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

the dynamics of motion of the south american plate observations of the intraplate stress field as compiled and evaluated in the context of the world stress map project can be compared with stress modelling results to increase our understanding of the dynamics of motion of the lithospheric plates in this paper we investigate the driving and resistive plate tectonic forces acting on the south american plate in order to establish a detailed basis for future stress modelling our basic assumption is that the plate is in mechanical equilibrium that is the net torque of the forces acting on the plate vanishes the complexity of the forces exerted along the margins of the south american plate requires the dynamical analysis to be performed step wise incorporating the forces which are expected to be of major importance a first order model is defined the effect of additional plate boundary forces is subsequently examined in terms of superpositions onto the first order model with the first order model the relation between three resistive forces and shear stress on the base of the lithosphere is derived the ridge push force driving the motion of the south american plate can be quantified independently and is included in the calculations as a known quantity solutions are obtained using four different determinations of the euler vector of absolute motion of the south american plate which is taken to define the orientation of the basal shear stress the results demonstrate that the concept of a basal shear stress actively driving plate motion cannot be discarded on the basis of a dynamical analysis further with the assumption that basal drag is entirely concentrated below the continental pan of the lithosphere estimates of the maximum amount of continental basal shear resistance are derived evaluation of an alternative model for the south american plate excluding the northernmost pan of the andes demonstrates the significance of the resistive force exerted on the south american plate due to convergence with the caribbean plate it is to be expected that comparison of theoretical distributions of intraplate stress computed on the basis of the present dynamical analysis with observations of the actual state of stress will further constrain the range of feasible force models

change from late tertiary compression to quaternary extension in southern tibet during the india asia collision field studies in southern tibet show that subsequent to folding resulting from the india asia collision 50 40 ma reverse faults and backthrusts formed as a result of nne ssw and nnw sse trending compressions they affect conglomerates of oligocene miocene age and are probably contemporaneous with shearing on the himalayan main central thrust and folding of paleogene basins along the northern border of southern tibet 25 16 ma rift systems postdate compressional structures they were active during the pleistocene and probably during the pliocene 5 2 ma change from a compressional to an extensional regime in tibet probably occurred during the late miocene 10 5 ma this change is tentatively correlated with uplift inferred by chinese workers from paleogene to present day variations of the tibetan vegetation recent and active faulting in the high plateau results from a wnw ese striking extension this is in agreement with tibetan focal mechanisms the himalayan lowlands are subjected to a nne ssw trending compression as demonstrated by superficial deformations and seismicity therefore the horizontal maximum compressive stress trajectory is roughly parallel to india asia convergence hmax being 1 in the lowlands and 2 in the high plateau this change results from the increase of the vertical principal stress value due to the weight of the elevated mass zz is 3 in the lowlands and 1 in the high plateau thus the third principal stress axis is 2 in the lowlands and 3 in the high plateau allowing extension to occur in the wnw ese direction extensional tectonics are significantly different in southern and northern tibet southern tibet being constrained between the himalayan syntaxes the e w extensional principal deviatoric stress value 3 is of rather small magnitude and the n s trending 2 value is extensional as demonstrated by analysis of recent faults normal components prevail along all the azimuths in the field this favors formation of rift systems of large axial extension on the contrary north of the himalayan syntaxes northern tibet flows easily eastward the e w extensional principal deviatoric stress value 3 is of higher magnitude and the n s stress 2 is compressional as shown by inversion of focal mechanism data this favors strike slip components on faults which deviate from the n s direction as expressed in the field by shorter n s trending graben linked with normal strike slip faults these faults permit some n s shortening in northern tibet without thickening in response to india asia collision in high plateaus which are subjected to lateral compression such as the andean altiplano and tibet the extensional direction is basically controlled by the convergence direction being roughly orthogonal to the latter

forces controlling the present day state of stress in the andes the present day state of stress in the andes is expected to be controlled primarily by two different types of forces 1 the resistive force exerted on the western overriding margin of the south american plate and 2 forces that arise from the thickened crust of the andes i e the effect of topography and its compensating crustal root we have studied these forces on the basis of a model for the dynamics of the entire south american plate in which the andes are embedded in this model a given set of forces is constrained by the criterion that the net torque on the plate should vanish a thin elastic shell representation is used to calculate the intra plate stress field associated with the various force distributions we define a reference model for the present study that incorporates a uniform magnitude for the resistance associated with convergence along the western plate margin fpcr plate contact resistance and does not include the effects of topography subsequently we investigate the effect of lateral variation in the magnitude of fpcr and add the topography related forces the main results are 1 a uniform magnitude of fpcr leads to a better match with the observations than a magnitude that is a function of the dip of the lithosphere subducting below the western plate margin 2 the amount of horizontal compression across the andes found in the case that ridge push is considered to be the only force driving the south american plate is small compared to the value required to sustain the andes

ridge torques and continental collision in the indian australian plate the excess gravitational potential energy of a collisional orogen that may be supported by focusing distributed ridge torques in a plate depends on the ratio l of the effective lengths of ridge systems and collisional fronts in the indian australian plate the net torque due to the asymmetric distribution of mid ocean ridges and young oceanic lithosphere along the southern boundary is 8 5 10 25 n m and is capable of supporting an excess potential energy of the himalayan collision zone of up to 7 5 10 12 j m 2 the concept that collision in the indian australian plate may be sustained by ridge torques suggests that collisional driving stresses may be sensitive to l in the indian australian plate l changed significantly during progressive collision from 55 to 45 ma during amalgamation of the indian and australian plates at 45 ma and during the development of the new guinea orogen through the late oligiocene and miocene such changes in l may help explain the termination of spreading between india and australia following the onset of the himalayan collision and the early miocene normal faulting in the high himalaya

mountain building in the central andes the central andes is the middle part of the andean chain between about 13 s and 27 s characterized by the parallel running high mountain chains the western and eastern cordilleras at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km the altiplano puna from the examination of geophysical and geological data in this area including earthquakes deformation gravity anomaly volcanism uplift history and plate motion we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the central andes we propose that the present cycle of mountain building has continued in the cenozoic with the most active phase since the miocene and that the present subduction angle 30 is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the miocene because of the young and buoyant slab involved this situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab addition of volcanic materials was not restricted to the western edge where active volcanoes of the western cordillera exist but extended to the western and central portion of the altiplano puna the western half of the central andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there in the eastern edge the thermal effect is small and the crust is strongly pushed by the westward moving south american plate this caused the shortening of crustal blocks due to reverse faulting and folding in the eastern cordillera and amazonian foreland the magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west these two processes are superposed between the two mountain chains and form high plateaus there the altiplano of bolivia and peru and the puna of argentina this interpretation is supported by the observation that 1 neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the altiplano and the western cordillera 2 no significant reverse fault systems are observed on the altiplano 3 neogene volcanic rocks and volcanic centers since the miocene are not restricted to the western cordillera but are widely distributed over most of the altiplano 4 most of the altiplano is in a zone of high heat flow values 5 thick paleozoic rocks are strongly folded and faulted in the eastern cordillera with little volcanism and no large scale plutonism in the cenozoic age 6 crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the eastern cordillera and amazonian foreland and 7 the crustal thickness suddenly decreases at the junction of the eastern cordillera and the amazon basin exactly at the place of reverse earthquakes

effect of the altiplano puna plateau south america on the regional intraplate stresses abstract a previous compilation of stress data for the sub andean region of northwest argentina is complemented with eight additional earthquake focal mechanisms seven events 5 3 5 5 mb had their mechanisms determined by inversion of teleseismic body waves short period p waves and long period sh waves all events had predominantly reverse faulting mechanisms five had almost pure dip slip motion and two had a component of oblique motion the other mechanism was a composite fault plane solution of the hualfin catamarca series of july 1986 maximum 4 mb determined with data from a local temporary network and the permanent inpres regional network the local data shows that the hualfin events occurred on a single fault plane oriented nw se with predominantly strike slip motion the stress data in the sub andean region suggests a radial pattern around the altiplano puna plateau with compressional stresses tending to be perpendicular to the eastern front of the plateau this indicates that local spreading stresses due to lateral density variations crustal thickening or lithospheric thinning of the plateau may be comparable or even exceed the plate wide regional stresses at the southeastern border of the puna plateau where a stress rotation of about 30 may be detected preliminary order of magnitude calculations indicate that the local plateau induced compression can be about 1 2 times greater than the regional horizontal deviatoric stress the present predominance of the local stress over the regional stress is consistent with the decrease of the nazca south america convergence rate observed since about 10 20 ma

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

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

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

the mechanical state of the lithosphere in the altiplano puna segment of the andes abstract information about topography the shape of the geoid seismicity neogene deformation and volcanism in the region of altiplano puna of western south america is used to analyse the state of stress across the convergent plate margin in terms of the effects of topography and simple models of its compensation an average elevation near 4 km is consistent with compensation by a yet unresolved combination of crustal root and hot uppermost mantle producing a geoid high of 22 27 meters average horizontal compressive stress in excess of a reference sea level lithostatic value of 390 bars in a 150 km thick lithosphere and an average shear stress of 170 bars along a 30 dipping interplate boundary the basis for these estimates is evidence for a neutral to extensional stress regime within the high plateau contrasted with a compressional regime on the eastern slopes and along the interplate boundary itself comparison with other plateaus in a convergent plate tectonic setting suggests an evolutionary sequence from compressional to extensional tectonics as elevation of the plateau increases

analogies to oceanic behaviour in the continental breakup of the western woodlark basin the woodlark basin d entrecasteaux island region off northeast papua new guinea fig 1 offers a rare glimpse of the propagation of active rifting and continental breakup1 into orogenically thickened and subsequently extended lithosphere2 10 rifting continental breakup and the subsequent formation of oceanic lithosphere by sea floor spreading often involve rupture controlled by propagation of a rift tip11 20 we recognize several tectonic elements in the evolving western woodlark intracontinental system that are close geometric analogues of wholly oceanic propagators and a kinematic development that can be viewed as an extrapolation of oceanic tectonics the response to deformational stresses in the final stages of breakup in a thick relatively hot and hence weak continental lithosphere appears akin to that of oceanic lithosphere

possible orogeny parallel lower crustal flow and thickening in the central andes the relatively low elevation and thick crust in the altiplano in comparison to the higher elevation but thinner crust in the puna plateau together with geophysical data suggests that isostatic equillibrium is achieved by cooler and denser lithospheric mantle in the altiplano excess density in the altiplano mantle could create differential horizontal stress in the order of 25 mpa between both lithospheric columns numerical models accounting for pressure and temperature dependent rheology show that such stress can induce horizontal ductile flow in the lower crust from the puna towards the altiplano with a minimum viscosity of 1019 pa s this flow reaches 1 cm year displacing more than 50 km of material within 5 ma if the lower crust viscosity is smaller the amount of orogeny parallel lower crustal flow can be even greater such a mechanism of channel flow may explain that different amounts of crustal material have been accommodated by shortening in the altiplano and in the puna because of the strength of the elastic brittle upper crust this channel flow does not necessitate large amounts of surface deformation except vertical uplift making it difficult to detect from the geology

upper mantle s velocity structure of central and western south america we have used broadband seismograms recorded at portable and permanent seismic stations in south america to image the three dimensional s velocity structure beneath central and western south america we have interactively fitted the waveforms of s and surface wave trains in 500 seismograms from 90 earthquakes and 60 different stations the constraints on upper mantle s velocity structure provided by the waveform fits have been combined with independent estimates of depth to the mohorovicic discontinuity yielding model sa99 the s velocity structure of sa99 reveals a highly heterogeneous upper mantle with a maximum peak to peak difference of 1 km s above 100 km the heterogeneity seems to be correlated with tectonic and geologic domains sa99 shows high lithospheric s velocities for the precambrian cratons the amazon basin is also underlain by high s velocities while three other intraplatform basins are underlain by low s velocities at various depths sa99 shows extremely low velocities in the mantle wedge beneath the volcanically active andes we explain these low s velocities as originating from realistic amounts of volatiles released by the subducting nazca crust and resulting amounts of partial melt within the peridotitic mantle wedge without requiring elevated temperatures while sa99 could be consistent with diffuse influx for corner flow in the mantle wedge it does not support models that require influx of subcontinental upper mantle into the mantle wedge through a laterally uniform low viscosity channel

accurate source depths and focal mechanisms of shallow earthquakes in western south america and in the new hebrides island arc synthetic seismograms are matched to long period p waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms accurate depths are obtained for a large sample of moderate sized 6 ms 7 events which produced relatively simple p waveforms and do not require intensive analysis the estimated error in the determination of depth is about 5 km or less for most of the events considered the procedure is applied to two suites of events one in south america 83 events and one in the new hebrides 61 events in these two areas of contrasting styles of subduction the accurate depths provide new information on the bending of descending plates near trenches the seismicity and tectonics of the upper plate and the geometry and structure of the zone of contact between the upper and descending plates depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with chapple and forsyth s 1979 preferred model of a bending elastic perfectly plastic plate however an unusually deep event occurred beneath the fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late miocene time the calculated depth of the event 48 km places it below the lithosphere asthenosphere boundary derived from thermal models vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin 10 km thick curved zone of contact dipping arcward or landward from the trench axis however a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact these events would be accommodated by an interplate zone 15 km thick in both subduction zones interplate events occur at depths between about 15 and 50 km however the downdip width of the south american interplate zone is 2 to 3 times larger than that in the new hebrides most of the intraplate events near the interplate boundary are located in the descending plate in the upper plate seismicity is more pronounced in the back arc regions of the two subduction zones thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc depths of shallow earthquakes in the eastern andes and sub andean regions of south america range between about 9 and 36 km indicating that both the upper and middle crust are seismically active about 75 of the events occurred in the upper 25 km furthermore the focal mechanisms indicate that from ecuador to argentina the crust of the eastern side of the andean cordilleran is in a state of east west compression oriented nearly parallel to the direction of convergence between the nazca and south american plates the depths the orientations of the earthquake nodal planes and the close relationships to quaternary tectonics imply basement involved crustal shortening in much of the eastern andes and sub andean zones of peru and argentina

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

stress modeling in the andes constraints on the south american intraplate stress magnitudes regions of high topography which exhibit both compressional and extensional deformation provide a natural laboratory to place constraints on horizontal intraplate stress magnitudes an elastic finite element analysis of the lithospheric stress field in the cordillera bianca region of peru has been used to evaluate the lithospheric stress state and constrain the south american intraplate stress magnitudes the orthogonal orientation of the andean high topography to the east west regional stress field in conjunction with the well documented change in deformation style from compression to extension across the cordillera provides a unique setting for the stress modeling a two dimensional finite element grid consisting of an assembly of isotropic elastic quadrilateral elements in a state of plane strain was used to represent the lithosphere an isostatically balanced crustal density model was used to calculate the stresses associated with the lateral density variations between the andean topography and the brazilian shield in some models horizontal stress was concentrated at midcrustal depths by including vertical variation in young s modulus the modeling results indicate bounds of 10 and 75 mpa for the magnitude of the average horizontal stress averaged over a 100 km thick lithosphere a preferred value of 25 mpa is obtained on the basis of fitting all of the data for the rheological models considered and predicting a transition from extension to compression near the 3000 m elevation contour such a far field average stress is consistent with a ridge push origin because the modeling performed in this study assumes that the topographic load is entirely compensated by the buoyancy of the deflected moho these estimates represent upper bounds the effect of incorporating thermal support for the topographic load of the cordillera bianca would be to further reduce our estimate of the intraplate stress magnitudes analysis of the distribution of calculated stress with depth indicates that previous assumptions about the vertical plane shear stress within the lithosphere being negligible are acceptable for the geometry of the andes

do the billecocha normal faults ecuador reveal extension due to lithospheric body forces in the northern andes active compressional and transcurrent structures are widely reported for the entire northern andes whereas there is no clear evidence of extensional tectonics in that region in contrast both active compressional and extensional structures occur extensively in the central andes extensional tectonism in the central andes occurs mainly in high plateaus of large wavelength 100 km and is interpreted to have resulted from the effect of compensated high topography at the lithospheric scale the observation of holocene normal faults on the low wavelength 15 km billecocha high plateau in the northern ecuadorian andes raises a problem could the lithospheric body forces be the cause of the normal faulting in a chain with a mean altitude of 2800 m compared to the central andes where normal faulting is widely distributed across the chain normal faulting on the billecocha plateau is concentrated in a restricted 4 km wide zone in addition normal faulting does not extend further than a few kilometres eastward where compressive structures have been observed considering the local character of the extension body forces at the lithospheric scale cannot be responsible for this normal faulting and thus we can preclude a significant lowering of the magnitude of hmax in the northern andes with respect to the central andes analysis of these normal faults shows that normal faulting occurred between 10 000 yr and 6000 yr b p in addition the relationship between the geometrical parameters vertical throw and rupture length of the major normal fault differs significantly from that given empirically by methods based on scale laws in such a case the billecocha normal faults are unlikely to have a tectonic origin various local processes e g gravity gliding sackung that may be possible are contemplated for triggering the normal faulting in addition a theoretical simple 2 d model is proposed in where local conditions peculiar to the plateau structure glaciation and the surrounding areas are taken into account in this model normal faulting occurs by shearing along the planes of weakness bedding planes in response to the elastic rebound of the fluid saturated plateau which prevailed after the last glaciation

extension related to a high topography results from a microearthquake survey in the andes of peru and tectonic implications summary a microearthquake study performed in 1985 october november in the cordillera blanca western cordillera of northern peru allowed us to make a precise mapping of its shallow seismicity 160 local events are accurately located in the vicinity of an active normal fault system showing a vertical displacement of 4500 m over the last 7 myr part of the seismic activity can be related to this west dipping system but a great number of events indicate a new seismic zone situated under the highest peaks of the chain by using an inversion algorithm of p wave polarities we show a regional deviatoric stress tensor which defines an extensional axis in a direction approximately orthogonal to the andean structures this result is in good agreement with the theoretical distribution of stresses expected in an elongated high topography bordered by tectonic convergent zones moreover we suggest that an absolute uplift of this young batholith occurs on a normal fault system and that this phenomenon is partly due to buoyancy forces originating in its crustal root

thermal control on post orogenic extension in collision belts abstract intracontinental extension develops in two different ways in the rhine graben the baikal or the east african rifts it is localized in a long and narrow zone along a few normal faults which often delineate a single graben in contrast in the basin and range province or the tibetan plateau it is distributed over a much broader area up to more than 1000 km across this difference arises from differences in rheology and applied forces widespread extension probably reflects the thinning of a previously thickened crust geological observations indicate that only wide orogenic belts undergo late or post orogenic thinning a related fact is that anatectic granites seem to be most abundant in wide orogenic belts the general correlation between belt width and occurrence of both extension and anatectic granites suggest that it reflects in part the characteristics of horizontal heat transfer between thickened crust and surroundings in wide orogenic plateaux the large amount of heat produced in the crust by radioactive decay and lack of erosion leads to high temperatures in narrow mountain belts horizontal heat transfer and high erosion rates concur to prevent temperatures from rising to values at which the crust may yield in extension we have tested this hypothesis using transient two dimensional numerical models of orogenic belts focussing on the effects of the shortening rate the final width of the belt and the rate of crustal radiogenic heat production in the range of geological values the shortening rate has little influence on the results crustal temperatures increase as a function of belt width we find that the sensitivity of crustal temperatures to belt width is most pronounced in the 100 300 km width range and for large values of crustal heat generation the pressure temperature paths followed by geological formations depend on the belt width which is consistent with available data the integrated shear strength of the lithosphere decreases with time as the belt is being built and then eroded this evolution is sensitive to belt width if extension occurs when the integrated shear strength drops below a critical value our results yield a framework for comparing different orogenic belts we predict that extension starts late in the history of wide belts and never occurs in narrow belts a final implication of this study is that the average rate of crustal heat production is probably close to 1 1 10 6 w m 3 which is in the upper range of possible values we present a simple and convenient diagram summarizing the thermal characteristics of orogenic belts as a function of width and shortening rate

lithospheric stress and uppermantle dynamics in mainland china due to mantle flow based on combination of global and regional scale seismic tomography abstract in order to explore the importance of mantle flow to lithospheric stress field in mainland china seismic tomography based mantle flow models are used to predict the most compressive principal horizontal stress directions mcphsds considered that regional scale seismic tomography models have higher horizontal resolution to map the mantle structure while global scale models can present the information out of the imaging domains of regional scale models although this information has relatively poor horizontal resolution the combined global and regional scale seismic tomography based mantle flow models hereafter called combined models are mainly used in this paper after the comparison of the observed and our predicted mcphsds it is suggested that 1 a combined model compared with a only global scale seismic tomography based model could improve greatly the predictions in some regions of mainland china such as sichuan yunnan south china and north china blocks 2 the mantle flow model driven by both plate motions and mantle density heterogeneity hereafter called plate density driven model compared with the flow model driven only by mantle density heterogeneity hereafter called density driven model has much better predictions in the eastern china 3 the presence of density variations above 250 km could better dramatically the predictions in the eastern china and 4 sublithospheric mantle flow causes the lithosphere under compression in mainland china and plays an important role in forming the lithospheric stress in alashan qaidam western tibetan and eastern tarim blocks as well as the east of the eastern china

arc stresses determined by slabs implications for mechanisms of back arc spreading there is an anti correlation in stress field between the back arc area and the shallow portion of the slab generally slabs showing down dip compression have tensional back arcs and the reverse we explain this anti correlation by a force balance in the fore arc wedge between the slab pull collision force in the upper plate and the ridge push back arc spreading can occur as a passive response to the down dip compressional slab the mariana kyushu and aegean arcs are the exceptions to the anti correlation in these arcs the back arc is tensional although the slab is in down dip tension this may be because the fore arc is driven by the mantle drag toward the trench resulting in compression balanced with the slab pull the flow in the mantle would cause back arc spreading in this case

a pliocene quaternary compressional basin in the interandean depression central ecuador summary the segment of the interandean depression of ecuador between ambato and quito is characterized by an uppermost pliocene quaternary basin which is located between two n s trending reverse basement faults the victoria fault to the west and the pisayambo fault to the east the clear evidence of e w shortening for the early pleistocene between 1 85 and 1 21 ma favours a compressional basin interpretation the morphology river deviations landslides folded and flexure structures demonstrates continuous shortening during the late quaternary the late pliocene quaternary shortening reached 3400 f 600 m with a rate of 1 4 f 0 3 mm yr the e w shortening is kinematically consistent with the current right lateral reverse motion along the ne sw trending pallatanga fault the quito ambato zone appears to act as a n s restraining bend in a system of large right lateral strike slip faults the compressive deformation which affects the interandean depression during the pliocene is apparently coeval to the beginning subduction of very young oceanic lithosphere north of the gulf of guayaquil the relatively buoyant new crust may have significantly increased the mechanical coupling in the subduction zone from pliocene to present

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

space time variations of stresses in the miocene quaternary along the calama olacapato el toro fault zone central andes abstract we describe the temporal and spatial changes in the tectonic state of stress occurred during the miocene quaternary in the trans orogen area of the calama olacapato el toro cot fault zone in the central andes at about 24 s within the northern portion of the puna plateau this work sheds new light on the complexity of stress pattern distribution in general and contributes to the analysis of the relationships between tectonics and volcanism and of the seismic hazard of the area field geological surveys along with kinematic analysis and numerical inversion of 140 new fault slip measurements have revealed that this portion of the cot zone previously considered a continuous long lived lineament in reality has been subjected to three different kinematic regimes 1 a miocene transpressional phase with the maximum principal stress 1 chiefly trending nne ssw 2 an extensional phase that started by 9 ma with a horizontal nw se trending minimum principal stress 3 and 3 a left lateral strike slip phase with a horizontal e w 1 and n s 3 dating to the late pliocene quaternary which decreases toward the westernmost part of the studied zone where it transitions to extension producing a n s trending graben structure hence even if transcurrence is still active in the eastern portion of the cot as focal mechanisms of crustal earthquakes indicate our study demonstrates that extension is becoming the predominant structural style of deformation at least in the western region the major changes in the tectonic regimes are attributed to changes in the magnitude of the boundary forces due to subduction processes whereas gravitational effect of a thickened crust might be responsible for the overall orogen perpendicular extension

extensional compressional tectonics associated with the aegean arc comparison with the andean cordillera of south peru north bolivia extensional compressional tectonics are analysed along the convergent boundaries of the aegean arc and of the andean cordillera of south peru and north bolivia this paper deals only with deformations of miocene pliocene and pleistocene ages the prevailing regime in the aegean basin is extensional yet two compressional tectonic phases of u miocene l pliocene and of l pleistocene age have been recorded they are also known in many places all around the mediterranean thus forces that induce compressional tectonics in the aegean seem to have their origin outside the region they seem to be linked with changes in the conditions of convergence change in rate between the arabo african and eurasian plates beneath the andean cordillera of south peru north bolivia the subduction zone plunges at an angle of 25 30 as it does under the aegean arc there as in the aegean sea tectonics are predominantly extensional and several short lived compressional phases break up this extensional regime the latter may also be induced by changes of the conditions of convergence here between the nazca and south american plates beneath central peru north of the nazca ridge the subduction zone plunges at a small angle about 10 there neotectonic deformations are different in the recent they are essentially compressional or expressed by strike slip faulting geological data from the aegean and andes seem to demonstrate that the compressional deformation in the crust above subduction zones is strongly controlled by the stress conditions along convergent boundaries

spatial distributions of secondary minerals in the martian meteorite mil 03346 168 determined by raman spectroscopic imaging miller range mil 03346 is a nakhlite meteorite that has been extensively studied due to its unique complex secondary mineral phases and their potential implications for the hydrologic history of mars we conducted a set of raman spectroscopic and raman imaging studies of mil 03346 168 focusing on the secondary mineral phases and their spatial distributions with a goal to better understand the possible processes by which they were generated on mars this study revealed three types of calcium sulfates a solid solution of k na jarosite and two groups of hydrated species with low crystallinity hslc in the veins and or mesostasis areas of the meteorite the most abundant ca sulfate is bassanite that suggests two possible paths for its direct precipitation from a ca s h2o brine either having low water activity or with incomplete development producing bassanite with gypsum microcrystals on mars the second most abundant ca sulfate is soluble caso4 which raises a new question on the origins of this phase in the martian meteorite since caso4 readily hydrates in the laboratory but is apparently stable in atacama desert the close spatial relationship of k na jarosite solid solutions with rasvumite kfe2s3 magnetite hslc and fine grained low crystallinity alkali feldspar in mesostasis suggests a potential in situ formation of mesostasis jarosite from these fe k na s o h2o species

crustal dynamics and active fault mechanics during subduction erosion application of frictional wedge analysis on to the north chilean forearc abstract the forearc region of the non accreting south american plate margin in northern chile is characterised by subduction erosion and regional uplift neotectonic deformation structures reflect simultaneous extensional and contractional fault kinematics in the outer forearc where the brittle crust directly overlies the subducting nazca plate the stress regime changes from extension in the upper part to compression at the base of the forearc wedge as seen in neotectonic surface structures and seismic data in the inner forearc surface structures indicate a compressional stress regime also affecting the western rim of the magmatic arc this stress regime is limited to a brittle crustal wedge segment which overlies the ductile part of the inner forearc lithosphere the shape of the two brittle forearc wedges at the leading edge of the south american plate is controlled by exogenetic surface processes internal deformation processes contemporaneous basal tectonic erosion and underplating mechanical parameter sets controlling and reflecting the recent tectonic processes and geometrical wedge segmentation within the forearc system are evaluated and applied to general frictional wedge models the states of stress within the crustal wedges are controlled by spatial variations of the basal mechanical parameters in the down dip direction of the forearc wedge base the new models illustrate the fundamental kinematics and dynamic processes of coulomb type basal tectonic erosion and mass transfer modes along active non accretive convergent margins the frictional wedge models explain the dynamics of the simultaneous and contrary deformation processes affecting the forearc crust at the north chilean convergent margin shaped by active basal tectonic erosion

martian tharsis bulge insight to mantle related processes abstract when examining the tectonics of the one plate planet mars it is possible to test and even to develop further some ideas with respect to earth s plate tectonic paradigm the definition of terrestrial plate movements does not unambiguously identify the actual mantle processes involved one way to leam more about the endogenous development of planets is to undertake comparative studies the main tectonic structures of mars are interpreted here to give information concerning mantle processes which may relate to those processes driving earth s plate tectonics but which in the case of mars have been insufficient to break martian lithosphere into plates but which have been sufficient to cause rift block tectonics and to build the tharsis bulge

continental drift what driving mechanism a subducting oceanic lithospheric slab acts as an efficient heat sink which strongly influences the convection pattern in the neighbouring sub continental mantle laboratory convection experiments show that this thermal coupling induces a roll about 5 times broader than deep underneath the continental lithosphere the experiments are performed in a tank with imposed top and bottom temperature and with in addition one cooled side wall the convective pattern is observed by differential interferometry for upper mantle convection one predicts rolls extending 3000 to 4000 km into the continental domain parallel to the active margin we show that for gondwana prior to the break up this simple convection pattern can explain the tensional state of the crust leading to the emission of flood basalts karroo parana and antarctica the thermal and mechanical effect of the large roll also provides a mechanism for the break up itself and subsequent drift of the continental fragments this motion of the continental plate leads finally to the closure of existing marginal basins along the pacific coast of gondwana the laboratory experiments throw some light into another phenomenon of possible geodynamical relevance within the large convection roll a smaller scale circulation is observed in the form of diapiric upwelling produced by thermal instabilities in the boundary layer this dynamics may apply to hot spot volcanicity

interaction between the nazca and south american plates and formation of the altiplano puna plateau review of a flexural analysis along the andean margin 15 34 s abstract the results of a two dimensional flexural analysis applied to the andean margin which is based on the correlation between topography and bouguer anomaly are here reviewed in order to characterize rigidity variations across and along the forearc arc transition of the central andes and to understand the role of the forearc in the formation of the altiplano plateau the forearc has maximum rigidities between 15 and 23 s forearc rigidity decreases gradually southward and sharply toward the plateau the main orogen elevations higher than 3000 m is very weak along the entire central andes a semi quantitative interpretation of these trends based on the relationship between flexural rigidity and the thermo mechanically and compositionally controlled strength of the lithosphere allows the following conclusions to be made 1 across strike rigidity variations are dominated by the thermal structure derived from the subduction process 2 the forearc constitutes a strong cold and rigid geotectonic element 3 southward weakening of the forearc is directly related to the decreasing thermal age of the subducted slab 4 very low rigidities along the main orogen are caused by the existence of a thick quartz rich crust with a low strain rate to heat flow ratio 5 the strength of the plateau lithosphere is localized in an upper crustal layer whose base at 15 km could be correlated with a p to s seismic wave converter trac1 of yuan et al 2000 yuan x sobolev s kind r oncken o et al 2000 subduction and collision processes in the central andes constrained by converted seismic phases nature v 408 21 28 diciembre p 958 961 6 the forearc plateau rigidity boundary corresponds to a zone of changing thermal conditions eastward increasing crustal thickness and felsic component in the crust and low strain rate deformation which correlates with a west verging structural system at the surface these conclusions suggest that the rigid forearc acts as a pseudo indenter against the weak plateau and allows the accumulation of ductile crustal material that moves westward from the eastern foreland this pseudo indenter is geometrically represented by a crustal scale triangular zone rooted at trac1 this model allows the integration of existing contradictory ideas on the dynamics of forearc plateau interaction that are related to the relative importance of upper crustal compressive structures and lower crustal accumulation below the forearc

the final galileo ssi observations of io orbits g28 i33 abstract we present the observations of io acquired by the solid state imaging ssi experiment during the galileo millennium mission gmm and the strategy we used to plan the exploration of io despite galileo s tight restrictions on data volume and downlink capability and several spacecraft and camera anomalies due to the intense radiation close to jupiter there were many successful ssi observations during gmm four giant high latitude plumes including the largest plume ever observed on io were documented over a period of eight months only faint evidence of such plumes had been seen since the voyager 2 encounter despite monitoring by galileo during the previous five years moreover the source of one of the plumes was tvashtar catena demonstrating that a single site can exhibit remarkably diverse eruption styles from a curtain of lava fountains to extensive surface flows and finally a 400 km high plume over a relatively short period of time 13 months between orbits i25 and g29 despite this substantial activity no evidence of any truly new volcanic center was seen during the six years of galileo observations the recent observations also revealed details of mass wasting processes acting on io slumping and landsliding dominate and occur in close proximity to each other demonstrating spatial variation in material properties over distances of several kilometers however despite the ubiquitous evidence for mass wasting the rate of volcanic resurfacing seems to dominate the floors of paterae in proximity to mountains are generally free of debris finally the highest resolution observations obtained during galileo s final encounters with io provided further evidence for a wide diversity of surface processes at work on io

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

minimum work mountain building minimum work methods may provide a useful way to examine mountain building processes here we present forward models of crustal deformation in which faults form along paths that minimize the total frictional and gravitational work associated with slip our numerical model produces thrust wedges analogous to the wedges produced by viscous models but with explicit fault paths rather than diffuse internal deformation the model reproduces the general geometry of real compressional belts highlighting the basic competition between friction and topography gravitational work which may drive the evolution of orogenic zones in particular we replicate observed geologic features such as dendritic faulting patterns thin skinned thrust systems and back thrusting while some modeled features differ from observations the results suggest that for many applications minimum work may be a good approximation for macroscale crustal deformation

island arc tectonics abstract tectonic stress states in arc backarc systems have been examined on simplified model structures corresponding to the chilean mariana and intermediate arc types two sources of tectonic stress are assumed the stress due to lateral density variations and the stress transmitted from the subducting oceanic plate to the forearc the average vertical stress zz of the former type has been calculated for each isostatically compensated model lithospheric plate using the difference from a reference structure from the sign of xx zz one can estimate whether the tectonic situation is extentional or compressional if we assume that xx is caused by mechanical coupling between the subducting oceanic slab and is laterally constant in one arc backarc system we can estimate at least the upper or lower boundary of the mechanical coupling for the chilean type arcs where the truly great interplate earthquakes occur and no backarc spreading takes place the transmitted average stress must be larger than 660 bar for the mariana type arcs where no large interplate earthquakes are recorded and spreading takes place in the backarc region xx should be smaller than 100 bar in any arc backarc system the most extentional feature is expected at the highest topography of the volcanic arc once spreading has started the viscosity under the backarc region drops dramatically and the spreading will continue in the backarc even if its topography is subdued in comparison with the now separated main and remnant arcs

a no lid zone in the central chang thang platform of tibet evidence from pure path phase velocity measurements of long period rayleigh waves pure path phase velocities of fundamental mode rayleigh waves in the period range 30 80s are obtained for different paths across the tibet plateau using a two event method with events located in tibet and aligned with world wide standard seismograph network stations at teleseismic distance evidence is found for significant lateral variations of structure of the crust and uppermost mantle in tibet in particular phase velocities obtained on paths contained in the chang thang platform are higher than average in tibet at periods shorter than 60s while remaining very low at longer periods when interpreting these phase velocities in terms of simple layered s velocity models it is found that contrary to other areas in tibet they are incompatible with the existence of a high velocity lid s velocities of 4 4 4 5 km s are found immediately beneath the moho and the crust is somewhat thinner 50 60 km than average in tibet this indicates a possible upwelling of hot asthenospheric material compatible with landsat evidence for subrecent basaltic volcanism and independent seismic evidence such as inefficient transmission of sn waves on the other hand a 70 km thick crust and very high upper mantle s velocities are found on the western border of tibet karakorum hindu kush compatible with previous finer scale results concerning the associated zone of intermediate depth seismicity

large scale gravity profiles across subducted plates summary the mean gravity profiles across central and south america and eurasia in the direction normal to the subduction zone are deduced from the gem 10b gravity model they have a typical shape a maximum close to the trench a negative slope towards the interior of the plate over a 3000 km wide distance usually followed by a local maximum it is found that large convective cells driven by the heat sink of the sinking slab have an associated gravity signal having such a typical shape a detailed comparison between observed and theoretical data supports this point of view and thus constrains the possible structure of the convective flow under these plates

basin and range large scale tectonics constraints from gravity and reflection seismology two geophysical informations constrain the mechanical state of the lithosphere in the basin and range province cocorp reflection seismic structure across nevada shows a rather smooth reflector at about 30 km depth which can be interpreted as indicative of a rather flat moho on the other hand the bouguer map exhibits two marked troughs in the same region and filtering techniques reveal a predominant 200 km wavelength of the deep compensating masses of the topography this paper attributes these deep heterogeneities to lateral thermal variations created by boudinage type instabilities of the stretched mantle lithosphere

effects of density contrasts on the orientation of stresses in the lithosphere relation to principal stress directions in the transverse ranges california the influence of stresses arising from horizontal density contrasts on the orientation and relative magnitudes of principal stresses in an otherwise uniform lithospheric stress field is investigated a simple model is constructed in which a local deviatoric stress due to a density anomaly embedded within or just below the lithosphere and a regionally constant deviatoric stress field are each approximated by biaxial tensors the net stress field is obtained from the sum of the two both the relative magnitudes of principal stresses and the magnitude of the angular difference in principal stress direction of the summed tensor compared with that obtained in the absence of buoyancy forces depend on two parameters the first is the ratio where is a measure of the magnitude of the regional deviatoric stress and is the magnitude of the stress arising from buoyancy forces associated with the density anomaly the second parameter is the angle between the trend of the density anomaly and the direction of maximum compressional stress that obtains in the absence of any perturbation by the local buoyancy forces the directions of the principal axes of the total stress field are found to differ by up to 90 from those of the reference stress field the model is applied to the transverse ranges california where the observed 23 difference in orientation of principal horizontal compressive stress compared with the principal compressive stress direction in central california constrains the predicted value of to be approximately 0 4 this is consistent with an independently calculated range of in which is inferred from seismological constraints on the magnitude of density variations underneath the transverse ranges and is inferred from observations of heat flow along the san andreas fault in central california the agreement between the two estimates of supports the hypothesis that the observed differences in horizontal principal stress orientation in california can be explained by the combined influence of a local negative buoyancy force under the transverse ranges and a regional stress field associated with transcurrent deformation within the pacific north american plate boundary zone the observed counterclockwise angular difference in principal horizontal stress direction in the transverse ranges compared with central california implies that the plane of maximum right lateral shear stress is also rotated counterclockwise relative to that in central california this supports the possibility that the big bend in the san andreas fault may be a consequence of the negative buoyancy forces acting in the transverse ranges and not the cause of transverse ranges formation as has often been assumed

faulting assisted lateral collapses and influence on shallow magma feeding system at ollague volcano central volcanic zone chile bolivia andes abstract the role played by the regional tectonics and relationships between volcanic structures and their shallow magmatic feeding system are of paramount importance in understanding multiple lateral collapse events and thus for hazard assessment in active volcanoes ollague 21 18 s 68 11 w andean central volcanic zone chile bolivia border is an active andesite dacite composite volcano developed on a regional nw striking extensional fault system that bisects the volcanic edifice geological and stratigraphic studies on the summit part of the cone together with new 40ar 39ar dating and petrographic and geochemical major trace elements analyses have allowed us to delineate the volcanic history of ollague and recognize multiple deformation and lateral collapse events the evolution of ollague appears to be characterized by four stages of volcano building separated by three main events of deformation of the cone these deformation events developed progressively by downthrow of the sw sector of the volcano 800 ka along the nw striking normal fault propagating from the substrate to a final catastrophic failure 300 ka of the sw sector and debris avalanche emplacement both nw trending regional tectonics and weakness of the edifice caused by the successive deformation and collapse events are responsible for both the geometric variations of the shallow magma feeding system and the coexistence of summit and parasitic dacitic vents central and lateral vents shifted in a sw trending direction i e perpendicular to the tectonic trend and along the axis of the collapse movement and developed along a nw trending alignment i e parallel to the main tectonic trend and collapse scarps

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

geophysical study of the valle fertil lineament between 28 45 s and 31 30 s boundary between the cuyania and pampia terranes abstract a gravimetric and magnetometric study was carried out in the north eastern portion of the cuyania terrane and adjacent pampia terrane gravimetric models permitted to interpret the occurrence of dense materials at the suture zone between the latter terranes magnetometric models led to propose the existence of different susceptibilities on either side of the suture the curie temperature point depth representing the lower boundary of the magnetised crust was found to be located at 25 km consistent with the lower limit of the brittle crust delineated by seismic data this unusually thick portion of the crust is thought to release stress producing significant seismicity moho depths determined from seismic studies near western sierras pampeanas are significantly greater than those obtained from gravimetric crustal models considering mass and gravity changes originated by the flat slab nazca plate along cuyania and western pampia terranes it is possible to reconcile moho thickness obtained either by seismic or by gravity data thus topography and crustal thickness are controlled not only by erosion and shortening but by upper mantle heterogeneities produced by a the oceanic subducted nazca plate with normal slope also including asthenospheric materials between both continental and oceanic lithospheres b flat slab subducted nazca plate as shown in this work without significant asthenospheric materials between both lithospheres these changes influence the relationship between topographic altitudes and crustal thickness in different ways differing from the simple airy system relationship and modifying the crustal scale shortening calculation these changes are significantly enlarged in the study area future changes in nazca plate slope will produce changes in the isostatic balance

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

depths and focal mechanisms of crustal earthquakes in the central andes determined from teleseismic waveform analysis and insar 1 we investigate the depths of crustal earthquakes mw 5 5 earthquakes from the global cmt catalog and previous work spanning 1944 2007 for 38 well recorded events we use teleseismic p and sh waveforms to model the strike dip rake focal depth and source time function we use insar observations of surface deformation from 9 earthquakes to compare inferred fault parameters with the waveform inversions and global catalogs to assess their accuracies while the depths from the insar and waveform analyses generally agree within error horizontal and depth errors in global catalogs are 10 to 50 km as found elsewhere as noted in previous work the majority of crustal earthquakes occur in the eastern cordillera and foreland regions of the central andes although a few normal and strike slip earthquakes occur beneath the altiplano plateau and in the forearc in southern peru and northernmost chile we propose a new interpretation of one of the basement thrusts shira mountain peru as a pop up block on the basis of our new earthquake depths we confirm that earthquakes in the flat slab areas of peru and argentina are within the sometimes aseismic lower crust lower crustal earthquakes are globally found in all types of tectonic settings only when the thermal lithosphere is more than 80 km thick and the amount of recent shortening extension is 30

a continuum model of continental deformation above subduction zones application to the andes and the aegean large scale continental tectonics of back arc extensional and andean type compressional environments are investigated by using the thin viscous sheet model to calculate the deformation within continental lithosphere that is subjected to horizontal forces on its plate boundaries and to basal drag from the asthenospheric flow beneath the shear tractions acting on the base of a deformable lithosphere are derived from a corner flow model that assumes a rigid subducting plate and a deformable overlying plate because the calculated shear tractions and the deformation within the overlying plate are interdependent the corner flow and the thin viscous sheet models are solved simultaneously we use a perturbation method to obtain analytical solutions for the velocity and strain rate fields within the overlying continental lithosphere the solutions depend on the angle of subduction the dimensionless thickness of the lithosphere and the ratio of asthenospheric to lithospheric viscosities which governs the viscous coupling between the asthenosphere and the lithosphere calculations are compared with observations from the andes and the aegean our results explain some of the features of the deformation in these regions that have heretofore not been explained by other models our model predicts that in a compressional environment a broad region of uplifted topography will tend to develop above a more steeply dipping slab 30o rather than above a shallower slab 10o 15o this is in accord with observations in the various segments of the central andes for an extensional environment the model predicts that a zone of compression can develop near the trench and that extensional strain rate can increase with distance from the trench as is observed in the aegean we also estimate the effective viscosities of 102o pa s for the asthenosphere 2x 1021 pa s for the aegean lithosphere and 1022 pa s for the andean lithosphere

laboratory convection experiments effect of lateral cooling and generation of instabilities in the horizontal boundary layers convection experiments are carried out in a tank with two isothermal heat sinks the top plate and one of the sidewalls heating is supplied either by an isothermal bottom plate or by generation within the fluid this situation is similar to that of the earth s subcontinental mantle in the presence of a neighboring subducting oceanic lithosphere there the mantle material loses heat not only to the base of the continental lithosphere but also to the cold dipping oceanic slab the thermal structure of the convective fluid is observed by a variety of techniques including differential interferometry and strioscopy two parameters characterize the observations the usual vertical rayleigh number and a newly defined lateral rayleigh number the lateral cooling induces a large roll with axis parallel to the cold wall the variation of its width relative to the values of the two rayleigh numbers has been determined an application to the earth s upper mantle would predict rolls 5 times wider than high the circulation in the large roll remains two dimensional for rayleigh numbers up to about 3 104 beyond this value boundary layer instabilities are observed within the persisting large roll their period has been determined the interferometric method is shown to be very useful for visualizing other time dependent processes such as the growth of the induced large rolls this growth is rapid enough to allow us to propose the existence of such large rolls in the earth and argue that their action could have led to continental break up

linking slab break off hellenic trench retreat and uplift of the central and eastern anatolian plateaus abstract the central and eastern anatolian plateaus are integral parts of the world s third largest orogenic plateau in the past decade geophysical surveys have provided insights into the crust lithosphere and mantle beneath eastern anatolia these observations are now accompanied by recent surveys in central anatolia and new data constraining the timing and magnitude of uplift along its northern and southern margins together with predictions from geodynamic models on the effects of various processes on surface deformation and uplift the observations can be integrated to identify probable mechanisms of anatolian plateau growth a changeover from shortening to extension along the southern margin of central anatolia that is coeval with the start of uplift can be most easily associated with oceanic slab break off and tearing this interpretation is supported by tomography deep seismicity or lack thereof and gravity data based on the timing of uplift geophysical and geochemical observations and model predictions slab break off likely occurred first beneath eastern anatolia in middle to late miocene time and propagated westward toward cyprus by the latest miocene alternatively the break off near cyprus could have occurred in late pliocene to early pleistocene time in association with collision of the eratosthenes seamount continental fragment with the subduction zone uplift at the northern margin of central anatolia appears to result from crustal shortening starting in the late miocene or early pliocene which has been linked to the broad restraining bend of the north anatolian fault the uplift history of the interior of central anatolia since the late miocene is unclear although shortening there appears to have ended by the late miocene followed by ne sw extension this change in the deformation style broadly coincides with faster retreat of the hellenic trench as well as uplift of the northern and southern margins of central anatolia these different events throughout the plateau may be linked as faster retreat of the hellenic trench has been predicted to occur after slab break off which could have induced extension of central anatolia and helped to form the north anatolian fault through accelerated westward movement of anatolia relative to eurasia correlative geochronologic evidence that we summarize here supports the hypothesis that the geodynamic activity throughout the aegean anatolian domain starting in latest miocene to early pliocene time defines a series of events that may all be linked to slab break off

tectonic stresses in the lithosphere various types of observables earthquake focal mechanisms in situ measurements and geological deformations give information about the large scale lithospheric stress field the latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates this approach ignores the role of mass heterogeneities within the lithosphere here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude the presence of internal sources makes the problem three dimensional we show however that it can be reduced to a two dimensional plane stress formulation whereby the edge forces are expressed by the non hydrostatic stresses and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress for the oceanic lithosphere we derive a compression that increases with age the comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag for the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans

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

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

mass anomaly structure of the earth gravitational interaction is the weakest among the four known forces in the universe the particular gravity equipotential field that coincides with sea level is called the geoid and satellite data have revealed anomalies in its pattern that are puzzling to explain dynamic topography solutions have offered the preferred explanation for the past 15 years but problems remain an alternative explanation presented here contends that very large mass anomalies lie deep in the earth presumably produced by topography at the coremantle boundary cmb and only much smaller mass anomalies occur at shallower depths the principal distinction is whether one is willing to accept the possibility that processes within the earth s core may generate the cmb topography an analysis of the south american regional geoid high spherical harmonic degrees 2 10 indicates that only mass anomalies shallower than 1200 km are its cause this single positive source is contrary to dynamic topography solutions that would generate negative deflections of both the surface and the cmb in response to the positive subducted mass for the earth s fourth greatest geoid high our alternate explanation is tested utilizing a set of 9274 subduction 5 cube slab bloblet center points developed from reconstructed plate history that provide estimates of the locations of material subducted into the earth s mantle two global mass solutions are offered utilizing 1 only those bloblets in the outer 800 km and 2 only those bloblets in the outer 1400 km four point masses at 3000 km depth to simulate cmb topography produced by processes within the core unrelated to mantle dynamic topography complete the two mass models both models show reasonable agreement with patterns and magnitudes of the regional geoid and its component degree parts the model cmb mass anomalies are only a factor of 3 greater than those initially estimated by gravity to geoid ratios from observed values at geoid anomaly center locations difficulties with the dynamic topography solutions matters of the density of the subducted material the proper hydrostatic flattening value to use for analyses and possible causes for the cmb topography are discussed the subduction mass models are also used to estimate the magnitude of the driving force for plate tectonics 2 78 1020 to 3 23 1021 n approximately equivalent to 7 0 1012 to 8 1 1013 n m 1 these force values are comparable to published slab force estimates based on thermal considerations

geoid height age relation from seasat altimeter profiles across the mendocino fracture zone twenty eight seasat altimeter profiles crossing the mendocino fracture zone are used together with seafloor ages determined from magnetic lineations to estimate the change in oceanic geoid height with age between ages of 15 and 135 m y an unbiased estimate of the overall geoid offset along each profile is determined from a least squares fit of the along track derivative of the geoid to the geoid slope predicted from a simple two layer gravitational edge effect model uncertainties based upon the statistical properties of each profile are also determined a geoid slope age relation is constructed by normalizing the geoid offsets and uncertainties by the age offsets the results are in agreement with geoid slope age relations determined from symmetrically spreading ridges sandwell and schubert 1980 however the fracture zone estimates have smaller uncertainties and show less scatter a comparison of these results with the geoid slope age prediction of the boundary layer cooling model shows that the thermal structure begins to deviate from this model at an early age 20 40 m y a plate cooling model with a thickness of 125 km is most compatible with the geoid slope age estimates although significant deviations occur these may indicate that the lithospheric thermal structure is not entirely age dependent

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

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

granitoid emplacement and deformation along a major crustal lineament the cordillera blanca peru abstract the cordillera blanca batholith and ignimbrites of the intermountain callejon de huaylas basin represent the last magmatic event in the andean cycle of central northern peru this most spectacular example of an extensional regime is characterised by acid magmatism spanning some 10 ma with both magma emplacement and subsequent deformation being fundamentally controlled by a ca 300 km lineament known as the cordillera blanca fault complex the western margin of the batholith has been strongly deformed by the cordillera blanca fault resulting in a series of ductile to brittle structures that increase with intensity away from the relatively undeformed core towards the margin the late stage brittle fabrics are coeval with extensional structures in the callejon de huaylas basin and relate to uplift and extension along the fault zone the more ductile fabrics as indicated by combined k ar and 40 ar 39 ar mineral dates formed earlier in the intrusion history and are consistent with emplacement into an active dextral strike slip shear zone the rapid uplift and high erosion rates seen from the late miocene onwards and the current state of stress in this region of thickened crust are due to gravitational instability body forces set up at high topographical levels and buoyancy forces resulting from the intrusion of young acid magmas at high crustal levels

tomography on pp p waves and its application for investigation of the upper mantle in central siberia abstract we develop the method of tomographic inversion called rr r scheme which is based on joint use of teleseismic p or s refracted rays and corresponding pp or ss rays with bounce points located within a study region this scheme allows imaging the deep seismic structure beneath blank areas where there are neither recording stations nor earthquakes utilization of differential travel times makes it possible to avoid the difficulty of source and station corrections which cause problems in teleseismic tomography the rr r scheme has been applied to more than 10 000 ray pairs from the isc database to investigate a large region from the north arctic ocean to the northern part of china and mongolia inversion was carried out in several separate windows covering the study area down to the depth of 530 km velocity anomalies were computed in grid nodes distributed in the study 3d area according to ray density the tomography results from different windows were combined into one general map of the lithospheric structure in central siberia principal positive p wave velocity anomalies 1 3 down to 400 km are associated with the precambrian siberian craton negative velocity anomalies are related to the thin lithosphere of the west siberian plate the altai mountains and the hangai plateau 3 1 in the depth range 30 to 430 km a strong negative anomaly beneath the precambrian hangai block is apparently associated with a mantle plume positive velocity anomalies in sedimentary basins at southern siberia are believed to have the crustal origin

neogene uplift of central eastern patagonia dynamic response to active spreading ridge subduction the chile triple junction ctj episodically migrated northward during the past 14 ma from 54 s to its present day position at 46 30 s as different almost trench parallel spreading segments entered in subduction this migration resulted in the opening of an asthenospheric window below patagonia inducing a disturbance in the regional mantle convection on the overriding plate the middle miocene time corresponds to a major change in the central patagonian basin dynamics with a transition from subsidence to generalized uplift the detailed mapping and the morphological study of post middle miocene terraces evidence regional scale trench parallel tilt histories that differ depending on latitude south of 46 30 s the slopes of the fluvial terraces indicate a change from northward to southward tilt during late miocene terraces younger than the early pliocene do not show any trench parallel tilt north of 46 30 s in contrast only northward tilt active until the pleistocene is recorded we use a semianalytical model of dynamic topography which suggests that the northward migration of the chile triple junction should be accompanied by a dynamic uplift of the central patagonian basin uplift would cause both north directed and south directed tilt north and south of the triple junction respectively with values of 0 1 0 2 500 km east of the trench tilt measured in the southeastern part of the central patagonian basin 0 1 is comparable to values predicted using the semianalytical model the dynamic topography associated to the chile ridge subduction may have exerted a major control on the neogene dynamics of patagonia

plio quaternary geodynamic evolution of a segment of the peruvian andean cordillera located above the change in the subduction geometry the cuzco region abstract the cuzco region which is located above a change in subduction geometry appears to be characterized by a variable plio quaternary tectono sedimentary evolution essentially located along the major fault system that separates the high plateaux from the eastern cordillera after the higher surface formation of the high plateaux a set of neogene basins were filled by miocene fluvio torrential series and by plio pleistocene fluvio lacustrine deposits the neogene series have been affected by compressional tectonic forces attributed to the late miocene this compression is followed by roughly e w trending syn sedimentary extensional tectonics attributed to the pliocene it is related to reactivation of the pre existing major faults basin evolution and volcanic activity concentrated along the faults in the early pleistocene fluvio lacustrine deposits are affected by syn and post sedimentary compressional tectonism it is characterized by shortening that trends both n s and e w and produces folding and faulting of the sedimentary cover extensional tectonism trending roughly n s has been taking place from the middle pleistocene to the present it is coeval with shoshonitic volcanic activity and with sedimentation of fluvio lacustrine terraces torrential fans and moraines quaternary and active normal faults due to this tectonism are located in a narrow zone more than 100 km long between the high plateaux and the eastern cordillera and two 15 km long fault sectors in the eastern cordillera characteristic pleistocene scarps 400 m or more high are due to the cumulative normal offset and there are also little scarps with heights ranging between 2 and 20 m which are related to holocene fault reactivations recent fault reactivation on the cuzco fault system during the april 5 1986 earthquake m b 5 3 is due to the n s trending extension this state of stress located at a mean elevation of roughly 3730 m is generally homogeneous to different scales the active cuzco normal faults may be a consequence of adjustment between the compensated western cordillera and the undercompensated eastern cordillera this latter being uplifted higher than its isostatic equilibrium due to compression acting on its eastern edge the variation of the state of stress during the plio quaternary is in agreement with the variations of the compressional boundary forces it may be explained by variation of the convergence rate or by the variation of pull slab forces

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

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

variations of slab dip and overriding plate tectonics during subduction insights from analogue modelling we present small scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions the oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate in this configuration the long term process of subduction is not steady state slab interaction with the upper mantle lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes followed by periods of slab steepening the overriding plate tectonic regime is influenced by the dynamics of subduction slab anchoring favouring trench perpendicular shortening when the slab is anchored slab flattening further favours shortening while slab steepening favours extension or smaller shortening rates non steady state long term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present day subduction zones experiments suggest that despite boundary conditions applied on the converging plates do not change tectonics pulses within the overriding plate may be caused by this non steady state behaviour

a zone of very low seismicity in the vicinity of the chang thang platform tibet abstract we note that the area of no sn transmission and no lid of previous researchers in the central chang thang platform of tibet has low seismicity whereas the kun lun and lhasa blocks lying north and south of the chang thang platform are seismically active an area bounded by the nujiang kun lun fault and kokoxili suture in the north the bangong nujiang suture and kang ting fault in the south and longitudes 87 and 100 e encompassing a large portion of the chang thang platform in central tibet has low seismicity we also note that portions of this area have a signature in topography as well the elevations are higher by several hundred metres here than over the lhasa and kun lun blocks these observations and the reported extensive quaternary volcanism in the chang thang platform suggest upwelling of hot mantle material below this area as the most likely explanation

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

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

horizontal subduction zones convergence velocity and the building of the andes we discuss the relationships between andean shortening plate velocities at the trench and slab geometry beneath south america although some correlation exists between the convergence velocity and the westward motion of south america on the one hand and the shortening of the continental plate on the other hand plate kinematics neither gives a satisfactory explanation to the andean segmentation in general nor explains the development of the bolivian orocline in paleogene times we discuss the cenozoic history of horizontal slab segments below south america arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive we argue that the existence of horizontal slab segments below the central andes during eocene oligocene times and below peru and north central chile since pliocene resulted 1 in the shortening of the continental plate interiors at a large distance from the trench 2 in stronger interplate coupling and ultimately 3 in a decrease of the trenchward velocity of the oceanic plate present day horizontal slab segments may thus explain the diminution of the convergence velocity between the nazca and south american plates since late miocene

tectonic evolution of high plateaus abstract the striking contrast in tectonic style between highlands like tibet with its active grabens and low altitude flanks like the thrusting structures in the foothills of the himalayas shows that a disorder of the tectonic stresses correlates with the topography the same is true for gravity in particular for the geoid as clearly observed in the case of the altiplano these observations can be explained physically by means of simple relationships between the dipolar mass distribution of the high topography and its compensating root this analysis leads to a proposed scenario for plateau evolution in three stages 1 1 build up with thrusting and folding 2 2 neutral state or strike slip deformation and finally 3 3 relaxation with normal faulting the basin and range represents a mature illustration of phase 3 its bouguer anomaly suggests the existence of an undulation of the lithosphere astenosphere boundary with a 200 km wavelength physical models of lithospheric boudinage are shown for one to compare with the observed topographic and gravimetric signatures

plate scale potential energy distributions and the fragmentation of ageing plates abstract geoid anomalies associated with mid ocean ridge systems and a number of continental margins imply that on the scale of individual plates the old ocean lithosphere represents a gravitational potential energy sink since lateral variations in potential energy contribute to deviatoric stresses in the lithosphere the changing potential energy distributions in individual plates associated with the growth and ageing of the oceanic lithosphere may be expected to result in changes in the intraplate stress field analytical models for simple plate geometries using lithospheric density models consistent with small positive 6 m geoid anomalies across continental margins show that the growth of oceanic lithosphere over a period of 200 ma may contribute to a decline in the mean plate potential energy u p of about 1 10 12 n m 1 and thus contribute a mean extensional stress difference zz xx in continental lithosphere of up to about 8 mpa averaged over a 125 km thick lithosphere these estimates are sensitive to the assumed mean continental potential energy u c about which there is some uncertainty for higher u c approaching that of the mid ocean ridges u mor the net decline in u p may be as much as 1 7 10 12 n m 1 whereas for significantly lower u c approaching that of old ocean lithosphere plate growth may increase u p transiently by up to 2 7 10 12 n m 1 leading to compression in the continents in the african and antarctic plates the ageing of the ocean lithosphere since the late jurassic is estimated to have contributed to a decline in u p of about 0 6 10 12 and 0 95 10 12 n m 1 respectively contributing a mean stress difference of about 5 mpa and 7 5 mpa in the respective continents whereas the predicted stress changes associated with ageing of the oceanic lithosphere are significantly smaller than most laboratory based estimates of the extensional strength of the lithosphere they may provide an important contribution to the stress fields that eventually lead to the fragmentation of ageing plates

viscosity estimates for the crust and upper mantle from patterns of lacustrine shoreline deformation in the eastern great basin the deformed shorelines of lake bonneville constitute a classic source of information on lithospheric elastic thickness and upper mantle viscosity we describe and apply a new model to a recently augmented data set new data better constrain both the complex spatio temporal pattern of the lake load and the crustal deformation response to that load the history of lake level fluctuations has been significantly refined and somewhat modified this is due to both more radiocarbon dates from within the bonneville basin and to an improved calibration of the radiocarbon timescale itself the data which constrain the crustal deformation pattern consist of ages and shoreline elevations from several hundred points which sample three major levels of lake bonneville and corresponding elevations from the high stands of three smaller lakes situated to the west of lake bonneville the data from the smaller lakes help elucidate the pattern of deflection which occurred beyond the edge of the big lake the geometry of the earth model incorporates an arbitrary number of layers overlying a half space and the rheology of each level can accommodate an arbitrary number of maxwell viscoelastic elements in parallel the inverse modeling comprises three complementary approaches for the simplest configurations we performed a direct search of the parameter space and delineated the irregular boundary of the subspace of acceptable models for more complex configurations we constrained the elastic parameters to their seismically determined values and then solved for viscosity versus depth profiles by either expressing the log viscosity versus log depth profile as a series of specially constructed orthogonal polynomials or by allowing each of 8 10 layers plus the half space to have an independently determined viscosity we found that the data do not strongly support nor can they conclusively exclude a more complex rheology than simple maxwell viscoelasticity the orthogonal polynomial solution exhibits an essentially monotonic decrease in viscosity with depth the most rapid change occurs at shallow depths decreasing from 1023 pa s at 3 km to 102o pa s at 30 km the decrease is much more gradual below with only another factor of 5 decrease between 30 and 300 km depth the unconstrained solution exhibits a rapid decrease in viscosity with depth from 2x 10 24 pa s in the top 10 km to 4x 1017 pa s at a depth of 40 km a nearly isoviscous asthenospheric region extends from 40 to 150 km and is underlain by a mantle lithospheric region with increased viscosity 2x 102o pa s extending from 150 to 300 km depth and by a uniform viscosity 1019 pa s half space below

the state of stress in an overriding plate situated above a flat slab the andes of central peru in the andes of central peru that are situated above a flat dipping slab analysis of both structural data collected in the field and available focal mechanisms show that the quaternary and recent state of stress is characterized by n s trending extension in the western cordillera and along the pacific coast whereas e w trending compression prevails in the sub andean lowlands and at the contact between the nazca and south american plates the eastern cordillera deformations suggest an intermediate state of stress i e strike slip faulting characterized by e w trending compression and n s trending extension thus major fault zones striking nw se i e parallel to the central peruvian andes are normal with a sinistral component in the western cordillera e g cordillera bianca and reverse sinistral in the eastern cordillera e g cordillera de huaytapallana this state of stress displays similarities with that of southern peru and may be also interpreted as an effect of compensated high topography in this model the vertical stress zz increases with the topography and hmax is considered as fairly constant trending e w roughly parallel to the convergence on both edges of the andes tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then hmax is 2 allowing hmin striking n s to be 3 along the pacific coast tectonics is not compressional as expected and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep peru chile trench however a conspicuous difference occurs in the andes of central peru in respect to the southern peru andes compressional strike slip tectonics is observed in the eastern cordillera indeed the mean elevation of the eastern cordillera is 3 7 km whereas that of the western cordillera is 4 2 km allowing an intermediate state of stress to occur therefore in central peru a lower mean andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern peru superficial deformations related to the subduction of the aseismic nazca ridge are restricted to the coastal domain opposite to this ridge they support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting

a modelling study of vertical surface displacements at convergent plate margins summary during the evolution of a subduction zone system variations are likely to occur in for example surface plate velocities and buoyancy of the subducting lithosphere we quantify vertical surface displacements at convergent plate margins resulting from such imposed variations for this purpose we use a 2 d numerical model in which the lithospheric plates have an effective elastic thickness we first define a model in which the subducting plate is driven by its negative buoyancy and a velocity at its surface side boundary its equilibrium topography after around 2 myr is the reference level for examination of surface displacements resulting from variations in buoyancy velocity of the surface plates friction along the interplate contact and subduction zone roll back we find that a decrease increase in buoyancy of the subducting material leads to a deepening uplift of the plate margins an increase in friction along the subduction fault deepens the overriding plate margin subduction zone roll back due to sinking of the negatively buoyant subducting plate induces subsidence of the overriding plate margin this subsidence is reduced when roll back takes place in a land locked basin setting trench retreat forced by the motion of the overriding plate is characterized by higher topography of the overriding plate margin than the case of retreat due to the sinking of the negatively buoyant slab in the first case in plane stress in the back arc region is compressive while it is tensional for roll back due to the sinking of the slab we conclude that vertical surface displacements during ongoing subduction may reach a magnitude of a few kilometres on the overriding and subducting plate margins