unraveling 470 m y of shortening in the central andes and documentation of type 0 superposed folding in the famatina ranges central andes of western argentina 470 m y of protracted shortening is recorded through episodic coaxial coplanar fold amplification ordovician anticlines within the east vergent thrust belt were refolded and tightened during later paleozoic and cenozoic shortening folding episodes are recorded in four major unconformities that can be measured on both flanks of one of these map scale anticlines superposed folding type 0 interpreted from mapping and bedding plots shows similar axial orientation in older and younger rocks throughout the different stratigraphic layers of the shortening 30 is pre cenozoic and allows the recording of episodic deformation during continuous plate convergence at the gondwana south american margin

stable isotope composition of middle miocene carbonates of the frontal cordillera and sierras pampeanas did the paranaense seaway flood western and central argentina abstract the geographic extent interconnectedness and chronology of the miocene paranaense epeiric seaway have been the subject of considerable debate understanding the timing and location of this marine incursion is significant for documenting sea level changes paleoclimatic changes and surface uplift or subsidence stable isotope analyses of carbonate strata in the flat slab segment of the sierras pampeanas and high andean cordillera previously purported to be of marine origin provide evidence that the parana seaway did not inundate this portion of west and central argentina no unambiguously marine facies or isotopic signatures were recognized for five representative stratigraphic units the saguion formation ese of salinas grande in cordoba province anta formation in the quebrada de la yesera and lerma valley of salta province del buey and del abra formations within the famatina ranges la rioja province and chinches formation in the manantiales foreland basin in the frontal cordillera san juan province paleontologic and lithologic features in support of a marine origin are reconsidered herein instead a lacustrine origin is inferred for these formations their contemporaneity perhaps related to concurrent global climate conditions and broad tectonic setting until more substantive evidence of marine deposition is found we reject correlation of these units with the parana seaway preserved in the chaco pampean plain subsurface and discourage mapping of the seaway as extending into the central sierras pampeanas or andean foreland our findings suggest the miocene andean foreland was elevated above sea level although it remained as a largely flat lying area west of the pampas and chaco plains which were inundated by the parana seaway

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

regional deformation of the sierra de san luis argentina implications for the paleozoic development of western gondwana 1 metamorphic and ductile deformation fabrics within the sierra de san luis central argentina provide evidence for the early to middle paleozoic development of the paleo pacific margin of gondwana presumed vendian aged metasedimentary rocks within both the sierra de san luis and the sierras de cordoba preserve early pressure solution cleavage development cambrian peak metamorphism in the sierras de cordoba and ordovician peak metamorphism in the sierra de san luis indicate that juxtaposition of these two terranes did not occur prior to the late ordovician peak metamorphism and regional folding of the sierra de san luis metasedimentary rocks is equated with early stages of the development and shortening of the famatinian magmatic arc along the western margin of gondwana extensive nne trending zones of upper greenschist to amphibolite facies ductile deformation within the sierra de san luis and along the western margin of the sierra de comechingones record the dextral oblique juxtaposition of the sierra de san luis terrane against the southwestern margin of the sierras de cordoba coincident with or closely followed by the suture of the exotic precordillera terrane to the western margin of gondwana by the early devonian middle to late devonian restricted lower greenschist grade reactivation of ductile faults may record the accretion of the chilenia terrane the final stage of paleozoic convergent tectonism along this segment of western gondwana

slope apron deposition in an ordovician arc related setting the vuelta de las tolas member suri formation famatina basin northwest argentina abstract the ordovician suri formation is part of the infill of the famatina basin of northwest argentina which formed in an active setting along the western margin of early paleozoic gondwana the lower part of this formation the vuelta de las tolas member records sedimentation on a slope apron formed in an intra arc basin situated on a flooded continental arc platform the coincidence of a thick arenig llanvirn sedimentary succession and volcanic plutonic arc rocks suggests an extensional or transtensional arc setting and is consistent with evidence of an extensional regime within the volcanic are in the northern puna region the studied stratigraphic sections consist of volcanic rocks and six sedimentary facies the facies can be clustered into four facies associations association 1 composed of facies a laminated siltstones and mudstones and b massive mudstones and siltstones is interpreted to have accumulated from silty muddy high and low density turbidity currents and highly fluid silty debris flows with subsequent reworking by bottom currents and to a lesser extent hemipelagic suspension in an open slope setting facies association 2 is dominated by facies c current rippled siltstones strata these deposits are interpreted to record overbank sedimentation from fine grained turbidity currents facies e matrix supported volcanic breccias interbedded with andesitic lava units comprises facies association 3 deposition was contemporaneous with subaqueous volcanic activity and accumulated from cohesive debris flows in a coarse grained wedge at the base of slope facies association 4 is typified by facies d vitric fine grained sandstones and siltstones and f channelized and graded volcanic conglomerates and breccias deposits these strata commonly display thinning and fining upward trends indicating sedimentation from highly concentrated volcaniclastic turbidity currents in a channelized system the general characteristics of these deposits of fresh pyroclastic detritus suggest that their accumulation was contemporaneous with or post dated shallow water or subaereal explosive volcanism the vuelta de las tolas member tends to show an overall random facies patterns reflecting the strong influence of non cyclical episodic processes related to arc volcanism and slope sedimentation the scarcity of resident ichnofaunas and the presence of thick packages of uniform mudstones suggest deposition under oxygen depleted conditions in a topographically confined ponded sub basin interbasinal correlations favor comparison with middle arenig slope apron successions formed in the northern puna basin and suggest a southward prolongation of the arenig volcanic arc

deformation resulting from regional extension during pluton ascent and emplacement central sierra nevada california solid state foliation lineation small scale folds and domainal shear zones have developed in preexisting granitic and minor metasedimentary wallrock during a combined deformation involving a regional extensional strain and ascent and emplacement of the mt givens pluton mgp mylonite is common throughout much of the 1 2 km wide 10 km long shear zone with ultramylonite best developed near the contact with the mgp which itself lacks significant solid state deformation migmatization accompanies ultramylonite formation in the northern half of the zone but both these features are poorly developed or absent in its southern half where the shear zone is distributed over a wider area strain estimates across the shear zone using microgranitic enclaves as markers show a positive gradient and an increasing ratio of simple shear pure shear towards the mgp microprobe analyses on hornblende and plagioclase yield pressure and temperature estimates of 3 5 kb and 680 c respectively during shear zone formation at least at its late stages of development zircon pb u and 40 ar 39 ar ages constrain timing of the high temperature movement on the shear zone to 90 ma essentially the age of the mgp although movement immediately prior to that time appears likely we speculate that a regional extensional shear zone was developing prior to the emplacement of the mgp which as it ascended heated the wallrock facilitating both further strain in the zone as well as buoyant rise of the pluton along the zone the mgp was near its critical melt fraction during the last several kilometers of its ascent and could have possessed sufficient viscosity strength to impose a weak shear strain on the shear zone rocks although most of the foliation and extensional features in the zone are probably related to the regional strain field late stage folding of the foliation is attributed to shouldering aside of the wallrock by the mgp during the last increment of its ascent and final emplacement

proterozoic early paleozoic evolution in western south america a discussion in tectonophysics 354 121 137 2002

mafic enclaves in granitoid intrusions a catastrophic model of rheological behaviour the significance of mafic enclaves as indicators of magma mixing processes between juxtaposed felsic and mafic magmas is evaluated from a rheological point of view a qualitative model for explaining strain and morphological changes in the enclaves has been developed on the basis of the catastrophe theory mafic enclaves in intrusive granitoids commonly behave as physical systems that can be described using a cusp catastrophe model their behaviour is characterized by bimodality divergence and sudden changes which are properties typical of this model accordingly the presence of mafic enclaves showing variable strain and morphology within the same granitoid intrusion would be indicarive of mutual interaction and mingling between mafic and felsic magmas due to the characteristics of these processes it is not possible to establish unambiguous age relationships between the two magmatic components

late cretaceous paleocene synorogenic sedimentation and kinematic history of the sevier thrust belt northeast utah and southwest wyoming integration of cross cutting structural relationships overlapping sedimentary units new conglomerate provenance data and radiometric and palynological dates provides a basis for reinterpretation of the distribution and timing of late cretaceous through paleocene thrust faulting in the northeast utah southwest wyoming part of the sevier thrust belt these data indicate a general eastward progression of deformation that was punctuated by local out of sequence and hinterlandward verging events provenance data delimit a sequential restoration of a regional cross section the principal thrust systems in northeast utah and southwest wyoming are the willard ogden crawford absaroka and hogsback thrusts the willard is the westernmost structurally highest and oldest of the thrusts it carries a unique section of thick proterozoic sedimentary rocks as well as paleozoic rocks and was folded during displacement on younger thrust systems the next youngest thrust system is the ogden which comprises several basement rooted imbricate thrusts that together form a large antiformal stack with a structural culmination in the wasatch range growth of the wasatch culmination took place during coniacian through paleocene time contemporaneous with sequential displacement on the frontal crawford absaroka and hogsback thrusts total shortening in this part of the thrust belt was 100 km and structural relief of 25 km developed in the area of the wasatch culmination during coniacian through paleocene time thrusting followed an overall eastward progression that was interrupted by local out of sequence and hinterlandward verging events several episodes of synchronous displacement on two or more thrusts can be demonstrated shortening occurred in three main episodes the first episode 89 84 ma involved 33 km of shortening on the crawford thrust and its footwall imbricates approximately 19 km of structural relief on the basement cover contact developed in the area of the wasatch culmination the second episode 84 62 ma with a break between 75 69 ma involved 30 km of shortening mainly on the absaroka thrust and development of an additional 6 km of structural relief in the culmination the third episode of shortening 56 50 ma took place on the hogsback thrust involved 21 km of horizontal shortening and produced no significant increase in structural relief in the culmination long term rates of shortening ranged between 3 0 mm yr and 6 6 mm yr these three episodes of shortening produced three large accumulations of synorogenic conglomerate totaling 3 km in thickness the henefer formation and echo canyon and weber canyon conglomerates were deposited during crawford thrusting the evanston formation was deposited during and after absaroka thrusting and the lower conglomeratic part of the wasatch formation was deposited during and after hogsback thrusting most of the sediment in these synorogenic units however was derived from repeated uplift of the willard thrust sheet and from the eastern flank of the wasatch culmination in the rear part of the thrust belt only local minor accumulations were derived from the frontal ramp anticlines sediment accumulation and structural deformation were generally out of phase periods of regional shortening and uplift were marked by development of unconformities and sediment bypassing to distal parts of the foreland basin periods of structural inactivity were marked by accumulation of aerially widespread braided river conglomerate on top of the thrust belt one exception to this pattern is the henefer echo canyon weber canyon conglomerate deposit which contains evidence of progressive deformation in close proximity to the tip of the crawford thrust comparison of the sequential restoration with the late cretaceous subsidence history and isopach patterns in the distal foreland basin of western wyoming demonstrates that the principal controls on regional subsidence and sediment supply were the growth and erosion of the wasatch culmination growth of the duplex beneath the culmination may have been a means of maintaining critical taper in the thrust wedge

some aspects of handling velocity inversion and hidden layer problems in seismic refraction work there are two types of masked layers in seismic refraction work the velocity reversal low velocity layer and the hidden layer insufficient velocity contrast or layer thickness on the basis of an analytical formulation of the general case of a masked layer under an overburden of plane and parallel multiple refractors the two limiting cases are discussed the solution resulting from an uncritical interpretation of the measured time distance curve and the blind zone solution between these two limiting cases there is a variety of possible masked layer solutions these no blind zone solutions as well as the blind zone solution itself are formulated separately for the velocity inversion and the hidden layer case for the evaluation of some no blind zone solution a diagram is presented which can be used for any case of multiple refractors in the overburden of the masked layer however it is only for the three and the four layer case that a blind zone interpretation by use of diagrams is advisable such diagrams are presented together with the basic sets of formulae which contain as parameters only ratios of velocities and layer thicknesses as the velocity of the masked layer is usually unknown the diagrams are principally constructed to show the dependence on the masked layer velocity this is useful for estimation of the largest possible error

role of viscous plate coupling in the late tertiary andean tectonics 1 plate coupling between oceanic and continental plates in convergent margins of andean type is analyzed from the continuum mechanics approach we postulate a simple mechanism that accounts for the compressive regime in andean type environments in this mechanism deformation in the continental lithosphere is split into two distinctive domains the forearc domain and the arc foreland domain the forearc deformation is controlled by the balance between buoyancy forces associated with the trench and continental slope relief and the stress transferred from the convergence velocity through the age and velocity dependent slip zone the arc foreland deformation is controlled by the absolute plate velocity of the continental plate and the resistance at the slip zone strength of the coupling zone is determined by analyzing the dynamic trench topography along the active margin of south america between 0 and 55 s using this approach we found strength values in the range of 20 95 mpa in strong direct correlation with the age of the subducting plate the slip layer strength observation has been successfully tested against a thermal and strain rate dependent rheological model from this theoretical result we define an empirical relationship between strength of the slip zone and the age and convergence velocity applying this plate coupling model we reproduce shortening rates in the order of 1 10 km myr in agreement with those reported for the late tertiary evolution of the andes model results reproduce some first order features of the geological evolution of the margin such as the shape of the trench the overall andes relief the altiplano buildup and block rotation patterns in addition the model provides a mechanism to explain the evolution of the central depression the inversion of tertiary basins under slow convergence rates during the miocene and the segmentation of the margins tectonic erosion

oblique diapirism the criffel granodiorite granite zoned pluton southwest scotland abstract strain trajectories strain gradients and quartz c fabrics have been used to propose an emplacement history of the criffel zoned pluton strain has been estimated from the microdioritic xenoliths included in the outer granodiorite textural criteria have been used to complete the pattern of strain variations throughout the pluton strain gradients are found to be related to the granodiorite granite boundary zone with highest strain towards the northeast side of the pluton giving an asymmetric horseshoe pattern quartz c fabrics allow two strain histories to be distinguished within the pluton an early coaxial flattening in the granodiorite followed by a non coaxial deformation in the granite and the adjacent granodiorite these features are interpreted as the result of an oblique diapiric intrusion of the inner granite into the partially emplaced and crystallized granodiorite

lithospheric modeling of the ordovician foreland basin in the puna of northwestern argentina on the influence of arc loading on foreland basin formation in a qualitative sedimentological model of the ordovician basin in the puna of northwestern argentina the temporal and spatial distribution of predominantly marine volcanoclastic facies was interpreted to indicate a basin evolution from extensionally related subsidence to that driven by crustal down flexure sedimentological data were taken to reflect a middle ordovician eastward thrusting of an extinct early ordovician arc over part of its back arc basin in the course of the collision of the allochthonous arequipa massif terrane with the south american margin to test the original qualitative sedimentological interpretation of the puna basin an infinite beam elastic plate loading model was applied using the constraints of the field data contrary to the original interpretation results show that the puna basin can be modeled as resulting entirely from the effects of lithospheric loading caused by the progressive contruction of a static load i e a volcanic arc two loading events one in the early ordovician and one in the llanvirnian with tectonic load thicknesses of 6000 and 2000 m respectively best explain the observed basin geometry the onlap patterns on the distal margin on the peripheral bulge are reproduced well using a flexural rigidity of 1022 n m representing an equivalent elastic thickness of 14 km of the loaded plate higher values for the flexural rigidity are considered unlikely as they result in an enhanced peripheral bulge which uplifts marine near shore deposits on the distal margin to unrealistic subaereal positions the model results fit a foreland basin development where crustal thickening was caused by the construction of a static volcanic arc on a previously passive margin modeling the movement of thrust loads over parts of the basin according to traditional models of foreland basin formation in this case leads to model results which are inconsistent with field data even when moderate thrust rates are considered

multiple injections of magmas along a hercynian mid crustal shear zone sila massif calabria italy abstract syn tectonic late hercynian granitoids emplaced at mid crustal level through multiple dyke injections are well exposed in a 1 km long section in the southern sila massif calabria italy the fieldwork study permitted us to reconstruct the following order of emplacement i granodiorites with euhedral kfs megacrystals grd1 in the main text and heterogranular granodiorites with coarse rounded pl and kfs in a fine grained biotite rich matrix grd2 ii granodiorites with a nearly homogeneous grain size grd3 iii tonalite and iv granite both in the wall rock represented by migmatitic paragneisses and in the deformed granitoids the foliation attitude is substantially homogeneous striking about n140 with a dip of 50 60 to the ne the magmatic lineation is defined by the preferred alignment of euhedral feldspars parallel to the mineral lineation observed in the host rocks micro and meso structural analyses indicate that foliation evolved from melt present to solid state conditions feldspar tiling s c structures type structures c axis quartz orientations commonly suggest a top to the west sense of shear during magma injection and deformation under decreasing temperatures age of magma emplacement is constrained by u pb datings at 304 300 ma coeval with the regional metamorphic peak foliated granitoids and wall rock are intruded by poorly foliated hbl gabbro and finally by undeformed leucogranite pegmatite and felsic porphyritic dykes new u pb datings of the last intrusion event indicate an age of emplacement of 281 8 ma providing a minimum estimate for the end of the shear strain at mid crustal level

a solution to the blind zone problem by modeling of postcritical reflections an approach for solving the blind zone problem in seismic refraction prospecting is investigated hidden layer s if present in blind zones can be identified with the help of postcritical reflections these arrivals carry high energy and are easily identifiable on seismograms forward modeling techniques may be employed to determine the hidden layer parameters even in complex situations

ordovician back arc foreland and ocloyic thrust belt development on the western gondwana margin as a response to precordillera terrane accretion 1 the famatina belt southern central andes records a circa 470 m y shortening episode ocloyic orogeny affecting the peri gondwanan back arc basin in response to the accretion of the precordillera terrane collision created distinct features across and along the margin some of which persisted into the present andean structure from east to west the ocloyic deformation is recorded across the telescoped region of famatina represented by 1 an east vergent fold thrust belt evftb involving a major angular unconformity between early and middle ordovician rocks 2 a highly deformed volcano sedimentary belt and 3 an east vergent ductile shear zone affecting famatinian granites axis attitudes of ordovician refolded anticlines in the evftb show west east tectonic transport in agreement with kinematics of ocloyic high strain belts indicating shortening orthogonal to the present pacific margin eastward displacement of the ordovician volcanic arc at 27 s latitude together with a documented sinistral slip zone across the proto andean margin suggests tectonic indentation

structural aspects of suspect terranes and accretionary tectonics in western north america abstract the regional structural relationships between the cordilleran suspect terranes and cratonic north america and within the suspect terranes themselves are reviewed for eastern alaska western canada and united states and northern mexico a distinctive characteristic of the relationships between the suspect terranes and the north american craton is that although much modified by post accretionary strike slip faulting thrust faulting and extensional detachment faults the generally more distal and or oceanic terranes are frequently found as rootless nappes or thin thrust sheets sitting upon more inboard terranes or upon the craton itself the amount of tectonic transport implied is large strike slip faulting has been a very significant aspect of the structural evolution of the north american cordillera with total distributed displacements of over 2000 km documented on geologic relationships alone and much more implied by paleomagnetic evidence at least since later mesozoic time to the present movements were such that the terranes moved northward post accretionary intraplate telescoping along thrust faults transpressional strike slip faulting and delamination at mid to upper crustal levels were important processes in cordilleran tectonic evolution

early gondwanan connection for the argentine precordillera terrane abstract the precordillera of argentina is widely accepted as an exotic terrane of laurentian north american affinity newly acquired u pb ages on individual detrital zircons from lower cambrian and upper ordovician quartz sandstone beds in the argentine precordillera indicate a gondwanan provenance not associated with any known part of laurentia accordingly the precordillera terrane is likely underlain by basement rock of gondwanan affinity in addition detrital zircons from the upper ordovician sandstone bed provide no evidence for a mid ordovician position against the inboard famatina arc these results demand critical re evaluation of widely held assumptions regarding the paleogeography of the argentine precordillera

the oligocene xanthi pluton northern greece a granodiorite emplaced during regional extension the xanthi pluton is one of a series of oligocene subduction related granodiorites in northern greece its emplacement was controlled by major faults it is located on the ene trending kavala komotini fault zone which probably originated as a strike slip fault geophysical data show that the pluton is laccolith shaped extending many kilometres south of the fault andesitic dykes several million years older than the pluton indicate a ne sw directed extensional stress field mineral lineations plunging to the sw reflect continued extension during cooling of the pluton subsequent jointing and dykes of aplite and lamprophyre reflect continued extension although this could result from extensional pull apart at a bend during dextral strike slip motion on the kavala komotini fault there is no evidence for subsequent strike slip deformation furthermore two similar laccolithic plutons are unrelated to the kavala komotini fault and the regional extent of thick oligocene sediments suggests an extensional environment during the extension that created a ramp space along the listric fault bounding the xanthi basin into which the granite was intruded the kavala komotini fault acted as a transfer fault reactivation of early joints occurred during late oligocene early miocene compression faults within the pluton parallel to the kavala komotini fault accompanied extension during neogene basin formation emplacement of subduction related magma into ramp space developed along listric faults to produce laccolith like plutons may be a common feature of back arc extension

cenozoic provenance history of synorogenic conglomerates in western argentina famatina belt implications for central andean foreland development in famatina at the modern northern flat slab segment of the central andean belt synorogenicconglomeratefaciesprovideaunique opportunity to reconstruct paleogeography and tectonic setting stratigraphically constrained provenance analyses record a multi event history of the andean foreland and a complex pattern of exhumed uplands which in turn controlled the depocenter history western and eastern sources can be differentiated the western domain was composed of equigranular granitoids and lower and upper paleozoic volcano sedimentary and sedimentary units whereas the eastern domain was mainly formed by porphyrytic granitoids and minor slightly metamorphosed lower paleozoic clastic rocks the surrounding regions expose medium to high grade metamorphic rocks which are not recorded in any cenozoic conglomerate of famatina unroofing sequences can be deduced from the compositional trends conglomerate composition and paleocurrent data show input from intraba sinal sources and important participation of basement since the beginning of the neogene we infer an early miocene broken foreland during deposition of the del creston formation a significant source inversion with clasts supplied from the eastern basement domain is recorded 2000 m from the base within the middle miocene del abra formation reactivation of western granitic thrust sheets occurred by the late middle miocene and late miocene as indicated by provenance studies of the santo domingo and el durazno formations in contrast to simple asymmetric foreland models provenance analysis documents early participation of crystalline rocks suggesting early development of intrafore land basement highs within the southern central andes at a regional scale basement thrusting and associated intermontane basin development were diachronous this interpretation is supported by broad propagation of basement thrusting from the northern sierras pampeanas and famatina toward the central and eastern sierras pampeanas between early middle miocene and late miocene pliocene time early miocene reconstructions of the juan fernandez ridge position on the central andean margin suggest that during this interval shallow subduction was likely unrelated to oceanic plateau subduction under the sierras pampeanas

controls of surface erosion on the evolution of the alps constraints from the stratigraphies of the adjacent foreland basins the combined information about the stratigraphies from the foreland basins surrounding the swiss alps exhumation mechanisms and the structural evolution of the alpine orogenic wedge allow an evaluation of the controls of erosion rates on large scale alpine tectonic evolution volumetric data from the molasse basin and fining upward trends in the gonfolite lombarda indicate that at 20 ma average erosion rates in the alps decreased by 50 it appears that at that time erosion rates decreased more rapidly than crustal uplift rates as a result surface uplift occurred because of surface uplift the drainage pattern of the alpine hinterland evolved from an across strike to the present day along strike orientation furthermore the decrease of average erosion rates at 20 ma coincides with initiation of a phase of thrusting in the jura mountains and the southern alpine nappes at 50 km distance from the pre 20 ma thrust front coupled erosion mechanical models of orogens suggest that although rates of crustal convergence decreased between the oligocene and the present the reduction of average erosion rates at 20 ma was high enough to have significantly influenced initiation of the state of growth of the swiss alps at that time

normal and reverse unroofing sequences in syntectonic conglomerates as evidence of progressive basinward deformation when a new mountain belt is generated by tectonics the denudation of the mountain front may cause the development of alluvial fans at its base the retreat of the mountain front by denudation processes and a widening of the drainage network in the source area create adjacent alluvial fan conglomerates with an inverted vertical clast distribution this vertical clast distribution represents a normal unroofing sequence renewed uplift of a previous inverted vertical clast distribution can produce a reverse unroofing sequence if continued shortening occurs in the same area a rejuvenation of the source area and basin margin may produce a new generation of clasts from the source area and a partial or total reworking of previously deposited conglomerates to supply second generation clasts if this type of reworking process is repeated as fault uplift steps basinward several new inverted and stratigraphic vertical clast distributions may arise

magnetostratigraphy and paleomagnetism of early and middle miocene synorogenic strata basement partitioning and minor block rotation in argentine broken foreland magnetostratigraphic and paleomagnetic studies on early andean synorogenic strata del creston fm in the famatina belt 28 7 s 67 5 w clarify details of chronology that permit calculation of sedimentation rates within the broken foreland of west argentina the del creston fm represents the first record of broken foreland sedimentation within the southern central andean belt and the earliest retroarc volcanic rocks exposed several hundred kilometers from the trench twenty five out of 49 sites collected along the succession presented a primary remanence as determined through positive fold and reversal tests correlation of the local magnetic polarity section with the global polarity time scale indicates that the sedimentation of del creston fm started at 16 7 ma and continued until 14 5 ma the youngest strata are represented by conglomerates bearing abundant lower paleozoic granite boulders indicating unroofing of the crystalline basement within the nw sierras pampeanas this result supports the hypothesis of an early broken foreland stage at these latitudes of the andes with involvement of the basement in deformation and coeval retroarc volcanism common attributes of flat subduction regimes a mean site paleomagnetic direction of dec 6 3 inc 43 6 95 8 0 n 24 confirm our earlier intrepretation that the central part of the famatina belt within the sierras pampeanas did not undergo large vertical axes rotations since the middle miocene

dynamics of deformation and sedimentation in the northern sierras pampeanas an integrated study of the neogene fiambala basin nw argentina reply the thick skinned sierras pampeanas morphotectonic domain of western and northwestern argentina 27 s 33 s is characterized by reverse fault bounded basement blocks that delimit internally deformed neogene sedimentary basins foreland basin evolution in this part of the andes is still not very well understood for example challenging questions exist as to how thick skinned deformation develops if there are distinct spatiotemporal trends in deformation and exhumation how such deformation styles influence sedimentation patterns and whether or not broken foreland basins are related to regional plate tectonic processes such as flat slab subduction the fiambala basin of the northwestern sierras pampeanas is the largest of several intermontane basins in the transition to the southern margin of the puna plateau this basin preserves a thick continental neogene sequence that provides information on the dynamics of thick skinned deformation and resulting sedimentation the fiambala basin contains 4 km of fluvial alluvial sedimentary rocks that comprise the tamberia guanchin and punaschotter formations u pb geochronology of ashes intercalated within the fiambala stratigraphic sequence demonstrates that these sedimentary rocks are late miocene to pliocene 8 2 0 3 ma to 3 05 0 4 ma in age sedimentology and provenance data indicate that the source of the tamberia formation was located to the west of the modern western basin bounding range the guanchin and punaschotter formations record input from local sources including the modern basin bounding range to the west and the southern puna plateau to the north suggesting reorganization of the catchment area at ca 5 5 ma the coarsening upward trends recorded by the fluvial tamberia and guanchin formations indicate enhanced tectonics and relief during sedimentation the punaschot ter conglomerates record alluvial fan sedimentation and local sources fault kinematic data document a contractional regime characterized by e w and ne sw shortening active throughout the middle late miocene and pliocene furthermore a comparison between the fiambala basin and similar sedimentary basins in the sierras pampeanas e g bermejo foreland basin and the eastern cordillera leads us to propose that the study area originally constituted an integral part of a continuous and more extensive foreland basin system thin skinned for much of its early history our data suggest coeval intrabasin deformation along strike from the bermejo region northward to the eastern cordillera the coeval change at ca 6 ma from a regional to more compartmentalized thick skinned tectono sedimentary environment in the regions adjacent to the eastern cordillera the southern puna margin and other sectors within the sierras pampeanas domain may thus reflect a regional tectonic process related to flat subduction our data combined with existing sedimentological and petrological evidence imply that the passage from steep to flat subduction occurred synchronously from 30 s to 26 s

a numerical assessment of slab strength during high and low angle subduction and implications for laramide orogenesis a finite element computer program for one dimensional heat flow was modified to apply to two dimensional subduction where conductive heat transport parallel to the upper and lower plates was neglected one dimensional finite element columns were cut displaced and reassembled to model heat advection calculated temperature depth dependent pressure stress levels associated with dislocation creep in olivine at constant strain rate and a 1 gpa deviatoric stress maximum were used to calculate the form of the brittle plastic failure envelope for profiles across subducted slabs the failure envelopes were then used to calculate maximum sustainable compression parallel to the slab model results suggest the following 1 in high angle subduction without frictional heating and horizontal heat flow temperatures along the interplate contact zone remain below about 120 c which is far below temperatures necessary for olivine plasticity without substantial frictional heating or lateral heat flow from the adjacent magmatic arc the subducted slab will gain rather than lose strength before its upper surface contacts asthenosphere in addition viscous coupling due to olivine plasticity and associated ablation of the upper plate will be unimportant slab strength reduction due to heating occurs rapidly after the top of the slab comes into contact with asthenosphere 2 in low angle subduction the top of the slab remains in contact with overlying lithosphere and does not contact asthenosphere slab strength reduction due to conductive heating from above occurs slowly because the upper plate is cooled by the lower plate and is the source of progressively less heat temperatures along the interplate contact zone may be sufficient for olivine plasticity at large distances from the trench and viscous coupling between plates and associated ablation of the upper plate may be important in this tectonic setting 3 low angle subduction for distances of over 1500 km during the laramide orogeny in southwestern north america appears viable at 50 ma slab strength beneath the black hills of south dakota is predicted to have been comparable to or greater than strength at the trench

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

dynamics of deformation and sedimentation in the northern sierras pampeanas an integrated study of the neogene fiambala basin nw argentina comment and discussion in a recent contribution to the geological society of america bulletin carrapa et al 2008 1 provided an interesting geochronological and thermochronological database for the fiambala basin northern end of the sierras pampeanas which together with stratigraphic sedimentological and

proterozoic early paleozoic evolution in western south america a discussion abstract the hypothesis of exotic terranes in peru bolivia argentina and chile generated discussions on the mode of transfer and extent of accretional events that may have occurred in the southern andes during the late proterozoic early paleozoic initially a tectogenesis based on autochthonous mobile fold belts was discussed following ideas emphasised the fragmentation of the supercontinent rodinia laurentia moving along the west gondwana border and colliding with the gondwana western margin the most important effect of this laurentia gondwana relationship was attributed to the argentine precordillera or cuyania terrane splitting off from laurentia and docking to gondwana in the early paleozoic in this study the most cited arguments for this laurentia precordillera relationship are discussed emphasising paleontological considerations it is shown that these arguments do not exclude a close original vicinity of the precordillera terrane to gondwana the precordillera terrane is suggested to be part of a hypothetical platform which developed between south america africa and antarctica safran platform and which was displaced to its actual position by transcurrent faults the collisional events in the sierras pampeanas ensued from strike slip movements and were responsible for the s and i type transpressional magmatism along the pampean and famatinian terranes the final result of this continent parallel movement of terrane slices is similar to that of a terrane split off from laurentia but the first named way of formation easier explains the general continuity of plate convergence at the western border of gondwana than the laurentia precordillera connection does

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

mid crustal shear zone evolution in a syn tectonic late hercynian granitoid sila massif calabria southern italy by means of petrogrological meso and microstructural analyses the fabric of a syn tectonic late hercynian k feldspar megacryst bearing granodiorite is described in this paper the granodiorite was emplaced at 293 ma within migmatitic paragneisses which had reached the regional peak metamorphic conditions at 304 300 ma the granodiorite and the migmatitic paragneisses are both affected by the same ductile shear zone in the core of the shear zone mylonites show a clear grain size reduction and microstructures related to deformation at high to medium temperature conditions migmatitic paragneisses foliated granodiorites and mylonites mostly show concordant lineation and foliation orientations in addition the preferred orientation of euhedral feldspars in granodiorites indicates that the fabric anisotropy started to develop in the magmatic state these features strongly suggest that shear deformation was active during crystallisation of granitoids and continued under subsolidus conditions in wall rocks and mylonites kinematic indicators such as and type porphyroclasts s c fabrics shear bands and quartz c axis orientations suggest a top to the w sense of shear this is similar to the magma flow direction indicated by the tiling of euhedral feldspar megacrysts in granodiorites shear deformation developed preferentially by partitioning of strain in the granodioritic crystal mush geobarometry indicates that deformation took place at middle crustal levels p 400 500 mpa whole rock white mica rb sr geochronological analysis of an undeformed pegmatite crosscutting the mylonitic foliation provided an age of 265 ma timing of deformation is therefore bracketed between 293 ma and 265 ma

provenance analysis and tectonic setting of the ordovician clastic deposits in the southern puna basin nw argentina provenance studies on early to middle ordovician clastic formations of the southern puna basin in north western argentina indicate that the sedimentary detritus is generally composed of reworked crustal material tremadocquartzrich turbidites tolar chico formation mean composition q t 89 f7 l4 are followed by volcaniclastic rocks and greywackes tolillar formation mean q t 33 f42 l25 these are in turn overlain by volcaniclastic deposits mean qt24 f30 l46 of the diablo formation late arenig early llanvirn that are intercalated by lava flows all units were deformed in the ocloyic orogeny during the middle and late ordovician sandstones of the tolar chico formation are characterized by th sc ratios 1 la sc ratios 10 whereas associated fine grained wackes show slightly lower values for both ratios lree light rare earth elements enrichment of the arenites is 50 chondrite eu eu values are between 0 72 and 0 92 and flat hree heavy rare earth elements patterns indicate a derivation from mostly felsic rocks of typical upper crustal composition the e n d t s e d values scatter around 11 to 9 the calculated nd t d m residence ages vary between 1 8 and 2 0 ga indicating contribution by a palaeoproterozoic crustal component the th sc and la sc ratios of the tolillar formation are lower than those of the tolar chico formation normalized ree rare earth elements patterns display a similar shape to paas post archaean average australian shale but with higher abundances of hrees eu eu values range between 0 44 and 1 17 where the higher values reflect the abundance of plagioclase and feldspar bearing volcanic lithoclasts average e n d t s e d values are less negative at 5 1 and nd t d m are lower at 1 6 ga this is consistent with characteristics of regional rocks of upper continental crust composition which most probably represent the sources of the studied detritus the rocks of the diablo formation have the lowest th sc and la sc ratios lower lree abundances than the average continental crust and are slightly enriched in hrees eu eu values are between 0 63 and 1 17 the nd isotopes e n d t s e d 3 to 1 t d m 1 2 ga indicate that one source component was less fractionated than both the underlying early ordovician and the overlying middle ordovician units synsedimentary vulcanites in the diablo formation show the same isotopic composition our data indicate that the sedimentary detritus is generally composed of reworked crustal material but that the diablo formation appears to contain 80 of a less fractionated component derived from a contemporaneous continental volcanic arc there are no data indicating an exotic detrital source or the accretion of an exotic block at this part of the gondwana margin during the ordovician

reconstructing the thermal and exhumation history of the sierras pampeanas through low temperature thermochronology a case study from the sierra de velasco

bedrock incision rock uplift and threshold hillslopes in the northwestern himalayas the topography of tectonically active mountain ranges reflects a poorly understood competition between bedrock uplift and erosion dating of abandoned river cut surfaces in the northwestern himalayas reveals that the indus river incises through the bedrock at extremely high rates 2 12 mm yr 1 in the surrounding mountains the average angles of hillslopes are steep and essentially independent of erosion rate suggesting control by a common threshold process in this rapidly deforming region an equilibrium is maintained between bedrock uplift and river incision with landsliding allowing hillslopes to adjust efficiently to rapid river down cutting

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

use of traveltime skips in refraction analysis to delineate velocity inversion1 first arrival refraction data does not normally provide any indication of the velocity inversion problem however under certain favorable circumstances when the low velocity layer lvl is considerably thicker than the overlying higher velocity layer hvl the velocity inversion can be seen in the form of a traveltime skip model studies show that in such cases the length of the hvl traveltime branch can be used to determine the thickness of the hvl and the magnitude of the traveltime skip in order to determine the thickness of the lvl this is also applicable in the case of field data

first late ordovician paleomagnetic pole for the cuyania precordillera terrane of western argentina a microcontinent or a laurentian plateau abstract time and tectonic processes involved in docking of the argentine precordillera cuyania terrane against sw gondwana has been a matter of much debate a paleomagnetic study on the early caradoc pavon formation exposed in the san rafael block province of mendoza argentina is presented after detailed thermal and alternating field demagnetizations two geologically significant magnetic components were defined a widespread post tectonic component a is present in most sites of the pavon formation with dual polarities and is coincident with the characteristic remanence isolated from a permo triassic rhyolitic dome intruding the sediments its pole position 83 7 s 271 0 e dp 6 8 dm 9 0 n 11 sites falls on the late permian early triassic south american reference poles suggesting that this component was acquired during the choiyoi magmatic phase a second component b also shows dual polarities and a positive fold test suggesting a primary origin unblocking temperatures and rock magnetic experiments indicate that b is carried either by hematite or magnetite at different sites anisotropy of magnetic susceptibility results suggest a depositional fabric and no remanence distortion due to deformation or compaction a paleomagnetic pole computed from this remanence pv falls on 3 6 n 346 4 e dp 2 9 dm 4 6 n 22 samples it indicates a paleolatitude around 26 s for deposition of pavon sediments and constrains the paleogeographic evolution of cuyania during the ordovician which was still at subtropical latitudes by the early caradoc pv is consistent with the laurentian late ordovician reference pole if cuyania remains attached to se laurentia for the early caradoc while it shows a significant cw rotation with no paleolatitude anomaly respect to the gondwana reference pole when kept in its present position in sw south america these comparisons are interpreted in three possible alternatives for the paleogeographic and tectonic setting of cuyania in the late ordovician

the effect of inherited paleotopography on exhumation of the central andes of nw argentina differential exhumation in the puna plateau and eastern cordillera of nw argentina is controlled by inherited paleostructures and resulting paleotopography related to the cretaceous salta rift paleomargins the ceno zoic deformation front related to the development of the andean retro arc orogenic system is generally associated with 4 km of exhumation which is recorded by cenozoic apatite fi ssion track aft and u th sm he ages he ages in the eastern cordillera of nw argentina new aft ages from the top of the nevado de cachi document oligocene ca 28 ma cooling which combined with existing data indicates exhumation of this range between ca 28 ma and ca 14 ma however some of the highest ranges in the eastern cordillera preserve cretaceous ages indicative of limited cenozoic exhumation samples collected from an similar to 3 km elevation transect along the northern part of the sierra de quilmes paleorift fl ank laguna brava show aft ages between ca 80 and ca 50 ma and he ages between ca 45 and ca 10 ma another set of samples from an similar to 1 km elevation transect farther to the southwest la quebrada shows cretaceous aft ages between ca 116 ma and ca 76 ma and mainly cretaceous he ages in agreement with aft data analysis of existing aft and he ages from the area once occupied by the salta rift reveals a pattern characterized by cretaceous ages along paleorift highs and cenozoic ages within paleorift hanging wall basins and later foreland basin depocenters this pattern is interrupted by the sierras pampeanas at similar to 28 degrees s which record mid cenozoic ages our data are consistent with a complex inherited pattern of pre andean paleostructures likely associated with paleotopography which was beveled by the cenozoic regional foreland basin and reactivated during the late neogene ca 10 ma strongly controlling the magnitude of cenozoic uplift and exhumation and thus cooling age distribution this combined with variable lithologic erodibility resulted in an irregular distribution of themochronological ages

provenance of the liuqu conglomerate in southern tibet a paleogene erosional record of the himalayan tibetan orogen abstract the liuqu conglomerate exposed along the yarlung zangbo suture zone in southern tibet has been interpreted as a paleogene molasse deposited during india asia collision or the product of collision between india and an intra oceanic arc detailed petrographic studies and analyses of u pb and hf isotopes of detrital zircon indicate multiple sources for the conglomerate including the sedimentary succession within the xigaze forearc basin to the north of the suture the yarlung zangbo ophiolite within the suture itself and the langjiexue group and tethyan himalayan sedimentary rocks to the south of the suture the occurrence of clasts of both indian and asian origin suggests that the conglomerate was deposited after india asia collision the present data suggest that the onset of himalayan tibetan orogenic erosion occurred as early as the middle eocene when the yarlung zangbo suture zone and xigaze forearc succession were uplifted and became sources for the liuqu conglomerate thus the results are inconsistent with the proposal that the provenance and age of the liuqu conglomerate support the hypothesis that india asia collision did not occur until the oligocene it is further suggested that eocene neotethyan slab breakoff which may result in accelerated exhumation and uplift of the orogen provided a deep level dynamic control on the deposition of the liuqu conglomerate

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

the seismic refraction method a viable tool for mapping shallow targets into the 1990 s geometrical considerations show that first arrivals can be recorded from below hidden layers a certain minimum amount of data must be collected in order to resolve lateral versus vertical subsurface changes and thereby to determine the interpretation method field procedures therefore are independent of the interpretation method the optimum xy parameter in the generalized reciprocal method grm of processing refraction seismic data is significant as a quality control factor in refraction data interpretation by comparison of the optimum xy value that is recovered through velocity analysis and time depth function analysis with a value for optimum xy that is calculated from the migrated depth section the hidden layer condition can be recognized in addition to identifying the hidden layer condition on the basis of first arrivals alone the grm allows the hidden layers to be accommodated and depth precisions of less than 5 are possible

structural evolution of the argentine precordillera the rio san juan section abstract the argentine precordillera ap represents a high level fold and thrust belt which was formed during the andean tertiary crustal shortening in a w e transect studied mostly e directed imbrications are combined with folding and involve a pile of cambrian to tertiary sediments the thrust belt is detached above a main decollement within the ordovician to lower devonian strata to the east it is bounded by a back thrust zone directed westwards restored sections allow an estimate of crustal shortening and also provide insight into the structural development during pre tertiary times carbonate platform sequences of cambrian to ordovician age grade into a volcani clastic succession with transitional melanges representing deposits along a western continental slope in the western and easternmost parts of the ap cambrian to silurian deposits are affected by a first folding which to the west is combined with a slight metamorphic overprint during carboniferous to permian times intensified mobility along the eastern rim and in the western parts of the ap leads to crustal extension which partly was triggered by the formation of synsedimentary normal faults in the central ap no angular unconformities at the base of carboniferous permian deposits were found there was no field evidence found to postulate a terrane boundary within the ap area and an onset of thrust tectonics during cretaceous to early tertiary times

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

corrigendum to early gondwanan connection for the argentine precordillera terrane earth planet sci lett 205 2003 349 359

lithologic provenance modeling applied to the late cretaceous synorogenic echo canyon conglomerate utah a case of multiple source areas lithologic provenance modeling is a deductive technique for identifying matches between actual compositions of conglomerates and modeled conglomerate compositions from hypothetical source sections the method is applied to the echo canyon conglomerate in northeastern utah a coniacian santonian synorogenic conglomerate generally assumed to have been derived from the crawford thrust sheet the echo canyon is problematic because it is composed of abundant quartzite clasts most of which could have been derived from any of several source formations ranging in age from proterozoic through jurassic lithologic provenance modeling paleocurrent data and regional structural relations indicate that the echo canyon was derived from three separate stratigraphic sections exposed on the inactive willard thrust sheet and the eastern flank of the wasatch basement culmination

chemical and isotopic evidence for major mass transfer between mafic enclaves and felsic magma abstract in both the strontian and criffell zoned granitoid plutons certain chemical elements notably k rb ca sr and pb display a correlation between their concentration in microdiorite enclaves and their concentration in the immediately enclosing host granitic rock this appears to represent the effects of post incorporation mixing and exchange between the enclave and its host magma enclave initial 87 sr 86 sr ratios are indistinguishable from those of the host while enclave initial 143 nd 144 nd ratios are systematically more radiogenic establishing that these enclaves are not melt segregations nor restite the isotopic data are in agreement with the chemical data in indicating that sr is almost entirely derived from the host despite clear evidence of major mass transfer of alkali metals alkali earths and transition metals a comparison of the concentrations and ratios of hfse and ree and nd isotopic ratios with those of a number of potential starting protoliths allows the enclave precursors to be positively discriminated in this regard enclave hfse and ree compositions may allow for models of granitoid production by the injection of hot mantle derived magmas into the crust to be tested

laramide thrust generated alluvial fan sedimentation sphinx conglomerate southwestern montana the uppermost cretaceous lower tertiary sphinx conglomerate crops out over an area of approximately 20 km2 8 mi2 in the madison range of southwestern montana the sphinx consists of more than 1 000 m 3 300 ft of synorogenic boulder and cobble conglomerate derived from a late cretaceous laramide uplift that was located in the area presently occupied by the madison river valley palynological and radiometric age data indicate that the sphinx was deposited 75 58 ma and that thrusting and folding of the deposit had largely ceased by 56 ma compositions of sphinx clasts and paleocurrent data indicate that the sphinx was produced by uplift and unroofing of mesozoic and paleozoic rocks located on two thrust sheets to the west and southwest the lower s hinx was deposited on the distal portions of an eastward prograding alluvial fan system clast assemblages and lithofacies indicate that deposition of the middle sphinx was controlled by a combination of progradation in response to ongoing thrusting and an influx of resistant clasts derived from middle paleozoic carbonates in the source area deposition of the upper sphinx was probably controlled by source lithology as the influx of very coarse resistant clasts from middle and lower paleozoic carbonates overwhelmed the fan system s ability to organize its load of sediment by normal fan processes a preliminary facies model for thrust generated alluvial fan deposits predicts intraformational deformation cannibalization of proximal synorogenic fan facies and abrupt compositional breaks in response to episodes of thrusting

magnetic anomaly interpretation across the southern central andes 32 34 s the role of the juan fernandez ridge in the late tertiary evolution of the margin marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of chile between 32 34 s three magnetic domains have been identified oceanic continental margin and subaerial the oceanic domain has seafloor spreading anomalies 16 37 ma to 18 39 5 ma disturbed by anomalies of the juan fernandez hot spot chain in the continental margin the most prominent fabric are e w anomalies in the upper slope corresponding to onshore e w anomalies of large intrusive bodies onshore a n s lineament of short wavelength anomalies defines the roots of a cretaceous volcanic arc a resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to 50 km landward of the trench absolute cenozoic plate motion for nazca and south american plates and dating of the juan fernandez chain provide a kinematic model of ridge continent collision the reconstruction indicates rapid southward migration of the collision point along 1400 km of the margin from 20 to 11 ma 20 cm yr 1 from 11 ma to present the collison point has migrated at a slower rate along 375 km of the margin 3 5 cm yr 1 the predicted location of the subducted portion of the juan fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship in the last 10 myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates these observations indicate that subduction of the juan fernandez chain plays a major role in arc forearc tectonics

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

kinematic history of the meade thrust based on provenance of the bechler conglomerate at red mountain idaho sevier thrust belt with up to 45 km of slip the meade thrust fault in southeastern idaho is one of the major thrusts in the sevier thrust belt yet its age of displacement has remained enigmatic because of the lack of a derivative synorogenic deposit we propose that the synorogenic conglomerate produced by initial meade thrusting crops out on red mountain in southeastern idaho 1 5 km east of the present trace of the meade thrust this conglomerate historically has been considered part of the ephraim formation of the lower cretaceous gannett group but we suggest that it is a conglomeratic facies of the bechler formation localized to the area of red mountain regional stratigraphic considerations and paleontological dates from underlying and overlying strata indicate that the bechler conglomerate facies bcf is aptian in age the bcf is 850 m thick and consists of pebble to boulder conglomerate sandstone and mudrock deposited by fluvial and mass flow processes on medial to distal parts of an alluvial fan paleocurrent data indicate an eastward fan shaped dispersal pattern the bcf contains clasts of micritic limestone chert pebble conglomerate and cherty litharenite that were derived from the ephraim formation in addition the conglomerate contains abundant clasts of ordovician carboniferous and lower mesozoic rocks that crop out on both the meade thrust sheet and the paris thrust sheet 25 30 km to the west the base of the bcf is marked by a local 27 angular unconformity on top of the ephraim formation the bcf contains intraformational progressively rotated angular unconformities internal growth folds and minor dip discontinuities that were produced by simultaneous folding and sediment accumulation on the proximal footwall of the meade thrust provenance modeling indicates that the bcf was derived from mesozoic strata that were exposed along the frontal part of the meade thrust sheet and from paleozoic strata that were coevally exposed in the hindward located paris thrust sheet topography on the paris sheet was rejuvenated as it was carried passively over a major ramp in the meade thrust provenance and structural data are combined to produce an incremental bulk rock retrodeformation of initial meade displacement clasts of jurassic twin creek limestone with pressure solution cleavage in the lower part of the bcf indicate that an episode of layer parallel shortening occurred in the meade hanging wall prior to meade displacement this may have been related to emplacement of the paris thrust sheet the bcf recorded 6 km of initial meade related shortening by thrust slip and fault propagation folding during aptian time illite crystallinity systematic fracture sets in the ephraim formation and bcf finite strain in meade footwall and hanging wall rocks and thermal models by previous workers indicate that the meade thrust sheet ultimately overrode the bcf and other footwall rocks probably during albian cenomanian time the bcf and the meade thrust were folded during slip on thrust faults related to the northern crawford thrust during coniacian time a revised interpretation of gannett group provenance throughout the thrust belt suggests that the ephraim formation was derived from both the paris thrust sheet and an older hindward located thrust sheet the bechler was derived from the meade and paris sheets

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

deformation of mylonites in palm canyon california based on xenolith geometry abstract an analysis of xenoliths in granodiorite and mylonites of the peninsular ranges batholith in southern california documents large deformation with von mises equivalent strains ranging from 1 to 4 from the geometry of xenoliths in mylonites it is concluded that much of the deformation occurred by flattening if deformation had been homogeneous the 800 m thick mylonite sequence would have extended originally over more than 10 km contrary to xenoliths associated with ballooning diapirs these in santa rosa mylonites show much larger strains and are elongated in the lineation direction it is proposed that the overall deformation pattern is consistent with a top to the west shear displacement but most of the strain in the santa rosa mylonites was produced in flattening during emplacement of the partially solidified batholith the large deformation may have produced folds that have coalesced with the foliation to accommodate the strain

temporal constraints on the tectonic evolution of sierra de famatina northwestern argentina using the fission track method to date tuffs interbedded in synorogenic clastic sedimentary strata fission track dates from seven volcaniclastic units allow time constraints to be placed on synorogenic clastic strata exposed along the flanks of sierra de famatina northwest argentina four formations are exposed along the western margin of sierra de famatina the el abra conglomerate is composed of clasts eroded from the basement exposed just to the west of cadena de paiman the el buey formation contains sediments probably derived from the precordillera fold and thrust belt to the west fission track dates give a maximum age of 6 ma for the deposition of both of these formations the el durazno formation deposited between 6 and 4 ma contains proximal volcaniclastic and other clastic rocks derived from the core of sierra de famatina the santa florentina conglomerate was derived from the rising sierra de famatina and deposited in the past 4 m y sierra de famatina probably become a positive topographic feature approximately 6 ma prior to that time this region was receiving fine grained sediments fro

re examining pluton emplacement processes abstract previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement in fact the only means of making space during emplacement of mantle derived magmas in the crust are 1 lowering the moho or 2 outward displacement of the earth s surface other pluton emplacement mechanisms are material transfer processes mtps that do not increase the volume of the crust one frequently cited mtp for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall rocks around the plutons the expected structures and total strains near such plutons are dependent on the three dimensional pluton shape the width of the deforming aureole whether the pluton is a piercing or non piercing diapir and in the latter case the number of body radii the pluton travels we have re examined a few such plutons and calculated the amount of material transfer caused by ductile flow in no case is more than 40 and in most cases only 15 35 of the required material transfer accommodated by wall rock flow the contact aureoles around these plutons are too narrow and or strains too low indicating that the plutons are predominantly discordant bodies with narrow 0 1 0 4 pluton radii concordant aureoles a comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains we suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models are too simplistic instead we argue that the ascent and emplacement of magmas require multiple near field and far field mtps these mtps will show gradients with depth distance from the magma and with time rates of near field mtps must be rapid whereas far field rates are probably nearer to long term average rates of orogenic processes

right lateral shear and rotation as the explanation for strike slip faulting in eastern tibet the convergence of two continents can be accommodated by crustal thickening and by lateral transport of crust out of the path of the converging continents1 3 the principal features suggesting that lateral transport is important are major strike slip faults striking roughly orthogonally to the orientation of convergence between the continents displacement on such faults is imagined to allow lateral transport of material with respect to the converging continents possibly accounting for a large fraction of the convergence between the two continents the most spectacular example of such strike slip faulting is in the eastern part of the tibetan plateau where three major left lateral faults with slip rates of 10 mm yr 1 and several smaller roughly parallel faults dominate the regional strain field2 3 cobbold and davy4 suggested that these faults may be rotating in a clockwise fashion here we put bounds on that rate of rotation and conclude that the image of lateral transport on such faults2 known also as continental escape extrusion or expulsion is an illusion and that instead the left lateral slip on east striking planes in eastern tibet is a manifestation of north striking right lateral simple shear if this conclusion is correct the east striking left lateral faults and the crustal blocks between them are rotating clockwise at 1 2 myr 1 the east west dimension of eastern tibet is shortening at 10 20 mm yr 1 and little material is moving eastward out of india s path into eurasia by left lateral simple shear

a method for the determination of the initial shapes of deformed xenoliths in granitoids abstract measuring strain from deformed xenoliths is problematic due mainly to the large initial shape variations of these markers a method is described which allows mean initial shape to be determined for a number of xenolith populations by displaying their logarithmic ranges log rfmax log rfmax on a range diagram the diagram contains a check on validity and allows bad samples to be recognised xenolith data from a deformed granite in northwest ireland is analysed using the method

evidence for significant granite space creation by the ballooning mechanism the example of the ardara pluton ireland the emplacement mechanism of the ardara granite a caledonian pluton located on the northwestern seaboard of ireland has been a widely cited and a key point of reference in the debate about the granite space problem for nearly half a century early models related its subcircular tailed shape concentric foliation pattern and concentrically deformed envelope to diapirism it was subsequently reinterpreted using strain data as a ballooning pluton that was created by 70 in situ expansion at the emplacement site and was later cut by a regional shear zone recently it was the leading example in a wide ranging review of forcefully emplaced plutons of this type and was reinterpreted as a series of nested diapirs associated with only modest amounts 30 of forceful space creation but a large component of inferred early block stoping we report the results of the complete remapping and extensive collection of new structural data from this 8 km diameter normally zoned three phase calc alkaline pluton we confirm the existence of the steeply inclined internal foliation pattern and that it was formed in the magmatic state although this and the outline of the pluton are modified by late stage high temperature solid state regional transcurrent shear zones along two of the pluton contacts structural mapping has failed to identify a macroscopic lineation in the magmatic foliation planes and this is confirmed by strain determinations using mafic enclave populations and magmatic crystal separations for the magmatic state deformation that show nearly pure flattening type strains k 0 strains measured by these techniques show an apparently smooth gradient increasing outward toward the pluton contacts from an inner low strain zone we argue that these features together with the simple normal petrographic zonation a lack of small scale structures that would indicate the pluton had moved up relative to the wall rocks and an exposure level that is not in the roof zone are inconsistent with diapirism either singly or multiply but are most easily explained by a ballooning emplacement mechanism synmagmatic shear sense data from both the pluton and the adjacent country rocks indicate however that within the overall context of radial ballooning there was a limited amount of late stage nonradial northward injection of magma an inversion of the deformed enclave and crystal spacing strain determinations and strain determinations within the country rocks suggest that more than 80 of the volume of the pluton was accommodated by the ballooning mechanism

the significance of microgranitoid enclave shapes and orientations abstract enclaves are often incorrectly used to measure magmatic strains in plutons we emphasize that microgranitoid enclaves are not like other ellipsoidal markers used to determine strain for the following reasons 1 adjacent enclaves may form at different times and different places and initially have non spherical shapes with axial ratios up to 2 7 2 the final shapes and orientations of enclaves are a complex function of a initial shape and temperature of enclaves b subtle changes in composition melt percents volatiles grain sizes and thus temporally variable viscosity contrasts between the enclave and magma c a competition between strain and interfacial energies and d deformation path which may include internal strain and rigid rotations caused by magma flow during ascent convection expansion chamber boundary processes and tectonism 3 enclaves spend much of their time in magma as relatively rigid objects and thus rigidly rotate and potentially break apart rather than strain at matrix strain rates 4 in some instances enclaves do not record or track finite strain because of the above final enclave populations are heterogeneous and the use of single enclaves or enclave populations as strain markers violates many assumptions needed to complete strain analyses on the other hand a comparison of the preserved characteristics of igneous layering mineral fabrics and carefully evaluated enclave fabrics including internal mineral alignment in enclaves may provide qualitative data on the changing magnitude and kinematics of magmatic strains

mio pliocene volcaniclastic deposits in the famatina ranges southern central andes a case of volcanic controls on sedimentation in broken foreland basins abstract a well constrained record of miocene pliocene explosive volcanism is preserved within the broken foreland of western argentina along the famatina ranges this paper focuses on the volcaniclastic record known as the rio blanco member of the el durazno formation three facies can be recognized in the study area 1 massive tuffs 2 volcaniclastic conglomerates and 3 pumiceous sandstones these facies are interpreted as primary pyroclastic flow deposits ignimbrites and reworked volcanogenic deposits within interacting volcanic fluvial depositional systems alternation between ignimbrites and volcanogenic sandstones and conglomerates suggest a recurrent pattern of sedimentation related to recurrent volcanic activity considering the facies mosaic and relative thicknesses of facies short periods of syn eruption sedimentation volcaniclastic deposits seem to have been separated by longer inter eruption periods where normal stream flow processes were dominant the volcaniclastic component decreases up section suggesting a gradual reduction in volcanic activity the mean sedimentation rate of the rio blanco member is higher 0 44 mm year than those obtained for the underlying and overlying units this increase cannot be fully explained by foreland basement deformation and tectonic loading hence we propose subsidence associated with volcanic activity as the causal mechanism volcanism would have triggered additional accommodation space through coeval pyroclastic deposition modification of the stream equilibrium profile flexural loading of volcanoes and thermal processes these mechanisms may have favored the preservation of volcaniclastic beds in the high gradient foreland system of famatina during the mio pliocene thus the rio blanco member records the response of fluvial systems to large volcanism induced sediment loads

devonian sedimentation in the xiqingshan mountains implications for paleogeographic reconstructions of the sw qinling orogen abstract the qinling orogen between the north china and yangtze plates comprises various accreted and collisional terranes recording the evolution of the proto tethyan ocean and the formation of east asia knowledge of the provenance and tectonic setting of a thick succession of e w striking devonian sedimentary rocks sandwiched tectonically between the shangdan and mianlue ophiolitic suture zones in this belt is essential to understanding the evolution of the qinling orogen and the assembly of the north china and yangtze plates during the late paleozoic systematic studies of the devonian sedimentary facies in the xiqingshan mountains indicate the northern margin of the south qinling belt was uplifted prior to the early devonian lower devonian sandstones are dominated by feldspathic litharenite q 12 31 f 18 37 l 40 65 indicative of a continental arc derivation detrital zircon u pb data demonstrate that the lower and middle devonian sandstones were derived respectively from ca 770 ma and ca 400 ma felsic arc related magmatic sources devonian sedimentation in the xiqingshan mountains is interpreted to have occurred in a foreland basin between the yangtze plate and the north qinling continental arc

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

middle miocene tectonic development of the transition zone salta province northwest argentina magnetic stratigraphy from the metan subgroup sierra de gonzalez magnetostratigraphy isotopic dating and sandstone petrography establish age limits on the depositional history of 2100 m of foreland basin strata in the neogene metan subgroup of northwest argentina the strata were deposited between ca 15 1 and 9 7 ma in the eastern sistema de santa barbara the region is positioned above the cretaceous salta rift basin in the transi tion zone between modern relatively steep and flat subducting segments of the nazca plate formations within the subgroup are shown to be diachronous over a 60 km dis tance the younger ages are in the east changes in paleocurrent flow directions and the lithic clast component of sandstones collected from the arroyo gonzalez section suggest that basal fluvial strata were de rived from the craton to the east beginning in middle miocene time just prior to 15 1 ma by ca 14 5 ma the paleocurrent flowed from a source in the west and sedi ment accumulation rates increased dramat ically these changes correlate with con temporaneous tectonism in the west a local increase in basin accommodation may be partly related to a zone of weakness near the eastern boundary of the salta rift uplift in the western cordillera oriental apparently began by 13 7 ma and thrusting rapidly migrated eastward the eastern cordillera oriental ranges began to rise be tween 25 and 26s ca 10 ma as thrusting migrated eastward low energy depositional environments were overwhelmed ca 13 7 ma above an erosional unconformity that removed strata to an age of ca 9 7 ma bas al strata from the overlying jujuy sub group were deposited beginning after 9 ma sandstones from ro o yacones suggest that the cordillera oriental uplift contin ued for several million years longer be tween 24 and 25s uplift of the sistema de santa barbara in the distal portion of the foreland did not begin until after ca 9 ma

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