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

new thermochronometric constraints on the rapid palaeogene exhumation of the cordillera darwin complex and related thrust sheets in the fuegian andes thermal modelling of new fission track and u th sm he data from the fuegian andes reveals rapid cooling 12 48 c ma 1 during the middle and late eocene followed by slow cooling 1 5 c ma 1 to the recent we interpret the rapid cooling as a result of exhumation from contractional uplift within the crystalline interior of the orogen this interpretation is consistent with independent evidence of eocene shortening flexural subsidence and provenance changes in the study area and is approximately coeval with marine geochemical evidence of the onset of drake passage opening in light of the palaeogene history of nazca south american plate convergence and the differences in shortening magnitudes and exhumation histories between the fuegian and patagonian andes our data support eocene development of the patagonian orocline which also provides a plausible explanation for early opening of drake passage

fractionation of bulk rock composition due to porphyroblast growth effects on eclogite facies mineral equilibria pam peninsula new caledonia chemically zoned porphyroblasts in metamorphic rocks indicate that diffusional processes could not maintain equilibrium conditions on a grain scale during porphyroblast growth or establish it afterwards an effect of this inability to maintain equilibrium is the progressive removal of elements forming garnet cores from any metamorphic reaction that occurs at the porphyroblast boundaries or in the matrix of the rock to examine this effect on mineral assemblages the bence albee matrix correction was applied to x ray intensity maps collected using eclogite samples from northern new caledonia in order to determine the chemical composition of all parts of the sample the manipulation of these element maps allows a quantitative analysis of the fractionation of the bulk rock composition between garnet cores and the matrix a series of calculated equilibrium volume compositions represents the change in matrix chemistry with progressive elemental fractionation as a consequence of prograde garnet growth under high p conditions pressure temperature pseudosections are calculated for these compositions in the cao na2o feo mgo al2o3 sio2 h2o system assemblages modal proportions and mineral textures observed in the new caledonian eclogites can be closely modelled by progressively removing elements forming garnet cores from the bulk rock composition the pseudosections demonstrate how chemical fractionation effects the peak metamorphic assemblage prograde textures and the development of retrograde assemblages

thermal history of australian passive margin cover sequences accreted to timor during late neogene arc continent collision indonesia abstract paleotemperature indicators and apatite fission track analysis of australian continental margin cover sequences accreted to the active banda arc continent collision indicate little to no heating during rapid late neogene uplift and exhumation thermal maturation patterns of vitrinite reflectance conodont alteration and illite crystallinity show that peak paleotemperatures ppt increase with stratigraphic and structural burial the highest ppt is found in the northern hinterland of the accretionary wedge which was beneath progressively thicker parts of the upper plate towards the north major discontinuities in the pattern of ppt are associated with the position of major thrust ramps such as those forming the ramelau kekneno arch rka ppt for upper triassic to neogene strata south of the rka are 60 80 c which are similar to and in many cases lower than correlative and age equivalent units drilled on the nw australian shelf permian to lower triassic sedimentary strata thrust over younger units within and north of the rka have ppt of 100 220 c thrust sheets accreted beneath the upper plate have ppt approximately 90 c higher than those frontally accreted metamorphism of the northernmost units of these sequences yield ppt of 300 c thrust stacking yields an inverted thermal profile of ppt decreasing discontinuously downward and to the south towards the foreland the timing of ppt is constrained by apatite fission track ages from mostly triassic continental margin cover sequences ages of upper triassic units are primarily coeval with deposition and show little evidence of thermal annealing whereas those of lower triassic units are almost completely annealed and range from 1 8 0 5 19 2 9 7 ma the clustering of apatite fission track ages into two distinct groups indicates that the upper boundary of the partial annealing zone has remained for some time at a triassic stratigraphic interval in the slope and rise of the nw australian continental margin the position of this zone on the present shelf is higher in the stratigraphic column due to the greater thickness of post breakup shelf facies units thrust stacking of rise slope and shelf units produces an inverted vertical profile of increasing apatite fission track age with depth lack of any long confined track lengths in apatite from all of the units requires rapid and recent exhumation of the thrust stack which is coincident with rapid phases of pliocene pleistocene exhumation documented throughout timor these data preclude pre late miocene tectonic burial or pre pliocene exhumation of the nw australian continental margin

origin of atoll garnets in eclogites and implications for the redistribution of trace elements during slab exhumation in a continental subduction zone detailed electron and ion microprobe analyses were carried out on atoll shaped and normal garnets in ultrahigh pressure uhp metamorphic eclogite from dabie east central china compositional profiles of both normal garnets and rings of atoll garnets show well preserved growth zoning with a decrease in mn ca and heavy rare earth elements hrees and an increase in mg toward rims manganese and middle rare earth element mree enrichments are observed near garnet rims island and peninsula shaped garnet inside atolls are homogeneous in major elements and show the same composition as garnet rims whereas the hree concentrations are similar to those of the normal garnet cores electron back scatter diffraction ebsd analyses show that the island and peninsula shaped garnet fractions inside atolls have crystallographic orientations identical to that of the atoll rings these observations suggest that the atoll garnets were formed by the consumption of earlier formed cores by fluid released from both hydroxyl exsolution from the nominally anhydrous minerals nams and lawsonite decomposition at the onset of exhumation i e garnet breakdown was from the inside and re growth from outside to inside though somewhat restricted this study reveals that because garnet and zircon act as sinks for hrees and zr respectively the majority of released hrees and zr were likely re incorporated into newly grown garnet and zircon during the dabie uhp metamorphic slab exhumation

calculation of partial melting equilibria in the system na2o cao k2o feo mgo al2o3 sio2 h2o nckfmash a thermodynamic model for haplogranitic melts in the system na2o cao k2o al2o3 sio2 h2o nckash is extended by the addition of feo and mgo with the data for the additional end members of the liquid incorporated in the holland powell 1998 internally consistent thermodynamic dataset the resulting dataset with the software thermocalc is then used to calculate melting relationships for metapelitic rock compositions the main forms for this are p t and t x pseudosections calculated for particular rock compositions and composition ranges the relationships in these full system pseudosections are controlled by the low variance equilibria in subsystems of nckfmash in particular the solidus relationships are controlled by the solidus relationships in nkash and the ferromagnesian mineral relationships are controlled by those in kfmash however calculations in nckfmash allow the relationships between the common metapelitic minerals and silicate melt to be determined in particular the production of silicate melt and melt loss from such rocks allow observations to be made about the processes involved in producing granulite facies rocks particularly relating to open system behaviour of rocks under high grade conditions

a method for applying matrix corrections to x ray intensity maps using the bence albee algorithm and matlab matlab scripts that apply the bence albee 1968 matrix correction algorithm to x ray intensity data collected as element maps on a cameca sx 50 microprobe are used to produce two dimensional maps of oxide weight percent and cation proportions for sio2 a l 2o3 feo mno mgo cao na2o and k2o once generated large data sets of mapped oxide weight percent values or cation numbers that retain spatial information can be used petrologically the technique is used to evaluate the compositional range of barroisitic amphibole in an eclogite from new caledonia to examine aspects of equilibration during the partial hydration of the eclogite facies mineral assemblage

u pb zircon geochronology of palaeozoic units in western and central guatemala insights into the tectonic evolution of middle america

detrital zircon shrimp u pb age study of the cordillera darwin metamorphic complex of tierra del fuego sedimentary sources and implications for the evolution of the pacific margin of gondwana abstract the cordillera darwin metamorphic complex in the southernmost andes includes a basement of probable palaeozoic age a mid jurassic and younger volcano sedimentary cover and a suite of jurassic granites all of which were jointly metamorphosed during the cretaceous detrital zircon ages presented here show that some of the amphibolite facies metamorphic rocks previously mapped as basement have a jurassic protolith overall the detrital zircon age patterns for samples of the cordillera darwin basement differ from those of the madre de dios terrane of the western patagonian andes with which they had been correlated instead they are more comparable with those from the eastern andes metamorphic complex which apparently developed in a passive margin setting the paucity of cambrian detrital zircons indicates that the meta igneous basement of the magallanes foreland basin of central and northern tierra del fuego was not the main source of detritus for the protolith of the cordillera darwin metamorphic complex the possibility is envisaged that the magallanes fagnano transform fault boundary between the scotia and south america plates resulted from reactivation of an older pre jurassic suture zone between the basement terranes of north central tierra del fuego and cordillera darwin supplementary material geographical coordinates of analysed samples sensitive high resolution ion microprobe u th pb analytical data and representative cathodoluminescence images of zircon grains analysed are available at http www geolsoc org uk sup18397

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

40ar 39ar dating of volcanic rocks of the shyok suture zone in north west trans himalaya implications for the post collision evolution of the shyok suture zone abstract 40 ar 39 ar geochronological studies carried out on the khardung volcanics of ladakh india and our earlier ar ar results from the volcanics of the shyok suture along with the available geological and geochemical data provide good constraints for post collision evolution of the shyok suture zone whole rock samples from the shyok volcanics yielded disturbed age spectra and we have demonstrated earlier that the youngest tectonic event in the shyok suture zone responsible for the thermal disturbance of these samples is karakoram fault activation at 14 ma contrastingly whole rock samples from the khardung volcanics which are in tectonic contact with these shyok volcanics and are exposed in the form of thick rhyolitic and ignimbritic flows yielded undisturbed age spectra and good plateau ages the whole rock plateau ages of two rhyolite samples are 52 8 0 9 and 56 4 0 4 ma we interpret these ages to be the time and duration of emplacement of these volcanics over thickened margin of the continental crust which appears to be coeval with the initiation of the collision between the indian and asian plate the lesser extent of post emplacement isotopic re equilibration in these samples unlike the shyok volcanics indicate that these samples were present in different tectonic settings away from the karakoram fault at the time of deformation in the shyok suture zone we propose that the two volcanic belts of contrasting nature were brought together in juxtaposition by the karakoram strike slip faulting at 14 ma

orogenesis at the southern tip of the americas the structural evolution of the cordillera darwin metamorphic complex southernmost chile abstract new detailed lithologic and structural data are presented from three separately mapped areas along the southern boundary of the cordillera darwin metamorphic complex of southernmost chile cordillera darwin is a unique uplift because it exposes the highest grade rocks in the andes south of peru and averages 1 km higher in elevation than adjacent areas the structural data indicate that cordillera darwin experienced mid late cretaceous trans pressional deformation with a partitioned strike slip component localized along the beagle channel that forms the southern boundary to the range foliation lineation and fold axis trends indicate ne sw directed contraction and nw se strike slip shearing present directions during progressive d1 d2 andean deformation d2 deformation is marked by outcrop to 10 km scale south southwest vergent folds late cretaceous tertiary brittle ductile and brittle left lateral strike slip faults and shear zones crosscut all d1 d2 structures although limited structural evidence for extensional tectonics was documented in this study apparent normal offsets across both arms of the beagle channel and previously documented field evidence for extension from other areas in cordillera darwin suggest that transtensional displacements also may have occurred in southern cordillera darwin during the late cretaceous early tertiary cordillera darwin s position within the evolving patagonian orocline adjacent to an evolving mesozoic cenozoic left lateral transform boundary between the south american and antarctic plates and later the south american and scotia plates necessitates consideration of the possible effects of regional counterclockwise rotation on development of structures regional counterclockwise rotation of cordillera darwin may have controlled the temporal and spatial transition of deformational regimes within cordillera darwin exhumation of the metamorphic core of cordillera darwin during the late cretaceous early tertiary is interpreted to have been due to rapid uplift and normal rates of erosion rapid uplift may have been due to migration of contraction to northern cordillera darwin and crustal relaxation in southern cordillera darwin as contractional structures rotated into parallelism with 1 and either ceased to be active or were reactivated as strike slip or oblique slip faults rapid isostatic rebound of cordillera darwin may have also been a response to aborted continental subduction of southern cordillera darwin and detachment of a subducted mafic root during mid cretaceous closure of the rocas verdes marginal basin

contrasting styles of superposed deformation in the southernmost andes brittle fault arrays regional structural patterns and marine multichannel seismic reflection profiles across the magallanes basin near 53 s latitude show contrasting styles of superposed extensional contractional and transtensional deformation below the northeastern end of a ne sw transect of the basin seismic reflection profiles show deepening levels of pre late jurassic basement and upper jurassic to lower cretaceous volcanic horizons toward the southwest this trend is associated with apparent normal fault offsets that may be attributed to a widespread late jurassic early cretaceous extensional event contrastingly below the southwestern end of magallanes basin subsurface depths of basement and volcanic horizons shallow to the southwest integration of seismic reflection data with published seismic refraction data geologic mapping and satellite gravity anomaly data suggest that this shallowing trend is controlled structurally by craton vergent basement involved contractional deformation this style of shortening contrasts with thin skinned styles of thrusting in the northern and central parts of the basin on tierra del fuego structural relationships stratigraphic data and previously published metamorphic data indicate that basement involved contraction occurred during latest cretaceous to early tertiary time overprinting all contractional structures steep tertiary strike slip and oblique slip faults in the central and southern parts of the magallanes basin display oblique normal mostly down to the southwest offsets across wnw ese trending segments of the south american scotia transform plate boundary on the south american continent newly discovered features along the plate boundary segment include sediment filled grabens and half grabens up to 7 km wide that occur between subparallel fault splays in the magallanes fault zone kinematic analyses of fault slip data indicate that sinistral transtensional motion occurred within this fault zone between the eastern arm of the straits of magellan and central tierra del fuego together these data indicate that 1 the southernmost andes were under compression during rapid cooling and exhumation of unique high pressure 8 kbars basement rocks now exposed in the darwin metamorphic complex dmc near 55 s latitude and 2 final exhumation of basement rocks in the dmc occurred during a shift away from compression toward transtension during late tertiary crustal relaxation we suggest that preexisting structural anisotropies formed during basement involved contractional deformation and the initiation of seafloor spreading south of south america during the oligocene controlled late tertiary sinistral transtensional patterns in the magallanes fault zone

origin and mechanical significance of honeycomb garnet in high pressure metasedimentary rocks from the tauern window eastern alps high pressure schists 2 2 5 gpa from the eclogite zone in the tauern window contain honeycomb garnet in which fine webs of garnet surround strain free quartz carbonate grains high resolution x ray computed tomography shows that the garnet webs form a cellular structure that coats all surfaces of the inclusions electron backscatter diffraction analysis shows that the garnet cells are crystallographically continuous with more massive garnet regions and that the quartz carbonate inclusions have random orientations in contrast matrix quartz exhibits a prominent crystallographic preferred orientation cpo high resolution transmission electron microscopy shows few dislocations in either the garnet or the inclusion quartz most honeycomb garnet is chemically homogeneous but some displays asymmetric core rim zoning taken together these observations are most consistent with formation of the garnet sheets via precipitation from a wetting fluid along quartz quartz grain boundaries or possibly via wholesale precipitation of garnet quartz carbonate from a fluid in either case a silicate rich aqueous fluid must have been present the likelihood that a fully wetting fluid existed at high pressure has important implications for rheology during subduction of metasedimentary rocks strain may be accommodated by grain rotation and sliding in an aqueous silicate slurry rather than via dislocation creep mechanisms at high pressures the absence of a cpo in early quartz may thus point to involvement of a pervasive grain boundary fluid rather than requiring low differential stresses during subduction

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

eocene arc continent collision in new caledonia and implications for regional southwest pacific tectonic evolution new caledonia preserves evidence that constrains models for the tectonic evolution of the southwest pacific region onland geology reflects four main tectonic phases 1 early mesozoic development of subduction related terranes and their accretion to the gondwana margin 2 cretaceous passive margin development and sea floor spreading during the gondwana breakup 3 foundering of an oceanic basin and the eocene arrival of thinned gondwana margin crust at a southwest facing subduction zone resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast and 4 detachment faulting during extensional collapse resulting in unroofing of metamorphic core complexes the last phase explains supposedly anomalous metamorphic gradients in the northeast of the island

zircon upb and hornblende 40ar39ar ages for the dufek layered mafic intrusion antarctica implications for the age of the ferrar large igneous province abstract new high precision zircon u pb ages for the dufek intrusion establish the crystallization age for the only layered mafic intrusion sensu stricto known in the ferrar igneous province antarctica one of the largest igneous provinces in the world three concordant zircon fractions from the capping lexington granophyre yield an age of 183 9 0 3 ma 2 which we interpret as the crystallization age for the dufek intrusion concordant zircon fractions from a silicic dike crosscutting the upper layered gabbroic sequence yield a slightly younger crystallization age of 182 7 0 4 ma 2 for both samples agreement within errors between the two u pb and 207 pb 206 pb ages for three fractions each with a unique set of physical and chemical characteristics demonstrates that inheritance of older zircon components from the country rocks was imperceptible it also suggests that assimilation or anatexis of the lower paleozoic country rocks was not the dominant mechanism in the formation of the granophyre these new u pb dates agree within analytical errors with recent zircon and baddeleyite u pb ages for dolerite sills in both the ferrar and karoo igneous provinces which demonstrates the vastness of the rift axis along which simultaneous magmatic activity occurred contemporaneity between the dufek intrusion and other ferrar group rocks supports the possibility that this pluton acted as a conduit for magmas of the widely distributed ferrar sills and flows hornblende 40 ar 39 ar ages for the lexington granophyre 175 2 ma barely overlap with those for a silicic dike 179 2 ma in the layered gabbro section just below the granophyre the younger of the two hornblende ages may be explained by degassing of inclusions or by protracted hydrothermal alteration preventing the ar ar geochronometer form becoming a closed system in the higher level granophyre until after it had done so in the more deeply positioned silicic dike the 179 2 ma age of the dufek silicic dike is interpreted to represent cooling through the ar retention temperature for hornblende of about 500 c combining this cooling age with the u pb crystallization age of 182 7 0 4 ma yields a cooling rate for the dike of approximately 100 c m y inasmuch as this rate is not rapid it can be inferred that the bulk of the dufek intrusion was still fairly hot when the dated silicic dike was emplaced

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

transition from back arc to foreland basin development in the southernmost andes stratigraphic record from the ultima esperanza district chile during the mesozoic evolution of the southernmost andes back arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting a stratigraphic record of this tectonic transition is described from jurassic and cretaceous rock units exposed in the patagonian fold thrust belt at 51 s latitude initiation of an extensional deep marine trough in late jurassic time is documented by interstratified rhyolites and marine mudstones in the tobifera formation development of the early cretaceous rocas verdes back arc basin is recorded by zapata formation submarine slope deposits which draped the west facing passively subsiding cratonic margin of the basin the onset of andean compressional orogenesis and creation of the magallanes foreland basin is marked by the influx of coarse grained sandstone turbidites of the albian cenomanian punta barrosa formation during late cretaceous time the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east this geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back arc basin margin beneath the load of the obducted rocas verdes basin floor changes in depositional regime and sediment dispersal patterns in the latest cretaceous together with an eastward shift in the magallanes basin depocenter record cratonward migration of deformation in the patagonian fold thrust belt loading of the thicker cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin

continental underthrusting and obduction during the cretaceous closure of the rocas verdes rift basin cordillera darwin patagonian andes 1 the patagonian andes record a period of cretaceous neogene orogenesis that began with the compressional inversion of a late jurassic rift called the rocas verdes basin detrital zircon ages from sediment that filled the southern part of the basin provide a maximum depositional age of 148 ma suggesting that the basin opened approximately simultaneously along its length during the late jurassic structural data and u pb isotopic ages on zircon from granite plutons near the beagle channel 55 s show that basin inversion involved two stages of shortening separated by tens of millions of years an initial stage created a small 60 km wide thrust wedge that placed the basaltic floor of the rocas verdes basin on top of adjacent continental crust prior to 86 ma structures and metamorphic mineral assemblages preserved in an exhumed middle to lower crustal shear zone in cordillera darwin suggest that this obduction was accompanied by south directed subduction of the basaltic crust and underthrusting of continental crust to depths of 35 km beneath a coeval volcanic arc a subsequent stage of out of sequence thrusting culminating in the paleogene shortened basement and upper jurassic igneous rock in the internal part of the belt by at least 50 km forming a bivergent thrust wedge this latter period coincided with the exhumation of rocks in cordillera darwin and expansion of the fold thrust belt into the magallanes foreland basin this orogen provides an important example of how orogenesis initiated and led to continental underthrusting and obduction of basaltic crust during closure of a quasi oceanic rift basin

modified gulf of california model for south georgia north scotia ridge and implications for the rocas verdes back arc basin southern andes new field mapping and an integrated trace element and nd isotopic study have been undertaken on the jurassic cretaceous larsen harbour complex on the island of south georgia recognized as a displaced part of the southern andes ensialic autochthonous rocas verdes marginal basin five basalt groups groups i v are identified from trace element data initial nd isotopic compositions suggest that they are derived from only two sources magmas produced during early stages of continental lithospheric attenuation groups i iii were derived by varying degrees of partial melting and fractional crystallization from a large ion lithophile element lile enriched low epsilon sub nd mantle source relative to normal n type mid ocean ridge basalt morb magmas produced during later stages of rifting groups iv and v were however derived from a high epsilon sub nd asthenospheric mantle source similar to n type morb unaffected by earlier lile enrichment our data do not occur with previous suggestions that the larsen harbour complex and by implication the rocas verdes formed in a supra subduction zone setting on the basis of geologic and geochemical evidence we favor basin formation along an oblique slip margin akin to that of the gulf of california

magallanes fagnano continental transform fault tierra del fuego southernmost south america 1 multichannel seismic reflection profiles gravity measurements and bathymetric soundings in conjunction with field geological reconnaissance and remote sensing images reveal with unprecedented detail the morphostructure of a major segment of the south america scotia plate boundary in the tierra del fuego region this segment known as the magallanes fagnano fault system is a continental transform margin arranged in an en echelon geometry along which prominent asymmetric basins were developed data acquired off the atlantic coast of isla grande the main island of tierra del fuego in its central and eastern part and in the central and western magallanes strait image the surface and subsurface structure of the transform fault and its associated basins the magallanes fagnano fault system is composed of distinct tectonic lineaments that are segments of the transform system and are represented by mostly near vertical faults in the atlantic sector the fault system trends broadly n70 e and seems to be composed by a single master fault along which a highly asymmetric basin has formed at around 63 w the fault terminates by splaying into secondary normal faults that dissipate the horizontal displacement along the system in the central eastern part of isla grande the fault segments have been principally identified from analyses of remote sensing images on the basis of their morphological expression these segments are located within river valleys and are generally associated with localized gravity minima lago fagnano a 105 km long e w trending depression is a large mostly asymmetric pull apart basin developed within the principal displacement zone of the magallanes fagnano fault system restraining bends and overlapping step over geometry characterize the central part of the magallanes strait along the western part of the fault system in the vicinity of the pacific entrance of the magallanes strait asymmetric sedimentary basins have also developed the sedimentary architecture of the basins formed within the principal displacement zone of the fault in which the thick end of the depositional wedge abuts the transform segment suggest simultaneous strike slip motion and transform normal extension a common feature found in other continental transtensional environments strike slip faulting in the tierra del fuego region is also documented along other prominent lineaments which parallel the magallanes fagnano fault system along at least two of these lineaments characterized by a remarkable morphological expression widespread quaternary activity occurs the present day motion between the south america and scotia plates is slow 5 mm yr also the modern seismicity monitored in the tierra del fuego region is low individual events 3 5 in magnitude the low seismicity may be explained by the slow relative motion between plates and may be further affected by slip partitioning along the different segments which make up the magallanes fagnano fault array and along the subsidiary wrench lineaments that traverse the region

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

structural geology and geochronology of subduction complexes along the margin of gondwanaland new data from the antarctic peninsula and southernmost andes subduction complexes along the andean margin in central and southern chile yield mid paleozoic to lower mesozoic ages yet they crop out within 100 km of the modern trench that shows evidence of active accretion along much of its length the scarcity of uplifted subduction complex rocks younger than mid mesozoic along the chilean margin and in parts of the scotia arc suggests to us that these old crystalline rocks uplifted in the triassic and jurassic represent a boundary in the forearc beyond which tectonic erosion does not easily occur greenschist blueschist and amphibolite facies subduction complex rocks from the scotia arc were originally thought to be a simple continuation of the subduction complexes in chile based on new 40 ar 39 ar ages the scotia arc subduction complexes reveal a complex history related to distinct local tectonic events and are not a simple continuation of the old accretionary prism in chile structural and metamorphic analysis indicates the earliest and most penetrative deformation in the subduction complexes around the scotia arc occurred at some depth in a subduction zone or zones certainly below the brittle ductile transition and in some cases under blueschist facies conditions we believe that the early subduction related deformation and metamorphism in the greenschist and blueschist facies rocks of the scotia arc to be overprinted by mid cretaceous transpression along the south america antarctica plate boundary in the case of elephant island transpression and subsequent localized transtension in the earliest cenozoic in the case of the darwin complex a mid cenozoic spreading rate change in the case of smith island and early neogene initiation of drake passage opening shackleton fracture zone formation in the case of the gibbs island subgroup

the magallanes and deseado fault zones major segments of the south american scotia transform plate boundary in southernmost south america tierra del fuego the 600 km long magallanes fault zone and the newly discovered 100 km long deseado fault zone represent large but little understood segments of the south american scotia sam sco transform on the south american continent 50 56 s latitude kinematic analyses of fault populations near mount hope on tierra del fuego indicate that these fault zones have accommodated sinistral strike slip motion with a probable component of normal slip on vertical subvertical faults during cenozoic time regional geological data and overprinting relationships between the strike slip oblique slip faults and mid cretaceous to oligocene contractional structures suggest that sinistral motion in the mount hope segments of the two fault zones occurred significantly since 60 ma and dominantly since 30 ma with the most recent episode of activity occurring during the quaternary results from mapping show 20 25 km of cumulative sinistral separation of a thrust contact across a well exposed segment of the magallanes fault zone north and east of mount hope although the exact amount of displacement accommodated by this segment of the magallanes fault zone remains uncertain the 20 25 km of cumulative sinistral separation combined with an estimate of 3 km of vertical separation is consistent with small magnitudes of displacement of the order of tens of kilometers this result apparently contrasts with predictions based on plate reconstructions that large amounts of strike slip displacement 200 km have been accommodated internally within southernmost south america since early cenozoic time a low magnitude of displacement along the mount hope segment of the magallanes fault zone suggests that 1 strike slip motion has been partitioned across the continent since about 30 ma and or 2 the mount hope segment of the magallanes fault zone represents a young part of the modern plate boundary that migrated to its present location during the last few million years