thrust belts of the southern central andes along strike variations in shortening topography crustal geometry and denudation the andean fold and thrust belts of west central argentina 33 s and 36 s above the normal subduction segment present important along strike variations in mean topographic uplift structural elevation amount and rate of shortening and crustal root geometry to analyze the controlling factors of these latitudinal changes we compare these parameters and the chronology of deformation along 11 balanced crustal cross sections across the thrust belts between 70 w and 69 w where the majority of the upper crustal deformation is concentrated and reconstruct the moho geometry along the transects we propose two models of crustal deformation a 33 40 s model where the locus of upper crustal shortening is aligned with respect to the maximum crustal thickness and a 35 40 s model where the upper crustal shortening is uncoupled from the lower crustal deformation and thickening this degree of coupling between brittle upper crust and ductile lower crust deformation has strong influence on mean topographic elevation in the northern sector of the study area an initial thick and felsic crust favors the coupling model while in the southern sector a thin and mafic lower crust allows the uncoupling model our results indicate that interplate dynamics may control the overall pattern of tectonic shortening however local variations in mean topographic elevation deformation styles and crustal root geometry are not fully explained and are more likely to be due to upper plate lithospheric strength variations

kinematics of the active west andean fold and thrust belt central chile structure and long term shortening rate west verging thrusts synthetic with the nazca south america subduction interface have been recently discovered at the western front of the andes at 33 30 s the active san ramon fault stands as the most frontal of these west verging structures and represents a major earthquake threat for santiago capital city of chile here we elaborate a detailed 3d structural map and a precise cross section of the west andean fold and thrust belt based on field observations satellite imagery and previous structural data together with digital topography we then reconstruct the evolution of this frontal belt using a trishear kinematic approach our reconstruction implies westward propagation of deformation with a total shortening of 9 15 km accumulated over the last 25 myr an overall long term shortening rate of 0 1 0 5 mm yr is deduced the maximum value of this shortening rate compares well with the rate that may be inferred from recent trench data across the san ramon fault and the slip associated with the two past mw 7 earthquakes this suggests that the san ramon fault is most probably the only presently active fault of the west andean fold and thrust belt and that most if not all the deformation is to be released seismically

petrogenesis and assembly of the don manuel igneous complex miocene pliocene porphyry copper belt central chile the 4 0 3 6 ma don manuel igneous complex dmic central chile provides a window into igneous processes involved in magma genesis associated with porphyry style copper mineralization this study uses petrographic petrological geochemical and isotopic data to examine the evolution of magmas from the mid to lower crustal source region to shallow emplacement the data provide evidence for progressive oxidation of magma during differentiation and ascent fractionation of cl from s through degassing and the late stage near solidus removal of cl from the system magmas of basaltic andesite to rhyolite composition were produced by polybaric differentiation of hydrous parental mafic magmas variations in crustal differentiation depths led to variable suppression of plagioclase saturation that is recorded in distinctive strontium versus anorthite evolution patterns hydrous derivative magmas generated over a wide range of pressures were episodically emplaced into the shallow crust at depths between 3 5 and 5 km intermediate porphyry dikes closely associated with copper mineralization contain diverse crystal cargoes indicating significant magma mixing these crystal cargoes represent samples of crystal mush entrained from different depths as well as crystals originating in different magmas and crystals grown in situ from hybridized magmas mafic enclaves containing plagioclase and amphibole compositions that match those of the basaltic andesites occur within biotite tonalite testifying to magma mingling during ascent sulfur and chlorine contents of apatite within the different dmic units record variable degassing and decoupling of volatile components with sulfur showing variations of three orders of magnitude compared with one order of magnitude for chlorine the hypabyssal nature of the dmic affords a detailed integrated record of magmatic differentiation processes occurring within trans crustal magmatic systems of the sort thought to characterize many crustal arc settings and play a fundamental role in driving porphyry style copper mineralization

the west andean thrust the san ramon fault and the seismic hazard for santiago chile 1 the importance of west verging structures at the western flank of the andes parallel to the subduction zone appears currently minimized this hampers our understanding of the andes altiplano one of the most significant mountain belts on earth we analyze a key tectonic section of the andes at latitude 33 5 s where the belt is in an early stage of its evolution with the aim of resolving the primary architecture of the orogen we focus on the active fault propagation fold system in the andean cover behind the san ramon fault which is critical for the seismic hazard in the city of santiago and crucial to decipher the structure of the west andean thrust wat the san ramon fault is a thrust ramp at the front of a basal detachment with average slip rate of 0 4 mm yr young scarps at various scales imply plausible seismic events up to mw 7 4 the wat steps down eastward from the san ramon fault crossing 12 km of andean cover to root beneath the frontal cordillera basement anticline a range 5 km high and 700 km long we propose a first order tectonic model of the andes involving an embryonic intracontinental subduction consistent with geological and geophysical observations the stage of primary westward vergence with dominance of the wat at 33 5 s is evolving into a doubly vergent configuration a growth model for the wat altiplano similar to the himalaya tibet is deduced we suggest that the intracontinental subduction at the wat is a mechanical substitute of a collision zone rendering the andean orogeny paradigm obsolete

role of basin width variation in tectonic inversion insight from analogue modelling and implications for the tectonic inversion of the abanico basin 32 34 s central andes

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

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

neogene paleoelevation of intermontane basins in a narrow compressional mountain range southern central andes of argentina the topographic growth of mountain ranges at convergent margins results from the complex interaction between the motion of lithospheric plates crustal shortening rock uplift and exhumation constraints on the timing and magnitude of elevation change gleaned from isotopic archives preserved in sedimentary sequences provide insight into how these processes interact over different timescales to create topography and potentially decipher the impact of topography on atmospheric circulation and superposed exhumation this study uses stable isotope data from pedogenic carbonates collected from seven different stratigraphic sections spanning different tectonic and topographic positions in the range today to examine the middle to late miocene history of elevation change in the central andes thrust belt which is located immediately to the south of the altiplano puna plateau the world s second largest orogenic plateau paleoelevations are calculated using previously published local isotope elevation gradients observed in modern rainfall and carbonate formation temperatures determined from clumped isotope studies in modern soils calculated neogene basin paleoelevations are between 1 km and 1 9 km for basins that today are located between 1500 and 3400 m elevation considering the modern elevation and 18o values of precipitation at the sampling sites three of the intermontane basins experienced surface uplift between the end of deposition during the late miocene and present the timing of elevation change cannot be linked to any documented episodes of large magnitude crustal shortening paradoxically the maximum inferred surface uplift in the core of the range is greatest where the crust is thinnest the spatial pattern of surface uplift is best explained by eastward migration of a crustal root via ductile deformation in the lower crust and is not related to flat slab subduction

southeast pacific tectonic evolution from early oligocene to present plate tectonic reconstructions of the nazca antarctic and pacific plates are presented from late oligocene to present these reconstructions document major plate boundary reorganizations in the southeast pacific at dirons 6c 24 ma 6 o 20 ma and 5a 12 ma and a smaller reorganization at chron 3 o 5 ma during the chron 6 o reorganization it appears that a ridge propagated into crust north of the northernmost pacific antarctic ridge between the chiloe fracture zone fz of the chile ridge and agassiz fz of the pacific nazca ridge which resulted in a northward jump of the pacific antarctic nazca pac ant naz mid ocean triple junction during the chron 5a reorganization the chile ridge propagated northward from the valdivia fz system to the challenger fz through lithosphere formed roughly 5 myr earlier at the pacific nazca ridge during this reorganization a short lived microplate the friday microplate existed at the pac ant naz triple junction the pac ant naz triple junction jumped northward 500 km as a result of this reorganization from a location along the valdivia fz to a location along the challenger fz the chron 5a reorganization also included a change in spreading direction of the chile and pacific antarctic ridges the reorganization at chron 3 o initiated the formation of the juan fernandez and easter microplates along the east pacific rise the manner of plate boundary reorganization at chron 6 o and chron 5a and possibly today at the juan fernandez microplate included a sequence of rift propagation transfer of lithosphere from one plate to another microplate formation and microplate abandonment and resulted in northward migration of the pac ant naz triple junction the associated microplate differs from previously studied microplates in that there is no failed ridge

cenozoic deformation and tectonic style of the andes between 33 and 34 south latitude 1 the andes of argentina and chile between latitudes 33 and 34 s are composed from west to east of an oligocene to miocene volcanic arcs and the neogene east vergent aconcagua fold and thrust belt of the cordillera principal and the basement block faulted cordillera frontal a regional cross section suggests that shortening across the andes was achieved by thrusting along detachments at several levels in the crust while thin skinned deformation along newly formed thrusts occurred in mesozoic sequences of the eastern cordillera principal reactivation of preexisting jurassic and oligocene normal faults has resulted in additional hybrid thick and thin skinned structures in the western cordillera principal five major thrusting events are recognized in this part of the andes 1 early to middle miocene tectonic inversion of the extensional faults in the western cordillera principal 2 middle to late miocene development of the aconcagua fold and thrust belt 3 late miocene uplift of cordillera frontal 4 late miocene early pliocene out of sequence thrusting in the cordillera principal and 5 pliocene to present deformation of the foreland

revisiting the crustal structure and kinematics of the central andes at 33 5 s implications for the mechanics of andean mountain building the andean belt is the only present day active case example of a subduction type orogeny however an existing controversy opposes classical views of andean growth as an east verging retro wedge against a recently proposed bi vergent model involving a primary west vergent crustal scale thrust synthetic to the subduction we examine these diverging views by quantitatively re evaluating the orogen structural geometry and kinematics at the latitude of 33 5 s we first provide a 3d geological map and build an updated section of the east vergent aconcagua fold and thrust belt aconcagua ftb which appears as a critical structural unit in this controversy we combine these data with geological constraints on nearby structures to derive a complete and larger scale section of the principal cordillera within the forearc region we restore our section and integrate published chronological constraints to build an evolutionary model showing the evolving shortening of this forearc part of the andes the proposed kinematics implies uplift of the frontal cordillera basement since 20 25 ma supported by westward thrusting over a crustal ramp that transfers shortening further west across the principal cordillera the aconcagua ftb is evidenced as a secondary east verging roof thrust atop the large scale basement antiform culmination of the frontal cordillera we estimate a shortening of 27 42 km across the principal cordillera of which only 30 is absorbed by the aconcagua ftb finally we combine these findings with published geological data on the structure of the eastern back arc andean mountain front and build a crustal scale cross section of the entire andes at 33 5 s we estimate a total orogenic shortening of 31 55km mainly absorbed by crustal west vergent structures synthetic to the subduction our results provide quantitative key geological inferences to revisit this subduction type orogeny and compare it to collisional alpine type orogenic belts

reply to the comment by r a astini and f m davila on the west andean thrust the san ramon fault and the seismic hazard for santiago chile 1 we have proposed earlier a new tectonic model for the evolution of the andes mountain belt as a bivergent orogen here to reply to a comment by astini and davila 2010 we discuss briefly the protracted diachronic evolution over tens of million years by propagating deformation at the large scale over 10 10 km its influence on basin formation in the back arc region retroarc foreland basin and the mechanical implications of the bivergence in the tectonics of the fore arc region particularly the possible effects of the underthrusting of the coastal crustal scale rigid block the marginal block beneath the west andean thrust 2 the comment by astini and davila 2010 criticizes our new model presented by armijo et al 2010 suggesting that theandes is a fundamentally bivergent or doubly vergent orogen and defends the conventional model of andean orogeny which we think obsolete involving crustal shortening only by development of retroarc thrusts in the back arc region e g as discussed among many others by isacks 1988 our model is based on the structural study of the san ramon fault system and the principal cordillera at the front of the western flank of the andes which is used to characterize the crustal scale west andean thrust wat a major fold thrust system in the fore arc region synthetic to the subduction zone astini and davila raise cursorily a large number of important questions which cannot be fully addressed in this reply 3 to summarize the main argument 1 astini and davila think that according to the critical taper wedge model e g davis et al 1983 the dominant thrusting of the andes cannot have shifted from an initial westward vergence to an eastward vergence as we propose 2 they claim that the well known eastward cratonward migration of thrusting associated with foreland basin formation in argentina cannot be explained if the initial dominant thrusting of the andes was in the fore arc with westward directed vergence and 3 astini and davila argue that no continental block in the fore arc region as the marginal or coastal block that we have defined can be considered as a western foreland of the andes because there is no well developed western foreland basin the miscellaneous final remarks by astini and davila express their doubts that our model may fit currently accepted models of mass transfer sediment flow across orogens and tectonic climatic forcing because according to our model a major topographic slope would have been created to the west of the andes a feature that as anyone can check is not hypothetical but a matter of fact last but not least the andes at 33 5 s latitude would not be in an early stage of its evolution as we claim because sediments in the argentinean foreland record its development since more than 20 ma our reply intended to identify first order conflicting issues is as follows

first cosmogenic ages for glacial deposits from the plata range 33 s new inferences for quaternary landscape evolution in the central andes this paper presents the first numerical cosmogenic ages of glacial deposits from the eastern slope of the plata peak 6000 in the arid central andes 32 s the moraines are distributed along three different valleys vallecitos angostura and las mulas in the first only one moraine was identified and no blocks could be sampled for dating the other two have evidence of two glacial advances angostura i and angostura ii the advances loma de los morteritos i and ii were recognised in the las mulas valley in both cases first and second advances are preserved at a similar altitude surface boulders on the top of moraines were sampled for be10 obtained ages are all similar and late pleistocene in age they fit very well with local stratigraphy and regional paleoclimate evidence the loma de los morteritos moraine ii located at 3000 masl dated to 25 000 years ages of 8 000 and 12 000 years were found for the angostura ii deposit 3300 masl an age was obtained for the outwash deposit identified along the el salto valley 69 000 years associated with a late pleistocene drift quantitative datings presented here improve knowledge about the glacial chronology of the plata range and quaternary stratigraphy of the central andes even though they represent a maximum age for these glacial advances new findings support the occurrence of two late pleistocene glacial advances and an older one in the blanco river valley adjusted ages around 8 000 and 25 000 years for younger stage could be evidence of the last glacial maximum lgm period for this region

geochemistry of fluid discharges from peteroa volcano argentina chile in 2010 2015 insights into compositional changes related to the fluid source region s abstract this study presents the first geochemical data of fluid discharges collected from february 2010 to march 2015 from the planchon peteroa azufre volcanic complex ppavc located in the transitional southern volcanic zone tsvz at the border between argentina and chile during the study period from january 2010 to july 2011 peteroa volcano experienced phreatic to phreatomagmatic eruption possibly related to the devastating maule earthquake occurred on february 27 2010 the compositional dataset includes low temperature from 43 2 to 102 c gas discharges from i the summit of peteroa volcano and ii the se flank of azufre volcano both marked by a significant magmatic fluid contribution as well as bubbling gases located at the foothill of the peteroa volcanic edifice which showed a chemical signature typical of hydrothermal fluids in 2012 strong compositional changes affected the peteroa gases from the summit area the acidic gas species especially so 2 increased suggesting an input of fluids from magma degassing nevertheless the r ra and 13 c co 2 values decreased which would imply an enhanced contribution from a meteoric hydrothermal source in 2014 2015 the chemical and isotopic compositions of the 2010 2011 gases were partially restored the anomalous decoupling between the chemical and the isotopic parameters was tentatively interpreted as produced by degassing activity from a small batch of dacitic magma that in 2012 masked the compositional signature of the magmatic fluids released from a basaltic magma that dominated the gas chemistry in 2010 2011 this explanation reliably justifies the observed geochemical data although the mechanisms leading to the change in time of the dominating magmatic fluid source are not clear at this regard a geophysical survey able to provide information on the location of the two magma batches could be useful to clarify the possible relationships between the compositional changes that affected the peteroa fluid discharges and the 2010 2011 eruptive activity

evolution of shallow and deep structures along the maipo tunuyan transect 33 40 s from the pacific coast to the andean foreland fil giambiagi laura beatriz consejo nacional de investigaciones cientificas y tecnicas cientifico tecnologico mendoza instituto argentino de nivologia glaciologia y ciencias ambientales argentina

tectonic interaction between mesozoic to cenozoic extensional and contractional structures in the preandean depression 23 25 s geologic implications for the central andes abstract a multidisciplinary study supported by field data and 2 d seismic information was conducted to understand the tectonic interactions between mesozoic to cenozoic extensional and contractional structures along the preandean depression 23 25 s in northern chile which have been largely debated this work gives some new ideas about the control that ancient basement extensional faults exert on the crustal shortening and growth of those orogenic belts located over normal subduction zones similar to the central andes in this work two sedimentary basins were analyzed the salar de atacama and the salar de punta negra both of which correspond to intra montane basins bounded by the domeyko cordillera and the current volcanic arc of the central andes the interpretation of a series of west to east oriented 2 d seismic profiles revealed three types of structural interaction i pure positive reactivation ii decapitation of previous normal faults by reverse faults and iii transport of previous normal faults by thrust ramps inversion anticlines and basement involved contractional structures were the most common observed structures indicating that the geometry and distribution of inherited normal faults of the paleozoic and mesozoic basement played an important role in basin contraction the different modes of tectonic interaction frequently created variations on the geometry and kinematic of the final structural styles present in the region commonly the upper cretaceous to cenozoic synorogenic tectonosequences recorded the ages of contractional deformation that affected the region similar to other regions of northern chile basin inversion occurred during the late cretaceous and subsequent propagation of reverse faults occurred from the upper late cretaceous to the present the basement involved reverse fault structures usually create the most important positive topographic relief in this part of the western central andes using as example our study area we finally conclude that the original position and distribution of ancient basement pre orogenic normal faults in the orogenic belts condition the final vergence and kinematic of the subsequent thrust systems related to shortening episodes

response of the low latitude d region ionosphere to extreme space weather event of 14 16 december 2006 the response of the d region low latitude ionosphere has been examined for extreme space weather event of 14 16 december 2006 associated with a x1 5 solar flare and an intense geomagnetic storm dst 146 nt using vlf signals from northwest cape australia nwc 19 8 khz and lualualei hawaii callsign npm 21 4 khz transmitters monitored at suva geographic coordinates 18 10 s 178 40 e fiji modeling of flare associated amplitude and phase enhancements of nwc 3 6 db 223 and npm 5 db 153 using long wave propagation capability code shows reduction in the d region reflection height h by 11 1 km and 9 4 km and enhancement in ionization gradients described by increases in the exponential sharpness factor by 0 122 and 0 126 km 1 for the nwc and npm paths respectively during the storm the daytime signal strengths of the nwc and npm signals were reduced by 3 2 db on 15 and 16 december for about 46 h and recovered by 17 december modeling for the nwc path shows that storm time values of h and were reduced by 1 2 km and 0 06 km 1 respectively morlet wavelet analysis of signal amplitudes shows no clearly strong signatures of gravity wave propagation to low latitudes during the main and recovery phases the reduction in vlf signal strength is due to increased signal attenuation and absorption by the earth ionosphere waveguide due to storm induced d region ionization changes and hence changes in d region parameters the long duration of the storm effect results from the slow diffusion of changed composition ionization at d region altitudes compared with higher altitudes in the ionosphere

cenozoic plate kinematics subduction and magmatism south american andes revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the andean subduction zone these include age of subducted plate convergence rate and interaction with hotspot traces from 50 to 25 ma in the andes the age of oceanic plate subducted progressively decreased apparently resulting in a9gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity a major reorganization of plate motions at 25 ma resulted in an increased convergence rate normal to the chilean andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of bolivia and argentina convergence rates changed only slightly across the peruvian margin and no clear effect on subsequent magmatism is indicated interaction of the andean subduction zone with the juan fernandez and easter nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks the current gaps in igneous activity correspond with contemporary subduction of the two traces and this association apparently occurred in the past as well

miocene deformation in the orogenic front of the malargue fold and thrust belt 35 30 36 s controls on the migration of magmatic and hydrocarbon fluids abstract the integration of surface observations and sub surface data wellbore and seismic from the orogenic front of the malargue fold and thrust belt allows us to study its kinematics and to interpret the local stress field and its control over fluid magmatic and hydrocarbon migration reverse faults correspond to inverted nnw striking mesozoic normal faults and n s striking cenozoic low angle thrusts parallel to the orogen oblique structures with strike slip movement are also present the magmatic activity in the study area was strongly controlled by this structural framework and the in situ stress field miocene dykes and sills were emplaced in relation to strike slip and reverse faults respectively we propose an evolution of the study region from a foredeep sector in the early middle miocene to a peak in deformation in the late miocene and finally a waning of deformation from the pliocene to the present our structural model suggests that during the evolution of the thrust front the in situ stress field changed from a compressional to strike slip compressional stress field favouring the synchronous emplacement of sills and dykes this alternation of stress regimes favours hydrocarbon migration through both thrusts and subvertical strike slip faults this exchange between both stress regimes is likely related to the similar values of the minimum 3 and intermediate 2 principal stress with an e w oriented maximum principal stress 1 according to the plate convergence vector

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

late oligocene early miocene compressional tectosedimentary episode and associated land mammal faunas in the andes of central chile and adjacent argentina 32 37 s abstract a reassessment of the geologic and land mammal fossil evidence used in attribution of a tectosedimentary episode in the andes between 32 and 37 s to the middle eocene incaic tectonic phase of peru indicates that the episode occurred during late oligocene early miocene times 27 20 ma from west to east three structural domains are recognized for this time span in the study area a volcanic arc chile a thin skinned e verging fold thrust belt cordillera principal chile argentina border strip and a foreland basin argentina initiation of thrusting in the cordillera principal fold thrust belt produced the coeval initiation of sedimentation in the foreland basin of adjacent argentina this onset of foreland deposition postdates strata bearing a divisaderan land mammal age fauna i e 35 30 ma and is marked at 36 30 s by the base of the rodados lustrosos conglomerates which are conformably overlain by sedimentary rocks containing a deseadan land mammal age fauna i e 29 21 ma geologic relationships between the thick volcanic abanico coya machali and farellones formations also demonstrate that this tectosedimentary episode practically ended at 20 ma at least in the volcanic arc and was therefore roughly coeval with the major tectonic crisis 27 19 ma known in northwestern andean bolivia some 1500 km to the north this strongly suggests that a long outstanding tectonic upheaval affected at least an extended 12 37 s segment of the andean margin of south america during late oligocene and early miocene times

matrix deformation in a basement involved fold and thrust belt a case study in the central andes malargue argentina abstract an analysis of the anisotropy of magnetic susceptibility ams is used to investigate the matrix strain record of weakly deformed clastic rocks including sandstones and shales along a cross section of the malargue fold and thrust belt mftb argentina we collected 339 oriented cores from the tres esquinas sandstones the red beds of the tordillo and neuquen formations as well as from shales silts of the mendoza group overall 90 of the magnetic fabrics are well defined and are related to the strain imprint of the layer parallel shortening among the tectonic related fabrics 40 show the development of a magnetic lineation at right angle of the shortening type ii 38 display an apparent loss of magnetic foliation type iii and 21 develops a magnetic foliation at right angle of the shortening type iv there is no lithological control on magnetic fabric typology despite contrasting ferromagnetic mineralogy between the red sandstones and dark shales thin section observations highlight the good match between the orientation of both the susceptibility ellipsoid and microstructure orientation which enables us to infer the preferred orientations of the petrofabric i e finite strain directions in this way two trends can be recognized in the study area shortening directions trending n110 in the north and trending n80 in the south in addition the spatial distribution of magnetic fabric typology and burial data suggests an atypical pattern of deformation from the foreland to hinterland such a pattern contrasts with previous studies in fold thrust belt settings suggesting that at the matrix scale the deformation even in a pre folding setting is compartmentalized and controlled by the occurrence of deep seated thrusts localized along inherited structures from the analysis of cross sections we can distinguish several domains each of them reflecting distinct states of matrix deformation

detrital thermochronology reveals major middle miocene exhumation of the eastern flank ofthe andes that predates the pampeanflat slab 33 33 5 s fil lossada ana clara consejo nacional de investigaciones cientificas y tecnicas oficina de coordinacion administrativa ciudad universitaria instituto de estudios andinos don pablo groeber universidad de buenos aires facultad de ciencias exactas y naturales instituto de estudios andinos don pablo groeber argentina universidad de buenos aires facultad de ciencias exactas y naturales departamento de geologia argentina

the lifetime of shoreface nourishments in fields with nearshore sandbar migration abstract migration of nearshore sandbars results from nonlinear interacting hydro and morphodynamic processes in the surf zone of wave dominated sandy shorelines to gain new insights in the largely varying temporal effects of shoreface nourishments within these areas the principle component analysis is applied to sixteen data sets with i natural sandbar migration on different timescales and ii interfering shoreface nourishments the statistical methodology is able to separate the non stationary effects of shoreface nourishments from the natural stationary migration of the sandbars depending on other long term morphodynamic changes and nourishment interactions in the data the duration of these interferences the nourishment lifetimes can be quantified relating these nourishment lifetimes l n with respect to the bar cycle return periods t r of these areas e g the relative nourishment lifetime of twenty one shoreface nourishments with several design parameters and parameters of the sandbar system in which they were placed remained inconclusive a negative exponential fit with the bar cycle return period itself was the most significant r 2 0 41 assuming a linear system of the parameters increases the predictive capability r 2 0 67 the most influential parameters in this relation are the nourishment concentration the nourishment depth the concentration of the migrating sandbars and the bar cycle return period thereby indicating the prominent roles of both nourishment design and natural sandbar system in the inter site variability of shoreface nourishment lifetimes

large rockslides in the southern central andes of chile 32 34 5 s tectonic control and significance for quaternary landscape evolution abstract the distribution and age of large 0 1 km2 pliocene to recent rockslides in the chilean cordillera principal 32 34 5 s the southern central andes has been analyzed to determine the rockslide triggering mechanisms and impact on regional landscape evolution most of the rockslides appear in the western cordillera principal and cluster along major geological structures variographic analyses show spatial correlation between rockslides geological structures and shallow seismicity a relative chronosequence was calibrated with existing 14c and 40ar 39ar dates and new cosmogenic nuclide exposure ages for selected rockslides rockslide induced sediment yield was estimated with empirical relations for rockslide area distributions throughout the quaternary rockslides have delivered sediment to streams at rates equivalent to denudation rates of 0 10 0 06 mm a 1 while estimates using short term 20 a seismicity records are 0 3 0 2 0 6 mm a 1 the estimates of sediment transfer and the spatial distribution of rockslides reflect a landscape in which tectonic and geological controls on denudation are more significant than climate

the relationships between tectonics climate and exhumation in the central andes 18 36 s evidence from low temperature thermochronology abstract the central andes between 18 s and 36 s latitude strike north south for 2000 km along the chilean subduction margin cross several climate zones from hyperarid to humid and exhibit mean elevations in excess of 4000 m a s l here we investigate the relationships between tectonics climate and exhumation by inverting low temperature thermochronological data compiled from the literature 824 ages from 549 samples and new data 238 ages from 146 samples to quantify the exhumation rate history of the central andes since 80 ma our inferred exhumation rates west of the drainage divide and between 18 and 32 s did not exceed 0 25 km ma such low exhumation rates are consistent with low shortening rates and arid conditions in this region local pulses of exhumation occurred only during the eocene as response to active deformation and during the miocene probably as response to uplift of the western andean slope east of the drainage divide between 18 and 28 s the observed exhumation pattern reflects the onset and eastward propagation of deformation here exhumation occurred locally since the middle to late eocene in the eastern cordillera and the altiplano puna and subsequently affected larger parts of these regions and the north western sierra pampeanas during the oligocene in the early miocene 20 ma the interandean zone started exhuming and at 12 10 ma exhumation propagated into the subandean zone enhanced shortening rates and intensified precipitation along the eastern deformation front associated with the onset of the south american monsoon led to increased exhumation rates in the eastern interandean and the subandean zones in the plio pleistocene 0 6 km ma higher pleistocene exhumation rates are also observed in the northern sierra pampeanas 1 5 km ma that can be related to rock uplift along steep reverse faults coupled with high precipitation south of 32 s on the western side exhumation rates in the principal cordillera increased from ca 0 25 km ma in the miocene to rates locally exceeding 2 km ma in the pleistocene whereas the tectonic regime in the southern principal cordillera remained unchanged since the late miocene these higher rates are likely associated with enhanced erosion resulting from intensified pleistocene precipitation and glacial growth in this region reinforced by isostatic rock uplift and active tectonics our study shows that the onset of exhumation correlates mainly with the initiation of horizontal shortening and crustal thickening whereas the magnitude of exhumation is largely set by the amount of precipitation and glacial erosion and by the style of deformation which is controlled by inherited structures and the amount of sediments in the foreland

arc oblique fault systems their role in the cenozoic structural evolution and metallogenesis of the andes of central chile abstract the evolution of the main cordillera of central chile is characterized by the formation and subsequent inversion of an intra arc volcano tectonic basin the world s largest porphyry cu mo deposits were emplaced during basin inversion statistically the area is dominated by ne and nw striking faults oblique to the n striking inverted basin margin faults and to the axis of cenozoic magmatism this structural pattern is interpreted to reflect the architecture of the pre andean basement stratigraphic correlations syn extensional deposits and kinematic criteria on fault surfaces show several arc oblique structures were active as normal faults at different stages of basin evolution the geometry of syn tectonic hydrothermal mineral fibers in turn demonstrates that most of these structures were reactivated as strike slip reverse faults during the middle miocene early pliocene fault reactivation age is constrained by 40 ar 39 ar dating of hydrothermal minerals deposited during fault slip the abundance and distribution of these minerals indicates fault controlled hydrothermal fluid flow was widespread during basin inversion fault reactivation occurred under a transpressive regime with e to ene directed shortening and was concentrated around major plutons and hydrothermal centers at the margins of the former intra arc basin deformation was largely accommodated by reverse faulting whereas in its central part strike slip faulting was predominant

neogene erosion of the andean cordillera in the flat slab segment as indicated by petrography and whole rock geochemistry from the manantiales foreland basin 32 32 30 s abstract the miocene manantiales foreland basin is defined as a thick succession of sedimentary rocks belonging to the chinches formation in the frontal cordillera between 32 s and 32 30 s located over the flat slab segment of the andean cordillera it has been interpreted as a foreland basin that mainly received contributions from several mountain ranges such as the principal cordillera and frontal cordillera to the west in this paper we present a detailed study of its provenance based on the petrography and whole rock geochemistry of sedimentary rocks from the chinches formation our results indicate that the manantiales basin was fed by igneous provinces in the hinterland that contain sub alkaline intermediate rocks andesitic and acidic rocks dacitic to rhyodacitic and granitic arranged in a clear stratigraphic progression these signatures may be assigned to cenozoic rocks farellones formation in the principal cordillera and permo triassic choiyoi group in the frontal cordillera in contrast to a previous result we found a continuous geochemical trend in the chinches formation between ca 19 and 10 ma opposing the idea of a tectonic repetition in the sedimentary succession

surface uplift and erosion of the southernmost argentine precordillera article i nfo article history the timing of uplift of the precordillera is important for understanding the linkages if any between slab dynam ics shortening and topography the study region between 32 and 33 s latitude liesat the southern end of the flat slab where the subducting nazca plate is nearly horizontal south of the study region subduction occurs at normal subduction angles of around 30 while north of the study region the slab subducts at around 5 we use the geomorphology of the region to date the initiation of surface uplift and the ensuing landscape adjustment the topography of the precordillera of the argentine andes consists of both remnants of a low relief miocene landscape developed when the region was at a lower elevation and rapidly eroding fluvial systems that have been dissecting this surface since uplift this study utilizes 26 al and 10 be concentrations in stream sediment quartz to calculate erosion rates of the miocene remnant paleo landscape as well as incision rates within the ac tively incising post uplift fluvial system the remnant landscape is eroding at b10 m my while the surrounding landscape is eroding an order of magnitude faster approaching 100 m my these values show the transition from a rapidly eroding system adjusting to uplift to a system where rates of uplift approach those of erosion we used a locally determined relationship between average upstream slope and erosion rate to model erosion across the landscape dem analyses of modern river profiles are used to reconstruct paleo river profiles which suggest an average of 1 3 km of uplift uplift of the southernmost argentine precordillera is constrained to have initiated by 10 ma demonstrating that under the arid conditions typical of the southernmost precordil lera millions of years are needed for a landscape to reach equilibrium

inversion of an oligocene volcano tectonic basin and uplifting of its superimposed miocene magmatic arc in the chilean central andes first seismic and gravity evidences abstract a new structural model is presented for the cenozoic part of the main range east of santiago a chain made up of thick miocene ignimbrites and andesitic lavas overlying primitive oligocene volcanoclastics deposited on a thin continental crust because the latter coya machali formation locally show tight folding close to the margins of the miocene volcanics they had previously been described as deformed prior to construction of the arc our field observations analysed along two profiles at 33 and 34 15 s however support the conclusion hinted at by ongoing geochronological and geochemical studies deformation of both formations is synchronous and related to late miocene inversion of the coya machali intra arc basin uplift of the rigid arc rocks and part of its volcanoclastic basement took place mainly along two northward shallowing boundary structures that seem to share a lower decollement the eastern fierro out of sequence thrust and a series of western back thrusts that resemble triangle zones seismic data obtained from el teniente porphyry copper mine 34 s rockbursts and two gravimetric profiles at the western end of the southern profile are compatible with a decollement located at that locality 2 3 km under the surface

widening of the andes an interplay between subduction dynamics and crustal wedge tectonics shortening of the continental lithosphere is generally accommodated by the growth of crustal wedges building above megathrusts in the mantle lithosphere we show that the locus of shortening in the western margin of south america has largely been controlled by the geometry of the slab numerical models confirm that horizontal subduction favors compression far from the trench above the asthenospheric wedge and steeply dipping segment of the subducting slab as a result a second crustal wedge grows in the hinterland of the continent and widens the andes in the bolivian orocline this wedge corresponds to the eastern cordillera whose growth was triggered by a major episode of horizontal subduction when the slab returned to a steeper dip angle shortening and uplift pursued facilitated by the structural and thermo chemical alteration of the continental lithosphere we review the successive episodes of horizontal subduction that have occurred beneath south america at different latitudes and show that they explain the diachronic widening of the andes we infer that the present day segmented physiography of the andes results from the latitudinally variable transient interplay between slab dynamics and upper plate tectonics over the cenozoic we emphasize that slab flattening or absence thereof is a major driving mechanism that sets the width of the andes at any latitude

magnetic anomaly interpretation across the southern central andes 32 34 s the role of the juan fernandez ridge in the late tertiary evolution of the margin marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of chile between 32 34 s three magnetic domains have been identified oceanic continental margin and subaerial the oceanic domain has seafloor spreading anomalies 16 37 ma to 18 39 5 ma disturbed by anomalies of the juan fernandez hot spot chain in the continental margin the most prominent fabric are e w anomalies in the upper slope corresponding to onshore e w anomalies of large intrusive bodies onshore a n s lineament of short wavelength anomalies defines the roots of a cretaceous volcanic arc a resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to 50 km landward of the trench absolute cenozoic plate motion for nazca and south american plates and dating of the juan fernandez chain provide a kinematic model of ridge continent collision the reconstruction indicates rapid southward migration of the collision point along 1400 km of the margin from 20 to 11 ma 20 cm yr 1 from 11 ma to present the collison point has migrated at a slower rate along 375 km of the margin 3 5 cm yr 1 the predicted location of the subducted portion of the juan fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship in the last 10 myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates these observations indicate that subduction of the juan fernandez chain plays a major role in arc forearc tectonics

quaternary tectonics and seismic potential of the andean retrowedge at 33 34 s the andean retrowedge located between 338s and 348s lies in the transition region of the pampean flat slab subduction zone to the north and a normal subduction zone to the south neo tectonic structures and shallow seismicity are very common north of this segment and become pro gressively less frequent southwards the frontal cordillera and the cerrilladas pedemontanas are the main morphostructures involved in the quaternary deformation of this region the frontal cor dillera is a thick skinned fold and thrust belt uplifted since late miocene time the cerrilladas pedemontanas are low relief hills that represent the mild inversion of the cuyo triassic rift depo centre since pliocene time middle miocene holocene synorogenic strata cover the cuyo basin and surrounding foreland areas the quaternary tectonic evolution of this area has been established through integration of new data from fieldwork in the frontal cordillera piedmont with subsurface information and previously published data mean late pleistocene uplift rates ranging between 0 21 and 0 92 mm a 21 and earthquake minimum moment magnitudes mw o fc 6 4 6 7 have been estimated for the morphostructural units analysed in this manuscript

apatite u th he thermochronology and re os ages in the altar region central andes 31 30 s main cordillera of san juan argentina implications of rapid exhumation in the porphyry cu au metal endowment and regional tectonics altar is a large porphyry cu au deposit located in the main cordillera of argentina 20 km to the north of the giant los pelambres el pachon porphyry copper cluster at the southern portion of the pampean flat slab segment of the andes although this region hosts telescoped porphyry epithermal deposits the precise temporal relationship between porphyry emplacement mineralization cooling and regional orogenic uplift are still poorly understood new re os molybdenite ages indicate that altar orebodies are associated with two magmatic hydrothermal centers altar east 11 16 0 06 ma and altar central 10 38 0 05 ma formed at temporally distinct periods new u th he ages from the early permian and late eocene plutons and the middle miocene subvolcanic stocks associated with cu au mineralization of the altar region reflect a rapid cooling pulse during the middle miocene 15 02 to 10 66 ma coeval with a major phase of tectonic shortening and regional uplift the main pulse of rapid cooling and related tectonic uplift in the altar region was synchronous with the formation of the hydrothermal systems and resulted in an increased focused metal endowment au cu grades due to the telescoping of epithermal mineralization over the rapidly uplifted porphyry system this 11 10 ma tectonically triggered exhumation event coincides with the collision of the e trending segment of the juan fernandez ridge with the peru chile trench at this latitude collision and ensuing ridge subduction may have driven a localized pulse of rapid cooling and exhumation of the main cordillera that has not been well documented to the north or south of the altar los pelambres region

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

magnetostratigraphy of neogene andean foreland basin strata lat 33 s mendoza province argentina neogene nonmarine strata in the southernmost part of the precordillera thrust belt of the andes of argentina accumulated during the miocene and pliocene in response to eastward advance of thrust sheets located to the west to link the sequence of deformational events with the sedimentary record five stratigraphic sections in the mendoza province were dated using magnetic polarity stratigraphy calibrated with 40 ar 39 ar dates from interbedded tephras these data establish age control on tectonic activity volcanism and deposition the existence of a backarc foreland basin that developed prior to or ca 16 ma is recorded by the oldest strata the ca 15 7 12 2 ma baksi time scale marino formation deposition of this unit is linked to a major phase of thrust activity in the central part of the aconcagua fold and thrust belt in the principal cordillera initiation of deposition coincides with an eastward shift of the subduction related magmatic foci to the argentine principal cordillera the overlying ca 11 7 9 0 ma la pilona formation is thought to record the initiation of exhumation of the frontal cordillera to the northwest of the study region an extensive ash rich unit the 8 9 locally 9 5 to 8 7 ma tobas angostura formation is correlated with the latest and probably the largest extrusive episode in the principal cordillera chronologic data constrain the timing of oil generation in the cuyo basin and development of prospective closures the new data also require that land mammalian fossils found in the rio de los pozos formation are late miocene rather than pliocene as previously believed

tectono stratigraphic evolution of the andean orogen between 31 and 37 s chile and western argentina fil charrier reynaldo universidad de chile facultad de ciencias fisicas y matematicas departamento de geologia chile universidad andres bello chile

extension and basin formation in the southern andes caused by increased convergence rate a mid cenozoic trigger for the andes the southern andes between 33 and 45 s latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin which itself was characterized by volcanic activity the basins are filled with thick up to 3000 m accumulations of interbedded sedimentary and volcanic strata of late oligocene early miocene age we interpret that these basins developed during a phase of moderate extension within the plate margin system triggered by an increased rate of convergence of the farallon nazca and south american plates between 28 and 26 ma this history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling although extensional basins of this age are only well described in the southern andes the convergence history and volcanic chronology are similar farther north in the central andes 18 33 s leading to the speculation that extension may have characterized the late oligocene early miocene interval regionally we hypothesize that this extension was a necessary condition to subsequent building of the modern andes mountains

cenozoic tectonics of the high andes of west central argentina 30 36 s latitude abstract the structure of the central andes shows three distinctive segments characterized by different geometries these geometries are superimposed on the present large scale plate tectonic setting characterized by distinct subduction segments the northern la ramada segment is a thick skinned fold and thrust belt formed by tectonic inversion of a late triassic rift the central aconcagua segment consists of a thin skinned fold and thrust belt while the southern malargue segment like the first one is a thick skinned fold and thrust belt developed by tectonic inversion of a late triassic early jurassic rift system during late cenozoic times the amount of shortening gradually decreases from north to south as indicated by the crustal roots of the central andes the different geometries along the principal cordillera controlled the abrupt changes in the shortening among segments the structure of precordillera and sierras pampeanas has also been considered in order to account for the total shortening in the la ramada segment the main shortening occurred in the precordillera in the aconcagua segment in the principal cordillera while in the malargue segment the shortening is widely distributed in a broader principal cordillera because south of the flat slab subduction segment the precordillera and sierras pampeanas are missing

the control of pre existing extensional structures on the evolution of the southern sector of the aconcagua fold and thrust belt southern andes abstract the aconcagua fold and thrust belt located in the andean mountains at 32 30 to 34 s has been described as a classic model of a thin skinned thrust belt however new structural data from its southern sector have shown that it has a complex structural framework reflected in multiple mesozoic extensional phases overprinted by structural inversion as well as thin and thick skinned tectonics two major superimposed extensional structural styles have been identified for the mesozoic characterized by distinctly oriented stress fields a key role in the evolution of this part of the fold and thrust belt was played by a late triassic to early jurassic depocentre and by late jurassic block faulting shortening was accommodated by a combination of inversion of pre existing normal faults development of footwall short cuts and both thin and thick skinned thrusting synrift and postrift sedimentary rocks were uplifted by reactivation of normal faults with further shortening along newly formed thin skinned thrust faults the geometry of thin skinned fault systems is controlled by the architecture of the rift basin competent footwalls forming barriers to the lateral propagation of detachments

the crustal seismicity of the western andean thrust central chile 33 34 s implications for regional tectonics and seismic hazard in the santiago areathe crustal seismicity of the western andean thrust central chile 33 34 s