evolution of the san jorge basin argentina the san jorge basin although small is the most important hydrocarbon producing basin in argentina remaining untested potential is high because of the presence of good source rock favorable structural complexity and multiple reservoirs reservoir quality is commonly low because of the highly tuffaceous sandstones the sedimentary fill of the basin is closely related to its tectonic history northwest southeast trending grabens formed and filled during a triassic and early jurassic early rift phase climaxing with a pervasive middle jurassic volcanic episode continued growth and filling of the basin occurred during a late jurassic earliest cretaceous late rift phase and cretaceous early and late sag phases late cretaceous early tertiary extension set up many of the present day structural traps along normal faults middle tertiary andean compression produced the narrow north south san bernardo structural belt which exhibits reversed movement on older normal graben bounding faults and on local low angle thrust faults marked early to middle tertiary erosion produced a significant unconformi y within cretaceous beds around basin margins origin of upper jurassic and lowermost cretaceous sedimentary fill is primarily lacustrine or fluvial in origin lacustrine organic rich black shales are fringed by oolitic and other limestones and fluvial deltaic sandstones derived mostly from the north a significant southern source of sand existed during the valanginian interbedded marine shales occur mostly to the west toward a presumed marine seaway connection to the northern magallanes basin middle to upper cretaceous sedimentary rocks sourced mostly from the north are mainly fluvial sandstone shale successions with some minor lacustrine influence reservoir quality glauconitic sands were deposited during a late cretaceous early tertiary marine incursion from the atlantic after development of a lower tertiary regional unco formity relatively undisturbed younger tertiary sediments completed the basin fill lower cretaceous lacustrine shales of the d 129 formation are the best source rocks yet identified however most hydrocarbon production occurs above the source in middle and upper cretaceous sandstones along the basin flanks the most obvious hydrocarbon emplacement mechanism is vertical migration along reactivated graben faults

new paleomagnetic data from the antarctic peninsula and their tectonic implications new paleomagnetic data presented here further constrain the relative motion of the antarctic peninsula relative to east antarctica during the mesozoic development of the southern ocean basins the antarctic peninsula ap is one of four crustal blocks that define west antarctica the others being the ellsworth whitmore mountains ewm thurston island eights coast ti and marie byrd land mbl a jurassic pole 155ma 124 e 64 s a95 7 1 n 10 vgps was obtained from the ap block which suggests that the ap block rotated clockwise between 175 and 155 ma due to significant early opening in the weddell sea basin a new early cretaceous paleomagnetic pole 182 e 74s a95 5 9 n 6 site mean vgps indicates that the ap block was in or near to its present day position with respect to east antarctica by 130 ma between 155 and 130 ma counterclockwise rotation of the ap ti blocks together with the southward motion of east gondwanaland probably resulted in subduction of weddell sea ocean floor beneath the southern ap block and initiated the palmer land deformational event a 130 ma pole from the ti block requires clockwise rotation of the ti and possibly the ewm blocks between 130 and 110 ma producing sinsistral strike slip motion between the ewm block and east antarctica and dextral transpressional motion between the ti ewm blocks and the ap block new ap block 110 ma and 85 ma poles from this study 199 e 74 s a95 6 9 n 13 vgps 152 e 86 s a95 7 5 n 6 vgps respectively are similar to equivalent age poles from east antarctica and suggest little or no relative motion between the antarctic peninsula and east antarctica northern and southern antarctic peninsula mid cretaceous poles are very much alike suggesting that the s shape of the antarctic peninsula is not due to oroclinal bending since 110 ma

igneous history of the andean cordillera and patagonian plateau around latitude 46 degrees s from the middle jurassic onwards persistent igneous activity in the southern andes around 46 s was controlled by easterly dipping subduction along the pacific margin cogenetic plutonic rocks belonging to the patagonian batholith and calc alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences for example in the cretaceous lower tertiary divisadero formation the coyhaique region marks the northerly extension of a narrow back arc basin in which the marine neocomian successions accumulated volcaniclastics from the island arc which presumably lay to the west are intercalated with the sediments although the marine basin was short lived a mildly extensional back arc regime may have existed through much of mesozoic recent times widespread basalt rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment remnants of the patagonian basalt plateau at latitude 45 47 s extend from the argentine chile frontier to lago colhue huapi four principal age and compositional groups have been distinguished in the lavas i the oldest which are about 80 ma occur in sections at senguerr and morro negro they are almost exclusively tholeiitic but show some calc alkaline affinities and resemble in other respects basalts from marginal basins ii the second group 57 43 ma occur in the lower part of the chile chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite iii the upper part of the main plateau sequence where the flows are in the range 25 9 ma are dominantly of alkali basalt composition iv post plateau flows from small cinder cones on the surface of the plateau range in age from ca 4 ma to 0 2 ma or less they are mostly highly undersaturated basanites with occasional leucite basanites enriched in incompatible elements a few of the earlier tholeiites with calc alkali traits may have been closely associated with subduction or marginal basin processes the younger lavas are more alkalic intraplate types generated in the remote back arc extensional zone

a tectonostratigraphic synthesis of the sub andean basins implications for the geotectonic segmentation of the andean belt the tectonic structural and depositional history of 19 sub andean foreland and eight southern caribbean basins has been synthesized and is summarized by regional geohistory charts these charts recapitulate the subsidence regimes recognized and have been used to evaluate correlations between genetically related stratigraphic sequences based on spatial and temporal changes in palaeo depositional setting the sub andean region can be subdivided longitudinally into several tectonostratigraphic domains the differential amount of subsidence between two adjacent tectonostratigraphic provinces or sub provinces relies on the presence of a transverse zone of structural accommodation the location of these transfer zones correlates with a number of known and several newly identified zones of intracontinental deformation combined with changes in the structural geometry of the andean fold and thrust belt and the relative dominance of basement fault systems along the length of the andean foreland it is suggested that the andean belt can be separated into five tectonic domains which are themselves bounded by transverse structural accommodation zones representing the multiphase reactivation of pre existing basement fault systems the influence of these broad zones of transcontinental deformation is most apparent during the triassic jurassic period suggesting that they were intimately associated with accommodating intraplate stresses during the breakup of the gondwana supercontinent

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

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

nomenclature of amphiboles additions and revisions to the international mineralogical associations amphibole nomenclature the introduction of a fifth amphibole group the na ca mg fe mn li group defined by 0 50 b mg fe 2 mn 2 li 1 50 and 0 50 b ca na 1 50 apfu atoms per formula unit with members whittakerite and ottoliniite has been required by recent discoveries of b lina amphiboles this and other new discoveries such as sodicpedrizite which is herein slightly but significantly changed from the original idealized formula necessitate amendments to the ima 1997 definitions of the mg fe mn li calcic sodic calcic and sodic groups the discovery of obertiite and the finding of an incompatibility in the ima 1997 subdivision of the sodic group requires further amendments within the sodic group all these changes which have ima approval are summarized

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

tectonic evolution of the north patagonian andes 41 44 s through recognition of syntectonic strata abstract the north patagonian fold thrust belt 41 44 s is characterized by a low topography reduced crustal thickness and a broad lateral development determined by a broken foreland system in the retroarc zone this particular structural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment here new field and seismic evidence of syntectonic strata constrain the timing of the main deformational stages evaluating the prevailing crustal regime for the different mountain domains through time growth strata and progressive unconformities controlled by extensional or compressive structures were recognized in volcanic and sedimentary rocks from the cordilleran to the extra andean domain these data were used to construct a balanced cross section whose deep structure was investigated through a thermomechanical model that characterizes the upper plate rheology our results indicate two main compressive stages interrupted by an extensional relaxation period the first contractional stage in the mid cretaceous inverted jurassic lower cretaceous half graben systems reactivating the western canadon asfalto rift border 500 km away from the trench at a time of arc foreland expansion for this stage available thermochronological data reveal forearc cooling episodes and global tectonic reconstructions indicate mid ocean ridge collisions against the western edge of an upper plate with rapid trenchward displacement widespread synextensional volcanism is recognized throughout the paleogene during plate reorganization retroarc paleocene eocene flare up activity is interpreted as product of a slab rollback and fore to retroarc oligocene slab asthenospheric derived products as an expression of enhanced extension the second stage of mountain growth occurred in miocene time associated with nazca plate subduction reaching nearly the same amplitude than the first compressive stage extensional weakening of the upper plate predating the described contractional stages appears as a necessary condition for abnormal lateral propagation of deformation

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

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

cretaceous slab segmentation in southwestern gondwana the mesozoic austral basin of patagonia in southwestern gondwana experienced a major tectonic segmentation during aptian times sometime between 121 and 118 ma aptian the northern part of the austral basin known as the aisen basin or rio mayo embayment was inverted with the sediments overlain by calc alkaline subaerial volcanic rocks of aptian to maastrichtian age in the southern segment of the austral basin known as the magallanes basin predominantly marine sediments accumulated until cenozoic times in a back arc position relative to a magmatic arc located to the west the subduction related n s trending volcanic chains of both segments were geographically displaced during aptian to late cretaceous times in the aisen segment north of 49 50 s the volcanic chain was located further east than the coeval arc in the magallanes segment a transform fault connected the trenches of both segments with the aisen segment dipping at a shallower angle than the magallanes segment

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

benthic foraminiferal salinity index in marginal marine environments a case study from the holocene guadalquivir estuary sw spain abstract here we developed and validated a new benthic foraminiferal salinity bfs index from marginal marine environments by analysing benthic foraminifera from the holocene guadalquivir estuary sediments sw spain this index is formulated utilising only four species ammonia tepida and haynesina germanica with higher tolerance to brackish waters and indicating lower salinity and elphidium translucens and elphidium granosum indicative of greater marine influence and pointing to higher salinity thus the bfs index is calculated easily and rapidly and therefore it makes it possible to analyse a higher number of samples in less time the bfs index values from the studied cores enabled the detailed description of subtle changes in the guadalquivir estuary restriction during the holocene for this purpose three degrees of salinity depending on marine influence were defined higher bfs index 0 0 0 4 high marine influence moderate bfs index 0 4 0 7 moderate marine influence and lower bfs index 0 7 1 0 low marine influence before 2000 bce the estuary was moderately open and well connected to the atlantic ocean from 2000 bce the estuary experienced a greater marine influence increasing in extension as a consequence of a sea level rise and subsidence immediately afterwards it began to experience restriction processes due to southward shoreline progradation related to the growth of littoral spits and sediment supply from 1400 to 1000 bce gradual restriction transformed the open estuary into a semiclosed estuary a last phase of estuary restriction occurred from 1000 bce to the present day leading to the lowest salinity and the highest estuary restriction finally the bfs index was successfully applied in two other marginal marine environments a pleistocene lagoon in northern italy and a pliocene coastal bay in southeastern spain the index allowed assessment of the degree of restriction in these different environments supporting its utility in different regions environments and timescales

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

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

paleomagnetism of the patagonian plateau basalts southern chile and argentina a total of 505 paleomagnetic samples were collected from 65 sites volcanic flows of the patagonian plateau basalts at four locations in southern chile and argentina k ar analyses indicate that 38 flows from two locations form a late cretaceous group 64 79 ma while the remaining 27 flows are eocene in age 42 56 ma progressive demagnetization indicates that a characteristic remanent magnetization chrm has been successfully isolated from 59 flows rock magnetic properties and analysis of chrm directions within and between sites allow the secure inference that the chrm is a thermoremanent magnetization acquired during original cooling the eocene flows yield 15 independent samplings of the paleomagnetic field however these data fail the reversals test and probably do not adequately average geomagnetic secular variation the late cretaceous flows yield 18 independent samplings of the paleomagnetic field six of normal polarity and 12 of reversed polarity these data pass the reversals test and have angular dispersion of virtual geomagnetic poles vgps consistent with adequate sampling of geomagnetic secular variation the mean of these 18 vgps yields a late cretaceous paleomagnetic pole for south america latitude 78 7 s longitude 358 4 e a95 6 3 k 31 6 s 14 5 this pole is consistent with similar age poles from north america and africa when the atlantic is reconstructed to 70 ma contrary to previous indications the south american apparent polar wander path does not reach the present rotation axis until after late cretaceous time

stratigraphic record across a retroarc basin inversion rocas verdes magallanes basin patagonian andes chile the mesozoic evolution of the andean cordillera of southern patagonia is recorded in two formations that are now part of the fold thrust belt the zapata formation and the punta barrosa formation extension in the ultima esperanza region began in the late jurassic with deposition of the marine volcaniclastic tobifera formation and eventually resulted in the full fledged oceanic rocas verdes basin the zapata formation was deposited over a time span of 50 m y in an irregular basin ultimately bordered by an early cretaceous arc to the west it is characterized by interbedded shale and siltstone mostly deposited in shallow water and in areas of oceanic crust emplacement also as deep water hemipelagic deposits sediments of the zapata formation were derived initially from local oceanic upwarps and tobifera highs and later from the ande sitic cover of the juvenile arc and or exhumed oceanic crust the paucity of sandstone in the zapata formation in the ultima esperanza region indicates a highly irregular basin partitioned by prominent horsts with sand deposition confined to sub basins closer to the arc changes in depositional regimes and sediment dispersal patterns related to the onset of andean contraction and formation of the magallanes foreland basin are recorded by sediments of the overlying punta barrosa formation this formation records the evolution of a fold thrust belt on the basis of the multimodal mineralogical and geochemical character of its sandstone and shale the presence of an arc is indicated but nearby andean metamorphic terranes are more significantly represented in punta barrosa sediments stratigraphic details across the zapata and punta barrosa formations indicate that deformation and development of a proto andean cordillera in southern south america was initiated in the late mesozoic and involved conspicuous crustal shortening

new angles on south atlantic opening existing models of relative motion between the south america and africa plates significantly misrepresent the azimuth of lower cretaceous seafloor spreading in the south atlantic ocean an improved model is derived from fits of fracture zones magnetic reversal anomalies and the edges of a rifted large igneous province at the northeast georgia rise and agulhas plateau an absolute date for this rifting event is not known but by assuming least change in spreading rates during magnetic chron c34 rifting can be estimated to have occurred at 100 ma this modelling demands a plate divergence history that involves diachronous opening of the south atlantic consistent with published estimates of the ages of breakup from sedimentary basins on the south american and african passive margins the diachronous opening lasts approximately 40 million years during which time it must be accommodated by significant intracontinental deformation a reconstruction using the intra c34 rotation also illustrates the earliest possibility for direct deep water connection between the central and south atlantic oceans one further consequence of this model is that a total reconstruction derived from it and closure of the central atlantic between north america and africa suggests that the venezuelan and gulf of mexico basins may be conjugates formed during the earliest opening of the central atlantic and not separate marginal basins

a geodynamic model linking cretaceous orogeny arc migration foreland dynamic subsidence and marine ingression in southern south america abstract this study synthesizes the tectonomagmatic evolution of the andes between 35 30 s to 48 s with the aim to spotlight early contractional phases on andean orogenic building and to analyze their potential driving processes we examine early tectonic stages of the different fold thrust belts that compose this andean segment additionally we analyzed the spatio temporal magmatic arc evolution as a proxy of dynamic changes in andean subduction during critical tectonic stages of orogenic construction this revision proposes a hypothesis related the existence of a continuous large scale flat subduction setting in cretaceous times with a similar size to the present largest flat slab setting on earth this potential process would have initiated diachronically in the late early cretaceous and achieved full development in late cretaceous to earliest paleocene times constructing a series of fold thrust belts on the retro arc zone from 35 30 s to 48 s moreover we assess major paleogeographic changes that took place during flat slab full development in maastrichtian danian times at this moment an enigmatic atlantic derived marine flooding covered the patagonian foreland reaching as far as the andean foothills based on flexural and dynamic topography analyses we suggest that focused dynamic subsidence at the edge of the flat slab may explain sudden marine ingression previously linked to continental tilting and orogenic loading during a high sea level global stage finally flat subduction destabilization could have triggered massive outpouring of synextensional intraplate volcanic rocks in southern south america and the arc retraction in late paleogene to early neogene times

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

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