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

role of subduction erosion in the generation of andean magmas subduction erosion or scraping off of the continental crust by subducting oceanic lithosphere is an important tectonic process along parts of the andean convergent plate boundary estimated rates of subduction erosion increase northward north of the southem andes due to decreasing subduction angle and decreasing sediment supply to the trench west of the central andes subduction erosion may remove as much as 535 km 3 of crust per 1 km of trench per 1 m y or 7 vol of the subducting oceanic lithosphere the higher 87 sr 86 sr ratios of mafic magmas erupted north of the southern andes may be explained by the higher rates of subduction erosion and increased amounts of contamination of the sub arc mantle magma source

miocene quaternary structural evolution of the uyuni atacama region andes of chile and bolivia abstract we describe the miocene quaternary geological structural evolution of the region between the salar de uyuni and de atacama andes of chile and bolivia we recognized four main tectonic events based on fold geometry fault kinematics and stratigraphic data the oldest event of miocene age is characterized by folding and reverse faulting of the sedimentary successions with an e w direction of shortening in the northern part of the studied area and a wnw ese shortening in the southern part the following two events of pliocene age are characterized by lower shortening amounts they occurred first by reverse faulting with a nw se trending greatest principal stress 1 computed with striated fault planes and a vertical least principal stress 3 followed by pervasive strike slip faulting with the same nw se trending 1 and a horizontal ne sw 3 the fourth event dating to the late pliocene quaternary is characterized by normal faulting the 3 still trends ne sw whereas the intermediate principal stress 2 exchanged with 1 volcanism accompanied both the contractional transcurrent and extensional tectonic phases the mio pliocene compression appears directly linked to a rapid convergence and an apparently important coupling between the continental and oceanic plates the e w to wnw ese direction of shortening of the miocene structures and the nw se 1 of the pliocene structures seem to be more linked to an intra andean re orientation of structures following the wnw directed absolute motion of the south american plate the extensional deformations can be interpreted as related to gravity forces affecting the highest parts of the volcanic belt in a sort of asymmetrical sw ward collapse of the belt

the relation between ground acceleration and earthquake source parameters theory and observations abstract a simple relation between the root mean square rms of the ground acceleration and earthquake spectral or source parameters is introduced in which 0 is the low frequency displacement spectral plateau f 0 is the corner frequency is an attenuation parameter and t is the data interval this result uses the omega square model for far field radiation and accounts for site specific attenuation the main advantage of the new relation with respect to that of hanks 1979 is that it relaxes the simplifying assumption that the spectral corner frequency is much smaller than the maximum corner frequency resulting from attenuation and that the spectrum may be approximated as being perfectly flat between the two frequencies the newly proposed relation is tested using a composite dataset of earthquake records from japan california mexico and taiwan excellent agreement is found between observed and predicted ground acceleration for any combination of corner frequencies thus use of the above relation enables the extrapolation of ground motion prediction equation inferred from the frequent small magnitude earthquakes to the rare large magnitudes this capacity is extremely useful near slow slip plate boundaries where the seismic moment release rates are low

tectonomagmatic characteristics of the back arc portion of the calama olacapato el toro fault zone central andes 1 post 20 ma magmatism in the central andes is either localized in the magmatic arc or distributed east of it on the altiplano puna plateau here there is a distinct concentration of magmatic centers on nw se trending lineaments such as the calama olacapato el toro cot that extends into the eastern cordillera to the east of the puna understanding the possible genetic relationship between prominent structures and magmatic centers on these lineaments is important to elucidate the tectonomagmatic evolution of the central andes we investigated the back arc area of the cot using remote sensing geological structural and petrochemical data our study demonstrates that this portion of the cot consists of nw se striking faults formed under overall left lateral transtension that decreases in activity toward the cot termini deformation on the cot occurred during and after activity of prominent n s striking transpressive fault systems and is coeval with magmatism which is focused on the central cot the most evolved magmatic rocks with an upper crustal imprint are exposed on the central cot whereas more primitive mantle derived mafic to moderately evolved magmatic rocks are found toward the cot termini this points to a genetic relationship between upper crustal deformation and magmatic activity that led to enhanced magma storage in the central cot cot magmas may result either from slab steepening or episodic delamination of the asthenospheric mantle

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

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

regional o sr and pb isotope relationships in late cenozoic calc alkaline lavas of the andean cordillera quaternary recent volcanism in the andes has occurred in three regions 45 33 s 28 16 s and 2 s 5 n each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics isotope ratios of o and sr are lowest and those of pb least variable in the southern volcanic zone svz where medium k lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent the svz lavas were probably derived from an asthenospheric mantle source above a shallow benioff zone parental basaltic magmas rose largely unmodified through thin continental crust where differentiation occurred by low pressure crystal fractionation without concurrent modification of isotopic composition the slight enrichment of 206 pb in the northern volcanic zone nvz where the crust is thin and the benioff zone deep suggests a greater subduction zone component for parental medium k magmas the slight 18 o enrichment suggests a small amount of lower crustal interaction isotope ratios of o and sr are highest and those of pb most variable in the central volcanic zone cvz parental magmas in the cvz were probably generated within the same mantle source region as those in the svz and nvz subsequently during transit through the exceptionally thick continental crust of the central andes high k magmas were produced by a combination of bulk contamination in the lower crust and later low pressure fractional crystallization assimilation processes in the upper crust which altered both chemical and isotopic compositions

recent rapid uplift in the bolivian andes evidence from fission track dating apatite and zircon fission track cooling ages constrain the tertiary cooling and uplift history of the eastern cordillera and altiplano of bolivia fission track data are from two triassic plutons and surrounding paleozoic metasedimentary rocks in the eastern andes north of la paz zircon cooling ages indicate that the roof of the huayna potosi pluton was emplaced in the zircon partial annealing zone and that the zongo pluton was emplaced entirely in the zircon total annealing zone apatite cooling ages for both plutons record uplift in the past 5 15 m y and zircon cooling ages from the zongo pluton reflect uplift in the past 25 45 m y uplift rates calculated by these apatite and zircon cooling ages suggest that uplift rates were 0 1 0 2 mm yr between 20 and 40 ma and increased significantly between 10 and 15 ma by 3 ma uplift rates may have been as high as 0 7 mm yr

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

structural control on geothermal circulation in the cerro tuzgle tocomar geothermal volcanic area puna plateau argentina abstract the reconstruction of the stratigraphical structural framework and the hydrogeology of geothermal areas is fundamental for understanding the relationships between cap rocks reservoir and circulation of geothermal fluids and for planning the exploitation of the field the cerro tuzgle tocomar geothermal volcanic area puna plateau central andes nw argentina has a high geothermal potential it is crossed by the active nw se trans andean tectonic lineament known as the calama olacapato toro cot fault system which favours a high secondary permeability testified by the presence of numerous springs this study presents new stratigraphic and hydrogeological data on the geothermal field together with the analysis from remote sensed image analysis of morphostructural evidences associated with the structural framework and active tectonics our data suggest that the main geothermal reservoir is located within or below the pre palaeozoic ordovician basement units characterised by unevenly distributed secondary permeability the reservoir is recharged by infiltration in the ridges above 4500 m a s l where basement rocks are in outcrop below 4500 m a s l the reservoir is covered by the low permeable miocene quaternary units that allow a poor circulation of shallow groundwater geothermal fluids upwell in areas with more intense fracturing especially where main regional structures particularly nw se cot parallel lineaments intersect with secondary structures such as at the tocomar field away from the main tectonic features such as at the cerro tuzgle field the less developed network of faults and fractures allows only a moderate upwelling of geothermal fluids and a mixing between hot and shallow cold waters the integration of field based and remote sensing analyses at the cerro tuzgle tocomar area proved to be effective in approaching the prospection of remote geothermal fields and in defining the conceptual model for geothermal circulation

andean age structure of eastern cordillera province of la paz bolivia a moho root beneath the bolivian andes 40 km deep is consistent with 230 km of overlap of neogene age on a single trenchward dipping transcrustal thrust fault with 10 of finite ramp cutoff main andean thrust mat only 10 of the andean crustal volume is ascribable to magmatic addition the mat is located within south american crust of full thickness it intersects the basement top 450 km inland from the neogene crustal margin it is not a collision suture as shown by persistent pre neogene facies continuity thrusting is not accompanied by terrane accretion the present bilaterally symmetrical thrust belt responds to elastic line loading and to coulomb rheology in the hanging wall of the mat a deep high stress wedge base builds a steep critical slope in the footwall the foredeep response is fast subcritical growth by progradation and blind thrusting on a low stress decollement interaction is maintained by out of sequence renewal of movement on the mat

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

the late miocene quaternary antofalla volcanic complex southern puna nw argentina protracted history diverse petrology and economic potential abstract the antofalla volcanic complex avc is located on the southern puna plateau of nw argentina 100 km east of the main axis of the cordillera occidental volcanic arc it lies on the nw se archibarca lineament one of several transverse structures that cut across the andes from chile and divert arc magmatism in se trending fingers across the puna 40ar 39ar dating of volcanic products from the avc reveals a protracted magmatic history from 10 9 to 1 6 ma initial volcanism was characterized by eruption of voluminous rhyolitic ignimbrites deposited directly onto exposed crystalline basement and supracrustal sedimentary rocks these rhyolites record significant geochemical evidence for crustal contamination or derivation by crustal melting which decreases upwards through the pyroclastic sequence they were followed by extrusion of potassic lavas of the shoshonite latite trachydacite suite from 10 1 to 9 5 ma subsequent magmatism 9 1 1 6 ma was of basaltic andesite andesite dacite composition with localized smaller volume dacitic and rhyolitic ignimbrites monogenetic basaltic andesite cinder cones and lava flows were erupted in the quaternary in response to a change to transtensional tectonics in the puna the volcanic sequence is interpreted to record the early development of a lower crustal mash zone at 11 ma where mantle derived arc magmas interacted extensively with felsic lower crustal rocks to produce evolved shoshonitic compositions and crustal melts that erupted to form rhyolitic ignimbrites common at this time throughout the puna after establishment of a steady state mash zone by 9 ma subsequent basaltic andesite andesite magmas show less evidence of felsic crustal contamination and may have developed largely by interaction with previous ultramafic mafic fractionates of early magmas in the lower crust more felsic dacitic magmas appear to have evolved largely by fractional crystallization from basaltic andesites and andesites with widespread evidence of mixing and mingling between magmas of different degrees of differentiation some contamination by upper crustal components is also evident in these dacites but is shown more clearly by small volume rhyolites some of which may again represent predominantly crustal melts epithermal deposits of au and pb zn ag occur within the avc apparently associated with the formation of rhyolite domes the mineralization style is typical of other andean volcanic centres and may be underlain by deeper seated porphyry styles of mineralization extensive blanketing by younger lavas and limited erosion restricts exposure of these altered and mineralized sequences however although the potential for further discoveries beneath cover or at depth in this system appears high

space time variations of stresses in the miocene quaternary along the calama olacapato el toro fault zone central andes abstract we describe the temporal and spatial changes in the tectonic state of stress occurred during the miocene quaternary in the trans orogen area of the calama olacapato el toro cot fault zone in the central andes at about 24 s within the northern portion of the puna plateau this work sheds new light on the complexity of stress pattern distribution in general and contributes to the analysis of the relationships between tectonics and volcanism and of the seismic hazard of the area field geological surveys along with kinematic analysis and numerical inversion of 140 new fault slip measurements have revealed that this portion of the cot zone previously considered a continuous long lived lineament in reality has been subjected to three different kinematic regimes 1 a miocene transpressional phase with the maximum principal stress 1 chiefly trending nne ssw 2 an extensional phase that started by 9 ma with a horizontal nw se trending minimum principal stress 3 and 3 a left lateral strike slip phase with a horizontal e w 1 and n s 3 dating to the late pliocene quaternary which decreases toward the westernmost part of the studied zone where it transitions to extension producing a n s trending graben structure hence even if transcurrence is still active in the eastern portion of the cot as focal mechanisms of crustal earthquakes indicate our study demonstrates that extension is becoming the predominant structural style of deformation at least in the western region the major changes in the tectonic regimes are attributed to changes in the magnitude of the boundary forces due to subduction processes whereas gravitational effect of a thickened crust might be responsible for the overall orogen perpendicular extension

volcanism in a compressional andean setting a structural and geochronological study of tromen volcano neuquen province argentina we document evidence for growth of an active volcano in a compressional andean setting our data are surface structures and 39ar 40ar ages of volcanic products on tromen volcano tromen is an active back arc volcano in the andean foothills of neuquen province argentina its volcanic products are unconformable upon mesozoic strata of the neuquen basin the volcano straddles a n s trending pop up which formed during e w shortening the main underlying structures are eastward verging thrusts their traces curve around the eastern foot of the volcano minor folds and faults also occur in the volcanic cover of tromen as a result of e w shortening new 39ar 40ar ages for these volcanic rocks are younger than 2 27 0 10 ma and show that tromen has been active almost continuously from the late pliocene to the holocene we conclude that volcanism and thrusting have been coeval and that magma must have reached the surface in a tectonic setting of horizontal compression our results have wider implications for magmatic processes in such settings

non explosive magma water interaction in a continental setting miocene examples from the eastern cordillera central andes nw argentina the middle upper miocene las burras almagro el toro bat igneous complex within the eastern cordillera of the central andes 24 s nw argentina has revealed evidence of non explosive interaction of andesitic magma with water or wet clastic sediments in a continental setting including peperite generation we describe and interpret lithofacies and emplacement mechanisms in three case studies the las cuevas member 11 8 ma comprises facies related to i andesite extruded in a subaqueous setting and generating lobe hyaloclastite lava and ii marginal parts of subaerial andesite lava dome s in contact with surface water comprising fluidal lava lobes hyaloclastite and juvenile clasts with glassy rims the lampazar member 7 8 ma is represented by a syn volcanic andesite intrusion and related peperite that formed within unconsolidated water saturated coarse grained volcaniclastic conglomerate and breccia the andesite intrusion is finger shaped and grades into intrusive pillows pillows are up to 2 m wide tightly packed near the intrusion fingers and gradually become dispersed in the host sediment 50 m from the parent intrusion the almagro a member 7 2 ma shows evidence of mingling between water saturated coarse grained volcaniclastic alluvial breccia and intruding andesite magma the resulting intrusive pillows are characterized by ellipsoidal and tubular shape and concentric structure the high level penetration of magma in this coarse sediment was unconfined and irregular magma was detached in apophyses and lobes with sharp contacts and fluidal shapes and without quench fragmentation and formation of a hyaloclastite envelope the presence of peperite and magma water contact facies in the bat volcanic sequence indicates the possible availability of water in the system between 11 7 ma and suggests a depositional setting in this part of the foreland basin of the central andes characterized by an overall topographically low coastal floodplain that included extensive wetlands

geomorphological evolution of the chimpa stratovolcano in the back arc region of the central andes abstract the morphological evolution of a volcano records the main events of its formation and its complexity is a function of overlapping constructive and destructive phases typical morphological evolutionary trends have been established simple cones grow into larger cones or they widen into sub cones or massifs however there are many exceptions to these general sequences one exception is represented by chimpa volcano an andesitic stratovolcano located in the central puna of the andes in order to evaluate the morphological evolution of chimpa volcano its morphology was characterized analyzed and quantified using a digital elevation model dem chimpa volcano is morphologically classified as a sub cone and three constructive stages of evolution and one destructive event have been recognized the current morphology is related to the formation of an elongated depression flanked by scarps named quebrada casana generated by a destructive event morphology and stratigraphy were linked using a parameter named emplacement balance e which is defined as the volumetric ratio between the volcanic products emplaced far away from the vent and those which remain near vent chimpa volcano s morphological evolution is directly related to the type of volcanic products emitted the evolution began with the emission of pyroclastic density currents pdcs where e remains high favoring the development of a plateau stage i then lava dome emplacement in a central position and its destruction reduced the value of e and a massif was created stage ii finally e reached the lowest value when the volcano began to emit lava flows forming a sub cone stage iii the presented methodology to evaluate volcanic products in relation to their morphology can be applied to other volcanic edifices in the central andes and elsewhere

the geological and structural evolution of the cerro tuzgle quaternary stratovolcano in the back arc region of the central andes argentina abstract the aim of our paper is to contribute to a better knowledge of the volcanism in the back arc region of the central andes and its relationships with the basement geology the stress field and the tectonic evolution by studying in detail the stratigraphy and the structure of the quaternary cerro tuzgle stratovolcano in the puna plateau field mapping and remote sensing analysis reveal the stratigraphic architecture the geological evolution and the volcanotectonic interactions in the cerro tuzgle area for the first time in a volcano of the puna plateau synthemic units bounded by unconformity surfaces have been defined unrevealing the temporal and spatial relationships between constructive and destructive phases of the volcano history our study indicates that after the emplacement of a small ignimbrite deposit and of few scattered lava domes the central cerro tuzgle volcano built up throughout three distinct phases of edifice construction the first of these constructive phases ended with a previously unreported destructive event consisting of 0 5 km 3 catastrophic sector collapse of the volcanic edifice whose stratigraphic position and main characteristics have been identified the study suggests that the regional stress regime and the topography of the substrata are the main non magmatic factors controlling the constructive and destructive phases of the volcano including the directions of magmatic intrusions faulting and gravitational sector failure of the volcano the integration of synthemic stratigraphy and volcanotectonic analysis in the study of volcanic edifices showed to be an effective methodological approach for the understanding of the magmatic and tectonic evolution of the puna plateau

late cenozoic magmatism of the bolivian altiplano small basalt to dacite volcanic centers are distributed sparsely over the bolivian altiplano behind the andean volcanic front most are pliocene to recent in age and are characterized by textural and mineralogical disequilibrium with abundant xenoliths and xenocrysts true phenocrysts are rare in the more mafic samples compared with recent volcanic rocks from andean stratovolcanoes the bolivian centers overlap in major element trends incompatible element contents tend to be higher particularly in the eastern altiplano the ranges of isotopic compositions reflect ubiquitous crustal contamination pb isotope compositions are dominated by pb from isotopically heterogeneous basement resulting in a wide scatter of data lying between inferred crustal compositions and showing little overlap with possible mantle sources in the region rocks sampled from the bolivian altiplano were probably derived from asthenospheric mantle and subjected to extensive open system differentiation during ascent through the 70 km thick crust of the region major element trends are largely controlled by the fractionating phase assemblage olivine clinopyroxene and amphibole trace element and isotope systematics however defy realistic attempts at modeling due to the geographic scatter of samples the uniformity of compositions at a given center and the heterogeneity of the contaminant nevertheless there are first order systematic trace element variations that appear to relate to the geometry of the subduction zone in particular life hfse exemplified by ba nb and zr nb ratios decrease from the arc front eastward into the altiplano these variations are not easily reconciled with control by crustal contamination alone a model is proposed in which the asthenospheric source is fluxed by high ba nb slab derived fluid to induce melting beneath the arc high fluid flux increases the ba nb ratio of the asthenosphre and leads to high degrees of partial melting high zr nb to the east lower or no fluid flux leads to low ba nb and low degrees of partial melting low zr nb melting in the back arc region of the altiplano may be facilitated by lithospheric delamination that leads to decompression melting of counter flowing asthenosphere there is no unequivocal evidence that requires a significant role for the lithospheric mantle

tectonic shortening and crustal thickness in the central andes how good is the correlation exceptional thickening of continental crust beneath the central andes is believed to be mainly a result of tectonic shortening of the south american plate in neogene time this shortening has been estimated to have contributed as much as 70 80 of the present crustal volume a compilation of published shortening values and our own estimates based on balanced cross sections from the central andes between lat 3 s and 40 s suggest that 70 80 is a maximum rather than an average value for this part of the andes tectonic shortening and the crustal cross section area are only loosely correlated variations in tectonic shortening are more abrupt than those of crustal areas particularly near the northern and southern ends of the altiplano puna high plateau where thick crust is associated with relatively small amounts of shortening shortening there may account for no more than about 30 of the present crustal cross section area the processes that created the remaining crustal area are not clear but are likely to involve poorly constrained pre neogene tectonic shortening moderate magmatic additions to the crust tectonic underplating of material derived from the forearc and possible flow of ductile lower crust along strike

glacial chronology and production rate cross calibration of five cosmogenic nuclide and mineral systems from the southern central andean plateau abstract glacial deposits on the high altitude arid southern central andean plateau cap the puna in northwestern argentina document past changes in climate but the associated geomorphic features have rarely been directly dated this study provides direct age control of glacial moraine deposits from the central puna 24 s at elevations of 3900 5000 m through surface exposure dating with cosmogenic nuclides our results show that the most extensive glaciations occurred before 95 ka and an additional major advance occurred between 46 and 39 ka the latter period is synchronous with the highest lake levels in the nearby pozuelos basin and the minchin inca huasi wet phase on the altiplano in the northern cap none of the dated moraines produced boulder ages corresponding to the tauca wet phase 24 15 ka additionally the volcanic lithologies of the deposits allow us to establish production ratios at low latitude and high elevation for five different nuclide and mineral systems 10be 21ne and 26al from quartz 11 or 12 samples and 3he and 21ne from pyroxene 10 samples we present production ratios for all combinations of the measured nuclides and cross calibrated production rates for 21ne in pyroxene and quartz for the high sub tropical andes the production rates are based on our 10be normalized production ratios and a weighted mean of reference 10be production rates calibrated in the high tropical andes 4 02 0 12 at g 1 yr 1 these are 21neqtz 18 1 1 2 at g 1 yr 1 and 21nepx 36 6 1 8 at g 1 yr 1 en 88 94 scaled to sea level and high latitude using the lal stone scheme with 1 uncertainties as 3he and 26al have been directly calibrated in the tropical andes we recommend using those rates finally we compare exposure ages calculated using all measured cosmogenic nuclides from each sample including 11 feldspar samples measured for 36cl and a suite of previously published production rates

generation of a crust mantle magma mixture magma sources and contamination at cerro panizos central andes cerro panizos a large caldera in the central andes mountains produced two large dacitic ignimbrites at 7 9 ma and 6 7 ma and many andesitic and dacitic lava flows and domes the older rhyodacitic cienago ignimbrite represents the most silicic magma erupted by the system the younger much larger volume dacitic cerro panizos ignimbrite is very crystal rich containing up to 50 biotite plagioclase and quartz crystals in the pumice it is weakly zoned with most of the zoning apparent between two main cooling units major and most trace elements show little variation through the cerro panizos ignimbrite but the small range of composition is consistent with typical fractionation trends sr nd and pb isotopic ratios are very crustal with initial 87sr 86sr values of 0 711 to 0 715 nd values of 7 5 to 10 2 and nearly invariant pb isotopic ratios 206pb 204pb 18 85 207pb 204pb 15 67 and 208pb 204pb 38 80 the limited zonation observed in the cerro panizos ignimbrite is explained by impeded crystal settling due to high crystal content the magma body was a crystal liquid mush before ascent to the pre eruption crustal levels crystals formed but did not separate easily from the magma limited fractionation of plagioclase and biotite may have occurred but the composition was largely controlled by lower crustal mash processes afc modeling shows that the cerro panizos magmas resulted from a mixture of roughly equal proportions of late miocene mantle derived basalts and melts from 1 0 ga grenville age lower crust this occurred in a mash zone in the lower crust and set the crustal isotopic ratios observed in the cerro panizos magmas the great thickening of the crust beneath the central andes mountains sent upper and middle crustal rock types to lower crustal and deeper depths and this explains the upper crustal isotopic signatures of the cerro panizos rocks minor upper crustal assimilation of early miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system the nearly invariant high pb isotopic values and high pb concentrations indicate that pb came almost entirely from the crustal source and was little altered by any subsequent upper crustal assimilation this pb signature is isotopically similar to that of the southern bolivian tin belt suggesting a widely distributed pb source the great difference between compositions of miocene and quaternary central andean volcanic rocks is explained by crustal thickening in early miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions the lower crust dried out considerably by quaternary time so less crustal component is present

molluscan response to early pleistocene rapid warming in the sea of japan major changes occurred in the benthic molluscan fauna of the sea of japan continental shelf during the transition from pleistocene glacial to interglacial stages owing to rapid warming associated with the inflow of the warm tsushima current molluscan associations representing this transition occur in the lower pleistocene omma formation in central japan and suggest that there were two patterns of faunal change the first was when warm water species migrated into the sea of japan and lived along with cold water species accompanied by a northward shift in species ranges the second pattern involved the migration of warm water mollusks shortly after the local extinction of cold water species in the latter it is possible that benthic molluscan communities with very low diversity and density existed temporarily and locally at inner shelf depths 100 m during the warming phase such a community has no modern analogue but may have resulted from a marine climate with a higher seasonality than occurs today these findings show that episodes of rapid warming have severe impacts on offshore benthic communities and molluscan species that include patterns not represented by modern faunal distributions

faulting assisted lateral collapses and influence on shallow magma feeding system at ollague volcano central volcanic zone chile bolivia andes abstract the role played by the regional tectonics and relationships between volcanic structures and their shallow magmatic feeding system are of paramount importance in understanding multiple lateral collapse events and thus for hazard assessment in active volcanoes ollague 21 18 s 68 11 w andean central volcanic zone chile bolivia border is an active andesite dacite composite volcano developed on a regional nw striking extensional fault system that bisects the volcanic edifice geological and stratigraphic studies on the summit part of the cone together with new 40ar 39ar dating and petrographic and geochemical major trace elements analyses have allowed us to delineate the volcanic history of ollague and recognize multiple deformation and lateral collapse events the evolution of ollague appears to be characterized by four stages of volcano building separated by three main events of deformation of the cone these deformation events developed progressively by downthrow of the sw sector of the volcano 800 ka along the nw striking normal fault propagating from the substrate to a final catastrophic failure 300 ka of the sw sector and debris avalanche emplacement both nw trending regional tectonics and weakness of the edifice caused by the successive deformation and collapse events are responsible for both the geometric variations of the shallow magma feeding system and the coexistence of summit and parasitic dacitic vents central and lateral vents shifted in a sw trending direction i e perpendicular to the tectonic trend and along the axis of the collapse movement and developed along a nw trending alignment i e parallel to the main tectonic trend and collapse scarps

the geochemical variations of the upper cenozoic volcanism along the calama olacapato el toro transversal fault system in central andes 24 s petrogenetic and geodynamic implications in this paper we present new geochemical and sr nd isotopic data for several upper miocene volcanic centres aligned along one of the most extensive transcurrent lineament in the central andes the calama olacapato el toro cot the transversal volcanic belt along cot is constituted by large composite volcanoes and a caldera structure they are from nw to se puntas negras rincon tul tul del medio and pocitos tumepo quevar aguas calientes and tastil in order to compare chemical data from the different centres along the cot transect differentiation effects were minimised by using data extrapolated at 60 sio2 with least square regression method in the western sector of the cot the volcanic products of puntas negras and rincon show relatively high k2o and 87sr 86sr and low rb cs ta th la yb 143nd 144nd to the east the tumepo products have high sr and 143nd 144nd la yb and ba rb and low y 87sr 86sr in the easternmost cot sector quevar aguas calientes and tastil volcanic complexes exhibit low la yb high87sr sr86 and low 143nd 144nd on the basis of these data we propose a petrogenetic and geodynamical model for central andes at 24 s in correspondence of miocene quaternary volcanic arc puntas negras and rincon the magmas inherited a calcalkaline signature partly modified by upper crustal and or sediment assimilation in the central eastern sector melting assimilation storage and homogenisation mash processes occurred at the base of a thickened crust in this cot sector tumepo products show an evident lower crust signature and could be considered representative for mash derived magmas in the easternmost sector quevar aguas calientes and tastil products could represent magmas generated by partial melting of underthrusted brasilian shield and mixed with magmas derived by mash processes

structural control on volcanoes and magma paths from local to orogen scale the central andes case abstract assessing the parameters that control the location and geometry of magma paths is of paramount importance for the comprehension of volcanic plumbing systems and geo hazards we analyse the distribution of 1518 monogenic and polygenic volcanoes of miocene quaternary age of the central volcanic zone of the andes chile bolivia argentina and reconstruct the magma paths at 315 edifices by analysing the morphostructural characteristics of craters and cones then we compare these data with outcropping dykes tectonic structures and state of stress most magma paths trend n s nw se and ne sw in decreasing order of frequency the n s and nw se paths coexist in the northern and southern part of the study area whereas n s paths dominate east of the salar de atacama outcropping dykes show the same trends the regional holocene stress state is given by an e w greatest horizontal principal stress n s and nne ssw reverse faults and folds affect deposits of 4 8 3 2 and 1 3 ma bp especially in the central and southern study areas a few nw se left lateral strike slip faults are present in the interior of the volcanic arc part of which belong to the calama olacapato el toro fault the volcanic chain is also affected by several n s and nw se striking normal faults that offset pliocene and quaternary deposits the results indicate different scenarios of magma tectonic interaction given by n s normal and reverse faults and n s fold hinges that guide volcano emplacement and magma paths magma paths are also guided by strike slip and normal nw se faults especially in the northern part of the study area zones with verticalized strata with bedding striking ne sw also acted as preferential magma paths these data suggest that at convergence zones with continental crust shallow magma paths can be more sensitive to the presence and geometry of upper crustal weakness zones than to the regional state of stress

contemporary recent extension and compression in the central andes abstract although extension in the high andes vs compression in the lowlands has already been widely discussed in the literature for the first time we recognized both extensional and contractional structures that developed contemporaneously during late pliocene quaternary times in a wide area of the central andean chain about 90 000 km 2 where crustal earthquake data are missing this area comprises north eastern chile south western bolivia and north western argentina and extends from the puna plateau to the altiplano volcanic belt late pliocene quaternary folds with hinge lines trending nne ssw to n s are mostly located along the westernmost part of the volcanic belt and the eastern part of the western cordillera locally there are coeval reverse faults parallel to the folds which reach up to the surface particularly the miscanti ridge tolocha fault and la casualidad ridge may be the morphostructural expression of tens km long fault propagation folds which locally show topographic scarps hundreds of meters high north and east of the contractional structures we found evidence of late pliocene quaternary normal faults striking n s in the southern part of the study area and nw se in the northern part well developed grabens are present in the higher areas of the volcanic belt and in the transition zone with the puna plateau the surface rupture zones of normal fault swarms range 8 24 km in length with single fault strands up to 18 km long which are typical of tectonic structures the distribution in space and time of the studied contractional and extensional structures indicates that they originated in the same time period we thus address the challenging question regarding the possible origin of the stress sources by analysing possible causes such as volcanotectonics high topography orogeny collapse and gravitational spreading of the orogen in relation also with the role played by inherited structures we finally analyse the relations between the different structures and magma upwelling and the potential for seismic hazard

lower bound on the amount of crustal shortening in the central bolivian andes balanced cross sections across the cordillera oriental and subandean zone of the central bolivian andes indicate that crustal shortening probably has played the dominant role in orogeny in this convergent margin setting a minimum amount of shortening 210 km is documented which can account for two thirds of the present day crustal cross sectional area along a transect spanning the entire mountain range substantial crustal shortening may also require loss of the lower lithosphere to the asthenosphere a large minimum amount of crustal shortening in the bolivian andes shows contrary to common assumptions about orogeny that 1 magmatic addition may be volumetrically less important in orogeny in andean type margins and 2 crustal shortening is not uniquely associated with continental or island arc collision

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

an inversion approach to assimilation and fractional crystallisation processes a numerical inversion program has been developed to investigate isotope and trace element variations in suites of related rocks in terms of assimilation and fractional crystallisation processes major element compositions constrain the degree of fractionation and the program may be used to calculate the isotope and trace element composition of either the crustal contaminant or of the parental magmas once the contaminant is known the ratio of the rate of assimilation to the rate of fractional crystallisation and the bulk partition coefficients can be varied with changes in magma composition the approach is illustrated with a suite of calc alkaline rocks from the cerro galan centre in nw argentina and a suite of continental flood basalts from the southern parana in brazil both exhibit striking increases in 87sr 86sr with increasing sio2 consistent with progressive contamination during differentiation within the continental crust in detail both suites contain a subset of samples which have undergone relatively little contamination and a subset characterised by relatively high sio2 and 87sr 86sr reflecting larger crustal contributions individual samples within the more contaminated subset can be modelled in terms of assimilation with fractional crystallisation afc processes although no clear progressively contaminated liquid lines of descent are preserved at both cerro galan and the southern parana the associated dacites and rhyodacites appear to have similar compositions to the crustal contaminants thus the inversion program has been used to investigate the amount of contamination the relationship between the amount of contamination and the amount of crystals removed by fractional crystallisation and the isotope and trace element contents of the parental magmas prior to afc the latter are estimated to have had 87sr 86sr 0 7055 and 143nd 144nd 0 51256 at cerro galan and 0 7090 and 0 51226 respectively in the southern parana

comparative probabilistic assessment of the hydrological performance of reconstructed and natural watersheds the oil sands industry has committed to returning the mine sites to a productive condition the reconstructed soil covers must have sufficient available water holding capacity awhc to supply enough moisture over the growing season to promote vegetation in order to assess the sustainability of various soil cover alternatives a generic system dynamic watershed model entitled gsdw was used along with the available historical meteorological records to estimate the maximum soil moisture deficit and annual evapotranspiration fluxes a probabilistic framework was adopted consequently frequency curves of the maximum annual moisture deficit values are constructed and used to assess the probability that various reconstructed and natural watersheds can provide the associated moisture demands in general the study showed a tendency for the reconstructed watershed to provide less moisture for evapotranspiration than natural systems watersheds of various soil types layering thicknesses and topography were studied the gained knowledge was used to predict the possible performance of a hypothetical reclamation cover the results indicated that the hypothetical cover performed in a similar manner to the thickest existing soil cover which confirmed a high probability of that cover to survive under the same existing climatic conditions moreover this probabilistic framework was found to be useful for integrating information gained from natural watersheds e g the canopy of mature natural systems and transfer the results to the reconstructed system the results show that the canopy influenced the moisture deficit regime positively which signifies a greater possibility that reconstructed covers will adapt to vegetation type in brief the adopted approach enables better understanding of the response of reconstructed systems via multiple simulations of what if scenarios using different soil vegetation alternatives copyright 2010 john wiley sons ltd

igneous origin of co2 in ancient and recent hot spring waters and travertines from the northern argentinean andes abstract thermal travertines are an archive of co2 sources and sinks in hydrothermal systems two major regional factors control travertine precipitation water availability and co2 supply thus travertines form a valuable archive of hydrodynamic variability and sources of main contributions to dissolved inorganic carbon dic it is relevant to determine the main dic sources of thermal waters i e organic matter degradation recycled from older carbonates emission of deep seated magmatic co2 as they are key inputs to calculate the lithosphere atmosphere co2 budget the antuco travertine from the central andes represents one of such archives with a 500 ky record of accretionary periods dated as 425 320 260 and 155 ky bp related to high hydrothermal activity of hot springs it consists of two travertine units 1 a lower massive unit displaying large calcite pseudomorphs after aragonite that precipitated in proximal ponds with abundant water and 2 an upper stratified unit showing more distal facies bearing siliciclastics and manganese and iron oxides the replacement of aragonite by calcite in the lower unit was related to the decrease of water salinity in the thermal system through time in the antuco travertine dic 13c values of travertine parental waters of around 9 suggest that co2 was related to igneous processes and volcanic activity and released along deep seated faults relative water rock ratios derived from 18o values and fluid inclusion microthermometric data from travertine carbonates are consistent with an interpretation of greater water availability in the hydrothermal system during the late pleistocene than at present the different petrographic features and isotopic signatures are interpreted to reflect increased water availability during more humid periods in the altiplano which triggered precipitation of travertine bodies travertine growth took place during interglacial humid climate periods between marine isotope stages mis 3 and 9 which correspond to highstand events in large lakes of the andean altiplano results of this study illustrate that volcanic activity furnishing rather constant co2 and heat fluxes were the key controls of thermalism in the altiplane region during the quaternary whilst climatic changes humid vs arid periods in the late pleistocene controlled mineralogy facies and architecture of the travertines the combined use of 13c and 87sr 86sr signatures in carbonate precipitates has been proven to be of major relevance to evaluate the co2 sources along fault zones in this study

predictive mapping for copper gold magmatic hydrothermal systems in nw argentina use of a regional scale gis application of an expert guided data driven approach and comparison with results from a continental scale gis geographic information systems giss are very useful tools for managing checking and organizing spatial information from many sources and of many types in thematic layers processing of these data enables exploration oriented giss to produce potential and predictive maps for a given commodity which constitute documents of real use in decision making integration of all information in a single reference system enables a better understanding of the parameters controlling a region s metallogeny in terms of both time and space but what scale should be used for developing a mineral exploration gis should preference be given to systems with high spatial resolutions scale 1 500 000 or to more general systems with scales of around 1 1 500 000 or 1 2 000 000 will the gain be worthwhile relative to the additional work generated by compilation at a higher scale in order to make greater use of previous predictive studies performed on gold rich epithermal and porphyry systems at the scale of the entire andes an expert guided data driven approach is now applied to a regional scale gis of nw argentina between the puna and the sierrras pampeanas where known deposits like bajo de la alumbrera agua rica and others account for a metal potential of over 10 mt cu and 750 t au in developing this new predictive map three criteria that were likely to be connected to the mineralizing event were selected and quantified i lithostratigraphy because of its role as a favourable environment for the development of mineralization based on its physico chemical properties ii lithostratigraphic contacts based on the rheological properties of the formations in contact and iii the orientation of structural discontinuities which channel source magmas and encourage the circulation of hydrothermal fluids assigning a score enables classification of the favourabilities calculated for each of the criteria considered this approach is employed here to check and standardize the statistical results obtained by methods such as weight of evidence modelling or an algebraic approach for each criterion four classes were distinguished very favourable score 3 favourable score 2 slightly favourable score 1 and unfavourable score 0 the predictive map is obtained by adding the scores for the three favourable criteria defined above the regional scale work identified 20 anomalous envelopes with cumulative scores greater than 5 they correspond to mining areas that are active e g bajo de la alumbrera under development e g agua rica or abandoned e g la mejicana or to new areas e g the vicuna pampa volcanic complex structural analysis of the region integrating the orientation of the favourable envelopes suggests that the mineralizing fluids were emplaced under extensional conditions sub parallel to the principal directions of shortening i wnw ese found along the southern edge of the puna and ii e w seen in the sierra de famatina it appears that a regional scale information system is a tool that is well suited to the definition of areas for mineral prospecting and to the study and confirmation of metallotects usable for mineral exploration comparison with work conducted on the basis of a 1 2 000 000 geological compilation shows that the principal mining districts can indeed be found at continental scale on the other hand the lack of detail inherent at a scale of 1 2 000 000 may lead to inaccuracies in particular fictitious favourabilities assigned to formations that are genetically unrelated to the mineralization but that contain for example small tertiary intrusive bodies that cannot be recorded at this scale this comparison therefore shows that the use of a continental scale gis is effective and well suited to the definition of prospective areas at a strategic level

the nevados de payachata volcanic region 18 s 69 w n chile ii evidence for widespread crustal involvement in andean magmatism volcanism extending over 11 ma is represented in the rocks of the nevados de payachata region culminating in the formation of two large composite stratocones within the last 500 000 years chemically distinct mafic magmas are erupted at a number of parasitic centers these cannot be related to each other by crystal fractionation and do not appear to be direct parents for the differentiated suites of the composite cones two distinct trends are defined by the intermediate and evolved rocks a high lile large ion lithophile element tio2 and ce yb lineage among the youngest rocks including the two major stratocones and a more typical calc alkaline trend among the older 1 ma rock types within individual volcanic centers differentiation involves fractionation of plagioclase pyroxene and hornblende with biotite and k feldspar in the more evolved rock types isotopic compositions sr pb nd o vary little with differentiation from basaltic andesite to rhyolite or with age contamination during differentiation from basalt to rhyolite may occur but the most mafic rocks erupted in the region are already enriched in incompatible trace elements and therefore may be insensitive to the effects of interaction with the crust the majority of data are similar to baseline compositions cenozoic parental magmas from other parts of the central andes and may reflect a relatively homogeneous magma source or source mixture throughout this central volcanic zone cvz which is distinct from the southern and northern andes and from island arc volcanic rocks

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

the relations between faulting and mafic magmatism in the altiplano puna plateau central andes mafic lava flows and scoria cone on the altiplano puna plateau correlate spatially and temporalily with late pliocene quternary strike slip faulting thrust faulting dominated deformation in the plateau and adjacent foreland during the miocene pliocene the relations between faulting and mafic magmatism may be explained by a change from vertical to horizontal least principal compression during the pliocene quaternary a model in which mantle derived mafic magma ascends through the thick less dense continental crust of the altiplano puna plateau suggests that nonlithostatic stress in the crust may have controlled the depth and geometry of intrusion horizontal nonlithostatic compression favored the formation of horizontal mafic intrusions in the lower crust which may have thermally weakened the crust localized crustal thickening and caused crustal anatexis horizontal nonlithostatic tension favored vertical intrusions in the crust and surface eruption

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

oligocene to present shallow subduction beneath the southern puna plateau abstract the southern puna plateau is a conspicuous example of a high elevation orogenic plateau in a non collisional setting this orogenic sector is currently located above an anomalously shallow subduction segment in which timing and relation to upper plate tectonics have been widely overlooked this subduction segment here referred to as the southern puna shallow subduction spss is characterized by a 200 km wide shallow area located at 300 km from the trench at a depth of 100 120 km and dipping 10 12 to the east to determine the onset of the spss and its link to the tectonic and magmatic activity in this region we analyzed the tectonomagmatic record of the southern puna plateau from preexisting datasets also we present a new approach based on global subduction data that provides a straightforward methodology to extract potential paleo slab angles from the bedrock arc record this analysis reveals that a pronounced eastward arc front migration and magmatic broadening took place at 26 ma and was preceded by 4 ma of reduced magmatic activity which we link to the inception of the spss as expected in shallow subduction settings a change to basement cored distributed deformation south of 25 s in the southern puna plateau coincides with the beginning of shallow subduction also the spss is coincident with the enigmatic post eocene intraplate deformation of the otumpa hills located at 950 km from the trench we suggest that this succession of events is not fortuitous and that the development of the spss impacted directly the overriding plate since the oligocene contributing to the building of one of the largest topographies 3 km and thickest orogenic crusts 70 60 km on earth the shallow subduction would have acted jointly with cenozoic changes in plate kinematics and climate enhancing andean orogenesis at studied latitudes

geodynamical evolution of central andes at 24 s as inferred by magma composition along the calama olacapato el toro transversal volcanic belt abstract miocene to recent volcanism on the puna plateau central andes developed in three geological settings a volcanic arc in the western cordillera miocene recent b trans arc along the main nw se transverse fault systems miocene and c back arc mainly monogenic volcanic centres pliocene quaternary we have studied the evolution of the arc trans arc volcanism along one of the most extensive transverse structures of central andes the calama olacapato el toro at 24 s compositional variations from arc to trans arc volcanism provide insights into petrogenesis and magma source regions puntas negras and rincon volcanic centres are arc type and have typical calc alkaline geochemical and sr nd pb isotopic characteristics east of the arc lavas of the tul tul del medio and pocitos complexes tumepo are heavy rare earth element depleted and could be derived from 20 30 of partial melting of a lower crustal garnet bearing metabasite these liquids could be variably mixed with arc magmas at the base of the crust mash this suggests important contributions from lower crustal sources to tumepo centres products at the quevar and aguas calientes volcanic complexes to the east of tumepo show a prominent upper crustal signature high 86sr 87sr low 143nd 144nd and could represent mixtures of 20 30 tumepo type liquids with up to 70 80 of upper crustal melts we propose a geodynamic model to explain geochemical variations for the arc trans arc transverse volcanism from the upper miocene to recent in our model arc volcanism is linked to dehydration of the subducting nazca plate which produces typical calc alkaline compositions during the upper miocene 10 5 ma lithospheric evolution in the puna plateau was dominated by thickening of ductile lower crust and thinning of the lithosphere lower crustal melting was promoted by concomitant asthenospheric upwelling and water release from the amphibolite eclogite transformation yielding tumepo magmas with lower crustal signatures at the same time the eastern sector of the puna plateau experienced westward underthrusting of the brazilian shield and upper crustal brittle deformation partial melts from underthrust upper crustal wedges mixed with lower crustal magmas to produce lavas at aguas calientes and quevar during pliocene to quaternary delamination of the lithosphere and lowermost crust promoted a widespread monogenetic ocean island basalt type and shoshonitic volcanism

implications of quaternary volcanism at cerro tuzgle for crustal and mantle evolution of the puna plateau central andes argentina the high k tuzgle volcanic center 24 s 66 5 w along with several small shoshonitic centers developed along extensional quaternary faults of the el toro lineament on the east central puna plateau 275 km east of the main front of the andean central volcanic zone cvz these magmas formed by complex mixing processes in the mantle and thickened crust 50 km above a 200 km deep scismic zone tuzgle magmas are differentiated from shoshonitic series magmas by their more intraplate like ti group element characteristics lower incompatible element concentrations and lower 87sr 86sr ratios at a given end underlying mio pliocene volcanic rocks erupted in a compressional stress regime and have back arc like calc alkaline chemical characteristics the tuzgle rocks can be divided into two sequences with different mantle precursors a an older more voluminous rhyodacitic ignimbrite to mafic andestitic 56 to 71 sio2 sequence with la yb ratios 35 la yb ratios are controlled by the mafic components low ratios result from larger mantle melt percentages than high ratios shoshonitic series lavas 52 to 62 sio2 contain small percentage melts of more isotopically enriched arc like mantle sources some young tuzgle lavas have a shoshonitic like component variable thermal conditions and complex stress system are required to produce the tuzgle and shoshonitic series magmas in the same vicinity these conditions are consistent with the underlying mantle being in transition from the thick mantle lithosphere which produced rare shoshonitic flows in the altiplano to the thinner mantle lithosphere that produced back are calc alkaline and intraplate type flows in the southern puna substantial upper crustal type contamination in tuzgle lavas is indicated by decreasing end 2 5 to 6 7 with increasing 87sr 86sr 0 7063 to 0 7099 ratios and sio2 concentrations and by negative eu anomalies eu eu 0 78 in lavas that lack plagioclase phenocrysts trace element arguments indicate that the bulk contaminant was more silicic than the tuzgle ignimbrite and left a residue with a high pressure mineralogy crustal shortening processes transported upper crustal contaminants to depths where melting occurred these contaminants mixed with mafic magmas that were fractionating mafic phases at high pressure silicic melts formed at depth by these processes accumulated at a mid to upper crustal discontinuity decollement the tuzgle ignimbrite erupted from this level when melting rates were highest subsequent lavas are mixtures of contaminated mafic magmas and ponded silicic melts feldspar and quartz phenocrysts in the lavas are phenocrysts from the ponded silicic magmas

reply to comment on the relation between ground acceleration and earthquake source parameters theory and observations by itzhak lior and alon ziv by j enrique luco

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

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

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

fluid geochemistry of hydrothermal systems in the arica parinacota tarapaca and antofagasta regions northern chile we investigate the chemical and isotopic composition of water and gas thermal discharges from six hydrothermal systems in the tarapaca and antofagasta regions northern chile surire puchuldiza tuja pampa lirima pampa apacheta el tatio and torta de tocorpuri to determine the chemical physical conditions at the fluid source the chemical facies of the thermal discharges vary from na cl el tatio puchuldiza tuja and part of surire where so42 rich waters also occur to na ca2 cl so42 pampa lirima ca2 so42 hco3 torta de tocorpuri and ca2 so42 pampa apacheta the gas seeps are characterized by the dominance of co2 h2s and ch4 with n2 ar ratios that occasionally are up to 450 significant amounts of so2 and hcl are present at pampa apacheta water and gas geothermometry suggests that el tatio and puchuldiza tuja fluid reservoirs have relatively high reservoir equilibrium temperature up to 270 c gases from pampa apacheta apparently equilibrated at unusually high temperature for hydrothermal fluids up to 350 c being likely related to an active magmatic system as testified by the presence of highly acidic gas compounds such as so2 and hcl on the contrary low equilibrium temperatures were calculated for the surire fluids 200 c suggesting interactions of deep originated fluids with meteoric water at shallow depth shallow low temperature aquifers mask any signal of deep fluids in the pampa lirima and torta de tocorcupi thermal discharges despite the variations in temperatures he isotopic compositions and redox conditions of the fluid reservoirs consistently indicate a significant contribution from magmatic degassing although geophysical studies are necessary to better constrain the geothermal potential of northern chile the geothermometric estimations carried out in the present study indicate that el tatio pampa apacheta surire and puchuldiza tuja can be regarded as the most promising areas and deserve more detailed geological investigations

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

tectonic development southeastern border of the puna plateau northwestern argentine andes the puna altiplano a 500 000 km2 area at an average elevation of 3 7 km in the central andes occurs in a non collisional plate setting over a 30 east dipping segment of the benioff zone and is bounded to the north and south by nearly flat segments of the subducted nazca plate a 120 km long portion of the southeastern boundary of the puna in northwestern argentina was studied to gain insight into the structures associated with uplift of the plateau and into the nature of structures over the transition from flat to steeper subduction the structural geometry and kinematic evolution of the region was determined from geologic mapping and kinematic analysis of minor fault arrays the present southeastern margin of the puna was uplifted between 5 10 and 2 ma northwest southeast horizontal shortening and vertical extension during uplift were produced by northeast striking en echelon thrust faults dipping 17 55 northwest four to seven en echelon structural blocks each 20 30 km long constitute the border of the puna their structural geometry precludes large scale overthrusting as an important mechanism of plateau uplift along the studied segment in the past 2 m y there has been a 45 change in the direction of shortening and locally a 90 change in the extension direction indicating that the structures accompanying uplift are no longer active this change could be due to a change either in the geometry of the subducted plate or in convergence directions although neither of these hypotheses is testable given presently available data