granite ascent and emplacement during contractional deformation in convergent orogens based on a case study in the central maine belt of west central maine u s a it is proposed that crustal scale shear zone systems provide an eaective focussing mechanism for transfer of granite melt through the crust in convergent orogens during contractional deformation flow of melt in crustal materials at depths below the brittle plastic transition is coupled with plastic deformation of these materials the flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses within the oblique reverse central maine belt shear zone system stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain which suggests percolative flow of melt to form the migmatite leuco somes and viscous flow of melt channelized in sheet like bodies possibly along fractures cyclic fluctuations of melt pressure may cause instantaneous changes in the eaective permeability of the flow network if self propa gating melt filled tensile and or dilatant shear fractures are produced due to melt enhanced embrittlement in homogeneous migmatite and schlieric granite occur in zones of lower strain which suggests migration of partially molten material through these zones en masse by granular flow and channelized flow of melt carrying entrained residue founded on the central maine belt case study we develop a model of melt extraction and ascent using the driving forces stress conditions and crustal rheologies in convergent especially transpressive orogens ascent of melt becomes inhibited with decreasing depth as the solidus is approached for intermedi ate a h2o muscovite dehydration melting the water saturated solidus occurs between 400 and 200 mpa near the brittle plastic transition during high t low p metamorphism where the balance of forces favors sub horizontal fracture propagation emplacement of melt may be accommodated by ductile flow and or stoping of wall rock and inflation may be accommodated by lifting of the roof at shallower crustal levels and or sink ing of the pluton floor the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduits although these plutons may have locally discordant relations with country rock structures when viewed at the crustal scale granite ascent and emplacement in convergent orogens are syn tec tonic processes 1998 elsevier science ltd all rights reserved

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

a numerical method for determining the state of stress using focal mechanisms of earthquake populations application to tibetan teleseisms and microseismicity of southern peru the numerical method described in this paper enables the study in terms of stress of the kinematics of seismic faults provided by focal mechanisms this method assumes a mean state of stress in the source region and is based on the simple mechanical model used for fault population analysis which supposes slip in the direction of the resolved shear stress acting on the fault plane the proposed algorithm first defines compressional and tensional zones resulting from superimposition of the compressional and tensional quadrants limited by the nodal planes this enables one to test the data homogeneity furthermore this restricts the space where the principal stress axes have to be searched then for each principal stress reference whose location is constrained by above confined zones the r value chosen equal to 2 1 3 1 is computed which fits the slip vector on each nodal plane this permits one to select a set of preferred seismic fault planes from a set of auxiliary planes finally a state of stress is computed from the preferred seismic fault plane set using the non linear simplex method already applied to fault populations this algorithm is constructed so as to avoid two major difficulties misleading estimation of the deviatoric stress tensor which may result from excessive emphasis by minimizing the residuals by a least squares method if some of the data are wrong and a lengthy prospection of the stress references over all the space of directions this calculation does not take into account possible changes of the strike and dip of the nodal planes however we use rotations of nodal planes in agreement with the first arrival data to test the compatibility of these data with the computed state of stress tibetan teleseisms and southern peruvian microseisms are analyzed and results are compared with kinematics of superficial recent and active faults measured in field in the same regions this suggests that major seismic events may give a good evaluation of the regional mean state of stress focal mechanisms of the small events are also modelled by a mean stress deviator but this differs significantly from the regional state of stress in southern peru deduced from quaternary faulting this may be due to imprecise location of the small events however some movements are strongly different from those predicted by the computed mean stress deviator this suggests that small fault kinematics surely depend on the regional state of stress but that local effects may control the small deformations

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

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

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

paleomagnetism of cretaceous volcanic rocks from central chile and implications for the tectonics of the andes the mean direction of remanent magnetization in well dated structurally and petrologically simple cretaceous ash flow tuffs from the central valley of chile near santiago indicates that the region has undergone a small about 14 clockwise rotation two other paleomagnetic studies on cretaceous rocks from coastal chile give a similar result we tentatively attribute these rotations to dextral shear caused by oblique subduction of the nazca plate beneath south america in contrast cretaceous rocks from peru seem from paleomagnetic evidence to have been rotated counterclockwise relative to the stable interior of the continent this sense of apparent rotation agrees with the sense of deflection of geologic trends from north south to north northwest at about the peru chile border the arica deflection on the basis of these observations several authors have suggested that the arica deflection is an orocline however uncertainty in all such paleomagnetic interpretations is introduced by what seems to be a very streaked distribution of cretaceous reference poles from the south american craton the cause of this streaking is not known

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

quaternary normal and reverse faulting and the state of stress in the central andes of south peru field studies in the andes of southern peru show that in the high andes and pacific lowlands quaternary and recent faults are normal this extensional tectonics postdates compressional deformations of pliocene early quaternary age in the sub andes the observed deformations are compressional they affect early quaternary deposits some of the faults separate quaternary deposits from the bedrock and thus are clearly of tectonic origin and not landslide effects striations on the fault planes indicate n s trending extension in the high andes and pacific lowlands the total amount of crustal stretching is small probably of the order of 1 during the last 1 2 m y in the sub andes folds and faults affecting neogene and early quaternary deposits result from n s shortening nevertheless it is supposed that this n s shortening is of early quaternary age the present day compression probably strikes e w judging from focal mechanisms in the sub andes of central peru southern bolivia and northwest argentina data from structural analysis of faults and from earthquake focal mechanisms allow us to surmise the state of stress in the andes of southern peru the high andes and pacific lowlands subjected to n s trending extension are bounded by two zones of e w trending compression the sub andes to the east and the contact between the convergent nazca and south america plates to the west in our model the maximum horizontal compressive stress trajectory hmax is roughly parallel with the e w convergence between the two plates hmax corresponds to 1 in the sub andes and to 2 in the high andes the latter situation is caused by the elevated mass of the high andes where zz the vertical stress is inferred to be 1 thus the third principal stress axis being orthogonal to the other two axes it is oriented n s allowing extension to occur in that direction on the other hand in the sub andes zz is 3 and horizontal e w shortening occurs the state of stress in the andean continental crust above the 30 dipping slab appears to be different from that in the andes of central peru situated above the flat subducting segment in this region compressional deformantion affect a wider part of the cordillera

palaeostress determinations from fault kinematics application to the neotectonics of the himalayas tibet and the central andes this paper concerns the determination of the stress deviator from a set of striated faults or of focal mechanisms of earthquakes southern tibet and the cuzco region of the high andes illustrate the applied methodology then the spatial and temporal changes in the tectonic regimes of tibet and central andes are briefly examined the quaternary present day stress patterns in these two high plateaus favour models explaining the spatial changes in the state of stress at a large wavelength as a result of a compensated elevated topography major temporal changes in the tectonic regimes are attributed tentatively to uplift for the tibetan plateau and for the already elevated andean plateau to changes in the magnitude of the boundary forces due to subduction processes

the ancash peru earthquake of 1946 november 10 evidence for low angle normal faulting in the high andes of northern peru summary the ancash peru earthquake is the largest and best documented intracontinental normal fault earthquake to have occurred in historically recorded time along the andean margin of south america the fault geometry and the rupture processes of this earthquake provide information on how normal faulting develops in a predominantly compressional regime and on the hazards associated with normal faulting in other parts of the andes a re evaluation of first motion data intensity data and epicentre locations coupled with waveform modelling suggests that the earthquake occurred along a fault that dips 30 10 at a hypocentral depth of 15 17 km geological information also suggests that the earthquake reactivated an older thrust fault that was favourably orientated with respect to the present regional stress regime

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

paleoseismicity and neotectonics of the cordillera blanca fault zone northern peruvian andes the cordillera bianca fault zone is a major west dipping normal fault that bounds the west side of a 120 to 170 km wide zone of active extension along the crest of the northern peruvian andes the fault is approximately 210 km long and exhibits continuous geomorphic evidence of repeated late pleistocene and holocene displacements but has not been the source of historical or teleseismically recorded earthquakes trenching and mapping of fault scarps provide new information on earthquake recurrence slip rate timing of the most recent events and andean neotectonics at quebrada queroccocha 55 km from the south end of the zone an 11 000 to 14 000 year old moraine is displaced 12 15 m and younger valley fill lacustrine and fluvial deposits are displaced 7 5 8 m scarp profiles tectonic terraces and trench exposures indicate five to seven scarp forming earthquakes of 2 3 m per event during the past 11 000 14 000 years at this location considering uncertainties in the number of events and in the age of the displaced moraine the average recurrence interval is estimated to be 2440 1060 years detrital charcoal from colluviums that predate and postdate the most recent event yielded ages of 2480 65 and 750 80 14c years b p respectively and stratigraphic relationships suggest that the event may be closer to the older date the late quaternary slip rate at quebrada queroccocha is 0 86 1 36 mm yr and appears to be representative of much of the fault zone at pachma bajo located 30 km from the north end of the zone on a separate segment displacement during each of the past two events was 2 to 3 m the morphology of scarps in alluvial fan and debris flow deposits indicates recurrence intervals of about 1000 3000 years relationships between pre inca walls and faulted deposits suggest that 1500 2000 years have elapsed since the most recent event fault behavior parameters particularly displacement per event and fault segment length show that the cordillera bianca fault zone is similar to seismogenic normal faults that nucleate m 7 7 earthquakes at depths of approximately 15 km the quebrada queroccocha slip rate and analysis of other normal faults in the region suggest cumulative late quaternary extensional strain rates of about 1 4 mm yr across the complete zone of andean crestal extension

regional state of stress and characteristic fault kinematics instabilities shown by aftershock sequences the aftershock sequences of the 1978 thessaloniki greece and 1980 campania lucania italia earthquakes as examples abstract usual analysis of fault kinematics in terms of stress supposes that the slip occurs in the direction of the shear stress resolved on the fault plane due to an homogeneous state of stress however as a block moves this yields perturbations of the static stress conditions the resulting displacement may produce interactions of blocks and may induce complex fault motions such as those demonstrated by the aftershock sequences of the thessaloniki and campania lucania earthquakes each of which shows a main group of focal mechanisms whose normal fault motions are modelled using a stress deviator tm in agreement both with the motion of the main seismic fault and with the regional stress pattern deduced from quarternary and recent faults it is therefore considered as representative of the mean state of stress acting in the epicentral area but two other families of fault motions are also demonstrated one shows reverse motions explained by a compressional stress deviator tc the other involving normal motions is explained by a tensional stress deviator te these deviators are not representative of the mean regional state of stress the compressional 1 axis of tc and the tensional 3 axis of te are roughly orthogonal to the 3 axis of the mean state of stress tm these tc and te deviators may be deduced from the mean state of stress tm by adding respectively td1 and td2 deviators to this latter this means that on each fault plane having a reverse c or a normal e motion this motion may be deduced from the normal kinematics m it should have occurred in the mean regional state of stress tm by adding local pushes d1 and d2 respectively the deviator td2 being nearly axisymmetric around a vertical 3 axis the local reverse pushes d1 are nearly dip slip and td2 having a high angle dipping 2 axis the local pushes d2 are essentially strike slip approximated solutions of these td1 and td2 deviators have been calculated by taking tm td1 tc and tm td2 te the mean state of stress tm being divided into two parts tv and ts which respectively produce the dip slip and essentially strike slip components of the stress resolved on the fault plane a first approximation of the data shows that td1 kv tv and td2 ks ts with kv and ks equal to 2 this approximation supposes that tm tc and te are coaxial and that td1 and td2 are respectively axisymmetric around the vertical 3 axis and planar in the horizontal plane actually the data give kv 2 and ks in the order of 4 5 it is suggested that the local reverse and strike slip pushes are induced by the reaction of the rocky body surrounding small blocks moving as a consequence of a motion on a major fault particularly as there are incompatibilities of motion of adjacent small blocks however data are not sufficient to demonstrate the mechanical and geometric conditions which control the observed different behaviors of the small seismic faults numerical simulations or experiments could probably precise these conditions whatever under certain conditions the stable solution m is replaced by kinematics instabilities e and c however it is usually observed that faults studied in the field in a simple way often yield a single stress deviator this is probably due to the nature of the measured striations which are often growth fibers and oblique stylolithes resulting from creep fault motions in this case incompatibilities of block motions are probably accommodated by a pressure dissolution phenomena therefore it is suggested that the bifurcation parameter which controls the occurrence of the characteristic instabilities possibly depends on a ratio of the deformation rate versus the pressure dissolution rate of the material a question remains unanswered are these instabilities scale invariant or is there a changeover in the behavior between minor and major faults

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

rotation of the peruvian block from palaeomagnetic studies of the central andes palaeomagnetic studies were performed on the rocks of the central andes to test the hypothesis of oroclinal bending of the andes around the peru chile border natural remanent magnetization of mesozoic volcanic and sedimentary rocks of peru shows a counterclockwise shift of declination by several tens of degrees relative to the field directions of the same ages in the stable south american craton two mesozoic dyke swarms from arica region northernmost chile also indicate about 20 of declination shift in a counterclockwise sense these results suggest a considerable amount of counterclockwise rotation of large area north of arica that is oroclinal bending of the andes around some hinge near the peru chile border one neogene dyke swarm sampled in central peru also gave declination which deviates counterclockwisely by 15 if this declination shift is a result of the wide area tectonic rotation as suggested by the mesozoic rocks oroclinal bending appears to have continued until relatively recent times

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

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

the role of transcurrent intra arc tectonics in the configuration of a volcanic arc summary field investigations show that plio quaternary mexican and quaternary ecuadorian volcanic arcs are coeval with intra arc transcurrent or transtensional tectonics with motions along faults parallel or subparallel to the arc this relationship can also be observed in the quaternary kamchatka arc these arcs have an anomalously wide distribution of volcanic centres which cannot be explained simply by a low dip in the subducting lithosphere comparison with the intra arc tectonics and geometry of other quaternary arcs reveals that strike slip fault zones parallel to the arc favour magma rising in a broad surficial zone and may channel magma feeding mainly huge stratovolcanoes

recent state of stress change in the walker lane zone western basin and range province united states the nw to north trending walker lane zone wlz is located along the western boundary of the northern basin and range province with the sierra nevada this zone is distinguished from the surrounding basin and range province on the basis of irregular topography and evidence for both normal and strike slip holocene faulting inversion of slip vectors from active faults historic fault offsets and earthquake focal mechanisms indicate two distinct quaternary stress regimes within the wlz both of which are characterized by a consistent wnw 3 axis these are a normal faulting regime with a mean 3 axis of n85 9 w and a mean stress ratio r value r 2 1 3 1 of 0 63 0 74 and a younger strike slip faulting regime with a similar mean 3 axis n65 70 w and r values ranging between 0 1 and 0 2 this younger regime is compatible with historic fault offsets and earthquake focal mechanisms both the extensional and strike slip stress regimes reactivated inherited mesozoic and cenozoic structures and also produced new faults the present day strike slip stress regime has produced strike slip normal oblique slip and normal dip slip historic faulting previous workers have explained the complex interaction of active strike slip oblique and normal faulting in the wlz as a simple consequence of a single stress state with a consistent wnw 3 axis and transitional between strike slip and normal faulting maximum horizontal stress approximately equal to vertical stress or r 0 in both regimes with minor local fluctuations the slip data reported here support previous results from owens valley that suggest deformation within temporally distinct normal and strike slip faulting stress regimes with a roughly constant wnw trending 3 axis zoback 1989 a recent change from a normal faulting to a strike slip faulting stress regime is indicated by the crosscutting striae on faults in basalts 300 000 years old and is consistent with the dominantly strike slip earthquake focal mechanisms and the youngest striae observed on faults in plio quaternary deposits geologic control on the timing of the change is poor it is impossible to determine if there has been a single recent absolute change or if there is rather an alternating or cyclical variation in stress magnitudes our slip data in particular the cross cutting normal and strike slip striae on the same fault plane are inconsistent with postulated simple strain partitioning of deformation within a single regional stress field suggested for the wlz by wesnousky and jones 1994 the location of the wlz between the deep seated regional extension of the basin and range and the right lateral strike slip regional tectonics of the san andreas fault zone is probably responsible for the complex interaction of tectonic regimes in this transition zone in early to mid tertiary time the wlz appears to have had a similarly complex deformational history in this case as a back arc or intra arc region accommodating at least part of the right lateral component of oblique convergence as well as a component of extension

neogene and quaternary ignimbrites in the area of arequipa southern peru stratigraphical and petrological correlations abstract this study focuses on the correlation of four newly identified rhyolitic ignimbrites located around arequipa which were previously described as a single ignimbrite sheet 1 the rio chili ignimbrite ca 13 33 ma crops out in the rio chili canyon and consists of a cooling unit of non welded to partially welded massive crystal rich lapilli tuff this ignimbrite represents the base of the neogene ignimbrite succession in the arequipa area and may be stratigraphically equivalent to the 14 13 ma huaylillas ignimbrites in southern peru 2 the second voluminous 20 km3 and widespread la joya ignimbrite sheet ca 4 87 ma filled the arequipa depression its probable source is now buried underneath the chachani volcanic complex the la joya ignimbrite covers both the arequipa batholith toward the rio vitor piedmont and also part of the altiplano to the east and north of the chachani complex 3 the arequipa airport ignimbrite aai sheet ca 1 6 ma 18 km3 the third and latest infill of the arequipa basin consists of a lower white columnar jointed massive lapilli tuff indurated by vapor phase recrystallization and is overlain by an upper pink lithic rich non welded massive lapilli tuff the source of this ignimbrite is also buried underneath the chachani complex as indicated by anisotropy of magnetic susceptibility measurements and component lithology 4 the yura non welded pumice flow deposits ca 1 02 ma 1 5 km3 are restricted to the northern and western flanks of the chachani complex and may correlate with tuffs of the capillune formation on the altiplano all ignimbrites are high k calc alkaline to alkaline rhyolites which contain variable amounts of plagioclase biotite quartz sanidine and fe ti oxides the modal composition of mineral phases and chemistry of amphiboles and biotites are useful for distinguishing the ignimbrites specific geochemical fingerprints such as sr y rb ba cs ree and sr isotopes were effective discriminator elements magmatic differentiation crustal residence time and contamination and source melting processes are discussed in order to explain compositional variations the older la joya ignimbrite and rio chili ignimbrite reflect initial wetter crustal melts closer to granite composition whereas with time the younger arequipa airport ignimbrite and yura tuffs initial melts are slightly less evolved and dryer due to more extensive crustal melting the voluminous 40 km3 neogene ignimbrites of the arequipa area were produced by three major explosive volcanic pulses in the central volcanic zone of southern peru whose genetic relation with tectonic uplift and crustal thickening of the andean cordillera are still a matter of debate

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

thrust and extensional faulting under the chilean coast 1965 1971 aconcagua earthquakes summary earthquakes under coastal northern chile from 18 s to 33 s occur at two very different depths the shallower interplate events have the typical thrust mechanisms associated with subduction of the nazca plate under south america the deeper close to 70 km events have normal fault mechanisms with a tension axis approximately parallel to the dip of the downgoing slab we study in detail a couple of ms 7 5 events that took place about 60 km north of valparaiso chile the 1971 july 9 event was a shallow thrust interplate event at 40 km depth while the 1965 march 28 earthquake had a normal fault mechanism and a depth of 72 km we carry out a detailed analysis of long period p and surface waves observed in the wwssn network of stations in order to improve fault plane solutions and depth determinations for the 1971 event we find a seismic moment of 5 6 1027 dyne cm from both p and surface waves the source area for this event is relatively small compared to that of other underthrust events with similar moments assuming a circular fault shape and a rigidity of 7 1011 dyne cm 2 we find a stress drop of 38 bar the 1965 event is more complex seismic moments of 1 8 and 1 0 1027 dyne cm are found from surface and body waves respectively the stress drop estimated from p waves and a circular fault model is close to 91 bar the existence of earthquakes at two different depths near the chilean coast may be interpreted in terms of a double layered benioff zone as has been found in honshu kuriles or central peru unfortunately the hypo central determinations of events in this region are not precise enough to separate the two layers an alternative method to demonstrate the existence of a double benioff zone is to look for compressional events associated with the upper layer of the down going slab a search among available fault plane solutions is ambiguous three shallow coastal events are found in stauder s catalogue with horizontal pressure axes normal to the trench but they might be intra continental plate events

plate boundary forces at subduction zones and trench arc compression abstract viscoelastic finite element modelling has been used to study the state of stress in the overriding and subducting plates meeting at a subduction zone the subduction fault is included using the dual node technique it is demonstrated that substantial horizontal deviatoric compressive stress occurs in the trench arc region as a result of the downpull of the dense slab and the associated surface depression including the trench and other downflexing of the plates this may be masked at the trench by bending stress it is the lack of significant shearing stress along an unlocked subduction fault in the presence of this compressive stress that gives rise to the slab pull and trench suction plate boundary forces slab pull and trench suction were found to be of comparable magnitude within the range 1 0 to 4 0 10 12 n m in models studied with vertical subduction and there are indications that this may also apply when the slab dips at 45 as a result of viscous flow induced by rollback when the slab dips beneath the arc backarc region it is shown that horizontal deviatoric compression can occur in this region contemporaneous with plate interior tension produced by trench suction this suggests that backarc tension associated with marianas type trench arch systems may be related to the nearly vertical slab whereas backarc compression in the chilean type may result from the low dip and small downpull of the slab it is also shown that successive locking and unlocking of the subduction fault may give rise to large variations of stress in plate interiors

on crustal seismicity of the amazonian foothill of the central peruvian andes a microearthquake survey of crustal seismicity conducted in 1985 on the eastern cordillera and the amazonian foothills in central peru gives a description of the present tectonic activity related to the uplift of the andes hypocenters on the huaytapallana fault are shallower than 10 km and their focal mechanism is in agreement with the fault trace corresponding to the earthquakes of 1969 and suggests a reverse strike slip movement on a plane striking n130 an active tectonic zone is evidenced along a nw se direction passing through the amauta subandean region the vertical distribution of hypocenters and the comparison of shallow and deep focal mechanisms suggest a reverse fault dipping to the west depths vary between 0 and 20 km and never exceed 32 km the huaytapallana cordillera is uplifted by thrust faults on both sides

the configuration of the seismic zone and the downgoing slab in southern peru using data from temporary networks of portable seismographs in southern peru we located 888 shallow and intermediate depth events near a proposed discontinuity in the seismic zone there these events reveal a prominent contortion instead of a discontinuity that trends approximately n80 e parallel to the direction of relative plate motion north of about 15 s the seismic zone beneath peru is nearly horizontal but south of about 15 5 s it dips at about 25 volcanoes lie above the more steeply dipping zone where earthquakes occur between 120 and 140 km and the volcanic line in southern peru stops abruptly at the contortion

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

cenozoic plate kinematics subduction and magmatism south american andes revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the andean subduction zone these include age of subducted plate convergence rate and interaction with hotspot traces from 50 to 25 ma in the andes the age of oceanic plate subducted progressively decreased apparently resulting in a9gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity a major reorganization of plate motions at 25 ma resulted in an increased convergence rate normal to the chilean andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of bolivia and argentina convergence rates changed only slightly across the peruvian margin and no clear effect on subsequent magmatism is indicated interaction of the andean subduction zone with the juan fernandez and easter nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks the current gaps in igneous activity correspond with contemporary subduction of the two traces and this association apparently occurred in the past as well

quaternary extension in southern tibet field observations and tectonic implications we summarize evidence for quaternary and active faulting collected in the field during three sino french expeditions to southeastern tibet 1980 1982 detailed mapping of quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the himalayas in the last 2 0 5 m y the maximum horizontal principal stress in south tibet appears to be only the intermediate principal stress 2 1 being vertical south of the chord joining the eastern and western syntaxes of the himalayan arc extensional strains are principally localized within seven regularly spaced rift zones three of which have been studied in some detail the extension direction is determined to be n96 e 7 mainly from statistical averaging of strikes of newly formed normal faults throw rates on normal faults are evaluated for different time spans 2 0 5 m y 60 40 kyr and 10 2 kyr b p using structural and topographic reliefs as well as synglacial and postglacial vertical offsets the rate of quaternary extension is about 1 m y 1 along an 1100 km long ese traverse across south tibet this corresponds to a spreading rate of 1 0 6 cm yr this rate and the divergent horizontal projections of slip vectors of earthquakes along the himalayan front constrain the rate at which rigid india underthrusts southern tibet to be 2 1 cm yr although most of the normal faults appear to be independent of and nearly orthogonal to the e w mesozoic tertiary tectonic fabric the yadong gulu rift appears to be guided for over 130 km by the older oblique ne sw nyainqentanglha range and fault zones along it this reactivated zone is the most prominent left lateral strike slip fault system in se tibet excepting this zone and the vicinity of the se extremity of the karakorum fault quaternary strike slip faulting is rare in south tibet i e south of the chord between the syntaxes of the himalayan arc north of the chord the tectonic style is different there minor conjugate strike slip faulting is widespread and appears to control quaternary normal faulting which is more diffuse and subdued than in south tibet along the chord the presence of a major zone of active right lateral en echelon strike slip faults karakorum jiali fault zone probably reflects the greater facility of eastward extrusion in north central tibet in response to the northward push of india the eastern himalayan syntaxis may be an obstacle to such extrusion movements south of the chord

cause and effect among thrust and normal faulting anatectic melting and exhumation in the himalaya the transition from compression to extension in a regime of overall convergence in a narrow mountain belt requires a change with time in at least one of the following horizontal normal stress applied to the belt by its surroundings the distribution of shear traction on the base of the belt the constitutive relation between stress and strain rate of material in the belt itself the surface height of the belt the following processes could have caused one or other of these changes in the himalaya in miocene time and thus could have triggered the onset of extension by normal faulting on planes parallel to thrust faults that were active at the same time the shear stress on the thrust fault at the base of the himalaya could have decreased for instance because of melting on the fault plane alternatively the creation of a new deeper thrust fault and hence a new locus of convergence could have initiated extensional faulting if the distribution of shear stress on the new thrust fault were unable to support the himalaya above it the himalaya could have been weakened by an increase in pore fluid pressure for instance owing to metamorphic dehydration reactions within the wedge the himalaya could have been elevated for instance following the detachment of a tongue of mantle lithosphere from beneath it or by the underthrusting of a lens of material beneath its base the apparent simultaneity of thrust and normal faulting on nearly parallel planes of anatexis and of exhumation in the himalaya in miocene time can be explained if normal faulting was responsible for most of the exhumation and through decompression melting for anatexis existing data suggest that the most probable cause for miocene normal faulting was either uplift of the himalaya or the cessation of slip on the main central thrust at the base of the orogenic wedge and the creation of a new deeper thrust fault and a new base to the deforming himalaya 1993 the geological society

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

comment on tectonic erosion of the peruvian forearc lima basin by subduction and nazca ridge collision by p d clift et al

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

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

tectonic implications of the microearthquake seismicity and fault plane solutions in southern peru using recordings from 15 portable instruments and six permanent stations operated during two field investigations of microearthquakes in southern peru we determined locations of 888 shallow and intermediate depth earthquakes and 56 fault plane solutions a series of tests evaluating the effects of elevation corrections erroneous velocity structure and random errors in arrival times on calculated locations suggest that the locations of most of the events are uncertain by less than 7 km at shallow depths 60 km the seismicity is concentrated in three belts one offshore one beneath the coast and one beneath the western cordillera there is also scattered but infrequent shallow activity near the altiplano the belts offshore and beneath the coast are apparently associated with slip along the nazca south american plate boundary although the style of deformation is complicated one can infer that beneath the coast the active boundary between the nazca and south american plates is within the mantle rather than at the base of the south american crust the belt beneath the western cordillera is about 30 km in width and dips southwest from near the surface to a depth of 60 km fault plane solutions suggest that crustal shortening is taking place in a northwest southeast direction in this region the significance of this activity is difficult to explain but these earthquakes provide no clear evidence of tectonic erosion in this area a composite fault plane solution from the few events near the altiplano shows normal faulting which is typical of the active tectonics of that region the results of the investigation of intermediate depth earthquakes shows that the downgoing slab in southern peru is warped and not torn in the region where the dip changes from nearly horizontal to a dip of about 30 they also suggest that the axis of the contortion is not perpendicular to the coast but parallel to the direction of plate motion moreover the volcanic line ends at the contortion which suggests that a relatively thick wedge of aesthenospheric material is necessary for the formation of subduction related volcanism fault plane solutions for events between 60 and 100 km depth show a large scatter but some suggest a detachment of a sliver of the downgoing slab solutions for deeper events are more consistent with one another with the t axes for most events being nearly horizontal and generally normal to the projections of the seismic zone

relating rapid plate motion variations to plate boundary forces in global coupled models of the mantle lithosphere system effects of topography and friction abstract recent high resolution models of past plate motions and their comparison with plate motion models inferred from space geodetic techniques reveal a number of short term variations in global plate velocities over the past 10 myrs such variations serve as powerful probe into the nature and magnitude of plate boundary forces because they are unlikely to originate from changes in mantle buoyancy forces which evolve on longer time scales here we explore the constraints of the velocity record using a novel coupled modeling approach of global neo tectonic simulations combined with realistic plate driving forces obtained from mantle circulation models mcms to arrive at simple global budgets of mantle lithosphere and plate boundary forces we focus on three plate boundary systems along the nazca south america plate margin the aleutian trench and the india australia plate boundary to show that gravitational spreading from high topography in the andes and tibet contributes substantially to the global plate tectonic force balance and that this contribution is sufficient to explain some 35 of recent velocity changes over the earth s surface including among others the observed 30 convergence reduction between the nazca south america plates our models make a number of specific predictions such as significant lateral variations in plate coupling forces along a given margin revealed by trench parallel gravity and bathymetry anomalies and the occurrence of large earthquakes as well as differences by as much as a factor of five from margin to margin they also support the notion of a relatively young plate boundary separating the india and australia plates which has been previously suggested based on independent observations importantly we find that the modeled nazca south america convergence reduction explains recent spreading rate variations in the south atlantic and south pacific which points to the importance of far field effects on the adjacent continents in explaining the spreading record of oceanic basins our numerical results demonstrate a that detailed budgets of forces acting upon plates can be obtained and b support the notion of strong forcing along weak plate boundaries

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

evidence for strain rate variation and an elevated transient geothermal gradient during shear zone evolution in the cordillera blanca peru abstract the cordillera blanca shear zone cbsz occupies the footwall of the cordillera blanca detachment fault an active low moderate angle detachment fault extending for 200 km along the western margin of the cordillera blanca in central peru recrystallized quartz paleopiezometry two feldspar thermometry on asymmetric strain induced myrmekite and structural analysis from the cbsz provide evidence for shear zone evolution within a transient elevated geotherm during syn convergent extension shear zone width varies by a factor of six along strike with widths of up to 400 m within the central segment of the shear zone microstructurally derived strain rates calculated from quartz paleopiezometry and two feldspar thermometry results are on the order of 10 14 to 10 12 s 1 and are broadly consistent with macrostructurally derived strain rates of 10 14 to 10 13 s 1 calculated from shear zone thickness and previously determined exhumation rates within the context of existing thermochronological data constraints on batholith emplacement and previously determined offset along the cordillera blanca detachment our data suggest that ductile deformation in the cbsz occurred within a transient elevated geothermal gradient of at least 50 c km and potentially as high as 100 c km which deviated from a 30 c km gradient for an unspecified duration of time we discuss the evolution of the cordillera blanca where observed deformation fabrics are related to strain localization along an exhuming fault as well as thermal weakening from the interplay between faulting and magmatism

soil liquefaction during the arequipa mw 8 4 june 23 2001 earthquake southern coastal peru abstract the arequipa june 23 2001 earthquake with a moment magnitude of mw 8 4 struck southern peru northern chile and western bolivia this shallow 29 km deep interplate event occurring in the coupled zone of the nazca subduction next to the southeast of the subducting nazca ridge triggered very localized but widely outspread soil liquefaction although sand blows and lateral spreading of river banks and road bridge abutments were observed 390 km away from the epicenter in the southeast direction nearing the town of tacna close to the chile border liquefaction features were only observed in major river valleys and delta and coastal plains in the meizoseismal area this was strongly controlled by the aridity along the coastal strip of southern peru from the sand blow distribution along the coastal area a first relationship of isolated sand blow diameter versus epicentral distance for a single event is ever proposed the most significant outcome from this liquefaction field reconnaissance is that energy propagation during the main june 23 2001 event is further supported by the distribution and size of the isolated sand blows in the meizoseismal area the sand blows are larger to the southeast of the epicenter than its northwestern equivalents this can be stated in other words as well the area affected by liquefaction to the northwest is less spread out than to the southeast implications of these results in future paleoliquefaction investigations for earthquake magnitude and epicentral determinations are extremely important in cases of highly asymmetrical distribution of liquefaction features such as this one where rupture propagation tends to be mono directional it can be reliably determined an epicentral distance between earthquake and liquefaction evidence and an earthquake magnitude only if the largest sand blow is found therefore magnitude estimation using this uneven liquefaction occurrence will surely lead to underrating if only the shortest side of the meizoseismal area is unluckily studied which can eventually be the only part exhibiting liquefaction evidence depending on the earthquake location and the distribution of liquefaction prone environments

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

active detachment faulting above the peruvian flat slab the cordillera blanca detachment fault in the peruvian andes is to our knowledge the first active detachment to be documented above a modern flat slab crustal detachment has unroofed the ca 8 ma cordillera blanca batholith now the backbone of the highest mountain range in peru large magnitude slip along the fault was thermally enhanced by emplacement of the batholith the penultimate magmatic event prior to flattening of the nazca slab however extensional models based on arc magmatism and crustal thickening alone do not adequately explain the scale or structural asymmetry of a series of young deep seated west dipping normal faults across peru here we show that the onset of detachment faulting coincided with subduction of the aseismic nazca ridge and consequent flattening of the nazca slab we propose that slab buoyancy from ridge subduction triggered extensional collapse of the prethickened continental crust and that this buoyancy drove footwall uplift that exceeds basin subsidence the west dipping asymmetry of late cenozoic extensional faults in peru may be controlled by a preexisting crustal anisotropy older thrusts and or formation of riedel like shears kinematically linked to the flat nazca slab

quaternary state of stress in the northern andes and the restraining bend model for the ecuadorian andes inversion of shallow focal mechanisms in the northern andes together with a neotectonic analysis of the ecuadorian andes shows that the state of stress is homogeneous in most of the northern andes e w trending 1 however north of 5 n 1 is roughly nw se trending this difference in state of stress is due to the force balance between the nazca caribbean and south american plates south of 5 n the state of stress appears to be mainly controlled by the nazca south american plate interaction while north of 5 n it appears to be mainly controlled by the caribbean south american plate interaction this state of stress difference from south to north is consistent with dextral motions along faults trending parallel to the chain south of 5 n and sinistral motions along faults trending parallel to the chain north of 5 n this inversion of shallow focal mechanisms also illustrates the incomplete strain partitioning of the oblique convergence between the nazca and south american plates along the ecuadorian southern colombian trench the convergence obliquity that increases northward controls the active transcurrent fault slip rate in the upper plate finally this convergence obliquity is responsible for the location of the seismicity in the upper plate in addition the andean block is considered in this paper as a large triple zone under constriction with one of its borders caribbean behaving partly as a free border

paleomagnetism of the puente piedra formation central peru abstract paleomagnetic samples were collected from 15 sites in the early cretaceous puente piedra formation near lima peru this formation consists of interbedded volcanic flows and marine sediments and represents the oldest known rocks of the andean coastal province in this region the puente piedra formation is interpreted as a submarine volcanic arc assemblage which along with an overlying sequence of early cretaceous clastic and carbonate rocks represents a terrane whose paleogeographic relationship with respect to the peruvian miogeocline in pre albian time is unknown moderate to high coercivities blocking temperatures below 320 c and diagnostic strong field thermomagnetic behavior indicate that pyrrhotite is the dominant magnetic phase in the puente piedra formation this pyrrhotite carries a stable crm acquired during an event of copper mineralization associated with the intrusion of the santa rosa super unit of the coastal batholith at about 90 5 m y b p the tectonically uncorrected formation mean direction of d 343 2 i 28 6 95 3 4 is statistically concordant in inclination but discordant in declination with respect to the expected direction calculated from the 90 m y reference pole for cratonic south america the observed declination indicates approximately 20 of counterclockwise rotation of the puente piedra rocks since about 90 m y this is consistent with other paleomagnetic data from a larger crustal block which may indicate modest counterclockwise rotation during the cenozoic associated with crustal shortening and thickening in the region of the peru chile deflection

stress variability and brittle tectonic structures the problem of stress variability both in space and time during brittle deformation of rocks is faced in this note several possible causes of the temporal and spatial variations of the stress tensor are reviewed and discussed a genetic partitioning of the stress tensor is proposed showing that any stress field within a crustal volume can be seen as the sum of relatively simple stress tensors associated to specific genetic components the genetic components discussed in this note are the gravitational the tectonic the fluid the thermal and the diagenetic one each genetic component is a function of several parameters but for all of them the possible important role played by the variable time is emphasised according to numerous researches based on theoretical and experimental grounds brittle deformation in a rock mass occurs when some critical conditions are reached starting from initial equilibrium conditions and leading to even different stress conditions as a consequence of failure field examples are presented and data from literature are discussed to exhibit evidences from complex tectonic structures and to argue about small and large scale spatial variations as well as about short and long term temporal variations of the stress trajectories and of the stress magnitudes that continuously occur during a brittle deformational event the relationships between local and temporal stress variations and what is commonly defined the regional stress field are discussed adding recommendations for future researches

geological constraints on the kinematics and fault plane solution of the quiches fault zone reactivated during the 10 november 1946 ancash earthquake northern peru the ancash earthquake of 10 november 1946 mw 6 8 produced surface faulting along the quiches normal slip fault field studies reported discontinuous nw se to nnw sse striking seismic scarps that displace quaternary glacial moraines and outwash as well as mesozoic sedimentary rocks structural analyses show that the 1946 normal displacement was accompanied by a small left lateral slip component morphological analysis of the fault scarp and observations in a trench excavated across the scarp show evidence of a single normal fault displacement before the 1946 seismic event that offset late pleistocene glacial moraines this indicates an approximate recurrence interval of more than 13 000 1 000 years for the major events suggesting a rate of net vertical displacement on the fault of less than 0 25 mm yr kinematic analysis along the quiches fault in comparison with other regional studies suggests that the state of stress in the high andes is regionally and locally homogeneous with only minor changes in the stress directions adjacent to major structures such as the maranon thrust and fold belt the minimum horizontal stress 3 axis is oriented n10 25 e the intermediate horizontal stress 2 axis is oriented n100 115 e and the maximum stress 1 axis is vertical we suggest that the 60 sw dipping quiches surface fault plane is connected at depth to a low angle dipping fault plane this seismic fault is a reactivated thrust fault of the tertiary maranon thrust and fold belt

joint statistical determination of fault plane parameters abstract joint treatment of first motion data for sets of earthquakes has great advantages over determinations of individual solutions an algorithm is used to find maximum likelihood estimates of the orientations of the p and t axes and of the fault planes and their variances together with two precision parameters i that weights each event according to its agreement with the group solution and j that weights each station according to its performance the method has been applied to two sets of earthquakes having different characteristics the first is formed by 19 earthquakes in bear valley ca with magnitudes of 3 and recording distances less than 200 km and the second by 29 microearthquakes with magnitudes of 1 5 and recorded at distances less than 60 km in the pyrenees the data from bear valley form a single homogeneous group having a mechanism related to motion on the san andreas fault the data from the pyrenees are divided into five groups having different mechanisms

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

geologic framework of the veladero high sulfidation epithermal deposit area cordillera frontal argentina located in the northern part of the el indio belt of chile argentina the 12 6 million ounce moz gold late miocene veladero high sulfidation epithermal deposit is hosted principally in late oligocene and middle miocene volcanic and volcaniclastic rocks and to a lesser extent in the underlying permian rhyolitic pyroclastic rocks the principal host rocks are poorly bedded volcaniclastic breccia and minor air fall tuff of the middle miocene cerro de las tortolas formation which unconformably overlies deformed and structurally interleaved late oligocene and late paleozoic rocks these latter rocks form a steeply dipping fold and thrust belt that was bevelled by the regional miocene frontera deidad paleogeomorphic surface the cerro de las tortolas formation deposited across the frontera deidad surface represents at veladero an 16 ma vent complex characterized by domes and their coeval subvolcanic equivalents infiernillo intrusive unit surrounded by coarse poorly sorted volcaniclastic breccia that grades eastward onto a volcaniclastic apron of bedded and reworked detritus with interbedded air fall tuff and local lacustrine tuffaceous sandstone and siltstone gold at veladero is hosted in a tabular north northwest elongated silicified horizon that cuts across the host stratigraphy and has a subhorizontal top that ranges between 4 200 and 4 400 m above sea level the silicified horizon lies at the approximate elevation to slightly beneath the 14 5 to 12 ma azufreras torta surface which controlled formation of the economic el indio tambo and pascua lama high sulfidation gold deposits elsewhere in the el indio belt published late miocene 40ar 39ar ages between 11 and 10 ma on alunite from the filo federico zone at veladero and from uneconomic prospects nearby indicate that advanced argillic alteration and likely gold deposition is 5 m y younger than the 16 ma host rocks the veladero deposit formed after a change in the local tectonic environment from one of shortening that preceded deposition of the host volcaniclastic sequence to one of extension localized within the vicinity of veladero that episodically disrupted the host volcanic sequences reactivated the steeply dipping thrust faults and displaced the frontera deidad and azufreras torta paleogeomorphic surfaces regional tectonic relationships suggest that this transition did not reflect a fundamental change in the overall shortening that dominates the cenozoic of the chilean argentinean andes instead local extension around veladero resulted from the inherent gravitational instability created by topography in excess of 4 000 m above sea level

magma emplacement during arc perpendicular shortening an example from the cascades crystalline core washington it is increasingly inferred by others that active arcs at all crustal levels are undergoing arc perpendicular extension and that magma utilizes this extension or extension along arc parallel strike slip faults during emplacement however a review of stress fields in modern arcs and deformation which occurred during magmatism in ancient arcs and our own emplacement studies of individual plutons indicate that magma emplacement typically occurred during arc perpendicular shortening without the aid of strike slip faulting this conclusion raises the question of how host rock is displaced during magma emplacement in shortening and thickening orogens evaluation of host rock material transfer processes that operated during emplacement of the 93 ma 10 to 14 km deep mount stuart batholith washington provides one example and includes the following 1 ductile downward flow in a narrow 0 2 body radii structural aureole 2 stoping and of lesser importance 3 regional folding thrusting and subhorizontal arc parallel extension we see no evidence that arc perpendicular extension or local extension along strike slip faults made space for this pluton nor for most other plutons in contractional arcs instead we argue that both localized and rapid around plutons and regional and slower downward flow during growth of crustal roots is a common process in such arcs

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

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

slope instabilities at an active continental margin large scale polyphase submarine slides along the northern peruvian margin between 5 s and 6 s a mega submarine slide was evidenced along the peruvian margin during a seabeam survey of the r v charcot bourgois et al 1986 the debris slide was initially interpreted as the result of the slope failure occurring along a seaward curved scarp and extending from east to west along the line of maximum slope complementary bathymetrie data obtained from the r v sonne survey conducted to the north and to the south of the previous surveyed area has resulted in the identification of large scale polyphase submarine slides involving a total surface of about 1000 km2 between latitudes 5 15 s and 6 05 s using seabeam and hydrosweep multibeam echosounder data in combination with deep sea submersible observations three distinct slope failure types related to three main stages have been revealed the three sliding phases occurred roughly along the same trend orientated n230 and are therefore mainly controlled by the n80 orientation of the subducting nazca plate 1 the first phase of slope failure is documented by a debris avalanche deposit which extends from the lower slope down to 5 km within the trench floor the deposit originates from the northern wall of a wide valley located along the upper slope 2 the second phase of slope failure is characterized by a debris avalanche with a crescent shaped scar located along the middle slope and a hummocky deposit covering the lower slope and extending up to 10 km across the trench the volume of rock involved in this event is estimated to some 250 km3 the slope failure is assumed to be related to an oversteepening of the middle slope induced by a rollover deformation 3 the third phase of slope failure corresponds to a translational sliding block and a toppling block with volumes of 6 km3 and 13 km3 respectively the seismic energy produced during the seismic cycle has greatly increased facturation and fluids buildup along the area previously weakened by a rollover fold a restricted n s folding is observed in the vicinity of the trench to the north of the two debris avalanche deposits it may have formed in relation to the local compression limited to the subduction of the nazca plate sliding and folding thus document the paradox between the compressive regime in the lower plate and the extensional regime in the upper part of the upper plate

analyses of the stress field in southeastern france from earthquake focal mechanisms summary owing to the apparent deformation field heterogeneity the stress regimes around the provence block from the fronts of the massif central and alpine range up to the ligurian sea have not been well defined to improve the understanding of the se france stress field we determine new earthquake focal mechanisms and compute the presentday stress states by inversion of the 89 available focal mechanisms around the provence domain including 17 new ones calculated in the current study this study provides evidence of six distinct deformation domains around the provence block with different tectonic regimes on a regional scale we identify three zones characterized by significantly different stress regimes a western one affected by an extensional stress normal faulting regime a southeastern one characterized by a compressional stress reverse to strike slip faulting regime with nnw to wnw trending s1 and a northeastern one namely the digne nappe front marked by a ne trending compression note that the digne nappe back domain is controlled by an extensional regime that is deforming the western alpine core this extensional regime could be a response to buoyancy forces related to the alpine high topography the stress regimes in the southeast of the argentera massif and around the durance fault are consistent with a coherent nnw trending s1 implying a left lateral component of the active reverse oblique slip of the moyenne durance fault in the rhone valley an e trending extension characterizes the tectonic regime implying a normal component of the present day no mes fault displacement this study provides evidence for short scale variation of the stress states which arises from abrupt changes in the boundary force influences on upper crustal fragments blocks these spatial stress changes around the provence block result from the coeval influence of forces applied at both its extremities namely in the northeast the alpine front push and in the southeast the northward african plate drift in addition to these boundary forces the mantle plume under the massif central influences the western block boundary

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

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

using the size and position of drumlins to understand how they grow interact and evolve drumlins are subglacial bedforms streamlined in the direction of ice flow common in deglaciated landscapes they have been widely studied providing rich information on their internal geology size shape and spacing in contrast with bedform investigations elsewhere in geomorphology aeolian and fluvial dunes and ripples for example most drumlin studies derive observations from relict and thus static features this has made it difficult to gain information and insights about their evolution over time which likely hampers our understanding of the process es of drumlin formation here we take a morphological approach studying drumlin size and spacing metrics unlike previous studies which have focussed on databases derived from entire ice sheet beds we adopt a space for time substitution approach using individual drumlin flow sets distributed in space as proxies for different development times periods framed and assisted by insights from aeolian and fluvial geomorphology we use our metric data to explore possible scenarios of drumlin growth evolution and interaction we study the metrics of the size and spacing of 36 222 drumlins distributed amongst 71 flow sets left behind by the former british irish ice sheet and ask whether behaviour common to other bedform phenomena can be derived through statistical analysis through characterising and analysing the shape of the probability distribution functions of size and spacing metrics for each flow set we argue that drumlins grow and potentially migrate as they evolve leading to pattern coarsening furthermore our findings add support to the notion that no upper limit to drumlin size exists and to the idea that perpetual coarsening could occur if given sufficient time we propose that the framework of process and patterning commonly applied to non glacial bedforms is potentially powerful for understanding drumlin formation and for deciphering glacial landscapes

coseismic and postseismic deformation of the 2016 mw 6 2 lampa earthquake southern peru constrained by interferometric synthetic aperture radar

the regional intraplate stress field in south america a compilation of lithospheric stress directions for continental south america and the inferred major patterns of the regional intraplate stress field are presented stress orientations are based primarily on earthquake focal mechanisms and quaternary fault slip inversion published in the literature four new focal mechanisms based on short period p wave modeling and selected centroid moment tensor solutions published by u s geological survey consistent with p wave first motions at south american and other world wide standard seismograph network stations are also included in the data base the observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the south american plate in the andean plateau altitudes greater than 3000 m n s extensional stresses predominate e w compressional stresses are observed in the sub andean and platform regions extending up to about 1000 km east of the andes the maximum horizontal stress shmax is uniformly oriented in the e w direction throughout western south america averages of the shmax direction at grid points spaced 2 5 o were taken as representing the regional field the e w direction of this regional field is not affected by the change of strike of the andean chain nor by the contact of a flat subducted slab underneath the regional shmax direction is oriented 15 o more clockwise than the direction of the nazca plate convergence the difference between the regional shmax orientations and the absolute plate motion may be only about 6 o this may indicate that contact forces with the nazca plate may not be the only major contributor to the intraplate stresses in western south america the eastern limit of this andean stress province seems to coincide with aseismic regions in the upper amazon basin and in the paraml basin in the central amazonian region seismicity and stress data suggest a seismic province with n s compressional stresses the origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle amazon basin in northeastern brazil seismicity is characterized by upper crustal strike slip earthquakes bordering the potiguar marginal basin a model is proposed for this region in which the stress field is the result of a superposition of regional e w compressional stresses and local extensional stresses oriented perpendicular to the continental margin possibly related to density contrasts and sediment loading in the continental shelf

multiple pulses of neogene compressive deformation in the ayacucho intermontane basin andes of central peru strata of the neogene ayacucho intermontane basin central peru can be subdivided into three tectonostratigraphic units the larampuquio volcanics consisting largely of intermediate lavas and coarse conglomerates of similar lithology the unconformably overlying huanta formation consisting of a lower part formed predominantly of lacustrine and volcaniclastic sedimentary rocks and an upper part composed largely of intermediate lavas tuffs and conglomerates and the unconformably overlying ayacucho formation consisting of complexly interfingering volcaniclastic sedimentary rocks silicic tuffs and lavas of intermediate to silicic composition radiometric ages of 17 3 0 2 m y and 18 3 0 6 m y have been obtained on tuffs from the upper and lower parts of the larampuquio volcanics determinations on 2 rocks from the lower member of the huanta formation indicate an age of 11 4 0 5 m y and a rock from the upper member is dated at 9 3 0 3 m y b p units of the ayacucho formation yield ages showing that it was deposited between 7 7 and 6 m y ago the intervening molinoyocc and puchcas volcanics were erupted between 9 9 and 7 4 m y ago these units may postdate the huanta formation or may be associated in part with the huanta and in part with the ayacucho formation coarse conglomerates of the larampuquio volcanics represent debris eroded from highlands uplifted during the first and most intense pulse of the late cenozoic quechuan phase of andean compressive tectonism that began in the early miocene folding of the larampuquio volcanics may have taken place during this or a subsequent pulse of compression a marked unconformity between the huanta and ayacucho formations demonstrates a period of deformation between 8 5 and 9 5 m y ago compression also was active during at least part of the deposition of the ayacucho formation as shown by inter fingering wedges of coarse clastic debris along the northeastern margin of the basin and by angular unconformities within the formation folded rocks of the ayacucho formation and its equivalents to the west are overlain by undeformed volcanic units dated at 3 8 0 4 m y b p and 5 5 m y b p bracketing a well defined pulse of deformation slightly younger than 6 m y these subsequent episodes of compressive deformation represent recurrent pulses of the quechuan phase folding related to these late quechuan pulses is restricted in the andes to northwest trending belts corresponding to reactivated fault zones in the pre neogene substratum a system of north trending dikes in the huachocolpa district 70 km west of ayacucho suggests relaxation of compression in the pliocene and compression of quaternary age has been recognized in the huancayo area 170 km northwest of ayacucho taken together with the first pulse of quechuan deformation in the early and middle miocene these data indicate at least four pulses of crustal compression during the late cenozoic separated by periods during which compressional stresses were at least partly relaxed and at times possibly replaced by extension

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

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

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