strike slip faulting thrusting and related basins in the cenozoic evolution of the southern branch of the bolivian orocline abstract the evolution of the cenozoic deformation of the cordillera oriental and of the contact zone with the altiplano in southern bolivia is well documented in the tupiza nazareno and estarca basins the tectonic evolution started at about 29 ma the period between 29 and about 22 21 ma is marked by development of a pull apart basin related to n s trending left lateral strike slip faulting during this period initial deposition consisted of andesitic lavas and detrital sediments breccias matrix supported red conglomerates sands and silts of the catati and tupiza formation deposited in alluvial fan and flood plain environments from 21 to 20 ma the tectonic setting evolved to n s oriented thrusts which had a dextral component this event caused the tupiza basin to evolve into a full ramp basin and triggered the development of two piggy back basins nazareno and estarca both basins record detrital deposition nazareno formation in an alluvial fan environment at 10 9 ma deformation in this area stopped allowing peneplanation during which time the san juan de oro erosional surface was formed and the fluvial conglomerates of the oploca formation were deposited this segment of the andes was then tectonically transported to the east while uplift due to thrusting continued in the eastern andean front and in the subandean zone

lithospheric structure and along strike segmentation of the central andean plateau seismic q magmatism flexure topography and tectonics the central andean plateau is a 300 km wide nearly 4 km high plateau which is situated above a 30 e dipping segment of the subducted nazca plate systematic changes in the topography the upper mantle seismic attenuation q structure the lithospheric flexural rigidity and the distribution and chemistry of back arc lavas indicate that the upper mantle structure changes along strike beneath the plateau south of 23 s the upper mantle beneath the plateau becomes hotter and the lithosphere becomes thinner and weaker this lateral variation in upper mantle structure coincides with variations in the tectonic style and timing of deformation in two physiographically distinct segments of the plateau and its adjacent foreland thrust belt to the east the bolivian altiplano and subandean ranges in the north and the argentine puna and santa barbara system in the south the segments different mantle lid thicknesses result in an altiplano supported mainly by crustal thickening and a puna supported by a combination of a crustal root and a thermal mantle root a thicker stronger foreland lithosphere in the north favors thin skinned underthrusting of the foreland beneath the plateau while to the south the thinner weaker lithosphere allows more basement involved shortening within the foreland and less underthrusting beneath the plateau a weaker lithosphere results in a longer period of compressional deformation and greater local and structural relief in the puna than in the relatively stable altiplano to the north ultimately this observed n s directed segmentation on the plateau and in the foreland may reflect the different responses of strong and weak continental lithosphere to an andean type subduction margin

paleomagnetic evidence of an early paleozoic rotated terrane in northwest argentina a clue for gondwana laurentia interaction paleomagnetic results from three lower ordovician units in northwest argentina are reported when assessed with available geologic evidence these data permit the definition of the puna oriental famatina rotated terrane paleogeographic reconstructions based on these paleomagnetic results place puna oriental famatina in the south iapetus ocean close to the gondwana margin and near some of the lower middle ordovician central mobile belt terranes of the northern appalachians the accretion of puna oriental famatina may have resulted from the closure of this ocean in middle ordovician time

crustal balance and crustal flux from shortening estimates in the central andes abstract the central andes of south america form the second largest high elevation plateau on earth extreme elevations have formed on a noncollisional margin with abundant associated arc magmatism it has long been thought that the crustal thickness necessary to support andean topography was not accounted for by known crustal shortening alone we show that this may in part be due to a two dimensional treatment of the problem a three dimensional analysis of crustal shortening and crustal thickness shows that displacement of material towards the axis of the bend in the central andes has added a significant volume of crust not accounted for in previous comparisons we find that present day crustal thickness between 12 s and 25 s is accounted for 10 to 3 with the same shortening estimates and the same assumed initial crustal thickness as had previously led to the conclusion of a 25 35 deficit in shortening relative to volume of crustal material we suggest that the present day measured crustal thickness distribution may not match that predicted due to shortening and substantial redistribution of crust may have occurred by both erosion and deposition at the surface and lower crustal flow in regions of the thermally weakened middle and lower crust

upb geochronology of the central ribeira belt brazil and implications for the evolution of the brazilian orogeny abstract the ribeira belt in southeastern brazil was generated during the brazilian orogeny the south american equivalent of the pan african orogeny 700 450 ma the central sector of the belt is characterised by a series of northwest vergent crustal scale thrust stacks emplaced under amphibolite grade conditions major shear zones separate four lithotectonic domains andrel ndia to the northwest juiz de fora paraiba and coastal domains analyses of zircon monazite and titanite from a variety of rock types from the four domains lead to the following conclusions all domains record an important tectono thermal event at 590 565 ma represented by partial melting granitoid intrusion and remobilisation of older basement gneisses and corresponding to thrusting and development of dextral shear zones also recorded are older metamorphic ages 611 604 ma of unknown significance only the paraiba and coastal domains in the south record a younger metamorphic event m2 at 535 520 ma most likely associated with another episode of thrusting and shear zone development the absence of m2 metamorphism in the andrel ndia and juiz de fora domains can be explained by post 535 520 ma emplacement of the paraiba domain late to post tectonic activity is represented by 503 492 ma metamorphism precursors of the basement gneisses are 2185 2134 ma old rocks generated during the transamazonian orogeny some of which contain a significant archean component with minimum ages of 2981 2846 ma the ages presented above define specific metamorphic events for the brazilian orogeny for the first time in southeastern brazil in addition together with previously published data they indicate that the basement rocks found in the allochthonous domains of the central ribeira belt represent an extension of the units present in the southern sao francisco craton and indicate that the foreland was more extensive than the presently defined craton

paleomagnetism of the sierra de las animas complex southern uruguay its implications in the assembly of western gondwana global paleogeographies in the neoproterozoic are poorly known in particular the assembly of gondwana at the end of the proterozoic is controversial in order to constrain the kinematic evolution of the rio de la plata craton a paleomagnetic study was carried out in the vendian to cambrian sierra de las animas magmatic complex exposed in southeastern uruguay 34 7 s 55 3 w one hundred and forty seven samples from 23 sites were studied these included two sites on the vendian glaciogenic sediments of the playa hermosa formation after stepwise af and thermal cleaning a characteristic remanence was isolated at several sites according to the available isotopic data for the complex and the correlation between remanence directions and lithologic type and age two paleomagnetic poles were computed for the sierra de las animas complex sa1 338 1 e 5 9 n dp 19 6 dm 26 7 n 7 sites mean declination 44 5 mean inclination 58 0 95 18 1 with a likely age around 520 ma shows a distribution of vgps that suggests a remanence acquisition that spanned a long time during a period of fast apparent polar wander for gondwana sa2 250 9 e 16 9 s dp 15 9 dm 21 5 n 6 sites mean declination 95 3 mean inclination 58 5 95 14 5 corresponds to a likely age around 550 ma and falls close to a pole of similar age from the congo craton this suggests that the rio de la plata craton was at that time part of gondwana and confirms previous postulates that a single apwp can be defined for the supercontinent since 550 ma a preliminary mean geomagnetic pole obtained from the playa hermosa formation ph 198 4 e 43 0 s dp 8 6 dm 16 0 n 6 samples mean declination 226 0 mean inclination 24 2 95 15 0 is consistent with a previous pole of 595 ma from the campo alegre ca lavas and may suggest if confirmed that the glaciogenic deposits were produced at low to intermediate latitudes despite high uncertainties a new speculative model on the process of assembly of gondwana is proposed

attenuation of high frequency seismic waves beneath the central andean plateau observed patterns of high frequency seismic wave attenuation suggest that near 22 s the upper mantle structure beneath the central andean plateau changes along strike to the south contrasting regions of high and low upper mantle seismic wave attenuation beneath the plateau are identified based on striking azimuthal variations in the character and frequency content of p and s waves propagating beneath the plateau to a portable seismic network deployed in jujuy province argentina 24 s 65 w ray paths from intermediate depth earthquakes located north and northwest of the network transmit seismic waves with a higher frequency content than ray paths from earthquakes at similar depths and distances but located west and south of the network the estimated apparent q values fall into two categories qp 500 and qs 350 for the high q paths and qp 350 and qs 200 for the low q paths in addition sn phases from regional crustal earthquakes in the subandean foreland fold thrust belt to the north propagate efficiently to the jujuy network while sn is not observed from foreland earthquakes located at similar distances to the south of the network these observations combined with data from previously reported wave propagation studies suggest that south of about 22 s the upper mantle beneath the plateau and its adjacent foreland thrust belt is more highly attenuating than the upper mantle farther north forward modeling of the q measurements made at jujuy indicates that the observations can be explained either by a variable thickness high q upper plate beneath the plateau or by a thin variable width very low q zone in the asthenospheric wedge above the subducted slab this lateral variation in upper mantle structure coincides with two physiographically distinct segments of the central andean plateau and its adjacent foreland thrust belt to the east the bolivian altiplano and subandean ranges in the north and the argentine puna and santa barbara system in the south we interpret the north south change in upper mantle attenuation and the corresponding changes in physiography topography and tectonic style at the surface to be due to a mantle lid that is thicker beneath the altiplano and the subandean belt than beneath the puna and the santa barbara ranges

revised deformation history of the central andes inferences from cenozoic foredeep and intermontane basins of the eastern cordillera bolivia 1 investigation of cenozoic deformation sediment accumulation and provenance in the eastern cordillera of bolivia at 17 21 s indicates major shortening 60 140 km and foredeep development followed by limited internal shortening and intermontane basin development contrasting histories of shortening deposition sediment dispersal and detrital composition distinguish a formerly extensive paleogene foredeep exposed along an eastern belt of synclines from a zone of principally neogene intermontane basins in the central eastern cordillera new 40ar 39ar ages of 25 17 ma for interbedded tuffs reveal latest oligocene early miocene accumulation in intermontane basins at rates 80 m myr several times lower than andean foredeeps although poorly dated foredeep evolution probably coincided with middle eocene oligocene deformation and denudation in adjoining regions to the west a mid cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement involved tectonic wedge beneath the eastern cordillera in this interpretation wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 ma whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas regardless of the regional subsurface structural geometry 40ar 39ar ages define a 25 21 ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the eastern cordillera and possibly the age of initial shortening in the interandean and subandean zones to the east in contrast to several popular models the revised tectonic history presented here suggests significant pre neogene shortening in the eastern cordillera and underscores the large uncertainties in estimates of long term shortening rates in the andes

andean foreland evolution and flexure in nw argentina chaco parana basin the chaco parana basin was initiated during the late oligocene as a result of shortening in response to nazca south america plate convergence which produced a fold and thrust belt in the backarc region of the central andes foreland basins are the result of the flexural isostatic response of an elastic plate to orogenic and or thrust sheet loading we carried out flexural modeling along a w e profile 22 5 degrees s to determine flexural crustal and chaco parana foreland basin development using available information on ages elastic and sedimentary thicknesses topographic and structural histories we also developed a 3d flexural model considering present day topography and elastic thickness values in order to assess the possible existence of major differences with 2d models it is possible to reproduce present day elevation gravity anomaly moho depth elastic thicknesses foreland sedimentary thicknesses and the basin geometry the 2d modeling also permits to reproduce foreland sedimentary thicknesses documented at 10 ma and 12 ma the new results indicate that any decrease of the elastic thickness beneath the eastern puna eastern cordillera would have occurred during the 14 12 10 ma interval suggesting a major change in the thermal state of the lithosphere during such time span coincidently other authors determined the occurrence of a rapid elevation increase of the southern altiplano eastern cordillera at 13 9 ma possibly related to piecemeal removal of lower lithosphere in spite of the numerous limitations and simplifications mentioned above our flexural model has shown to be a useful tool for the better understanding of the relationships between the development of the chaco parana foreland basin and the spatiotemporal evolution of the andean orogen

lithospheric flexure and bending of the central andes gravity anomaly and topography data are used to define the effective elastic thickness of the lithosphere t in the bend region of the central andes values of z increase from nearly zero north and south of the bend to values greater than 50 km at the bend in bolivia there is a close correlation between t and the style and magnitude of the shortening in the central andes since the late miocene in the bend region where the lithosphere is flexurally strong with large t foreland deformation is concentrated into a thin skinned fold and thrust belt above a basal decollement which has absorbed more than 100 km of shortening further north and south where the lithosphere is flexurally weak with low t foreland deformation is more complex involves basement and has absorbed less shortening the along strike gradients in foreland shortening have accommodated both clockwise and anticlockwise rotations about a vertical axis of almost the entire width of the bolivian andes we speculate that the observed variation in t is related to the proximity of the brazilian shield and has been an important factor in controlling the nature and amount of foreland deformation and hence bending of the central andes the flexural properties of the lithosphere may play an important role in determining the large scale evolution of mountain belts

deep reflection seismic imaging in se poland using extended correlation method applied to polandspan data abstract here i apply the extended correlation technique to a 156 km long regional seismic profile pl1 5100 from the polandspan survey located in se poland this method allows the extension of the nominal record length of the survey 12 s in this case to much longer times 22 s tested here given that in the field the raw uncorrelated data were stored and a broadband sweep was used processing of the ground force correlated data compared to pilot sweep correlated one produced qualitatively similar results based on the consistency of the ground force measured along the profile data were correlated with the mean of the ground force estimates results of the processing were validated against the coincident wide angle reflection refraction warr profile celebration 2000 cel05 profile and a deep reflection profile located further to the se polcrust 01 profile i found a good correspondence between the wide angle moho signature and the near vertical incidence nvi moho reflectivity both in terms of the depth and the reflection strength with the exception of the area of the crustal keel moho 50 km interpreted from the warr data where the nvi reflectivity is observed at depth ca 47 km similarly to polcrust 01 line pl1 5100 is portraying the attenuated crust of the east european platform margin with the dipping crystalline basement down to ca 20 km depth and the reflective lower crust two important fault zones izbica zamo c fault and wilczopole fault zone are interpreted as the deeply rooted crustal features but there is no evidence for the teisseyre tornquist zone itself

post collisional transition from calc alkaline to alkaline magmatism during transcurrent deformation in the southernmost dom feliciano belt braziliano pan african uruguay abstract in the southernmost dom feliciano belt of uruguay highly fractionated calc alkaline granites mildly alkaline granites shoshonitic volcanics and peralkaline intrusions and volcanics are spatially and temporal associated with the evolution of shear zones four representative magmatic unites of this diverse association were petrographic and geochemically investigated the solis de mataojo complex a medium to high k2o calc alkaline granite with signature typical of mature continental arcs and post collisional settings the maldonado granite highly fractionated calc alkaline to alkaline with characteristics that are transitional between both types of series the pan de azucar pluton with characteristics typical of post collisional alkaline granites and the las flores shoshonitic basalts geochemistry and geotectonic setting point out that slab breakoff was most likely the mechanism associated with the generation of high k calc alkaline magmas solis de mataojo and maldonado shortly after collision extension associated to the formation of molassic basins and emplacement of dolerites and basalt flows with shoshonitic affinity las flores 15and finally a shift to magmas with alkaline signatures pan de azucar simultaneous with a second transpressional phase were probably linked with lithospheric thinning through delamination this evolution took place between 615 and 575 ma according to available data contrary to previous proposals which considered this magmatism to represent the root of a continental magmatic arc a post collisional environment transitional from orogenic to anorogenic during transcurrent deformation is proposed

tectonic setting of the late proterozoic lavalleja group dom feliciano belt uruguay abstract the lavalleja group exposed along the dom feliciano orogenic belt is located in the southeast of uruguay this group consists of volcano sedimentary rocks developed during the neoproterozoic brasiliano cycle the geochemical signature of the igneous rocks of the lavalleja group mainly metagabbros and basic and acidic metavolcanic rocks indicates a back arc basin tectonic setting the metamorphic grade increases to the southeast from very low grade lower green schist facies in the minas formation to a medium grade amphibolite facies in fuente del puma and zanja del tigre formations the metamorphic mineral assemblages correspond to a low pressure regional metamorphism associated with a high thermal gradient a compressive deformational event that probably corresponds to the closure of the lavalleja basin during a continental collision was recognized the petrology geochemistry metamorphic grade and tectonic setting are consistent with a back arc basin setting for the lavalleja group

structure and cenozoic kinematics of the eastern cordillera southern bolivia 21 s 1 the eastern cordillera is a main morphotectonic unit of the central andes in southern bolivia it is predominantly composed of ordovician sedimentary rocks as thick as 10 km revision of the ordovician stratigraphy and new structural fieldwork resulted in detailed balanced cross sections improved estimates of crustal shortening and a new kinematic model for the eastern cordillera new estimates of total shortening in the eastern cordillera are up to 95 km 41 44 higher than previous estimates and sufficient to thicken the andean crust to its present state between the eastern foreland and the magmatic arc after initial tertiary uplift at 58 ma first thrusts developed in the middle eocene to early oligocene 40 30 ma exhuming the central part of the eastern cordillera tectonic shortening culminated in late oligocene and early miocene time 25 20 ma when west facing thrust systems developed in the central and western part of the eastern cordillera thrusting terminated in the western part by the end of the early miocene and middle miocene tectonic activity was restricted to minor strike slip faulting in the central part thrusting remained active until the late miocene by 10 ma shortening in the eastern cordillera essentially ceased kinematic fault analyses indicate clockwise rotation of the shortening direction from nne ssw or ne sw in eocene early oligocene time over e w to wnw ese or even nw se in late oligocene to middle miocene time nevertheless wnw ese directed shortening and subvertical extension predominate sinistral oblique motion on many thrusts probably reflects plan view flexural slip in the clockwise rotating southern limb of the bolivian orocline it is proposed that space problems in the oroclinal hinge slowed the rotation there and caused decoupling of the still rotating south limb along ne trending transfer zones

crustal scale structural architecture in central chile based on seismicity and surface geology implications for andean mountain building 1 we document a crustal scale structural model for the central chile andes based on seismicity and surface geology which consists in a major east verging ramp detachment structure connecting the subduction zone with the cordillera the ramp rises from the subducting slab at 60 km depth to 15 20 km below the western edge of the cordillera extending eastward as a 10 km depth flat detachment this structure plays a fundamental role in the andean orogenesis because most of the shortening has been accommodated by structures rooted in it and allows the distribution of crustal thickening in a simple shear deformation mode indeed despite shortening distribution being very asymmetric 16 km versus 70 km in the western and eastern side respectively the western side is higher and thicker than what is expected yield strength envelopes show strong rheological control on this structure vp and vp vs variations in the upper mantle and in the deepest limit of the seismogenic interplate contact mark the intersection of the ramp with the slab which coincides with the blueschist eclogite transition therefore subduction processes would control the depth where the major east verging structure may merge with the slab such a ramp flat structure is observed in other parts of the chilean margin hence it seems to be a first order feature in the andean subduction zone this structure delimitates upward the rocks transmitting part of the plate convergence stress from the plate interface and controls mountain building tectonics thus playing a key role in the andean orogeny

the paranapanema lithospheric block its importance for proterozoic rodinia gondwana supercontinent theories abstract this work considers the tectonics of the southeastern portion of the south american platform based on new geological and geophysical grounds for the last decade only three amazonic sao francisco and la plata of the many other cratonic blocks have been attributed remarked to the south america portion for most of the usual rodinia reconstitutions the possibility of the existence of other blocks has rarely been mentioned the postulation of the presence of a considerable paleoproterozoic pre brasiliano fragment as part of parana basin basement is highly probable in order to infer the basement structure of parana basin previous to the sedimentation process an isostatic modeling was applied to a large scale gravity survey looking to correlate topographic and gravity anomalies caused by sub surface loads the bouguer anomaly obtained from the gravity survey represents the crustal contribution of crystalline basement in addition to the sedimentary and volcanic layers of the basin following the isostatic modeling and the basin load stripping the residual anomaly allows observing similarities between the basement gravity signature and outcropping units besides the stress pattern of the two earlier events obtained through the back stripping analysis presents a geographically coincident maximum and a new e se high emerging for the second event suggesting continuous change of the stress field as a precursor for south american plate rotation the evident correlation between gravity highs and main attenuation suggests the presence of some pre existing suture zones the weakened lithosphere during ordovician and carboniferous provided the magma conduits to form in early cretaceous tectonic stress field pattern the resultant mosaic of gravity blocks and the main faults site give support to the presence of this cratonic proterozoic unit here on referred to as the paranapanema block which had been neglected in most of the models reported for the reconstruction of gondwana and rodinia

high level surfaces plateau uplift and foreland development bolivian central andes newly dated tertiary strata in the bolivian central andean plateau and synthesis of the tertiary record in the adjacent subandean fold thrust belt constrain the age of deformation in both regions age of deformation within the plateau is determined by dated crosscutting relations associated with a regionally extensive high level surface known as the san juan del oro surface new 40 ar 39 ar dates on undeformed strata above the high level surface preclude significant upper crustal shortening within the eastern cordillera after 10 ma tertiary strata within the adjacent subandean region demonstrate that formation of the fold thrust belt occurred after 10 ma on the basis of these data we propose a two stage model of late cenozoic andean growth that links plateau uplift to the development of the fold thrust belt in the first stage early plateau uplift occurred in response to widespread compressional deformation of the plateau eastern cordillera and altiplano during the second stage beginning after 10 ma upper crustal deformation within the plateau terminated and the subandean fold thrust belt developed crustal scale eastward thrusting along the eastern margin of the eastern cordillera drove subandean folding and thrusting the eastern cordillera served as the bulldozer for the deforming subandean wedge

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

distal magmatic hydrothermal origin for the camaqua cu au ag and santa maria pb zn cu ag deposits southern brazil abstract the camaqua cu au ag and santa maria pb zn cu ag deposits are the largest base metal deposits discovered so far in the sedimentary clastic sequences of the neoproterozoic age camaqua basin the origin of the camaqua santa maria deposits has been the centre of dispute with three alternative genetic hypotheses proposed a syngenetic model a diagenetic model and a magmatic hydrothermal model in detail this mineralization has been suggested to be related to sedimentary clastic diagenetic processes volcanic related processes or deep granitic magmatism reevaluation of previous data and new studies in the area yield the following conclusions 1 mineralization is fracture controlled and magmatic hydrothermal in origin rather than stratiform syngenetic or diagenetic 2 the temperature of deposition of the main ore minerals was 210 to 300 c 3 the 34 s of sulphides of around 0 indicates an external magmatic hydrothermal source of sulphur 4 pb isotope ratios of sulphides indicate that metals were derived at the end of the brasiliano cycle from a large crustal source with very primitive pb and that 5 the age of mineralization is 594 ma as constrained by u pb shrimp sensitive high resolution ion microprobe determinations on zircons of the lavras granite thus the camaqua and santa maria deposits are interpreted to be of magmatic hydrothermal origin with the metals derived from an old crustal basement source during the end of dom feliciano collisional orogeny at 594 ma late in the brasiliano cycle the interpretation above is critical for base metal exploration in the sul riograndense shield previous exploration methodologies mainly followed models based on a sedimentary hypothesis for the origin of the deposits however the occurrence of mineralization along fractures within specific wall rocks requires consideration of alternative exploration parameters these include 1 ancient ew and nw trending regional fractures and their intersections which are potential structural sites for base metal mineralization and 2 preferential wall rock sites either rocks with high initial porosity or secondary fracture induced porosity or soluble rocks which are susceptible to replacement processes post collisional plutonism of the dom feliciano orogeny was the most likely heat source and also the source of sulphur and base metals gravity surveys and alteration studies would be useful to determine the presence of intrusive bodies at depth which may have the potential to host porphyry style cu au deposits

extraction of deep crustal reflections from shallow vibroseis data using extended correlation lower crustal seismic reflections can be extracted from shallow crustal seismic profiles through the application of extended correlation to uncorrelated vibroseis seismic data fixed bandwidth extended correlation shortens the correlation operator before crosscorrelation producing reflections over an increased correlation time range all with lowered bandwidth self truncating extended correlation preserves the full bandwidth in the original seismic reflection times but loses bandwidth in a predictable manner at the additional later arrival times correlation wavelet shape and extra correlation time are directly related and can be calculated for specific acquisition parameters pre correlation tapering is necessary to avoid undue wavelet distortion at extended correlation times seismic data collected in the basin and range province illustrate the application of the method the results are verified with conventional correlations of long sweep records and with impulsive source data

paleogeography and andean structural geometry northwest argentina neogene tectonic segmentation of the andean cordillera in northwestern argentina is a function of both the dip of the subducted plate and of major paleogeographic structures and sedimentary basins vergence fault dip and lateral continuity of neogene structures can be related to the character and pre existing geometries and structures of the materials in which they form south of 27 s the thick skinned sierras pampeanas which occur mostly over a flat segment of the subducted nazca plate formed where little sedimentary cover overlay crystalline basement north of 24 s the thin skinned subandean belt overlying an inclined slab segment formed within a thick elongate eastward tapering trough of paleozoic and mesozoic sediments that may represent a complex of pre andean foreland basin deposits the transitional region between 24 27 s the santa barbara system and cordillera oriental is characterized paleogeographically by complex cross trending structures and irregular basin geometries and the neogene structures are equally complex and variable the paleogeographic features of nw argentina are a product of compressional tectonism during much of the paleozoic that resulted in an orogenic belt oriented obliquely to the modern andes this earlier orogen was disrupted by several zones of transverse structures and by cretaceous extensional or pull apart basins these features clearly influence the geometry of the superposed andean structures more intriguing however is the possibility that preexisting structures of the continental lithosphere may in some way help to modify the geometry of the subducted plate

eocene exhumation and basin development in the puna of northwestern argentina 1 the puna is part of the larger puna altiplano plateau also known as the central andean plateau characterized by high elevation low relief and aridity located in the central andes of bolivia and argentina tertiary sedimentary rocks preserved within the puna contain a unique archive of information regarding the paleogeography depositional environments and timing of sediment source exhumation during the early stages of andean mountain building the eocene geste formation in the salar de pastos grandes area within the central puna of northwestern argentina consists of deposits that are the result of confined to unconfined flows in a sandy to gravelly braided fluvial system and alluvial fans proximal to the source terrane paleocurrent data document an overall eastward flow direction up section coarsening of the geste formation suggests that topographic relief in the source area increased through time possibly owing to enhanced tectonic activity and source terrane unroofing sandstone petrography and conglomerate clast count data document quartzose and phyllitic compositions typical of ordovician rocks preserved just west of the salar de pastos grandes area paleocene eocene detrital apatite fission track age populations p1 35 52 ma p2 52 65 ma of the geste formation and their consistent trends up section suggest moderate to rapid 0 4 mm a to 1 mm a exhumation of western sediment sources during the early to mid tertiary stages of andean mountain building sedimentation rates increase up section from 0 1 mm a to 1 mm a our data when combined with other structural stratigraphic and seismic evidence from surrounding regions suggest that the geste formation was deposited in response to crustal shortening and resulting erosion and sedimentation which started as early as cretaceous in the chilean cordillera de domeyko and in the salar de pastos grandes area by eocene time the geste formation could be interpreted either as a local wedge top accumulation on the eastward propagating central andean orogenic wedge or as a local intermontane basin the similarities between wedge top deposits preserved in bolivia and eocene deposits in northwestern argentina south of 25 s lead us to favor the wedge top scenario for the geste formation if correct this implies that the deformation front of the andean orogenic wedge incorporated both thin and thick skinned structures as it migrated possibly unsteadily from the cordillera de domeyko during the cretaceous paleocene to areas within the puna and eastern cordillera by mid late eocene time contemporaneously a regional scale foreland basin system developed over an along strike distance of at least 650 km

southernmost exposures of the arroyo del soldado group vendian to cambrian uruguay palaeogeographic implications for the amalgamation of w gondwana abstract the occurrence of the vendian to lowermost cambrian arroyo del soldado group asg is reported from an area located to the sw of minas which was formerly mapped as part of the lavalleja group the yerbal polanco and cerro espuelitas formations of the asg occur there as a rather continuous microfossil rich sedimentary cover intruded by granites of probable cambrian age minas granite organic walled microfossils recovered from the yerbal and polanco formations include bavlinella faveolata leiosphaeridia tenuissima soldadophycus bossii soldadophycus major siphonophycus solidum glenobotrydion aenigmatis and filament mats the acritarch dyctiotidium sp is described for the first time from the yerbal formation essentially the same microfossil assemblage occurs in five samples of this unit collected immediately to the north of minas no identifiable microfossils were found in fifteen samples of organic rich lithologies of the lavalleja group based on detailed geological mapping litho and biostratigraphic data we show that the contact between the arroyo del soldado and lavalleja groups is of tectonic nature arroyo la plata lineament the latter being pre vendian in age the villa serrana block is defined and consists of the carape and lavalleja groups las ventanas and playa hermosa formations an unnamed siliciclastic ultrabasic succession showing low grade metamorphism and various granitic intrusions mostly of neoproterozoic to cambrian age its boundaries are the sarandi del yi piriapolis shear zone to the w the sierra ballena shear zone to the e the arroyo la plata lineament to the n and the rio de la plata to the s the villa serrana block could be an allochthonous terrane or alternatively a part of the nico perez terrane that was thrusted onto the asg no continuity exists between the brusque and porongos groups of southern brazil and the lavalleja group according to available data these units are probably not coeval and do not represent the suture between the cratonic areas to the w rio de la plata craton and the remnants of a neoproterozoic magmatic arc to the e cuchilla dionisio pelotas terrane accretion of these blocks was controlled by sinistral continental scale megashears the geology of the area is far too complex to be explained in terms of a single orogeny following one wilson cycle

on the strength of interplate coupling and the rate of back arc convergence in the central andes an analysis of the interseismic velocity field 1 we interpret the interseismic crustal velocity field of the central andes using a simple three plate model in which the andean mountain belt is treated as a rigid microplate located between the nazca and south american soam plates we assume that the euler vectors associated with these plates are strictly coaxial and that the surface velocity field can be decomposed into an elastic loading field driven by locking of the main plate boundary and a smaller contribution associated with back arc convergence we obtain our best fit to the geodetic velocities if the main plate boundary is fully 100 locked between depths of 10 50 km and 8 5 of nazca soam plate convergence is achieved in the back arc by underthrusting of the brazilian shield beneath the subandean zone

tectonic inversion in the santa barbara system of the central andean foreland thrust belt northwestern argentina 1 the santa barbara system sbs of northern argentina is a 400 km long segment of the subandean foreland thrust belt it is characterized by predominantly west verging relatively high angle thrust faults many of these faults are reactivated normal faults from one branch of a complex cretaceous to paleogene rift system heteroaxial folding in parts of the sbs is probably due to a slight component of range parallel dextral strike slip motion acting on preexisting faults striking north and ne during inversion regional balanced cross sections along two transects across the northern sbs indicate that the major faults flatten into a detachment in the basement at about 10 km depth neogene e w contraction in the sbs is of the order of 21 26 km rift extension is not very well constrained but was probably less than 10 km the structural style of the sbs differs from the thin skinned subandean thrust belt to the north and from the large wavelength sierras pampeanas basement uplifts to the south the changes between the different styles are sharp and coincide with the northern and southern boundaries of the rift in the foreland suggesting that crustal or lithospheric heterogeneities exert an overriding control on foreland structural style

geochemistry and crustal evolution of volcano sedimentary successions and orthogneisses in the sao gabriel block southernmost brazil relics of neoproterozoic magmatic arcs abstract precambrian metaplutonic rocks of the sao gabriel block in southernmost brazil comprise juvenile neoproterozoic calc alkaline gneisses cambai complex the connection with associated ultra mafic metavolcanic and metasedimentary rocks palma group is not well established the whole complex was deformed during the brasiliano orogenic cycle both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the cambai complex have been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and to establish a tectonic model for the sao gabriel block and its role during the assembly of west gondwana the major element compositions of the igneous rocks palma group and cambai complex indicate a subalkaline character most orthogneisses have a calc alkaline chemistry many metavolcanic rocks of the palma group show signatures of low k tholeiitic volcanic arc basalts trace element data especially ti zr y nb of most igneous samples from both the lower palma group and the cambai complex indicate origin at plate margins i e in a subduction zone environment this is corroborated by relative enrichment in lree low contents of nb and other high field strength elements and enrichment in lile like rb ba and th the data indicate the possible existence of two suites an oceanic island arc and a continental arc or active continental margin however some ultramafic samples of the lower palma group in the western sao gabriel block indicate the existence of another volcanic suite with intra plate character which possibly represents relics of oceanic island basalts oib trace element data indicate contributions from andesitic to mixed felsic and basic arc sources for the metasedimentary rocks the patterns of chondrite and n morb normalized spider diagrams resemble the patterns of the igneous rocks i e lile and lree enrichment and hfs depletion the geochemical signatures of most igneous and metasedimentary samples and their low 87sr 86sr t ratios suggest only minor contribution of old continental crust a geotectonic model for the sao gabriel block comprises east ward subduction and following accretion of an intra oceanic island arc to the eastern border of the rio de la plata craton at ca 880 ma and westward subduction beneath the newly formed active continental margin between ca 750 and 700 ma the sao gabriel block represents relics of an early brasiliano oceanic basin between the rio de la plata and kalahari cratons this ocean to the east of the rio de la plata craton might be traced to the north and could possibly be linked with neoproterozoic juvenile oceanic crust in the western brasilia belt goias magmatic arc

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

chemostratigraphy of the lower arroyo del soldado group vendian uruguay and palaeoclimatic implications abstract new c and o isotopic determinations from the vendian lower arroyo del soldado group are reported and combined with sedimentologic and biostratigraphic data on the basis of different geochemical and petrographic criteria the primary nature of the c and sr isotopic signature is shown positive d 13 c values characterize the mainly siliciclastic upper yerbal formation which contains oxide facies bif and a diverse assemblage of skeletal fossils including cloudina riemkeae a series of positive and negative d 13 c excursions occur up section in the overlying polanco formation mainly composed of limestones and limestone dolostone rhythmites the transition to the overlying barriga negra and cerro espuelitas formation which consists of conglomerates and shales cherts bif respectively is marked by a further negative excursion on the basis of sedimentary structures a correlation of d 13 c with palaeobathymetry is established positive d 13 c peaks are associated with high sea level stand high organic carbon burial and relatively higher microfossil diversity while negative d 13 c excursions occur in carbonates with less organic matter less microfossil diversity and are always associated to regressions these observations can be readily explained by palaeoclimatic models which postulate that enhanced bioproductivity due to higher availability of nutrients p n fe was the key factor controlling neoproterozoic glaciations the mentioned models are discussed in view of the new data the occurrence of at least four cold periods in the upper vendian is envisaged these cold periods led to sea level fall and possibly glaciation at higher latitudes the absence of glacigenic rocks associated to negative d 13 c excursions in the arroyo del soldado group is probably due to the tropical setting of the basin finally the upper vendian age of the lower arroyo del soldado group is confirmed on the basis of c and sr isotopes the onset of carbonate deposition at the base of the polanco formation is estimated at 580 ma by comparison with existing global isotopic curves

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

dynamics of deformation and sedimentation in the northern sierras pampeanas an integrated study of the neogene fiambala basin nw argentina reply the thick skinned sierras pampeanas morphotectonic domain of western and northwestern argentina 27 s 33 s is characterized by reverse fault bounded basement blocks that delimit internally deformed neogene sedimentary basins foreland basin evolution in this part of the andes is still not very well understood for example challenging questions exist as to how thick skinned deformation develops if there are distinct spatiotemporal trends in deformation and exhumation how such deformation styles influence sedimentation patterns and whether or not broken foreland basins are related to regional plate tectonic processes such as flat slab subduction the fiambala basin of the northwestern sierras pampeanas is the largest of several intermontane basins in the transition to the southern margin of the puna plateau this basin preserves a thick continental neogene sequence that provides information on the dynamics of thick skinned deformation and resulting sedimentation the fiambala basin contains 4 km of fluvial alluvial sedimentary rocks that comprise the tamberia guanchin and punaschotter formations u pb geochronology of ashes intercalated within the fiambala stratigraphic sequence demonstrates that these sedimentary rocks are late miocene to pliocene 8 2 0 3 ma to 3 05 0 4 ma in age sedimentology and provenance data indicate that the source of the tamberia formation was located to the west of the modern western basin bounding range the guanchin and punaschotter formations record input from local sources including the modern basin bounding range to the west and the southern puna plateau to the north suggesting reorganization of the catchment area at ca 5 5 ma the coarsening upward trends recorded by the fluvial tamberia and guanchin formations indicate enhanced tectonics and relief during sedimentation the punaschot ter conglomerates record alluvial fan sedimentation and local sources fault kinematic data document a contractional regime characterized by e w and ne sw shortening active throughout the middle late miocene and pliocene furthermore a comparison between the fiambala basin and similar sedimentary basins in the sierras pampeanas e g bermejo foreland basin and the eastern cordillera leads us to propose that the study area originally constituted an integral part of a continuous and more extensive foreland basin system thin skinned for much of its early history our data suggest coeval intrabasin deformation along strike from the bermejo region northward to the eastern cordillera the coeval change at ca 6 ma from a regional to more compartmentalized thick skinned tectono sedimentary environment in the regions adjacent to the eastern cordillera the southern puna margin and other sectors within the sierras pampeanas domain may thus reflect a regional tectonic process related to flat subduction our data combined with existing sedimentological and petrological evidence imply that the passage from steep to flat subduction occurred synchronously from 30 s to 26 s

geochronological constraints on the evolution of the southern dom feliciano belt uruguay abstract new u pb sensitive high resolution ion microprobe k ar and ar ar data from the southernmost dom feliciano belt allow the identification of four major events orthogneisses from the punta del este terrane indicate a magmatic episode at c 770 ma and high grade metamorphism at 641 17 ma granitoid emplacement at 627 23 ma was roughly coeval with peak metamorphism volcaniclastic rocks of the las ventanas formation dated at 573 11 ma can be correlated with the peripheral foreland basin 571 8 ma transpression and coeval high k calc alkaline magmatism is recorded in the maldonado granite dated at 564 7 ma the following events are postulated 1 magmatism at 850 750 ma related to rifting 2 metamorphism and granite emplacement at 650 600 ma 3 molasse sequences and foreland basins at c 573 ma 4 late magmatism at 580 560 ma associated with transpression the data indicate that 1 the punta del este terrane could be a portion of the coastal terrane of the kaoko belt 2 granitoid emplacement at 650 600 ma in the punta del este and nico perez terranes favours westward subduction and 3 widespread post collisional synkinematic magmatism occurred in the dom feliciano and kaoko belts between 580 and 550 ma supplementary material analytical methods and data are available at http www geolsoc org uk sup18369

subandean thrust and fold belt of northwestern argentina geometry and timing of the andean evolution the subandean ranges of northwestern argentina are an active thin skinned fold and thrust belt the main detachment level in silurian shales dips 2 3w and all the major east verging faults detach from it important intermediate detachment levels in the devonian shales generate lift off structures and the decoupling of the lower and upper structural levels the subandean thrust belt has a minimum shortening of about 60 km 36 at about 2240 latitude the deformation started at about 8 5 9 ma with the uplift of the el pescado range and the formation of an important back thrust at the cinco picachos range fault generation gets young to the east the pintascayo range uplift started at 7 6 ma while the baja orn range uplift began at about 6 9 ma these two ranges continued growing simultaneously until at least 4 7 ma at san antonio range fault movement began at approximately 4 4 ma and the aguarage uplift started at about 2 7 ma an important stage dominated by out of sequence movements spans from about 4 5 ma to present for both proposed models of shortening the quaternary rates of shortening between 8 and 11 mm yr coincide well with global positioning system results from the area the hydrocarbon generation and migration is contemporaneous with the deformation enhancing the possibilities of hydrocarbon entrapment in the area

the kinematic history of the central andean fold thrust belt bolivia implications for building a high plateau this paper presents a model for the kinematic evolution of the central andean plateau based on balanced cross sections across the bolivian andes the proposed model links the formation of the andean plateau to the development of the andean fold thrust belt through the creation and propagation of two large basement mega thrusts support for large basement involved thrust sheets is found in significant steps in both the topography and the exposed structural elevation of the andean fold thrust belt the structurally highest basement thrust raised folds and faults in predominantly lower paleozoic rocks of the eastern cordillera with respect to tertiary rocks in the broad internally drained basin of the altiplano to the west and east verging folds and faults in upper paleozoic rocks of the interandean zone to the east the interandean zone was in turn raised both structurally and topographically with respect to the frontal folds and faults of the fold thrust belt the subandean zone by a second structurally lower basement thrust sheet thus these two megathrusts divide the andean fold thrust belt into four areas of markedly different structural elevations the eastern cordillera can be further subdivided into two zones of west and east vergent folds and thrusts shortening accommodated by the fold thrust belt can be divided among these tectono structural zones and linked to shortening accommodated by the inferred basement megathrusts the proposed kinematic model suggests that the eastward propagation of the structurally highest basement thrust fed 105 km of slip into the eastern cordillera along east vergent and west vergent faults this structure also fed 90 km of eastward slip into the interandean zone the initiation and eastward propagation of a lower basement thrust structurally elevated the interandean zone with respect to the foreland while feeding 65 km of slip into the subandean zone out of sequence basement thrusting to the west is proposed to have elevated the western edge of the plateau and accommodated 40 km of shortening within the altiplano total cumulative shortening within the cover rocks of the andean fold thrust belt 300 330 km can be balanced by an equivalent amount of shortening along two basement megathrusts to the first order the eastern margin of the central andean plateau defined by the 3 km topographic contour is contiguous with the leading edge of the upper basement megathrust this relationship between the basement highs and the physiographic boundaries of the andean plateau suggests that extensive megathrust sheets involving strong rocks such as crystalline basement or quartzite play an important role in the formation of the central andean plateau and a similar link between megathrust sheets and plateaus may be found in other orogens

new constraints on the chronology magnitude and distribution of deformation within the central andean orocline 1 constructing an accurate kinematic model for crustal thickening in the central andes is hampered by the inability of 2 d balanced cross sections to resolve the 3 d displacement field within the poorly studied orocline core this study presents new structural and thermochronometer data from the orocline axis in the central andes that constrains the magnitude and timing of deformation through 1 a balanced cross section quantifying shortening perpendicular to strike 2 field observations that constrain translational displacements parallel to strike and 3 thermochronology that brackets the age of exhumation and deformation mapping results show that north directed right lateral motion has been accommodated on at least one major strike slip fault in the eastern cordillera ec shortening perpendicular to fault strike is 179km 41 for the ec and inter andean zone ia and 86km 39 for the sub andes sa apatite fission track cooling ages indicate rapid ec ia exhumation from 46 to 18ma and 15 to 0ma in the sa zircon and apatite u th he ages show that rapid exhumation began 10 5ma in the sa these results are integrated with existing paleomagnetic and gps rotation data in map view reconstructions to quantify the magnitude of translational faulting related to oroclinal bending the reconstructions suggest that at least 85km of north directed translational displacement has been accommodated within the orocline core orogen parallel displacement and vertical axis rotations have focused crustal thickening at the orocline core and facilitated development of a high elevation plateau suggesting a genetic link between large scale oroclines and plateaus

comment on the beginnings of hydrous mantle wedge melting cb till tl grove ac withers contributions to mineralogy and petrology doi 10 1007 s00410 011 0692 6 the paper by till et al 2011 reports new experiments on high pressure melting in the system lherzolite water in comparison with earlier published work several appropriate methods of phase characterisation and experimental techniques have not been reported a result is that the authors conclusions are not substantiated for two important issues the position in pressure temperature p t space of the hydrous vapour saturated solidus and the compositions of near solidus melts the application of the experimental results to discussion of melting in the mantle wedge overlying subduction zones is therefore questionable

the araa uaa west congo orogen in brazil an overview of a confined orogen formed during gondwanaland assembly abstract the neoproterozoic adamastor brazilide ocean was generated during the breakup of the rodinia supercontinent and remnants of its oceanic lithosphere have been found in the brasiliano pan african orogenic system that includes the aracuai west congo brasilia ribeira kaoko dom feliciano damara and gariep belts the aracuai and the west congo belts are counterparts of the same neoproterozoic orogen the first belt comprises two thirds of the aracuai west congo orogen this orogen is rather unique owing to its confined nature within the embayment outlined by the sao francisco and congo cratons in spite of this the presence of ophiolitic remnants and a calc alkaline magmatic arc indicate that the basin orogen evolution comprise both oceanic spreading and consumption it is assumed that coeval paramirim and sangha aulacogens played a key role by making room for the aracuai west congo basin sedimentary successions record all major stages of a basin that evolved from continental rift when glaciation related sedimentation was very significant to passive margin rifting started around 1 0 0 9 ga the oceanic stage is constrained by an ophiolitic remnant dated at 0 8 ga if the cratonic bridge that once linked the sao francisco and congo palaeocontinental regions did not hinder the opening of an ocean basin it certainly limited its width as a consequence only a narrow oceanic lithosphere was generated and it was subducted afterwards this is also suggested by orogenic calc alkaline granitoids occuping a small area of the orogen geochronological data for pre syn and late collisional granitoids indicate that the orogenic stage lasted from 625 ma to 570 ma a period of magmatic quiescence was followed by intrusion of postcollisional plutons at 535 500 ma the features of the aracuai west congo orogen suggest the development of a complete wilson cycle in a branch of the adamastor ocean which can be interpreted as a gulf with limited generation of oceanic lithosphere

3d density model of the central andes abstract we developed a 3d density model of the continental crust the subducted plate and the upper mantle of the central andes between 20 29 s and 74 61 w through the forward modelling of bouguer anomaly the goal of this contribution is to gain insight on the lithospheric structure integrating the available information geophysical geologic petrologic and geochemical in a single model the geometry of our model is defined and constrained by hypocentre location reflection and refraction on and offshore seismic lines travel time and attenuation tomography receiver function analysis magnetotelluric studies thermal models and balanced structural cross sections the densities allocated to the different bodies are calculated considering petrologic and geochemical data and pressure and temperature conditions the model consists of 31 parallel e w vertical planes where the continental crust comprises distinct bodies which represent the different morphotectonic units of the central andes we include a partial melting zone at midcrustal depths under the altiplano puna low velocity zone and consider the presence of a rheologically strong block beneath the salar de atacama basin according to recent seismic studies contour maps of the depth of the continental moho the thickness of the lower crust and the depth to the bottom of the lithosphere below south america are produced the possible percentage of partial melt in the central andes low velocity zone is estimated the residual anomaly is calculated by subtracting from the bouguer anomaly the gravimetric effect of the modelled subducted slab and of the modelled moho isostatic anomalies are calculated from regional and local isostatic mohos calculated with and without internal loads derived from our gravity model which are then compared to the modelled continental moho this study contributes to a more detailed knowledge of the lithospheric structure of this region of the andes and provides an integrated 3d density model which may be used by the geoscientific community as a tool that can help to understand and interpret the geodynamic features and processes acting along the central andes

the cuchilla dionisio terrane uruguay an allochthonous block accreted in the cambrian to sw gondwana abstract the cuchilla dionisio terrane cdt of uruguay is a tectonostratigraphic unit defined as the block to the east of the sierra ballena shear zone sbsz it is composed of a palaeo to mesoproterozoic metamorphic basement cerro olivo and chafalote complexes intensely reworked and intruded by syncollisional to late orogenic granites between 680 and 550 ma dacitic and rhyolitic volcanism is recorded around 570 575 ma cerros de aguirre and sierra de rios formations the cdt is correlated with the pelotas terrane of southern brazil the western boundary of the block in brazil is defined by the cangucu and major gercino shear zones which are the northward extension of the sbsz the present position of the cdt is not a consequence of orthogonal collision after completion of a wilson cycle but of lateral sinistral accretion along the above mentioned megashear zones the allochthonous nature of the cdt is postulated on the grounds that a magmatism in the cdt is ca 120 ma younger than metamorphism of neighbouring volcanosedimentary successions e g porongos group in brazil and b while intense volcanism occurred in the cdt in the late vendian represented by the cerros aguirre and sierra de rios formations a passive continental margin deepening to the e existed on the eastern edge of the rio de la plata craton represented by the arroyo del soldado group therefore the cdt and rio del la plata craton were separated hundreds or thousands of kilometers by late vendian times accretion of the cdt took place at 530 ma by tangential collision and was one of the last events in the complicated amalgamation of w gondwana in uruguay the cdt collided with the nico perez terrane generating the sbsz and sinistrally reactivating the sarandi del yi piriapolis shear zone in brazil the cdt pelotas terrane collided with the parana block and a series of parautochthonous terranes curitiba apiai characterized by a transamazonian basement strongly reworked in the neoproterozoic the provenance of the cdt is still uncertain considering that both transamazonian and namaqua natal kibaran ages have been reported from its basement previously reported nd model ages tdm strongly suggest an african affinity the dom feliciano belt is thus a collage of diachronous units formed in different geotectonic settings and not the product of a single orogenic cycle terrane docking at different stages mainly by lateral accretion was the dominant process therefore usage of the name dom feliciano belt should be restricted only to descriptive purposes

west gondwana amalgamation based on detrital zircon ages from neoproterozoic ribeira and dom feliciano belts of south america and comparison with coeval sequences from sw africa

deepest exposed crust of brazil geochemistry of paleoproterozoic depleted santa maria chico granulites abstract the deepest crustal processes of brazil are investigated in the santa maria chico granulitic complex of rio grande do sul state depth of ca 30 km is inferred from garnet bearing mafic granulite geobarometry for the deformation of the bimodal basic acid sequence of late archean age this low k tholeiitic magmatism occurred in an island arc environment which accreted to the atlantica supercontinent in a collisional episode at ca 2 1 ga during the transamazonian cycle although this magmatism is strongly depleted in lithophile elements its geochemistry is still magmatic and shows that the metamorphism did not disturb the original distribution of the elements in the host rocks significantly the associated trondhjemites could have been the differentiated portions of the basic magmas but more likely were the result of the partial melting of the garnet bearing mafic residue this suite of high grade rocks is one of several protoliths for the generation of the abundant k granites of the 600 ma brasiliano cycle from southern brazil this high grade rock association is rather comparable to the lewisian complex of scotland

seismic imaging of a convergent continental margin and plateau in the central andes andean continental research project 1996 ancorp 96 1 a 400 km long seismic reflection profile andean continental research project 1996 ancorp 96 and integrated geophysical experiments wide angle seismology passive seismology gravity and magnetotelluric depth sounding across the central andes 21 s observed subduction of the nazca plate under the south american continent an east dipping reflector nazca reflector is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km we interpret parts of the nazca reflector as a fluid trap located at the front of recent hydration and shearing of the mantle the fluids being supplied by dehydration of the oceanic plate patches of bright quebrada blanca bright spot to more diffuse reflectivity underlie the plateau domain at 15 30 km depth this reflectivity is associated with a low velocity zone p to s wave conversions the upper limits of high conductivity and high vp vs ratios and to the occurrence of neogene volcanic rocks at surface we interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during neogene shortening and plateau growth the imaging properties of the continental moho beneath the andes indicate a broad transitional character of the crust mantle boundary owing to active processes like hydration of mantle rocks in the cooler parts of the plate margin system magmatic underplating and intraplating under and into the lowermost crust mechanical instability at moho etc hence all first order features appear to be related to fluid assisted processes in a subduction setting

late cenozoic uplift of the eastern cordillera bolivian andes abstract this paper analyses late cenozoic uplift in the bolivian andes using the morphology of well preserved regional paleosurfaces in the eastern cordillera that define three axially draining braided river catchments that formed between 12 and 9 ma rock uplift since the formation of the paleodrainage systems which has been quantified using four different methods is 1705 695 m with a mean erosion of 230 90 m as a consequence of entrenchment of the drainage systems the lack of faulting or tilting in the regions immediately farther west strongly suggest that rock uplift of the paleodrainage systems also extends to the western margin of the eastern cordillera balanced structural cross sections require crustal deformation at depth beneath the eastern cordillera in order to accommodate underthrusting of the brazilian shield beneath the thin skinned fold and thrust belt on the eastern margin of the bolivian andes in this case the observed uplift of the eastern cordillera is easily explained by sliding up a ramp in the major decollement dipping in the range 4 16 w as a consequence of the observed 60 110 km of shortening farther east in the subandean zone contemporaneous uplift of the western margin of the eastern cordillera is easily explained in terms of crustal thickening as a result of ductile squeezing in the lower crust accommodating at depth the subandean shortening it remains unclear how this uplift relates to that of the regions farther west in the altiplano and volcanic arc except that uplift in the eastern cordillera coincides with a phase of intense shortening in the northern altiplano commencing at 9 5 ma and continuing to 2 7 ma and possibly younger

kinematic variations across eastern cordillera at 24 s central andes tectonic and magmatic implications abstract the eastern cordillera central andes 24 s consists of a basement involved thrust system resulting from miocene quaternary eastward migrating compression separating the puna plateau from the santa barbara system foreland the inferred tertiary strains arising from shortening in the eastern cordillera and santa barbara system are similar higher than in the puna slip data collected on the major n s trending faults of eastern cordillera show a westward progression from dip slip contraction to dextral and sinistral motions this consistently with established tectonic models may result from partitioning due to the oblique mio quaternary underthrusting of the brazilian shield north of 24 s this strain partitioning has three main implications 1 as the dextral and sinistral shear in the eastern cordillera are 62 and 29 of the compressive strain respectively the eastern cordillera results more strained than santa barbara system foreland contrary to previous estimates 2 the partitioning in the eastern cordillera may find its counterpart in that to the west of the central andes giving a possible structural symmetry to the central andes 3 the easternmost n s strike slip structures in the eastern cordillera coincide with the easternmost mio pliocene magmatic centres in the central andes at 24 s provided that further to the east the crust is partially molten the absence of magmatic centres may be explained by the presence of pure compressive structures in this portion of the eastern cordillera

multiple accretion at the eastern margin of the rio de la plata craton the prolonged brasiliano orogeny in southernmost brazil the neoproterozoic eoplalaeozoic brasiliano orogeny at the eastern margin of the rio de la plata craton in southernmost brazil and uruguay comprises a complex tectonic history over 300 million years the southern brazilian shield consists of a number of tectono stratigraphic units and terranes the sao gabriel block in the west is characterized by c 760 690 ma supracrustal rocks and calc alkaline orthogneisses including relics of older c 880 ma old igneous rocks both igneous and metasedimentary rocks have positive end t values and neoproterozoic tdm model ages they formed in magmatic arc settings with only minor input of older crustal sources a trondhjemite from the sao gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation d3 yield an u pb zircon age la icp ms of 701 10 ma giving the approximate minimum age of the sao gabriel accretionary event the encantadas block further east containing the supracrustal porongos belt and the pelotas batholith is in contrast characterized by reworking of neoarchean to palaeoproterozoic crust the 789 7 ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the porongos belt provides a time marker for the basin formation zircons of a sample from tonalitic gneisses constituting the palaeoproterozoic basement of the porongos belt form a cluster at 2 234 28 ma interpreted as the tonalite crystallization age zircon rims show ages of 2 100 2 000 ma interpreted as related to a palaeoproterozoic metamorphic event the porongos basin formed on thinned continental crust in an extensional or transtensional regime between c 800 700 ma the absence of input from neoproterozoic juvenile sources into the porongos basin strongly indicates that the encantadas and sao gabriel blocks were separated terranes that became juxtaposed next to each other during the brasiliano accretional events the tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the rio de la plata craton i accretion of an intra oceanic arc at c 880 ma passinho event and ii accretion of the 760 700 ma cambai vila nova magmatic arc sao gabriel event the latter event also includes the collision of the encantadas block with the rio de la plata craton to the west collision and crustal thickening was followed by sinistral shear along sw ne trending orogen parallel crustal scale shear zones that can be traced from southern brazil to uruguay and have been active between 660 and 590 ma voluminous granitic magmatism in the pelotas batholith spatially related to shear zones is interpreted as late to post orogenic magmatism possibly assisted by lithospheric delamination it marks the transition to the post orogenic molasse stage localized deformation by reactivation of preexisting shear zones continued until c 530 ma and can be assigned to final stages of the amalgamation of west gondwana

strain accumulation controls failure of a plate boundary zone linking deformation of the central andes and lithosphere mechanics 1 we make use of observations on orogenic strain accumulation and deformation partitioning in the central andes to explore the backarc strength evolution at the lithospheric scale in plan view the altiplano puna plateaux experienced rapid initial increase of surface area undergoing active deformation during the cenozoic beyond the maximum lateral extent reached around 10 15 ma 40 50 of entire proto andes undergoing deformation at 10 20 total strain rapid localization initiated at the eastern flank of the altiplano inter and subandean thrust belt but not at the puna latitude localization was associated with a significant increase in bulk shortening rate average fault slip rates equally increased by an order of magnitude following a protracted period of stable average rates estimates of strength evolution based on force balance calculations and critical wedge analysis suggest significant backarc weakening driving this change after the middle miocene strain accumulation led to localization and weakening with development of a detachment propagating through crust and upper mantle we find that lithosphere scale failure resulting from strain weakening beyond a critical strain threshold c 20 and fault coalescence with formation of a weak detachment in shales effective coefficient of friction 0 1 plays a key role in the evolution of the andes strain related lithosphere weakening appears to dominate over the impact of external forcing mechanisms such as variations of plate convergence mantle assisted processes or erosion comparison of these orogen scale observations with experimental rock rheology indicates substantial similarity of deformation behavior with similar weakening thresholds across a wide range of scales

tearing up rodinia the neoproterozoic palaeogeography of south american cratonic fragments rodinia was initially defined as a long lived supercontinent that assembled all the continental fragments around laurentia and remained stable from 1000 up to 750 ma nonetheless recent work has cast doubt on the rodinia palaeogeography and even on the timing of its assembly and break up the geochronological and palaeomagnetic databases accumulated for south america and africa in the last decade show that most of these continental fragments were not part of rodinia a wide brasiliano ocean separated most of the south american and african cratons from the laurentia amazonia rio apa west africa margin this ocean was closed between 940 and 630 ma along the pampean paraguay araguaia pharusian mobile belts moreover accretion along the south american and african platforms was a diachronous and long lived process that involved several intra oceanic and continental magmatic arcs and microcontinents this evolution started at around 1000 ma and ended at around 520 ma with the final assembly of gondwana

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

origin of the high plateau in the central andes bolivia south america the bolivian altiplano in the central andes of south america is part of the second largest high plateau on earth it is an elongate region of subdued relief 1 2 105 km2 and 4 km above sea level bounded by the eastern cordillera and volcanic arc western cordillera here the crust is up to 75 km thick we describe the cenozoic geological evolution of this region using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure crustal shortening and magmatic addition and locally sedimentation are the main mechanisms of cenozoic crustal thickening leading to nearly 4 km of surface uplift since the paleocene addition of mafic melts appears to be a first order mechanism of cenozoic crustal growth contributing 40 of the crustal thickening beneath the volcanic arc removal of the basal part of the lithosphere may have caused two episodes of widespread arc and behind arc mafic volcanism at 23 ma and 0 5 ma contributing to the surface uplift the altiplano originated as a sedimentary basin several hundred kilometers wide between the proto western cordillera and a narrow zone of uplift proto eastern cordillera farther east the latter zone formed by inversion of the center of a wide lacustrine or marine cretaceous paleocene basin close to sea level at 45 ma a thickness of 2 4 km of paleogene continental elastics accumulated in the proto altiplano basin subsequently in the oligocene we estimate that this region and the western margin of the eastern cordillera were technically shortened 22 65 km resulting in 9 km of average crustal thickening the altiplano basin was rejuvenated at 25 ma and subsequently flooded with up to 8 km thickness of detritus eroded from the uplifting eastern and western cordilleras between 25 and 5 ma folding and thrusting in the western margin of the eastern cordillera migrated westward into the center of the altiplano basin essentially terminating deposition except in local subbasins and accommodating 13 30 km of shortening and an estimated 7 km of average crustal thickening subsequently there has been strike slip deformation and limited local thrusting 5 km of shortening geomorphological and geochronological evidence for 1 5 2 km of surface uplift of this region since the late miocene suggests 14 km of lower crustal thickening beneath an essentially rigid lid and can be explained by 100 150 km of underthrusting of the brazilian shield and adjacent regions beneath the eastern margin of the central andes the present subdued relief in the altiplano may be a result of ductile flow in the lower crust and sedimentation and erosion in an internal drainage basin

sm nd isotope geochemistry of metamorphic volcano sedimentary successions in the sao gabriel block southernmost brazil evidence for the existence of juvenile neoproterozoic oceanic crust to the east of the rio de la plata craton abstract juvenile neoproterozoic dioritic tonalitic trondhjemitic and granodioritic gneisses in the sao gabriel block southern brazil have been identified by geochronologic studies age proposals for associated ultra mafic metavolcanic and metasedimentary rocks however range from archean to neoproterozoic whole rock sm nd analyses presented here support a neoproterozoic age for these rocks tdm model ages of the ultra mafic metavolcanic rocks range between 0 65 and 1 35 ga with nd t positive values between 3 16 and 6 87 tdm model ages of metasedimentary and metavolcanoclastic rocks vary between 0 77 and 1 19 ga with nd t values between 1 2 and 6 23 tonalitic calc alkaline gneisses show nd t values of 4 34 and 6 3 and tdm model ages of 0 89 and 0 72 ga respectively a late kinematic granite santa zelia granite display slightly negative nd t values 1 6 and a higher tdm model age of about 1 4 ga these data support the existence of meso neoproterozoic juvenile oceanic crust and island arc rocks during the brasiliano orogenic events the main source rocks of the metasedimentary units are previously formed juvenile rocks the data also indicate minor assimilation of older crustal material and or contamination of the melts by radiogenic nd released from older rocks on the subducting slab existence of widespread old sialic crust in the subduction zone environment however can be ruled out indicating important orogenic accretion between 0 9 and 0 7 ga a geotectonic model for the sao gabriel block and the eastern margin of the rio de la plata craton comprises eastward subduction and following accretion of an intra oceanic island arc between 0 9 and 0 8 ga and a subsequent westward subduction with formation of an active continental margin at the eastern margin of the rio de la plata craton between 0 8 and 0 7 ga we postulate that the juvenile rocks of sao gabriel block represent relics of a neoproterozoic ocean between the rio de la plata craton and a continental block encantadas block possibly derived from the kalahari craton subduction and arc accretion began roughly coeval with the initial stages of the break up of rodinia 0 9 ga and indicate a peripheric rio de la plata craton in relation to the rodinia supercontinent with evolution from a passive margin to an active margin in the beginning of the neoproterozoic brasiliano orogenic events

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

eocene to recent variations in erosion across the central andean fold thrust belt northern bolivia implications for plateau evolution abstract quantifying the erosional and kinematic evolution of orogenic plateaus has been limited by insufficient age constraints on their deformation and erosion histories palinspastic restorations suggest the central andean fold thrust belt and plateau evolved concurrently in bolivia we present an analysis that synthesizes 19 new and 32 previous apatite and zircon fission track and u th he mineral cooling ages along a 200 km traverse across the plateau margin and entire thrust belt in northern bolivia the new apatite fission track data are interpreted using a grain age deconvolution algorithm with inverse thermal modeling of track lengths grain ages and mineral composition proxy data results suggest 1 eo oligocene 40 25 ma initial rapid erosion of the plateau margin 2 accelerated distributed erosion across the entire thrust belt since the early to mid miocene 15 ma and 3 the magnitude of erosion decreases eastward from 10 to 4 km we compare these results with two end member models of the central andes that contrast in duration and magnitude of deformation the rapid eo oligocene 40 25 ma erosion is only consistent with the end member that emphasizes the long duration and large magnitude deformation controlled by the sequential stacking of basement thrust sheets however the distributed miocene 15 ma to recent erosion is consistent with both end members because the recorded cooling could have resulted from active deformation protracted erosion or both if the long duration model is correct the time between the two phases of accelerated cooling brackets the cessation of the first basement thrust sheet and implies the early development of the andean plateau analogous to its modern width but unknown elevation by the early miocene 20 ma

basement involved blind thrusting in the eastern cordillera oriental southern bolivia evidence from cross sectional balancing gravimetric and magnetotelluric data abstract a neogene to recent thrust belt forms the subandean ranges and cordillera oriental along the eastern slope of the central andes in bolivia over much of its length there is an area of intermediate topographic height and structural level between the subandean zone and the cordillera oriental proper cross sectional balancing suggests that in southern bolivia the basal detachment of this tightly folded zone links up with a blind thrust that dips westward beneath the high standing eastern margin of the cordillera oriental and rides piggy back on a large buried thrust sheet which forms the backstop for the thin skinned subandean belt magnetotelluric and gravimetric data indicate that this thrust sheet is made up of basement rocks probably including intrusives and higher grade metamorphics such blind basement thrusts in a relatively external position may characterize much of the eastern andean thrust belt and their geometry is possibly related to preexisting structural patterns linked to the configuration of early palaeozoic basins and their margins

a balanced model of the southern central andes to explain the actual crustal thickness of the southern central andes a balanced cross section at 21 s latitude was modeled a forward modeling procedure was used on a crustal scale to explain the gross crustal structure obtained by seismic refraction investigations in the model 320 km of shortening since late cretaceous based on balanced geological cross sections are assumed for the whole andes from the trench to the foreland and the initial crustal thickness varies between 30 and 40 km it results that about 20 of the actual crustal volume must be explained by other sources than crustal shortening structures of high velocity material in the upper crust of the eastern cordillera can be explained by detached and overthrust material from the crustal base thus producing a crustal doubling in the back arc underplating of continental material that was removed by subduction erosion and magmatic addition of derivates of the subducted nazca plate may contribute to the lower crust of the forearc and arc regions crustal thickening of the eastern cordillera and parts of the altiplano was undoubtedly produced by tectonic thickening whereas the sources of thickening in the western cordillera and forearc region are uncertain

isolating active orogenic wedge deformation in the southern subandes of bolivia a new gps derived surface velocity field for the central andean backarc permits an assessment of orogenic wedge deformation across the southern subandes of bolivia where recent studies suggest that great earthquakes mw 8 are possible we find that the backarc is not isolated from the main plate boundary seismic cycle rather signals from subduction zone earthquakes contaminate the velocity field at distances greater than 800 km from the chile trench two new wedge crossing velocity profiles corrected for seasonal and earthquake affects reveal distinct regions that reflect 1 locking of the main plate boundary across the high andes 2 the location of and loading rate at the back of orogenic wedge and 3 an east flank velocity gradient indicative of decollement locking beneath the subandes modeling of the subandean portions of the profiles indicates along strike variations in the decollement locked width wl and wedge loading rate the northern wedge decollement has a wl of 100 km while accumulating slip at a rate of 14 mm yr whereas the southern wedge has a wl of 61 km and a slip rate of 7 mm yr when compared to quaternary estimates of geologic shortening and evidence for holocene internal wedge deformation the new gps derived wedge loading rates may indicate that the southern wedge is experiencing a phase of thickening via reactivation of preexisting internal structures in contrast we suspect that the northern wedge is undergoing an accretion or widening phase primarily via slip on relatively young thrust front faults

composition and thickness of the southern altiplano crust bolivia slant stacking of broadband seismograms recorded in the western united states for two 1993 intermediate depth earthquakes that occurred near the bolvia argentina chile borders reveals small but clear precursors to both the compressional p wave and shear s wave depth phases we interpret and model these precursors as underside reflections from the thickened andean altiplano crust we model the crustal structure of the altiplano with a grid search technique to match the timing and amplitudes of the depth phases and precursors in the waveforms of both the p wave and s wave our best fit model has an average crust velocity of 5 9 6 0 km s a crustal v sub p v sub s of 1 6 a crustal thickness of 75 80 km and a high velocity v sub p 8 4 km s high v sub p v sub s 1 9 mantle wedge assuming isotropy the low v sub p v sub s ratio of 1 6 for the crust corresponds to an anomalously low poisson s ratio of 0 18 such a low value in conjunction with the low average v sub p estimate is consistent only with a felsic composition and high upper crustal temperatures the finding of a thick felsic crust overlying a high velocity mantle supports models of altiplano uplift due predominantly to crustal shortening as opposed to mafic magmatic additionmore and is inconsistent with recent mantle delamination 19 refs 3 figs 1 tab less

anomalous crust of the bolivian altiplano central andes constraints from broadband regional seismic waveforms a one year deployment of broadband seismographs in the bolivian altiplano recorded numerous intermediate depth earthquakes at near regional distances we modeled the associated broadband waveforms of two earthquakes to estimate an average crustal structure for the altiplano the resulting model is characterized by an anomalously low mean p velocity of 6 0 km s a low poisson s ratio of 0 25 and a crustal thickness of 65 km the combination of the low mean velocity and low poisson s ratio can be explained only by a predominantly quartz rich felsic bulk composition this constraint precludes significant volumes of magmatic addition from the mantle contributing to the great thickness of the altiplano crust but is consistent with thickening by compressive shortening concentrated in a weak felsic layer

transition from basement involved to thin skinned thrusting in the cordillera oriental of southern bolivia the eastern flank of the central andes in southern bolivia and northern argentina is underlain by a fold and thrust belt of neogene age it comprises an external thin skinned segment with basal detachment in silurian shales subandean ranges and an internal segment where units as deep as precambrian metasedimentary and igneous rocks are involved in thrusting eastern cordillera oriental between the cordillera oriental and the subandean ranges an intermediate segment interandean zone intervenes north of the argentinian bolivian border 22 s the structure of the interandean zone is characterized by folded and thrust faulted cambrian to triassic strata balanced cross sections across the western subandean ranges the interandean zone and the eastern cordillera oriental are based on previously available data and recent field work they suggest that the geological structure can be interpreted in terms of two major basement involved thrusts here referred to as interandean and subandean thrusts the more internal interandean thrust caused the development of broad basement anticlines in the eastern cordillera oriental however at shallower levels its displacement was accommodated by thin skinned folding and thrusting in the interandean zone in sequence activation of the subandean thrust uplifted the interandean zone on top of an imbricate basement sheet and caused the development of the thinskinned subandean ranges farther east published radiometric data and subsidence history of the subandean foreland basin suggest that the interandean thrust was active between about 10 ma and 5 ma the subandean thrust is still active southward displacement transfer from the subandean thrust onto the interandean thrust and a southward dipping lateral ramp of the interandean thrust result in the gradual transition from the interandean zone north of 22 s to the cordillera oriental of northern argentina a similar but larger scale transfer may be involved in the southern termination of the thin skinned subandean ranges of argentina at 23 s shortening in the western part of the thrust belt is of the order of 80 90 km total shortening in the eastern andean thrust belt from the undeformed foreland to the eastern margin of the cordillera oriental is 140 150 km

the footwall ramp of the subandean decollement northernmost argentina from extended correlation of seismic reflection data abstract the link between thin skinned foreland shortening and thickening of the lower crust beneath the hinterlands of mountain belts has seldom been observed we have reprocessed yacimientos petroliferos fiscales ypf s a seismic reflection data from the eastern cordillera of the central andes at 22 s latitude using vibroseis extended correlation the result we tentatively interpret as an image of the crustal scale footwall ramp of the subandean belt decollement three crossing lines extended from 5 to 15 s 45 km provide excellent 3d control a band of strong mid crustal reflections between 4 5 and 6 s 9 to 13 km displays imbricate thrust geometry it may be a shallow zone of decollement but is 5 10 km too shallow for the subandean belt decollement at 7 8 s 16 to 20 km a band of reflections dips steeply nw at about 35 migrated and strikes parallel to the andean structure underlying the camargo syncline the dipping band correlates closely with the westward projection of the subandean decollement east of the dipping event scattered horizontal reflections are present to 12 s 32 km reflections above the dipping event have geometries similar to hanging wall anticlines and may truncate against it west of the band reflections occur to 15 s 41 km none of these lines have sufficient penetration to image moho the footwall ramp of the subandean belt apparently extends to near the base of the crust and would mark the western limit of under thrusting of undeformed craton beneath the orogen based on lateral variation in surface geology existing balanced cross sections and the geometry imaged on the reprocessed seismic lines we suggest that the subandean belt decollement steepens its ramp from north to south as shortening dies out southward at 24 s foreland structures are locally compensated and have seismicity to 25 km

pure and simple shear plateau uplift altiplano puna argentina and bolivia abstract the concepts of pure and simple shear strain provide a meaningful way of discussing the alternative modes of lithospheric shortening which produced uplift of the central andean plateau since 10 ma discrete segments of the plateau exemplify pure or simple shear modes characteristics of the simple shear mode are a flanking thin skinned thrust belt and subdued relief and lack of significant late miocene to recent deformation within the plateau the pure shear mode is characterized by more rugged internal relief and active neotectonic deformation within the plateau and thick skinned foreland deformation involving seismic deformation of the crust to 30 km or more these modes of crustal thickening correlate with changes in lithosphere thickness modes of isostatic compensation broad wavelength topography and back arc magmatism the correlation of the boundary between pure and simple shear segments with major changes in pre existing anisotropies in the foreland of the orogen suggests that events which long pre date the andean orogeny indirectly controlled the along strike variation in the magnitude and style of shortening in the central andes and may have influenced the development of flat subduction

coupled tectonic evolution of andean orogeny and global climate abstract the largest tectonic relief breaking the earth s surface 13 km vertically is at the subduction margin of the andes which generates routinely megathrust earthquakes mw 8 5 and drives the paradigmatic andean orogen here we present key geologic evidence to reassess first order features of geomorphology and tectonics across the central andes where the orogen includes the altiplano plateau and attains its maximum integrated height and width the andean subduction margin has a stepped morphology dominated by the low relief atacama bench which is similar to a giant uplifted terrace slopes gently over a width of 60 100 km from the andes to the pacific and runs over more than 1000 km of coastal length we find that the genesis of stepped morphology at the andean seaboard is due to concomitant development of large west vergent thrusts parallel to the subduction interface and increasing aridity in the atacama desert which keeps an unprecedented large scale record of interplaying tectonics and cenozoic climate change incorporating our results with published geological knowledge demonstrates that andean orogeny is characterized by trench perpendicular bivergent and trench parallel bilateral growth over the past 50 myr associated with positive trench velocity toward the continent trench advance and subduction of a wide slab under south america we hypothesize that a global plate tectonic reorganization involving long lasting viscous mantle flow has probably forced both andean orogeny and global climate cooling since 50 ma in contrast two important stepwise pulses of increasing aridity and trench perpendicular andean growth appear to be results of changes in erosion rates due to global late eocene and middle miocene cooling events

neoproterozoic igneous complex emplaced along major tectonic boundary in the kaoko belt nw namibia ion probe and la icp ms dating of magmatic and metamorphic zircons granitoid intrusions of the boundary igneous complex separate segments with different ages of high grade metamorphism in the kaoko belt nw namibia two granitoids of this complex were dated at 575 10 ma secondary ionization mass spectrometry sims or 571 9 ma laser ablation inductively coupled plasma mass spectrometry la icp ms and 562 11 ma sims or 572 4 ma la icp ms respectively the age of granulite facies metamorphism in the eastern part of the western kaoko zone was established at 549 5 ma sims by analysing metamorphic overgrowths of older c 1850 1000 ma zircons from melt segregations in amphibolites the coastal part of the western kaoko zone consists of horizons of migmatitic metasedimentary rocks that are intercalated with fine grained orthogneisses and amphibolites resembling metamorphosed sequences of bimodal volcanic rocks zircons from felsic members of two bimodal suites have sims ages of 805 4 ma and 810 840 ma respectively that are interpreted as dating their respective igneous protoliths melt segregations in the mafic member of the lower bimodal suite contain two populations of zircon dated at 650 5 ma sims or 645 5 ma la icp ms and 629 6 ma sims or 630 5 ma la icp ms respectively the later age is indistinguishable from the age of 630 4 ma sims or 625 10 ma la icp ms obtained from melt patches present in overlying metagreywackes the available age data suggest that the boundary igneous complex masks the suture between the coastal terrane and the rest of the kaoko belt ages of granitoid intrusions in this igneous complex are indicative of magmatic activity between 580 and 550 ma

distribution of active faulting along orogenic wedges minimum work models and natural analogue abstract numerical 2 d models based on the principle of minimum work were used to examine the space time distribution of active faulting during the evolution of orogenic wedges a series of models focused on thin skinned thrusting illustrates the effects of arid conditions no erosion unsteady state conditions accretionary influx greater than erosional efflux and steady state conditions accretionary influx balances erosional efflux on the distribution of fault activity for arid settings a general forward accretion sequence prevails although a significant amount of internal deformation is registered the resulting fault pattern is a rather uniform spread along the profile under fixed erosional efficiency settings the frontal advance of the wedge front is inhibited reaching a steady state after a given forward propagation then the applied shortening is consumed by surface ruptures over a narrow frontal zone under a temporal increase in erosional efficiency i e transient non steady state mass balance conditions a narrowing of the synthetic wedge results a rather diffuse fault activity distribution is observed during the deformation front retreat once steady balanced conditions are reached a single long lived deformation front prevails fault activity distribution produced during the deformation front retreat of the latter scenario compares well with the structural evolution and hinterlandward deformation migration identified in southern bolivian subandes ssa from late miocene to present this analogy supports the notion that the ssa is not in steady state but is rather responding to an erosional efficiency increase since late miocene the results shed light on the impact of different mass balance conditions on the vastly different kinematics found in mountain ranges suggesting that those affected by growing erosion under a transient unbalanced mass flux condition tend to distribute deformation along both frontal and internal faults while others under balanced conditions would display focused deformation on a limited number of steady structures

crustal wedging triggering recent deformation in the andean thrust front between 31 s and 33 s sierras pampeanas precordillera interaction 1 we document a new model of crustal structure of the andean front in argentina where numerous historic earthquakes destroyed the cities of mendoza in 1861 ms 7 and san juan in 1944 mw 7 0 the cerro salinas anticline is formed above the west directed cerro salinas thrust an east facing monocline with an amplitude of about 2 km folds the cerro salinas thrust and overlying neogene succession this monocline is formed above a blind crustal thrust in the basement its dip of 14 west is inferred from fold geometry this thick skinned east directed blind thrust and the thin skinned west directed cerro salinas thrust define a tectonic wedge the wedge tip occurs at a depth of 5 4 km growth of the monocline after 8 5 ma is revealed on multichannel seismic msc profile 31017 repsol ypf rates of cerro salinas thrust displacement are of the order of 1 mm yr whereas vertical uplift of 0 45 mm yr results from the combined displacement on the cerro salinas thrust and growth of east facing monocline the lateral extent of the east directed crustal blind ramp corresponds with the along strike extent of the eastern precordillera when combined with the low displacement rate a long earthquake recurrence interval is implied smaller magnitude earthquakes however indicate that segments of the blind thrust ramps ruptured in historic events if all the segments of the blind thrust ruptured together the fault area is 7000 km 2 and could produce a mw 7 7 earthquake the crustal wedge model provides new constraints on the origin and potential size of earthquakes that threaten the densely populated region

geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing example from the foreland thrust system of the southern bolivian subandean zone the subandean zone of bolivia is a foreland fold and thrust belt which forms the eastern edge of the central andes mountains between 19 s and 22 s latitude the construction of five balanced cross sections shows that the n s trending subandean zone is characterized by the existence of passive roof duplexes these complex structures can be distinguished by the lithotectonic unit within which duplexing occurs the five balanced cross sections permit the geometric and kinematic analyses of these passive roof duplexes the sequential restorations of certain cross sections reveal a possible development of a piggy back sequence of three passive roof duplexes apparently these passive roof duplexes propagated toward the foreland from deeper and deeper lithotectonic units while a passive roof duplex was developing the sole thrust stuck and the major horizontal displacements were then transferred either to out of sequence thrusts or to a new sole thrust in a deeper detachment horizon therefore each passive roof duplex would correspond to the orogenic front of the andean range at one very particular time in the history of the subandean zone of southern bolivia from south to north the quantitative analysis by cross section balancing shows a transfer of displacement from the hinterland structures to the passive roof duplex that forms the present orogenic front available data do not permit us to explain completely this phenomenom

seismicity and tectonics in jujuy province northwestern argentina the portable array for numerical digital analysis panda network a digitally recorded seismic array operated for nine months in jujuy province of northwestern argentina the network was deployed along the eastern edge of the altiplano puna plateau in a major n s thrust belt that is transitional in style between the thin skinned deformation of the bolivian foreland to the north and basement involved deformation of the pampean region to the south teleseismic locations of crustal earthquakes in the region indicate that seismicity is associated with compressional structures found near the eastern deformation front no crustal seismicity was detected beneath the puna plateau to the west peak seismicity levels beneath the foreland occurred between 20 and 25 km depth a sharp decrease in seismicity was observed below 25 km an estimate of 42 km for the thickness of the jujuy foreland crust was inferred from wide angle moho reflections observed on the digital seismograms the highest concentration of crustal seismicity was located beneath sierra de zapla a broad anticline immediately east of san salvador de jujuy many of the earthquakes in the 20 25 km depth range have a shallow west dipping nodal plane as does the focal mechanism solution for a moderately large 1973 earthquake inversion of focal mechanism data for the orientation of principal stresses shows that maximum compression is oriented at azimuth 74 closely paralleling both the current nazca south america convergence direction and the shortening direction derived from regional quaternary fault slip data we interpret the earthquakes as occurring on planes of weakness first produced during cretaceous rifting and later reactivated by neogene compressive stresses crustal seismicity patterns and fault plane solutions suggest the presence of a midcrustal detachment along which significant late cenozoic e w shortening has occurred

neogene shortening contribution to crustal thickening in the back arc of the central andes to illustrate the neogene shortening distribution in the back arc units of the central andes and to estimate the contribution of the shortening to crustal thickening two balanced crustal cross sections have been constructed across the northern and southern branches of the bolivian orocline total neogene shortening which varies from 191 to 231 km is accommodated by a crustal duplex below the cordillera oriental but is insufficient to produce the 70 km of crustal thickness evidenced by geophysical data below the altiplano the best explanation for this anomalous thickening seems to be crustal underplating by material tectonically eroded from the continental margin this process probably caused the altiplano uplift the subduction of oceanic lithosphere coupled with this underplating and a brief episode of gravity spreading of the altiplano constituted the driving forces that produced neogene shortening and development of the central andes

granites and the geodynamic history of the neoproterozoic brasilia belt central brazil a review abstract recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the neoproterozoic brasilia belt in central brazil this is an orogenic belt developed in response to the convergence between the amazon sao francisco congo and parana continental blocks the presence of neoproterozoic juvenile arc rocks and syn collisional peraluminous granites challenged previous intracontinental evolution models for the belt the granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt the main field and isotopic characteristics of four granite suites associated with the brasilia belt are reviewed i 1 77 1 58 ga old rift related a type granite intrusions ii ca 0 8 0 7 syn collisional granitoids iii arc metatonalites and metagranodiorites ca 0 9 to 0 63 ga and iv bimodal post orogenic suite ranging in age from ca 0 59 to 0 48 ga these rocks suggest that during most of the neoproterozoic the western margin of the sao francisco continent faced a large oceanic basin where subduction and oceanic lithosphere consumption started at ca 0 9 ga roughly coeval with the initial stages of the break up of rodinia final ocean closure happened at ca 0 63 0 60 ga with crustal thickening uplift and erosion post orogenic extension related magmatism took place between ca 0 6 and 0 5 ga and was partially contemporaneous with the deposition of the paraguay and tucavaca sedimentary successions resulting from the rifting event related to the break up of laurentia from southwestern gondwana

a connection between the neoproterozoic dom feliciano brazil uruguay and gariep namibia south africa orogenic belts evidence from a reconnaissance provenance study abstract a provenance study of neoproterozoic siliciclastic successions in the stratigraphically and tectonically lowermost and uppermost parts of the pan african gariep belt stinkfontein subgroup and oranjemund group respectively in southwestern africa as well as in the rocha group of the punta del este terrane dom feliciano belt in uruguay revealed that the oranjemund and rocha groups can be correlated and most likely formed in the same basin thus the rocha group is considered to represent the fill of the westernmost part of a re activated ediacaran gariep basin the lower parts of the oranjemund and rocha groups reflect erosion of mafic rocks whereas the upper parts are derived from a predominantly felsic source area first major and trace element geochemical data are consistent with oceanic islands of within plate geochemistry in the immediate vicinity as the most likely source of the mafic input into the lower part of the oranjemund group whereas a combination of continental margin and island arc is indicated as source for the bulk of the younger sediments in the upper oranjemund group age spectra obtained by shrimp u pb analyses of detrital zircon grains from the stinkfontein subgroup port nolloth group the oranjemund group and the rocha group are very similar except for a lack of the youngest age group around 600 ma in the stinkfontein subgroup in all three units zircon grains of 1000 1200 ma dominate with a further peak in the age distribution between 1700 and 2000 ma these ages compare well with the pre gariep basement geology in southwestern africa where the former age range corresponds to magmatic and high grade metamorphic activity in the mesoproterozoic namaqua natal belt and the latter to an extensive palaeoproterozoic andean type volcanic arc richtersveld terrane comparable ages are conspicuously absent in the basement of the rio de la plata craton in south america derivation of the rocha group sediments from a similar source as the contemporaneous oranjemund group sediments is therefore suggested the most likely source of the youngest detrital zircon grains in these two stratigraphic units is the 640 590 ma magmatic arc of the dom feliciano belt this interpretation explains the continental island arc component in the sediment provenance and takes into account the absence of comparable magmatism in the gariep belt two stages of basin evolution are distinguished for the gariep basin an early failed continental rift that subsequently became re activated as back arc basin related to the cryogenian dom feliciano arc consequently the gariep basin does not represent the main suture between the rio de la plata and the kalahari cratons this suture is now suspected to be west of the dom feliciano volcanic arc and might be connected with the sierra ballena major gercino lineament