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

variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

miocene quaternary structural evolution of the uyuni atacama region andes of chile and bolivia abstract we describe the miocene quaternary geological structural evolution of the region between the salar de uyuni and de atacama andes of chile and bolivia we recognized four main tectonic events based on fold geometry fault kinematics and stratigraphic data the oldest event of miocene age is characterized by folding and reverse faulting of the sedimentary successions with an e w direction of shortening in the northern part of the studied area and a wnw ese shortening in the southern part the following two events of pliocene age are characterized by lower shortening amounts they occurred first by reverse faulting with a nw se trending greatest principal stress 1 computed with striated fault planes and a vertical least principal stress 3 followed by pervasive strike slip faulting with the same nw se trending 1 and a horizontal ne sw 3 the fourth event dating to the late pliocene quaternary is characterized by normal faulting the 3 still trends ne sw whereas the intermediate principal stress 2 exchanged with 1 volcanism accompanied both the contractional transcurrent and extensional tectonic phases the mio pliocene compression appears directly linked to a rapid convergence and an apparently important coupling between the continental and oceanic plates the e w to wnw ese direction of shortening of the miocene structures and the nw se 1 of the pliocene structures seem to be more linked to an intra andean re orientation of structures following the wnw directed absolute motion of the south american plate the extensional deformations can be interpreted as related to gravity forces affecting the highest parts of the volcanic belt in a sort of asymmetrical sw ward collapse of the belt

the mediaeval warm period drought recorded in lake huguangyan tropical south china the geochemistry of dated sediment cores from lake huguangyan 21o9 n 110o17 e tropical south china reveals distinct stratigraphical patterns in total organic and inorganic carbon toc tic biogenic silica bs and total nitrogen tn over the past 1400 years in this hydrologically closed lake tic variations may reeect changes in the precipitation evaporation ratio which controls the evaporative enrichment of carbonate toc bs and tn in the sediment are proxy indicators of lake productivity and nutrient input which we believe are linked to local precipitation high tic content correlates with low concentrations of toc bs and tn and indicates two drought episodes dated to ad 670 760 and ad 880 1260 in the sediments of lake huguan gyan local historical chronicles support these data suggesting that the climate of tropical south china was dry during the mediaeval warm period mwp and wet during the little ice age lia the detected mwp drought is temporally correlated with evidence for lower precipitation on the guliya china and quelccaya peru ice caps and with increased salinity in moon lake us great plains

an empirical estimate of near source ground motion for a major mb 6 8 earthquake in the eastern united states abstract three empirical techniques are used to estimate ground motion amplitudes in the near source region of a major earthquake hypothesized to occur along the east coast of the united states these techniques include the estimation of strong motion parameters from the statistical analysis of selected strong motion records from empirical and theoretical attenuation relations and from modified mercalli intensity source parameters for the hypothesized earthquake were selected to be similar to those inferred for the 1886 charleston south carolina earthquake the epicentral region is characterized by a modified mercalli intensity of x from which magnitudes of m b 6 8 m s 7 2 and m 7 2 were estimated inferred fault mechanisms for the hypothesized event vary from strike slip on a vertical fault to reverse or reverse oblique slip on a fault dipping as shallow as 45 the median estimate of peak horizontal acceleration for a point located 15 km from the surface projection of fault rupture is 0 30 g independent of site geology corresponding estimates of peak horizontal velocity are 35 cm sec for soil and soft sedimentary rock sites and 18 cm sec for crystalline basement rock sites strong motion parameters for a point located 15 km from the epicenter corresponding to a point located directly above the rupture zone of the hypothesized earthquake are estimated to be approximately 80 per cent higher than these estimates or about 0 55 g for peak acceleration and about 63 and 33 cm sec for peak velocity on soil soft rock and crystalline rock respectively corresponding velocity acceleration ratios are 115 cm sec g for soil soft rock sites and 60 cm sec g for crystalline rock sites standard deviations associated with the natural logarithm of peak horizontal acceleration and velocity are estimated to be 0 5 and 0 7 respectively representing an 84th percentile estimate or equivalently an upper 68 per cent confidence limit that is approximately 65 per cent higher than the median for acceleration and 100 per cent higher than the median for velocity while these strong motion values are generally higher than those currently used in the seismic design of most structures in the eastern united states their impact on the seismic safety of specific sites in the region will depend on an assessment of the probability of occurrence of an earthquake of this size within the near source region of these sites

significance of first order faults in folding mechanically isotropic layers evidence from the sudbury basin canada abstract the sudbury basin is a non cylindrical fold basin demarcated by the layered sudbury igneous complex sic the eastern part of which is transected by prominent curved faults folding of the sic and adjacent rock units occurred in the brittle field and is peculiar due to its petrographically distinct but initially mechanically similar layers overall the layers are characterized by low levels of solid state strain raising the question how the layer contacts acquired their curvature we addressed this question by developing a g i s based workflow to analyze the orientation and slip vectors of the faults slip vectors form clusters of normal and reverse slip along a given fault the clustering is best interpreted in terms of successive slip events during folding of the sic as the faults formed most likely as planar reverse faults prior to folding of the sic they subsequently served as mechanically anisotropic elements to fold the sic the results contribute to 1 better understand the folding mechanisms of thick melt sheets in the upper crust 2 explain apparently incompatible principal strain axes during progressive deformation and 3 efficiently analyze the orientation and kinematics of fault zones close to the earth s surface

tectonomagmatic characteristics of the back arc portion of the calama olacapato el toro fault zone central andes 1 post 20 ma magmatism in the central andes is either localized in the magmatic arc or distributed east of it on the altiplano puna plateau here there is a distinct concentration of magmatic centers on nw se trending lineaments such as the calama olacapato el toro cot that extends into the eastern cordillera to the east of the puna understanding the possible genetic relationship between prominent structures and magmatic centers on these lineaments is important to elucidate the tectonomagmatic evolution of the central andes we investigated the back arc area of the cot using remote sensing geological structural and petrochemical data our study demonstrates that this portion of the cot consists of nw se striking faults formed under overall left lateral transtension that decreases in activity toward the cot termini deformation on the cot occurred during and after activity of prominent n s striking transpressive fault systems and is coeval with magmatism which is focused on the central cot the most evolved magmatic rocks with an upper crustal imprint are exposed on the central cot whereas more primitive mantle derived mafic to moderately evolved magmatic rocks are found toward the cot termini this points to a genetic relationship between upper crustal deformation and magmatic activity that led to enhanced magma storage in the central cot cot magmas may result either from slab steepening or episodic delamination of the asthenospheric mantle

geochemistry of daihai lake sediments inner mongolia north china implications for provenance sedimentary sorting and catchment weathering abstract to advance the understanding of sediment distribution catchment weathering hydraulic sorting and sediment provenance in a tectonically stable basin the geochemistry of surface sediment samples from daihai lake in north china is presented mud bulk sediments were analyzed for 10 major and 30 trace elements organic carbon and nitrogen and for 87sr 86sr ratios in silicate fraction acid insoluble ai and carbonates acid soluble as enrichment in fe2o3 cao mno mgo k2o and p2o5 and their positive correlation with al2o3 rb sr ratios and bivariate discrimination diagrams of k vs rb and th u vs th suggest low chemical weathering intensity under semiarid conditions in the daihai catchment accumulation of clays in deeper water sites by sedimentary sorting results in an evident distribution of sediment composition with the lake water depth although similar chondrite normalized ree rare earth element patterns of lake sediment samples with lree light rare earth element enrichment negative eu anomalies and an almost flat hree heavy rare earth element pattern may suggest similar sedimentary source rocks original sediment provenance can be recognized on the basis of their distinctive 87sr 86sr ai ratios and immobile th zr ti and nb trace element signature separating from weathering and sedimentary processes as well as from grain size effect

interdecadal changes in summertime typhoon tracks abstract the present work examines interdecadal variations of typhoon tracks in the western north pacific wnp during the boreal summer june september for the period 1951 2001 typhoon tracks are expressed as percentage values of the total number of typhoon passages into a 5 5 latitude longitude grid box with respect to the total number of typhoons formed in the wnp the analysis period is divided into two interdecadal periods id1 1951 79 and id2 1980 2001 from id1 to id2 typhoon passage frequency decreased significantly in the east china sea and philippine sea but increased slightly in the south china sea the time series of typhoon passage frequency over the east china sea and south china sea further reveal a regime shift in the late 1970s while those over the philippine sea indicate a continuous downward trend of 9 decade 1 the interdecadal changes in typhoon tracks are associated with the westward expansion of the subtropical northwestern pacific high snph in the late 1970s the e

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

assessing sedimentary records of paleohurricane activity using modeled hurricane climatology patterns of overwash deposition observed within back barrier sediment archives can indicate past changes in tropical cyclone activity however it is necessary to evaluate the significance of observed trends in the context of the full range of variability under modern climate conditions here we present a method for assessing the statistical significance of patterns observed within a sedimentary hurricane overwash reconstruction to alleviate restrictions associated with the limited number of historical hurricanes affecting a specific site we apply a recently published technique for generating a large number of synthetic storms using a coupled ocean atmosphere hurricane model set to simulate modern climatology thousands of overwash records are generated for a site using a random draw of these synthetic hurricanes a prescribed threshold for overwash and a specified temporal resolution based on sedimentation rates observed at a particular site as a test case we apply this monte carlo technique to a hurricane induced overwash reconstruction developed from laguna playa grande lpg a coastal lagoon located on the island of vieques puerto rico in the northeastern caribbean apparent overwash rates in the lpg overwash record are observed to be four times lower between 2500 and 1000 years b p when compared to apparent overwash rates during the last 300 years however probability distributions based on monte carlo simulations indicate that as much as 65 of this drop can be explained by a reduction in the temporal resolution for older sediments due to a decrease in sedimentation rates periods of no apparent overwash activity at lpg between 2500 and 3600 years b p and 500 1000 years b p are exceptionally long and are unlikely to occur above 99 confidence under the current climate conditions in addition breaks in activity are difficult to produce even when the hurricane model is forced to a constant el nino state results from this study continue to support the interpretation that the western north atlantic has exhibited significant changes in hurricane climatology over the last 5500 years

late cenozoic tectonism collapse caldera and plateau formation in the central andes abstract the evolution of andean volcanism including the formation of late miocene to recent collapse calderas on the puna plateau is generally interpreted in terms of the kinematic framework of the nazca and south american plates we present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first order nw se striking zones of left lateral transtension on the local and regional scales consequently ages of collapse calderas indicate activity of these fault zones which initiated at about 10 ma on the puna plateau the onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen parallel stretching upon reaching maximum crustal thickness and critical surface elevation horizontal magma sheets that formed at mid crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub vertical faults this accounts well for the observed tectono magmatic phenomena at surface it follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the andean plateau rather than to changes in the geometry of the wadati benioff zone or plate boundary kinematics

cenozoic stratigraphy and basin tectonics of the andes mountains 20 28 south latitude clastic sedimentary basins have evolved during the past 40 m y in the central andes lat 20 28 s in response to shifting patterns of magmatism and deformation the distribution of these basins and their genetic relations to uplifted areas are analogous to the basins and mountain belts of the north american rocky mountains during the late cretaceous and early cenozoic petroleum exploration has focused on zones underlying the upper cenozoic strata along the eastern margin of the andes mountain belt between about 40 and 25 ma a nonmarine basin extended across the region that is now the andes mountains between about 25 and 10 ma the western part of the former basin became the site of a volcanic arc sediment accumulation continued in the east where marine intercalations demonstrate the low elevation of the basin after 10 ma the volcanic arc remained active and locally widened and crustal shortening caused regionally important thrust and reverse faulted ranges during the past 10 m y up to 4 000 m of coarse clastic debris accumulated in a foreland basin on the eastern flank of the mountains meanwhile in the interior of the mountains over 4 000 m of fine grained strata and evaporites accumulated in local depocenters

reconstruction of environmental changes using a multi proxy approach in the ulleung basin sea of japan over the last 48 ka based on elemental geochemical data we reconstructed the sediment provenance surface productivity and bottom water redox conditions for the last 48 ka in the ulleung basin sea of japan and inferred the factors controlling them al2o3 tio2 ratio and chemical index of alteration cia suggest that sediment provenance changed during the glacial period 48 18 ka compared to the deglacial ca 18 11 ka and holocene mass accumulation rates of total organic carbon toc caco3 phosphorus cadmium and excess barium reveal low paleoproductivity during low sea stand during 18 11 ka productivity increased due to increasing inflow of nutrient rich water masses the oyashio and the east china sea coastal water in tandem with the rising sea level maximum productivity occurred during younger dryas and pre boreal periods when sea level was at 60 m and then gradually decreased as the tsushima warm current inflow kicked off at ca 9 3 ka consistent with other paleoredox proxies which reveal the presence of anoxic bottom water during ca 12 9 ka with the changes in paleoredox proxies and their ratios toc mo u mn c s ratio and uauthigenic and mo contents we hypothesized that the redox changes were mainly ventilation driven and were superimposed on the influence of circulation induced productivity changes the global climate and sea level changes on a millennial timescale play a major role in enhancing paleoproductivity and restrict bottom water advection subsequently driving the oxygenation of bottom water in the ulleung basin copyright 2012 john wiley sons ltd

early to middle tertiary foreland basin development and the history of andean crustal shortening in bolivia a 2 5 km thick succession of tertiary strata in the eastern cordillera of southern central bolivia consists of predominantly fluvial and lacustrine deposits the age of the base of the succession is middle paleocene and its upper part which is erosionally truncated is probably late oligocene early miocene the lower 50 130 m of the succession consists of interbedded fluvial sandstone and mudstone including minor paleosols of the santa lucia formation these strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the santa lucia formation and lower part of the impora formation that may represent much of late paleocene eocene time above the paleosol zone is a thin zone 10 m of lacustrine carbonate and clastic rocks in the uppermost impora formation and this in turn is overlain by a 2000 m thick upward coarsening succession of clastic fluvial deposits in the cayara camargo and suticollo formations paleocurrent data indicate that fluvial channels that deposited the santa lucia impora and cayara formations flowed mainly westward whereas channels responsible for the camargo and suticollo formations flowed generally eastward modal sandstone petrographic data show a long term evolution from subarkosic during santa lucia deposition to quartz arenitic during impora and cayara deposition to sublitharenitic during camargo and suticollo deposition compositions we argue that the lithofacies and sediment accumulation history of this succession are most consistent with deposition in an eastward migrating foreland basin system e mail decelles geo arizona edu critical to this interpretation is the zone of extreme stratigraphic condensation in the lower impora formation the inferred upper paleocene eocene part of the succession the abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous postrift thermal subsidence resulted in a gradual continuous decay of early tertiary sediment accumulation rates stratigraphic condensation is however consistent with passage of the forebulge through the region an interpretation that also can be reconciled with the record of foreland basin development in the altiplano to the west and gross sediment accumulation patterns in the subandean zone and modern foreland basin to the east simple flexural modeling and palinspastic reconstruction of the complete cenozoic migration history of the foreland basin system suggest that 1000 km of foreland lithosphere has been underthrust westward beneath the andean orogenic belt at the latitude of southern bolivia the amount of middle and lower crust that would have been added to the andean infrastructure is sufficient to explain the present crustal thicknesses in bolivia

regional modification of acceleration attenuation functions abstract strong ground motion attenuation functions developed on the basis of data from one physiographic region do not generally apply to other regions due to both differences in source characteristics and seismic wave attenuation at the same time many regions with known seismic hazard lack sufficient strong motion data to develop regionally unique attenuation functions a method is proposed for the estimation of peak ground acceleration attenuation functions for these regions the technique makes use of regional variations in relationships between event magnitude and epicentral intensity and event magnitude and radius of the felt area to correlate peak ground accelerations from a region which has empirical strong motion data to one lacking this data the necessary relationships to make the modifications require only limited instrumental observations the method is an improvement over previously suggested schemes in that the fundamental assumptions are more restricted than those previously used

the geological and structural evolution of the long lived miocene pleistocene la hoyada volcanic complex in the geodynamic framework of the central andes argentina abstract the long lived la hoyada volcanic complex lhvc is located in the east of cordillera de san buenaventura in argentina this region is important for understanding of the geodynamic evolution of the southern puna since it is located in its southern edge where the transition between normal and low angle subduction occurs the aim of this work is to contribute to the knowledge of the volcanism of the southern puna through the analysis of the stratigraphy and geological evolution of lhvc fieldwork and image analysis have led to a new lithostratigraphic scheme that identifies lithosomes and unconformity bounded stratigraphic units defining the stratigraphic architecture of the lhvc new 40ar 39ar ages further constrain the geological evolution of this complex the resulting stratigraphic scheme combines the lithostratigraphic units into lithosomes and synthemic units with the identification of at least six superimposed eruptive centers and their products during the lhvc evolution a new stratigraphic framework for the eastern cordillera de san buenaventura composed by four supersynthems separated by unconformities that reflect periods of quiescence of the volcanic activity catastrophic destructive events and changes in the magmatic plumbing system is proposed 1 san buenaventura supersynthem ca 9 7 ma including mainly explosive rhyolitic volcanism 2 lower la hoyada supersynthem ca 7 d pyroclastic and epiclastic deposits and 4 purulla supersynthem

exploring typhoon variability over the mid to late holocene evidence of extreme coastal flooding from kamikoshiki japan sediment cores from two coastal lakes located on the island of kamikoshiki in southwestern japan lake namakoike and lake kaiike provide evidence for the response of a backbarrier beach system to episodic coastal inundation over the last 6400 years sub bottom seismic surveys exhibit acoustically laminated parallel to subparallel seismic reflectors intermittently truncated by erosional unconformities sediment cores collected from targeted depocenters in both lakes contain finely laminated organic mud interbedded with coarse grained units with depths of coarse deposits concurrent with prominent seismic reflectors the timing of the youngest deposit at kamikoshiki correlates to the most recently documented breach in the barrier during a typhoon in 1951 ad assuming that this modern deposit provides an analog for identifying past events paleo typhoons may be reconstructed from layers exhibiting an increase in grain size a break in fine scale stratigraphy and elevated sr concentrations

mineralogical and geochemical investigations of sediment source region changes in the okinawa trough during the past 100 ka images core md012404 abstract we present a mineralogical and geochemical study of core md012404 retrieved from the central okinawa trough ot of the east china sea our studies reveal that the sediment sources of the core have been changed through time during the past 100 ka our mineralogical proxies indicate that the sediments source from the yangtze river correlates well sea level changes before 24 ka our ti al ratios otherwise indicate an increase of sediment supply from eastern taiwan after 26 ka the cooler climate of the last glacial maximum lgm 23 19 ka led to a reduction in fluvial sediments from the yangtze river however subsequent climate warming after 19 ka resulted in an abrupt increase in fluvial sediments after the lgm the kuroshio intrusion flow into the ot may have increased we also infer anomalous changes in eolian sources transported by winter monsoons during the lgm and at 80 ka based on an eolian mineralogical indicator feldspar we conclude that the sediment source of core md012404 is primarily of terrestrial origins but influenced by sea level changes and variations in the east asian monsoon

cenozoic post collisional brittle tectonic history and stress reorientation in the high zagros belt iran fars province abstract the zagros fold and thrust belt of sw iran is among the youngest continental collision zones on earth collision is thought to have occurred in the late oligocene early miocene followed by continental shortening the high zagros belt hzb presents a neogene imbricate structure that has affected the thick sedimentary cover of the former arabian continental passive margin the hzb of interior fars marks the innermost part of se zagros trending nw se that is characterised by higher elevation lack of seismicity and no evident active crustal shortening with respect to the outer sw parts this study examines the brittle structures that developed during the mountain building process to decipher the history of polyphase deformation and variations in compressive tectonic fields since the onset of collision analytic inversion techniques enabled us to determine and separate different brittle tectonic regimes in terms of stress tensors various strike slip compressional and tensional stress regimes are thus identified with different stress fields brittle tectonic analyses were carried out to reconstruct possible geometrical relationships between different structures and to establish relative chronologies of corresponding stress fields considering the folding process results indicate that in the studied area the main fold and thrust structure developed in a general compressional stress regime with an average n032 direction of 1 stress axis during the miocene strike slip structures were generated under three successive strike slip stress regimes with different 1 directions in the early miocene n053 late miocene early pliocene n026 and post pliocene n002 evolving from pre fold to post fold faulting tensional structures also developed as a function of the evolving stress regimes our reconstruction of stress fields suggests an anticlockwise reorientation of the horizontal 1 axis since the onset of collision and a significant change in vertical stress from 3 to 2 since the late stage of folding and thrusting a late right lateral reactivation was also observed on some pre existing belt parallel brittle structures especially along the reverse fault systems consistent with the recent n s plate convergence however this feature was not reflected by large structures in the hzb of interior fars the results should not be extrapolated to the entire zagros belt where the deformation front has propagated from inner to outer zones during the younger events

the negra muerta volcanic complex southern central andes geochemical characteristics and magmatic evolution of an episodically active volcanic centre abstract this petrologic analysis of the negra muerta volcanic complex nmvc contributes to understanding the magmatic evolution of eruptive centres associated with prominent nw striking fault zones in the southern central andes specifically the geochemical characteristics and magmatic evolution of the two eruptive episodes of this complex are analysed the first one occurred as an explosive eruption at 9 ma and is represented by a strongly welded fiamme rich andesitic to dacitic ignimbrite deposit the second commenced with an eruption of a rhyolitic ignimbrite at 7 6 ma followed by effusive discharge of hybrid lavas at 7 3 ma and by emplacement of andesitic to rhyodacitic dykes and domes both explosive and effusive eruptions of the second episode occurred within a short time span but geochemical interpretations permit consideration of the existence of different magmas interacting in the same magma chamber our model involves an andesitic recharge into a partially cooled rhyolitic magma chamber pressurising the magmatic system and triggering explosive eruption of rhyolitic magma chemical or mechanical evidence for interaction between the rhyolitic and andesitic magma in the initial stages are not obvious because of their difference in composition which could have been strong enough to inhibit the interaction between the two magmas after the initial explosive stages of the eruption at 7 6 ma the magma chamber become more depressurised and the most mafic magma settled in compositional layers by fractional crystallisation restricted hybridisation occurred and was effective between adjacent and thermally equivalent layers close to the top of the magma chamber at 7 3 ma increments of caldera formation were accompanied by effusive discharge of hybrid lavas through radially disposed dykes whereby andesitic magma gained in importance toward the end of this effusive episode in the central portion of the caldera assimilation during turbulent ascent ata is invoked to explain a conspicuous reversed isotopic signature 87sr 86sr and 143nd 144nd in the entire volcanic series therefore the 7 6 to 7 3 ma volcanic rocks of the nmvc resulted from synchronous and mutually interacting petrological processes such as recharge fractional crystallization hybridisation and assimilation during turbulent ascent ata geochemical characteristics of both volcanic episodes show diverse type and or depth in the sources and variable influence of upper crustal processes and indicate a recurrence in the magma forming conditions similarly other minor volcanic centres in the transversal volcanic belts of the central andes repeated their geochemical signatures throughout the miocene

fault slip inversions their importance in terms of strain heterogeneity and kinematics of brittle deformation abstract heterogeneous deformation is intrinsic in natural deformation but often underestimated in the analysis and interpretation of mesoscopic brittle shear faults based on the analysis of 11 222 faults from two distinct tectonic settings the central andes in argentina and the sudbury area in canada interpolation of principal strain directions and scaled analogue modelling we revisit controversial issues of fault slip inversions collectively adhering to heterogeneous deformation these issues include the significance of inversion solutions in terms of 1 strain or paleo stress 2 displacement notably plate convergence 3 local versus far field deformation 4 strain perturbations and 5 spacing between stations of fault slip data acquisition furthermore we highlight the value of inversions for identifying the kinematics of master fault zones in the absence of displaced geological markers a key result of our assessment is that fault slip inversions relate to local strain not paleo stress and thus can aid in inferring the kinematics of master faults moreover strain perturbations caused by mechanical anomalies of the deforming upper crust significantly influence local principal strain directions thus differently oriented principal strain axes inferred from fault slip inversions in a given region may not point to regional deformation caused by successive and distinct deformation regimes this outcome calls into question the common practice of separating heterogeneous fault slip data sets into apparently homogeneous subsets finally the fact that displacement vectors and principal strains are rarely co linear defies the use of brittle fault data as proxy for estimating directions of plate scale motions

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

topography and subduction geometry in the central andes clues to the mechanics of a noncollisional orogen the central andean orogen between 12 and 32 s latitude exhibits a high degree of spatial order principally an extraordinary bilateral symmetry that is common to the earth s surface the underlying wadati benioff zone and the nazca south america plate kinematics which has been stable since the mid tertiary this spatial order must reflect the physical mechanisms of mountain building in this noncollisional orogen the shapes of the topography and subduction zone can be reduced to symmetric and antisymmeric components relative to any vertical symmetry plane the particular plane which minimizes the antisymmetry and maximizes the symmetry is well resolved and is essentially coincident with the stable euler equator of nazca south america relative motion since the mid tertiary that the topography subduction geometry and persistent mid tertiary plate kinematics share common spatial and geometric elements suggests that the distribution of topography in this orogen depends strongly on the dynamics of subduction other factors that might affect the topography and underlying tectonics such as climate and inherited structural fabric which have different spatial characteristics must be of less significance at a continental scale furthermore the small components of asymmetry among the various elements of the orogen appear to be mutually related in a simple way it is possible that this coupled asymmetry is associated with a late tertiary change in plate kinematics these observations suggest that there is a close connection between plate tectonics and the form of the earth s surface in this noncollisional setting it follows that the distribution of topography near convergent plate boundaries may provide a powerful constraint for understanding the dynamics of subduction

pleistocene mafic volcanoes in the puna cordillera oriental boundary nw argentina abstract los gemelos and el saladillo are both monogenetic strombolian basaltic shoshonitic volcanoes that constitute the easternmost recognized examples of mafic plio quaternary volcanism in the southern central andes two regional faults delimit the borders of the calchaqui valley as thrusts with opposite vergence the eastern calchaqui fault and the western toro muerto fault while los gemelos are set in the hanging wall of calchaqui back thrust fault el saladillo are set in the footwall of toro muerto fault as los gemelos volcanoes have well preserved morphological features we highlight some relationship between them and their tectonic setting kinematic data and one new measurement that indicate right strike slip movement in the vicinity of los gemelos during the pleistocene holocene the emplacement of these volcanoes should be related to a transpression zone parallel to the valley where the alignment of the cones is outlining the trend of conjugated faults the magmas were derived from a small degree of partial melting of an enriched garnet bearing mantle source the analysed rocks have primitive signature high ni cr co and mgo concentrations presence of chromite and forsteritic olivine and evidence for crustal contamination with felsic rocks quartz plagioclase k feldspar xenocrysts with coronas reaction rims and or embayments high 87sr 86sr ratios negative end values the high cr and ni content high mg and low crystal content suggest that no major fractional crystallization occurred therefore precluding long residence periods rapid magma ascent across 60 km of continental crust was guided by magmatic overpressure favoured by important tectonic stresses also avoiding significant residence time at upper crust depth thus we invoke a process of assimilation during turbulent ascent ata to explain the contamination at crustal levels los gemelos volcanoes were formed around 35 000 yr as inferred from the age of lacustrine sediments overlying the lava flows these ages agree well with the age of paleolakes of tectonic origin in nw argentina suggesting a common tectonic cause for both effects

tectonic significance of fault slip data abstract the statistical analysis of populations of minor faults is commonly used by structural geologists working in areas of brittle rock it is based on measurements of fault attitude direction and sense of slip at individual sampling localities results are classically interpreted as indicators of stress or strain rate fields assuming a homogeneous stress or strain rate tensor respectively however fault patterns are expected to vary with time because of displacements rigid rotations and internal strains which generally occur along boundaries of fault bounded blocks as deformation proceeds thus in the general situation where early faults accumulate displacements and rigid rotations and where new faults develop during progressive deformation fault slip data can be rather complex and variable in space and reflect neither local stress or strain rate tensors nor finite strains and finite rotations in a simple way we use two examples of faulted regions to illustrate the spatial variability of fault slip data this can be due to local complications at the edges of fault blocks or to complex kinematic conditions at regional boundaries such complications may make it difficult to deduce a consistent and simple pattern of either stresses or strain rates instead our results suggest that information contained in fault slip data can be pertinent to finite deformation in particular the principal axes that we have deduced from the analysis of fault slip data are consistent with the finite displacements at block boundaries that we have calculated by numerical restoration

cenozoic kinematic evolution of southwestern central iran strain partitioning and accommodation of arabia eurasia convergence abstract the yazd block of southwestern central iran is located in the back of the zagros fold thrust belt and monitors accommodation of cenozoic shortening within the arabia eurasia collision zone the saghand area within the yazd block exposes a basin and range type morphology which results from ne respectively e w direction of shortening and a major tectonic change in structural trends within the southwestern central iranian block the nne structural trend of various lithotectonic units including the eocene chapedony metamorphic core complex in the north is parallel to prominent cenozoic nne trending chapedony and posht e badam strike slip faults in the south the nnw trending anar kharanagh zarrin behabad and kuhbanan faults are parallel to the zagros fold thrust belt and represent dextral strike slip respectively ne directed thrust faults the thrust component representing a back thrust component to the zagros fold thrust belt results of interpretation of satellite images and structural and geomorphic field observations show a distributed deformation pattern covering a wide domain within southwestern central iran morphotectonic features like linear mountain ranges and intervening basins kavirs are interpreted to result from neogene to recent crustal scale folding with uplifting mountain ranges and subsiding basins active thrust faults and evidence for block tilting are common on the eastern side of mountain blocks five distinct regional deformation phases respectively paleostress tensor groups have been recognized in the central iran from the major structures and through using paleostress inversion techniques for fault slip data the first phase occurred in the late paleocene to oligocene and is characterized by a subvertical 1 and ca n s oriented 3 which indicate n s extension associated with late stages of normal faulting possibly due to post orogenic collapse during exhumation of the chapedony metamorphic complex the second phase occurred from late oligocene time onwards with an nne ssw to ne sw trending subhorizontal 1 and wnw ese to nw se trending 3 patterns due to transpressional deformation associated with a combination of thrusting and strike slip faulting the ne sw to ene wsw compressional deformation phase is spectacular evidence of thrust faulting and block tilting affecting the region up to holocene this deformation phase is consistent with geodetic gps based kinematic data indicating ca n10 e motion which implies strain partitioning with ne sw shortening and dextral displacement along n and nw trending strike slip faults

bending of the bolivian orocline and growth of the central andean plateau paleomagnetic and structural constraints from the eastern cordillera 22 24 s nw argentina 1 we report new paleomagnetic and structural data from late cretaceous to mio pliocene continental sandy silty sedimentary rocks from the eastern cordillera central andes here n s to nne ssw ridges hosting paleozoic basement and upper cretaceous continental red beds overthrust thick adjacent cenozoic basins pretilting and likely primary reliable directions gathered at 15 sites document 45 9 9 4 30 1 23 9 and 15 4 19 3 clockwise cw rotations with respect to south america occurring after the late cretaceous 80 ma oligo miocene 20 30 ma and late miocene pliocene 5 10 ma respectively conversely four upper cretaceous sites from the walls of a n s left lateral strike slip fault yavi abra pampa fault yield a null rotation about 20 km to the west flower structures and subvertical syntectonic strata dated at 14 26 0 19 ma are exposed along the subparallel abra moreta left lateral strike slip fault relying on data from the literature on the period when deformation began we suggest that since eo oligocene times 30 40 ma the eastern cordillera has undergone a regional cw rotation of 40 50 synchronous with crustal shortening and large scale bending of the andean salient the cw rotation is possibly still active today as documented by regional gps data from the andes since 15 ma ago the activity of n s left lateral strike slip faults induced counterclockwise rotations along the fault zone locally annulling the regional cw rotation in agreement with a previous model we speculate that mid miocene strike slip activity accommodated the progressive southward spreading of the altiplano puna high altitude plateau laterally migrating from the overthickened crustal region of the salient apex

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

plio quaternary stress states in ne iran kopeh dagh and allah dagh binalud mountain ranges abstract ne iran including the kopeh dagh and allah dagh binalud deformation domains comprises the northeastern boundary of the arabia eurasia collision zone this study focuses on the evolution of the plio quaternary tectonic regimes of northeast iran we present evidence for drastic temporal changes in the stress state by inversion of both geologically and seismically determined fault slip vectors the inversions of fault kinematics data reveal distinct temporal changes in states of stress during the plio quaternary since 5 ma the paleostress state is characterized by a regional transpressional tectonic regime with a mean n140 10 e trending horizontal maximum stress axis 1 the youngest modern state of stress shows two distinct strike slip and compressional tectonic regimes with a regional mean of n030 15 e trending horizontal 1 the change from the paleostress to modern stress states has occurred through an intermediate stress field characterized by a mean regional n trending 1 the inversion analysis of earthquake focal mechanisms reveals a homogeneous transpressional tectonic regime with a regional n023 5 e trending 1 the modern stress state deduced from the youngest fault kinematics data is in close agreement with the present day stress state given by the inversions of earthquake focal mechanisms according to our data and the deduced results in northeast iran the arabia eurasia convergence is taken up by strike slip faulting along ne trending left lateral and nnw trending right lateral faults as well as reverse to oblique slip reverse faulting along nw trending faults such a structural assemblage is involved in a mechanically compatible and homogeneous modern stress field this implies that no strain and or stress partitioning or systematic block rotations have occurred in the kopeh dagh and allah dagh binalud deformation domains the plio quaternary stress changes documented in this paper call into question the extrapolation of the present day seismic and gps derived deformation rates over geological time intervals encompassing tens of millions of years

development of the southern eastern cordillera nw argentina constrained by apatite fission track thermochronology from early cretaceous extension to middle miocene shortening 1 for the puna plateau and eastern cordillera of nw argentina the temporal and spatial pattern of deformation and surface uplift remain poorly constrained analysis of completely and partially reset apatite fission track samples collected from vertical profiles along an ese trending transect extending from the plateau interior across the southern eastern cordillera at 25 s reveals important constraints on the deformation and exhumation history of this part of the andes the data constrain the neogene andean development of the eastern cordillera as well as rift related exhumation for some of the sampled locations in the late jurassic early cretaceous an intervening eocene oligocene exhumation episode in the southern eastern cordillera was probably related to crustal shortening subsequent reburial of the area by andean foreland basin strata commenced between 30 and 25 myr magnitude and duration of sedimentation revealed by thermal modeling differ between the sample locations pointing to an eastward propagating basin system in the southern eastern cordillera andean deformation commenced at 22 5 21 myr predating both the inferred formation of significant topography by 5 7 5 myr and preservation of sediments in the adjacent cenozoic basins by 6 5 8 myr comparing the calculated structural depth of partially reset samples suggests that newly formed west dipping reverse faults along the former salta rift margin accommodated most of the neogene tectonic movement late cenozoic deformation at the southern eastern cordillera began earlier in the west and subsequently propagated eastward the lateral growth of the orogen is coupled with a foreland basin system developing in front of the range and then becomes subsequently compartmentalized by later emergent topography

east asian monsoon variability since the mid holocene recorded in a high resolution absolute dated aragonite speleothem from eastern china abstract paleoclimatic conditions associated with the east asian monsoon are reconstructed for the mid to late holocene using 18o values 819 measurements from a precisely dated 10 230th dates aragonite stalagmite from lianhua cave hunan province china 18o values 1 6 to 7 0 are interpreted to reflect the precipitation amount effect related to the strength of summer monsoonal circulation throughout this period the intensity of east asian summer monsoonal circulation declined in response to changes in insolation and a southward shift in the intertropical convergence zone itcz punctuating this first order trend are abrupt millennial to decadal scale fluctuations the most prominent of which occurred around 3337 5 yr bp and records a dramatic weakening in summer monsoonal circulation that coincides with the beginning of neoglacial conditions comparisons between the 18o values of stalagmite a1 and previously published data from stalagmite hs 4 from heshang cave and stalagmites d4 and da from dongge cave china demonstrate a similar regional response to changes in east asian monsoonal circulation several spectral analysis techniques and wavelet analysis were applied to these stalagmite records to evaluate the nature and character of cyclicity in the east asian monsoon during the holocene multi taper method mtm lomb scargle and single spectrum analysis ssa results reveal common periodicities that are unlikely to be artifacts of any single technique the periodicities expressed in the stalagmite 18o records occur at both solar and non solar frequencies and support previous interpretations that variability in monsoonal circulation responds to both solar forcing and internal climatic mechanisms wavelet analysis demonstrates that some of these frequencies are intermittent and recur at semi regular intervals which may indicate long term cyclicity that is unconstrained by the chronology of the record

cenozoic dextral transpression and basin development at the southern edge of the puna plateau northwestern argentina abstract in northwestern argentina the southern edge of the altiplano puna is the most remarkable positive topographic anomaly average altitude 3700 m of the andean chain to the south are the sierras pampeanas a less elevated domain made of alternating compressive basins and ranges developed during the cenozoic at about 27 s the transition between the puna and the sierras pampeanas coincides with a substantial narrowing of the high chain and with a change in the dip of the pacific subduction zone nazca plate from 30 e below the plateau to subhorizontal below the sierras pampeanas we argue that this structural transition which can be identified at the scale of the andean topography is a major dextral transpressional transfer zone the tucuman transfer zone in this region ene wsw to e w directed subhorizontal shortening related to bulk convergence between the nazca and south american plates interferes with nw se directed subhorizontal shortening strains due to lateral influence of changes in the amount of crustal thickening on intraplate kinematics structural observations digital mapping spot satellite images and a microtectonic analysis of fault populations provide general insights into the neogene kinematic evolution of transpressional basins and ranges at the edge of the puna

a criterion for identifying asian dust events based on al concentration data collected from northern taiwan between 2002 and early 2007 1 we attempted to identify asian dust ad events between february 2002 and february 2007 in northern taiwan using aerosol al measurements we subsequently used the results to propose a criterion for defining ad events a total of 30 ad events were detected based on al spiking concentrations and these occurred frequently in 2002 and 2006 the dust plumes that were identified occurred between november and april which were concentrated in february march and april there were 35 dusty days which accounted for 1 89 of the whole study period and for 3 75 when considering only november through april the seasonality of atmospheric al is quite evident reaching a maximum in spring and winter and a minimum in summer this pattern is related to the long range transport of ad in the northeasterly monsoon and the cease of ad contributions the efficient removal by heavy rain and effective mixing in summer according to air mass trajectory analysis the primary dust regions are located in and around the gobi desert and loess plateau the geometric mean concentration of the data set excluding the ad cases is 502 ng m 3 with a geometric standard deviation gsd of 2 8 on the basis of our success in identifying the major ad events the geometric mean 2 gsd i e 2800 ng m 3 in our study area is proposed as a straightforward criterion to define the significant ad event as such the statistic based criterion could be applicable for other areas

bimodal volcanism in a tectonic transfer zone evidence for tectonically controlled magmatism in the southern central andes nw argentina abstract this field based and analytical laboratory study focuses on the genetic relationship between bimodal volcanic centres and fault types of an important tectonic transfer zone in the southern central andes the nw se striking calama olacapato toro cot volcanic belt more specifically tectono magmatic relationships are examined for the 0 55 ma tocomar the 0 78 ma san jeronimo and the 0 45 ma negro de chorrillos volcanic centres in the tocomar area 66 30 w 24 15 s structures of the cot volcanic belt notably nw se striking strike slip faults and ne sw trending normal faults accommodated differential shortening between major n s striking thrust faults on the puna plateau we present evidence that bimodal volcanism was contemporaneous with activity of these fault types in the cot volcanic belt whereby eruption and composition of the volcanic rocks in the tocomar and san jeronimo negro de chorrillos areas appear to have been controlled by the kinematics of individual faults more specifically rhyolitic centres such as the tocomar are associated with normal faults whereas shoshonitic andesitic monogenetic volcanoes e g the san jeronimo and negro de chorrillos centres formed at strike slip dominated faults thus the eruption of higher viscous rhyolite magmas appears to have been facilitated in tectonic settings characterized by horizontal dilation whereas ascent and effusive volcanic activity of less viscous and hot basaltic andesites to shoshonites were controlled by subvertical strike slip faults while the tocomar rhyolites are interpreted to be derived from an anatectic crustal source geochemical characteristics of the san jeronimo and negro de chorrillos shoshonitic andesites are in agreement with a deeper source this suggests that the composition of erupted volcanic rocks as well as their spatial distribution in the tocomar area is controlled by the activity of specific fault types such volcano tectonic relationships are also evident from older volcanic centres of the cot volcanic belt this points to a close genetic relationship between bimodal volcanism and the upper crustal deformation regime whereby episodes of increased volcanic activity correlate with deformation episodes in the central andes

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

a multiproxy lake record from inner mongolia displays a late holocene teleconnection between central asian and north atlantic climates in order to study how the holocene central asian climate is coupled to the global climate system a 4 24 m long lake core from western inner mongolia in china was studied using a multiproxy approach sedimentology and geochemical parameters such as gypsum and dolomite content presence of lakeshore sand changing to aeolian sand and changes in paleomagnetic properties bear witness to a trend toward a generally drier climate over the late holocene aridification is linked to the southward retreat of the northern boundary of the asian summer monsoon leaving central asia under the influence of the westerly belt the weakening of the asian summer monsoon in turn was caused by an orbitally driven decrease in summer insolation the weakening summer insolation also likely increased the intensity of the siberian high pressure system further promoting aridification of central asia on a shorter time scale the multiproxy record shows the climate to have been relatively dry during the medieval warm period ad 800 1100 with the ensuing humid environment at the end of this period gradually turning to become extremely dry ad 1100 1550 at the little ice age maximum switches in the north atlantic oscillation caused these changes through a teleconnection in the form of westerlies these westerlies provided most of central asia s moisture after the retreat of the asian summer monsoon the central asian climate therefore corresponds closely with late holocene european climate changes

late cenozoic and modern stress fields in the western fars iran implications for the tectonic and kinematic evolution of central zagros 1 the zagros iran developed during mio pliocene times in response to arabia eurasia convergence the western fars highlights a major bend of the deformation front and displays a remarkable set of nearly n s right lateral strike slip faults the kazerun borazjan karebass sabz pushan sarvestan faults oblique at high angle to the belt the region likely plays a major kinematic role by accommodating the change in shortening modes from partitioned in the western central zagros to nonpartitioned in the eastern zagros the inversion of focal mechanisms from small and moderate earthquakes shows a consistent n020 030 compression with a low ratio between differential stresses this regime accounts for the combination of strike slip and thrust type mechanisms through likely 2 3 permutations fault slip analysis reveals two successive late cenozoic regional compressional trends ne sw then n020 the latter is in good agreement with the present day stress the significance of the ne sw compression is discussed alternatively in terms of stress deviations or block rotations in relation to the strike slip fault system fieldwork and satellite imagery suggest that these faults behave first as transfer faults during folding of the cover and later as strike slip faults in agreement with the succession of stress regimes and the evolution of the dominant deformation style from thin skinned to thick skinned the first order stability of the collision related state of stress since 5 ma supports that the arabia eurasia convergence did not give rise to partitioning in the western fars but rather was and is still accommodated by distributed deformation involving both shortening and strike slip motion throughout the cover and the basement

normal faulting along the southern margin of the puna plateau northwest argentina 1 though orogen parallel shortening and vertical extension have dominated the tectonic evolution of the central andes a significant kinematic shift from horizontal contraction to extension appears to have occurred within the high puna altiplano plateau with the establishment of extension oblique to the orogen since late miocene time we present data from the southern margin of the puna plateau nw argentina where new normal faults have been documented in the fiambala punta negra and la quebrada areas the unifying characteristics of these areas are that young normal faults reactivate or crosscut older thrust and reverse faults the relationship between the faults and the late miocene pliocene punaschotter conglomerate suggests that the extensional faulting must be younger than 3 5 to 7 ma existing data are incomplete but indicate that similar horizontal extension has occurred in many regions throughout the puna altiplano plateau while shortening continues along the plateau margins given the spatial and temporal distribution of this late miocene to pliocene kinematic shift both lithospheric loss in the puna plateau and plateau wide gravitational extensional spreading enhanced by slowing of plate convergence rate could be responsible the young disorganized horizontal extension in the andes today may be the precursor to more pronounced extension such as observed on the tibetan plateau since mid miocene time

the calama olacapato el toro fault system in the puna plateau central andes geodynamic implications and stratovolcanoes emplacement abstract the structural evolution of the puna plateau is characterized by the activity of both orogen parallel and orogen oblique faults understanding the possible relationship between these two structural styles their geodynamic implications and the influence on the migration of magmas is important to get insights into the tectonic and magmatic evolution of the central andes in this study we present a structural analysis of the orogen oblique calama olacapato el toro fault system and the surrounding orogen parallel thrust faults in the central eastern puna plateau morphostructural analysis and field mapping reveal the geometry kinematics and dynamics of the tectonic features in the studied area we propose a three dimensional geometrical reconstruction of the main fault planes showing their attitude and intersections at depth the study indicates that the crust underwent simultaneous deformation along both the vertical transcurrent calama olacapato el toro fault system and the low angle thrust faults and that the back arc portion of the calama olacapato el toro fault system developed as a transfer zone among the main n striking thrusts our model considers that both orogen parallel and orogen oblique fault systems should be regarded as parts of the same tectonic system accommodating crustal shortening of a thickened crust the study suggests that the tectonic control on the magma and fluid circulation in the crust is mainly related to the geometry of the fault planes and the orientation of the stress field with a previously unrecognized important role played by the orogen parallel thrust faults on the emplacement of the stratovolcanoes

a rb sr record of catchment weathering response to holocene climate change in inner mongolia variation in the rubidium to strontium rb sr ratio of the loess palaeosol sequences has been proposed to reflect the degree of pedogenesis and weathering in the northwestern region of china to characterize the rb sr ratio of the dissolved loads of a single catchment we analysed a 12 08 m sediment core from daihai lake in inner mongolia north china dating control was provided by 210pb 137cs and ams 14c sequential extraction experiments were conducted to investigate the concentrations of rb and sr on various chemical fractions in the lake sediments down core variation in the rb sr ratios provides a record of holocene weathering history from 9 to 3 5 ka bp accelerated chemical weathering was experienced throughout the daihai catchment under mainly warm and humid conditions and this reached a maximum at c 5 ka bp however weathering was reduced between c 8 25 and 7 90 ka bp which may reflect the global 8 2 ka cooling event after c 2 5 ka bp increased rb sr ratios with higher frequency of fluctuations indicate reduced weathering within the daihai catchment the highest rb sr ratios in the little ice age lake sediments indicate the weakest phase of holocene chemical weathering resulting from a marked reduction in sr flux into the basin the rb sr record also shows an enhancement of chemical weathering under today s climate but its intensity is less than that of the medieval warm period increased rb sr in lake sediment corresponding to reduced catchment weathering is in striking contrast to rb sr decrease in the glacial loess layers in the loess palaeosol sequence copyright 2005 john wiley sons ltd

the 2011 sikkim earthquake mw 6 9 of the himalaya estimates of earthquake source parameters and representability of attenuation characteristics abstract the empirical accelerograms of the september 18 2011 sikkim earthquake m w 6 9 have been used in the present study to estimate its source parameters and to identify a representative relation for the attenuation of peak ground acceleration with distance the spectra of high energy packets observed in the sh waves approximated by transverse components of accelerograms has been modeled using brune s model a two step procedure has been adopted for this purpose the estimated source parameters of the 2011 sikkim earthquake have been found to be as corner frequency f c 0 09 hz seismic moment m 0 2 43 0 5 10 26 dyne cm stress drop 50 4 9 28 bars and the moment magnitude m w 6 87 0 05 these estimated values have been found to be consistent with those of reported values obtained using different data sets and procedures the high frequency earthquake ground motions are governed by stress drop at the source therefore a value of stress drop estimated here is useful for the simulation of earthquake strong ground motions in the region the attenuation characteristics of observed peak ground acceleration as a function of distance have been compared with those of obtained using five empirical relations it has been found that a relation given by mcguire 1978 represents the attenuation characteristics of peak ground acceleration is relatively better this relation may be used for estimating the peak ground acceleration of similar magnitude for future earthquakes in the region this relation is also useful for the simulation of earthquake strong ground motions in the region using envelope technique the outcome of the present study is useful for the estimation of the seismic hazard of the sikkim region of the himalaya

recent slow normal and strike slip faulting in the pasto ventura region of the southern puna plateau nw argentina 1 recent normal and strike slip faulting on the puna plateau of nw argentina has been linked to lithospheric foundering gravitational spreading plate boundary forces and a decrease in crustal shortening from north to south however the timing kinematics and rate of extension remain poorly constrained we focus on the pasto ventura region nw argentina located on the southern puna plateau and recent deformation 1 ma field mapping and kinematic analysis across offset volcanic cinder cones show that the overall extension direction is subhorizontal is oriented ne sw to nne ssw and occurs at a slow time integrated rate of 0 02 to 0 08 mm yr since at least 0 8 0 5 ma a regional compilation from this study and existing data shows that recent extension across the puna plateau is subhorizontal but varies in azimuthal orientation dramatically data from the pasto ventura region are consistent with a number of models to explain normal and strike slip faulting on the puna plateau all of which likely influence the region some role for lower lithospheric foundering through dripping appears to be seen based on the regional extension directions and ages of mafic volcanism in the southern puna plateau

kinematic link between episodic trapdoor collapse of the negra muerta caldera and motion on the olacapato el toro fault zone southern central andes a combined geochronological and structural analysis of the miocene negra muerta caldera was designed to better understand caldera formation associated with prominent faults on the central andean plateau rb sr ages of the caldera outflow facies indicate that caldera formation occurred in two volcano tectonic episodes the first episode commenced with explosive eruption of the 9 0 0 1 ma andesitic acay ignimbrite followed by a period of volcanic quiescence and moderate tectonic activity dominant volcanic and tectonic activity occurred during the second episode which is bracketed by eruption of the 7 6 0 1 ma rhyolitic toba 1 ignimbrite and effusive discharge of the 7 3 0 1 ma rhyodacitic to andesitic lava flows structural relationships between rocks of the negra muerta volcanic complex and collapse induced normal faults notably ne striking normal faults agree with simultaneous volcanic activity and floor subsidence of the caldera during the second episode floor subsidence was achieved by tilting on an outward dipping reverse fault to the northwest of the caldera floor around a hinge zone located south of the caldera floor this induced horizontal extension of the caldera floor and was accomplished by fragmentation of and intrusion of dikes into the floor collapse induced and post collapse fault populations of the caldera do not differ significantly in the directions of their axes of maximum extension and are in this respect kinematically compatible with left lateral slip on the nearby olacapato el toro fault zone this furnishes evidence for a kinematic control by prominent faults on the formation of collapse calderas in the central andes the structural analysis of the negra muerta caldera shows that collapse calderas can serve as deformation markers that contribute in elucidating the regional kinematic regime and the time of activity of prominent dislocations genetically related to collapse calderas

postrift stress field inversion in the potiguar basin brazil implications for petroleum systems and evolution of the equatorial margin of south america rifting and related normal stress regime in the equatorial continental margin of brazil ceased during the late cretaceous when the stress regimes in eastern south america and west africa changed to induce strike slip or reverse motion in this study we explore the postrift tectonic geomorphic magmatic and sedimentary responses to stress changes using the potiguar basin the easternmost basin in the equatorial margin of brazil as a case study we use field and topographic data 2d seismic reflection lines vertical electric soundings and geochronological and borehole data to constrain the stress evolution of the potiguar basin from the late cretaceous to the quaternary discussing the role of basin inversion on sedimentation and landforms our results indicate the presence of two strike slip stress regimes after rifting the first stress field sf1 occurred from late cretaceous to the middle miocene and consisted of a n s oriented maximum subhorizontal compression and an e w oriented extension the second stress field sf2 took place from the middle miocene to the present day and included subhorizontal e w to nw se compression combined with n s and ne sw subhorizontal extension emplacement of volcanic rocks occurred along transtensional faults with a principal peak during sf1 at 20 30 ma and a subordinate peak during sf2 at 5 10 ma in response to shortening during sf2 a 70 km long and 50 km wide dome formed where marine miocene strata were uplifted to 250 m asl this uplift induced the displacement of alluvial channels away from the dome anticlines formed by transpression along the main ne sw striking faults during both sf1 and st2 acted as traps in the petroleum system similar shifts and stress field inversions documented in other areas of the brazilian continental margin are consistent with the neogene rise of the andes and may have implications for reconstructing the tectonic history of the equatorial atlantic margin of south america

tropical cyclone activity in the northwest pacific in relation to the el nino southern oscillation phenomenon abstract the interannual variations in tropical cyclone activity in the northwest pacific nwpac and their relationships with the el nino southern oscillation enso phenomenon were studied using the method of spectral analyses time series of a southern oscillation index soi defined as the sea level pressure difference between easter island and darwin and tropical cyclone activity in the entire nwpac ocean basin as well as in different regions of the nwpac were analyzed two spectral peaks are apparent in all these time series one corresponds to the generally accepted southern oscillation with a period of 3 to 3 5 years and another at the quasi biennial oscillation qbo frequency cross spectral analyses between the soi and tropical cyclone activity show significant coherence in these two spectral peaks the dominant peak is at the southern oscillation frequency with the soi leading typhoon activity by almost a year at the qbo frequency the two series are almost in phase cyclone activity in t

changes in the tectonic regime above a subduction zone of andean type the andes of peru and bolivia during the pliocene pleistocene this paper adresses the changes in the tectonic regime in the peruvian and bolivian andes that have occurred since the upper miocene when the present day elevation of the cordillera above sea level has been almost reached the stress patterns are deduced essentially from a field study of fault kinematics and a numerical inversion of the slip vector data measured on the fault planes the cuzco fault system in southern peru is chosen as an example to illustrate the methodology used in this region striations on both active and holocene faults are in agreement with a n s extension but faults affecting early pleistocene deposits exhibit two families of striations the younger results from the previous n s extension the older involving reverse motions results from either an e w or a n s compression faults affecting pliocene formations often show an oldest family of striations resulting from a ne sw or an e w trending extension thus three tectonic regimes are demonstrated which are also supported by regional unconformities and sedimentological data 1 a pliocene extensional regime 2 a lower pleistocene compressional regime and 3 a mid pleistocene present day extensional regime similar analyses conducted in the pacific and sub andean lowlands allow sketching of the successive pliocene pleistocene stress patterns in the central andes the quaternary and present day stress pattern is characterized by a n s extension in the high andes and in the pacific lowlands and by an e w compression in the sub andean lowlands and at the contact between the nazca and south american plates this stress pattern is interpreted at a large wavelength 100 km as an effect of compensated topography this model supposes that the vertical lithospheric stress zz increases with the topography the crustal thickness and the low density mantle beneath and that the lithospheric maximum compressional horizontal stress hmax trending e w roughly parallel to the convergence is fairly constant on both edges of the andes the tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then the e w trending hmax is 2 and hmin trending n s is 3 allowing extension to occur in this direction the pliocene stress pattern was characterized by a ne sw or an e w trending extension in the high andes in the pacific lowlands and possibly in the sub andean lowlands this stress pattern was clearly different from the present day one because the e w trending stress was hmin this required a weak push or eventually tractional boundary forces acting on the south american lithosphere it is suggested that this might result from a strong slab pull due to a long steeply dipping slab which decreased the value of the xx stress transmitted to the overriding plate the early pleistocene state of stress was compressional since the elevation of the andes had not markedly decreased during this period this required an increase of the e w trending stress value this resulted from a strong coupling between the two lithospheric plates possibly due to a rupture of a long slab under its own weight other spatial changes in the stress pattern are related to the particular situation of the forearc to the subduction of the buoyant nazca ridge and to the different dips of the slab extension in the high andes is of small magnitude of the order of 1 during the last 1 2 m y in a few basins it may have attained 40 during the pliocene 5 3 m y

extensional collapse of the northern taiwan mountain belt as an active collision zone between the luzon arc and the china continental margin the taiwan mountain belt particularly its south central part is undergoing crustal shortening and rapid uplift the northern part of the orogen is however subjected to crustal stretching and rifting as a result of flipping of subduction from the northwest facing luzon arc system to the south facing ryukyu arc system taiwan and its neighboring southern ryukyu used to be part of the rifted china continental margin before the collision started in late miocene time engineered by the northwest movement of the philippine sea plate in the past 5 m y the north south trending luzon arc obliquely overrode the northeast southwest trending continental margin and tectonized it into a collision orogen that progressively enlarged and migrated from northeast to southwest following the southwest propagating collision the north dipping philippine sea plate which was subducting beneath the ryukyu arc also extended westward and caused flipping of subduction in the northern part of the collision orogen in consequence the orogen lost the compressive support by the colliding plates and became subjected to lithospheric stretching induced by the trench suction the collision orogen collapsed as a result of crustal thinning and its central part foundered into a deep submerged basin a whole spectrum of active tectonic scenarios ranging from orogenic buildup by collision to orogenic collapse by rifting and subsidence can be readily observed in the taiwan ryukyu area and provides a vivid example for the process of orogenic collapse

late miocene early pliocene onset of n s extension along the southern margin of the central andean puna plateau evidence from magmatic geochronological and structural observations abstract extensional faulting typifies many of the earth s cenozoic orogenic plateaus the altiplano puna plateau in the southern central andes is the world s second largest orogenic plateau it contains a rich array of extensional fractures and normal faults with closely associated mafic volcanic flows dykes and cones this is in stark contrast to the foreland areas which record protracted shortening and continuous eastward growth of the mountain belt within the puna area a kinematic change over from shortening to extension has previously been inferred to have occurred during the quaternary however our research in the southern puna together with earlier petrologic investigations in the south central part of the plateau indicates an earlier onset for the extensional tectonism our analysis has focused on the eastern portion of the cordillera de san buenaventura in the southernmost part of the plateau where we mapped two upper miocene pliocene volcanic units associated with extensional faulting the aguada alumbrera ignimbrite and the la hoyada volcanic complex the structural inventory of these units records n s extensional deformation in the puna associated with the development of e w oriented fractures dykes normal and strike slip faults and folds new 40ar 39ar dating of magmatic rocks together with their relationships with the mapped structures reveal that widespread extension in the southern puna had already commenced by 5 ma and possibly as early as 7 ma although the extensional structures on the puna are typically associated with mafic volcanism the upper miocene pliocene la hoyada volcanic complex hosts dykes of intermediate compositions mafic to intermediate magmatism and extensional tectonism in this region is thus contemporaneous with and structurally akin to that in the central and northern parts of the plateau the ubiquitous cogenetic relationship between mafic volcanism and extensional tectonism both within the puna plateau and along its eastern flanks clearly indicates that the normal faulting is a regional phenomenon that is broadly coeval with an inferred rapid and large scale uplift of the altiplano puna plateau and a decrease in plate convergence rates these observations are compatible with a mantle delamination scenario in which the delamination of lithospheric mantle may have produced an ensuing weakening of the crust and regional uplift which may in turn have generated the conditions necessary to trigger gravitational extensional collapse beginning in the late miocene to early pliocene

analysis of geomorphic indices in the southern central andes 23 28 s evidence for pervasive quaternary to recent deformation in the puna plateau abstract satellite data based morphotectonic analyses are popular methods to identify active crustal deformation of large and partly remote regions such as the central andes for the southern central andes between 23 to 28 s the mountain front sinuosity smf index was estimated for 189 first order mountain fronts as well as valley shape vf ratios and basin symmetry t factors for 3366 second order tributary drainage basins the correlation between known fault ages and smf indices shows that the indices can be used as a proxy to infer the ages of faults for which radiometric ages do not exist vf ratios and smf indices point to pronounced quaternary to recent surface deformation on first order reverse faults delimiting mountain fronts by contrast analysis of the transverse topographic symmetry t factor which has been used to assess rotational components of drainage basins points to stochastic variations of erosion possibly due to local lithological heterogeneity rather than to a tectonic influence on basin geometry we also examined 32 mainly nne ssw trending first order basin margins for tilt of alluvial fan deposits and lateral stream migration the symmetry of first order basins in the southern central andes seems to be strongly controlled by quaternary deformation on bivergent first order reverse or thrust faults quaternary deformation of the upper crust in the puna plateau is apparently more pervasive than previously appreciated and has important ramifications for the neogene to recent morphotectonic evolution of the southern central andes

the role of inversion tectonics in the structure of the cordillera oriental nw argentinean andes the salta basin of cretaceous age in the andes of nw argentina was inverted during the development of the cordillera oriental between the neogene and recent the different orientations of the extensional faults of the salta rift system and the subsequent w e shortening resulted in a great variety of inversion structures extensional faults dipping ne to ese were preferentially reactivated as contractional faults the initial stepped geometry of these faults gave rise to a distinctive sigmoidal pattern of basement involved thrusts and related folds these contractional structures tended to run parallel to the extensional faults and acquired a n s trend across the previous accommodation zones resulting in folding interference patterns other interference patterns are attributed to the superposition of contractional folds on previous rollovers and to the constrictional deformation imposed by the reactivation of adjacent extensional faults with different orientations the nnw to wsw dipping extensional faults show no evidence of reactivation despite the fact that some of them are folded the inversion at crustal scale is limited owing to the presence of the cretaceous salta rift across the different andean structural units and foreland

a plea for more caution in fault slip analysis abstract the analysis of fault slip data is a widely used tool in structural geology this method consists of three major steps a data acquisition by measurements in outcrops b data separation into homogeneous subsets and calculation of the principal stress or strain axes with one of several available methods and c data interpretation in a regional tectonic context which includes the assignment of ages and the derivation of regional patterns each one of these three steps may introduce artefacts and misinterpretations the major problem during step a is sampling bias due to unfavourable outcrop geometry or due to neglecting faults with subtle slip sense indicators step b contains numerous possibilities to influence the calculation results e g by arbitrary separation of data into subsets choice of the analysis method and uncritical use of software age assignment in step c can in most cases be made only indirectly e g by the assumption that measured outcrop scale faults have the same age as parallel map scale faults or by the inference that subsets from different sites with parallel oriented principal axes are syngenetic neither assumption is necessarily valid with this paper we want to sharpen researchers awareness of the problems related to fault slip analysis

strengthening of paleo typhoon and autumn rainfall in taiwan corresponding to the southern oscillation at late holocene the purpose of this study is to improve prehistoric knowledge of paleo typhoon and winter monsoon activity in the western north pacific after 8 cal ka bp we examine two sediment cores derived from lakes located to the south and north of the ilan plain in northeastern taiwan seasonal and typhoon rainfall data over the past several decades are collected for this study region we also use mineral indicators k rb ratios and grain size to estimate sediment sources intensity of chemical weathering and rainfall intensity the results show that the chemical weathering index k rb ratio of meihua lake indicates the continuing effects of autumn rainfall while coarse grain sizes of dahu lake sediments suggest increased intensity in typhoon rainfall grain size results reveal that increasing el nino southern oscillation intensity in the tropical pacific has intensified the activity of super typhoons in the western north pacific finally we find that frequent activity of super paleo typhoons occurred in taiwan during the la nina like state and the strengthening winter monsoon activity and higher sea surface temperature enhanced plentiful autumn rainfall in el nino like state after comparing our results with the paleo typhoon data in japan and south china we suggest that the tracks of super typhoon are mainly affected by variations in the warm pool and subtropical northwestern pacific high in the western pacific copyright 2012 john wiley sons ltd

basement involved blind thrusting in the eastern cordillera oriental southern bolivia evidence from cross sectional balancing gravimetric and magnetotelluric data abstract a neogene to recent thrust belt forms the subandean ranges and cordillera oriental along the eastern slope of the central andes in bolivia over much of its length there is an area of intermediate topographic height and structural level between the subandean zone and the cordillera oriental proper cross sectional balancing suggests that in southern bolivia the basal detachment of this tightly folded zone links up with a blind thrust that dips westward beneath the high standing eastern margin of the cordillera oriental and rides piggy back on a large buried thrust sheet which forms the backstop for the thin skinned subandean belt magnetotelluric and gravimetric data indicate that this thrust sheet is made up of basement rocks probably including intrusives and higher grade metamorphics such blind basement thrusts in a relatively external position may characterize much of the eastern andean thrust belt and their geometry is possibly related to preexisting structural patterns linked to the configuration of early palaeozoic basins and their margins

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

kinematics of tertiary to quaternary intracontinental deformation of upper crust in the eastern cordillera southern central andes nw argentina 1 to elucidate factors controlling the geometry of and kinematics associated with prominent upper crustal structures in the eastern cordillera of nw argentina structural analyses of key areas were complemented by fault slip analysis remote sensing and 3d representation of prominent faults the analyses revealed that deformation during tertiary to quaternary times was accomplished mostly by prominent orogen parallel reverse faults the geometry of which was significantly influenced by paleozoic and cretaceous planar structures locally this deformation was preceded by kilometer scale doming of upper crustal rocks analysis of 767 brittle faults at 67 stations in the studied areas indicates that local upper crustal doming and orogen parallel reverse faults formed chiefly under nw se and e w shortening shortening directions inferred from fault slip data portray the kinematics of first order faults and folds more specifically shortening directions are mostly uniform with respect to first order structures age and kinematics of deformation inferred from brittle fault analysis in the study areas is consistent with equivalent data compiled from other parts of the eastern cordillera puna and pampean ranges collectively the data is at variance with hypotheses relating late tertiary to quaternary deformation in the eastern cordillera to plate boundary kinematics the kinematics of intracontinental deformation in the southern central andes points to deformation partitioning of upper crust as a consequence of bulk e w shortening we therefore caution the usage of brittle fault kinematics in continental interiors as an indication for plate kinematic changes

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

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

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

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

late quaternary mass wasting records and formation of fan terraces in the chen yeo lan and lao nung catchments central southern taiwan abstract development of fan terraces in incision dominated river settings documents episodic increases of sediment supply from upland catchments in the non glaciated active mountains of taiwan such increases in sediment supply result typically from increases in landslide debris flow activity studying fan terraces therefore helps understand the history and nature of mass wasting processes eroding the mountains this study explores fan terrace successions developed at tributary mouths of the chen yeo lan and lao nung rivers in central southern taiwan judging from the spatial variations of sizes heights sequences and sedimentation of the fan terraces the magnitude and frequency of mass wasting activities appear to vary from tributary catchment to catchment two types of tributary catchment are differentiated which can be located in close proximity to each other one has been subject to extremely large but infrequent mass wasting events as a result of which high and or large fan terraces with 100 200 m thick debris flow fluvial gravels have been formed seven examples of this type of fan terrace have yielded radiocarbon dates of 40 ka 18 4 ka 13 9 ka 8 9 9 5 ka 3 9 4 8 ka 3 3 ka and 1 4 ka in age with the availability of multiple dates this type of fan terrace can be shown to have been created in no more than 1000 years from the aggradation of thick gravel sequences to subsequent incision the second type of catchment has been subject to relatively minor but frequent mass wasting events due to repeated incision after each such event the resultant fan terraces are relatively low and limited in size but have formed over lengthy periods although located in the same geological and geomorphic setting the dated fan forming mass wasting events in the chen yeo lan catchment do not appear to have occurred synchronously with those in the lao nung catchment nor could the succession of these events or their origins be clearly linked to known paleo climate data these findings highlight the importance of internal factors e g rock mass strength in determining the timing magnitude and frequency of mass wasting events in the studied mountains which are subject to active rock uplift highly active seismicity and intense rainfall

use of geostatistics for accurate mapping of earthquake ground motion summary the geostatistical method of kriging is applied for the analysis estimation and mapping of earthquake ground motion no modification to this method was required for this new application along with kriging the concept of the semi variogram is introduced and shown to be important for estimation and mapping in geostatistical analyses of earthquake ground motion the ground motion is modelled by a random function this random function model is shown to be accurate for providing estimates of earthquake ground motion at unsampled locations kriging is applied in this paper for the mapping of peak horizontal acceleration recorded during the 1971 san fernando and the 1983 coalinga california earthquakes these results are compared to maps predicted using attenuation expressions and differences are analysed results demonstrate the utility of kriging for the analysis of earthquake ground motion

sedimentary aspects and paleoenvironmental evolution of a rift basin salta group cretaceous paleogene northwestern argentina the rift history of the salta basin is related to the evolution of the central andes and to the activity of the pacific margin owing to its geographic location sedimentation occurred from the neocomian to the paleogene with deposits reaching up to 5 000 m in thickness paleoenvironmental analysis reveals an evolutionary history controlled by tectonic and climatic changes isolated grabens characterized the early synrift stage differential subsidence provoked distinct environments in the southern and northern subbasins in the southern subbasins alluvial fan fluvial fan and lacustrine deposits prevail whilst in the northern subbasins eolian and fluvial environments dominate during the maastrichtian two major factors controlled the basin fill the decrease in tectonic subsidence and a relative sea level rise as recorded in south america an extensive and shallow atlantic marine ingression installed a carbonate system coincident with mainly humid conditions until the danian until the middle eocene the fluvial and lacustrine environmental evolution of the sag basin was controlled especially by the alternation of temperate with dry and humid periods paleontological records reflect these climatic changes and show their relationship to the sedimentation regime

tectonic evolution of the western margin of gondwana inferred from syntectonic emplacement of paleozoic granitoid plutons in northwest argentina abstract the cerro durazno brealito and cobres granitoid plutons belong to the eastern ordovician magmatic belt of northwest argentina and are associated with a network of nw and n striking ductile shear zones a structural analysis of these plutons and their metamorphic host rocks was conducted to elucidate mid to upper crustal magmatism and tectonism at the western margin of gondwana pluton emplacement took place at 480 455 ma toward the end of a ca 75 ma long episode of regional deformation and high temperature metamorphism the cerro durazno and brealito plutons were probably emplaced as ne sw striking vertical magma sheets that coalesced into large magma bodies under overall e w shortening that generated sw dipping metamorphic foliations at a later stage of pluton formation these fabrics were superimposed to the magmatic structures in the granite plutons and to the high grade metamorphic structures of the host rocks at the northern pluton margins ductile high strain zones notably the agua

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

tectonic development southeastern border of the puna plateau northwestern argentine andes the puna altiplano a 500 000 km2 area at an average elevation of 3 7 km in the central andes occurs in a non collisional plate setting over a 30 east dipping segment of the benioff zone and is bounded to the north and south by nearly flat segments of the subducted nazca plate a 120 km long portion of the southeastern boundary of the puna in northwestern argentina was studied to gain insight into the structures associated with uplift of the plateau and into the nature of structures over the transition from flat to steeper subduction the structural geometry and kinematic evolution of the region was determined from geologic mapping and kinematic analysis of minor fault arrays the present southeastern margin of the puna was uplifted between 5 10 and 2 ma northwest southeast horizontal shortening and vertical extension during uplift were produced by northeast striking en echelon thrust faults dipping 17 55 northwest four to seven en echelon structural blocks each 20 30 km long constitute the border of the puna their structural geometry precludes large scale overthrusting as an important mechanism of plateau uplift along the studied segment in the past 2 m y there has been a 45 change in the direction of shortening and locally a 90 change in the extension direction indicating that the structures accompanying uplift are no longer active this change could be due to a change either in the geometry of the subducted plate or in convergence directions although neither of these hypotheses is testable given presently available data