implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary non collisional and collisional suturing orogens

pacific ocean and cenozoic evolution of climate 1 the pacific ocean has played a major role in climate evolution throughout the cenozoic 65 0 ma it is a fundamental component of global heat transport and circulation the dominant locus of primary productivity and consequently the largest reservoir for carbon exchange between the oceans and the atmosphere a satisfactory understanding of the cenozoic evolutionary history of the pacific and its impact on global climate is currently data limited nevertheless the large dynamic range of cenozoic conditions sets the stage to greatly expand our understanding of global climate and biogeochemical cycles past earth experiments are particularly useful to understand interactions between climate and geosystems under different greenhouse gas loads we highlight in this review four important problems in which the pacific played a major role the effect of changing geographic boundary conditions on ocean circulation interactions between the carbon cycle and climate the pacific ocean s influence on north american climate and its water cycle and the gradual evolution of climate systems

neogene sedimentary deformation in the chilean forearc and implications for andean basin development seismicity and uplift neogene sediments in the northern chilean forearc display a wide range of near syndepositional structures analysis of the origin and distribution of these structures in space and time offers new insights into the development of the forearc basins the structures are described in detail and show many features associated with soft sediment deformation pseudo diapirism and slope failure synsedimentary deformation reached a peak in the late miocene to early pliocene while the sediments were saturated in a largely plastic state and many of the structures were probably triggered by seismic shock late stage tilting of the forearc generated shear stresses in the sediments leading to slumping and sliding base level revision and drainage incision led to sediment bypass and cessation of lacustrine sedimentation that was not necessarily linked to climate change compaction and dewatering of the basins caused transition of the sediments from a plastic to a brittle state the age and distribution of structures associated with seismicity appears to correlate with increasing subduction erosion and westward drift of south america but not with basin subsidence shortening rates or plate convergence this suggests that upper crustal deformation is at least partly decoupled from plate movement

sedimentary features of tsunami backwash deposits in a shallow marine miocene setting mejillones peninsula northern chile abstract miocene shoreface sandstones in the caleta herradura half graben northern chile contain an exceptionally coarse deposit that based on sedimentologic and stratigraphic features is regarded as having been laid down during a tsunami event by non cohesive and sediment laden subaqueous density flows interpretations of the principal sediment depositing mechanisms effective in the tsunami surges rely largely on field observations of deposit geometry and internal sedimentary characteristics this example comprises two erosively based sedimentation units that were probably deposited by successive waves in the tsunami wave train the lower unit consists of a clast supported polymodal boulder bearing breccia composed mostly of angular clasts and fewer well rounded clasts framework components are mostly chaotic but may also exhibit either inverse to normal grading or crude normal grading laterally changes in characters of depositional facies are common and abrupt the sand sized bioclastic rich matrix is poorly sorted and very similar to the underlying lower shoreface bioclastic sandstone implying that soft sediments eroded at the lower erosional surface contributed to the tsunami deposit the bulk of the upper unit is a poorly sorted breccia bearing sandstone pebbles and cobbles are scattered massive or normally graded sporadic outsized boulders emplaced as debris fall deposits may occur along the erosional base an array of signatures such as unusually coarse grain size in comparison to the surrounding deposits erosional bases the mixed sources of sediments multiple erosional and depositional events normal size grading or massive texture are all considered distinctive features of tsunamigenic deposits backwash deposition is indicated by the incorporation within the tsunami deposits of sediments derived from mixed sources such as angular clasts from nearby subaerial settings rounded clasts reworked from beach gravels and bioclastic sand eroded from older and unconsolidated shoreface deposits notwithstanding the absence of soft sediment deformation features the marked facies change from a lower shoreface into an upper shoreface environment through the tsunamiites provides directional information about the origin of the tsunami wave it was most likely produced by a sea floor fault displacement associated with an episode of sudden probably coseismic coastal uplift

paleomagnetism in the precordillera of northern chile 22 30 s implications for the history of tectonic rotations in the central andes abstract widespread clockwise rotations in mesozoic and lower tertiary rocks of northern chile have been interpreted as the sum of two rotational events separated in time an early rotation related to local deformation plus a late rotation related to wholesale rotation of northern chile linked to late cenozoic oroclinal bending in the central andes in this paper we report new paleomagnetic data from cretaceous upper oligocene and miocene sedimentary rocks in the precordillera of northern chile the results suggest that all these rocks acquired their remanence at or close to the time of deposition the lack of rotation in undeformed lower miocene strata clearly indicates that clockwise rotations found in underlying faulted and folded cretaceous rocks were completed before the late cenozoic results from nearby localities in deformed upper oligocene strata would argue for little 5 rotation since the late oligocene data from widely separated miocene localities covering an area of about 5000 km2 in the calama basin strongly suggest that northern chile did not undergo significant wholesale rotation during the late cenozoic this together with previous paleomagnetic evidence against neogene rigid body like rotation of the southern peruvian forearc suggests that the curved shape of the central andean forearc was not significantly enhanced during the late cenozoic by inference all of the rotation in most mesozoic and lower tertiary rocks of northern chile was accomplished in the cretaceous and or early cenozoic when the locus of deformation in the central andes was localized in the present forearc region

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

cenozoic evolution of the northwestern salar de atacama basin northern chile 1 since 90 ma the nonmarine salar de atacama basin has been the largest deepest and most persistent sedimentary basin of northern chile integration of 200 km of two dimensional seismic reflection data with surface geological data clarifies oligocene and neogene evolution of the northern part of the basin a normal fault with 6 1 km of vertical separation controlled the western boundary of the basin during the accumulation of the oligocene lower miocene paciencia group the combination of this structure a similar one in the calama basin and regional structural data suggests that localized extension played an important role within a tectonic environment dominated by margin perpendicular compression and margin parallel strike slip deformation seismic data substantiate the surface interpretation that much of the cordillera de la sal ridge resulted from diapiric flow of the paciencia group diapiric flow initiated during the late early miocene or middle miocene associated with a deep reverse fault

using gemorphological markers to discriminate neogene tectonic activity in the precordillera of north chilean forearc 24 25 s abstract we study the neogene tectonic activity in a sector of the precordillera in the andean forearc analysing aerial photographs satellite images and fieldwork data the interpretation of alluvial landforms drainage organisation and evolution of intermittent river networks affecting post lower miocene deposits allow us to recognize low intensity tectonic processes controlling the landscape evolution all these geomorphological markers indicate no strike slip offsets but repeated and small tectonic pulses that reactivate previous structures originated under a transpressive context the observed deformation pattern is the consequence of e w orthogonal compression resulting in limited shortening related to the accommodation of deformation in the chilean forearc of the neogene uplift of the altiplano puna

the andean subduction zone between 22 and 25 s northern chile precise geometry and state of stress abstract one year of seismicity recorded by a local network is used to obtain more precision about the geometry and the stress regime of the andean subduction between 22 and 25 s in the northern chile seismic gap a sharp image of the wadati benioff zone wbz is obtained down to 270 km in depth a seismically quasi quiescent zone is observed in the wbz below the volcanic arc between 150 and 210 km in depth hypocentres of distant intermediate depth earthquakes located with the local network are compared with worldwide seismic network hypocentres in order to evaluate the accuracy of the wbz image at depth greater than 100 150 km in depth no shallow microearthquakes have been observed in the continental crust but some seismic activity is likely to occur locally at the deep root of the atacama fault the stress field and the characteristics of faulting along the subducted slab are investigated underthrusting and localized reverse faulting earthquakes define the seismically coupled plate interface from 20 to 50 km in depth locked zone downdip intra slab normal faulting prevails tensile zone but some strike slip faulting is observed a transition between normal faulting with variable fault azimuth and normal faulting with nearly homogeneous nnw to nw oriented fault plane is found at about 80 km in depth it is found that the stress axes 1 and 3 in the locked zone are oriented in the convergence direction 75 80 e downdip in the tensile zone 3 has a mean azimuth 60 65 e there the slab is hence submitted to a tensional force slab pull oblique relatively to the convergence the transition between seismic underthrusting and intraplate normal faulting downdip occurs at the depth where the continental moho encounters the wadati benioff zone suggesting that a relationship exists between seismic coupling and the presence of continental crust at the plate interface the pre seismic state of this segment of the andean subduction zone is confirmed by the occurrence of strong earthquakes located by the global network around the presumed rupture area and by the stress regime found along the wadati benioff zone

internal dynamics of a paleoaccretionary wedge insights from combined isotope tectonochronology and sandbox modelling of the south central chilean forearc forearc accretionary wedges are cyclic systems in which material is frontally and or basally accreted material cycling involves underthrusting subduction underplating exhumation erosion transfer to the trench and underthrusting again in this study we present a novel tectonochronologic approach to constrain long term exhumation rates of basally accreted wedge complexes based on isotopic dating of structural features on petrological data and sandbox analogue simulations congruence between the structural inventory in nature and structures generated in scaled sandbox experiments allows detailed insights into wedge dynamics for the present day surface material of the paleoaccretionary wedge of south central chile valdivia area 40 s published u pb ages of detrital zircon place a maximum age of 278 ma for subduction prograde metamorphism at transitional greenschist to blueschist facies conditions 420 c 8 9 kbar was immediately followed by progressive penetrative deformation associated with basal accretion dated at 250 245 ma using rb sr internal mineral isochrons the accretion process involved duplex tectonics and antiformal stacking with formation of near horizontal mylonitic shear zones at around 241 ma continuous basal accretion at depth gave rise to an extensional tectonic regime at higher structural levels both semi ductile small scale extensional shear zones and post kinematic vein mineralizations yield rb sr ages of 235 ma tension gashes representing the latest isotopically dateable stage of structural evolution were formed at 210 ma at conditions of 230 c at 1 5 3 kbar as constrained by fluid inclusion data zircon fission track data indicate final cooling to below 200 c at 186 24 ma the results suggest continuous basal accretion for at least 50 ma with long term average exhumation rates of 0 6 0 2 mm a most probably outbalanced by similar long term average erosion rates changing plate boundary conditions at about 210 200 ma terminated the accretion process as evident from a dramatic decrease of exhumation rates at that time since then the paleoaccretionary wedge remained stably in place despite its delicate geotectonic position within the andean active margin the tectonochronologic approach complements thermochronologic and geomorphologic methods of exhumation research as it provides direct constraints on mass flux rates even for high temperature increments of p t trajectories

the nature of the fluids associated with the monte rosa gold district nw alps italy recent o isotope and fluid inclusion studies have provided evidence on the nature of the fluids associated with the late alpine quartz gold deposits of the monte rosa district the most abundant inclusions in quartz from these deposits contain a low salinity aqueous fluid about 2 to 10 wt nacl eq and a co2 phase usually less than 20 mol in places with minor methane co2 densities and total homogenization temperatures vary widely throughout the district reflecting diverse conditions of trapping p 1 to 3 kb t 300 to 450 c at miniera dei cani unmixing between co2 rich and h2o rich fluids possibly occurred a second type of inclusion contains an aqueous brine without recognizable co2 and is especially abundant at val toppa o isotope studies suggest that fluids were largely equilibrated in a metamorphic environment it is concluded that the gold related fluids in the district were mainly of a metamorphic nature at val toppa both isotopic and fluid inclusion data point to contributions of unexchanged meteoric waters mechanisms of gold transport and precipitation are less contrained a possible model involves transport of gold as bisulfide complexes and precipitation due to one or more of the following processes decrease of sulfur activity due to precipitation of sulfides wall rock reaction cooling dilution and or fluid unmixing

cenozoic stratigraphy and basin tectonics of the andes mountains 20 28 south latitude clastic sedimentary basins have evolved during the past 40 m y in the central andes lat 20 28 s in response to shifting patterns of magmatism and deformation the distribution of these basins and their genetic relations to uplifted areas are analogous to the basins and mountain belts of the north american rocky mountains during the late cretaceous and early cenozoic petroleum exploration has focused on zones underlying the upper cenozoic strata along the eastern margin of the andes mountain belt between about 40 and 25 ma a nonmarine basin extended across the region that is now the andes mountains between about 25 and 10 ma the western part of the former basin became the site of a volcanic arc sediment accumulation continued in the east where marine intercalations demonstrate the low elevation of the basin after 10 ma the volcanic arc remained active and locally widened and crustal shortening caused regionally important thrust and reverse faulted ranges during the past 10 m y up to 4 000 m of coarse clastic debris accumulated in a foreland basin on the eastern flank of the mountains meanwhile in the interior of the mountains over 4 000 m of fine grained strata and evaporites accumulated in local depocenters

paleomagnetic data from the precordillera of northern chile a multiphase rotation history related to a multiphase deformational history abstract one of the most conspicuous features of the andean chain is the change in its trajectory from nw se to n s at 18 s known as the bolivian orocline although the central andes rotation pattern carp agrees roughly with the geometry of the orocline large variations and greater than expected rotation magnitudes cannot be explained by a progressive oroclinal bending history alone in this work 621 oriented samples from mesozoic and cenozoic rocks in three different areas in the precordillera of northern chile north of calama were analyzed the samples were subjected to different demagnetization processes to isolate the characteristic magnetic remanence and calculate the tectonic rotations by sampling pretectonic syntectonic and post tectonic units the rotation history of different structural blocks was determined and related to specific deformational events the results obtained from lower cretaceous units intruded or overlain by upper cretaceous units not rotated or recording smaller rotations indicate the existence of 20 clockwise and 20 counterclockwise rotated sites related to the late cretaceous peruvian orogenic event data obtained from upper cretaceous lowermost paleocene rocks show areas slightly clockwise rotated 10 15 and not rotated during the early paleocene k t deformation event remanence data from upper cretaceous eocene units across the area indicate as much as 20 clockwise rotation in the hanging wall of the eocene chintoraste quetena rabonas jaspe reverse fault system whereas the footwall was unaffected this eocene rotation occurred during the early stages of the eocene oligocene incaic orogenic phase prior to emplacement of the 40 37 ma fortuna granodiorite complex current models propose that rotations in carp were entirely cenozoic and largely accomplished in a single paleogene event our data indicate that each of the peruvian k t and incaic deformation events contributed to carp with each event generating a non uniform pattern of rotations which was superimposed on prior rotations in a non uniform manner

the mejillonia suspect terrane northern chile late triassic fast burial and metamorphism of sediments in a magmatic arc environment extending into the early jurassic the mejillonia terrane named after the mejillones peninsula northern chile has been traditionally considered an early paleozoic block of metamorphic and igneous rocks displaced along the northern andean margin in the mesozoic however u pb shrimp zircon dating of metasedimentary and igneous rocks shows that the sedimentary protoliths were triassic and that metamorphism and magmatism took place in the late triassic norian field evidence combined with zircon dating detrital and metamorphic further suggests that the sedimentary protoliths were buried deformed foliated and folded and metamorphosed very rapidly probably within few million years at ca 210 ma the metasedimentary wedge was then uplifted and intruded by a late arc related tonalite body morro mejillones at 208 2 ma only a short time after the peak of metamorphism the mejillones metamorphic and igneous basement represents an accretionary wedge or marginal basin that underwent contractional deformation and metamorphism at the end of a late permian to late triassic anorogenic episode that is well known in chile and argentina renewal of subduction along the pre andean continental margin in the late triassic and the development of new subduction related magmatism are probably represented by the early jurassic bolfin punta tetas magmatic arc in the southern part of the peninsula for which an age of 184 1 ma was determined we suggest retaining the classification of mejillonia as a tectonostratigraphic terrane albeit in this new context

structural characteristics of an active fold and thrust system in the southeastern atacama basin northern chile abstract the western south american margin is one of the most active plate boundaries in the world using various remote sensing data sets we mapped the neotectonic characteristics of an area at the southeastern corner of the atacama basin northern chile in the andean forearc there one major n s trending ridge is clearly visible both in the satellite images and in the field this ridge reaches 250 m above the basin floor in its middle part and is asymmetrical with a steep eastern slope and a much gentler western slope the geometry of the ridge indicates that it formed as an asymmetrical anticline this anticline is likely formed as a shear fault bend fold with a major decollement at a depth of about 2 5 km in the naranja formation we suggest that this decollement is a major structure of the atacama basin area from the ages of the ignimbrites and lake deposits that were deformed by this anticline we obtained a long term shortening rate of the major underlying structure at about 0 2 mm yr this thin skinned fold and thrust system appears to be active since at least about 3 ma and could be as long as since middle miocene therefore crustal structures may play important roles in the neogene development of the western andean margin

thermal and exhumation history of the coastal cordillera arc of northern chile revealed by thermochronological dating abstract the thermal and erosional history of convergent plate boundaries is important for understanding the links between subduction arc magmatism genesis of ore deposits topography and climate of orogenic belts unlike the continent continent collision that formed many of the largest orogenic belts known today the central andes of south america is a unique case where an oceanic continent collision has given rise to the earth s longest and second tallest orogenic belt over the last thirty years a plethora of models have been suggested in an attempt to explain how a plateau type orogen formed at the leading edge of western south america in the central andes most research have focussed attention on the study of the evolution of the arc and backarc since continuous subduction erosion of the forearc has left little trace of the interplate dynamics that initiated the orogenic belt in this article we present a new insight into the thermal and exhumation history of the forearc along the coastal cordillera of northern chile based on biotite k ar apatite fission track and apatite zircon u th he dating we collected diorite samples in a 2 km thick crustal section at the coastal cliff 22 s and a sea level isoelevation profile between 21 and 27 s results from all three dating methods show that the cooling of coastal cordillera took place shortly after emplacement during a period of rifting in jurassic times cooling took place in two episodes mainly in late jurassic early cretaceous 118 152 ma but also during late cretaceous 60 80 ma due to the resumption of compression rift closure arc uplift exhumation eastward migration of magmatic arc activity and thermal relaxation the youngest apatite u th he ages reveal a cooling event never reported previously between 40 and 50 ma eocene this thermal event affected a 500 km long and 1 km thick section of the coastal cordillera in northern chile rock cooling recorded in the eocene cannot be explained by the thermal effect of the magmatic arc we explore two scenarios that can provide an explanation for the observed cooling 1 forearc uplift and exhumation and 2 changes in plate subduction dynamics erosion rates of 0 24 to 0 36 km myr for a period of 10 myr are necessary to explain the cooling event alternatively the subduction of a ridge could explain significant cooling by flat slab dewatering based on recent sea floor spreading reconstruction we suggest the subduction of the farallon phoenix ridge as a possible candidate the subduction of this ridge can account for the cooling event at 40 50 ma but also for some important aspects of the eocene development of the central andes notably the onset of uplift an abrupt decrease in magmatic activity and porphyry copper mineralization

implication of dating of early tertiary volcanic rocks from the north chilean precordillera abstract since the early jurassic subduction induced volcanic activity in the north chilean andes 21 23 5 s has migrated 200 300 km eastward from the present coast jurassic magmatic arc to the western cordillera miocene recent volcanic rocks due to the paucity of chronological data however the exact age and extent of various magmatic sub belts within this area are not well constrained in the precordillera a sequence of 1500 5000 m thick red bed sediments is partly intercalated with and mostly covered by predominantly intermediate calc alkaline tuffs and lavas presently the age of this sequence is considered to be upper jurassic and or cretaceous from this sequence eleven biotites and seven hornblendes were analyzed and yielded 40 ar 39 ar plateau ages of between 48 and 38 ma indicating upper eocene volcanic activity in the eastern part of the precordillera in contrast volcanic activity in the western part of the precordillera has been dated at 92 2 5 ma indicating an earlier onset of volcanism and therefore sedimentation of the red beds in the late eocene the intense volcanism ceased in this area contemporaneous with decreasing relative convergence rate of the nazca plate and the south american continent this final phase of eocene volcanism coincided with a period of important deformation the incaic phase biotites from two ash fall tuffs separated by a distinct angular unconformity yield identical ages of 38 50 0 90 ma and 38 45 0 60 ma these dates provide the best age estimate of the incaic phase in the north chilean precordillera

sedimentation and preservation of the miocene atacama gravels in the pedernales chanaral area northern chile climatic or tectonic control in recent years longitudinal changes on the thin thick skinned tectonic styles of the central andes has been intensively discussed while other studies have considered the role of mass transfers on the unloading the orogen and on the stress regime along the plate interface arising from changes on the volume of sediment arriving into the peru chile trench the search for paleo climate records is therefore crucial for our understanding of the history of the central andes in this paper we focus on the atacama gravels an extensive blanket of miocene continental deposits filling a neogene paleo valley system along the southern atacama desert in northern chile an east west transect between pedernales and chanaral 26o30 s enabled us to carry out a sedimentological and tectonic study of the atacama gravels based on logging and field observations along the rio salado canyon new 39ar 40ar ages obtained on intercalated and overlying ignimbrites date the beginning of the atacama gravels sedimentation at around the oligocene miocene boundary and cessation of sedimentation in the late miocene thirteen lithofacies included within five facies associations a1 to a5 were identified depositional environments vary from proximal alluvial fan a1 a2 in the precordillera through ephemeral fluvial a3 a4 to distal playa lake a5 in the coastal cordillera no evidences of syn sedimentary deformation have been found showing that the change from sediment removal to sediment preservation cannot be explained by tectonic causes and climate change appears to be the dominant controlling factor of sediment preservation a progressive change from semi arid towards hyper arid climatic conditions during the miocene led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleovalley system formed during the oligocene

recently active contractile deformation in the forearc of southern peru abstract geomorphic and structural features of southern peru 14 18 s provide strong evidence for distributed crustal deformation along range sub parallel contractile structures we use in situ produced cosmogenic radionuclides in conjunction with field and remote mapping to determine the ages of geomorphic features and find 1 ancient surfaces 1 ma preserved as a result of very low surface erosion rates 2 young 30 ka low relief pediment surfaces developed during recent landscape modifications 3 active tectonic structures accommodating compressional stresses and 4 pleistocene river incision rates of 0 3 mm yr consistent with longer term rates in this region of southern peru the steep western wedge trench to arc area of the andean margin presently maintains the high topography of the altiplano through a combination of uplift and contractile deformation along steep east dipping faults and isostatic responses to the focused removal of large amounts of crustal material through canyon incision

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

petrology and u pb geochronology of footwall porphyritic rhyolites from the wolverine volcanogenic massive sulfide deposit yukon canada implications for the genesis of massive sulfide deposits in continental margin environments porphyritic rhyolite sills form an important component of the footwall of the wolverine volcanogenic massive sulfide vms deposit yukon canada and occur proximal to mineralization in the immediate deposit area wolverine lynx zone and at similar stratigraphic levels along strike fisher puck and sable zones porphyritic rhyolites are of two types an older quartz feldspar porphyritic qfp rhyolite suite and a younger feldspar porphyritic fp suite both the qfp and fp suites of intrusions are semiconcordant suggesting a silllike morphology and are altered and crosscut by veinlet mineralization suggesting that they are pre to synmineralization the margins of qfp suite of intrusions contain minor xenoliths of surrounding shales and poorly developed chilled margins suggesting emplacement into partially consolidated sedimentary rocks whereas the fp suite of intrusions shows well developed chilled margins indicative of emplacement into fully solidified sedimentary rock these features suggest that the qfp suite of intrusions represents an older phase of rhyolitic magmatism whereas the fp suite represents a younger event this is supported by u pb zircon ages which indicate a 352 4 1 5 ma emplacement age for the fp suite and a 347 to 346 ma emplacement for the fp suite two ages at 347 8 1 3 and 346 0 2 2 ma both suites of porphyries have inherited proterozoic zircon and have ratios of la smucn 1 and nb thucn 1 ucn upper continental crust normalized indicating derivation from and or extensive interaction with ancient upper continental crustal materials the fp suite however has elevated high field strength element hfse and rare earth element ree contents high zircon saturation temperatures and higher nb ta ratios and lower ti sc ratios than the qfp suite these features are interpreted to reflect that the fp suite of magmas was hotter 900 c melts with a greater mantle component in their genesis both suites however are interpreted to have formed due to basaltic upwelling crustal melting and crust mantle mixing during ensialic back arc basin activity the presence of mantle heat within the wolverine basin from 352 to 347 to 346 ma a minimum of 5 m y suggests that sustained mantle heat flow was critical to the genesis of the wolverine porphyries it is also suggested that this sustained mantle heat was responsible for the wolverine hydrothermal system and that upwelling mantle may be essential in providing the heat to drive hydrothermal systems even in continental margin type vms environments e g bathurst iberian pyrite belt the wolverine porphyries are among the most hfse and ree enriched felsic rocks associated with vms mineralization globally the high hfse and ree concentrations in these rocks are interpreted to be due to

extensional tectonics in convergent margin basins an example from the salar de atacama chilean andes the salar de atacama basin of northern chile preserves stratigraphic evidence for the evolution of the andean cycle it has evolved from a non arc related rift through back arc and inter arc stages to a neogene fore arc basin accumulation of the sedimentary succession was mainly due to extensional faulting important but short duration contractional episodes do link to known fast order plate margin changes but their stratigraphic effect appears to be restricted to uplift erosion rather than creation of significant flexural subsidence the salar de atacama basin originated as part of a regional rift system during permian time and contains 2 km of permo triassic continental detritus and volcanic rocks the area remained above depositional base level throughout late triassic to late cretaceous time syn rift continental red beds were deposited to a thickness of at least 2 km on a parallel rift segment in the domeyko area to the west continued triassic jurassic extension in the domeyko basin resulted in a classic continental to marine transition with deposition of a 2 km jurassic mixed carbonate clastic sequence in latest cretaceous eocene time the salar basin was an arc related basin and accommo dated some 4 km of continental detritus purilactis group due to back arc extension sediment being derived from the domeyko cordillera and arc rocks to the west late eocene right lateral strike slip faulting and associated restraining bend uplift were driven by a high rate of oblique convergence between the farallon and south american plates this deformation complicated the stratigraphy of the purilactis group and inverted the western basin margin which formed the dominant provenance area for a 2 km thick oligocene continental basin fill component paciencia group the oligocene basin was an extensional to trans tensional basin the miocene holocene salar basin is a continental fore arc basin this latest segment of the basin fill comprises pyroclastic and continental sedimentary rocks thrust over quaternary gravels in many places the cordillera de la sal is an intrabasinal uplift initiated as a thin skinned contractional feature thus the superposed basin fill components represent responses to distinctly different geodynamic settings

slow rates of subduction erosion and coastal underplating along the andean margin of chile and peru marine seismic and drilling data from the andean margin of peru and northern chile show trench retreat and tectonic erosion since at least 20 ma however the onshore coastal zone has undergone modest regional uplift since ca 2 ma this argues against subduction erosion of the margin by steady state retreat such as recognized in tonga or south sandwich instead we identify a new style of slow tectonic erosion uplifted forearc sediments indicate underplating of subducted material beneath the base of the coastal forearc synchronous with tectonic erosion under the trenchward part of the forearc these observations require slow rates of forearc crust subduction in the region during this time period averaging no more than 13 km3 m y km since 20 ma as the forearc wedge taper increased isostatic volume estimates suggest that as much as 85 of subducted material was underplated under the forearc since 330 ka and does not contribute to crustal growth under the andes the switch in tectonic style parallels that in the southern andes but cannot be triggered by increased sediment flux to the trench as proposed for that area

cordillera formation mantle dynamics and the wilson cycle we propose that the andes cordillera formed by a collision between the trenchward moving south american plate and the nazca slab and subslab mantle resistance to trenchward motion in the form of horizontal compressive normal stress deforms south america9s leading edge the advancing continent also perturbs the subslab mantle flow field as mantle is displaced this flow field detected via shear wave splitting includes trench parallel flow away from a central stagnation point and corner flow around the northern and southern ends of the continent the corresponding spatial variation in applied stress accounts for the large scale characteristics of andean deformation buckling at the stagnation point maximum normal stress forming the central andes and bolivian orocline and corner flow associated with the eastward motion of the caribbean and scotia plates the high stresses required to form and maintain the andes imply that south america9s trenchward motion is primarily driven by deep mantle flow coupled to its base no other driving force of sufficient magnitude is available similarities between north and south american plate motion history and large scale cordillera structure indicate that the western north american laramide rockies formed in a similar manner and that the north american plate is also driven westward by deep mantle flow we infer that atlantic spreading itself is similarly driven by contrast slab rollback in a closing ocean basin opens marginal basins at convergent margins as in the western pacific we suggest that there is a causal link between the opening and closing of ocean basins and the formation of cordilleras and marginal basins cordillera formation occurs at the leading continental margin during the spreading phase of the wilson cycle whereas marginal basins form during the closing phase

response of supergene processes to episodic cenozoic uplift pediment erosion and ignimbrite eruption in the porphyry copper province of southern peru the jurassic to middle eocene porphyry copper deposits and prospects exposed on the pacific slopes of the central andean cordillera occidental of southern peru between latitudes 16 309 and 18 s record a protracted ca 30 m y history of supergene processes that were fundamentally controlled by the evolving local geomorphologic environment itself a response to successive regional tectonic events including the late eocene incaic the late oligocene to earliest miocene aymara and the middle to late miocene quechuan events weathering of the porphyry centers also overlapped temporally with the local resumption of arc volcanism in southern peru at 25 5 ma following a 27 m y amagmatic interval and supergene processes were variously interrupted or terminated by ignimbrite blanketing although in several locations supergene profiles were preserved by such cover the landform chronology for the area surrounding the cuajone quellaveco and toquepala deposits ca 17 s is revised and extended northwestward through field mapping to the cerro verde santa rosa district ca 16 309 s the 40 ar 39 ar incremental heating dates of supergene alunite group minerals from the angostura 38 1 and 38 8 ma and posco 38 8 ma prospects and the cerro verde deposit 36 1 38 8 ma demonstrate that supergene processes were underway in the late eocene beneath a subplanar topography resulting from uplift and erosion during the incaic orogeny now represented by a regional unconformity in the cenozoic volcanic sedimentary rock succession broadly contemporaneous supergene processes were probably active in the cuajone quellaveco toquepala district slow erosion and the accumulation of clastic sediments through the tectonically quiescent early to mid oligocene are envisaged to have caused a rise in the water table and the widespread preservation of the incaic supergene profiles aymara uplift subsequently led to the incision of the 23 8 to 24 ma altos de camilaca and the 18 8 to 19 1 ma pampa lagunas pediplains and their regional correlatives the ensuing water table lowering was associated with intense leaching and sulfide enrichment from the late oligocene 24 4 28 ma natroalunite at cerro verde 26 27 ma natroalunite at santa rosa and 28 6 ma jarosite at la llave to the early miocene 23 ma alunite and 21 ma natroalunite at cerro verde and 19 2 ma jarosite at la llave and was plausibly responsible for much of the upgrading of the cuajone and toquepala deposits and thr quellaveco prospect which are intersected by both the altos de camilaca pediplain and erosional features representing upslope extensions of the pampa lagunas pediplain the younger supergene profiles were widely superimposed on the remnants of those generated during the incaic orogeny middle miocene 14 2 ma biotite age chuntacala formation flows protected the cuajone supergene profile from destruction by erosion but at 13 0 ma interrupted supergene processes at quellaveco revision of volcano stratigraphic relationships in the latter area reveals that subsequent erosion of the chuntacala formation ignimbrites and part of the supergene profile took place prior to the deposition of a 10 1 ma ash flow tuff of the asana formation elsewhere supergene activity persisted at the cachuyito prospect through 11 4 ma and minor jarosite development occurred at least until 4 9 ma both there and at cerro verde during and following the multiple pediment landform stage ca 7 9 15 0 ma the occurrence of relics of late eocene alunite group minerals within considerably younger late oligocene to late miocene supergene alteration profiles suggests that the overall physiographic configuration of the pacific piedmont of southern peru remained remarkably consistent from the late eocene to the middle miocene moreover the new age data confirm that as in northern chile semiarid climatic conditions prevailed along much of the plate boundary from the mid eocene until the late miocene or early pliocene onset of hyperaridity the local geomorphologic and volcanic conditions in southern peru however conspired to generate more complex supergene profiles with lower aggregate enrichment factors relative to the strongly enriched profiles in the late eocene to early oligocene porphyry copper belt of northern chile which underwent supergene upgrading over relatively brief periods

the precordilleran fault system of chuquicamata northern chile evidence for reversals along arc parallel strike slip faults abstract the chilean precordillera situated between the longitudinal valley and the western cordillera of northern chile was the site of the andean magmatic arc from the late cretaceous to the eocene oligocene boundary magmatism came to an end during the incaic tectonic phase 38 ma which caused arc normal shortening and the development of longitudinal dextral strike slip faults precordilleran fault system this magmatic arc tectonism is also related to the formation of the chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the precordillera structural investigations in and around the chuqicamata open pit mine have shown that wrench tectonics determined the kinematics of the area the n s striking west fissure which separates a 35 ma old non mineralised pluton to the west from a central late paleozoic basement ridge containing the mineralization became a sinistral fault along with other parallel faults in the area the central part is separated from similar paleozoic rocks to the east by the messabi este fault and a narrow faulted and sheared syncline of mesozoic cenozoic sediments this fault bears structures indicating dextral movements which probably are of an age that is similar to the mylonites 34 8 ma in the western pluton the dextral movements preceded the sinistral shear thus the fault system of chuqicamata displays a reversal of arc parallel shear movements according to the orientation of quartz veins in the mineralized body it is presumed that the sense of displacement of these strike slip motions reversed when mineralization started at about 32 ma during this time the stress field must have changed fundamentally the incaic phase dextral transpression is supposed to have been induced by the oblique vector of plate motion the following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling as however the vector of plate motion remained unchanged during that time oblique subduction cannot be used as an argument for arc parallel sinistral shear movements

interaction between the nazca and south american plates and formation of the altiplano puna plateau review of a flexural analysis along the andean margin 15 34 s abstract the results of a two dimensional flexural analysis applied to the andean margin which is based on the correlation between topography and bouguer anomaly are here reviewed in order to characterize rigidity variations across and along the forearc arc transition of the central andes and to understand the role of the forearc in the formation of the altiplano plateau the forearc has maximum rigidities between 15 and 23 s forearc rigidity decreases gradually southward and sharply toward the plateau the main orogen elevations higher than 3000 m is very weak along the entire central andes a semi quantitative interpretation of these trends based on the relationship between flexural rigidity and the thermo mechanically and compositionally controlled strength of the lithosphere allows the following conclusions to be made 1 across strike rigidity variations are dominated by the thermal structure derived from the subduction process 2 the forearc constitutes a strong cold and rigid geotectonic element 3 southward weakening of the forearc is directly related to the decreasing thermal age of the subducted slab 4 very low rigidities along the main orogen are caused by the existence of a thick quartz rich crust with a low strain rate to heat flow ratio 5 the strength of the plateau lithosphere is localized in an upper crustal layer whose base at 15 km could be correlated with a p to s seismic wave converter trac1 of yuan et al 2000 yuan x sobolev s kind r oncken o et al 2000 subduction and collision processes in the central andes constrained by converted seismic phases nature v 408 21 28 diciembre p 958 961 6 the forearc plateau rigidity boundary corresponds to a zone of changing thermal conditions eastward increasing crustal thickness and felsic component in the crust and low strain rate deformation which correlates with a west verging structural system at the surface these conclusions suggest that the rigid forearc acts as a pseudo indenter against the weak plateau and allows the accumulation of ductile crustal material that moves westward from the eastern foreland this pseudo indenter is geometrically represented by a crustal scale triangular zone rooted at trac1 this model allows the integration of existing contradictory ideas on the dynamics of forearc plateau interaction that are related to the relative importance of upper crustal compressive structures and lower crustal accumulation below the forearc

major forearc subsidence and deep marine miocene sedimentation in the present coastal cordillera and longitudinal depression of south central chile 38 30 s 41 45 s neogene marine strata crop out in the present coastal cordillera and longitudinal depression of south central chile between 38 30 s and 41 45 s indicating the onset of a major marine transgression that covered most of the forearc in this area in order to determine the sedimentary environment paleobathymetry and age of these deposits we carried out integrated sedimentologic ichnologic and micropaleontologic studies on samples from oil wells and outcrops in the region our results indicate that these successions were deposited at lower bathyal depths 2000 m during the middle to late miocene shallow marine deposition followed in the southwestern part of the study area during the pliocene we attribute deep marine sedimentation in this area to a major event of subsidence in the miocene that affected the entire forearc and that was caused by basal subduction erosion we suggest that the anomalously thin crust that characterizes this area may have facilitated forearc subsidence and allowed the miocene transgression to advance much farther inland here than in other regions of chile subsequent uplift of the forearc is ascribed to basal accretion or underplating of sediments our conclusions contradict previous studies that favor a stable margin at these latitudes since the jurassic deep marine sedimentation in this area during the miocene implies that the present coastal cordillera and longitudinal depression were probably submerged during that epoch

150 million years of climatic stability evidence from the atacama desert northern chile the sedimentary succession in the atacama desert records deposition under an arid to semiarid climate from the late jurassic 150 ma to the present day palaeomagnetic data indicate no significant latitudinal movement of this area since the late jurassic the present day location of the atacama within the dry subtropical climate belt is the principal cause of aridity this situation is likely to have prevailed since the late jurassic supplemented by 1 the continentality effect enhanced by the gondwanan landmass and 2 the presence offshore of a cold upwelling current from at least the early cenozoic onwards and possibly earlier resulting in conditions promoting climatic stability and desert development rapid and extreme climatic fluctuations during the plio pleistocene were not sufficient to destabilize the climate within the atacama comparison with other long lived deserts e g sw usa namib sahara and australia suggests that the atacama is the oldest extant desert on earth

slow rates of subduction erosion and coastal underplating along the andean margin of chile and peru comment and reply comment in their article clift and hartley 2007 1 propose the existence of two alternating modes of subduction erosion for north central chile and peru one fast with steady state retreat from 150 to 20 ma and the other slow with erosion constrained only in the trench domain from 20 ma onward the

seismic structure of the north central chilean convergent margin subduction erosion of a paleomagmatic arc we study the erosive convergent margin of north central chile at similar to 31 degrees s by using high resolution bathymetric wide angle refraction and multichannel seismic reflection data to derive a detailed tomographic 2 d velocity depth model in the overriding plate our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6 0 6 5km s which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the coastal cordillera beneath the lower and middle continental slope however the presence of a zone of reduced velocities 3 5 5 0km s is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion at the landward edge of the outermost fore arc the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp similar to 1km offset which overlies a strong lateral velocity contrast from similar to 5 0 to similar to 6 0km s this pronounced velocity contrast propagates deep into the continental crust and it resembles a major normal listric fault we interpret this seismic discontinuity as the volcanic continental basement contact of the submerged coastal cordillera characterized by a gravitational collapse of the outermost fore arc subduction erosion has most likely caused large scale crustal thinning and long term subsidence of the outermost fore arc

geochemistry and u pb dating of felsic volcanic rocks in the riotinto nerva unit iberian pyrite belt spain crustal thinning progressive crustal melting and massive sulphide genesis abstract we present new geochemical sm nd and u pb data on the felsic volcanic rocks containing the world class volcanic hosted massive sulphide deposits in the riotinto nerva unit iberian pyrite belt spain three new u pb ages from older plagioclase quartz phyric dacites and plagioclase phyric rhyolites to youngest plagioclase quartz phyric rhyolites indicate a time span for felsic volcanism ranging from 351 5 0 4 to 345 7 0 6 ma the youngest felsic rocks exhibit lower e nd as well as contrasting ti sr zr hf and eu eu values we interpret that in the riotinto nerva unit crustal melting successively affected shallower more evolved horizons in the crust progressive crustal melting is consistent with current interpretations of the iberian pyrite belt in terms of a late devonian early carboniferous transtensional setting coupled with underplating by basic magma we suggest that low e nd evolved crustal magmatism could be used as a proxy in studies of the genesis of these and possibly other phanerozoic massive sulphide deposits supplementary material details of the analytical methods and whole rock data for basalts are available at http www geolsoc org uk sup18452

geomorphology on geologic timescales evolution of the late cenozoic pacific paleosurface in northern chile and southern peru abstract the atacama desert on the western margin of the central andes is one of the driest and oldest deserts in the world it is defined by a distinct and ancient surface known as the pacific paleosurface pps or atacama paleosurface the age of this surface is determined as the time at which sediment deposition ceased and the surface was effectively abandoned early studies suggested that this abandonment took place between 14 and 10 ma and was related to both the uplift of the andes and the onset of hyperaridity in the region here we provide a regional re examination of the pps compiling existing work on the underlying geology sedimentology surface exposure dating and seismic profiling we also present new multispectral satellite maps of the pps and 45 new cosmogenic 3he and 21ne surface exposure ages in order to constrain the formation age and the preservation and incision history of the paleosurface we conclude that the pps is not a single paleosurface but instead is a mosaic of smaller surfaces that were formed by aggradational and degradational processes over 19 million years or more and should be termed collectively as the pacific paleosurfaces the time at which individual paleosurfaces formed is related to regional climate where the location of each is controlled by regional tectonic activity cosmogenic surface exposure ages suggest that the surfaces are a record of regional scale climate events

mid paleozoic initiation of the northern cordilleran marginal backarc basin geologic geochemical and neodymium isotope evidence from the oldest mafic magmatic rocks in the yukon tanana terrane finlayson lake district southeast yukon canada the fire lake formation of the yukon tanana terrane in the finlayson lake region yukon canada consists primarily of late devonian ca 365 360 ma mafic metavolcanic rocks and smaller volumes of mafic and ultramafic subvolcanic metamorphosed intrusions in this paper field geochemical and nd isotope attributes of these rocks are presented in an attempt to understand their tectonic setting the magmatic processes involved in their formation and their roles in cordil leran crustal growth the mafic rocks of the fire lake formation exhibit a wide diversity of geochemical signatures and are classified into seven chemically defined suites 1 back arc basin basalt 2 enriched mid oceanic ridge basalt e morb 3 oceanic island basalt oib 4 th rich oib 5 boninite 6 island arc tholeiite and 7 light rare earth element lree enriched island arc tholeiite the diversity of geochemical signatures is interpreted to represent variable mixtures of asthenospheric morb type mantle subarc mantle wedge and lithospheric oib type mantle with or without elemental contributions from the subducted slab and or continental crust these suites of rocks are also associated with fine grained basinal sedimentary facies variations in metavolcanic and metasedimentary unit thickness extensional synvolcanic faults and apparent extensional fault controlled emplacement of mafic intrusive rocks and hydrothermal volcanic hosted massive sulfide mineralization the suites also exhibit a broad spatial distribution those with arc signatures nb thmn 1 mn normalized to primitive mantle values are located primarily in the western parts of the formation and suites with nonarc signatures nb thmn 1 are located primarily within the eastern parts of the formation collectively these geologic and geochemical attributes are interpreted to stem from the transition from arc magmatism to the initiation of an extensional backarc basinal environment associated with an east dipping subduction zone the initiation of backarc basin magmatism recorded in the fire lake formation was part of a much larger late devonian backarc basinal system forming along the western edge of the margin of north america the fire lake formation is interpreted to represent 1 the commencement of yukon tanana arc rifting and separation from the north american cratonic margin and 2 the initiation of a marginal backarc basin now the slide mountain terrane inboard of the yukon tanana arc system this tectonic evolution likely occurred either as a result of slab rollback toward the west within the convergent margin responsible for yukon tanana arc activity or as a result of the propagation of the slide mountain backarc basin spreading ridges into the yukon tanana arc system this yukon tanana arc rifting episode was also broadly coincident with rifting and hydrothermal activity within rocks of the north american craton the geochemical and isotopic signatures of magmatic rocks in the fire lake formation have some features similar to intraoceanic arc rocks e g boninites island arc tholei ites and many have juvenile nd isotope signatures i e nd t 0 most have nd t 5 suggesting that the pericratonic terranes of the northern cordillera have a significant juvenile component if this is the case throughout the yukon tanana terrane then the pericratonic terranes may have contributed much more juvenile material to cordil leran crustal growth in the phanerozoic than has previously been considered

concept of continuum as opposed to periodic tectonism in the andes abstract results of a study of various andean sedimentary basins that are distinct in terms of age and geodynamic position allow us to postulate the idea of a compressional tectonic continuum as opposed to the short tectonic events which have so far been considered as one of the main features of the dynamic evolution of the andean belt in these basins the development of kilometre scale synsedimentary folded structures implies the existence of compressional stress over a period of several million years the existence of tectonic events of long duration allows us to modify the interpretation of the classic andean tectonic model the timing of the continuum tectonism is perhaps dependent on the variation in the convergence rate between the farallon nazca and south american plates

k ar and 40ar 39ar geochronology of supergene processes in the atacama desert northern chile tectonic and climatic relations new k ar and 40 ar 39 ar data obtained for 29 samples of supergene alunite jarosite and hollandite group minerals from the central depression of northern chile document almost continuous supergene oxidation from 33 to 9 ma the downward migration of supergene oxidation zones is used to calculate maximum denudation rates of 16 35 m ma 1 these calculated rates are sensitive to pre existing topography and show that the present relief observed in the central depression represents a modified miocene landscape the results presented here are in good agreement with previously published geochronological data for the atacama desert the combination of all published data including the results presented here defines a continuous skewed distribution with a maximum between 21 and 14 ma the relative paucity of ages greater than 30 ma reflects the removal of supergene profiles by erosion during and immediately after the incaic compressive deformation from 29 to 9 ma supergene oxidation profiles were generated under semi arid conditions throughout the atacama region ages between 9 and 5 ma on the northern and southern margins of the desert mark the end of supergene oxidation processes consistent with a pliocene onset of present day hyperaridity

petrogenetic evolution of felsic volcanic sequences associated with phanerozoic volcanic hosted massive sulphide systems the role of extensional geodynamics abstract felsic volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension however empirical geochemical links between felsic volcanism geotectonic environment and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic hosted massive sulphide vhms mineralization which is important considering most vhms deposits are hosted in felsic volcanic bearing sequences using felsic geochemical data from six different vhms mineralized phanerozoic geotectonic settings it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas which in turn helps in stratigraphic and geotectonic interpretations for detailed vhms exploration in extensional geodynamic settings bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral high stress regimes in calc alkaline arc building environments the empirical geochemical metallogenic vhms relationships evident in the archean felsic volcanic rocks in particular the low zr y n yb n 2 o conditions in different source areas and geotectonic environments they seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings because of the limited degree of assimilation and fractional crystallization afc affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment within plate felsic or a type anorogenic type and anomalous ocean ridge or type felsic magmas that have tholeiitic affinities relatively low alkali elements are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting in part a relationship to extension the high contents of high field strength elements zr ga lree hree y nb etc and other lines of evidence indicate they are relatively high temperature relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic simatic or continental sialic crust low p granulites or amphibolites but also in some cases possibly mantle rocks such that they are significantly water undersaturated during melting i e a h 2 o 1 reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels diking vs diapirism higher rates of heat advection from mantle mafic magmas into the thickened oceanic and or continental crust during lithospheric extension are needed to facilitate melting of these rock to form a type and or or type partial melts tholeiitic felsic this usually results in bimodal magmatism as magma mixing is not favored in extensional settings especially at high magma production intrusion and extrusion rates i e high heat flow this would inhibit ferromagnesian phase in particular hornblende and magnetite fractionation especially at low f o 2 in the felsic magmas that would deplete the melt in y hree as well as ore related elements particularly zn which is 2 to 3 times higher in a type and or type rocks compared to other magma types therefore the empirical felsic igneous geochemical metallogenic relationships observed by others low zr y n yb n

shallow intraplate seismicity related to the illapel 2015 mw 8 4 earthquake implications from the seismic source abstract the september 16 2015 mw 8 4 illapel chile earthquake is the first large event occurring in north central chile after the 1943 earthquake filling a known seismic gap in the region the earthquake took place in a complex tectonic region nearby an area where transition from erosive to accretionary margin occurs due to the collision of juan fernandez ridge jfr along the chilean margin we inverted the kinematic rupture process of the 2015 mw 8 4 illapel earthquake from the joint inversion of teleseismic body waves and near field data the relative weighting between datasets and the weighting of spatial temporal constraints are objectively estimated by applying the akaike s bayesian information criterion the coseismic slip model yields a total seismic moment of 4 92 1021 nm occurred over 120 s the rupture shows both downdip and updip propagation with slip extending along the thrust interface from 50 km depth to shallow near trench depths

the andean orogenic period and its major structures in central and northern peru the andean orogenic period began in the albian and consisted of at least 6 discrete compressional phases which are albian to late miocene in age deformation migrated progressively toward the amazonian foredeep the major andean structures in central and northern peru are related to the eocene incaic early miocene quechua 1 and late miocene quechua 3 phases the maranon thrust and fold belt mtfb which bounds the western cordillera to the ne rose in the eocene it was reactivated partly by the quechua 1 phase which also folded the altiplano farther e and possibly part of the eastern cordillera the sub andean thrust and fold belt stfb is related to the quechua 3 phase in both mtfb and stfb the imbricate thrusts merge at depth in large decollement faults gravity sliding does not play a dominant role for these faults have to be rooted westward in the pre andean basement the corresponding crustal shortening played a significant part in the creation of the sialic root of the andes

tectonic signatures in arid continental basins an example from the upper miocene pleistocene calama basin andean forearc northern chile abstract the calama basin a transtensional basin situated within the andean forearc and its extension to the pampa del tamarugal central depression is filled with 700 m of sediments detailed sedimentary logging and 40ar 39ar dates from interbedded volcanic deposits have established a lithostratigraphic and chronostratigraphic framework for the upper 95 m thick upper miocene pleistocene succession five facies associations fluvio lacustrine diatomite palustrine carbonate massive anhydrite fluvial and alluvial fan have been identified within this part of the basin fill the following events are recognised in the basin evolution 1 deposition of diatomaceous lacustrine sediments in two separate depocentres in the pampa del tamarugal and the central calama basin with alluvial fan accumulation along the basin margins at 8 ma 2 localised deformation within the central calama basin between 7 and 6 ma but continuous sedimentation elsewhere 3 palustrine carbonate deposition across the entire area from 6 to 3 ma except in the pampa del tamarugal where diatomaceous sedimentation continued until 5 ma before being replaced by palustrine carbonate deposition 4 widespread folding after 3 ma and entrenchment as the drainage system cut down to reach a new base level caused by the rio loa breaching the coastal cordillera and reaching the sea localised lacustrine deposition in topographic lows or tectonically created dams also occurred during this period comparison with similar age facies elsewhere in northern chile has shown changes such as these to be localised and diachronous indicating that tectonic controls on accommodation space and drainage patterns are the primary influence on sedimentation in this tectonic setting the lacustrine deposits do not appear to be a reliable recorder of low frequency climatic change this is exemplified by the change from lacustrine to palustrine deposition at 6 5 ma which would appear to record a drying of the climate however a synchronous change from ephemeral alluvial fan deposition to more permanent fluvial deposition at the basin margins indicates increased run off

the kinematic and geodynamic significance of the atacama fault zone northern chile abstract the atacama fault zone afz is the dominant feature in the structure of the north chilean coastal cordillera left lateral displacement took place along its system of longitudinal faults during the jurassic and early cretaceous this development was contemporaneous with arc magmatism and was later reactivated resulting in a steep normal fault strike slip movements along the afz consist of two sets of ductile shear zones of different ages one jurassic formed under amphibolite facies conditions the other early cretaceous with greenschist facies mylonites structural asymmetries point to a sinistral sense of shear in both sets the afz can be interpreted as a magmatic arc structure which accommodated the oblique subduction of an oceanic plate trench linked strike slip fault the sinistral sense of shear is consistent with reconstructions of late jurassic to early cretaceous plate configurations in the se pacific

sedimentological response of an alluvial system to neogene thrust tectonics atacama desert northern chile the llano de la paciencia is a thrust sheet top basin in which the sedimentological and topographic evolution can be linked to thrust tip propagation it is an elongate gravel plain which borders the salar de atacama a major intermontane basin in the andean forearc of northern chile the llano is bounded to the west by the cerros de purilactis a cretaceous paleocene sequence uplifted by the frontal domeyko thrust the eastern margin of the llano is formed by the cordillera de la sal which was uplifted by a linked back thrust frontal thrust system the salar de atacama is thus divided into a number of discrete sub basins the llano de la paciencia the pampa visachita and the western sub llano which from east west are bounded by the cordillera de la sal the northern imbricates and the ignimbrite back thrust two phases of sedimentological evolution can be distinguished within the llano on the basis of quaternary to recent sediment dispersal patterns initially phase 1 alluvial fan lobes prograded eastwards into the main salar de atacama basin subsequently uplift of the cordillera de la sal deflected drainage systems southwards parallel to the structural strike these phase 2 alluvial deposits drain into the salar de atacama at the lateral termination of the cordillera de la sal frontal thrust in places where thrust tip ramps are emergent within the llano gullies have been incised into the drainage pathways this has resulted in the reworking of the early phase 1 gravels and progradation of the phase 2 fan lobes the evolution of the llano de la paciencia illustrates the close link between topography and syntectonic alluvial drainage patterns in an arid intermontane basin

abrupt change in the dip of the subducting plate beneath north chile no large tsunamigenic earthquake has occurred in north chile since 1877 and the region has been largely recognized as a mature seismic gap1 2 3 4 5 6 7 8 9 at the southern end of the seismic gap the 2007 mw 7 7 tocopilla earthquake ruptured the deeper seismogenic interface whereas the coupled upper interface remained unbroken4 6 7 seismological studies onshore show a gently varying dip of 20 to 30 of the downgoing nazca plate3 6 which extends from the trench down to depths of 40 50 km here we study the lithospheric structure of the subduction zone of north chile at about 22 s using wide angle seismic refraction and reflection data from land and sea complemented by hypocentre data recorded during the 2007 tocopilla aftershocks7 our data document an abrupt increase in the dip of the subducting plate from less than 10 to about 22 at a depth of approximately 20 km the distribution of the 2007 aftershocks indicates that the change in dip acted as a barrier for the propagation of the 2007 earthquake towards the trench which in turn indicates that the subduction megathrust is not only segmented along the trench but also in the direction of the dip we propose that large magnitude tsunamigenic earthquakes must cross the barrier and rupture the entire seismogenic zone

evidence for active landscape evolution in the hyperarid atacama from multiple terrestrial cosmogenic nuclides the atacama desert is one of the driest places on earth multiple lines of evidence show that the atacama has been hyperarid since at least the late miocene among these are cosmic ray exposure ages indicating that individual clasts on some surfaces have been preserved for 9 ma and possibly since the oligocene although these remarkably old ages indicate slow landscape evolution it is not clear whether this pace is characteristic of the entire atacama or only of specific regions landforms or landscape elements to address this question we measured cosmogenic 10be 26al and 21ne from a wide variety of landscape elements in a transect across the central atacama where modern precipitation is at an extreme minimum but where the concentration of cosmogenic nuclides in stable landscape elements has not previously been recorded we find that the hyperarid core of the central atacama has substantially slower erosion rates than its eastern and western margins however even the driest part of this transect has erosion rates comparable to those of other deserts ranging from 0 2 0 4 m ma the most stable landscape elements are boulder fields with exposure ages of 1 5 2 6 ma the vast majority of samples in the central atacama desert however have cosmogenic nuclide concentrations corresponding to ages 5 ma documented elsewhere in the atacama were not found in our area and appear to be limited to exceptionally stable boulders or cobbles in either the northern or southern extremes of the atacama desert we suggest that the central atacama has been subject to episodic pliocene and pleistocene rainfall and geomorphic activity perhaps due to intrusion of pacific moisture

testing the potential of k feldspar pir irsl and quartz esr for dating coastal alluvial fan complexes in arid environments abstract coastal alluvial fans caf are important geo archives due to their sensitivity to both tectonic activity and climatic changes thus they can give key insights for geomorphic and sedimentary processes in this study we test the potential of k feldspar post infrared infrared stimulated luminescence pir irsl and quartz electron spin resonance esr methods for dating alluvial fan complexes in arid environments the existing chronological data from marine terraces that interact with caf make the atacama desert in northern chile an excellent setting for this research samples have been collected from alluvial marine and aeolian sediments embedded in the caf complexes allowing evaluation of the different signal properties and bleaching characteristics of the pir irsl and esr signals over late pleistocene time scales pir irsl dose distributions of clast rich alluvial fan samples are characterised by higher scatter and demonstrate heterogeneous bleaching while matrix rich alluvial fan deposits show rather homogeneous poor bleaching in the dose dispersion as indicated by a modern analogue sample in contrast marine and aeolian deposits are homogeneously well bleached supported by a modern littoral sample following the quartz multiple centre mc esr dating approach al and ti centres bleaching of the different centres prior to deposition has been achieved while the ti h centre provides mostly lower doses than the ti li centre in most cases the al centre provides the highest dose values this pattern is consistent with their respective bleaching kinetics and suggests that the ti centre signals most likely provide the closest estimate to the true burial dose for samples with doses 200 gy esr and pir irsl ages are consistent at 2 for the marine aeolian and clast rich debris flow deposits which is in agreement with existing chronological data in this area it appears that the mode of sediment transport on alluvial fans either as matrix or clast rich flows plays an important role in sediment bleaching while clast rich alluvial fan deposits are likely better bleached we cannot exclude insufficient bleaching during matrix rich alluvial fan flows our dating results suggest that both pir irsl and esr dating overestimate the true burial age the combination of pir irsl and mc esr dating can be considered as a promising tool for deciphering alluvial fan formation over late pleistocene time scales

tectonic controls on sequence stacking pattern and along strike architecture in the pleistocene mejillones formation northern chile implications for sequence stratigraphic models abstract in the mejillones formation a shallow marine pleistocene succession of northern chile the cyclic stratigraphic record is the result of the complex interaction of regional uplift glacio eustasy local tectonics sediment supply and sedimentary processes stratal geometries characteristics of sedimentary facies and nature of sequence bounding unconformities have been investigated to evaluate the influence of i intrabasinal short term normal faulting on both along strike variations in sequence architecture and genetic complexity of key stratal surfaces and ii long term regional uplift on sequence stacking pattern the stratigraphic succession dissected by small displacement few meters normal faults striking obliquely with respect to the palaeo shoreline trends displays systematic variations in sequence architecture and the nature of bounding surfaces across them indeed depending on position with respect to the fault plane two basic types of internal organisation can be recognised in the examined shallow marine almost clastic starved sequence within grabens it consists of a siliciclastic rich transgressive systems tract tst which is bounded beneath by a transgressively modified glossifungites demarcated sequence boundary sb rs overlain by a mollusc bearing falling stage systems tract fsst the erosional downlap surface that separates the tst from the fsst is the regressive surface of marine erosion rsme on the footwall crests the combination of marine regressive erosion during falls in relative sea level and uplift has resulted in complete removal of the sediments of the tst from these sites leading to the formation of a tectonically enhanced basal unconformity composed of the rsme superimposed onto the previous sb rs sb rs rsme the prominent lateral change in component units systems tracts and nature of bounding surfaces within the studied sequence is directly related to the presence of normal faults and indicates that fault activity had a major impact on the sequence stratigraphic evolution of the mejillones formation enhancing subsidence within the grabens and promoting unconformities in the horsts overall the mejillones formation records a long term sea level fall driven by the contemporaneous regional uplift punctuated by repeated high frequency eustatic sea level changes the effect of this superimposition was that glacio eustatic sequences were displaced progressively downward and basinward and stacked in a distinct downstepping tectonically enhanced falling stage sequence set which reflects basin wide loss in accommodation space the sequence set is underlain by a composite rsme that becomes progressively younger basinward and is made up by the lateral and down dip connection of a series of lower rank sequence boundaries including hanging wall sb rss and footwall sb rs rsmes of successive sequences

tectonic and climatic controls on the spatial distribution of denudation rates in northern chile 18 s to 23 s determined from cosmogenic nuclides in the arid region of northern chile the environmental conditions are favorable for measuring tectonic and climatic influences on catchment denudation rates in the absence of vegetation previous studies of denudation rates from cosmogenic 10be and 26al concentrations are limited to single drainages in this study we examine catchment to orogen scale spatial variation in denudation rates between 18 23 s in the coastal and western cordilleras of northern chile 10be and 26al data were obtained from 33 catchments to examine the relative roles of tectonics and climate on catchment averaged denudation rates at broader scales we examine whether denudation rates and orogen topography reflect the 3d plate geometry of the region cosmogenic nuclide derived denudation rates range from 0 4 0 5 to 20 6 1 5 m myr in the coastal cordillera and from 1 4 0 7 to 168 0 19 8 m myr in the western cordillera the controls on the denudation rates are evaluated using a statistical factor analysis of ten selected catchment parameters denudation rates indicate a strong linear relationship with channel steepness indices but insignificant correlations and covariation with mean annual precipitation rates drainage area stream order mean elevation mean local relief mean basin slope and analyzed grain size of the sampled sediments moreover denudation rates are better correlated with tectonic controls at catchment scale than orogen scale plate tectonics in the western and coastal cordillera

fold and thrust belt structures and strike slip faulting at the se margin of the salar de atacama basin chilean andes 1 a tectonic reinterpretation is reported for the southeastern margin of the salar de atacama basin of northern chile detailed structural mapping revealed the presence of an east vergent thin skinned fold and thrust belt affecting oligocene miocene paciencia group rocks and the overlying plio pleistocene volcanic rocks along strike segmentation of the main fold implies local foreland influence on footwall ramp geometry leading to local thrust sheet rotation to the east the adjacent western slope of the western cordillera displays two different structural domains probably controlled by preexisting basement structures the southern domain comprises two n s oriented sigmoidal belts of linear arranged pressure ridges indicating left lateral transpression in contrast the northern domain is characterized by east vergent fold and thrust belt structures and reactivated nw se striking sinistral strike slip faults governing clockwise block rotations an indenter driven deformation model is proposed to explain sinistral transpression and clockwise block rotations around vertical axes this variant of a small block rotation mechanism is discussed in the context of oroclinal bending of the central andes emphasizing the significance of ancient structures in controlling rotations

a geomorphological approach to determining the neogene to recent tectonic deformation in the coastal cordillera of northern chile atacama the large 10000 km2 and local scale 300 m sedimentary succession was deposited east of the afs the succession fills previously deep paleovalleys and it consists of gravel so called atacama gravels which passes laterally into fine grained playa related deposits near the afs we interpret the deposition of this succession as a result of a blocking closure of the valley flowing from the precordillera due to the activity on afs a pedimentation episode followed sediment deposition and is locally strongly re incised by the main modern day river valleys draining the precordillera incision may result from either regional uplift of the forearc and or from more localized activity on the afs furthermore recent quaternary tectonic activity on the afs has been observed which is consistent with a localized relative uplift of the crustal block west of the afs

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

development of a continental forearc a cenozoic example from the central andes northern chile in order to understand the response of a continental forearc to changes in subduction zone geodynamics we constructed a high resolution chronostratigraphic cross section across the central andean forearc of northern chile 21 24 s the tectono stratigraphic development of the forearc differs from established models no relationship was found between changes in rate of relative plate convergence and amount and style of deformation forearc response to continual compression since the oligocene has been uplift and segmentation into discrete tectono stratigraphic zones from west to east these zones are the extensional coastal cordillera the extensional and or transtensional central depression and the transpressional and or compressional precordillera preandean depression each area has recorded almost continuous sedimentation from oligocene eocene time to the present day accommodation space has been generated by basin margin uplift rather than active subsidence we propose a model in which uplift of the leading edge of the south american plate is driven by subcrustal accretion of material removed at the trench by subduction erosion uplift and subduction erosion result in the trenchward gravitational collapse of the plate edge the tectono stratigraphic complexity exhibited within the central andean forearc is likely to be representative of cordilleran type margins and would be difficult to recognize in an ancient continental forearc

neodymium isotope geochemistry of felsic volcanic and intrusive rocks from the yukon tanana terrane in the finlayson lake region yukon canada devonian mississippian felsic rocks from the finlayson lake region have variable geochemical and nd isotopic characteristics that provide insights into the tectonic and metallogenic evolution of the yukon tanana terrane ytt and the northern cordillera late devonian 365 360 ma calc alkaline and tholeiitic arc felsic rocks in the mafic dominated fire lake unit yield end350 4 8 and 0 1 respectively and have 1 49 1 94 ga depleted mantle model ages tdm devonian mississippian 360 356 ma felsic volcanic kudz ze kayah unit wolverine succession and intrusive rocks grass lakes suite associated with volcanogenic massive sulphide vms deposits have end350 7 8 to 9 5 with tdm 1 59 2 25 ga a granitoid sample from the early mississippian 350 345 ma simpson range plutonic suite has end350 12 9 and tdm 2 01 ga similar to previously reported values for this suite the vms associated grass lakes suite of granitoids has higher high field strength element hfse and rare earth element ree contents and higher zr sc zr tio2 nb la and zr la values relative to the simpson range plutonic suite these geochemical features are similar to coeval vms associated felsic volcanic rocks in the kudz ze kayah unit the identification of similar hfse ree enriched felsic volcanic and subvolcanic intrusive rocks may aid in delineating prospective regions for vms mineralization in the ytt and other continental margin arc to back arc environments the geochemical and nd isotopic data for these ytt felsic rocks suggest that they reflect episodic mid paleozoic arc fire lake unit simpson range plutonic suite and back arc magmatism kudz ze kudz unit wolverine succession built upon a transitional basement with variable but significant influence from evolved proterozoic crustal materials

upper cenozoic volcanism and tectonism in the andes of northern chile abstract recent geological and radiometric information of the upper cenozoic volcanic sequences of northern chile in conjunction with previous evidences point out the complex evolution of the central andes volcanism volcanic activity developed along the andean range during the last 23 m y appears to have been temporally and genetically related to tectonic movements that shaped the major morphostructural features of this region since the middle miocene 12 m y there has been a constant overlap both in time and space between lavas and ignimbrites however ignimbrite eruptions seem to predominate up to the upper miocene while lava eruptions are dominant from the pliocene to the quaternary small scale chronological and volumetric variations of the volcanic activity in the studied areas make difficult the correlations with volcanic episodes through long distances units older than pliocene have been folded and thrusted as a result of compressive movements that occurred ca 7 4 m y ago this deformation is mostly remarkable at the borders of major faulted blocks these blocks were structured along pre existing fractures caused by tectonic movements during the lower miocene pliocene and quaternary block movements developed mainly through n s fault systems these faults together with nw se and ne sw systems control the spatial distribution of most volcanic vents in this region locally the n s faults may correspond to reactivations of previous fault zones this allowed successive effusions of volcanic material at least during the last 17 m y

late oligocene early miocene major tectonic crisis and related basins in bolivia recent advances in biochronology geochronology magnetostratigraphy structural geology and basin analysis make untenable the traditional correlation of the first major andeanage deformational episode in bolivia with the middle eocene incaic tectonic phase defined in central peru and instead demonstrate that the episode took place in late oligocene and early miocene time this major tectonic crisis resulted in contemporary development of the subandean external foreland basin and altiplano intermontane basin which were separated by the initiation of thrusting in the present cordillera oriental area the deformation suggests that the bolivian orocline began to develop at that time it is likely that this tectonic upheaval is genetically linked to the marked increase in rate of plate convergence produced by the contemporaneous breakup of the farallon plate

cenozoic stratigraphic development in the north chilean forearc implications for basin development and uplift history of the central andean margin abstract analysis of the cenozoic stratigraphic development of the forearc of northern chile between 18 s and 23 30 s allows constraints to be placed on the timing and nature of basin formation and the uplift history of the central andes chronostratigraphic charts have been constructed from 20 lithostratigraphic sections distributed throughout the forearc sections were taken from the longitudinal valley central depression calama basin salar de atacama precordillera and the western flank of the western cordillera correlation and timing of events is largely based on the presence of dated volcanic horizons in all the studied sections three chronostratigraphic units are defined based upon the presence of regional unconformities deposition of the late eocene to early miocene chronostratigraphic unit 38 19 ma commenced across an irregular unconformity surface between 38 and 30 ma with alluvial fan and fluvial sediments derived from the east interbedded with rhyolitic ignimbrites aggradation after 25 ma resulted in development of a large broad basin over much of northern chile that expanded eastwards through onlap onto basement deposition terminated around 19 ma with the development of an angular unconformity over much but not all of the study area during deposition of the early to late miocene chronostratigraphic unit 18 10 ma emergent volcanic source areas to the east provided catchments for large fluvial systems that drained westwards into endorheic ephemeral lacustrine basins fold growth affected sedimentation restricting accommodation space to small intra thrust basins in the precordillera and localised disruption and unconformity development in the longitudinal valley the late miocene to present day chronostratigraphic unit 10 0 ma followed the development of a regional angular unconformity at 10 ma sedimentation was restricted to a series of thrust bounded endorheic basins in both the central depression and the precordillera sourced from the east with volcanic activity limited to the periodic eruption of extensive ignimbrite sheets alluvial fan fluvial and lacustrine sedimentation dominated within the endorheic basins from 8 to 3 ma after development of a regional unconformity at 3 ma a change to isolated evaporite sub basins took place in the central depression with small lacustrine basins developed along the flank of the western cordillera the scale and grain size recorded in the sedimentary systems indicates that a substantial source area was located in the present day area of the western cordillera by 30 ma and that this has persisted to the present day this area also shed material eastwards into the altiplano the presence of such a topographic feature by 30 ma suggests that a significant proportion of andean uplift had occurred prior to the late miocene this important uplift phase should be incorporated into any model of andean uplift the evidence from the basin fill succession suggests that sediments accumulated in a basin developed in front of a broad monocline between 38 and 19 ma and that a transition to a thrust bounded foreland style basin took place after the development of the unconformity at 19 ma

controls on supergene enrichment of porphyry copper deposits in the central andes a review and discussion the central andes host some of the world s largest porphyry copper deposits the economic viability of these deposits is dependent on the size and quality of their supergene enrichment blanket published models that have strongly influenced exploration policy suggest that supergene enrichment ceased at 14 ma due to an increase in aridity here we discuss these models using published geochronological geomorphological and geological data geochronological data indicate that supergene oxidation and enrichment has been active between 17 and 27 s across the forearc of northern chile and southern peru from 44 to 6 ma and on the bolivian altiplano and eastern cordillera of argentina from 11 ma to present there is evidence for cessation at 20 14 and 6 ma however a major problem is that as more geochronological data become available the age ranges and periods of enrichment increase this suggests that the full spectrum of enrichment ages may not have been sampled the relationship between supergene enrichment and the age of regional pediplain surface development is not well constrained only in two areas have surfaces related to enrichment been directly dated southern peru and south of 26 s in chile and suggest formation post 14 ma sedimentological data indicate that a fluctuating arid semi arid climate prevailed across the atacama desert until between 4 and 3 ma climatic conditions that are thought to be favourable for supergene enrichment the balance between uplift erosion burial and sufficient water supply to promote enrichment is complex this suggests that a simple model for controlling supergene enrichment is unlikely to be widely applicable in northern chile general models that involve climatic desiccation at 14 ma related to rainshadow development and or the presence of an ancestral cold upwelling humboldt current are not supported by the available geological evidence the integration of disparate sedimentological geomorphological and supergene age data will be required to fully understand the controls on and distribution of supergene oxidation and enrichment in the central andes

non steady long term uplift rates and pleistocene marine terrace development along the andean margin of chile 31 s inferred from 10be dating pleistocene uplift of the chilean coast is recorded by the formation of wave cut platforms resulting from marine erosion during sea level highstands in the altos de talinay area 31 s we have identified a sequence of 5 wave cut platforms using in situ produced 10be exposure ages we show that these platforms were formed during interglacial periods at 6 122 232 321 and 690 ka these ages correspond to marine isotopic stages mis or substages miss 1 5e 7e 9c and 17 shoreline angle elevations used in conjunction with our chronology of wave cut platform formation illustrate that surface uplift rates vary from 103 69 mm ka between 122 and 6 ka to 1158 416 mm ka between 321 and 232 ka the absence of preserved platforms related to the mis 11 13 and 15 highstands likely reflects slow uplift rates during these times we suggest that since 700 ka the altos de talinay area was predominantly uplifted during 2 short periods following mis 17 and miss 9c this episodic uplift of the chilean coast in the pleistocene may result from subduction related processes such as pulses of tectonic accretion at the base of the forearc wedge

trace element geochemistry of metabasaltic rocks from the yukon tanana upland and implications for the origin of tectonic assemblages in east central alaska we present major and trace element geochemical data for 27 amphibolites and six greenstones from three structural packages in the yukon tanana upland of east central alaska the lake george assem

a 3 d geodynamic model of lateral crustal flow during andean mountain building although the andes are believed to have resulted mainly from crustal shortening the shortening history remains debated and appears to require lateral along strike crustal flow three dimensional viscous flow modeling shows that within geological uncertainties the andes may have been produced by either neogene shortening alone or with significant pre neogene shortening these scenarios require major along strike crustal flow and predict significantly different histories of uplift and crustal motion

end member models for andean plateau uplift abstract diverse techniques have been applied over the past decade to quantify the uplift history of the central andean plateau ap in this study opposing models for surface uplift are evaluated including a rapid rise of 2 5 km 10 6 ma and a slow and steady rise since 40 ma these end member models are evaluated by synthesizing observations of the ap lithosphere and the history of deformation sedimentation exhumation magmatism uplift and fluvial incision structural and geophysical studies estimate variable shortening magnitudes 530 150 km involving cover to basement rocks an isostatically compensated thick crust 80 65 km high heat flow and zones of variable velocity and attenuation in the crust and mantle these observations have invoked interpretations such as a hot weak lithosphere partial melt crustal flow and perhaps current localized delamination but do not provide strong support for massive delamination required by the rapid uplift model deformation and associated exhumation began 60 40 ma and generally migrated eastward with consistent long term average shortening rates 12 8 mm yr in bolivia favoring the slow uplift model volcanic and helium isotope evidence show an ap wide zone of shallow mantle melting and thin lithosphere that has existed since 25 ma which is inconsistent with the rapid rise model that suggests lithospheric thinning occurred 10 6 ma paleoaltimetry data suggest a rapid 2 5 km elevation gain 10 to 6 ma but are equally consistent within error with a linear rise since 25 ma widespread fluvial incision 2 5 1 km occurred along the western flank since 11 8 ma and may be associated with surface uplift as proposed by the rapid rise model however the paleoaltimetry and incision data can also be explained by regional climate change associated with plateau uplift implications of these results for reconstructions of ap evolution are that 1 substantial deformation of a weak lithosphere is essential 2 ap growth has taken significantly longer 40 myr and was more uniform along strike 1500 km than previously appreciated and 3 the slow and steady uplift model is most consistent with available constraints we conclude that the rapid uplift model may be an overestimate and that a more protracted cenozoic uplift history is tenable

slow rates of subduction erosion and coastal underplating along the andean margin of chile and peru comment and reply reply we thank encinas and finger 2007 1 for their comments on our paper clift and hartley 2007 2 as they add detail to the model we presented while leaving intact the essential idea of neogene tectonic erosion along the andean margin being followed by accretion and underplating since 2 ma

late neogene lacustrine record and palaeogeography in the quillagua llamara basin central andean fore arc northern chile abstract the cenozoic quillagua llamara basin northern chile central andes is an asymmetrical intramassif fore arc basin with a relatively wide northern sector separated from a narrower southward extension by a basement threshold the northern sector was characterised by a noticeable oligocene late neogene alluvial fan and lacustrine dominated deposition which resulted in sequences up to 900 m thick whereas the southern sector was often a bypass zone with thinner fluvial and lacustrine sediment accumulation the basin infill includes two third order alluvial lacustrine unconformity bounded units which include other higher frequency 4th to 5th order sequences the evolution of the late miocene pliocene lacustrine episodes in the quillagua llamara basin was not only controlled by the regional variations from arid to hyperarid palaeoclimate conditions due to the combined influence of the pacific high pressure cell the rain shadow effect exerted by the rising andes and the northward flowing cold oceanic currents but also by a the extensional tectonics and evolution and uplift of the fore arc region which defined the location and size of the depocentres b the resulting basement palaeorelief which affected sediment thickness and facies distribution during the late basin infill episodes and c the tectonic modifications of watersheds water divides and drainage networks in the precordillera which caused considerable changes of water income in the lacustrine systems understanding of this regional tectonosedimentary evolution is a necessary first step before analysing of the low to high order lacustrine sequence changes in the region lacustrine water supply was very sensitive to tectonics even gentle tectonic tilting and uplifting in critical water divide zones could result in changes in water balance in the lacustrine basins and trigger variations in the depositional record the very conspicuous lacustrine regime changes recorded in the quillagua llamara basin infill cannot be considered in themselves conclusive proof of an exclusive climatic forcing since they took place close to either major regional drainage changes or to gentle but noticeable tectonic reactivation in the fore arc region