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

neotectonic reactivation of the western section of the malargue fold and thrust belt tromen volcanic plateau southern central andes abstract this study examines the neotectonic deformation and development of the tromen massif a quaternary retroarc volcanic field located in the western section of the malargue fold and thrust belt in the southern central andes the linkages between neotectonic deformation in the intra arc zone and the recent retroarc structures of the tromen volcanic plateau are not clearly understood these retroarc deformations affect the mid section of the fold and thrust belt leaving to the east a 200 km wide deformed zone that can be considered inactive over the last 12 10 ma this out of sequence deformation west of the orogenic front area has not been previously addressed in detail in this study exhaustive mapping is used to describe and discriminate structures with a neotectonic component from those fossilized by pleistocene strata two balanced cross sections are constructed showing the distribution of the youngest deformations and their relation to pre miocene structures an important means for evaluating this is the morphometric and morphological analyses that allowed identification of perturbations in the fluvial network associated with active structures in a broader perspective neotectonic activity in the fold and thrust belt is discussed and inferred to be caused by local mechanical weakening of the retroarc zone due to injection of asthenospheric material evidenced by magnetotelluric surveys thus deformation imposed by the oblique convergence between south american and nazca plates while to the south being limited to the liquine ofqui fault system that runs through the arc zone in the retroarc area is located at the site of magmatic emplacement presumably in association with a thermally weakened upper crust this control exemplifies the relationship that exists between surficial processes magmatic emplacement and upper asthenospheric dynamics in the southern central andes

uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

crustal attenuation in the southern andean retroarc 38 39 30 s determined from tectonic and gravimetric studies the lonco luan asthenospheric anomaly abstract the western retroarc of the southern andes between 38 and 40 s is formed by a nnw elongated ridge not associated with stacked thrust sheets on the contrary during the last 4 3 ma this ridge was affected by extensional deformation regional uplift and related folding on a very broad scale receiver function analysis shows that the drainage divide area and adjacent retroarc lie over an attenuated crust expected crustal thickness at these latitudes is around 38 km whereas in this part of the retroarc the thickness is less than 32 km the causes for such attenuation have been linked to a moderate steepening of the subducted nazca plate beneath the south american plate which is suggested by a westward shift and narrowing of the magmatic arc during the last 4 to 5 ma gravimetric studies show that the upper plate did not react homogeneously to slab steepening but ancient sutures and lithospheric discontinuities deeply buried under mesozoic to cenozoic sequences in the retroarc were locally reactivated these processes resulted in an asthenospheric anomaly that correlates at the surface with the area of pliocene to quaternary doming widespread extension and three radial troughs two of the troughs have accommodated substantial amounts of extension but the third was probably aborted at an early stage moreover the presence of an anomalous concentration of calderas and large volcanic centers over the proposed asthenospheric anomaly and their age distribution may indicate minor migration of the asthenospheric anomaly between 4 and 2 ma through the western south american plate

fluid and melt pathways in the central chilean subduction zone near the 2010 maule earthquake 35 36 s as inferred from magnetotelluric data

fission track analysis reveals character of collisional tectonics in new zealand during the late cenozoic the pacific plate has been obliquely converging with the australia plate in south island new zealand this has resulted in westward obduction of continental crust of the pacific plate and consequently a major mountain range the southern alps has formed east of the alpine fault and within the leading edge of the pacific plate we have applied the apatite and zircon fission track dating technique to a suite of outcrop samples from three transects across the southern alps starting at the alpine fault to better understand the uplift age and structure of the continental collision the results show the following 1 uplift was underway at 7 ma in the south at haast pass and by 5 ma in the north at arthur s pass 2 profiles of the total amount of late cenozoic uplift show an exponential increase toward the alpine fault adjacent to the alpine fault there has been 14 5 to 21 5 km of uplift in the vicinity of haast pass 14 5 to 21 km near mt cook and 12 5 to 15 5 km in the arthur s pass transect adjacent to the alpine fault in the mount cook region the average uplift rate from 5 5 ma to 1 ma was 2 3 to 2 5 mm yr between 1 ma and the present it has averaged 6 2 mm yr the strongly asymmetric uplift pattern is responsible for the exposure of a 13 25 km wide belt of greenschist to midamphibolite facies rocks immediately east of the alpine fault known as the alpine schists 3 late cenozoic uplift of torlesse greywackes and alpine schists was preceeded during the jurassic early cretaceous by 5 8 km of uplift the otago schist belt to the south was uplifted much more during this interval and in addition western parts of the otago schists were uplifted 5 6 km during the late cretaceous 4 the fission track data do not support the notion of significant late cenozoic shear heating about the alpine fault as previously suggested from spatial variations in k ar ages of the otago and alpine schists

cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

the early metamorphic history of the haast schists and related rocks of new zealand rare but widespread relics of sodic amphibole occur in metabasites and metacherts of the haast schists and related caples terrane rocks present main stage metamorphic assemblages are frequently chemically equivalent to earlier sodic amphibole bearing assemblages indicating that these rocks underwent an earlier higher p tmetamorphism prior to formation of the present pumpellyite actinolite and greenschist facies assemblages the earlier assemblages were stable during and after early isoclinal folding but were replaced by the present moderate p t assemblages prior to the last major fabric forming deformation the change in conditions was due to thermal relaxation probably accompanied by uplift and erosion and peak metamorphic temperatures were about 350 370 c in the pumpellyite actinolite zone of the caples terrane and near 390 c in the greenschist facies chlorite zone near queenstown according to henley 1975 these greenschist facies rocks attained a pressure of at least 6 4 0 4 kb during their history but a pressure of 4 6 0 6 kb has been estimated for a chlorite zone rock from middlemarch and so the 6 4 kb estimate probably refers to the maximum pressure attained during the earlier higher p t metamorphism similar changes in metamorphic facies series with time occur in some older and more complex metamorphic belts such as the caledonides and this study suggests that it may be possible to interpret particular elements in the metamorphic development of such belts in terms of specific circum pacific analogues

fission track lengths in the apatite annealing zone and the interpretation of mixed ages abstract measurements of confined fission tracks in apatites from deep boreholes show that their mean length is reduced and the length distribution becomes progressively broader through the fission track annealing zone at subsurface temperatures of around 100 c and above the length distribution characteristically becomes very broad without a pronounced peak the lengths of the longest tracks remain essentially constant at all stages of natural annealing observed in the boreholes this pattern is similar to that found in laboratory annealing of spontaneous fission tracks in apatite from an outcrop sample of the same formation these observations show that confined track lengths can be used as important indicators of the type of thermal history that a sample has experienced in the temperature zone of increasing track stability the length distributions reported here provide a basis for interpretation of fission track ages that might otherwise be ambiguous an apatite age which results from a uniform slow cooling history will have a broad negatively skewed length distribution while a bimodal distribution provides clear evidence of a two stage history involving partial annealing by a discrete thermal event the apparent fission track age associated with a bimodal length distribution will be a mixed age intermediate between the original age and that of the later event

a magnetotelluric study in the southern chilean andes long period magnetotelluric investigations were carried out along two transects traversing the southern chilean andes from the pacific ocean until the argentinian border near latitude 39 s to study the mid and deep crustal distribution of electrical conductivity dimensionality analysis showed that the mt data may be regarded as 2 d with the exception of areas in the longitudinal valley and the region east of the volcanic chain a subsequent multi site decomposition of magnetotelluric impedances yielded approx n s regional strike directions for most parts of the two profiles first 2 d inversion revealed a zone of moderately high conductivities approx 0 1 s m in the deep crust below the active volcanic arc extending eastwards to the argentinian border inversion results of both profiles are very similar except of a good mid crustal conductor in the longitudinal valley which only appears in the north and may image a fault zone running obliquely to the main south andean structures

plume overriding triggers shallow subduction and orogeny in the southern central andes abstract plate tectonic theory implies that mantle plumes may be eventually overridden by lithosphere during continental drift these events have particular tectonomagmatic consequences for active margins and hence related orogenic processes since the first documentation of plume overriding and the definition of the plume modified orogeny concept only few examples have been recognized in the geologic record in this study we analyze the neogene tectonic evolution of the southern central andes between 35 and 38 s and its potential relation to the subduction of the payenia plume as a recent analogue of this process through a series of tectonic reconstructions we show that progressive payenia plume overriding correlates with neogene arc front migrations linked to slab shallowing fold belt reactivation in the main cordillera and intraplate contraction in the san rafael block additionally nazca slab tear determined from tomographic analyses and subsequent diachronous steepening of the subducted plate may also be an aftermath of plume subduction as often described in the final stages of plume modified orogeny finally we propose a modern analogue for processes previously described dating back to the mesoproterozoic which provides further insights into these complex settings

evidence of quaternary tectonics along rio grande valley southern malargue fold and thrust belt mendoza argentina abstract the malargue fold and thrust belt is developed in the argentinian andes between 34 and 37 s through the tectonic inversion of upper triassic and lower jurassic depocenters of the neuquen basin with an uplift history since the cretaceous evidence of quaternary deformation has been described in the northern part of it 34 34 5 s potentially coeval to neotectonic activity along the eastern edge of san rafael block to the south compressional and extensional structures active during the quaternary were found in the dorso de los chihuidos along the agrio fold and thrust belt front 37 5 38 s contrastingly the southern segment of the malargue fold and thrust belt between these two areas with described neotectonic activity is partially covered by quaternary products of the payun matru volcanic field that may hide evidence of recent deformation in this 300 km gap of neotectonic information the landscape imprint of two individual structures aligned in the mountain front through the rio grande valley was analyzed new evidence of neotectonic deformation were recognized in particular over the western slope of the cara cura range expressed by faulting and folding of quaternary deposits and lava flows an 40ar 39ar age from a deformed lava flow at the flanks of an anticline in the foothills of the cara cura range may suggest at least an upper pleistocene compressional tectonic activity longitudinal river profile analysis revealed anomalies that show some correlation with the neotectonic structures described especially knickpoints and concavity index changes meanwhile normalized steepness index values showed a moderate response to recent deformation a proposed schematic geomorphic evolution for this segment of rio grande river is discussed to put the neotectonic activity into the context of landscape formation all together this evidence supports the idea of an active front through the rio grande valley during the quaternary coetaneous to an active broken foreland to the east in the southern central andes

fission track analysis of the late cenozoic vertical kinematics of continental pacific crust south island new zealand during the late cenozoic the pacific plate has been converging obliquely with the australia plate in south island new zealand a result of this convergence has been the growth of a major mountain range the southern alps at the leading edge of the pacific plate the results of fission track analysis of 140 samples from 13 transects across the alps reported here establish the late cenozoic vertical kinematics amount age and rate of rock uplift of the pacific crust underlying the alps the late cenozoic rock uplift of the pacific crust is asymmetrical with respect to the alpine fault being a maximum 19 km immediately east of the central part of the fault with lesser values at the eastern 3 km northern 10 km and southern 8 km extremities of the alps the age of the start of rock uplift varies spatially across the southern alps the earliest indications from fission track analysis being at 8 ma at the southern end of the alps decreasing to 5 ma at the northern end and 3 ma along the southeastern margin this age variation reflects the longer time over which the southern parts of the alps have been in collision the rate of propagation of rock uplift southeastward into the pacific plate has been 30 mm yr nearly 4 times the late cenozoic average rate of convergence normal to the plate boundary late cenozoic mean rock uplift rates range from a maximum of 2 8 mm yr at the alpine fault to a minimum of 1 0 mm yr in the east and have been sustained for periods of 3 8 m y accompanying denudation has exhumed amphibolite grade rocks immediately east of the alpine fault the rock uplift has been controlled by oblique slip displacement on the alpine fault a continental crustal section at least 19 km thick has been uplifted on the alpine fault comparison of the late cenozoic mean rock uplift rates with uplift rates derived from reset zircon data 2 10 mm yr near the alpine fault shows that uplift has accelerated over time but only significantly since 1 3 0 3 ma the amount of mesozoic uplift ranged from minimal amounts north of arthur s pass to 3 km near mount cook to 10 km in the south at lake wanaka

the neocomian of chachahuen mendoza argentina evidence of a broken foreland associated with the payenia flat slab fil sagripanti lucia 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 argentina

the loma seca tuff and the calabozos caldera a major ash flow and caldera complex in the southern andes of central chile a 26 14 km composite ring structure caldera of late pleistocene age has been discovered and mapped near the andean crest in central chile 35 30 s rhyodacitic to dacitic zoned ash flow sheets each representing 150 to 300 km 3 of magma were emplaced 0 8 0 3 and 0 15 m y ago the youngest of the associated collapses was closely followed by resurgent doming of the caldera floor and development of a longitudinal graben postcaldera eruptions of dacite and andesite have persisted into holocene time and active hot springs are abundant along caldera marginal and resurgent fault systems suggesting a significant geothermal energy resource the pleistocene eruption rate of this district and the abundance of older quaternary to miocene ash flow remnants in the 33 s to 36 s segment of the glaciated southern andes indicate that ash flow magmatism has been no less important here than in the arid central andes 16 s 28 s where ash flow sheets are far better preserved

inverse modelling of the reversely magnetized shallow plumbing system hosting oil reservoirs of the auca mahuida volcano payeina retroarc neuquen basin argentina

the southern andes between 36 and 40 s latitude seismicity and average seismic velocities abstract the project issa 2000 integrated seismological experiment in the southern andes consists of a temporary seismological network and a seismic refraction profile a network of 62 seismological stations was deployed across the southern andes at 38 s three hundred thirty three local seismic events were observed in a 3 month period p and s arrival times of a subset of high quality data were inverted simultaneously for 1 d velocity structure hypocentral coordinates and station delays seismic refraction data along a transect at 39 s provide further constraints on the crustal structure low crustal velocities beneath the forearc may be either due to subducted trench sediments or serpentinized mantle material of the continental lithosphere the continental moho is not clearly observed in this region average velocities of the crust beneath the arc are higher than those beneath the forearc crustal thickness is about 40 km crustal seismicity concentrates in the forearc region along the bio bio and gastre fault zones the area between these two prominent fault zones seems to be nearly devoid of crustal seismicity but shows highest uplift and topography in the forearc region benioff seismicity is observed down to 150 km depth resulting in the first accurate image of the benioff zone in the southern andes a maximum of seismicity at 60 km depth may be caused by dehydration embrittlement

analysis of the aeromagnetic anomalies of the auca mahuida volcano patagonia argentina we present the analysis of the subsurface structure of the auca mahuida volcano based on high resolution aeromagnetic data integrated with the available geological information most of the detected magnetic anomalies have a dipolar structure opposite to that of the present geomagnetic field according to the available geochronological data and paleomagnetic measurements the source bodies of auca mahuida mainly emplaced in the matuyama reverse polarity chron the reduction to the pole map confirms that the magnetization direction is mainly reverse with only few anomalies normally magnetized two opposite coexisting polarities do not allow to fully remove the dipolar character of the field in the reduction to the pole transformation therefore we model the measured anomaly field by applying analytical techniques that are independent of the magnetization direction the obtained anomaly strikes and source geometries indicate an emplacement of intrusive bodies controlled by the regional faults affecting the auca mahuida basement and the sedimentary successions of the neuquen basin magma upraised along these faults and fractures feeding the volcanic activity and subsequently crystallized the averaged power spectrum and euler deconvolution indicate source depths consistent with those of the intrusions recognized in wells borehole data highlight the widespread presence of intrusive bodies below the auca mahuida central crater and the peripheral sectors at depth of 2 km below sea level these bodies have played a major role in the thermal maturation of hydrocarbons and in the subsequent accumulation of oil below the volcano the obtained results shed light on the auca mahuida feeding system and on the intrusions geometry also pointing out the effectiveness of the magnetic prospecting in the oil industry even in presence of strong remanent magnetization

three dimensional mantle circulations and lateral slab deformation in the southern chilean subduction zone the along strike variation in plate properties is a common feature in the subduction zones the southern chilean subduction is a notable example where the slab age and length and the thickness of the overriding plate vary substantially in the trench parallel direction in this study the combined effects of these factors are examined using regional subduction zone models in the models lateral slab deformation and along arc pressure gradient in the mantle occur in response to the differential slab retrograde motion due to variations in slab buoyancy and slab strength the lower portion of the deep segments acts as a slab edge in part while rollback of the neighboring shallower segments influences the toroidal motion induced by the adjacent deep segments these factors together give rise to complex flow patterns and lead to a great extent of trench parallel components in the mantle wedge and subslab mantle and significant upwelling in the back arc mantle circulations are characterized by variable length scales for both toroidal and poloidal motions extending over 1500 km such three dimensional circulations may lead to complicated patterns of seismic anisotropy the upwelling may result in decompression melting to cause the extensive patagonian plateau basalts our model results indicate that certain segments of the antarctic slab reach at least 100 km depth the trajectories of passive tracers show intricate patterns such as helical streamlines and suggest that a recent input from the adjacent subduction zone for slab derived components of lavas from the chilean ridge cannot be ruled out

backstacking apatite fission track stratigraphy a method for resolving the erosional and isostatic rebound components of tectonic uplift histories measuring tectonic uplift of the mean surface of earth using thermochronological data to document uplift histories is often plagued by confusion arising from the independent nature of the thermal and sea level frames of reference the technique of backstacking apatite fission track stratigraphy differentiates between vertical movement of rocks toward earth9s surface due to erosional or tectonic denudation and tectonic uplift of earth9s surface apatite fission track analysis can also be used to identify episodes of erosion where no other evidence for the erosional event exists in addition to providing information about the timing magnitude and geographic distribution of erosion

an inverse method of modeling thermal histories from apatite fission track data abstract apatite fission track ft ages and track length distributions are important sources of information about the thermal histories of rocks recent advances in the understanding of track annealing in apatite provide a solution to the forward problem of predicting ft ages and track length distributions that result from a given thermal history in this paper a method is presented to address the inverse problem estimation of the thermal history from ft data the inverse method uses an iterative approach the downhill simplex to systematically modify a starting thermal history to achieve satisfactory statistical agreement between the predicted and observed ft age and track length distribution however because of analytical uncertainties a unique thermal history does not exist monte carlo simulations are used to take into account the uncertainties in the data and yield a spectrum of possible thermal histories the results are based on the isothermal annealing model of laslett et al 1 chem geol isot geosci sect vol 65 but because the method uses forward predictions and not an analytical formula specific to a single annealing model alternative annealing models could be used within the same framework this method is an improvement in interpreting fission track data because the range of thermal histories permitted by the data can be evaluated to demonstrate the method it is applied to several sets of test data created by forward modeling of idealized thermal histories the results show that the age at which the sample cooled through its closure temperature and the most general features of the thermal history are revealed however data from partially annealed samples have wide spectra that reflect large uncertainties in the form of the thermal history for such samples ft data alone are not likely to provide strong tests of geologic hypotheses conversely geologic information may play a crucial role in constraining the thermal history spectrum in application to uplift related cooling this inverse method can provide much more realistic assessments of the cooling rate and its uncertainty than are otherwise possible

u th age evidence from carbonate veins for episodic crustal deformation of central anatolian volcanic province abstract central anatolia represents one of the most outstanding examples of intraplate deformation related to both continental collision and back arc extension generating non uniformly distributed stress fields in this study we provide direct field evidence of various stress directions and investigate carbonate filled fracture systems in the central anatolian volcanic province using u th geochronology and isotope geochemistry for evaluating the episodes of latest volcanic activity under regional stress field data reveal two independent fracture systems in the region successive fracture development has been controlled by two different volcanic eruption centers hasandag composite volcano and acigol caldera trace element and stable c and o and radiogenic sr isotope compositions of carbonate veins indicate different fluid migration pathways for two different fracture systems the u th age data for carbonate veins of two independent fracture systems indicate that the crustal deformation intensified during 7 episodic periods in the last 150 ka the nne trending first fracture system was formed as a result of strain cycles in a period from 149 2 5 through 91 1 5 to 83 2 5 ka bp subsequent deformation events represented by the ene trending second fracture zone have been triggered during the period of 53 3 5 44 0 6 and 34 1 ka bp before the first fracture zone resumed the activity at about 4 7 0 15 ka bp although further studies are needed to evaluate statistical significance of age correlations the periods of carbonate precipitation inferred from u th age distributions in this study are comparable with the previous dating results of surrounding volcanic eruption events

eocene arc petrogenesis in central chile c 33 6 s and implications for the late cretaceous miocene andean setting tracking the evolving tectonic regime the scarce eocene geological record in the andean region of central chile c 33 6 s is represented by the estratos del cordon de los ratones unit this is a c 450 m thick mainly volcanic continental series with a bimodal character that hosts numerous subvolcanic intrusions the igneous products show compositional features indicating a co genetic character for the whole suite relatively juvenile mantle signatures and arc like affinities this along with age determinations of c 42 ma indicate that the unit corresponds to the middle eocene arc in the region the geochemical data obtained complement the evolving arc magmatic record throughout andean evolution this allows us to track 1 the earliest late cretaceous orogenic event 2 neutral tectonic conditions from the latest cretaceous to mid eocene times 3 the oligocene early miocene extensional episode and 4 the onset of the prevailing compression the record also indicates that the extensional event started no earlier than the mid eocene with the deposition of the estratos del cordon de los ratones and no later than the earliest oligocene in addition orogenic processes developed from early miocene times are highlighted relative to those developed during the early late cretaceous at least in the arc region supplementary material analytical methods item 1 the full dataset of u pb age determinations by shrimp item 2 and la icp ms item 3 and additional geochronological diagrams item 4 are available at https doi org 10 6084 m9 figshare c 4710305

potassium argon age studies of a low grade progressively metamorphosed greywacke sequence dansey pass south island new zealand 39 potassium argon age determinations from a progressively metamorphosed greywacke schist sequence in southern new zealand prehnite pumpellyite to greenschist facies range from 199 to 154 ma early to late jurassic they show an inverse correlation of age with metamorphic grade concordant ages from varied lithologies at all metamorphic grades show that degassing of detrital potassium bearing minerals was completed during metamorphism the lowest grade metamorphic rocks which yield the oldest ages about 200 ma have only just exceeded the argon closure temperature and hence their ages date closely their time of metamorphism in the earliest jurassic thus it is shown that the jurassic cretaceous rangitata orogeny in new zealand commenced earlier is more complex and of longer duration than previously supposed the remaining ages from pumpellyite actinolite and lower greenschist rocks range uniformly through the jurassic and are interpreted as cooling ages from these data a long term uplift rate of 0 23 mm yr is calculated for the jurassic cretaceous

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

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

paleomagnetic directions from mid latitude sites in the southern hemisphere argentina contribution to time averaged field models back arc volcanism located to the east of the andean cordillera was sampled in the argentina provinces of mendoza and neuquen for paleomagnetic time average field and paleosecular investigations the activity ranges from 2 ma to very recent time with a large variety of products from basalts to highly differentiated lavas after removal of sites affected by lightning those with 95 higher than 10 and combining of nearby sites displaying close directions we present new paleomagnetic results from 31 flows units belonging to two volcanic massifs the payun matru and the cerro nevado previous and new k ar age determinations constrain the volcanic activity of these massifs from 300 to 0 ka and from 1 9 to 0 9 ma respectively most paleomagnetic samples have nrm intensities between about 1 and 20 a m and depict progressive removal of magnetization components in a consistent fashion during stepwise af or thermal demagnetization nineteen flows yielded a normal direction declination 354 8 inclination 53 0 95 6 8 and 12 flows a reverse direction declination 181 0 inclination 52 3 95 5 9 the combined data yielded a mean direction declination 357 3 inclination 52 8 95 4 6 which is not statistically different from the axial dipole field g10 expected at this latitude 36 s the angular dispersion of virtual geomagnetic poles calculated from flows with normal directions asd 16 5 compares well with the observed value from global datasets for this site latitude but flows with reverse directions display a surprisingly low dispersion asd 12 5 since most reverse directions were sampled from flows ranging between 1 9 and 0 9 ma this can be interpreted as an interval of low paleomagnetic secular variation additional data also with accurate time constraints are obviously needed to better support this observation finally no convincing evidence for a complex time average field significantly different from the axial dipole can be supported by this study for the last 2 myr

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

shear heating associated with movement along the alpine fault new zealand abstract geothermometry based on the fe mg partitioning between biotite and garnet has revealed a temperature increase of circa 100 c associated with the development of schist derived mylonites that presently crop out within the alpine fault zone in new zealand calculations show that the temperature increase is compatible with a short shear heating event 1 5 10 5 years it has been tentatively associated with a possible high stress deformation 1 2 kbar which occurred when the then existent mylonites were unable to accommodate imposed strain rates because of the misorientation of their fabric elements during the change in the relative slip vectors between the pacific and australian plates in the late miocene early pliocene the high stress deformation resulted in the present mylonites which have their fabric elements orientated to accommodate the present oblique reverse slip movement between the two plates

el pozo volcanic complex evolution of a group of maars central mendoza province argentina abstract the el pozo volcanic complex in the south central mendoza province is formed by three maars the first to be described in the payenia volcanic province and one of the few examples in argentina the maars of el pozo volcanic complex 1 2 and 3 show large craters 0 97 km 1 2 km and 0 96 km respectively and their depth is between 10 and 57 m below the pre eruptive surface their deposits are described from the only outcrops that have been found called el pajarito and el visitante it was interpreted that the first maar to be formed would have been no 2 from which only the deposits corresponding to el pajarito 3 6 m and the lower section of el visitante 28 m are preserved we suggest that its shape is the result of lateral vent migration and coalescence of successive vents then after a short break represented by the angular unconformity in the el visitante section the active vent would have shifted more to the w and the maar no 3 is thought to have been formed the activity of this maar is represented in el visitante upper section with features resulting from the presence of water in the time of deposition maar no 1 was probably active simultaneously with maar no 3 with which coalesces but the lack of outcrops does not guarantee this affirmation towards the end of the eruptive cycle depletion of water produced a change in activity from phreatomagmatic to strombolian and effusive this gave rise to the nested pyroclastic scoria cone el pozo cone which lies in the maars 1 and 3 craters floors el pozo cone is related to lava flows with a tumulus and the deposit of a lapilli bearing tephra plume

crustal and mantle structure beneath the southern payenia volcanic province using gravity and magnetic data the auca mahuida volcanic field lies on the southernmost payenia volcanic province one of the broadest retroarc volcanic plateaux in the southern central andes 38 s this voluminous basaltic flooding of quaternary age was originated from a deep asthenospheric source interpreted as a mantle plume product of changing slab dynamics the geometry of this source is deduced from magnetotelluric data but the limited spatial coverage of this array does not allow a detailed resolution of this anomaly in order to present a detailed geometry of the conductive anomaly and related crustal magmatic bodies we used multiple data sources we combined magnetic and bouguer anomalies curie isotherm depth t c elastic thickness t e and moho depth derived from the global earth magnetic anomaly grid emag2 and terrestrial gravity measurements all together in a holistic geophysical analysis the magnetic data depict a nearly 200 km in diameter circular anomaly that would correspond to a dense body according to the bouguer anomaly geoid data from the gravity field model eigen 6c4 have been filtered in order to isolate deeper mass influences and visualize the asthenospheric upwelling previously described from magnetotelluric data moho inversion yields a crustal attenuation at 36 to 32 km depth coinciding with t e below 20 km depth and a shallow t c 15 km depth at the site where geoid positive undulation was calculated finally surface analysis allowed defining a topographic swell compatible with the dimensions of the identified magnetic anomaly where the main rivers deviated potentially due to a recent base level change 2018 american geophysical union all rights reserved

payenia volcanic province in the southern andes an appraisal of an exceptional quaternary tectonic setting abstract the southern volcanic zone of the andes has a quaternary basaltic province along the retroarc which has a unique tectonic setting the payenia volcanic province covers an area larger than 40 000 km2 between 33 30 and 38 south latitudes with an estimated volcanic volume of about 8387 km3 erupted through more than 800 volcanic centers in the last 2 ma the mainly basaltic province developed above the san rafael block is subdivided in three segments characterized by the cerro nevado llancanelo payun matru tromen and auca mahuida volcanic fields together with hundreds of minor monogenetic basaltic centers the analysis of the different segments shows the formation of a common basalt plateau with intraplate signature from south to north between 2 0 and 1 7 ma which reached the 35 s to the north above this plateau monogenetic centers as nihuil vn 1 433 ma and cerro chato at 1 352 ma are developed followed by the large polygenetic center of cerro nevado 3980 m a s l at 1 320 ma this plateau was broken by a series of normal faults that produced volcanic cone alignments such as the nnw trending mancha jarilla lineament in the central part at about 1 0 ma extension shifted to the eastern margin of the san rafael block which concentrates tens of monogenetic centers between 0 9 and 0 7 ma extension then migrated towards the foothills in the west where many monogenetic cones were erupted through nw trending normal faults between 0 5 and 0 435 ma the collapse of the large diamante caldera at 0 445 ma coincides with that period subsequent volcanism was concentrated in 1 the payun matru volcanic field with the eruption of cerro payen between 0 272 and 0 261 ma the payun matru shield volcano with polygenetic eruptions at least since the last 0 233 ma and with the caldera formation bracketed between 0 168 0 004 ma and 0 082 0 001 ma followed by several eruptions until 7000 yrs and even historical ones and in 2 the tromen volcano where younger than 0 2 ma eruptions took place and historical eruptions were reported the understanding of these eruptions in time and space combined with geophysical data indicates the geometry of an important crustal attenuation beneath payenia associated with a hot sublithosphere the late miocene uplifted san rafael block collapsed in the early pleistocene as a consequence of the steepening of the subducted slab and the injection of hot asthenosphere produced the quaternary payenia volcanic province melts of the lower crust along the principal cordillera at these latitudes are responsible for the quaternary calderas ignimbritic flows and rhyolitic volcanism that express the crustal delamination of the andes the payun matru volcanic field concentrates this asthenospheric flow in the present

cooling history of the nw himalaya pakistan fission track and 40ar 39ar cooling ages indicate that the late tertiary cooling history of the himalayan ranges of northern pakistan is largely a function of uplift and erosion interpretation of cooling ages which range from under 0 5 ma to over 80 ma suggests that during the late tertiary long term uplift rates at least doubled from under 0 2 mm yr to in some cases well over 0 5 mm yr uplift rates show strong and systematic regional variations as well which reflect the greater uplift of eastern and northern regions the association of very rapid uplift and erosion with the nanga parbat haramosh massif can be explained by a locally vigorous collision of india with eurasia near a promontory of indian crust the resultant rapid uplift of the nanga parbat haramosh massif reactivated the main mantle thrust melange zone with a reversed sense of motion discontinuities in the coolingage distribution along the main mantle thrust in the southern swat hazara region may be the result of the thermal effects of overthrusting

lithospheric and upper mantle structure of central chile and argentina summary the tectonics of central chile and argentina have been greatly affected by the shallow dips of the subducting nazca plate which controlled patterns of magmatism and deformation nearly 1000 km away from the plate boundary we calculate receiver functions from data recorded by the charge array which transected the andes and sierras pampeanas in central chile and argentina to better constrain the crustal structure of this region beneath the northern transect of the charge array where the nazca slab flattens near 100 km we find the crust is over 60 km thick beneath the andes and thin to the east the thick crust however extends 200 km to the east of the high elevations estimates of vp vs obtained from receiver functions vary along ancient terrane boundaries exhibiting higher values to the west interestingly we observe that the amplitude of the phase corresponding to the moho on receiver functions diminishes to the west complicating our images of crustal structure we proposed that the observations presented here of thickened crust within a region of low elevations diminished receiver function arrivals and reports of high shear wave speeds atop of the mantle wedge overlying the shallowly subducted nazca slab can be explained by partial eclogitization of the lower crust the moho appears simpler across the southern transect where it can be identified near 50 km depth at its deepest point beneath the andes and shallows eastwards volcanism remains active near the latitudes of our southern transect and we observe multiple crustal lowvelocity zones indicative of regions of partial melt near the centres of volcanism signals related to the nazca slab remain more elusive suggestive of a small impedance contrast between the slab and overlying mantle

geochemical constraints on the relationship between the miocene pliocene volcanism and tectonics in the palaoco and fortunoso volcanic fields mendoza region argentina new insights from 40ar 39ar dating sr nd pb isotopes and trace elements abstract new 40 ar 39 ar analyses constrain the formation of the volcanic succession of sierra de palaoco in the present back arc of the andean southern volcanic zone svz near 36 s to the late miocene and assigns them to the huincan ii formation the composition of major and trace elements sr nd and pb isotopes of the palaoco and nearby rio grande rocks require a strong arc like component in the mantle that is absent or weak in both early miocene fortunoso group and pleistocene alkaline lavas llancanelo group erupted in the same area we evaluate the relative roles of varying mantle source compositions and crustal contamination in the generation of geochemically very different lavas from the palaoco fortunoso and rio grande volcanic fields north of the payun matru volcano the source for the early miocene fortunoso i basalts was a oib type mantle devoid of subduction zone input this type of oib like volcanic activity terminated due to a change from an extensional to a compressional tectonic regime towards the end of the miocene renewed alkaline volcanism at fortunoso ii display a transition to arc type incompatible element enrichment shortly after the calc alkaline palaoco volcanism started with a very strong geochemical arc signature including ba la 60 and la nb 2 3 after a quiesence of 1 ma the major part of the voluminous late palaoco basalts were erupted around 7 5 ma over a few hundred ka these are less enriched in ba and sr and have compositions like many holocene rocks of the southern volcanic zone isotopically the fortunoso i and palaoco rocks are distinct regional volcanism of the charilehue huincan i and ii mostly has a moderate arc type enrichment indicating incipient arc developments however palaoco and la brea at c 35 s show full geochemical arc signature and we infer that a frontal arc was established the subsequent development in the palaoco rio grande area encompasses renewed late pliocene calc alkaline low volume volcanic eruptions rio grande group succeeded in the late pleistocene by alkaline oib type eruptions llancanelo group in the light of the course of volcanism to the east in the nevado area where late miocene pliocene calc alkaline volcanism was followed by late pliocene pleistocene alkaline volcanism we propose a scenario where the nazca plate developed an eastwards widening flat slab from which the east dipping slab before the late pliocene translated from palaoco to nevado and subsequently retreated passing rio grande in the late pliocene alkaline back arc volcanism was active east of the arc volcanism and expanded westwards during the late pliocene and pleistocene

hawaiian and strombolian style monogenetic volcanism in the extra andean domain of central west argentina abstract in the provinces of mendoza and la pampa central western argentina retroarc volcanism was active from the miocene through historic times there are approximately 40 basaltic eruptive centers between 36 s and 37 30 s to the east of 68 40 w these centers were studied in order to define their volcanological evolution the vents are constituted of seven lithofacies i e a 1 weakly welded lapilli and bomb beds a 2 agglutinated spatter and bomb beds a 3 agglutinated spatter beds b ash beds c lava flows d conduit fill and e dikes three eruptive episodes phases were identified the first one generated agglutinated spatter and bomb beds lithofacies a 2 which constitute the internal part of the edifices these are followed by weakly welded lapilli and bomb beds lithofacies a 1 and agglutinated spatter beds lithofacies a 3 the final episode includes lava flows lithofacies c minor agglutinated spatter deposits lithofacies a 3 and isolated lapilli and bomb clasts we postulate hawaiian style fountaining activity as responsible for the generation of agglutinated spatter and bombs beds and variations between strombolian and hawaiian activity for the formation of successions of weakly welded lapilli and bomb beds and agglutinated spatter beds the vent features and the homogeneity of the erupted magma suggest that the studied volcanoes were monogenetic

crustal model of the southern central andes derived from ambient seismic noise rayleigh wave tomography abstract the distribution and the interconnection of magmatic bodies beneath volcanic arcs is key to asses volcanic hazard and eruptive processes taking place at convergent margins we use ambient seismic noise rayleigh wave tomography to investigate the three dimensional shear wave crustal velocity structure of the southern central andes between latitudes 33 s and 38 s for the upper 30 km we investigate the occurrence of magmatic reservoirs in the upper crust and show how their geometry is affected by local tectonics the first prominent feature to be observed is the shear wave velocity contrast between the volcanic arc and the faster fore arc region we further identify areas of low shear wave velocity from 3 km to 10 km depth beneath the volcanic arc striking n s that we interpret as zones of fluid rich crust possibly characterised by confined regions of partial melts our results allow us to derive a model for the crustal structure of the southern central andes we propose that partial melts marked by lower shear wave velocity anomalies are connected to shallower structural levels and reservoirs by brittle regions where dikes and exsolved fluids may propagate

age and origin of the new zealand orocline in relation to alpine fault movement the alpine fault and the recurved arc structures orocline in the late palaeozoic mesozoic rangitata orogen are two of the first order structures of the new zealand subcontinent a commonly held view is that the orocline originated as a gigantic drag fold with alpine fault movement in view of the post oligocene age of the alpine fault however this concept involving a single recurved arc cannot reconcile the two published sets of palaeomagnetic data on tectonic rotations within the rangitata orogen and its cover rocks these data indicate at least 62 of dextral rotation during each of the mesozoic rangitata orogeny and the late cenozoic kaikoura orogeny a new model presented here postulates that there are actually two dextral recurved arcs a western arc which lies mainly outside the australia pacific plate boundary zone has a multiple origin having formed chiefly during the late triassic phase of the rangitata orogeny by the subduction accretion of the exotic rakaia terrane an eastern arc coincides with the area of the present plate boundary zone it formed as a mega brittle ductile shear zone with the late cenozoic development of the modern australia pacific plate boundary which includes the alpine fault this solves the problem of the nature and location north of the alpine fault of the relative plate movement evident as 480 km of displacement on the alpine fault a major implication of this model for plate tectonic reconstruc tions of the sw pacific is that only approximately 500 km of dextral displacement has occurred through new zealand during the late cretaceous cenozoic compared with the commonly adopted figure of 1000 km

thermal evolution of rifted continental margins new evidence from fission tracks in basement apatites from southeastern australia the palaeozoic granitic basement of southeastern australia shows a regular pattern of apatite fission track ages which is dominated by a rapid decrease in apparent age towards the rifted continental margin apparent ages away from the margin vary between about 230 and 360 myr but drop sharply to minimum values of around 80 myr near the coast just south of the sydney basin and 150 175 myr elsewhere along the coast fission track length information allows the age pattern to be interpreted as the result of partial to complete resetting of the older ages by a widespread thermal event at about 80 100 myr this event accompanied the rift phase of passive margin development and peaked just before the onset of sea floor spreading in the adjacent tasman sea the zone of reduced apatite ages is deeply embayed around the gippsland basin supporting the suggestion that it originated as a cretaceous failed arm of the tasman rift system the thermal event has affected apatites up to about 130 km from the margin and has produced a characteristic set of bimodal track length distributions in apatites with partially reset ages the bimodality gradually disappears towards the oldest apatite ages which have negatively skewed unimodal distributions the youngest apatites also tend towards unimodal length distributions indicating that thermal annealing of pre existing tracks in these samples went almost to completion maximum temperatures reached in samples presently exposed in the coastal area were thus probably about 110 120 c thermal activity was accompanied by considerable uplift across the region followed by gradual erosion to expose the present land surface post breakup erosion was generally greater towards the new coastline with near coastal samples being previously buried a probable 1 5 2 5 km

crustal collapse in the andean backarc since 2 ma tromen volcanic plateau southern central andes 36 40 37 30 s abstract analysis of seismic lines and gravity data shows the existence of quaternary extensional depocenters beneath thick covers of

continental underthrusting beneath the southern alps of new zealand both the inferred thermal regime and the variation in crustal thickness beneath the southern alps suggest that the mode of crustal shortening changes substantially along the length of the alpine fault north of mt cook crustal shortening is being accommodated predominantly by uplift and erosion of pacific plate continental crust as postulated in the wellman adams tectonic model however south of mt cook the absence of high heat flow and a large crustal root that is not in isostatic equilibrium may have resulted from underthrusting of as much as 100 km of indian plate continental crust

retroarc volcanism in the northern san rafael block 34 35 30 s southern central andes occurrence age and tectonic setting article i nfo one of the major retroarc volcanic provinces in the southern central andes 34 and 37 s is developed in the andean foothills of the san rafael region between the orogenic front and foreland basement uplifts of late miocene age here we present the first comprehensive geochronological study of the quaternary volcanism previously dated mainly on the basis of stratigraphy the new unspiked k ar radiometric and two radiocarbon determinations encompass many volcanic centers most of them monogenetic and of basaltic composition exposed between 34 and 35 30 s the data constrains the basaltic volcanism to between 1 8 ma and the holocene the spatiotemporal distribution of the ages indicates that eruption in the retroarc was episodic with some distinct patterns the orogenic front of the san rafael block is associated with 1 8 0 7 ma volcanic eruptions while the malargue fold and thrust belt front in the andean foothills is related to younger eruptions produced at 0 1 0 01 ma both areas are associated with late cenozoic normal faults that dismembered an uplifted a late miocene peneplain as indicated by younger over older fault relationships between paleozoic rocks and tertiary strata this linkage indicates a major relationship between pleistocene holocene retroarc eruptions of the basaltic centers and extensional collapse of the foreland region that shows a migration of the last volcanic activity towards the trench

dynamics of pacific plate crust in the south island new zealand zone of oblique continent continent convergence here we analyze topographic and fission track data to quantify the response of the surface of the pacific plate in south island new zealand to late cenozoic oblique continental convergence across the alpine fault over the central 350 km length of the southern alps mountain chain we derive and map the rates of mean surface uplift the rates of working associated with mean surface uplift during the late cenozoic mountain building the amounts and rates of denudation and consequent isostatic rebound and the tectonic component of rock uplift the rate of mean surface uplift ranges from 0 3 mm yr over most of the area east of the main divide the highest rates of mean surface uplift occur to the southeast of the regions of highest mean elevation and relief the rate of working against gravity during uplift of the mean surface ranges from 2 5 mw m 2 in the southwest to 10 mw m 2in the central eastern parts of the alps areas of lower mean elevation uplifted most recently have received rates of energy input similar to that of areas of higher mean elevation where uplift started earlier the amount of denudation is large compared with the mean surface uplift and ranges from 18 km adjacent to the alpine fault to 2 km along the southeast margin of the southern alps the rate of denudation ranges from 2 5 to 0 5 mm yr with increasing distance from the alpine fault across the alps to the southeast the amount of isostatic uplift ranges from a maximum of 14 km adjacent to the alpine fault to 2 km along the southeast margin of the alps the tectonic component of uplift varies from 4 km along the alpine fault to 1 km along the eastern margin of the alps

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

the role of phreatomagmatism in a plio pleistocene high density scoria cone field llancanelo volcanic field mendoza argentina abstract the plio pleistocene llancanelo volcanic field together with the nearby payun matru field comprises at least 800 scoria cones and voluminous lava fields that cover an extensive area behind the andean volcanic arc beside the scoria cones in the llancanelo field at least six volcanoes show evidence for explosive eruptions involving magma water interaction these are unusual in the context of the semi arid climate of the eastern andean ranges the volcanic structures consist of phreatomagmatic derived tuff rings and tuff cones of olivine basalt composition malacara and jarilloso tuff cones were produced by fallout of a range of dry to wet tephra the malacara cone shows more evidence for a predominance of wet emplaced units with a steep slump slope characterized by many soft sediment deformation structures such as undulating bedding planes truncated beds and water escape features the piedras blancas and carapacho tuff rings resulted from explosive eruptions with deeper explosion loci these cones are hence dominated by lapilli tuff and tuff units emplaced mainly by wet and or dry pyroclastic surges carapacho is the only centre that appears to have started with phreatomagmatic eruptions with lowermost tephra being rich in non volcanic country rocks the presence of deformed beds with impact sags slumping textures asymmetrical ripples dunes cross and planar lamination syn volcanic faulting and accretionary lapilli beds indicate an eruption scenario dominated by excessive water in the transportational and depositional regime this subordinate phreatomagmatism in the llancanelo volcanic field suggests presence of ground and or shallow surface water during some of the eruptions each of the tuff rings and cones are underlain by thick fractured multiple older lava units these broken basalts are inferred to be the horizons where rising magma interacted with groundwater the strong palagonitization at each of the phreatomagmatic cones formed hard beds resistant to erosion and therefore the volcanic landforms are well preserved

tectonic architecture of the mountain front foreland basin transition south island new zealand assessed by fission track analysis oblique continental convergence across the alpine fault since the mid miocene has loaded the australia plate with the leading edge of the pacific plate and formed a foreland basin the basin occurs mostly offshore beneath the continental shelf but remnants of the marine basin fill overlie ordovician basement in a narrow coastal strip 10 20 km wide in westland between the tasman sea and the alpine fault the results of fission track analyses of apatite and zircon separates from this basement integrated with the regional structure and stratigraphy indicate that the foreland basin formerly extended across the whole of westland and that its succession increased in thickness eastward toward the alpine fault reaching a maximum of about 4 km continuing covergence across the main thrust alpine fault has caused its footwall to thicken by reverse fauting in a transitional zone 10 20 km wide immediately west of the mountain front southern alps consequently the inner margin of the foreland basin has been inverted and the succession almost completely eroded in the southern part of westland upper crustal shortening in the australia plate amounted to about 2 km associated uplift started about 5 m y ago and involved only one major fault bounded block in northern westland upper crustal shortening amounted to about 12 km two major fault bounded blocks were involved and uplift was underway by 10 ma assuming a preuplift geothermal gradient of 25 c km the long term rate of uplift ranged between 0 8 and 2 0 mm yr in southern westland and between 0 4 and 1 2 mm yr in different parts of northern westland

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

three dimensional electrical conductivity in the mantle beneath the payun matru volcanic field in the andean backarc of argentina near 36 5 s evidence for decapitation of a mantle plume by resurgent upper mantle shear during slab steepening

emplacement history and inflation evidence of a long basaltic lava flow located in southern payenia volcanic province argentina abstract the el corcovo lava flow from the huanul shield volcano in the southern mendoza province central western argentina traveled a distance of 70 km and covered a minimum area of 415 km 2 the flow emplacement was controlled both by extrinsic e g topography and intrinsic e g lava supply rate lava physicochemical characteristics factors the distal portion of the lava flow reached the colorado river valley in la pampa province where it spread and then was confined by earlier river channels cross sections through the flow surveyed at several localities show two vesicular layers surrounding a dense central section where vesicles are absent or clustered in sheet shaped and cylindrical shaped structures lavas of the el corcovo flow are alkaline basalts with low values of viscosity the morphological and structural characteristics of the flow and the presence of landforms associated with lava accumulation are the evidence of inflation this process involved the formation of a tabular sheet flow up to 4 m of thick with a large areal extent in the proximal sectors while at terminal sectors frontal lobes reached inflation values up to 10 m the numerous swelling structures present at these portions of the flow suggest the movement of lava in lava tubes we propose that this aspect and the low viscosity of the lava allowed the flow travel to a great distance on a gentle slope relief

the temporal evolution of back arc magmas from the auca mahuida shield volcano payenia volcanic province argentina in order to better constrain the temporal volcanic activity of the back arc context in payenia volcanic province pvp argentina we present new k ar dating petrographic data major and trace elements from 23 samples collected on the auca mahuida shield volcano our new data coupled with published data show that this volcano was built from about 1 8 to 1 0 ma during five volcanic phases and that auca mahuida magmas were extracted from at least two slightly different oib type mantle sources with a low partial melting rate the first one containing more garnet was located deeper in the mantle while the second contains more spinel and was thus shallower the high mgo basalts or primitive basalts and the low mgo basalts or evolved basalts produced from the deeper and shallower lherzolite mantle sources respectively are found within each volcanic phase suggesting that both magmatic reservoirs were sampled during the 1 myr lifetime of the auca mahuida volcano however a slight increase of the proportion of low mgo basalts as well as of magmas sampled from the shallowest source can be observed through time similar overall petrological characteristics found in the pleistocene holocene basaltic rocks from los volcanes and auca mahuida volcano suggest that they originated from the same magmatic source consequently it can be proposed that the thermal asthenospheric anomaly is probably still present beneath the pvp finally our data further support the hypothesis that the injection of hot asthenosphere with an oib mantle source signature which was triggered by the steepening of the nazca subducting plate induced the production of a large volume of lavas within the pvp since 2 ma

shear wave velocities in the pampean flat slab region from rayleigh wave tomography implications for slab and upper mantle hydration 1 the pampean flat slab region located in central argentina and chile between 29 and 34 s is considered a modern analog for laramide flat slab subduction within western north america regionally flat slab subduction is characterized by the nazca slab descending to 100 km depth flattening out for 300 km laterally before resuming a more normal angle of subduction flat slab subduction correlates spatially with the track of the juan fernandez ridge and is associated with the inboard migration of deformation and the cessation of volcanism within the region to better understand flat slab subduction we combine ambient noise tomography and earthquake generated surface wave measurements to calculate a regional 3d shear velocity model for the region shear wave velocity variations largely relate to changes in lithology within the crust with basins and bedrock exposures clearly defined as low and high velocity regions respectively we argue that subduction related hydration plays a significant role in controlling shear wave velocities within the upper mantle in the southern part of the study area where normal angle subduction is occurring the slab is visible as a high velocity body with a low velocity mantle wedge above it extending eastward from the active arc where flat slab subduction is occurring slab velocities increase to the east while velocities in the overlying lithosphere decrease consistent with the slab dewatering and gradually hydrating the overlying mantle the hydration of the slab may be contributing to the excess buoyancy of the subducting oceanic lithosphere helping to drive flat slab subduction

vent distribution in the quaternary payun matru volcanic field western argentina its relation to tectonics and crustal structures abstract the payun matru volcanic field consists of two polygenetic and mostly trachytic volcanoes payun matru with a summit caldera and payun liso along with around 220 scoria cones and basaltic lava flows this volcanic field belongs to the payenia basaltic province 33 30 38 s a quaternary andean back arc basaltic province of the southern volcanic zone in western argentina the vent density distribution of the payun matru volcanic field is different from the other volcanic fields within payenia the payun matru volcano and the scoria cones are distributed in an e w oriented fringe about 15 km wide and 70 km long with the payun matru caldera in the middle of this fringe the structural framework in which the volcanic field is located allows to infer that this vent density distribution is strongly conditioned by pre existing crustal anisotropies the volcanic field is located in a transfer zone related to jurassic extensional structures of the neuquen basin which were inverted also as a transfer zone during the miocene compressive deformation that formed the malargue fold and thrust belt and in addition it is located in the southern margin of a neogene syn orogenic basin the analysis of vent center location and vent morphology is helpful to determine basaltic vent alignments within the payun matru volcanic field and to infer the syn eruptive stress field this analysis shows that vent alignments are compatible with the present day maximum horizontal stress as measured by break out of oil wells