cycle from base to top the lowermost outcropping deposits of the punta entrada member represent an inner shelf environment formed during a transgressive phase the zone of maximum flooding is marked by a high diversity of protoperidiniaceans heterotrophic and offshore dinoflagellate cysts an increase in the percentage and size of planktonic foraminifera and a decrease in shallow water benthic foraminifera regressive deposits in the upper part of the punta entrada member exhibit a progradational stratal stacking pattern and are characterized by an upward decrease in bioturbation and in the content of marine invertebrates the dinoflagellate cyst assemblages point toward shallower and more restricted marine conditions than in underlying deposits the cycle ends with the monte observacion member which contains an impoverished and mostly reworked fauna of invertebrates the presence of monospecific reefs of crassostrea orbignyi the decrease in dinoflagellate cysts and calcareous microfossils diversity and the increase of continental palynomorphs suggest progressively shallower conditions and increasing influence of freshwater discharge although eustatic controls could have contributed to the sedimentary evolution of the monte leon formation the upward regressive trend is interpreted as the result of tectonism linked to the andean orogeny which led to the uplift exhumation and erosion of the highlands in the west this is supported by the abundance of pyroclastic material together with reworked specimens of upper cretaceous forams and upper cretaceous and middle eocene dinoflagellate cysts in the upper part of the punta entrada member and in the monte observacion member

40ar 39ar dating of the jurassic volcanic province of patagonia migrating magmatism related to gondwana break up and subduction abstract the mesozoic large igneous province lip of patagonia southern south america which is one of the largest silicic provinces on earth has been investigated by the 40ar 39ar method twenty seven ages considered as valid including twenty plateau ages show that the volcanic activity ranging from 187 to 144 ma occurred between and contemporaneously with the initial break up of gondwana starting with the karoo antarctic tasmanian kat flood basalt province in the east and a subduction in the west the data display a regular decreasing of ages from the ene 187 ma to the wsw 144 ma along about 650 km apparently related to the tectonic structure in half grabens oriented nnw sse the good fitting of this trend with the opening of the rocas verdes sarmiento marginal basin favors a space time evolution of this continental volcanism culminating towards the ssw in a continental disruption behind the magmatic arc the observed age progression of volcanism may be the result of the variations of the physical characteristics of the subduction the spreading and thermal effect of the kat plume may have an additional effect and also could account for the unusually large volume of magma