strike slip faulting thrusting and related basins in the cenozoic evolution of the southern branch of the bolivian orocline abstract the evolution of the cenozoic deformation of the cordillera oriental and of the contact zone with the altiplano in southern bolivia is well documented in the tupiza nazareno and estarca basins the tectonic evolution started at about 29 ma the period between 29 and about 22 21 ma is marked by development of a pull apart basin related to n s trending left lateral strike slip faulting during this period initial deposition consisted of andesitic lavas and detrital sediments breccias matrix supported red conglomerates sands and silts of the catati and tupiza formation deposited in alluvial fan and flood plain environments from 21 to 20 ma the tectonic setting evolved to n s oriented thrusts which had a dextral component this event caused the tupiza basin to evolve into a full ramp basin and triggered the development of two piggy back basins nazareno and estarca both basins record detrital deposition nazareno formation in an alluvial fan environment at 10 9 ma deformation in this area stopped allowing peneplanation during which time the san juan de oro erosional surface was formed and the fluvial conglomerates of the oploca formation were deposited this segment of the andes was then tectonically transported to the east while uplift due to thrusting continued in the eastern andean front and in the subandean zone

balancing crustal thickening in arcs by tectonic vs magmatic means igneous intrusions were emplaced prior to and contemporaneous with horizontal shortening of the crust in the late cretaceous to late eocene magmatic arc in north chile 21 45 22 30 s temporally changing major and trace elements of magmatic rocks from this paleo arc system chronicled gradual crustal thickening prior to and substantial crustal thickening contemporaneously with crustal shortening balanced structural cross sections indicate a minimum of 9 km of arc normal shortening that occurred simultaneously with dextral arc parallel movements accounting for orogen parallel lengthening of 10 km this shortening produced 5 4 km of tectonic crustal thickening and resulted in a minimum of 42 km late eocene andean crustal thickness temporal and spatial geochemical changes diagnostic of crustal thickening indicate that the remainder 2 6 km was accommodated by basaltic underplating at or near the base of the arc crust prior to and during transpression the ratio of tectonic to magmatic crustal thickening is 2 1 whole crustal magmatic addition rates during the 12 m y duration of arc transpression are 35 km 3 per kilometer of model arc length per million years mafic underplating may have thickened the andean crust considerably but most pre neogene crustal thickening was due to discrete episodes of tectonic shortening

controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

revised deformation history of the central andes inferences from cenozoic foredeep and intermontane basins of the eastern cordillera bolivia 1 investigation of cenozoic deformation sediment accumulation and provenance in the eastern cordillera of bolivia at 17 21 s indicates major shortening 60 140 km and foredeep development followed by limited internal shortening and intermontane basin development contrasting histories of shortening deposition sediment dispersal and detrital composition distinguish a formerly extensive paleogene foredeep exposed along an eastern belt of synclines from a zone of principally neogene intermontane basins in the central eastern cordillera new 40ar 39ar ages of 25 17 ma for interbedded tuffs reveal latest oligocene early miocene accumulation in intermontane basins at rates 80 m myr several times lower than andean foredeeps although poorly dated foredeep evolution probably coincided with middle eocene oligocene deformation and denudation in adjoining regions to the west a mid cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement involved tectonic wedge beneath the eastern cordillera in this interpretation wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 ma whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas regardless of the regional subsurface structural geometry 40ar 39ar ages define a 25 21 ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the eastern cordillera and possibly the age of initial shortening in the interandean and subandean zones to the east in contrast to several popular models the revised tectonic history presented here suggests significant pre neogene shortening in the eastern cordillera and underscores the large uncertainties in estimates of long term shortening rates in the andes

slab pull and indentation tectonics insights from 3d laboratory experiments abstract we investigate using 3d laboratory experiments how the dynamics of indentation process are affected by the evolution at depth of the oceanic and continental subductions lithospheric plates are modelled by sand silicone plates floating on glucose syrup and the density contrast between oceanic and continental lithospheric plates and asthenosphere is reproduced analogue experiments model the convergence between two lithospheric plates a small continent indenting a large continental plate we show that the surface deformation in front of the indenter and above the oceanic subduction zone depends on the behaviour of the slab below the collision zone slab break off following the subduction of the small continent favours the indentation process because it results in an increasing compression in front of the indenter and extension above the neighbouring oceanic subduction both of them being responsible for the appearance of the indenter like geometry of the plate boundary when the slab does not deform significantly at depth in contrast the closure of the oceanic domain in front of the indenter is followed by a longer period of continental subduction during which the tectonic regime within the wide continent remains quite homogeneous comparing the presented analogue experiments with the subductions both part of the arabian indenter within eurasia our results suggest that the different tectonic regime on both sides of the arabia indenter may partly result from the probable occurrence of a detachment at depth under eastern anatolia

counterclockwise rotation of late eocene oligocene fore arc deposits in southern peru and its significance for oroclinal bending in the central andes the results of a paleomagnetic study along the fore arc of southern peru 15 18 s and northern chile 18 19 s are reported from middle to late miocene ignimbrites 7 sites late oligocene to early miocene ignimbrites 72 sites paleogene sediments 20 sites and mesozoic and paleocene volcanics and intrusions 31 sites comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5 2 11 3 clockwise to 55 6 7 0 counterclockwise spatially the magnitude of counterclockwise rotations increases northward from 0 within the chilean fore arc south of 18 30 s to 45 north of 16 30 s in southern peru paleomagnetic rotations recorded in paleogene red beds decrease from late eocene to late oligocene whereas miocene ignimbrites display no evidence of rotation these new results confirm that the rotations recorded in the fore arc of southern peru were acquired at least before 15 ma and probably before 25 ma and thus prior to the late neogene shortening of the sub andes the onset of major andean shortening in the eastern cordillera during the latest eocene earliest oligocene is interpreted to have triggered the bending of the peruvian fore arc the region of the peruvian fore arc with the largest rotations appears to be the fore arc counterpart of the abancay deflection a remarkable ne sw offset in the axis of the eastern cordillera induced by a major regional preorogenic structure we underline that the abancay deflection should be seen as the northwestern boundary and therefore as a key element of the bolivian orocline

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

late cenozoic deformation and uplift of the western flank of the altiplano evidence from the depositional tectonic and geomorphologic evolution and shallow seismic activity northern chile at 19 30 s 1 we analyze the west vergent thrust system wts along the western flank of the altiplano in northern chile 18 s 21 s in our study area 19 20 s 19 50 s the wts consists of three thrust propagation monocline folds flexures developing growth strata the relative uplift accommodated by the flexures is rapid between 26 and 8 ma 0 1 mm yr diminishing to 0 02 mm yr after 8 ma approximately 2000 m of relative surface uplift was accommodated by the flexures since the late oligocene sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 ma coeval with the shifting of deformation from the altiplano to the sub andean zone where the underthrusting of the brazilian craton would have resulted in crustal thickening surface uplift in the orogen and westward ductile subcrustal flow forearc tilting is accommodated by east vergent thrusts ets issued from the benioff zone beneath the central depression emerging into the western cordillera contributing 500 1400 m of surface uplift the wts connects the ets in the brittle ductile crustal transition 25 km depth continuing farther east as the altiplano low velocity zone configuring the western altiplano as a crustal scale fault bend fold forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ets meanwhile between 19 s and 21 s the wts accommodates dextral strike slip and 3 km of n s shortening occurred in the coastal cordillera transcurrence and strain partitioning are probably the result of slight plate convergence obliquity strong coupling within the interplate zone westward continental concavity and high elevation opposing horizontal contraction

the role of ridges in the formation and longevity of flat slabs flat slab subduction occurs when the descending plate becomes horizontal at some depth before resuming its descent into the mantle it is often proposed as a mechanism for the uplifting of deep crustal rocks thick skinned deformation far from plate boundaries and for causing unusual patterns of volcanism as far back as the proterozoic eon for example the formation of the expansive rocky mountains and the subsequent voluminous volcanism across much of the western usa has been attributed to a broad region of flat slab subduction beneath north america that occurred during the laramide orogeny 80 55 million years ago here we study the largest modern flat slab located in peru to better understand the processes controlling the formation and extent of flat slabs we present new data that indicate that the subducting nazca ridge is necessary for the development and continued support of the horizontal plate at a depth of about 90 kilometres by combining constraints from rayleigh wave phase velocities with improved earthquake locations we find that the flat slab is shallowest along the ridge while to the northwest of the ridge the slab is sagging tearing and re initiating normal subduction on the basis of our observations we propose a conceptual model for the temporal evolution of the peruvian flat slab in which the flat slab forms because of the combined effects of trench retreat along the peruvian plate boundary suction and ridge subduction we find that while the ridge is necessary but not sufficient for the formation of the flat slab its removal is sufficient for the flat slab to fail this provides new constraints on our understanding of the processes controlling the beginning and end of the laramide orogeny and other putative episodes of flat slab subduction

structure and cenozoic kinematics of the eastern cordillera southern bolivia 21 s 1 the eastern cordillera is a main morphotectonic unit of the central andes in southern bolivia it is predominantly composed of ordovician sedimentary rocks as thick as 10 km revision of the ordovician stratigraphy and new structural fieldwork resulted in detailed balanced cross sections improved estimates of crustal shortening and a new kinematic model for the eastern cordillera new estimates of total shortening in the eastern cordillera are up to 95 km 41 44 higher than previous estimates and sufficient to thicken the andean crust to its present state between the eastern foreland and the magmatic arc after initial tertiary uplift at 58 ma first thrusts developed in the middle eocene to early oligocene 40 30 ma exhuming the central part of the eastern cordillera tectonic shortening culminated in late oligocene and early miocene time 25 20 ma when west facing thrust systems developed in the central and western part of the eastern cordillera thrusting terminated in the western part by the end of the early miocene and middle miocene tectonic activity was restricted to minor strike slip faulting in the central part thrusting remained active until the late miocene by 10 ma shortening in the eastern cordillera essentially ceased kinematic fault analyses indicate clockwise rotation of the shortening direction from nne ssw or ne sw in eocene early oligocene time over e w to wnw ese or even nw se in late oligocene to middle miocene time nevertheless wnw ese directed shortening and subvertical extension predominate sinistral oblique motion on many thrusts probably reflects plan view flexural slip in the clockwise rotating southern limb of the bolivian orocline it is proposed that space problems in the oroclinal hinge slowed the rotation there and caused decoupling of the still rotating south limb along ne trending transfer zones

crustal scale structural architecture in central chile based on seismicity and surface geology implications for andean mountain building 1 we document a crustal scale structural model for the central chile andes based on seismicity and surface geology which consists in a major east verging ramp detachment structure connecting the subduction zone with the cordillera the ramp rises from the subducting slab at 60 km depth to 15 20 km below the western edge of the cordillera extending eastward as a 10 km depth flat detachment this structure plays a fundamental role in the andean orogenesis because most of the shortening has been accommodated by structures rooted in it and allows the distribution of crustal thickening in a simple shear deformation mode indeed despite shortening distribution being very asymmetric 16 km versus 70 km in the western and eastern side respectively the western side is higher and thicker than what is expected yield strength envelopes show strong rheological control on this structure vp and vp vs variations in the upper mantle and in the deepest limit of the seismogenic interplate contact mark the intersection of the ramp with the slab which coincides with the blueschist eclogite transition therefore subduction processes would control the depth where the major east verging structure may merge with the slab such a ramp flat structure is observed in other parts of the chilean margin hence it seems to be a first order feature in the andean subduction zone this structure delimitates upward the rocks transmitting part of the plate convergence stress from the plate interface and controls mountain building tectonics thus playing a key role in the andean orogeny

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

salar de atacama basin a record of compressional tectonics in the central andes since the mid cretaceous the salar de atacama basin lies in the inner fore arc of northern chile topographically and structurally it is a first order feature of the central andes the sedimentary fill of the basin constrains the timing and extent of crustal deformation since the mid cretaceous we have studied good exposures along the western edge of the basin and have correlated them with seismic reflection sections and data from an exploration well throughout most of its history the basin developed in a foreland setting during periods of thin skinned and thick skinned thrusting growth strata provide evidence for coeval sedimentation and thrust motions during mid cretaceous paleogene and neogene times pre neogene deformation was significant in the basin and in surounding areas of the early central andes models that attempt to explain the current thickness of the central andes should consider late cretaceous and paleogene shortening as well as the more obvious neogene and quaternary shortening

style and history of andean deformation puna plateau northwestern argentina topographically the puna plateau of northwestern argentina is the southern continuation of the bolivian altiplano its thickening and consecutive uplift result from the andean orogeny to better constrain the structural style and its progressive development we have studied field data topographic and satellite imagery balanced cross sections seismic reflection data kinematic analysis of fault slip data anisotropy of magnetic susceptibility ams paleomagnetic data and apatite fission track aft data across the puna plateau precambrian and paleozoic basement ranges bounded by high angle reverse faults dips 60 alternate with cenozoic intermontane basins major thrusts trend nne ssw and do not show a preferred vergence intermontane basins have various degrees of symmetry depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets there is a close correlation between the surface expression of a basin and the amount of internal deformation a line balanced cross section of the puna at 25 s has yielded a cenozoic shortening of 10 15 in a direction subperpendicular to the orogen by kinematic analysis of cenozoic fault slip data we have obtained principal directions of strain rate across the puna shortening axes are subhorizontal and trend on average wnw ese n110 stretching axes are subvertical and intermediate axes are subhorizontal and trend on average nne ssw strain ellipsoids are dominantly of plane strain type and they represent dip slip thrusting from paleomagnetic and ams data shortening axes form a radial pattern around the eastern edge of the central andes the pattern is attributed to an inhomogeneous stress field reflecting the eastward convex shape of the central andean thrust front from the history of burial and uplift andean shortening reached the northeastern part of the puna in the late eocene and the adjacent eastern cordillera in the late eocene or early oligocene this shortening was presumably due to the incaic phase of the andean orogeny in the eastern part of the orogen the onset of shortening was probably guided by preexisting paleozoic and mesozoic structures so that andean deformation propagated unevenly eastward

p and s velocity images of the lithosphere asthenosphere system in the central andes from local source tomographic inversion summary about 50 000 p and s arrival times and 25 000 values of t recorded at seismic arrays operated in the central andes between 20 s and 25 s in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3 d p s velocity and qp models the study volume in the reference model is subdivided into three domains slab continental crust and mantle wedge a starting velocity distribution in each domain is set from a priori information in the crust it is based on the controlled sources seismic studies in slab and mantle wedge it is defined using relations between p and s velocities temperature and composition given by mineral physics each iteration of tomographic inversion consists of the following steps 1 absolute location of sources in 3 d velocity model using p and s arrival times 2 double difference relocation of the sources and 3 simultaneous determination of p and s velocity anomalies p and s station corrections and source parameters by inverting one matrix velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double sided nodes at the domain boundaries the next iteration is repeated with the updated velocity model and source parameters obtained at the previous step different tests aimed at checking the reliability of the obtained velocity models are presented in addition we present the results of inversion for vp and vp vs parameters which appear to be practically equivalent to vp and vs inversion a separate inversion for qp has been performed using the ray paths and source locations in the final velocity model the resulting vp vs and qp distributions show complicated essentially 3 d structure in the lithosphere and asthenosphere p and s velocities appear to be well correlated suggesting the important role of variations of composition temperature water content and degree of partial melting

uplift of the western altiplano plateau evidence from the precordillera between 20 and 21 s northern chile 1 we present a quantitative reconstruction of uplift of the western flank of the altiplano plateau central andes one of the largest monoclines on the earth on the basis of an analysis of tectonic structures syntectonic deposits and geophysical data uplift occurred on a west vergent slowly propagating system of high angle reverse faults merging into a joint detachment that ramps down to midcrustal levels below the plateau edge the upper ramps determine local fold geometries while the lower ramp controls large scale surface tilting and uplift at 20 s this fault system was active between 30 ma and 5 10 ma with maximum shortening rates of 0 22 mm yr between 17 and 10 ma it generated some 2600 m of surface uplift with only minor shortening of 3000 m its activity was largely synchronous to eruption of large volume ignimbrites from a midcrustal source geophysical data indicate that the fault system localized deformation at the boundary between a cool strong forearc crust and a presumably fluid rich and or partially molten zone underneath the plateau area the systematic relation between crustal melting and shortening with uplift at the western plateau margin can be traced along most of the plateau flank with a stepwise decrease in age of deformation and magmatism toward the south indicating discontinuous addition of plateau segments crustal thickening to as much as 70 km from westward underthrusting in the back arc parts of the plateau isostatically compensated the tectonic surface uplift and monocline formation with respect to a stable forearc which only reacted with minor tilting

tectonic development of the north patagonian andes and their related miocene foreland basin 41 30 43 s 1 the northern patagonian andes have been constructed through multiple mechanisms that range from tectonic inversion of extensional structures of early to middle jurassic age in the main andes to oligocene in the precordilleran region these have acted during two distinctive orogenic stages first in late early cretaceous and later in miocene times late oligocene extension separates these two contractional periods and is recorded by half grabens developed in the retroarc region the last contractional stage coexists with an eastward foreland expansion of the late miocene arc whose roots are presently exposed as minor granitic stocks and volcanic piles subordinately in the main andes east of the present arc as a consequence of this orogenic stage a foreland basin has developed having progressed from 18 ma in the main north patagonian andes where the mountain front was flooded by a marine transgression corresponding to the base of the nirihuau formation to 11 ma in the foreland area cannibalization of this foreland basin occurred initially in the hinterland and then progressed to the foreland zone blind structures formed a broken foreland at the frontal zone inferred from growth strata geometries during pliocene to quaternary times most of the contractional deformation was dissipated in the orogenic wedge at the time when the arc front retracted to its present position

geomorphic evidence for post 10 ma uplift of the western flank of the central andes 18 30 22 s 1 the western andean mountain front forms the western edge of the central andean plateau between 18 5 and 22 s latitude the mountain front has 3000 m of relief over 50 km horizontal distance that has developed in the absence of major local neogene deformation models of the evolution of the plateau as well as paleoaltimetry estimates all call for continued large magnitude uplift of the plateau surface into the late miocene i e younger than 10 ma longitudinal river profiles from 20 catchments that drain the western andean mountain front contain several streams with knickpoint bounded segments that we use to reconstruct the history of post 10 ma surface uplift of the western flank of the central andean plateau the generation of knickpoints is attributed to tectonic processes and is not a consequence of base level change related to pacific ocean capture eustatic change or climate change as causes for creating the knickpoint bounded stream segments observed minor valley filling alluvial gravels intercalated with the 5 4 ma carcote ignimbrite suggest uplift related river incision was well under way by 5 4 ma the maximum age of river incision is provided by the regionally extensive approximately 10 ma el diablo altos de pica paleosurface the river profiles reveal that relative surface uplift of at least1 km occurred after 10 ma

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

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

influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge abstract geological observations indicate that along two active continental margins east asia and mediterranean major phases of overriding plate extension resulting from subduction hinge retreat occurred synchronously with a reduction in subducting plate velocity in this paper results of fluid dynamical experiments are presented to test the influence of the velocity of the subducting plate on the hinge migration velocity and on the geometry of the slab results show that hinge retreat decreases with increasing subducting plate velocity in addition phases of hinge retreat alternate with phases of hinge advance for relatively high subducting plate velocities due to interaction of the slab with the bottom of the box simulating the upper lower mantle discontinuity such slab kinematics could explain the episodic behaviour of back arc opening observed in convergent settings the geometry of the slab and the kinematics of subduction are significantly affected by the velocity of the subducting plate three subduction modes with accompanying slab geometry can be recognized a relatively low subducting plate velocity is accompanied by relatively fast hinge retreat with backward sinking of the slab and a backward draping slab geometry with increasing subducting plate velocity hinge migration is relatively small resulting in subvertical sinking of the slab and a folded slab piling geometry for a very high velocity the hinge migrates forward resulting in forward oriented subduction vectors and a forward draping slab geometry

the role of the overriding plate thermal state on slab dip variability and on the occurrence of flat subduction 1 slab dip varies significantly both between different and along single subduction zones provided that old subducting plates are colder and denser than young plates variations in the slab dip should correlate with slab age however recent statistical analyses do not show this expected correlation we present the results of non newtonian numerical dynamic models where subduction is driven by means of a kinematic boundary condition we systematically vary the age of both the overriding and subducting plates in order to test these effects on the slab dip at different depth ranges we find that colder overriding plates result in shallower slab dips and episodes of flat slab subduction as a result of the increased suction force in the mantle wedge the influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere modeling results are qualitatively compared to the large dip variability of the cocos slab including a flat slab segment we suggest that this variability is likely related to the change of the thermal state of the overriding plates with flat subduction occurring under cold lithosphere in southwestern mexico and steep subduction under the warmer lithosphere of the northwestern caribbean plate

exhumation history of the southern altiplano plateau southern bolivia constrained by apatite fission track thermochronology 1 although the structural geometry of the cenozoic altiplano puna plateau in the central andes is well defined the temporal evolution of this contractile deformation is poorly constrained to address this shortcoming we used apatite fission track thermochronology aft to quantify the cooling and exhumation history along a transect at 21 s in southern bolivia through the deformed intermontane altiplano basin the doubly vergent thrust belt of the eastern cordillera and the inner foreland thrust belt east of the plateau interandean zone thermal history modeling combined with published balanced cross sections and stratigraphic data constrain exhumation histories exhumation started during the late eocene 40 36 ma in the central eastern cordillera possibly due to bivergent thrusting and cretaceous rift structure inversion during the early oligocene 33 27 ma exhumation spread across the study area as the current boundary thrusts of the eastern cordillera were activated the inner west vergent thrust system became active in irregular order until circa 20 ma whereas the east vergent interandean thrust belt formed by eastward propagating deformation since circa 30 ma plateau exhumation continued at 0 2 mm yr until shortening terminated by 11 7 ma shortening within the plateau since circa 30 ma did not evolve by lateral accretion of thrust wedges the propagating deformation style is spatially confined to the foreland thrust belt which initiated coeval to plateau deformation interandean zone but propagated mainly after circa 10 ma subandean zone early oligocene plateau wide tectonically driven exhumation suggests that subduction related processes had already thermally weakened the continental lithosphere prior to the 27 25 ma onset of volcanic activity

the kinematic history of the central andean fold thrust belt bolivia implications for building a high plateau this paper presents a model for the kinematic evolution of the central andean plateau based on balanced cross sections across the bolivian andes the proposed model links the formation of the andean plateau to the development of the andean fold thrust belt through the creation and propagation of two large basement mega thrusts support for large basement involved thrust sheets is found in significant steps in both the topography and the exposed structural elevation of the andean fold thrust belt the structurally highest basement thrust raised folds and faults in predominantly lower paleozoic rocks of the eastern cordillera with respect to tertiary rocks in the broad internally drained basin of the altiplano to the west and east verging folds and faults in upper paleozoic rocks of the interandean zone to the east the interandean zone was in turn raised both structurally and topographically with respect to the frontal folds and faults of the fold thrust belt the subandean zone by a second structurally lower basement thrust sheet thus these two megathrusts divide the andean fold thrust belt into four areas of markedly different structural elevations the eastern cordillera can be further subdivided into two zones of west and east vergent folds and thrusts shortening accommodated by the fold thrust belt can be divided among these tectono structural zones and linked to shortening accommodated by the inferred basement megathrusts the proposed kinematic model suggests that the eastward propagation of the structurally highest basement thrust fed 105 km of slip into the eastern cordillera along east vergent and west vergent faults this structure also fed 90 km of eastward slip into the interandean zone the initiation and eastward propagation of a lower basement thrust structurally elevated the interandean zone with respect to the foreland while feeding 65 km of slip into the subandean zone out of sequence basement thrusting to the west is proposed to have elevated the western edge of the plateau and accommodated 40 km of shortening within the altiplano total cumulative shortening within the cover rocks of the andean fold thrust belt 300 330 km can be balanced by an equivalent amount of shortening along two basement megathrusts to the first order the eastern margin of the central andean plateau defined by the 3 km topographic contour is contiguous with the leading edge of the upper basement megathrust this relationship between the basement highs and the physiographic boundaries of the andean plateau suggests that extensive megathrust sheets involving strong rocks such as crystalline basement or quartzite play an important role in the formation of the central andean plateau and a similar link between megathrust sheets and plateaus may be found in other orogens

organic maturation in the french subalpine chains regional differences in burial history and the size of tectonic loads the french subalpine chains of southeastern france are the foreland fold thrust belt of the western alps formed in the mid to late tertiary as a result of the continental collision between the european and italian north african plates with the change from passive margin to destructive margin certain areas of the subalpine chains experienced enhanced temperatures through being buried to greater depths burial of these areas has been investigated using vitrinite reflectance r o levels of organic maturity of mesozoic and tertiary horizons have been established using vitrinite reflectance r o and spore fluorescence in the southwestern part of the chain values of r o for mesozoic units from the vercors and chartreuse massifs are generally low with all cretaceous units having values r o older jurassic horizons have higher values up to 1 25 for the mid jurassic oxfordian when compared to values from the same horizons and from younger tertiary units published data from the northeastern part of the chain it is clear that this part of the belt has encountered higher temperatures produced by greater depth of burial in the haut giffre and aravis massifs mesozoic units have r o values as high as 2 7 and in terms of hydrocarbon generation can be considered mature or overmature two causes of greater depth of burial of the northeastern part of the belt are indicated by field studies and by burial history modelling of specific localities of the subalpine chains first the earlier initiation of the tertiary foredeep in the northeast compared to the southwest and second differences in the size of tectonic loads emplaced onto the subalpine chains during the final stages of alpine compression with the greater structural deformation of the subalpine chains occurring in the northeast

new constraints on the chronology magnitude and distribution of deformation within the central andean orocline 1 constructing an accurate kinematic model for crustal thickening in the central andes is hampered by the inability of 2 d balanced cross sections to resolve the 3 d displacement field within the poorly studied orocline core this study presents new structural and thermochronometer data from the orocline axis in the central andes that constrains the magnitude and timing of deformation through 1 a balanced cross section quantifying shortening perpendicular to strike 2 field observations that constrain translational displacements parallel to strike and 3 thermochronology that brackets the age of exhumation and deformation mapping results show that north directed right lateral motion has been accommodated on at least one major strike slip fault in the eastern cordillera ec shortening perpendicular to fault strike is 179km 41 for the ec and inter andean zone ia and 86km 39 for the sub andes sa apatite fission track cooling ages indicate rapid ec ia exhumation from 46 to 18ma and 15 to 0ma in the sa zircon and apatite u th he ages show that rapid exhumation began 10 5ma in the sa these results are integrated with existing paleomagnetic and gps rotation data in map view reconstructions to quantify the magnitude of translational faulting related to oroclinal bending the reconstructions suggest that at least 85km of north directed translational displacement has been accommodated within the orocline core orogen parallel displacement and vertical axis rotations have focused crustal thickening at the orocline core and facilitated development of a high elevation plateau suggesting a genetic link between large scale oroclines and plateaus

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

central andean mantle and crustal seismicity beneath the southern puna plateau and the northern margin of the chilean pampean flat slab earthquake hypocenters recorded in the andean southern puna seismic array 25 28 s 70 65 w provide new constraints on the shape of the subducting nazca plate beneath the puna plateau the transition into the chilean pampean flat slab and the thermal state of the mantle and crust some 270 new mantle hypocenters suggest that the subducting slab under the puna shoals into the flat slab segment more abruptly and farther to the north than previously indicated the revised geometry is consistent with the central volcanic zone incapillo caldera being the southernmost center with pleistocene activity until reaching the southern side of the flat slab region evidence for the revised slab geometry includes three well defined hypocenter clusters in the pipanaco nest 27 5 29 s 68 66 w which are interpreted to reflect slab bending stresses a few low magnitude earthquakes with strongly attenuated s waves in the long recognized antofalla teleseismic gap 25 5 27 5 s support a continuous slab under the southern puna the paucity of gap earthquakes and the presence of mafic magmas are consistent with a hot mantle wedge reflecting recent lithospheric delamination evidence for a hot overlaying puna crust comes from new crustal earthquake hypocenters concentrated at depths shallower than 5 km two notable short duration swarms were recorded under the resurgent dome of the 2 ma back arc cerro galan caldera and the near arc cerro torta dome new crustal earthquake focal mechanisms from 17 events in the array along with two existing mechanisms have strike slip oblique reverse and oblique normal solutions fitting with regional e w compression and n s extension

seismic imaging of the cocos plate subduction zone system in central mexico broadband data from the meso america subduction experiment mase line in central mexico were used to image the subducted cocos plate and the overriding continental lithosphere beneath central mexico using a generalized radon transform based migration our images provide insight into the process of subducting relatively young oceanic lithosphere and its complex geometry beneath continental north america the converted and reverberated phase image shows complete horizontal tectonic underplating of the cocos oceanic lithosphere beneath the north american continental lithosphere with a clear image of a very thin low velocity oceanic crust 7 8 km which dips at 15 20 degrees at acapulco then flattens approximately 300 km from the middle america trench farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the trans mexican volcanic belt tmvb where the slab bends underneath the tmvb the migrated image depicts the transition from subducted oceanic moho to continental moho at 230 km from the coast neither of which were clearly resolved in previous seismic images the deeper seismic structure beneath the tmvb shows a prominent negative discontinuity fast to slow at 65 75 km within the upper mantle this feature which spans horizontally beneath the arc 100 km may delineate the top of a layer of ponded partial melt

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

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

seismic imaging of a convergent continental margin and plateau in the central andes andean continental research project 1996 ancorp 96 1 a 400 km long seismic reflection profile andean continental research project 1996 ancorp 96 and integrated geophysical experiments wide angle seismology passive seismology gravity and magnetotelluric depth sounding across the central andes 21 s observed subduction of the nazca plate under the south american continent an east dipping reflector nazca reflector is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km we interpret parts of the nazca reflector as a fluid trap located at the front of recent hydration and shearing of the mantle the fluids being supplied by dehydration of the oceanic plate patches of bright quebrada blanca bright spot to more diffuse reflectivity underlie the plateau domain at 15 30 km depth this reflectivity is associated with a low velocity zone p to s wave conversions the upper limits of high conductivity and high vp vs ratios and to the occurrence of neogene volcanic rocks at surface we interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during neogene shortening and plateau growth the imaging properties of the continental moho beneath the andes indicate a broad transitional character of the crust mantle boundary owing to active processes like hydration of mantle rocks in the cooler parts of the plate margin system magmatic underplating and intraplating under and into the lowermost crust mechanical instability at moho etc hence all first order features appear to be related to fluid assisted processes in a subduction setting

strain accumulation controls failure of a plate boundary zone linking deformation of the central andes and lithosphere mechanics 1 we make use of observations on orogenic strain accumulation and deformation partitioning in the central andes to explore the backarc strength evolution at the lithospheric scale in plan view the altiplano puna plateaux experienced rapid initial increase of surface area undergoing active deformation during the cenozoic beyond the maximum lateral extent reached around 10 15 ma 40 50 of entire proto andes undergoing deformation at 10 20 total strain rapid localization initiated at the eastern flank of the altiplano inter and subandean thrust belt but not at the puna latitude localization was associated with a significant increase in bulk shortening rate average fault slip rates equally increased by an order of magnitude following a protracted period of stable average rates estimates of strength evolution based on force balance calculations and critical wedge analysis suggest significant backarc weakening driving this change after the middle miocene strain accumulation led to localization and weakening with development of a detachment propagating through crust and upper mantle we find that lithosphere scale failure resulting from strain weakening beyond a critical strain threshold c 20 and fault coalescence with formation of a weak detachment in shales effective coefficient of friction 0 1 plays a key role in the evolution of the andes strain related lithosphere weakening appears to dominate over the impact of external forcing mechanisms such as variations of plate convergence mantle assisted processes or erosion comparison of these orogen scale observations with experimental rock rheology indicates substantial similarity of deformation behavior with similar weakening thresholds across a wide range of scales

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

ore genesis and hydrothermal evolution of the donggebi porphyry mo deposit xinjiang northwest china evidence from isotopes c h o s pb fluid inclusions and molybdenite re os dating

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

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

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

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

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

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

evolution of crustal thickening in the central andes bolivia abstract paleoelevation histories from the central andes in bolivia have suggested that the geodynamic evolution of the region has been punctuated by periods of large scale lithospheric removal that drive rapid increases in elevation at the surface here we evaluate viable times and locations of material loss using a map view reconstruction of the bolivian orocline displacement field to forward model predicted crustal thicknesses two volumetric models are presented that test assumed pre deformation crustal thicknesses of 35 km and 40 km both models predict that modern crustal thicknesses were achieved first in the northern eastern cordillera ec by 30 20 ma but remained below modern in the southern ec until 10 ma the altiplano is predicted to have achieved modern crustal thickness after 10 ma but only with a pre deformation thickness of 50 km including 10 km of sediment at the final stage the models predict 8 25 regional excess crustal volume compared to modern thickness largely concentrated in the northern ec the excess predicted volume from 20 to 0 ma can be accounted for by 1 crustal flow to the wc and or peru 2 localized removal of the lower crust or 3 a combination of the two only models with initial crustal thicknesses 35 km predict excess volumes sufficient to account for potential crustal thickness deficits in peru and allow for lower crustal loss however both initial thickness models predict that modern crustal thicknesses were achieved over the same time periods that paleoelevation histories indicate the development of modern elevations localized removal of lower crust is only necessary in the northern ec where crustal thickness exceeds modern by 20 ma prior to paleoelevation estimates of modern elevations by 15 ma in the altiplano crustal thicknesses match modern values at 10 ma and can only exceed modern values by 5 ma post dating when modern elevations were thought to have been established collectively these models predict that the timing of crustal thickening is consistent with paleoelevation data without requiring large scale removal of lower crust and mantle lithosphere

multiple pulses of neogene compressive deformation in the ayacucho intermontane basin andes of central peru strata of the neogene ayacucho intermontane basin central peru can be subdivided into three tectonostratigraphic units the larampuquio volcanics consisting largely of intermediate lavas and coarse conglomerates of similar lithology the unconformably overlying huanta formation consisting of a lower part formed predominantly of lacustrine and volcaniclastic sedimentary rocks and an upper part composed largely of intermediate lavas tuffs and conglomerates and the unconformably overlying ayacucho formation consisting of complexly interfingering volcaniclastic sedimentary rocks silicic tuffs and lavas of intermediate to silicic composition radiometric ages of 17 3 0 2 m y and 18 3 0 6 m y have been obtained on tuffs from the upper and lower parts of the larampuquio volcanics determinations on 2 rocks from the lower member of the huanta formation indicate an age of 11 4 0 5 m y and a rock from the upper member is dated at 9 3 0 3 m y b p units of the ayacucho formation yield ages showing that it was deposited between 7 7 and 6 m y ago the intervening molinoyocc and puchcas volcanics were erupted between 9 9 and 7 4 m y ago these units may postdate the huanta formation or may be associated in part with the huanta and in part with the ayacucho formation coarse conglomerates of the larampuquio volcanics represent debris eroded from highlands uplifted during the first and most intense pulse of the late cenozoic quechuan phase of andean compressive tectonism that began in the early miocene folding of the larampuquio volcanics may have taken place during this or a subsequent pulse of compression a marked unconformity between the huanta and ayacucho formations demonstrates a period of deformation between 8 5 and 9 5 m y ago compression also was active during at least part of the deposition of the ayacucho formation as shown by inter fingering wedges of coarse clastic debris along the northeastern margin of the basin and by angular unconformities within the formation folded rocks of the ayacucho formation and its equivalents to the west are overlain by undeformed volcanic units dated at 3 8 0 4 m y b p and 5 5 m y b p bracketing a well defined pulse of deformation slightly younger than 6 m y these subsequent episodes of compressive deformation represent recurrent pulses of the quechuan phase folding related to these late quechuan pulses is restricted in the andes to northwest trending belts corresponding to reactivated fault zones in the pre neogene substratum a system of north trending dikes in the huachocolpa district 70 km west of ayacucho suggests relaxation of compression in the pliocene and compression of quaternary age has been recognized in the huancayo area 170 km northwest of ayacucho taken together with the first pulse of quechuan deformation in the early and middle miocene these data indicate at least four pulses of crustal compression during the late cenozoic separated by periods during which compressional stresses were at least partly relaxed and at times possibly replaced by extension

paleogene building of the bolivian orocline tectonic restoration of the central andes in 2 d map view using available information on the magnitude and age of tectonic shortening as well as paleomagnetically determined tectonic rotations we have run a series of 2 d map view restorations of the central andes neogene shortening in the foreland belt induced only slight orogenic curvature of the central andes the constraints on the ages of the large observed fore arc rotations average of 37 counterclockwise in southern peru and 29 clockwise in northern chile indicate that the bolivian orocline formed during the eocene oligocene as a consequence of differential shortening focused in the eastern cordillera to minimize local block rotations along the fore arc the restoration that best fits the central andean rotation pattern requires about 400 km of total paleogene plus neogene shortening near the arica bend this value corresponds to the upper bound of estimates of maximum horizontal shortening for the central andes along strike variations in horizontal shortening in the back arc induced bending of the continental margin block rotations with fore arc along strike extension and or orogen parallel transport of upper crustal material toward the symmetry axis of the orocline

the role of slabs and oceanic plate geometry in the net rotation of the lithosphere trench motions and slab return flow 1 absolute plate motion models with respect to a deep mantle reference frame e g hot spots typically contain some net rotation nr of the lithosphere global mantle flow models for the present day plate setting reproduce similarly oriented nrs but with amplitudes significantly smaller than those found in some high nr pacific hot spot reference frames it is therefore important to understand the mechanisms of nr excitation which we attempt here with two dimensional cylindrical models of an idealized pacific domain we study the influence of slab properties oceanic ridge position continental keels and a weak asthenospheric layer on nr and trench migration fast slab return flow develops in models with stiff slabs and moderate slab dips rapid nrs comparable to the high nr pacific hot spot reference frames are primarily induced by asymmetric slab dips in particular a shallow slab beneath south america and a steep slab in the western pacific a scaling relationship links the amplitude of nr to plate size slab dip angle and slab viscosity asymmetric ridge positions also promote nr through asymmetric plate sizes continental keels have less impact in contrast to what has been found in earlier global studies several models yield unidirectional pacific trench motions such as slab advance in the western pacific and simultaneously slab retreat in the eastern pacific our model provides a physical explanation for nr generation in the present day pacific setting and hints at mechanisms for the temporal evolution of the basin