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

gravitational potential energy and regional stress and strain rate fields for continental plateaus examples from the central andes and colorado plateau abstract the commonly observed extension in areas of elevated and thickened crust is an expected consequence of having excess gravitational potential energy gpe compared to the low gpe of the surrounding crust while this conceptual model is well founded it is less clear how well gpe related stress orientations compare quantitatively to observed stress and strain rate orientations and what any inconsistency tells us about the presence of other competing forces we estimate the gpe distribution for the central andes and the greater colorado plateau area using topography and crustal thickness variations respectively and compare the related stress fields with the world stress map as well as with a geodetic strain rate field for the colorado plateau only in both areas deviatoric stresses associated with gpe variations alone cannot fully account for the observed deformation rate field for the central andes only a combination of deviatoric stresses associated with gpe and relative plate motions can account for the near n s tensional stress observed in the peruvian andes and the margin normal compressional stress along the eastern cordillera and sub andean fold and thrust belt the observed deformation field around the colorado plateau shows e w extension largely inconsistent with the deviatoric stresses associated with gpe variations except for the area east of the rio grande rift the ne sw oriented stress observed on the southwestern colorado plateau is consistent with the orientations of tensional deviatoric stresses associated with gpe variations we argue that this consistency could be haphazard stress observations may not reflect the current state of stress due to inherited structure or could result from the relative high strength of colorado plateau that allows for regional gpe variations and possibly basal shear to be more significant forces than far field plate interactions for the central andes and colorado plateau stresses associated with gpe variations have a strong influence on the total stress field and can thus be used to calibrate the overall level of deviatoric stress acting within the lithosphere

lithospheric structure and along strike segmentation of the central andean plateau seismic q magmatism flexure topography and tectonics the central andean plateau is a 300 km wide nearly 4 km high plateau which is situated above a 30 e dipping segment of the subducted nazca plate systematic changes in the topography the upper mantle seismic attenuation q structure the lithospheric flexural rigidity and the distribution and chemistry of back arc lavas indicate that the upper mantle structure changes along strike beneath the plateau south of 23 s the upper mantle beneath the plateau becomes hotter and the lithosphere becomes thinner and weaker this lateral variation in upper mantle structure coincides with variations in the tectonic style and timing of deformation in two physiographically distinct segments of the plateau and its adjacent foreland thrust belt to the east the bolivian altiplano and subandean ranges in the north and the argentine puna and santa barbara system in the south the segments different mantle lid thicknesses result in an altiplano supported mainly by crustal thickening and a puna supported by a combination of a crustal root and a thermal mantle root a thicker stronger foreland lithosphere in the north favors thin skinned underthrusting of the foreland beneath the plateau while to the south the thinner weaker lithosphere allows more basement involved shortening within the foreland and less underthrusting beneath the plateau a weaker lithosphere results in a longer period of compressional deformation and greater local and structural relief in the puna than in the relatively stable altiplano to the north ultimately this observed n s directed segmentation on the plateau and in the foreland may reflect the different responses of strong and weak continental lithosphere to an andean type subduction margin

late cenozoic bending of the bolivian andes new paleomagnetic and kinematic constraints 1 new paleomagnetic measurements of essentially undeformed late cenozoic volcanic rocks in the bolivian andes south america constrain rigid body rotations about vertical axes during the last 13 myr on both limbs of the bolivian orocline in the eastern cordillera thermal and alternating field demagnetization was carried out on samples from 52 sites in three major volcanic complexes 1 13 2 ma los frailes ignimbritic volcanics outcropping at 19 5 s 2 9 5 ma morococala ignimbritic volcanics outcropping at 18 s and 3 13 5 ma shoshonitic to acidic lavas and intrusives outcropping at 17 5 s well defined magnetic components were isolated which are interpreted to represent the earth s magnetic field at the time of volcanic activity the mean magnetic vector for site groupings suggests regional tectonic rotations about vertical axes with respect to stable south america of 10 8 clockwise for the 13 2 ma los frailes volcanic complex 8 9 clockwise for only the miocene 13 5 ma los frailes volcanics and 1 18 anticlockwise for the combined 13 5 ma morococala and eucalyptus volcanic complexes these data are consistent with observed shortening gradients on the eastern margin of the bolivian andes in the sub andean zone suggesting rotation and shortening are synchronous a joint inversion for both tectonic rotation and the amount of shortening assuming a linear variation in the amount of rotation along the length of the southern limb of the bolivian orocline shows that rotation of the eastern cordillera in the last 10 15 myr relative to stable south america varies from zero at the oroclinal hinge at 18 s to a maximum of 13 5 clockwise at 22 s decreasing to zero south of 23 s with an average rotation in the range 5 to 10 clockwise concomitant with this shortening in the sub andean zone decreases from a maximum of 86 km at the oroclinal hinge to 47 km at 22 s and then 33 km at 23 s the lack of evidence for large scale internal faulting in the eastern cordillera since 9 ma suggests either bending of the eastern cordillera was accommodated by more distributed small scale faulting or that relative rotation here mainly occurred prior to 9 ma the small average rotation of the eastern cordillera on the southern limb of the orocline is consistent with only very small rotation 10 of the forearc region in northern chile during the last 20 myr rotation of the northern limb is less well constrained but it is also likely to be small 5 and anticlockwise

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

uplift history of the central and northern andes a review the elevation of the andean cordillera is a crucial boundary condition for both climatic and tectonic studies the andes affect climate because they form the only barrier to atmospheric circulation in the southern hemisphere and they intrigue geologists because they have the highest plateau on earth formed at a noncollisional plate margin the altiplano puna yet until recently few quantitative studies of their uplift history existed this study presents both 1 a review of the quantitative paleoelevation estimates that have been made for the central and colombian andes and 2 an examination of the source and magnitude of error for each estimate in the central andes paleobotanical evidence suggests that the altiplano puna had attained no more than a third of its modern elevation of 3700 m by 20 ma and no more than half its modern elevation by 10 7 ma these data imply surface uplift on the order of 2300 3400 m since the late miocene at uplift rates of 0 2 0 3 mm yr paleobotanical and geomorphological data suggest a similar uplift history for the eastern cordillera namely no more than half the modern elevation present by 10 ma no evidence exists for an exponential increase in uplift rate as has been interpreted from fissiontrack data these uplift rates mostly reflect mean surface uplift rather than rock uplift that is uplift of material points because little dissection of the western eastern cordillera has occurred south of lat 19 s and of the altiplano puna thus the central andean plateau appears to be young in the colombian andes paleobotanical data imply rapid uplift of the eastern cordillera between 2 and 5 ma at rates on the order of 0 6 3 mm yr however some of this uplift is likely rock uplift due to erosion driven isostatic rebound rather than mean surface uplift

space geodetic observations of nazca south america convergence across the central andes space geodetic data recorded rates and directions of motion across the convergent boundary zone between the oceanic nazca and continental south american plates in peru and bolivia roughly half of the overall convergence about 30 to 40 millimeters per year accumulated on the locked plate interface and can be released in future earthquakes about 10 to 15 millimeters per year of crustal shortening occurred inland at the sub andean foreland fold and thrust belt indicating that the andes are continuing to build little 5 to 10 millimeters per year along trench motion of coastal forearc slivers was observed despite the oblique convergence

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

attenuation of high frequency seismic waves beneath the central andean plateau observed patterns of high frequency seismic wave attenuation suggest that near 22 s the upper mantle structure beneath the central andean plateau changes along strike to the south contrasting regions of high and low upper mantle seismic wave attenuation beneath the plateau are identified based on striking azimuthal variations in the character and frequency content of p and s waves propagating beneath the plateau to a portable seismic network deployed in jujuy province argentina 24 s 65 w ray paths from intermediate depth earthquakes located north and northwest of the network transmit seismic waves with a higher frequency content than ray paths from earthquakes at similar depths and distances but located west and south of the network the estimated apparent q values fall into two categories qp 500 and qs 350 for the high q paths and qp 350 and qs 200 for the low q paths in addition sn phases from regional crustal earthquakes in the subandean foreland fold thrust belt to the north propagate efficiently to the jujuy network while sn is not observed from foreland earthquakes located at similar distances to the south of the network these observations combined with data from previously reported wave propagation studies suggest that south of about 22 s the upper mantle beneath the plateau and its adjacent foreland thrust belt is more highly attenuating than the upper mantle farther north forward modeling of the q measurements made at jujuy indicates that the observations can be explained either by a variable thickness high q upper plate beneath the plateau or by a thin variable width very low q zone in the asthenospheric wedge above the subducted slab this lateral variation in upper mantle structure coincides with two physiographically distinct segments of the central andean plateau and its adjacent foreland thrust belt to the east the bolivian altiplano and subandean ranges in the north and the argentine puna and santa barbara system in the south we interpret the north south change in upper mantle attenuation and the corresponding changes in physiography topography and tectonic style at the surface to be due to a mantle lid that is thicker beneath the altiplano and the subandean belt than beneath the puna and the santa barbara ranges

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

erosional control on the geometry and kinematics of thrust belt development in the central andes long term erosion rates in the central andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt a drastic along strike erosional gradient exists in the modern central andes from a high erosion region directly north of the 17 5 s bend in the andes to a low erosion region south of the bend this gradient has existed since 10 15 ma based on fission track analyses of middle miocene to holocene denudation and qualitative evaluations of the preservation potential of middle late tertiary volcanic edifices and synorogenic sediment global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17 5 s bend whereas present day shortening in the thrust belt north of the bend is distributed over much of the width of the belt south of the bend it is concentrated near the eastern frontal margin structural data suggest a similar kinematic situation during late miocene to holocene shortening an out of sequence chronology of thrusting in the narrow 200 km wide thrust belt to the north versus a forward breaking sequence of thrusting in the relatively wide 350 km wide thrust belt to the south the long term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust wedge conditions induced by rapid erosion rates since 10 15 ma such conditions inhibit thrust front advance and favor distributed deformation within the thrust belt interior in the thrust belt south of the bend a progressive eastward migration of thrusting is interpreted to be the result of long term critical thrust wedge conditions promoted by extremely low rates of denudation since middle miocene time

bounds on the viscosity coefficient of continental lithosphere from removal of mantle lithosphere beneath the altiplano and eastern cordillera 1 the rapid rise of the central andean plateau between 10 and 6 8 ma implies that mantle lithosphere including eclogitized lower crust was removed from beneath the region in that time interval we infer from that removal that the average viscosity coefficient of mantle lithosphere was quite low when removal occurred using scaling laws for the growth of perturbations to the thickness of a dense layer over an inviscid substratum rayleigh taylor instability we place bounds on the average viscosity coefficient for central andean lithosphere when compared with laboratory measurements of flow laws for olivine and eclogite the allowed range of viscosity coefficients yields bounds on the temperature of 500 800 c at the moho beneath this region and suggests that mean stresses across mantle lithosphere during continental deformation are less than 50 mpa this range of temperature is comparable with if a slightly lower than we might expect for lithosphere approximately doubled in thickness and not yet equilibrated with the doubled crustal radioactivity the mean deviatoric stress is comparable to that associated with stresses that drive plates and hence shows that lithospheric material is not too strong to prevent removal of its mantle part

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

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

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

tectonic rotations within the bolivian altiplano implications for the geodynamic evolution of the central andes during the late tertiary paleomagnetic results from 61 sites in tertiary red beds and volcanic rocks from the bolivian andes allow quantification of tectonic rotations within the altiplano domain a total of 16 sites were also obtained in lower miocene ignimbrites that cover the forearc of the north chilean andes near arica in the southern altiplano lipez region a local clockwise rotation of up to 38 is recorded in lower miocene volcanic rocks farther north near the salar de uyuni there is no evidence of significant rotations counterclockwise rotations are observed in the northern altiplano the largest counterclockwise rotation 28 is found in eocene oligocene red beds tiwanaku formation along the eastern border of the corque syncline middle miocene sediments located within the center of the corque basin record a counterclockwise rotation of only 10 about 200 km north of the corque basin eocene oligocene sedimentary rocks near the locality of viacha record only 10 counterclockwise rotation paleomagnetic results in lower miocene ignimbrites from the forearc near the arica bend do not show evidence for late cenozoic rotation of the forearc these new results suggest that the 20 to 25 counterclockwise rotation of the southern peruvian forearc occurred prior to the miocene anisotropy of magnetic susceptibility ams data show a magnetic fabric mostly controlled by bedding but slight ams lineations are found almost parallel to the major regional structural trends such as fold axes the apparent relationship between tectonic rotations ams lineations and structural trends suggests that rotations occured after or during the final stage of folding paleomagnetic data obtained in paleozoic rocks show evidence of clockwise rotation of the southern sub andean ranges tectonic rotations during the neogene are mostly localized on the eastern side of the central andes the curvature of the occidental margin near arica was likely acquired prior to the last stage of andean deformation although the magnitude of the rotations may vary from one locality to the other there is a consistent pattern showing counterclockwise rotations to the north and clockwise to the south a large scale regional mechanism is needed to explain this pattern of tectonic rotations within the central andes we propose a model in which along strike segmentation of the andean foreland and indentation of the altiplano by a curved forearc are the major factors controlling tectonic rotations within the altiplano eastern cordillera and sub andean ranges

lithospheric flexure and bending of the central andes gravity anomaly and topography data are used to define the effective elastic thickness of the lithosphere t in the bend region of the central andes values of z increase from nearly zero north and south of the bend to values greater than 50 km at the bend in bolivia there is a close correlation between t and the style and magnitude of the shortening in the central andes since the late miocene in the bend region where the lithosphere is flexurally strong with large t foreland deformation is concentrated into a thin skinned fold and thrust belt above a basal decollement which has absorbed more than 100 km of shortening further north and south where the lithosphere is flexurally weak with low t foreland deformation is more complex involves basement and has absorbed less shortening the along strike gradients in foreland shortening have accommodated both clockwise and anticlockwise rotations about a vertical axis of almost the entire width of the bolivian andes we speculate that the observed variation in t is related to the proximity of the brazilian shield and has been an important factor in controlling the nature and amount of foreland deformation and hence bending of the central andes the flexural properties of the lithosphere may play an important role in determining the large scale evolution of mountain belts

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

lithospheric scale structure across the bolivian andes from tomographic images of velocity and attenuation for p and s waves we have developed a three dimensional lithospheric scale model across the bolivian andes at 20 s based on tomographic images of velocity and attenuation for both p and s waves observations of travel time and attenuation for this study are from regional mantle earthquakes in the subducted nazca plate recorded on a portable broadband seismic array broadband andean joint experiment and seismic exploration of the deep andes in bolivia and chile the shallow mantle under the altiplano from 18 s to 21 s is high velocity and moderately high q vp 8 3 vs 4 7 qp 500 and qs 200 suggesting lithospheric mantle high velocity material in the altiplano extends to a depth of 125 150 km the shallow mantle of the western cordillera is characterized by high vp vs 1 83 suggesting a correlation between vp vs and arc volcanism seismic velocity in the western cordillera mantle is on average only slightly reduced from global averages however velocity and attenuation anomalies are locally strong vp 7 8 vs 4 3 qp 200 and qs 100 consistent with partial melt conditions under the los frailes volcanic field in the eastern cordillera shallow mantle velocity and q decrease drastically from the neighboring altiplano vp 7 8 vs 4 3 qp 300 qs 100 however high vp vs is not as pervasive as it is in the western cordillera we believe that slab derived water and perhaps other volatiles strongly influence the western cordillera while the eastern cordillera low velocity region is more affected by partial melt and or compositional changes average velocity and q in the shallow mantle across the bolivian andes where the tomographic images are best resolved are significantly higher than in most mantle wedge environments where corresponding images are available this is likely the result of a compressional back arc setting in the andes this implies that lithospheric shortening and thickening associated with the formation of the andes has profoundly influenced the shallow mantle structure across the range shallow mantle structure is locally influenced by the subduction processes particularly under the western cordillera however the differing volcanism and seismic character under the two cordilleras suggest that the volcanic process in the eastern cordillera may be distinct from arc volcanism tertiary volcanism in the eastern cordillera is located in the region where mantle shortening is suspected to be greatest both the timing and location of volcanism are consistent with upward migration of mantle wedge asthenosphere following the removal of over thickened lithosphere

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

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

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

characterization of organic matter in late cretaceous black shales of the eastern alps kainach gosau group austria abstract bituminous marls occur at the base of the late cretaceous kainach gosau basin eastern alps they interfinger with local coaly layers and are overlain by a turbiditic sequence forty outcrop samples with different lithologies were studied by rock eval pyrolysis seven bituminous marls one coaly sample two siltstones from the turbiditic sequence and solid bitumen from the paleozoic basement have been characterized in detail by organic geochemical and petrographic techniques the organic matter from bituminous marls can be subdivided into allochthonous terrigenous material mainly recycled vitrinite which is present in all samples in similar amounts and autochthonous hydrogen rich organic matter alginite liptodetrinite which is most probably of lacustrine origin organic richness varies from 0 4 to 2 3 toc the amount of the autochthonous organic matter is controlled primarily by different oxygen concentrations sedimentological lack of bioturbation and geochemical data e g pristane phytane ratios homohopane indices c 30 c 29 ts ratios distribution of phenol and alkylphenols in the pyrolyzate suggest that the water column above the sediment surface was suboxic to dysoxic but probably not anoxic high energy events were responsible for short periods with enriched oxygen in the water column the organic matter in the resulting sediments is of a residual type drift wood was transported from the nearby shore into the lake paleontological evidence proves a temporary marine influence vitrinite reflectance and tmax indicate maturation levels ranging from the beginning to the peak stage of oil generation despite the fact that some hydrocarbons have already been generated hi values of kerogens of bituminous marls reach 605 mg hc g toc most analysed bituminous marls produce a high wax paraffinic oil during maturation and are good sources for oil pyrolysis gas chromatography and biomarker data indicate that changing wax contents of oils generated from bituminous marls might be a function of different oxygen concentrations in the water column coaly samples have a very good gas potential

constraints on present day shortening rate across the central eastern andes from gps data two years of continuous gps data from several sites in south america indicate that arequipa in the southern peruvian andes has a velocity of 13 3 mm yr two standard errors to the northeast with respect to stable south america we interpret these data as reflecting a combination of elastic strain accumulation associated with a locked nazca south america subduction zone and a small amount of crustal shortening across the fold and thrust belt on the eastern margin of the andes models of elastic strain accumulation for fully locked and partly locked subduction zones constrain shortening in the eastern andes to 0 3 mm yr fully locked and 0 12 mm yr partly locked slower than some geologic estimates averaged over millions of years

late cainozoic rotation of the peruvian western cordillera and the uplift of the central andes abstract a palaeomagnetic study was carried out in the region of lima peru on 55 sites 564 cores of middle cretaceous to late miocene age distributed along three northeast southwest transverses which cross the western cordillera of the central andes over the entire studied area the results consistently yield westerly declinations corresponding to post cretaceous and post eocene counterclockwise rotations with respect to stable south america of 16 6 and 13 7 8 1 respectively when compared to the ill defined reference poles of this continent these new results are in good agreement with previous ones from the central peruvian andes and support the model recently proposed for the uplift of the andean cordillera

relations between hinterland and foreland shortening sevier orogeny central north american cordillera the tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens the classic central cordilleran foreland fold and thrust belt in the united states late jurassic to early tertiary sevier belt and the more internal zones to the west central nevada thrust belt provide data critical to understanding the developmere of internal and external parts of orogens the garden valley thrust system part of the central nevada thnkst belt crops out in south central nevada within a region generally considered to be the hinterland of the jurassic to eocene sevier thrust belt the thrust system consists of at least four principal thrust plates composed of strata as young as pennsylvanian in age that are unconformably overlain by rocks as old as oligocene suggesting that contraction occurred between those times new u pb dates on intrusions that postdate contraction combined with new paleomagnetic data showing significant tilting of one area prior to intrusion suggest that regionally these thrusts were active before 85 100 ma the thrust faults are characterized by long relatively steeply dipping ramps and associated folds that are broad and open to close upright and overturned although now fragmented by cenozoic crustal extension individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike we correlate the structurally lowest thrust of the garden valley thrust system the golden gate mount irish thrust southward with the gass peak thrust of southern nevada this correlation carries the following regional implications at least some of the slip across jurassic to mid cretaceous foreland thrusts in southern nevada continues northward along the central nevada thrust belt rather than noaheastward into utah this continuation is consistent with age relations which indicate that thrusts in the type sevier belt in central utah are synchronous with or younger than the youngest thrusts in southern nevada this in turn implies that geometrically similar sevier belt thrusts in utah must die out southward before they reach nevada that slip along the southem nevada thrusts is partitioned

evolution of shallow and deep structures along the maipo tunuyan transect 33 40 s from the pacific coast to the andean foreland fil giambiagi laura beatriz consejo nacional de investigaciones cientificas y tecnicas cientifico tecnologico mendoza instituto argentino de nivologia glaciologia y ciencias ambientales argentina

composition and structural control of crustal domains in the central andes 1 present day ratios of pb isotopes 324 published samples 435 new and nd sr isotopes 150 published 180 new on proterozoic to holocene igneous metamorphic and sedimentary rocks define at high spatial resolution distinct isotopic domains of the crust in the central andes these domains correlate with the internal compositional structure of the crust as revealed by a three dimensional density model pb nd isotopic boundaries thus correspond to variations in crustal compositional structure and reflect proterozoic mafic dominated and paleozoic felsic dominated crustal lithologies age and composition mafic versus felsic of these domains have controlled the rheology of the andean crust have influenced crustal deformation patterns and correlate with the central andean plateau segmentation

petrogenesis of early neogene magmatism in the northern puna implications for magma genesis and crustal processes in the central andean plateau of uolm 2 and mm group rocks assuming the same endnew compositional data and petrogenetic models are presented for member compositions the modelling suggests genesis of the mm pre upper miocene volcanism in the northern puna of argentina magmas at higher pressure than the uolm 2 centres 10 kbar 22 s 24 s two phases of volcanism produced small dome vs 7 kbar which may reflect the influence of crustal thickening in complexes of mainly silicic andesite to low sio2 rhyolite the the plateau region by the mid miocene the felsic dome complexes upper oligocene lower miocene phase uolm 20 17 ma of this study are compositionally similar to the large volume calderaproduced two distinct groups of rocks the uolm 1 group is sourced felsic ignimbrites that dominated volcanism in the region metaluminous and mainly andesitic with isotopic compositions like from 10 to 2 ma and our results suggest that there is no fundamental those of the recent arc sr srt 0 706 ndt 3 the difference in magma genesis between them the differences in the uolm 2 group is more silicic and peraluminous and has isotopic volumes and the mode of eruption reflect changes in the stress and compositions indicating a substantial crustal contribution sr thermal regime with time srt 0 713 ndt 8 the mid miocene phase mm 15 12 ma produced rocks similar in composition to those of the uolm 2 group sr srt 0 710 ndt 7 but with higher incompatible element contents ratios of ba nb and zr

resistivity cross section through the southern central andes as inferred from magnetotelluric and geomagnetic deep soundings to investigate the upper lithospheric structure and subduction induced processes across an active continental margin magnetotelluric mt and geomagnetic deep sounding gds experiments were done in the southern central andes 21 25 s of northern chile southern bolivia and northwestern argentina using two dimensional modeling we have constructed an e w resistivity cross section at latitude 21 45 s that involves all the andean units from the pacific coast to the lowland plains the crust of the coastal cordillera has high resistivities of more than 5000 ohm m with some conductive structures embedded increased electrical conductivities between depths of 8 and 40 km may be related to the rise of fluids released from the subducted nazca plate the electrical resistivity of the normal andean crust seems to be low having values of only 200 ohm m a wide zone of extremely high electrical conductivity with an e w extent of more than 330 km was detected this high conductivity zone hcz is found at a depth of about 25 km under the western cordillera and has a total conductance of more than 23 000 s here it may be caused by partially melted crust an interpretation which would correlate with those from gravity and seismic data the hcz can be followed in the lower crust eastward below the altiplano where it has a total conductance of about 15 000 s in the western part of the eastern cordillera total conductance first increases and then drops abruptly at about longitude 65 10 w the latter hczs can be explained in terms of thrust tectonics where detachment zones may involve fluids as well as increased mineralizations the lowlands of the sub andean and the chaco are characterized by a cover of anisotropic low resistivity the resistivity of the crust and uppermost mantle increases from west to east to more than 1000 ohm m while the deeper mantle gets unexpectedly much more conductive

numerical simulations of a gravity wave event over ccope part iii the role of a mountain plains solenoid in the generation of the second wave episode abstract mesoscale model simulations have been performed of the second episode of gravity waves observed in great detail in previous studies on 11 12 july 1981 during the cooperative convective precipitation experiment the dominant wave simulated by the model was mechanically forced by the strong updraft associated with a mountain plains solenoid mps as this updraft impinged upon a stratified shear layer above the deep well mixed boundary layer that developed due to strong sensible heating over the absaroka mountains the gravity wave was created this wave rapidly weakened as it propagated eastward however explosive convection developed directly over the remnant gravity wave as an eastward propagating density current produced by a rainband generated within the mps leeside convergence zone merged with a westward propagating density current in eastern montana the greatly strengthened cool pool resulting from this new convection then generated a bore wave that appeared to be continuous with the move

tertiary provenance history of the northern and central altiplano central andes bolivia a detrital record of plateau margin tectonics abstract the internally drained altiplano plateau in the high elevation hinterland of the central andes contains an extremely thick up to 12 km succession of tertiary nonmarine strata that recorded the topographic evolution of eastern and western plateau margins in this study temporal and spatial variations in detrital composition based on 113 sandstone thin sections and 31 conglomerate clast counts combined with sediment dispersal patterns and regional stratigraphic correlations help delineate the tertiary history of altiplano basin development in relationship to thrust deformation and arc magmatism along plateau margins stratigraphic and provenance data suggest limited subsidence and west directed sediment dispersal during the early tertiary history of the altiplano region by late eocene time development of a rapidly subsiding basin probable foredeep was underway as recorded by extensive east directed fluvial systems of the 3 6 5 km thick potoco formation potoco compositional data are compatible with sediment sources in the incipient western cordillera and regions farther west including 1 a probable thrust belt source terrane composed of paleozoic mesozoic strata and precambrian granitic gneissic basement and 2 a subordinate magmatic arc source terrane composed of tertiary volcanic rocks increased abundances of plagioclase grains and volcanic lithic fragments upsection indicate a progressively greater arc influence due to either enhanced volcanism or eastward arc migration by late oligocene time west directed fluvial and alluvial fan systems of the upper potoco formation and the 3 km thick coniri formation reached the altiplano basin providing a minimum uplift age for the backthrust belt along the western flank of the eastern cordillera paleozoic clast conglomerate and relatively quartz rich sandstone containing sedimentary and metasedimentary lithic fragments show unroofing of predominantly paleozoic strata in the backthrust belt continued late oligocene to quaternary accumulation in the altiplano reveals that the basin persisted as a depositional region during a well documented period of neogene shortening and uplift in the andean thrust belt farther east this intra orogenic hinterland deposition between eastern and western highs during ongoing shortening suggests that the altiplano basin existed as a crustal scale piggyback basin possibly in an internally drained plateau like geomorphic setting since 25 ma provenance data and independent structural and thermochronological evidence for spatially and temporally varying tectonic histories along the eastern and western plateau margins imply that several modes of basin development were active during growth of the altiplano although flexural subsidence due to thrust loading along both plateau margins initially in the west and subsequently in the east may explain most tertiary subsidence possible closed basin conditions due to structural damming could have promoted additional sediment accumulation from late oligocene to quaternary time

along strike segmentation of the andean foreland causes and consequences abstract thrust belts of tertiary age characterize the east flank of the andes and part of the adjacent foreland from venezuela to the southern tip of south america this foreland deformation comprises three basically different structural styles 1 thin skinned thrust belts detached within the sedimentary cover 2 thick skinned thrust belts with an inferred basal detachment in the basement at 10 20 km depth and 3 foreland basement thrusts which possibly affect the entire crust alternating foreland structural styles along the orogen often with sharp boundaries produce a segmentation of the andean foreland the changes in structural style are interpreted to be primarily controlled by inherited stratigraphic and structural features of the south american plate for instance thin skinned thrust belts appear to require a more or less conformable sedimentary cover at least some 3 km thick over a metamorphic crystalline basement unaffected by mesozoic extension in areas of mesozoic rifting normal faults have been reactivated during tertiary contraction resulting in the formation of thick skinned thrust belts the basement becomes involved even if potential decollement levels are present deep seated foreland basement thrusts tend to form over basement arches with a thin sedimentary cover the three basic structural styles are unequally efficient in terms of shortening thin skinned belts are typically shortened by 40 70 thick skinned belts by 20 35 and areas of foreland thrusts by less than 10 the structural segmentation of the foreland therefore implies strong along strike variations in the amount of shortening available estimates of shortening along the central andes actually suggest that shortening does not vary steadily but has several maxima and minima strongly and weakly shortened foreland segments correlate poorly with the segmentation of the subducted plate in steeper and subhorizontal segments flat slabs but closely with lithospheric structure the most strongly shortened segment coincides with thick mantle lithosphere suggesting that shortening of the edge of the south american plate has resulted in substantial underthrusting of lithospheric mantle beneath part of the andes this underthrusting of thick lithosphere may have caused steepening of a formerly flat slab in the arica bend region to the normal 25 30 dip presently observed

basement reactivation in a sub andean foreland flexural bulge the pantanal wetland sw brazil the role of sub andean foreland peripheral bulges in controlling sedimentary sequence deposition and quaternary geomorphology is discussed in the case of the pantanal wetland sw brazil this shallow 0 5 km and wide 200 km depression was formed because of uplift and flexural extension in the brittle upper crust which mechanically reactivated the neoproterozoic paraguai fold thrust belt depth to the basement and structural information within the pantanal wetland were obtained from gravity reflection seismics drill hole and digital topography data the upper part of the basin is filled with pliocene quaternary unlithified sands the age of the oldest sediments is unknown and normal faults are mostly observed on the western border of the basin a two dimensional flexural model composed of a semi infinite thin elastic plate for the brazilian shield lithosphere with an effective elastic thickness of 125 150 km its western edge at 67 5 w under the altiplano and loaded on a free end by the andes topography predicts the forebulge to be presently located over the pantanal wetland flexural modeling was constrained using gravity data extending from the pacific ocean to the east of the pantanal wetland the width of the bulge is 700 km and its amplitude is 310 m the total amount of extension and subsidence observed in the pantanal basin is interpreted as due to a cumulative strain since late oligocene when the brazilian shield started its westward migration together with compression and uplift of the central andes the main phase of the wetland subsidence is very likely related to the last compressional pulse in the andes during the upper pliocene lower pleistocene 2 5 ma as inferred from field study of fault kinematics of the central andes

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

coupling of south american and african plate motion and plate deformation although the african plate s northeastward absolute motion slowed abruptly 30 million years ago the south atlantic s spreading velocity has remained roughly constant over the past 80 million years thus requiring a simultaneous westward acceleration of the south american plate this plate velocity correlation occurs because the two plates are coupled to general mantle circulation the deceleration of the african plate due to its collision with the eurasian plate diverts mantle flow westward increasing the net basal driving torque and westward velocity of the south american plate one result of south america s higher plate velocity is the increased cordilleran activity along its western edge beginning at about 30 million years ago

electrical and thermal anomalies in the central andean subduction zone an enhancement of the electrical conductivity in the upper layers in the subduction zones of southern peru and northern argentina is found by analyzing the solar quiet geomagnetic variations this feature appears in a zone associated with large subduction angles in this region high values of heat flow have been measured therefore a one dimensional thermal model is proposed using regional characteristic parameters to reproduce these values it is found that thermal convection necessarily plays a contributing role this effect may be produced by the ascending magma and fluids which can explain the enhancement of the electrical conductivity

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

megathrust locking and viscous mantle flow induce continental shortening in central andes crustal faults at subduction zones show evidence of activity over geological time but at the scale of earthquake cycle the mechanical behavior of these faults is not fully understood here we construct 2 d viscoelastic models constrained by both horizontal and vertical gps derived interseismic velocities to investigate the contribution and interrelation between subduction zone locking viscous mantle flow and upper plate faulting on surface deformation in central andes main pattern of horizontal velocities can be explained by a combination of locking degree and viscous flow whereas vertical signal is found essential for estimating the locking depth a sharp deformation gradient near the major back arc fault suggests an active interseismic shorting across this structure we further conduct mechanical viscoelastic models with a frictional back arc fault to analyze its displacement and activation conditions our results suggest that the back arc fault is creeping at 3 mm year and its motion is mainly driven by the interseismic viscous mantle flow which spreads plate tectonic stresses broadly across the continent moreover the frictional strength of the back arc fault must be remarkably weak and its mechanics re distributes the interseismic deformation and shortens the continental plate in central andes geological estimates suggest that the long term shortening rate at the back arc fault is 10 mm year suggesting that this structure can accumulate 7 mm year of slip deficit confirming the seismic potential of this structure

generation of a crust mantle magma mixture magma sources and contamination at cerro panizos central andes cerro panizos a large caldera in the central andes mountains produced two large dacitic ignimbrites at 7 9 ma and 6 7 ma and many andesitic and dacitic lava flows and domes the older rhyodacitic cienago ignimbrite represents the most silicic magma erupted by the system the younger much larger volume dacitic cerro panizos ignimbrite is very crystal rich containing up to 50 biotite plagioclase and quartz crystals in the pumice it is weakly zoned with most of the zoning apparent between two main cooling units major and most trace elements show little variation through the cerro panizos ignimbrite but the small range of composition is consistent with typical fractionation trends sr nd and pb isotopic ratios are very crustal with initial 87sr 86sr values of 0 711 to 0 715 nd values of 7 5 to 10 2 and nearly invariant pb isotopic ratios 206pb 204pb 18 85 207pb 204pb 15 67 and 208pb 204pb 38 80 the limited zonation observed in the cerro panizos ignimbrite is explained by impeded crystal settling due to high crystal content the magma body was a crystal liquid mush before ascent to the pre eruption crustal levels crystals formed but did not separate easily from the magma limited fractionation of plagioclase and biotite may have occurred but the composition was largely controlled by lower crustal mash processes afc modeling shows that the cerro panizos magmas resulted from a mixture of roughly equal proportions of late miocene mantle derived basalts and melts from 1 0 ga grenville age lower crust this occurred in a mash zone in the lower crust and set the crustal isotopic ratios observed in the cerro panizos magmas the great thickening of the crust beneath the central andes mountains sent upper and middle crustal rock types to lower crustal and deeper depths and this explains the upper crustal isotopic signatures of the cerro panizos rocks minor upper crustal assimilation of early miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system the nearly invariant high pb isotopic values and high pb concentrations indicate that pb came almost entirely from the crustal source and was little altered by any subsequent upper crustal assimilation this pb signature is isotopically similar to that of the southern bolivian tin belt suggesting a widely distributed pb source the great difference between compositions of miocene and quaternary central andean volcanic rocks is explained by crustal thickening in early miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions the lower crust dried out considerably by quaternary time so less crustal component is present

cenozoic foreland basin system in the central andes of northwestern argentina implications for andean geodynamics and modes of deformation 1 cenozoic strata in the central andes of northwestern argentina record the development and migration of a regional foreland basin system analogous to the modern chaco parana alluvial plain paleocene lower eocene fluvial and lacustrine deposits are overlain by middle upper eocene hypermature paleosols or an erosional disconformity representing 10 15 myr this supersol disconformity zone is traceable over a 200 000 km2 area in the andean thrust belt and is overlain by 2 6 km of upward coarsening eastward thinning upper eocene through lower miocene fluvial and eolian deposits middle miocene pliocene fluvial lacustrine and alluvial fan deposits occupy local depocenters with contractional growth structures paleocurrent and petrographic data demonstrate westerly provenance of quartzolithic and feldspatholithic sediments detrital zircon ages from cenozoic sandstones cluster at 470 491 522 544 555 994 and 1024 1096 ma proterozoic mesozoic clastic and igneous rocks in the puna and cordillera oriental yield similar age clusters and served as sources of the zircons in the cenozoic deposits arc derived zircons become prominent in oligo miocene deposits and provide new chronostratigraphic constraints sediment accumulation rate increased from 20 m myr during paleocene eocene time to 200 600 m myr during the middle to late miocene the new data suggest that a flexural foreland basin formed during paleocene time and migrated at least 600 km eastward at an unsteady pace dictated by periods of abrupt eastward propagation of the orogenic strain front despite differences in deformation style between bolivia and northwestern argentina lithosphere in these two regions flexed similarly in response to eastward encroachment of a comparable orogenic load beginning during late paleocene time

gps derived deformation of the central andes including the 1995 antofagasta mw 8 0 earthquake in order to study both the interplate seismic loading cycle and the distribution of intraplate deformation of the andes a 215 site gps network covering chile and the western part of argentina was selected monumented and observed in 1993 and 1994 a dense part of the network in northern chile and northwest argentina comprising some 70 sites was re observed after two years in october november 1995 the m w 8 0 antofagasta north chile earthquake of 30th july 1995 took place between the two observations the city of antofagasta shifted 80 cm westwards by this event and the displacement still reached 10 cm at locations 300 km from the trench three different deformation processes have been considered for modeling the measured displacements 1 interseismic accumulation of elastic strain due to subduction coupling 2 coseismic strain release during the antofagasta earthquake and 3 crustal shortening in the sub andes eastward displacement of the sites to the north and to the south of the area affected by the earthquake is due to the interseismic accumulation of elastic deformation assuming a uniform slip model of interseismic coupling the observed displacements at the coast require a fully locked subduction interface and a depth of seismic coupling of 50 km the geodetically derived fault plane parameters of the antofagasta earthquake are consistent with results derived from wave form modeling of seismolog ical data the coseismic slip predicted by the variable slip model reaches values of 3 2 m in the dip slip and 1 4 m in the strike slip directions the derived rake is 66 our geodetic results suggest that the oblique nazca south american plate convergence is accommodated by oblique earthquake slip with no slip partitioning the observed displacements in the back arc indicate a present day crustal shortening rate of 3 4 mm year which is significantly slower than the average of 10 mm year experienced during the evolution of the andean plateau

crustal structure of the altiplano from broadband regional waveform modeling implications for the composition of thick continental crust we have modeled the full waveforms from six intermediate depth and two shallow earthquakes recorded at regional distances by the banjo broadband andean joint experiment banjo and seismic exploration of the deep altiplano seda portable seismic networks in the central andes in this study we utilize data from those banjo and seda stations located within the altiplano and eastern cordillera we used reflectivity synthetic seismograms and a grid search to constrain four parameters of the altiplano eastern cordillera lithosphere crustal thickness average crustal velocity vp and crustal and upper mantle poisson s ratios crust and mantle using our grid search we investigated the crustal and upper mantle structure along 36 individual event station paths and applied forward modeling to 56 event station paths robust models for the altiplano that provide the best overall fit between the data and synthetic seismograms are characterized by an average vp of 5 75 6 25 km s crustal thicknesses of 60 65 km crust 0 25 and mantle 0 27 0 29 we find a north south variation in the structure of the altiplano with the crust south of the banjo transect characterized by either lower than average crustal p wave velocities or a slightly higher crust relative to crust north of the banjo transect these results are consistent with a model of crustal thickening caused predominantly by tectonic shortening of felsic crust rather than by underplating or magmatic intrusion from the mantle

vertical axis rotations across the puna plateau northwestern argentina from paleomagnetic analysis of cretaceous and cenozoic rocks between 10 s and 30 s the central andes are marked by both a major topographic anomaly the altiplano puna plateau and a westward concave geometry whose origin remains controversial the arcuate shape is accompanied by a remarkable pattern of rotations about vertical axes indeed in the central andes paleomagnetic studies have demonstrated counterclockwise rotations on the northern limb of the arc throughout peru northernmost chile and northern bolivia and clockwise rotations on the southern limb throughout southern bolivia northwestern argentina and northern chile to fill a gap in data from northern argentina and to contribute to the ongoing debate on the origin of rotations in the central andes we have undertaken a paleomagnetic study of 373 cores taken at 29 sites grouped into seven localities the samples are from sediments and lava flows of cretaceous to tertiary age located in intermontane basins of the puna plateau in northwestern argentina vertical axis rotations calculated from paleomagnetic declinations are clockwise for all localities and confirm the pattern of clockwise rotations associated with the southern central andes however significant variations in the amount of rotation occur from one locality to another suggesting that they are at least in part influenced by local tectonics as most faults in the puna plateau have reverse dip slip components we infer that the observed differential rotations between blocks are due to scissoring motions on thrust faults whether or not this mechanism has operated across the entire area of thickened crust in the central andes remains to be demontrated even if such faulting has locally influenced rotations cenozoic oroclinal bending is a likely cause of the remarkable pattern of vertical axis rotations across the central andes

lower bound on the amount of crustal shortening in the central bolivian andes balanced cross sections across the cordillera oriental and subandean zone of the central bolivian andes indicate that crustal shortening probably has played the dominant role in orogeny in this convergent margin setting a minimum amount of shortening 210 km is documented which can account for two thirds of the present day crustal cross sectional area along a transect spanning the entire mountain range substantial crustal shortening may also require loss of the lower lithosphere to the asthenosphere a large minimum amount of crustal shortening in the bolivian andes shows contrary to common assumptions about orogeny that 1 magmatic addition may be volumetrically less important in orogeny in andean type margins and 2 crustal shortening is not uniquely associated with continental or island arc collision

early eocene crustal thickening in southern tibet new age and geochemical constraints from the gangdese batholith abstract the lhasa terrane southern tibet is where the thickest part of the earth s crust occurs as a result of the india asia collision this paper reports zircon u pb age and geochemical data of samples collected from the gangdese batholith southern lhasa terrane a subset of which has already been dated at 65 41 ma using zircon u pb geochronology ji w q wu f y chung s l li j x liu c z 2009a zircon u pb chronology and hf isotopic constraints on the petrogenesis of gangdese batholiths southern tibet chemical geology 262 229 245 to constrain the timing and mechanism s responsible for crustal thickening beneath southern tibet our new data indicate that the gangdese magmatism lasted continuously from ca 65 to 34 ma during which geochemical evidence for crustal thickening is observed crustal thickening is best illustrated by the progressive increase in la yb and sr y of the early to latest eocene magmatic rocks associated with the decrease in hf isotopic ratios of paleocene to eocene magmatic zircons we also identify in this study the oldest ca 51 ma post collisional adakitic granitoids in the gangdese batholith interpreted as products from partial melting of underplated basaltic lower crust therefore a thickened crust likely existed by that time at least locally in the southern lhasa terrane tibet

crustal motion north and south of the arica deflection comparing recent geodetic results from the central andes 1 an intercomparison of recent velocity solutions for the global positioning system gps networks constructed by the south american nazca plate project snapp based in bolivia and peru and our central andes gps project cap based in chile and argentina indicates a velocity discontinuity of order 10 mm yr near the boundary between these networks we suggest that this velocity jump manifests measurement bias in the snapp velocity field our results indicate that no major slip partioning occurs within the forearc of northern chile in response to the obliquity of subduction but that nazca south america plate convergence is partitioned between the forearc and the backarc regions the present rate of shortening across the southern part of the sub andean belt in argentina is 8 9 1 6 mm yr

inversion of an oligocene volcano tectonic basin and uplifting of its superimposed miocene magmatic arc in the chilean central andes first seismic and gravity evidences abstract a new structural model is presented for the cenozoic part of the main range east of santiago a chain made up of thick miocene ignimbrites and andesitic lavas overlying primitive oligocene volcanoclastics deposited on a thin continental crust because the latter coya machali formation locally show tight folding close to the margins of the miocene volcanics they had previously been described as deformed prior to construction of the arc our field observations analysed along two profiles at 33 and 34 15 s however support the conclusion hinted at by ongoing geochronological and geochemical studies deformation of both formations is synchronous and related to late miocene inversion of the coya machali intra arc basin uplift of the rigid arc rocks and part of its volcanoclastic basement took place mainly along two northward shallowing boundary structures that seem to share a lower decollement the eastern fierro out of sequence thrust and a series of western back thrusts that resemble triangle zones seismic data obtained from el teniente porphyry copper mine 34 s rockbursts and two gravimetric profiles at the western end of the southern profile are compatible with a decollement located at that locality 2 3 km under the surface

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

geochemical evidences for quantifying crustal thickness over time in the urumieh dokhtar magmatic arc iran abstract geochemical data on igneous rocks of the urumieh dokhtar magmatic arc udma are used to reconstruct temporal variations in crustal thickness during the tertiary we use two approaches 1 whole rock sr y and la yb ratios and 2 nd isotopes and an assimilation model for magma genesis our results indicate that the crust was likely of normal thickness 35 km and remained constant during eocene to oligocene corresponding to slab roll back and back arc extension along the eurasian active margin crustal thickening began during the early miocene and culminated in the middle miocene 50 km this thickening is marked by a distinct increase in sr y and la yb ratios changing the stability of certain mineral phases and decrease in nd increasing crustal assimilation thickening was likely due to intrusion of magma as well as crustal shortening and was a consequence of the final roll back of the neo tethys slab and subsequent continental collision

quantifying crustal extension and shortening in the back arc region of northeast japan 1 the northeast japan nej arc has experienced strong extension in the early to middle miocene time and contractive deformation since pliocene time we determine the amounts of the miocene extension and pliocene contraction along four transects across the nej arc by area balancing restoration using seismic reflection gravity and surface geologic data we find that the miocene extension is as much as 31 56 km while the pliocene quaternary contraction is only 10 15 km the style of the miocene extension in the back arc region of nej is highly asymmetric the zone of concentrated extension is characterized by a breakaway fault on the west a rollover basement anticline on the east and abnormally deep 10 km basins and strongly rotated fault blocks in between suggesting the existence of a large scale detachment fault at a mid crustal level beneath the extended zone we reveal that the pliocene quaternary contractive surface deformation in the back arc region of nej resulted principally from positive tectonic inversion of the underlying detachment fault previous works have suggested that andean type orogens have often experienced negative and positive inversions alternately in their back arc regions although the former has been well documented for example for the cenozoic north american cordillera the latter has rarely been reported because subsequent contraction has obscured the older structures since the nej arc is still at an incipient stage and hence its older structures are well preserved our study provides an important example of positive tectonic inversion of andean type orogens

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

crustal shortening in the andes why do gps rates differ from geological rates gps data indicate 30 40 mm yr 1 present day crustal shortening across the andes whereas geological evidence shows crustal shortening concentrated in the sub andean thrust belt at a much lower rate 15 mm yr 1 we reconcile the discrepancy between the geodetic and the geological crustal shortening using geodynamic modeling that includes timescale dependent crustal deformation the gps velocities reflect the instantaneous deformation in the andes that includes both permanent deformation and elastic deformation that will be recovered during future earthquakes whereas the lower geological rates reflect only the permanent deformation the three dimensional viscoelastic model predicts nearly uniform short term velocity gradients across the andes similar to the gps results and concentrated long term crustal shortening in the sub andean thrust zone consistent with geological observations

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

partial melting below the magmatic arc in the central andes deduced from geoelectromagnetic field experiments and laboratory data abstract magnetotelluric and geomagnetic deep soudings in northern chile revealed a pronounced high conductivity zone hcz below the western cordillera which constitutes the present magmatic arc with active volcanism of the south american continental margin conductivities in the range of 1 s m are observed the anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km are interpreted in terms of partial melting other geophysical observations such as a zone of low seismic velocities lvz at similar depths high heat flow values 100 mw m2 and a pronounced negative anomaly in the residual gravity field are also considered impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities at sub solidus temperatures electrical behavior is described by defect electrons with an activation energy of 1 34 ev and a conductivity of 2 5 ms m at 900 c model calculations using a modified brick layer model mbl were compared with experimental observations a good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 s m at 1250 c assuming an activation energy of 1 ev the same mbl model is used to calculate melt proportions beneath the western cordillera between 14 and 27 vol of interconnected melt are necessary to explain the observed hcz the stability of the melt rich crust is explained by a dynamic melting crystallisation behavior during crustal anatexis and by magma filled dikes

seismic gravity and petrological evidence for partial melt beneath the thickened central andean crust 21 23 s on the basis of seismic refraction investigations and gravimetric data we have modelled the crustal structure of the southern central andes 21 23 s a pronounced variation in crustal parameters is seen in n s and w e crossing seismic profiles over the entire andean orogene characterized by a crustal thickness of up to 70 km under the magmatic arc and backarc strongly reduced seismic velocities and a bouguer minimum of 450 mgal anomalously low velocities of 5 9 6 0 km s in the deeper crust of the western cordillera and altiplano regions lead to an over compensation of the bouguer minima resulting in values of crustal densities higher than estimates based purely on seismic velocity measurements in an attempt to reconcile these differences the behavior of crystalline rocks based on published laboratory data was studied under varying pressure and temperature conditions up to the range of partial melting if the temperature is increased above the melting point a rapid decrease in seismic velocity is accompanied by a slow decrease in density for the central andes a good fit of the observed and calculated bouguer anomalies is obtained if the densities of the rocks from the low velocity zone lvz beneath the western cordillera and the altiplano are varied model calculations lead to a velocity density relation for partial molten rocks that allows the melt proportions of rocks to be estimated model calculations indicate that 15 20 vol of basaltic to andesitic melt at depth is necessary to explain the lvz and bouguer anomaly beneath the arc and parts of the backarc high heat flow values 100 mw m2 support the idea that large areas of the deeper andean crust are strongly weakened by the presence of partially molten rocks resulting in reduced seismic velocities with the western cordillera the active volcanic arc of the andean mountain range acting as a ductile buffer between the two more rigid crustal blocks of the forearc and backarc regions

paleomagnetism of mesozoic rocks from the central andes of southern peru importance of rotations in the development of the bolivian orocline a paleomagnetic study has been undertaken in jurassic volcanics and cretaceous intrusions from coastal southern peru a secondary magnetization in the jurassic volcanics was identified at 21 sites from three main localities along the southern coast of peru mean virtual geomagnetic pole 59 5 n 190 1 e k 111 95 3 0 this secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the peruvian coastal batholith and or to the first andean phase of compressional deformation in late cretaceous if we assume that the remagnetization is of late cretaceous age a regional counterclockwise rotation of about 20 is recorded by the forearc of southern peru from 16 s to 18 s primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations counterclockwise rotations of 50 to 80 are documented near the localities of chala eight sites ocona two sites and arequipa four sites preliminary results based on three sites of late permian to early triassic sediments from northern bolivia also show a large counterclockwise rotation the observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization perhaps this first episode of deformation partially structured the andean chain in the central andes and induced an incipient orocline during the cenozoic a differential shortening along the chain gave the present day shape of the bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20 as indicated by the remagnetization

crustal thickness variation in the andean foreland argentina from converted waves abstract local network three component digital data from the san juan area argentina provide the first seismological images of the deep crustal structure in the andean foreland above a horizontal segment of the subducted nazca plate we have identified s to p seismic phases converted on the moho by analysis of seismograms formed by taking the product of the radial and vertical components r z from intermediate depth earthquakes in the benioff zone directly beneath the network under the sierras pampeanas the moho is estimated to be at a depth of about 52 km beneath the eastern precordillera the moho deepens to 55 57 km and further west beneath the central precordillera to 60 km we therefore estimate a 5 to 10 westward dip to the moho under the andean foreland in both the sierras pampeanas and precordillera provinces the thickness of the ductile lower crust is comparable to that for a stable continental crust while the brittle upper crust implied from the spatial distribution of the seismicity shows an important thickness increase this rheological behavior is interpreted to be a consequence of the flattening of the dip of the subducted plate the east west deepening of the seismicity the moho and the depth to basement under the precordillera all suggest that the andean foreland lithosphere is underthrusting beneath the andes the associated shortening is estimated to be about 15 across the sierras pampeanas it is estimated to be at least 38 across the precordillera for the part of the crust beneath the decollement while it is up to 70 for the part above the decollement

transition from basement involved to thin skinned thrusting in the cordillera oriental of southern bolivia the eastern flank of the central andes in southern bolivia and northern argentina is underlain by a fold and thrust belt of neogene age it comprises an external thin skinned segment with basal detachment in silurian shales subandean ranges and an internal segment where units as deep as precambrian metasedimentary and igneous rocks are involved in thrusting eastern cordillera oriental between the cordillera oriental and the subandean ranges an intermediate segment interandean zone intervenes north of the argentinian bolivian border 22 s the structure of the interandean zone is characterized by folded and thrust faulted cambrian to triassic strata balanced cross sections across the western subandean ranges the interandean zone and the eastern cordillera oriental are based on previously available data and recent field work they suggest that the geological structure can be interpreted in terms of two major basement involved thrusts here referred to as interandean and subandean thrusts the more internal interandean thrust caused the development of broad basement anticlines in the eastern cordillera oriental however at shallower levels its displacement was accommodated by thin skinned folding and thrusting in the interandean zone in sequence activation of the subandean thrust uplifted the interandean zone on top of an imbricate basement sheet and caused the development of the thinskinned subandean ranges farther east published radiometric data and subsidence history of the subandean foreland basin suggest that the interandean thrust was active between about 10 ma and 5 ma the subandean thrust is still active southward displacement transfer from the subandean thrust onto the interandean thrust and a southward dipping lateral ramp of the interandean thrust result in the gradual transition from the interandean zone north of 22 s to the cordillera oriental of northern argentina a similar but larger scale transfer may be involved in the southern termination of the thin skinned subandean ranges of argentina at 23 s shortening in the western part of the thrust belt is of the order of 80 90 km total shortening in the eastern andean thrust belt from the undeformed foreland to the eastern margin of the cordillera oriental is 140 150 km

regional relative tectonic activity of structures in the pampean flat slab segment of argentina from 30 to 32 s abstract the pampean flat slab segment of the central andes in argentina is a broad active deformation zone subject to both plate boundary related and intraplate deformation to the west plate boundary related deformation is dominated by thin skinned fold and thrusts of the precordillera while to the east intraplate deformation involves thick skinned reverse faults of the sierras pampeanas gps data and absolute dating of displaced geomorphic features suggest that most of the active permanent deformation occurs between these two regions at the andean orogenic front and that there is a west to east trend of decreasing shortening rates however gps stations and sites with geomorphic slip rate measurements are currently too sparse and cover too short a time period to make reliable inferences about the longer term distribution and relative rates of deformation mountain range scale geomorphic indices on the other hand reflect tectonic activity since at least the pleistocene in this study we measured geomorphic indices hypsometric integral and curves basin elongation ratio basin volume to area ratio valley floor width to height ratio mountain front sinuosity and normalized steepness indices of rivers from 10 different n s striking range bounding faults that span both the precordillera and sierras pampeanas regions from west to east we use these data to assess the regional relative tectonic activity of major quaternary deformation features in the pampean segment of the central andes even when variations in climate and geology are considered mean values for each geomorphic index suggests uplift rates for the 10 mountain range bounding faults display local trends that are closely associated with the structural style and tectonic evolution of these structures although vertical uplift rates vary little horizontal shortening rates are higher for the precordillera fold and thrusts in the west than the thick skinned reverse faults of the sierras pampeanas in the east due to shallower fault dips for precordilleran structures the similarity with decadal slip rate gradients from gps studies could possibly indicate that the stress field has been constant since at least the pleistocene but further slip rate studies over broader time intervals in the late quaternary are necessary to confirm this variations observed in relative uplift rates along each fault point to sites that warrant further detailed study of slip rates and evaluation of associated seismic hazards

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

pure and simple shear plateau uplift altiplano puna argentina and bolivia abstract the concepts of pure and simple shear strain provide a meaningful way of discussing the alternative modes of lithospheric shortening which produced uplift of the central andean plateau since 10 ma discrete segments of the plateau exemplify pure or simple shear modes characteristics of the simple shear mode are a flanking thin skinned thrust belt and subdued relief and lack of significant late miocene to recent deformation within the plateau the pure shear mode is characterized by more rugged internal relief and active neotectonic deformation within the plateau and thick skinned foreland deformation involving seismic deformation of the crust to 30 km or more these modes of crustal thickening correlate with changes in lithosphere thickness modes of isostatic compensation broad wavelength topography and back arc magmatism the correlation of the boundary between pure and simple shear segments with major changes in pre existing anisotropies in the foreland of the orogen suggests that events which long pre date the andean orogeny indirectly controlled the along strike variation in the magnitude and style of shortening in the central andes and may have influenced the development of flat subduction

the nature of orogenic crust in the central andes 1 the central andes 16 22 s are part of an active continental margin mountain belt and the result of shortening of the weak western edge of south america between the strong lithospheres of the subducting nazca plate and the underthrusting brazilian shield we have combined receiver function and surface wave dispersion results from the banjo seda project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure the major results are as follows 1 the crust supporting the high elevations is thick and has a felsic to intermediate bulk composition 2 the relatively strong brazilian lithosphere is underthrusting as far west 65 5 w as the high elevations of the western part of the eastern cordillera ec but does not underthrust the entire altiplano 3 the subcrustal lithosphere is delaminating piecemeal under the altiplano ec boundary but is not completely removed beneath the central altiplano the altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation leading to lower crustal flow and flat topography in contrast in the high relief inland sloping regions of the ec and sub andean zone the upper crust is still strongly coupled across the basal thrust of the fold thrust belt to the underthrusting brazilian shield lithosphere subcrustal shortening between the altiplano and brazilian lithosphere appears to be accommodated by delamination near the altiplano ec boundary our study suggests that orogenic reworking may be an important part of the felsification of continental crust

moho geometry beneath the eastern margin of the andes northwest argentina and its implications to the effective elastic thickness of the andean foreland the seismically constrained shape of the moho beneath the eastern margin of the northwest argentine andes 24 s is interpreted in the context of regional isostasy to constrain the flexural rigidity of the lithosphere in this actively deforming orogen seismic travel time residuals from regional intermediate depth earthquakes are inverted for three dimensional variations in the moho depth beneath a portable seismic network spanning the eastern margin of the puna plateau and the santa barbara foreland system beneath the easternmost santa barbara ranges the moho is relatively flat and lies at 40 45 km depth farther west beneath the eastern margin of the puna the moho dips 10 15 to the west before flattening out at a depth of 60 65 km beneath the plateau the moho inversion model and coincident gravity data constrain the flexural rigidity of the foreland lithosphere to be between 1021 and 1022 m the corresponding effective elastic thickness of 6 12 km is a factor of 2 to 4 less than that estimated in previous published studies for the bolivian altiplano and subandean ranges farther to the north these results indicate that in contrast to the bolivian andes the foreland lithosphere in northwest argentina does not provide significant flexural support to the plateau margin and the eastern margin of the andes is nearly in a state of local isostatic compensation the north to south decrease in the effective elastic thickness of the andean lithosphere supports published seismic wave attenuation studies which indicate that south of 23 s the lithospheric thickness decreases beneath the plateau and foreland

deformation of the central andes 15 27 s derived from a flow model of subduction zones a simple viscous flow model of a subduction zone is used to calculate the deformation within continental lithosphere above a subducting slab our formulation accounts for two forces that dominate the deformation in the overriding lithosphere tectonic forces and buoyancy forces the tectonic forces arise from the subduction of oceanic lithosphere and produce minor extension in the lithosphere near the trench and compression farther inland buoyancy forces can induce horizontal pressure gradients due to variations in crustal thickness which cause lithospheric extension where the crust is thick numerical solutions obtained by using a finite element technique are compared with observations from the central andes 15 27 s the model predicts the observed deformation pattern of extension in the forearc compression in the western monocline corresponding to magmatic activity extension in the altiplano compression in the eastern monocline and subandes and no deformation in the brazilian shield by comparing the calculated solutions with the large scale tectonic observations we are able to evaluate the forces that govern the deformation in the central andes the approximately constant subduction velocity in the past 26 m y suggests that the rate of crustal shortening in the andes has decreased with time due to the thickening of the crust

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

neogene shortening contribution to crustal thickening in the back arc of the central andes to illustrate the neogene shortening distribution in the back arc units of the central andes and to estimate the contribution of the shortening to crustal thickening two balanced crustal cross sections have been constructed across the northern and southern branches of the bolivian orocline total neogene shortening which varies from 191 to 231 km is accommodated by a crustal duplex below the cordillera oriental but is insufficient to produce the 70 km of crustal thickness evidenced by geophysical data below the altiplano the best explanation for this anomalous thickening seems to be crustal underplating by material tectonically eroded from the continental margin this process probably caused the altiplano uplift the subduction of oceanic lithosphere coupled with this underplating and a brief episode of gravity spreading of the altiplano constituted the driving forces that produced neogene shortening and development of the central andes

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