thrust belts of the southern central andes along strike variations in shortening topography crustal geometry and denudation the andean fold and thrust belts of west central argentina 33 s and 36 s above the normal subduction segment present important along strike variations in mean topographic uplift structural elevation amount and rate of shortening and crustal root geometry to analyze the controlling factors of these latitudinal changes we compare these parameters and the chronology of deformation along 11 balanced crustal cross sections across the thrust belts between 70 w and 69 w where the majority of the upper crustal deformation is concentrated and reconstruct the moho geometry along the transects we propose two models of crustal deformation a 33 40 s model where the locus of upper crustal shortening is aligned with respect to the maximum crustal thickness and a 35 40 s model where the upper crustal shortening is uncoupled from the lower crustal deformation and thickening this degree of coupling between brittle upper crust and ductile lower crust deformation has strong influence on mean topographic elevation in the northern sector of the study area an initial thick and felsic crust favors the coupling model while in the southern sector a thin and mafic lower crust allows the uncoupling model our results indicate that interplate dynamics may control the overall pattern of tectonic shortening however local variations in mean topographic elevation deformation styles and crustal root geometry are not fully explained and are more likely to be due to upper plate lithospheric strength variations

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

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

crossing the several scales of strain accomplishing mechanisms in the hinterland of the central andean fold thrust belt bolivia depictions of structures at outcrop regional and tectonic scales enforce horizontal shortening and vertical thickening as the predominant style of deformation at all scales within the hinterland of the central andean fold thrust belt outcrop scale structures document a progression of strain that created 1 flexural slip folds 2 fold flattening via axial planar cleavage 3 vertical stretching via boudinage and late stage faulting and finally 4 kink folding these examples of intraformational deformation are generally concentrated just beyond the tip lines of thrust faults where fault propagation folds and related structures are well developed fault propagation folding accommodated the accrual of strain indicated by outcrop scale structures while the structures themselves indicate how deformation developed within each individual fold fault propagation fold geometries at a regional scale emerge from the construction of regional balanced cross sections the sections were drawn with careful attention to 1 known map relationships 2 field inspection of key contacts 3 bedding orientation data 4 local seismic control and 5 principles of balance the pervasive ese wnw shortening and vertical elongation seen at the outcrop and regional scales developed during the formation of the central andean backthrust belt in the hinterland of the andes the central andean backthrust belt is a large scale west vergent thrust system along the western side of the eastern cordillera in the generally east vergent andean fold thrust belt of bolivia strain accrual within this west verging zone of deformation is proposed to be a taper building mechanism that allowed the fold thrust belt to continue propagating eastward

rates of shortening propagation underthrusting and flexural wave migration in continental orogenic systems the rate of horizontal shortening in an orogenic wedge is the rate at which the length of undeformed crust decreases as it is incorporated into the orogen this rate equals the rate of convergence of the foreland lithosphere toward the central surface of the orogenic belt and the rate of subduction of foreland lithosphere beneath the central surface the rate of propagation of an orogenic wedge is the rate at which it elongates in the direction of horizontal shortening this rate is controlled by the rates of mass accretion to the orogenic wedge and erosion the orogenic belt drives a flexural isostatic wave through the foreland lithosphere at a velocity equal to the rate of propagation plus the rate of subduction or convergence or shortening in orogenic belts where the total amount of shortening cannot be reliably estimated from balanced regional cross sections it may be possible to determine total shortening from the distance of flexural wave migration in the foreland basin and the width of the orogenic wedge in addition orogenic wedges may accelerate solely in response to a reduction in taper

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

modern and long term evaporation of central andes surface waters suggests paleo archives underestimate neogene elevations abstract central andean paleoelevations reconstructed from stable isotope and paleofloral data imply a large magnitude 2 km miocene to modern surface uplift however the isotopic relationships between precipitation surface waters and soil waters upon which these reconstructions are based remain poorly constrained for both past and in many cases modern conditions we quantify the relationships between central andean precipitation and surface waters by measuring the isotopic composition of 249 stream water samples 18o and d collected between april 2009 and october 2012 the isotopic compositions of stream waters match precipitation along the eastern flank in contrast altiplano surface waters possess a lower d 18o slope 4 59 vs 8 for meteoric waters not observed in precipitation which signals heavy isotope evaporative enrichment in surface waters paleoclimate models indicate that highly evaporative conditions have persisted on the plateau throughout andean uplift and that conditions may have been more evaporative when the andes were lower thus more ancient proxy materials may have a greater evaporative bias than previously recognized and paleoelevation reconstructions from stable isotope based central andean plateau proxy materials likely overstate miocene to present surface uplift we propose altiplano paleoelevations of 1 2 km at 24 5 ma 1 5 2 9 km by 11 45 ma and modern elevations by 6 ma based on the lightest isotopic compositions observed in altiplano proxy materials which are least likely to be influenced by evaporation these constraints limit total late miocene to modern uplift to

physical models of magmatic intrusion during thrusting to investigate how magma rises through the brittle upper crust in a context of compressional tectonics we have performed experiments on scaled physical models powdered silica having a cohesion of 300 pa and an angle of internal friction of 38 was used to represent brittle crust a vegetable oil with a newtonian viscosity of 10 2 paae sa t 50c was used to represent magma a moving piston shortened the models in a box while oil was injected steadily at the base on cooling to room temperature the oil solidified the resulting intrusions were thin sills dykes and laccoliths their shapes and emplacement modes depended on the ratio r between rates of shortening and injection from shapes and orientations of intrusions we infer that hydraulic fracturing was one mechanism of emplacement unconsolidated intrusions strongly influenced thrust formation on the basis of our experiments we suggest that magmas in orogenic belts can rise along thrust faults

review of erosion dynamics along the major n s climatic gradient in chile and perspectives abstract chile is an elongated country running in a north south direction for more than 30 along a subduction zone its climate is progressively wetter and colder from north to south this particular geography has been used positively by a growing number of studies to better understand the relationships between erosion processes and climate land use slope tectonics volcanism etc here we review the erosion rates factors and dynamics over millennial to daily periods reported in the literature in addition 21 new catchment mean erosion rates suspended sediment and 10be are provided and previous suspended sediment derived erosion rates are updated a total of 485 local and catchment mean erosion rates are reported erosion rates vary between some of the smallest values on earth 10 5 mm a to moderate values 0 5 mm a compared to other active ranges this review highlights strong limitations concerning the quantification of local erosion factors because of uncertainties in sampling point location slope and rainfall data for the mean erosion rates e for the millennial and decennial catchments a model of the form e s 1 s 0 6 2 r with 0 3 0 8 accounts for 40 to 70 of the erosion variance confirming a primary role of slope s compared to precipitation rate r over this time scale over the long term this review points to the long 5 to 10 ma response time of rivers to surface uplift in north central arid chile over millennia data provide evidence for the progressive contribution of extreme erosion events to millennial averages for drier climates as well as the link between glacier erosion and glacier sliding velocity in this period of time a discrepancy exists between the long term offshore sedimentological record and continental decennial or millennial erosion data for which no single explanation appears still little information is available concerning the magnitude of variation of millennial erosion rates over centuries data show the variable role of groundwater in the dynamics of suspended load and document a decrease in erosion over hundreds of years probably associated with historical harvesting

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

evidence for active subduction beneath gibraltar comment and reply reply we welcome the opportunity to discuss the geodynamics of the rif betic region and will address the points in the order raised in the comment 1 there is no rigid alboran plate instead a portion of the upper plate the gibraltar block is pulled westward by hydrostatic suction induced by the

overriding plate shortening and extension above subduction zones a parametric study to explain formation of the andes mountains mountain building above subduction zones such as observed in the andes is enigmatic and the key parameter controlling the underlying dynamics remains a matter of considerable debate a global survey of subduction zones is presented here illustrating the correlation between overriding plate deformation rate and twelve physical parameters overriding plate velocity subducting plate velocity trench velocity convergence velocity subduction velocity subduction zone accretion rate subducting plate age subduction polarity shallow slab dip deep slab dip lateral slab edge proximity and subducting ridge proximity all correlation coefficients are low r 0 39 irrespective of the global reference frame relative plate motion model or overriding plate deformation model except for the trench velocity 0 33 0 68 exact value depends on adopted global reference frame and subduction velocity which shows an anticorrelation 0 55 0 57 this implies that no individual parameter can explain overriding plate deformation except that trench retreat generally corresponds to extension while an approximately stable trench or trench advance generally corresponds to shortening understanding of the variety of strain patterns is obtained when slab edge proximity and overriding plate velocity are combined orogenesis occurs in overriding plates bordering central regions of wide subduction zones 4000 km when the overriding plate is moving trenchward at 0 2 cm yr e g andes japan because the center of a wide slab offers large resistance to lateral migration the overriding plate effectively collides with the subduction hinge forcing the slab to attain a shallow dip angle e g nazca and japan slabs overriding plate extension is only found close to lateral slab edges or during overriding plate motion away from the center of a wide subduction zone but in the latter scenario maximum extension velocities are much lower than in the former scenario for subduction settings close to lateral slab edges overriding plate motion plays no significant role in overriding plate deformation thus for rapid overriding plate extension the key ingredient is rapid trench retreat which only occurs close to lateral slab edges while for overriding plate shortening the key ingredients are 1 the resistance to rapid trench and hinge retreat which occurs far from lateral slab edges and 2 trenchward overriding plate motion

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

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

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

short and long term effects of gps measured crustal deformation rates along the south central andes in this study we present the contemporary crustal deformation field along the central and southern andes 17 42 s estimated from four global positioning sys tem gps campaigns conducted in 1994 97 we find that the majority of the observed crustal deformation field is relatively homogenous roughly parallel to the plate conver gence direction with decreasing velocities away from the trench we attribute this type of deformation pattern to the inter seismic phase of an earthquake deformation cycle caused by 100 locking of the thrust interface between the subducting nazca and the overrid ing south american plates we have also detected a strong post seismic deformation sig nal in the vicinity of the 1995 mw8 0 antofagasta 22 26 s and 1960 mw9 5 chile 38 43 s earthquakes this type of deformation can be described as short term in nature compared to geological timescales the above conclusions are based on the results of the 3 d andean elastic dislocation model aedm by subtracting the aedm predicted deformation rates from the observations we obtained a residual velocity field that high lights the post seismic as well as more long term deformation eects for example we find a strong evidence for the continuing crustal shortening across the back arc reach ing its maximum 4 mm yr in the very north of our study area in addition between latitudes 29 34 s there is an indication of e w oriented extension within the fore arc in accordance with the recent geologic findings for the n s oriented normal faulting

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

overriding plate deformation and variability of fore arc deformation during subduction insight from geodynamic models and application to the calabria subduction zone in nature subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates such large plates can impose far field boundary conditions that influence the style of subduction and overriding plate deformation here we present dynamic laboratory models of progressive subduction in three dimensional space in which the far field boundary conditions at the trailing edges of the subducting plate sp and overriding plate op are varied four configurations are presented free both plates free sp fixed op fixed and sp op fixed we investigate their impact on the kinematics and dynamics of subduction particularly focusing on overriding plate deformation the results indicate that the variation in far field boundary conditions has an influence on the slab geometry subduction partitioning and trench migration partitioning our models also indicate that in natural narrow subduction zones assuming a homogeneous overriding plate the formation of back arc basins e g tyrrhenian sea aegean sea and scotia sea is generally expected to occur at a comparable location 250 700 km from the trench irrespective of the boundary condition in addition our models indicate that the style of fore arc deformation shortening or extension is influenced by the mobility of the overriding plate through controlling the force normal to the subduction zone interface trench suction our geodynamic model that uses the sp op fixed setup is comparable to the calabria subduction zone with respect to subduction kinematics slab geometry trench curvature and accretionary configuration furthermore the model can explain back arc and fore arc extension at the calabria subduction zone since the latest middle miocene as a consequence of subduction of the narrow calabrian slab and the immobility of the subducting african plate and overriding eurasian plate this setting induced strong trench suction driving fore arc extension and forced subduction to be accommodated almost entirely by slab rollback not trenchward subducting plate motion while trench retreat was accommodated almost entirely by back arc and fore arc extension not trenchward overriding plate motion comparable to our sp op fixed model

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

neoproterozoic high k granites produced by melting of newly formed mafic crust in the huangling region south china abstract the huangling igneous complex in the central yangtze block south china consists of the 850 ma tonalite and trondhjemite which were intruded by the 826 13 ma high k granite the granites are composed of plagioclase 30 60 alkali feldspar 10 30 quartz 20 30 biotite 2 63 1 73 3 wt k 2 o 2 58 4 79 wt and na 2 o 3 90 5 89 wt and low mgo 0 49 1 78 wt and are magnesian alkali calcic in composition their chondrite normalized ree patterns are characterized by enrichment of lree la yb n 14 5 32 8 and mostly negative eu anomalies eu eu 0 5 1 1 they have moderately high sr 202 698 ppm and low y 6 13 18 7 ppm with sr y ratios of 14 64 these large elemental variations probably resulted from fractional crystallization they also have negative whole rock nd 9 75 to 11 73 and zircon hf values 7 1 to 15 8 which are similar to those of the neoproterozoic mafic rocks in the same region suggesting that the granites may have been produced by melting of newly emplaced mafic rocks tonalites and trondhjemites of the huangling complex previously were considered as products by differentiation of water rich basaltic magmas and melting of the thickened archean continental crust in an arc setting respectively therefore the crustal growth and reworking of the yangtze block were probably wholly controlled by the subduction system which resulted in formation of the stable continent during the neoproterozoic

nonuniform surface uplift of the andean plateau revealed by deuterium isotopes in miocene volcanic glass from southern peru abstract proposals for rapid late miocene surface uplift driven by large scale lithospheric removal beneath the central andean plateau have been based largely on temperature sensitive paleoaltimeters both the magnitude and mechanism of this proposed pulse of uplift have been challenged first climatic general circulation models support protracted uplift with predicted temperature and isotopic shifts enhanced by attainment of threshold elevations second tectonic models in which surface elevations are compensated by regional contraction and crustal thickening question the need for lithospheric removal and predict broadly coeval uplift of the entire plateau we present hydrogen isotope data using a novel temperature insensitive volcanic glass proxy from continuous well dated lower to middle miocene basin fill in the western cordillera of the northern plateau that show a rapid decrease in d values 62 8 at 19 16 ma with extremely negative values continuing into the pliocene we propose that the basin reached its current elevation by 16 ma 6 myr earlier than proposed for the central plateau the rapid decrease in d values is consistent with punctuated surface uplift of 2 2 3 7 km between 19 and 16 ma whereas the 3 7 km upper estimate assumes a static climate similar to modern the 2 2 km lower estimate conservatively incorporates modeled changes in the isotopic composition of precipitation associated with elevation change comparison of these results to existing paleoelevation estimates from the andean plateau facilitates a tentative reconstruction of earliest middle miocene paleotopography showing a central depression with flanking hinterland and thrust belt highlands this apparent pattern of nonuniform plateau uplift contradicts shortening proportional topographic growth over tens of myr we propose that temporally and spatially irregular surface uplift may be linked to pronounced local variability in crustal shortening and or piecemeal removal of dense mantle lithosphere insofar as these irregularities reflect the scale of heterogeneity for surface uplift processes available paleoelevation estimates point to a principal uplift mechanism that varies over relatively short

a note on the isostatic compensation and origin of king s trough summary the spectral analysis of a 550 km long ship board gravity and topography profile across the central part of king s trough in the north eastern atlantic ocean shows that the topography is regionally compensated on lithosphere older than 20myr this feature with its enigmatic trend oblique to that of the mid atlantic ridge therefore dates from no earlier than the late oligocene and was formed by intra plate activity such a constraint to its age agrees with the recent analysis of geological samples taken on the northern flank of the trough close to the position of the profile it is not consistent with previous suggestions that king s trough was created either by the rotations of the iberian plate to form the pyrenees or by a readjustment in the strike of the mid atlantic ridge furthermore this analysis shows no evidence for either thin crust or high density mantle under the trough there is no structural evidence to suggest that it formed as part of a compressional boundary

structure of the crust and the lithosphere beneath the southern puna plateau from teleseismic receiver functions abstract we present a teleseismic p and s receiver function study using data from a temporary passive source seismic array in the southernmost puna plateau and adjacent regions the p receiver function images show the distribution of crustal thickness and v p v s ratio for this area over much of the southern puna plateau the crustal thickness is 50 55 km whereas to the west a thicker crust 60 to 75 km is observed beneath much of the andean volcanic arc region from the puna southward there is little obvious change in the crustal thickness across the border of the plateau south of 28 s the crust is seen to progressively thin towards the east in the pampean ranges where it is 35 40 km thick the southern puna plateau is characterized overall by a low crustal v p v s ratio less than 1 70 implying a felsic crustal composition an anomalously high v p v s ratio of 1 87 is observed beneath the cerro galan volcanic center in the region where a prominent crustal low velocity zone identified below 10 km depth probably extends into the lower crust the crustal thickness determined under the cerro galan area 59 km is close to that of the rest of the southern puna the prominent high v p v s ratio and low velocity zone beneath the galan region implies the presence of a zone of partial melt or a magma chamber is consistent with hypothesis calling for lithospheric delamination beneath the galan caldera a widespread crustal low velocity layer observed beneath much of the southern puna correlates well with crustal low velocity anomalies observed by teleseismic tomography the lithosphere asthenosphere boundary beneath the array can be clearly observed by both p and s receiver functions at depths of 70 90 km in agreement with previous studies suggesting a thin lithosphere beneath the high elevated plateau the mantle transition zone discontinuities appear at expected depths

possible flexural accommodation on the eastern edge of the altiplano in relation to focussed erosion in the rio la paz drainage system it has been proposed that erosional unloading by focused incision can be accommodated by flexural rebound which in turn potentially exerts a positive feedback on erosion this paper presents morphometric data from the la paz drainage basin in the bolivian andes to illustrate the effects of such a feedback mechanism between erosion and crustal bending the rio la paz is a prominent system with the headwaters on the altiplano and a trunk stream segment that cuts across the cordillera real thereby connecting large portions of the altiplano with the amazon basin the flexural feedback model explains why all drainages beyond the watershed disperse their waters to the altiplano it also provides an explanation for the presence of the highest peaks just next to the location where the la paz river cuts into the bedrock across the cordillera real

tectonic styles and crustal shortening of the central andes pampean flat slab segment in northern chile 27 29 s abstract the andean orogenic belt located in the central andes pampean flat slab segment in northern chile 27 29 s is composed of two major tectonic regions the coastal cordillera and the frontal cordillera to understand their internal tectonic styles history of growth and the shortening absorbed by the upper crustal structure of this segment we combined regional geological mapping data new ages obtained from radiometric u pb dating and a semibalanced and restored cross section 225 18 km in length the results as shown in the previous mesozoic extensional fault systems established in northern chile by the gondwana breakup have played a fundamental role in the orogenic buildup the central structure is characterized by an asymmetric basin upper cretaceous paleocene confined by a doubly vergent fault system composed of inverted faults related to the edges of the mesozoic chanarcillo and lautaro basins the u pb geochronological data obtained from synorogenic volcano sedimentary deposits and the angular unconformities recorded between the cenozoic geological units have revealed that the compressive deformation in this segment started at around 80 ma by tectonic inversion in the eastern coastal cordillera and western frontal cordillera however the presence of paleocene and miocene synorogenic successions at the footwall of the basement reverse faults of the frontal cordillera suggests a migration of andean deformation from the west to the east during the paleocene miocene by propagation of ramps involving inherited basement highs the pre compression restoration makes it possible to estimate 40 94 km of minimum shortening concentrated by inversion anticlines and fault controlled basement highs across the frontal cordillera

miocene forebulge development previous to broken foreland partitioning in the southern central andes west central argentina 1 in the distal region of the modern flat slab segment in the southern central andes an unusual stack of middle miocene paleosols together with regional upwarping and normal faulting indicate episodic aggradation and condensed sedimentation contemporaneous with the principal stage of foreland basin development associated with foreland flexure farther to the west these features are consistent with development of a forebulge zone during the early stages of a proposed asymmetric foreland basin system sedimentary thickness farther east and far from the cordilleran tectonic loads suggests accommodation and preservation driven by nonisostatic dynamic subsidence regional overlapping relationships and basin modeling suggest that the modern broken foreland present sierras pampeanas can be interpreted as a reactivation of a formerly partitioned broad forebulge

relationships among climate erosion topography and delamination in the andes a numerical modeling investigation cordilleran orogenic systems such as the andes are controlled by shortening rates climatically controlled erosion rates and in some cases eclogite production and delamination all of these processes are coupled however making it diffi cult to uniquely determine the relative importance of each process and the feedbacks among them in this paper we develop a massbalanced numerical model that couples an actively shortening orogen and crustal root with eclogite production delamination and climatically controlled erosion the model provides a fi rst order quantifi cation of the sources shortening and sinks erosion and eclogite production and delamination of crustal volume during the cenozoic in the andes as a function of latitude and time given reasonable estimates for the rates of eclogite production and the threshold size of the eclogitic root required for delamination the model suggests that in the central andes between 5 s and 32 s the orogen has grown to a suffi cient height to produce and maintain eclogite which in turn has promoted delamination in the lower crust and mantle in this region climatically controlled erosion rates infl uence the size of the orogen through two separate mechanisms by exporting mass via surface processes and by controlling the lithostatic pressure in the lower crust which modulates the rates of eclogite production and or delamination to the north and south of the central andes relatively low shortening rates and high precipitation and erosion rates have slowed eclogite production such that delamination likely has not occurred during the cenozoic

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

paleogeographic and tectonic evolution of the pampia terrane in the cambrian new paleomagnetic constraints abstract paleomagnetic magnetic fabric and rock magnetic studies were carried out in the late middle to early late cambrian campanario formation exposed in nw argentina the study also presents preliminary results from the early cambrian metasediments of the puncoviscana formation and from mojotoro intrusive a new paleomagnetic pole c2 was computed for the campanario formation 23 6 n 346 5 e a95 7 0 n 11 sites from the localities of tilcara t terma de reyes r and el perchel ep including previous data from santa victoria oeste m in northernmost argentina a positive fold test was obtained for tilcara locality while a positive regional tilt test was obtained for mean site directions for the four localities this pole can be considered as the representative pole position for the pampia terrane for the late middle to late cambrian previous paleomagnetic data for the same formation from the iruya matancillas locality show an in situ clockwise rotation of about 30 around a vertical axis when compared with the mean pampia pole the same is observed when comparing the early ordovician pole for the santa rosita formation at those localities with the recently obtained coeval pole of the la pedrera formation indicating that local tectonic rotations affected that area these results suggest that previous paleomagnetic interpretations for these late cambrian and early ordovician rocks in the region as recording displacement of the pampia terrane in cambrian times are probably incorrect additionally two virtual geomagnetic poles were computed for the puncoviscana formation tp1 34 5 n 52 3 e a95 5 0 n 22 specimens and tp2 38 3 n 33 6 e a95 5 7 n 14 specimens and one for the mojotoro intrusive mo 9 1 n 345 6 e a95 16 4 n 11 specimens these and previous early cambrian paleomagnetic data from the pampia terrane differ from gondwana coeval reference directions in inclination and or declination lacking a simple pattern

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

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

consistency of geologic and geodetic displacements during andean orogenesis displacement and convergence rates inferred from gps and marine magnetic data suggest that this trend may be continuing at present hence an increasing fraction of convergence is being absorbed by mountain building this change may reflect increased plate coupling due to decreased sediment supply younger subducting lithosphere or increased normal stress at the interface from the effects of uplift index terms 8102 tectonophysics continental contract orogenic belts 8157 evolution of the earth plate motions past 3040 8158 evolution of the earth plate motions present and recent 3040 8099 structural geology general or miscellaneous

three dimensional 3 d finite strain at the central andean orocline and implications for grain scale shortening in orogens three dimensional 3 d finite strain analyses from across the central andes are used to document the contribution of grain scale strain in quartzites and sandstones to the total shortening budget the results are compared to thermal stratigraphic and strain data from other fold and thrust belts to determine the influence of lithologic strength and deformation temperature on strain accommodation during orogenic evolution in the central andes 3 d best fit ellipsoids are inconsistently oriented relative to structural trends have short axes at high angles to bedding z mean plunge 78 21 and have bedding parallel long axes x mean plunge 6 24 ellipsoid shapes are dominantly oblate x y z indicate low natural octahedral shear strains e s 0 03 0 19 and have axial ratios that range from 1 02 1 0 81 e s 0 19 to 1 02 1 0 97 e s 0 03 highly variable r f data r f 1 0 5 0 fluctuations exceeding 100 indicate detrital grain shapes may overwhelm any measurable tectonic strain fabric recorded by grain geometry the best fit ellipsoids may reflect either weak compaction strain or they may be related to a depositional fabric at a minimum granular strain was insufficient to reset the detrital grain fabric and therefore grain scale strain in quartzites and sandstones is not a significant factor in deformation we suggest that the nonstrained nature of these stiffer lithologies indicates a lack of regional penetrative strain in the central andes like that quantified in similar lithologies in other orogens the regional lack of strain may be due to deformation temperatures

assessing uncertainties in balanced cross sections abstract balanced structural cross sections are models that are fit to incomplete data the models are under constrained with respect to any particular two dimensional line length model but enough data generally exists to yield a well constrained area balance solution furthermore the area balance encompasses all possible line length solutions therefore where the primary objective of section balancing is the determination of horizontal shortening magnitude area balancing provides an analytical solution we use this analytical solution to develop a comprehensive robust analysis of the uncertainty in shortening estimates resulting from cross section balancing the analytical solution allows us to propagate errors formally on the input parameters stratigraphic thicknesses depth to decollement eroded hanging wall cutoffs through the equations and produce the resulting uncertainty on the magnitude of shortening balanced cross sections from the subandean belt of the central andes are used to demonstrate the relative importance of stratigraphy and eroded hanging wall cutoffs in the contribution to the overall error

middle eocene deformation and sedimentation in the puna eastern cordillera transition 23 26 s control by preexisting heterogeneities on the pattern of initial andean shortening the quebrada de los colorados formation at the north end of calchaqui valley in salta province northwest argentina preserves evidence of syndepositional deformation since the middle eocene ca 40 ma that includes 1 an angular unconformity with the underlying salta group paleogene and 2 internal unconformities and changes in vertical facies succession and provenance its fossil record mammalian notoungulates middle eocene is correlatable to the casa grande formation which also unconformably overlies the salta group both units record middle eocene deformation along the eastern border of the puna plateau and outline previous fi rst order mechanical heterogeneities related to the cretaceous rift basin border along the western margin of the puna eocene deformation coincides with thermal magmatic arc and mechanical basin inversion heterogeneities thus the distribution of eocene deformation followed an irregular pattern as a consequence of the heritage of preexisting heterogeneities

the relationships between tectonics climate and exhumation in the central andes 18 36 s evidence from low temperature thermochronology abstract the central andes between 18 s and 36 s latitude strike north south for 2000 km along the chilean subduction margin cross several climate zones from hyperarid to humid and exhibit mean elevations in excess of 4000 m a s l here we investigate the relationships between tectonics climate and exhumation by inverting low temperature thermochronological data compiled from the literature 824 ages from 549 samples and new data 238 ages from 146 samples to quantify the exhumation rate history of the central andes since 80 ma our inferred exhumation rates west of the drainage divide and between 18 and 32 s did not exceed 0 25 km ma such low exhumation rates are consistent with low shortening rates and arid conditions in this region local pulses of exhumation occurred only during the eocene as response to active deformation and during the miocene probably as response to uplift of the western andean slope east of the drainage divide between 18 and 28 s the observed exhumation pattern reflects the onset and eastward propagation of deformation here exhumation occurred locally since the middle to late eocene in the eastern cordillera and the altiplano puna and subsequently affected larger parts of these regions and the north western sierra pampeanas during the oligocene in the early miocene 20 ma the interandean zone started exhuming and at 12 10 ma exhumation propagated into the subandean zone enhanced shortening rates and intensified precipitation along the eastern deformation front associated with the onset of the south american monsoon led to increased exhumation rates in the eastern interandean and the subandean zones in the plio pleistocene 0 6 km ma higher pleistocene exhumation rates are also observed in the northern sierra pampeanas 1 5 km ma that can be related to rock uplift along steep reverse faults coupled with high precipitation south of 32 s on the western side exhumation rates in the principal cordillera increased from ca 0 25 km ma in the miocene to rates locally exceeding 2 km ma in the pleistocene whereas the tectonic regime in the southern principal cordillera remained unchanged since the late miocene these higher rates are likely associated with enhanced erosion resulting from intensified pleistocene precipitation and glacial growth in this region reinforced by isostatic rock uplift and active tectonics our study shows that the onset of exhumation correlates mainly with the initiation of horizontal shortening and crustal thickening whereas the magnitude of exhumation is largely set by the amount of precipitation and glacial erosion and by the style of deformation which is controlled by inherited structures and the amount of sediments in the foreland

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

quantifying the energy dissipation of overriding plate deformation in three dimensional subduction models in a subduction system the force and the energy required to deform the overriding plate are generally thought to come from the negative buoyancy of the subducted slab and its potential energy respectively such deformation might involve extension and back arc basin formation or shortening and mountain building how much of the slab s potential energy is consumed during overriding plate deformation remains unknown in this work we present dynamic three dimensional laboratory experiments of progressive subduction with an overriding plate to quantify the force fopd that drives overriding plate deformation and the associated energy dissipation rate opd and we compare them with the negative buoyancy fbu of the subducted slab and its total potential energy release rate bu respectively we varied the viscosity ratio between the plates and the sublithospheric upper mantle with sp um 157 560 and the thickness of the overriding plate with top 0 5 2 5 cm scaling to 25 125 km in nature the results show that fopd fbu has average values of 0 5 2 0 with a maximum of 5 3 and opd bu has average values of 0 05 0 30 with a maximum of 0 41 the results indicate that only a small portion of the negative buoyancy of the slab and its potential energy are used to deform the overriding plate our models also suggest that the force required to deform the overriding plate is of comparable magnitude as the ridge push force furthermore we show that in subduction models with an overriding plate bending dissipation at the subduction zone hinge remains low 3 15 during steady state subduction

a scenario for late neogene andean shortening transfer in the camisea subandean zone peru 12 s implications for growth of the northern andean plateau precise knowledge of the timing of deformation in the subandean zone of the andean plateau is a prerequisite for deciphering the late neogene growth of the andean plateau in this paper we report new apatite fission track aft and vitrinite reflectance ro data for a regional balanced cross section of the camisea basin in the central peruvian subandean zone adjacent to the northern andean plateau the balanced cross section shows that the structure of this basin is characterized by a broad internal passive roof duplex and external thrust related anticlines the balanced cross section restoration shows 53 km 39 of total horizontal shortening we sampled paleozoic to cenozoic sedimentary strata for aft and ro analyses along the 4 km thick vertical profile of the mainique back thrust passive roof thrust the innermost preserved subandean structure young components of aft ages are spread between ca 6 ma and ca 24 ma a break in the slope in the aft ages determines the geometry of the miocene partial annealing zone and the exhumation of the mainique back thrust at ca 6 ma sequential restoration calibrated by aft and ro data indicates that the last 23 km horizontal shortening were accommodated by the camisea thrust system over the past 6 m y giving a mean shortening rate of 3 8 mm yr using this shortening rate for the first 30 km horizontal shortening we calculate that the andean shortening transfer into the peruvian subandean zone initially started at ca 14 ma this result suggests that the transfer of shortening from the northern andean plateau to the subandean zone occurred prior to the removal of dense lithosphere previously reported to have occurred between ca 10 ma and ca 7 ma we rather propose that the late neogene growth of the northern andean plateau mostly resulted from a continuous crustal shortening combined with lower crustal flow

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

rapid geodetic shortening across the eastern cordillera of nw argentina observed by the puna andes gps array exxonmobil through the convergent orogenic systems analysis achievement rewards for college scientists foundation peter j coney fellowship

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

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

the potential influence of subduction zone polarity on overriding plate deformation trench migration and slab dip angle abstract a geodynamic model exists the westward lithospheric drift model in which the variety of overriding plate deformation trench migration and slab dip angles is explained by the polarity of subduction zones the model predicts overriding plate extension a fixed trench and a steep slab dip for westward dipping subduction zones e g mariana and predicts overriding plate shortening oceanward trench retreat and a gentle slab dip for east to northeastward dipping subduction zones e g chile this paper investigates these predictions quantitatively with a global subduction zone analysis the results show overriding plate extension for all dip directions azimuth 180 to 180 and overriding plate shortening for dip directions with 90 to 110 the wide scatter in data negate any obvious trend and only local mean values in overriding plate deformation rate indicate that overriding plate extension is somewhat more prevalent for west dipping slabs west dipping subduction zones are never fixed irrespective of the choice of reference frame while east to northeast dipping subduction zones are both retreating and advancing in five out of seven global reference frames in addition westward dipping subduction zones have a range in trench migration velocities that is twice the magnitude of that for east to northeastward dipping slabs finally there is no recognizable correlation between slab dip direction and slab dip angle east to northeast dipping slabs 30 to 120 have shallow 0 125 km slab dip angles in the range 10 60 and deep 125 670 km slab dip angles in the range 40 82 while west dipping slabs 60 to 120 have shallow slab dip angles in the range 19 50 and deep slab dip angles in the range 25 86 local mean deep slab dip angles are nearly identical for east and west dipping slabs while local mean shallow slab dip angles are lower by only 4 7 8 1 for east to northeast dipping slabs it is thus concluded that overall there is no observational basis to support the three predictions made by the westward drift model and for some sub predictions the observational basis is very weak at most alternative models which incorporate and underline the importance of slab buoyancy driven trench migration slab width and overriding plate motion are better candidates to explain the complexity of subduction zones including the variety in trench migration velocities overriding plate deformation and slab dip angles

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

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

uplift of the central andes of nw argentina associated with upper crustal shortening revealed by multiproxy isotopic analyses this study contributes to the uplift history of the andes which has received increasing attention in recent years because of its implications for geodynamic models and climate feedbacks shortening resulting in crustal thickening and removal of gravitationally unstable mantle lithosphere has been proposed to control deformation and uplift of cordillera type orogenic systems such as the puna plateau of the central andes and its eastern margin the eastern cordillera we present new clumped isotope 47 18o and 2 data from carbonate nodules marlstone spring deposits and volcanic ashes from the puna plateau and eastern cordillera of nw argentina when combined with other geological evidence our data indicate that the puna plateau was near its present elevation since at least 10 ma whereas the eastern cordillera rose 1 5 km between 14 and 7 ma this history of uplift correlates with active shortening in the eastern cordillera and with incorporation of a regional foreland into the propagating orogenic wedge our study suggests that the elevation of the puna plateau changed little during the miocene pliocene whereas the margin experienced significant uplift associated with active deformation and crustal thickening

the topographic imprint of a transient climate episode the western andean flank between 15 5 and 41 5 s mountain range topography is determined by the complex interplay between tectonics and climate however often it is not clear to what extent climate forces topographic evolution and how past climatic episodes are reflected in present day relief the andes are a tectonically active mountain belt encompassing various climatic zones with pronounced differences in rainfall erosion and glacier extent under similar plate boundary conditions in the central to south western andes climatic zones range from hyperarid desert with mean annual rainfall of 5 mm a 22 5 s to year round humidity with 2500 mm a 40 s the andes thus provide a unique setting for investigating the relationship between tectonics climate and topography we present an analysis of 120 catchments along the western andean watersheds between 15 5 and 41 5 s which is based on srtmv3 90m data and new medium resolution rainfall tropical rainfall measurement mission trmm dataset for each basin we extracted geometry relief and climate parameters to test whether andean topography shows a climatic imprint and to analyze how climate influences relief our data document that elevation and relief decrease with increasing rainfall and descending snowline elevation furthermore we show that local relief reaches high values of 750 m in a zone between 28 s to 35 s during pleistocene glacial stages this region was affected by the northward shifting southern hemisphere westerlies which provided moisture for valley glacier formation and extended glacial coverage as well as glacial erosion in contrast the southern regions between 35 s to 40 s receive higher rainfall and have a lower local relief of 200 m probably related to an increased drainage density we distinguish two different climatically controlled mechanisms shaping topography 1 fluvial erosion by prolonged channel hillslope coupling which smoothes relief and 2 erosion by valley glaciers that generates relief finally our results suggests that the catchment scale relief of the andes between 28 s to 35 s is characterized by a pronounced transient component reflecting past climatic conditions copyright 2010 john wiley sons ltd

building the central andes through axial lower crustal flow 1 the orogen scale morphology of the central andes which are longitudinally symmetric at 19 s correlates with the modern geometry of the subducting nazca slab but is largely independent of local bedrock geology or precipitation crustal thickness as measured by the cross strike width of the orogen above 3000 m and the cross sectional area of the crust is not well correlated with observed magnitudes of upper crustal shortening in our interpretation the large scale morphology is instead correlated with and controlled by lithospheric scale processes and subduction dynamics using a semianalytic three dimensional newtonian viscous flow model we produce andean like topography and distribution of upper crustal shortening during subduction of an oceanic lithosphere eastward beneath the andes provided that 1 the more steeply dipping slab segment beneath the central andes is overlain by the weak lower or middle crust 2 the flat slab subduction segments to the north and south of this zone are overlain by the strong middle and lower crust and 3 the steeply dipping central slab segment is overlain by a narrow centrally localized zone of increased crustal shortening the resulting model orogen displays substantial orogen parallel flow in the weak lower crust above the steep slab zone orogen parallel flow does not penetrate into the regions of stronger lower crust above the flat slab segments resulting in a broad plateau topped orogen above the central slab segment and narrow but high orogenic segments above the flat slab region redistribution of material through lower crust flow can account for the observed mismatch between crustal volume and upper crustal shortening in the central andes and may explain young 10 ma crustal thickening and surface uplift that has been argued for in some regions

vertical axis rotation in the bolivian orocline south america 1 paleomagnetic analysis of cretaceous and cenozoic rocks the results of a paleomagnetic study are reported for 45 new sites or localities in the central andes of bolivia and northern chile 17 s 23 s south america in intrusive volcanic and sedimentary rocks ranging in age between 7 and 80 ma these outcrop across the width of the northern and southern limbs of the bolivian orocline from the sub andean zone in the east to the volcanic arc in the west for most localities a mean primary magnetization has been identified which is assumed to reflect a geocentric axial dipole field a comparison of directions of magnetizations with those predicted by the relevant paleomagnetic reference poles for stable south america reveals declination anomalies which are interpreted in terms of the rotation about a local vertical axis of rigid crustal blocks hundreds of meters to tens of kilometers across relative to stable south america these data when combined with previous paleomagnetic studies of upper cretaceous and cenozoic rocks show a pattern of vertical axis rotations which defines three regional domains extending across the width of the bolivian andes the differences between the various domains are best seen in rocks in the age range 20 to 80 ma younger rocks acquired a primary magnetization at later stages in the rotation history for rocks older than 20 ma crustal blocks in domain 1 on the northern flank of the bolivian orocline have undergone an anticlockwise rotation about a vertical axis in the range 8 to 33 with a geometric mean of 22 blocks from domain 2 immediately south of the main oroclinal bend have rotated between 19 anticlockwise and 40 clockwise with a geometric mean of 8 clockwise even farther south in domain 3b blocks have rotated between 3 and 57 clockwise with a geometric mean of 37 a localised area domain 3b in which blocks have undergone 32 clockwise rotation in the last 65 myr lies within the southeastern part of domain 1 the northern boundary of domain 1 and the southern boundary of domain 3a are as yet not well defined the amount and timing of cenozoic vertical axis rotation vary across the width of the andes and are interpreted to record both regional bending of both limbs of the bolivian orocline and small block rotation in zones of distributed sinistral or dextral shear during three stages in the cenozoic 1 45 to 25 ma 2 25 to 10 ma 3 10 ma to 0 bending of the bolivian andes during the last 25 myr was accommodated relative to stable south america by gradients of shortening along the eastern margin of the andes particularly in the sub andean zone

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

size dependence of iron solubility of asian mineral dust particles 1 asian mineral dust was sampled at hokkaido northern japan in spring 2004 and 2006 iron solubility of the bulk aerosol the size segregated aerosol 0 45 4 7 m we suggest that an iron solubility of around 0 4 is typical for asian mineral dust of large particles transported to hokkaido in the high nutrient low chlorophyll region of the western subarctic north pacific near the asian continent where the mineral dust deposition is dominated by large particles the iron solubility of the mineral dust entering the ocean is around 0 4

initial plate geometry shortening variations and evolution of the bolivian orocline comparisons of newly published cross sections across the bolivian andes with existing cross sections through argentina emphasize significant along strike changes in crustal shortening a sharp decrease in the magnitude of crustal shortening from 530 km to 150 km north to south occurs at 23 s a 20 40 m y difference in the ages at which deformation was initiated accompanies the abrupt decrease in the magnitude of shortening extending the western margin of south america to account for 530 km of shortening in the bolivian andes and 150 km of shortening in argentina produces a central andean salient that is perpendicular to the nazca plate shortening direction from 60 to 26 ma during this same time interval the chilean coast south of 23 s was in an orientation sufficiently oblique to oceanic convergence to allow for predominantly strike slip offset and backarc extension deformation within the andean mountain chain may be a function of plate convergence where the oblique nature of convergence south of 23 s inhibited mountain building whereas north of 23 s normal convergence to a central andean salient facilitated contractional deformation the magnitude of deformation north of 23 s is a consequence of both plate convergence direction providing a longer period of contractional deformation from ca 70 ma to the present and a thick phanerozoic sedimentary package that permitted large magnitudes of thin skinned deformation significant along strike changes in the shape of the south american margin allowing for convergence to change from compression to extension along the strike of the orogen may help explain the dramatic differences in timing amount and style of deformation in the andes

growth of the central andean plateau by tectonic segmentation is controlled by the gradient in crustal shortening abstract crustal deformation orocline formation and growth of the central andean plateau as well as the mutual relations of these processes with climate are a matter of international debate earthquake focal mechanism fault kinematic paleomagnetic and cross section balancing data are paramount for understanding the morphotectonic evolution of this noncollisional orogen but collectively await an explanation in terms of a comprehensive kinematic model we offer such a model that approximates simple shear in plan view and that predicts the kinematics of first order fault zones in the southern central andes the model indicates that segmentation of the upper crust into rhomb shaped domains confined by transpressive ranges resulted from the decrease in crustal shortening with distance to the central bend of the andes an anastomosing network of kinematically coupled reactivated orogen parallel and newly formed ne trending deformation zones encompassing components of sinistral and dextral strike slip res

simulation of late cenozoic south american flat slab subduction using geodynamic models with data assimilation abstract the formation mechanisms of flat slabs in south america remain unclear to quantitatively evaluate the earlier proposed mechanisms we simulate the post 100 ma subduction history below south america using 4 d geodynamic models by progressively incorporating plate kinematics seafloor ages and key tectonic features including the buoyant oceanic crust continental cratons oceanic plateaus i e the inferred inca plateau subducting nazca ridge and juan fernandez ridge as well as deformable trench profiles according to recent geological reconstructions we find that in the absence of an overriding plate and subducting buoyancy features the seafloor age affects slab dip angle by controlling the slab s mechanical strength i e the resistance to bending and negative buoyancy integrated positive density anomaly that enhances bending our models show that slab strength dominates its buoyancy at age 30 ma and the opposite for younger ages the existence of a thick overriding plate reduces the slab dip by increasing dynamic suction and individual cratonic roots further lead to along trench variations of dip angle reduction while dynamic suction from the overriding plate generates a permanent reduction of the long wavelength slab dip angle it is the final addition of subducting oceanic plateau and aseismic ridges that produces the transient and localized flat slabs as observed these results suggest that all mechanisms except the buoyancy features affect the slab dip only at large spatial scales our best fit model with all the above tectonic features included provides a good match to both the upper mantle benioff zones and the temporal evolution of volcanic arcs since the mid miocene the imperfect match of the peruvian flat slab is likely associated with the uncertain 3 d configuration of the amazonian craton

kinematics of neogene to recent upper crustal deformation in the southern central andes 23 28 s inferred from fault slip analysis evidence for gravitational spreading of the puna plateau abstract elucidating the tectono morphologic evolution of the central andes is important for understanding the mechanical behaviour of non collisional orogens at convergent plate boundaries fault kinematic data from the southern central andes are generally interpreted in terms of changes in plate boundary kinematics or vertical axis rotation associated with the formation of the andean orocline however none of these hypotheses accounts adequately for neogene to recent horizontal extension and vertical shortening of upper crust documented from the puna plateau and the adjacent eastern cordillera based on new and compiled fault kinematic data in total 4746 brittle faults at 317 stations we propose that strain axis configurations in the southern central andes are controlled by 1 local deformation kinematics 2 the far field deformation caused either by the orogen normal shortening component of plate convergence or by removal of lithospheric mantle and 3 differences in lithospheric thickness and respective surface elevation i e gravitational potential energy specifically the puna plateau and eastern cordillera have been undergoing horizontal shortening and gravitational spreading of the upper crust whereas the topographically lower pampean ranges continue to undergo horizontal shortening only although less apparent in the fault kinematic data than the e w and nw se extension orogen parallel extension is inherent to the puna plateau and eastern cordillera and compensates likely for overall wnw ese shortening and enhanced surface elevation the importance of orogen parallel extension in the deformation kinematics of topographically elevated parts of the southern central andes is underscored by the strike slip components on prominent reverse and thrust faults the data supports geodynamic interpretations in which the evolution of topography and lithospheric thickness influence significantly changes in upper crustal kinematics of the southern central andes

response of intracontinental deformation in the central andes to late cenozoic reorganization of south american plate motions new stratigraphic chronologic and fault kinematic data from the quebrada del toro in northwestern argentina provide a detailed view of late cenozoic kinematic reorganization in the southern central andes the quebrada del toro is a thrust bounded basin filled with 2 km of synorogenic clastic deposits ranging in age from early miocene to holocene as observed elsewhere in northwestern argentina faults in the quebrada del toro record an early phase of horizontal nw se contraction beginning between early and late miocene and ending after 0 98 ma and a later phase of horizontal ne sw contraction beginning between late miocene and 4 17 ma and still active these results require both kinematic regimes to have been active between 4 17 and 0 98 ma compatible with timing and kinematic data from the adjacent central andes and best explained by a temporal pattern of serial recurrence of the kinematic regimes a hypothesis that intraplate deformation in the central andes is driven by the absolute motion of the south american plate with respect to the hotspot reference frame is tested by comparing quebrada del toro kinematics to plate reconstructions and space based geodetic results nazca south america relative motion and absolute nazca plate motion show consistent azimuths throughout the past 20 5 myr in contrast absolute south american plate motion azimuths mimic the changing contraction directions determined from intraplate andean faults although the timing of the south american plate motion change is poorly resolved by current data it is compatible with the quebrada del toro data absolute upper plate motion might control intraplate kinematics along many of the earth s convergent margins

coupled tectonic evolution of andean orogeny and global climate abstract the largest tectonic relief breaking the earth s surface 13 km vertically is at the subduction margin of the andes which generates routinely megathrust earthquakes mw 8 5 and drives the paradigmatic andean orogen here we present key geologic evidence to reassess first order features of geomorphology and tectonics across the central andes where the orogen includes the altiplano plateau and attains its maximum integrated height and width the andean subduction margin has a stepped morphology dominated by the low relief atacama bench which is similar to a giant uplifted terrace slopes gently over a width of 60 100 km from the andes to the pacific and runs over more than 1000 km of coastal length we find that the genesis of stepped morphology at the andean seaboard is due to concomitant development of large west vergent thrusts parallel to the subduction interface and increasing aridity in the atacama desert which keeps an unprecedented large scale record of interplaying tectonics and cenozoic climate change incorporating our results with published geological knowledge demonstrates that andean orogeny is characterized by trench perpendicular bivergent and trench parallel bilateral growth over the past 50 myr associated with positive trench velocity toward the continent trench advance and subduction of a wide slab under south america we hypothesize that a global plate tectonic reorganization involving long lasting viscous mantle flow has probably forced both andean orogeny and global climate cooling since 50 ma in contrast two important stepwise pulses of increasing aridity and trench perpendicular andean growth appear to be results of changes in erosion rates due to global late eocene and middle miocene cooling events

lithospheric evolution of the andean fold thrust belt bolivia and the origin of the central andean plateau we combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central andean plateau between 188 and 208 s from late cretaceous to present by integrating geophysical results of upper mantle structure crustal thickness and composition with recently published structural stratigraphic and thermochronologic data we emphasize the importance of both the crust and upper mantle in the evolution of the central andean plateau four key steps in the evolution of the andean plateau are as follows 1 initiation of mountain building by 70 ma suggested by the associated foreland basin depositional history 2 eastward jump of a narrow early fold thrust belt at 40 ma through the eastward propagation of a 200 400 km long basement thrust sheet 3 continued shortening within the eastern cordillera from 40 to 15 ma which thickened the crust and mantle and established the eastern boundary of the modern central andean plateau removal of excess mantle through lithospheric delamination at the eastern cordillera altiplano boundary during the early miocene appears necessary to accommodate underthrusting of the brazilian shield replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the eastern cordillera and altiplano at 24 23 ma and further volcanism recorded by 12 7 ma crustal ignimbrites 4 after 20 ma deformation waned in the eastern cordillera and interandean zone and began to be transferred into the subandean zone long term rates of shortening in the fold thrust belt indicate that the average shortening rate has remained fairly constant 8 10 mm year through time with possible slowing 5 7 mm year in the last 15 20 myr we suggest that cenozoic deformation within the mantle lithosphere has been focused at the eastern cordillera altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination d 2005 elsevier b v all rights reserved

new geochemical and isotopic insights to evaluate the geothermal resource of the hydrothermal system of rosario de la frontera salta northern argentina abstract in this study the chemical and isotopic composition of thermo mineral springs from the rosario de la frontera hydrothermal system was used to construct a conceptual model describing the source regions the thermal fluids and the chemical physical processes controlling the chemistry of waters and dissolved gases during their underground circulation the main hydrothermal reservoir hosted within the cretaceous pirgua subgroup deposits is fed by meteoric water and shows a na hco 3 composition produced by water rock interactions involving sedimentary formations mostly consisting of conglomerates and sandstones which are interbedded with alkaline volcanic rocks and shales and limestone deposits this aquifer also receives significant contributions of crustal co 2 and he from mantle degassing the latter being likely favored by the regional tectonic assessment that is characterized by a deep detachment at about 10 km depth in the basement of the santa barbara thick skinned thrust system and a thinned lithosphere the uprising thermal fluids mix with a relatively high salinity na cl dominated aquifer produced by the interaction of meteoric water with the tertiary anta formation evaporite the temperatures of the hydrothermal reservoir estimated with water geothermometers are up to 130 c which are consistent with the thickness of the hydrothermal circuit 2700 3000 m and the relatively high local geothermal gradient 40 c km these results suggest that the heat stored in the fluid phase of rfhs is up to 1 10 18 j a value significantly higher 20 than that previously estimated assuming an average reservoir temperature of 90 c

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

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

the role of collisional tectonics in the metallogeny of the central andean tin belt abstract the inner arc of the central andes broadly corresponding to the eastern cordillera is the location of a rich tertiary and triassic sn w ag base metal metallogenic province commonly referred to as the bolivian tin belt we propose that the tertiary metallogeny which generated most of the tin ores was a direct consequence of discrete collisions between the south american plate and the nazca slab and sub slab mantle during the ongoing andean orogeny evidence supporting this proposal include 1 the coincidence of the tin province and the inner arc in a marked hump in the andean orogen which may represent tectonic indentation 2 the symmetry of the tin province with respect to the bolivian orocline the axis of which corresponds to the direction of highest compression 3 the relative symmetry of the magmatism and tin mineralization with respect to this axis 4 the concurrent timing of mineralization and compressional pulses 5 the similar host rock geochemistry and ore lead isotope data testifying to a common crustal reservoir and 6 the striking similarity of the igneous suites associated with the ore deposits to those from typical collisional orogens a number of studies have called upon a persistent tin anomaly to explain the metallogeny of the region we propose instead that the latter is better explained by periodic compressional interaction between the farallon nazca oceanic plate and the south american continent this led to the generation of peraluminous magmas which during fractional crystallization exsolved the fluids responsible for the voluminous sn w mineralization

vertical axis rotations determined from paleomagnetism of mesozoic and cenozoic strata of the bolivian andes 1 thermal demagnetization and principal component analysis allowed determination of characteristic remanent magnetization chrm directions from 256 sites at 22 localities in mesozoic and cenozoic sedimentary strata of the bolivian altiplano and eastern cordillera an inclination only fold test of site mean chrm directions from cenozoic units principally the santa lucia formation indicates optimum unfolding at 97 1 unfolding consistent with a primary origin for the chrm for mesozoic strata optimum unfolding occurred at 89 2 perhaps indicating secondary remagnetization at some locations for cenozoic units comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations from 33 counterclockwise to 24 clockwise euler pole analysis of along strike variation in crustal shortening within the subandean and interandean zones indicates 18 clockwise rotation south of the axis of curvature of the bolivian andes and 6 counterclockwise rotation northwest of the axis during the past 10 m y along strike variation of shortening within the eastern cordillera indicates 8 clockwise rotation south of the axis and 8 counterclockwise rotation northwest of the axis from 35 to 10 ma these vertical axis rotations produced by along strike variations in crustal shortening during development of the bolivian fold thrust belt agree well with observed rotations determined from paleomagnetism of cenozoic rocks in the eastern cordillera and in the subandean and interandean zones however local rotations are required to account for complex rotations in the cochabamba basin and within the altiplano the curvature of the bolivian andes has been progressively enhanced during cenozoic fold thrust belt deformation

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

chilean flat slab subduction controlled by overriding plate thickness and trench rollback how flat slab geometries are generated has been long debated it has been suggested that trenchward motion of thick cratons in some areas of south america and cenozoic north america progressively closed the asthenospheric wedge and induced flat subduction here we develop time dependent numerical experiments to explore how trenchward motion of thick cratons may result in flat subduction we find that as the craton approaches the trench and the wedge closes two opposite phenomena control slab geometry the suction between ocean and continent increases favoring slab flattening while the mantle confined within the closing wedge dynamically pushes the slab backward and steepens it when the slab retreats as in the peru and chile flat slabs the wedge closure rate and dynamic push are small and suction forces generate in some cases flat subduction we model the past 30 m y of subduction in the chilean flat slab area and demonstrate that trenchward motion of thick lithosphere 200 300 km currently 700 800 km away from the peru chile trench reproduces a slab geometry that fits the stress pattern seismicity distribution and temporal and spatial evolution of deformation and volcanism in the region we also suggest that varying trench kinematics may explain some differing slab geometries along south america when the trench is stationary or advances the mantle flow within the closing wedge strongly pushes the slab backward and steepens it possibly explaining the absence of flat subduction in the bolivian orocline

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

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

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

structure of the subduction transition region from seismic array data in southern peru data from three seismic arrays installed in southern peru were analysed using receiver functions from p pp and pkp wave phases in order to image the subducted nazca slab the arrays cover the transition region from flat slab subduction in central peru to normal subduction with an angle of about 30 further south a previous study used data from the first array in the normal subduction region to image the moho depth and slab and showed the existence of a mid crustal structure at 40 km depth that is suggested to be a possible underthrusting of the brazilian shield here we discuss new observations from the other two arrays that span the transition between the two subduction regimes and the flat subduction region the results provide an image of the flattened slab from the coast to approximately 300 km inland and also across the transition region from flat to 30 subduction which appears to be a bend rather than a tear in the slab in the flat slab region the slab is well defined near the coast and flattens out at 100 km depth beneath the altiplano the slab appears to start flattening some 400 km in advance of the subduction of the nazca ridge and the flattening is maintained for 1300 km after its passage the moho begins at a depth of around 30 km near the coast and has a maximum depth of 75 km beneath the altiplano consistent with the results of the other arrays both arrays also show a positive impedance mid crustal structure at 40 km depth which if explainable by underthrusting of the brazilian shield would add further support to the observations from the normal subduction region and show the northward and westward extent of the signal the underthrusting hypothesis would explain the missing crust from the shortening budget needed to support the altiplano the vp vs ratios for both arrays exhibit average values between 1 73 and 1 75 indicating a lesser likelihood that there is a high degree of partial melting or magma bodies at depth in this region the receiver function results provide new imaging of the flat slab and transition from normal to flat slab subduction which allows for comparison of different subduction regimes