paleogene synorogenic sedimentation in the altiplano plateau and implications for initial mountain building in the central andes sedimentologic data and palynological ages from paleogene clastic deposits of the northern and central altiplano plateau suggest foreland basin development in the central andes by mid paleocene time the nonmarine potoco formation 3000 6500 m thick constitutes the majority of cenozoic basin fill the potoco overlies the santa lucia formation 50 300 m thick previously dated as mid paleocene by mammal fossils and magnetostratigraphy new geochronologic data for the potoco formation include late eocene to oligocene palynomorph assemblages recovered throughout lower to upper stratigraphic levels these ages and published 40 ar 39 ar and k ar ages from overlying upper oligocene lower miocene volcaniclastic rocks indicate 1 nondeposition or greatly reduced deposition average sediment accumulation rates 10 m m y from mid paleocene to middle eocene time top santa lucia to lowermost potoco formation followed by 2 rapid deposition average sediment accumulation rates up to 500 m m y throughout late eocene and oligocene time majority of potoco formation lithofacies and paleocurrent data confirm both depositional phases a 20 100 mthick interval of superimposed paleosols in the basal potoco supports reduced sediment accumulation and low rates of subsidence during mid paleocene to middle eocene time the overlying main body of the potoco contains facies assemblages and stratigraphic architecture dominantly nonero

apatite feldspar u pb thermochronometer a reliable mid range 450 c diffusion controlled system the u pb systematics of apatite are commonly dominated by simple volume diffusion without secondary growth and recrystallization that can affect u pb systematics of sphene monazite and zircon although apatite is a relatively low u mineral both u and pb are lattice bound and thus the u pb systematics are not controlled by micro inclusions as appears to be the case for other low u minerals such as garnet and staurolite closure temperature estimates from both experimental and empirical studies range from 425 c to as high as 500 c for typical diffusion radii 25 m and cooling rates 2 100 c ma respectively this range can be exploited to constrain cooling and exhumation histories of both igneous and metamorphic rocks in addition the closure temperature for pb diffusion in apatite fills a gap between that for pb diffusion in sphene 650 c and diffusion of ar in hornblende 500 c muscovite 350 c and biotite 300 325 c apatite is a common accessory phase in many rock types and u pb apatite dating can be a reliable and powerful thermochronometer as it combines mid range cooling ages with the strength of the u pb system in testing for isotopic closure precisions of u pb apatite dates can range from 1 2 to as low as 0 3 for proterozoic or archean dates the main limitation to accuracy of u pb apatite dates is a moderate to low ratio of u to pb and corresponding dependence of concordia coordinates on the choice of initial pb isotopic composition for proterozoic aged samples with measured 206pb 204pb of 100 the range of geologically reasonable initial pb isotopic compositions can shift the calculated u pb age by 60 ma fortunately even though the diffusion rate of pb in alkali feldspar is less than in apatite it appears that the isotopic composition of pb in coexisting feldspar is a good approximation of the initial isotopic composition in apatite for many geologic settings examples are given from an archean terrane in se wyoming that was unroofed and cooled during proterozoic uplift and from proterozoic terranes in sw us that have contrasting thermal histories

seismicity deformation and seismic hazard in the western rift of corinth new insights from the corinth rift laboratory crl this paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of corinth greece the corinth rift laboratory crl is set up near the western end of the rift where instrumental seismicity and strain rate is highest the seismicity is clustered between 5 and 10 km defining an active layer gently dipping north on which the main normal faults mostly dipping north are rooting it may be interpreted as a detachment zone possibly related to the phyllade thrust nappe young active normal faults connecting the aigion to the psathopyrgos faults seem to control the spatial distribution of the microseismicity this seismic activity is interpreted as a seismic creep from gps measurements which shows evidence for fast continuous slip on the deepest part on the detachment zone offshore either the shallowest part of the faults is creeping or the strain is relaxed in the shallow sediments as inferred from the large ns strain gradient reported by gps the predicted subsidence of the central part of the rift is well fitted by the new continuous gps measurements the location of shallow earthquakes between 5 and 3 5 km in depth recorded on the on shore helike and aigion faults are compatible with 50 and 60 mean dip angles respectively the offshore faults also show indirect evidence for high dip angles this strongly differs from the low dip values reported for active faults more to the east of the rift suggesting a significant structural or rheological change possibly related to the hypothetical presence of the phyllade nappe large seismic swarms lasting weeks to months seem to activate recent synrift as well as pre rift faults most of the faults of the investigated area are in their latest part of cycle so that the probability of at least one moderate to large earthquake m 6 to 6 7 is very high within a few decades furthermore the region west to aigion is likely to be in an accelerated state of extension possibly 2 to 3 times its mean interseismic value high resolution strain measurement with a borehole dilatometer and long base hydrostatic tiltmeters started end of 2002 a transient strain has

refrigeration of the western cordilleran lithosphere during laramide shallow angle subduction the laramide orogeny has generally been attributed to a shift from normal angle to shallow angle subduction beneath the western margin of the north american plate in addition to important mechanical effects this shift may have had important thermal effects on the lithosphere in the western cordillera before the laramide geothermal gradients in the western cordillera were probably normal to high because of the presence below of a hot asthenospheric wedge with the shift to shallow subduction the wedge was probably expelled and replaced by a cold subducting slab that extended just below the cordilleran lithosphere this would shift the western cordillera into a cold forearc like thermal setting dominated by the refrigeration effects of the subducting slab thermochronologic data from the sierra nevada great basin mojave desert western arizona and perhaps the colorado plateau record latest cretaceous early tertiary cooling that may be evidence of such refrigeration if regional refrigeration occurred it would have several important implications 1 metamorphism in the western cordillera would have waned with the start of the laramide orogeny 2 the crust in the western cordillera would have strengthened and become much more resistant to deformation e g gravitational collapse and extension of thickened crust in the sevier hinterland would have been impeded and 3 latest cretaceous early tertiary isotopic cooling ages may not be valid indicators of a period of major regional uplift and unroofng

thermochronology geochronology and upper crustal structure of the cordillera real implications for cenozoic exhumation of the central andean plateau 1 structural mapping 40ar 39ar and fission track thermochronology u pb geochronology and basin analysis reveal rapid cooling during middle eocene late oligocene and late miocene pliocene exhumation in the central andean plateau of bolivia in the 4 6 km high cordillera real numerous granites and sw directed fold thrust structures define the central andean backthrust belt along the altiplano eastern cordillera boundary u pb zircon analyses indicate permo triassic granitic magmatism with less extensive magmatism of late oligocene age mapping reveals low magnitudes of slip 5 km depth the 26 ma quimsa cruz granite postdated most thrust structures suggesting that upper crustal shortening in the cordillera real had largely ceased by late oligocene time results of 40ar 39ar and fission track modeling help constrain the moderate to low temperature 350 c cooling history revealing two phases of rapid cooling from 45 40 ma to 26 ma and from 11 ma onward initial cooling coincided with middle eocene late oligocene deformation in the backthrust belt and associated deposition of coarse clastic sediments in the altiplano basin eocene oligocene exhumation of 7 5 km of upper crust is estimated on the basis of thermochronologic data rapid late miocene and younger cooling involved an estimated 3 5 km of exhumation and occurred in the apparent absence of upper crustal shortening these findings suggest that crustal shortening and resultant exhumation of middle eocene late oligocene age played a major role in construction of the central andes however for late miocene exhumation the importance of alternative nonshortening mechanisms is difficult to ascertain due to a poor understanding of subsurface structures we speculate that greater precipitation on the eastern edge of the central andean plateau north of 17 5 s was a key factor in driving rapid youthful exhumation of the cordillera real

sedimentary record of plate coupling and decoupling during growth of the andes geochronologic provenance and sediment accumulation records from the long lived 100 m y retroarc basin at the transition from the central to southern andes provide improved resolution to examine the duration and controls on mixed mode deformation and an enigmatic foreland depositional hiatus detrital zircon u pb ages for the malargue and neuquen basin systems of western argentina reveal shifts in exhumation and accumulation compatible with magmatic arc and thrust belt sources during unsteady cretaceous neogene deformation fully developed foreland basin conditions were only achieved during separate periods of late cretaceous and neogene shortening contemporaneous with possible episodes of enhanced coupling between a westward advancing south american plate and the subducting nazca slab separating these two contractional episodes is a 20 40 m y phase of reduced sedimentation and unconformity development potentially signifying a neutral to extensional mode across the retroarc hinterland to forearc region during diminished plate coupling we propose that the andean orogen and its foreland and forearc basins have always been sensitive to variations in subduction dynamics such that regional shifts in slab buoyancy and subduction geometry particularly slab dip superimposed on plate scale shifts in convergence have governed mechanical coupling along the plate boundary and resulting fluctuations among contractional extensional and neutral tectonic regimes

diagnosing an ancient shallow angle subduction event from cenozoic depositional and deformational records in the central andes of southern peru abstract the spectrum of responses exhibited by upper plates to changes in the subduction zone geometry remains poorly constrained in the paleogene the central andes in southern peru potentially experienced an episode of flat or shallow angle subduction presented here are new geochronologic thermochronologic basin subsidence backstripping analysis and 2d flexural modeling results these are synthesized with existing records on the temporal variation of the volcanic arc position basin fill chronostratigraphy deformation style and location and geophysical constraints on lithospheric architecture to investigate how this segment of the andes developed during a proposed ancient modification to the subduction zone dip basin chronostratigraphy confirms persistent contribution from volcanic arc products during the eocene miocene arguing against complete cessation of arc magmatism sediment accumulation was pulsed and generally more rapid during the late eocene oligocene event the subsidence curve is generally concave up atypical for a foreland basin situated within the compressional central andes the tectonic subsidence magnitude observed from backstripping analysis is at least 1 km less than the magnitude of subsidence predicted by 2d flexural model results this suggests a component of basin uplift potentially as a dynamic response to subduction shallowing may have opposed flexural subsidence that was driven by crustal thickening and topographic loading new geo and thermochronologic results constrain the timing of late triassic pluton emplacement and initial unroofing that predates the proposed timing of flat or shallow angle subduction this new timing suggests initial deformation with mixed thin and thick skinned deformation style in the eastern cordillera may be linked to reactivation of inherited structures rather than inboard deformation advance as a response to subduction flattening the lack of basement cored uplifts preservation of mantle lithosphere beneath this segment of the central andes and volcanic arc patterns argue against the occurrence of a flat slab event instead these results point to a shallow angle subduction event and highlight the diverse behaviors of cordilleran margins affected by variable subduction zone geometry and upper plates with inherited crustal heterogeneities

paleogeographic and tectonic controls on the evolution of cenozoic basins in the altiplano and western cordillera of southern peru abstract integrated studies of stratigraphy sedimentology paleogeography and tectonic controls on cenozoic basins provide the basis for a series of time slice reconstructions of basin evolution in the andes of southern peru the altiplano and adjacent margin of the western cordillera are characterized by several paleocene miocene synorogenic continental basins with thicknesses locally exceeding 10 km the evolution of these basins has been controlled by nw trending tectonic features that mark the altiplano western cordillera and altiplano eastern cordillera boundaries and the condoroma structural high sedimentary deposits of paleocene age preserved in the altiplano are the result of nonmarine sedimentation in a distal foreland basin during the early eocene predominantly dextral strike slip movements in the altiplano between the cusco lagunillas and urcos ayaviri fault systems created the transpressional kayra basin the soncco and anta basins middle eocene early oligocene are related to ne shortening 43 30 ma and represent proximal wedge top and foredeep basin environments preserved on the altiplano at 29 28 ma a change to predominantly e w shortening produced sinistral strike slip motion along nw striking faults resulting in intermontane transpressional basins in the altiplano the tinajani and punacancha 29 5 ma and paruro 12 6 ma basins were controlled by the cusco lagunillas and the urcos ayaviri fault systems the maure tincopalca huacochullo and condoroma basins 22 5 ma of the western cordillera developed between the condoroma high and the cusco lagunillas fault system oligocene miocene sedimentation commonly evolved from proximal alluvial facies along the borders to distal lacustrine facies these basins were linked to sinistral strike slip faults that evolved into reverse sinistral structures plate kinematics may play a role in andean basin evolution with deformation influenced by major preexisting faults that dictated paleogeographic trends but orientations of regional compression that appear to coincide with the plate convergence direction however the processes of slab flattening and steepening exerted a primary control on regional crustal shortening and filling of synorogenic basins

andean flat slab subduction through time the analysis of magmatic distribution basin formation tectonic evolution and structural styles of different segments of the andes shows that most of the andes have experienced a stage of flat subduction evidence is presented here for a wide range of regions throughout the andes including the three present flat slab segments pampean peruvian bucaramanga three incipient flat slab segments carnegie guanacos tehuantepec three older and no longer active cenozoic flat slab segments altiplano puna payenia and an inferred palaeozoic flatslab segment early permian san rafael based on the present characteristics of the pampean flat slab combined with the peruvian and bucaramanga segments a pattern of geological processes can be attributed to slab shallowing and steepening this pattern permits recognition of other older cenozoic subhorizontal subduction zones throughout the andes based on crustal thickness two different settings of slab steepening are proposed slab steepening under thick crust leads to delamination basaltic underplating lower crustal melting extension and widespread rhyolitic volcanism as seen in the caldera formation and huge ignimbritic fields of the altiplano and puna segments on the other hand when steepening affects thin crust extension and extensive within plate basaltic flows reach the surface forming large volcanic provinces such as payenia in the southern andes this last case has very limited crustal melt along the axial part of the andean roots which shows incipient delamination based on these cases a palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism the geological evolution of the andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system

comparison of crustal thickening budget and shortening estimates in southern peru 12 14 s implications for mass balance and rotations in the bolivian orocline two fundamental questions of interest with regard to the andean plateau are the mass balance of material needed to create and sustain a 3 4 km high plateau and the relationship between the plateau and the bending of the bolivian orocline the link be tween these two questions is the distribution of shortening through the central andes is crustal shortening suffi cient to support an isostatically compensated 60 70 km thick crust is differential shortening between the hinge of the orocline and its limbs suffi cient to account for both physiographic curvature and measured rotations three dimensional 3 d models of deformation within curved orogens require in addition to paleomagnetic data viable two dimensional estimates of dis placements to this end we present new map data and use those data to derive estimates of shortening across the northern margin of the andean plateau the cross section extent from the eastern edge of the volcanic arc to the foreland basin is approximately one half of the physiographic width of the andean plateau in peru cross sectional shortening estimates in southern peru 12 14 s provide a preferred estimate of 123 km or 40 short ening with a minimum estimate of 58 km or 25 shortening and maximum estimate of 333 km or 65 shortening the largest con trolling factor on the magnitude of shorten ing is where basement becomes involved in thrust belt deformation using our preferred shortening estimate the differential shorten ing between our cross section on the northern margin of the bolivian orocline 12 14 s and cross sections across the axis of the bolivian orocline 17 18 s is 177 km corresponding to 23 of rotation the differential shortening for the young younger than 15 ma defor mation of the altiplano and subandean re gion between 12 14 s and 17 18 s is 88 km correlating to 11 5 of rotation while dif ferential shortening can account for young younger than 15 ma vertical axis rotations it cannot completely account for early 45 35 ma rotations 37 15 this suggests that forearc rotations refl ect both bending due to differential shortening 12 as well as block rotations 25 30 which facilitated defor mation in a tightening core and transported material toward the center of the orocline the preferred estimate of shortening is well short of the required 240 300 km of shorten ing needed to account for a 60 70 km thick crust under the entire plateau this suggests that for an isostatically equilibrated crust either 1 there is a signifi cant amount of shortening 100 150 km in the western half of the plateau that is hidden by the volcanic arc or 2 crustal material is being added to the peruvian section of the andean plateau most likely through lower crustal fl ow

tectonic evolution of the central andean plateau and implications for the growth of plateaus current end member models for the geodynamic evolution of orogenic plateaus predict a slow and steady rise during crustal shortening and ablative subduction i e continuous removal of the lower lithosphere or b rapid surface uplift following shortening which is associated with punctuated removal of dense lower lithosphere and or lower crustal flow this review integrates results from recent studies of the modern lithospheric structure geologic evolution and surface uplift history of the central andean plateau to evaluate the geodynamic processes involved in forming it comparison of the timing magnitude and distribution of shortening and surface uplift in combination with other geologic evidence highlights the pulsed nature of plateau growth we discuss specific regions and time periods that show evidence for end member geodynamic processes including middle late miocene surface uplift of the southern eastern cordillera and altiplano associated with shortening and ablative subduction latest

nazca south america interactions and the late eocene late oligocene flat slab episode in the central andes 1 the most prominent features of the andean range are the altiplano and puna plateaus which were constructed by crustal shortening and uplift over the past 45 myr the early construction of these plateaus may have controlled subsequent growth of the orogen proposed models have suggested that an abrupt acceleration in relative motion between the nazca plate and the south american plate at 30 ma may have led to compression of the continent however the major plate motion change occurred at 25 23 ma and paleomagnetic rotations and crustal shortening of the andean forearc require that the arica bend formed prior to about 25 ma inferred history of flat slab subduction along the altiplano section of the andean margin and the structure of the adjacent south american cratonic shield combine to suggest an alternate scenario based partly upon geodynamic models of oceanic continental plate interactions in subduction zones we propose that central andean tectonism may have been controlled by two distinct regimes of subduction 1 oblique subduction along the central andean margin during the late eocene and oligocene accompanied by downdip alignment with the center of the amazonian shield flat slab activity in this phase of orogenesis may have been caused by a combination of cratonic root enhanced tectonics and oceanic plateau subduction and 2 an abrupt transition to trench normal subduction after 25 ma toward the more distal sao francisco craton was accompanied by a return to normal angle subduction similar interactions are hypothesized to have occurred during the laramide orogeny in western north america

shallowing and steepening subduction zones continental lithospheric loss magmatism and crustal flow under the central andean altiplano puna plateau

distribution of volcanoes in active margins we report a comprehensive study of the distribution of volcanoes in 16 active plate margins corresponding to a total of 479 volcanic systems the active volcanic arcs are found to have a ribbon geometry with an average length width ratio of around 10 the shape of the volcanic arc is compared to the shape of the associated trench by projecting one onto the other the projection direction agrees with the direction of plate convergence which shows that plate motion is the main factor constraining magma generation on the lithospheric scale there is no characteristic spacing between volcanoes in each arc the distribution of volcano spacing is best represented by a gamma distribution which corresponds to randomly generated points in the same geometrical conditions in order to explain how such distributions may be generated we have investigated in the laboratory the gravitational instability of a layer of buoyant liquid which is fed at a constant rate at the bottom of denser fluid different dynamic regimes are reached for different values of the viscosity ratio between the two fluids for low viscosity contrasts there is only one initial instability event with buoyant plumes organized in a periodic pattern these plumes are fed continuously by the source even at large times for high viscosity contrasts which is relevant to the geological problem plumes are produced intermittently by individual instability events they take the form of cavity plumes fed by narrow tails once the plumes have reached the surface they continue to be fed from below by material rising through their shrinking tail in successive instability events plume generation is not repeated at the same locations which leads to a complex plan form with time and hence with increasing number of instability events the cumulative distribution of plume spacing changes from periodic to random material from a single plume is able to sustain volcanic eruptions for a period longer than the time between two instability events and hence the distribution of active volcanic centers reflects the cumulative distribution of several instability events this is consistent with the observed distributions

thermochronometer record of central andean plateau growth bolivia 19 5 s 1 quantifying the timing magnitude and rates of exhumation and deformation across the central andes is a prerequisite for understanding the history of plateau rise we present 23 new apatite and zircon fission track thermochronometer samples to chronicle the exhumation and deformation across the entire 500 km andean fold thrust belt at 19 5 s in bolivia exhumation and deformation are constrained with inverse thermal modeling of the thermochronometer data regional stratigraphy geothermal gradients and mass deficits inferred from a balanced section results suggest the following 1 initial exhumation of the eastern cordillera ec fore thrust and back thrust belts began in the late eocene to early oligocene 27 36 ma and continued in a distributed manner in the late oligocene to early miocene 19 25 ma interandean zone ia exhumation began 19 22 ma followed by a third pulse of exhumation 11 16 ma in the ec back thrust belt and initial cooling in the westernmost subandes sa 8 20 ma finally exhumation propagated eastward across the sa during the late mio pliocene 2 8 ma 2 exhumation magnitudes are spatially variable and range from maximums of 8 km in the ec fore thrust belt to average values of 4 7 km across the ec 2 5 3 km in the altiplano 4 6 km in the ia and 3 km in the sa 3 exhumation rates range from 0 1 to 0 2 mm a in the ec from 0 1 to 0 6 mm a in the ia and from 0 1 to 0 4 mm a to locally 1 4 mm a or more in the eastern sa we synthesize similar constraints with sediments throughout bolivia and characterize plateau development by 1 distributed deformation throughout the altiplano and ec regions from 20 to 40 ma with minor deformation continuing until 10 ma 2 contemporaneous cessation of most ec deformation and exhumation of the ia 20 ma implying establishment of the modern plateau width with significant but unknown crustal thickness and elevation shortly thereafter by 15 20 ma and 3 dominantly eastward propagation of deformation from the ia since 20 ma with minor out of sequence deformation in the central to eastern sa

lithospheric structure beneath the northern central andean plateau from the joint inversion of ambient noise and earthquake generated surface waves the central andean plateau cap as defined by elevations in excess of 3 km extends over 1 800 km along the active south american cordilleran margin making it the second largest active orogenic plateau on earth the uplift history of this high plateau with an average elevation around 4 km above sea level remains uncertain as paleoelevation studies along the cap suggest a complex non uniform uplift history as part of the central andean uplift and the geodynamics of high topography caught project we image the s wave velocity structure of the crust and upper mantle using surface waves measured from ambient noise and teleseismic earthquakes to investigate the upper mantle component of plateau uplift we observe three main features in our s wave velocity model including 1 a positive velocity perturbation associated with the subducting nazca slab 2 a negative velocity perturbation below the subandean crust that we interpret as anisotropic brazilian cratonic lithosphere and 3 a high velocity feature in the mantle above the slab that extends along the length of the altiplano from the base of the moho to a depth of 120 km a strong spatial correlation exists between the lateral extent of this high velocity feature and the relatively lower elevations of the altiplano basin suggesting a potential relationship determining if this high velocity feature represents a small lithospheric root or foundering of orogenic lithosphere requires more integration of observations but either interpretation implies a strong geodynamic connection with the uppermost mantle and the current topography of the northern cap

plateau style accumulation of deformation southern altiplano 1 employing surface mapping of syntectonic sediments interpretation of industry reflection seismic profiles gravity data and isotopic age dating we reconstruct the tectonic evolution of the southern altiplano 20 22 s between the cordilleras defining its margins the southern altiplano crust was deformed between the late oligocene and the late miocene with two main shortening stages in the oligocene 33 27 ma and middle late miocene 19 8 ma that succeeded eocene onset of shortening at the protoplateau margins shortening rates in the southern altiplano ranged between 1 and 4 7 mm yr with maximum rates in the late miocene summing rates for the southern altiplano and the eastern cordillera we observe an increase from eocene times to the late oligocene to some 8 mm yr followed by fluctuation around this value during the miocene prior to shutoff of deformation at 7 8 ma and transfer of active shortening to the sub andean fold and thrust belt shortening inverted early tertiary graben and half graben systems and was partitioned in three fault systems in the western central and eastern altiplano respectively the east vergent fault systems of the western and central altiplano were synchronously active with the west vergent altiplano west flank fault system from these data and from section balancing we infer a kinematically linked western altiplano thrust belt that accumulated a minimum of 65 km shortening evolution of this belt contrasts with the eastern cordillera which reached peak shortening rates 8 mm yr in between the above two stages hence local shortening rates fluctuated across the plateau superimposed on a general trend of increasing bulk rate with no trend of lateral propagation this observation is repeated at the shorter length and time scales of individual growth structures that show evidence for periods of enhanced local rates at a timescale of 1 3 myr we interpret this irregular pattern of deformation to reflect a plateau style of shortening related to a self organized state of a weak crust in the central south american back arc with a fault network that fluctuated around the critical state of mechanical failure tuning of this state may have occurred by changes in plate kinematics during the paleogene initially reactivating crustal weak zones and by thermal weakening of the crust with active magmatism mainly in the neogene stage

late cretaceous early eocene laramide uplift exhumation and basin subsidence in wyoming crustal responses to flat slab subduction low angle subduction of the farallon oceanic plate during the late cretaceous early eocene is generally considered as the main driver forming the high rocky mountains in wyoming and nearby areas how the deformation was transferred from mantle to upper crust over the great duration of deformation 40 myr is still debated here we reconstruct basin subsidence and compile paleoelevation thermochronology and provenance data to assess the timing magnitude and rates of rock uplift during the laramide deformation we reconstruct rock uplift as the sum of surface uplift and erosion constrained by combining paleoelevation and exhumation with regional stratigraphic thickness and chronostratigraphic information the amount and rate of rock uplift of individual laramide ranges was less than 2 4 4 8 km 0 21 0 32 mm yr during the early maastrichtian paleocene stage 1 and increased to more than 3 km 0 38 0 60 mm yr during the late paleocene early eocene stage 2 our quantitative constraints reveal a two stage development of the laramide deformation in wyoming and an increase of rock uplift during stage 2 associated with enhanced intermontane basin subsidence exhumation and uplift during stage 1 is consistent with eastward migration of cordilleran deformation associated with low angle subduction whereas the change in exhumation during stage 2 seems to follow a southwestward trend which requires an alternative explanation we here suggest that the increase of rock uplift rate during the late paleocene early eocene and the southwestward younging trend of uplift may be a response to the rollback and associated retreating delamination of the farallon oceanic slab

thermal modelling of the laramide orogeny testing the flat slab subduction hypothesis abstract the laramide orogeny is the late cretaceous to palaeocene 80 55 ma orogenic event that gave rise to the rocky mountain fold and thrust belt in canada the laramide block uplifts in the usa and the sierra madre oriental fold and thrust belt in mexico the leading model for driving laramide orogenesis in the usa is flat slab subduction whereby stress coupling of a subhorizontal oceanic slab to the upper plate transmitted stresses eastwards producing basement cored block uplifts and arc magmatism in the foreland the thermal models presented here indicate that arc magma generation at significant distances inboard of the trench 600 km during flat slab subduction is problematic this conclusion is consistent with the coincidence of volcanic gaps and flat slab subduction at modern convergent margins lawsonite eclogite xenoliths erupted through the colorado plateau in oligocene time are inferred to originate from the subducted farallon slab and indicate that the laramide flat slab subduction zone was characterised by a cold thermal regime thermal modelling indicates that this regime can be produced by flat slab subduction of old 50 myr oceanic lithosphere at high convergence rates in the canadian and mexican portions of the laramide orogen the coeval development of a magmatic arc within 300 km of the trench refutes the existence of flat slab subduction in these regions it is proposed that subduction of an oceanic plateau aseismic ridge may have overcome the negative buoyancy inherent in old oceanic lithosphere and resulted in a spatially restricted zone of flat slab subduction in the usa these findings cast doubt on the flat slab model as a primary means of driving laramide orogenesis along its entire length and instead point to the need for an alternative mechanism for cordilleran wide laramide orogenesis

andean shortening inversion and exhumation associated with thin and thick skinned deformation in southern peru a balanced cross section spanning the eastern cordillera and subandean zone of southern peru 13 15 s constrains 130 km 38 of cenozoic orogen normal sw ne andean deformation accommodated by thick and thin skinned retro arc fold thrust belt shortening that overprinted pre andean triassic normal faults zircon and apatite u th he ages demonstrate continuous oligocene to miocene cooling of the permo triassic coasa pluton in the eastern cordillera zircon u th he ages 34 18 ma are reset and define a steep age versus elevation relationship apatite u th he results reveal reset ages that define two spatially separated groups with ages of 30 26 ma and 17 11 ma detrital zircon u pb geochronologic results from cretaceous cenozoic siliciclastic rocks from the altiplano eastern cordillera record andean fold thrust belt and magmatic arc sediment sources correlative subandean zone rocks preserve a cratonic sediment contribution with minor andean sediment appearing in some cenozoic rocks we propose that earliest andean deformation and structural compartmentalization of the eastern cordillera was linked to selective inversion of inherited permo triassic basement involved normal faults that guided subsequent thick and thin skinned deformation provenance variations between the hinterland and foreland depocentres reveal competing eastern and western sediment sources reflecting an axial zone in the eastern cordillera that coincided with the inherited triassic graben and impeded sediment source mixing our zircon and apatite u th he ages are consistent with published constraints along strike and support pulses of eocene to late miocene exhumation that were likely driven by normal fault reactivation and protracted eastern cordillera deformation

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

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

a teleseismic study of the altiplano and the eastern cordillera in northern bolivia new constraints on a lithospheric model the altiplano one of the world s great plateaus is the most significant geomorphological unit of the central andes knowledge of its lithospheric structure is essential to the understanding of the mountain building in this region in order to study this we performed a teleseismic field experiment in northern bolivia during a 4 month period 34 short period seismic stations were installed along a 320 km long profile crossing the altiplano and the eastern cordillera from the volcanic arc zone to the sub andean zone perpendicular to the main structural direction a careful study of relative travel time residuals led us to propose a qualitative model related to the structural patterns the maximum amplitude of these residuals along the profile reaches 3 s for p phases and have a strong azimuthal dependence the residuals show a noticeable sudden increase clearly associated with the fault system bordering the eastern cordillera to the west the cordillera real fault system the high quality of the data allows us to perform a velocity inversion and to calculate at what depth lateral variations occur the origin of the perturbations in the upper crust is reasonably accounted for by the depth variations of the sedimentary fill with the maximum thickness found under the altiplano basin the velocity perturbations in the lower crust are interpreted as variations of the moho depth which decreases from about 60 km below the altiplano to 50 km below the eastern cordillera in the upper mantle a high velocity zone is observed below the eastern cordillera the major feature of this model is a subvertical boundary dipping slightly to the southwest which separates two strongly contrasting velocity units throughout the model from the surface down to 140 km this boundary coincides with the cordillera real fault system and is interpreted as an old suture as the altiplano is characterized by low velocities in the crust the high velocities beneath the eastern cordillera down to 120 km may correspond to the brazilian craton in the region studied the western limit of the underthrusting of the craton corresponds to the western border of the eastern cordillera

basal continental mantle lithosphere displaced by flat slab subduction during the subhorizontal flat subduction of an ocean lithosphere plate the overlying continental plate is deformed far inland of the plate boundary in addition arc magmatism which is caused by melting in the asthenospheric wedge during steep subduction wanes or ends when the subduction is subhorizontal the observed upper plate deformation patterns have been explained by an end load exerted at the plate boundary or by stress transmitted into the overlying plate along the top of the flat slab here we present numerical thermal mechanical models of flat slab subduction that show the flattening of the slab results in a compression of the continental plate through end loading the advancing flat slab scrapes off the lowermost 20 50 km of continental mantle lithosphere the displaced continental mantle lithosphere fills the asthenospheric wedge ending arc type melting if the displaced material is buoyant it accumulates in a growing keel that migrates ahead of the slab if it is dense the displaced material sinks with the slab flat slab removal renews the asthenospheric wedge and arc magmatism and leaves a step in the lithosphere asthenosphere boundary and or a keel that consists of displaced continental mantle lithosphere a fossil keel and a fossil step formed during laramide flat subduction are preserved below the western united states continental mantle lithosphere is scraped from the base of the overriding plate by the underlying oceanic slab during flat subduction according to numerical thermal mechanical models

rapid regional surface uplift of the northern altiplano plateau revealed by multiproxy paleoclimate reconstruction the central altiplano is inferred to have experienced 2 5 1km surface uplift between 10 and 6 ma while the southern altiplano experienced a similar magnitude of surface uplift that began earlier between 16 and 9 ma to properly constrain the along strike timing of the altiplano plateau surface uplift it is necessary to know how and when the northernmost part of the altiplano plateau evolved we reconstruct the paleoclimate and infer the corresponding paleoelevation from the miocene pliocene deposits of the descanso yauri basin 14 15 s in the northernmost part of the altiplano plateau using 4 different proxies including carbonate clumped isotope composition i e 47 values carbonate 18 o c leaf wax d wax and pollen assemblages from paleosol lacustrine and palustrine carbonates and organic rich sediments the isotopic signatures reflect past climate conditions of mean annual air temperature 47 and meteoric water isotope values 18 o c d wax our results show that the northernmost plateau remained at low elevation 0 9 0 8 to 2 1 0 9km until late miocene time 9 ma characterized by 15 c warmer than modern temperature mean annual air temperature of 23 4 c 2 low elevation vegetation and precipitation signature with reconstructed 18 o mw vsmow of 8 3 2 0 2 from carbonate 18 o c and 8 6 1 8 2 from leaf wax d wax modern elevations of 4 km were not reached until 5 4 1 0ma as indicated by a negative shift in d wax vsmow from 143 4 12 8 2 to 209 2 21 1 2 between 9 1 0 7 and 5 4 1 0ma the timing of surface uplift of the northernmost altiplano is consistent with the evidence for late miocene surface uplift of the central altiplano 16 19 s between 10 and 6 ma and indicates that regional scale uplift in the northern central plateau significantly postdates the onset of surface uplift in the southern altiplano 19 22 s between 16 and 9 ma these results are consistent with piecemeal removal of the lower dense lithosphere combined with possible lower middle crustal flow from south to north in the plateau acting as the main mechanisms for the formation of the altiplano plateau

a review of the geodynamic evolution of flat slab subduction in mexico peru and chile abstract subducting plates around the globe display a large variability in terms of slab geometry including regions where smooth and little variation in subduction parameters is observed while the vast majority of subduction slabs plunge into the mantle at different but positive dip angles the end member case of flat slab subduction seems to strongly defy this rule and move horizontally several hundreds of kilometers before diving into the surrounding hotter mantle by employing a comparative assessment for the mexican peruvian and chilean flat slab subduction zones we find a series of parameters that apparently facilitate slab flattening among them trench roll back as well as strong variations and discontinuities in the structure of oceanic and overriding plates seem to be the most important however we were not able to find the necessary and sufficient conditions that provide an explanation for the formation of flat slabs in all three subduction zones in order to unravel the origin of flat slab subduction it is probably necessary a numerical approach that considers also the influence of surrounding plates and their corresponding geometries on 3d subduction dynamics

kinematics exhumation and sedimentation of the north central andes bolivia an integrated thermochronometer and thermokinematic modeling approach quantifying mountain building processes in convergent orogens requires determination of the timing and rate of deformation in the overriding plate in the central andes large discrepancies in both timing and rate of deformation prevent evaluating the shortening history in light of internal or external forcing factors geologic map patterns age and location of reset thermochronometer systems and synorogenic sediment distribution are all a function of the geometry kinematics and rate of deformation in a fold thrust belt foreland basin ftb fb system to determine the timing and rate of deformation in the northern bolivian andes we link thermokinematic modeling to a sequentially forward modeled balanced cross section isostatically accounting for thrust loads and erosion displacement vectors in 10 km increments are assigned variable ages to create velocity fields in a thermokinematic model for predicting thermochronometer ages we match both the pattern of predicted cooling ages with the across strike pattern of measured zircon fission track apatite fission track and apatite u th he cooling ages as well as the modeled age of fb formations to published sedimentary sections results indicate that northern bolivian ftb deformation started at 50 ma and may have begun as early as 55 ma acceptable rates of shortening permit either a constant rate of shortening 4 5 mm yr or varying shortening rates with faster rates 7 10 mm yr at 45 50 ma and 12 8 ma significantly slower rates 2 4 mm yr from 35 to 15 ma and indicate the northern bolivian subandes started deforming between 19 and 14 ma

large along strike variations in the onset of subandean exhumation implications for central andean orogenic growth abstract plate tectonics drives mountain building in general but the space time pattern and style of deformation is influenced by how climate geodynamics and basement structure modify the orogenic wedge growth of the subandean thrust belt which lies at the boundary between the arid high elevation central andean plateau and its humid low elevation eastern foreland figures prominently into debates of orogenic wedge evolution we integrate new apatite and zircon u th he thermochronometer data with previously published apatite fission track data from samples collected along four subandean structural cross sections in bolivia between 15 and 20 s we interpret cooling ages vs structural depth to indicate the onset of subandean exhumation and signify the forward propagation of deformation we find that subandean growth is diachronous south 11 3 ma vs north 6 2 ma of the bolivian orocline and that subandean exhumation magnitudes vary by more than a factor of two similar north south contrasts are present in foreland deposition hinterland erosion and paleoclimate these observations both corroborate diachronous orogenic growth and illuminate potential propagation mechanisms of particular interest is an abrupt shift to cooler more arid conditions in the altiplano hinterland that is diachronous in southern bolivia 16 13 ma vs northern bolivia 10 7 ma and precedes the timing of subandean propagation in each region others have interpreted the paleoclimate shift to reflect either rapid surface uplift due to lithosphere removal or an abrupt change in climate dynamics once orographic threshold elevations were exceeded these mechanisms are not mutually exclusive and both would drive forward propagation of the orogenic wedge by augmenting the hinterland backstop either through surface uplift or spatially variable erosion in summary we suggest that diachronous subandean exhumation was driven by piecemeal hinterland uplift orography and the outward propagation of deformation

the role of oceanic plateau subduction in the laramide orogeny the cause of the laramide phase of mountain building remains uncertain conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant hess or shatsky oceanic plateaux independent verification of these models has remained elusive because the putative ridges or plateaux are no longer at the earth s surface inverse convection models have identified two prominent seismic anomalies on the recovered farallon plate here we combine inverse convection models with reconstructions of plate motions to show that these seismic anomalies coincide palaeogeographically with the restored positions of the shatsky and hess conjugate plateaux as they subducted beneath north america specifically the distribution of laramide crustal shortening events tracked the passage of the shatsky conjugate beneath north america whereas the effects of the hess conjugate subduction were restricted to the northern mexico foreland belt we propose that continued subduction caused the oceanic crust to undergo the basalt eclogite phase transformation during which the shatsky conjugate lost its extra buoyancy and was effectively removed increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward followed by regional scale surface rebound we conclude that laramide uplift resulted from the removal rather than emplacement of the shatsky conjugate

zongo san gaban zone eocene foreland boundary of the central andean orogen northwest bolivia and southeast peru discordant muscovite and biotite k ar dates of samples taken in six transects through the cordillera oriental of southeast peru and northwest bolivia combined with sup 40 ar sup 39 ar age spectra reveal a northwest trending structurally cryptic tectonothermal zone of late eocene age ca 38 ma that overprints triassic and older igneous and metamorphic rocks the zone is at least 450 km long but is less than 25 km wide temperatures along its northeast boundary are estimated to have reached 350 sup 0 c no paleogene magmatism has been recognized that could account for the thermal resetting of the k ar isotopic system uplift erosion and southwest verging thrusting accompanied the development of this domain which defined the foreland boundary of the orogen prior to inception of northeast verging andean thrusting in miocene time it is proposed that this tectonothermal zone the zongo san gaban zone zsgz which involves thrusting and basement shortening is the southerly manifestation of the incaic compressional deformation which produced the thin skinned maranon thrust and fold belt mtfb north of the abancay deflection of central peru

dynamic uplift during slab flattening subduction exerts a strong control on surface topography and is the main cause of large vertical motions in continents including past events of large scale marine flooding and tilting the mechanism is dynamic deflection the sinking of dense subducted lithosphere gives rise to stresses that directly pull down the surface here we show that subduction does not always lead to downward deflections of the earth s surface subduction of young lithosphere at shallow angles flat subduction leaves it neutrally or even positively buoyant with respect to underlying mantle because the lithosphere is relatively warm compared with older lithosphere and because the thickened and hence drier oceanic crust does not undergo the transformation of basalt to denser eclogite accounting for neutrally buoyant flat segments along with large variations in slab morphology in the south american subduction zone explains along strike and temporal changes in dynamic topography observed in the geologic record since the beginning of the cenozoic our results show that the transition from normal subduction to slab flattening generates dynamic uplift preventing back arc marine flooding